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Opportunities in Chemistry (1985)

Chapter: III. Control of Chemical Reactions

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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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Suggested Citation:"III. Control of Chemical Reactions." National Research Council. 1985. Opportunities in Chemistry. Washington, DC: The National Academies Press. doi: 10.17226/606.
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CHAPTER III Control of Chemical Reactions IlI-A. New Processes A prime reason for wishing to understand and control chemical reactions is so that we can convert abundant substances into useful substances. When this can be done on an economically significant scale, the reaction (or sequence of reactions) is called a chemical process. A significant number of employed chemists are engaged in perfecting existing chemical processes and developing new ones. Their past success is attested by the vitality and strength of the U.S. chemical industry. It has manufacturers' shipments total- ling $175 billion (see Table A-2), a $12 billion positive international balance of trade (in both 1980 and 1981, see Table A-3) and more than a million employees. The industry makes billions of pounds of organic chemicals at Tow cost, in high yield, and with minimum waste products. For example, we produce 9.S billion pounds of syn- thetic fibers (such as polyesters), 28 billion L1J pounds of plastics (such as polyethylene), and 4.4 <' $20B billion pounds of synthetic rubber. Continued competitiveness in this multifaceted industry depends upon readiness to improve ex- isting processes and to introduce new ones. Our current position of world leadership can be attrib- uted to our strength in the field of chemical catalysis. The major role of industrial catalysis is signalled by estimates that 20 percent of the gross national product is generated through the use of catalytic processes that assist in satisfying such diverse societal needs as food production, energy conversion, defense technologies, environmental protection, $200 B IOOB E IOB LL IIJ O C' J -BOB ~CD ~ . 1 . _ ~m J 1~; Lid = 1 CHEMICALS: SECOND LARGEST POSITIVE TRADE BALANCE 21

22 CONTROL OF CHEMICAL REACTIONS and health care. On the horizon, the extensive use of catalysts will tap new energy sources (the subject of Section IlI-B). Our ability to remain in the forefront of the research frontiers of chemical catalysis will figure strongly in the health of our chemical industry and, hence, in the buoyancy of the U.S. economic condition. A catalyst accelerates chemical reactions without being consumed. Such accelerations can be as much as 10 orders of magnitude. A selective catalyst can have the same dramatic erect but on only one of many competing reactions. A stereoselective catalyst not only controls the product composition, it also favors a particular molecular shape, often with remarkable effects on the physical properties (such as tensile strength, stiffness, or plasticityJ and, for biologically active substances, on the potency. Catalysis can be subdivided according to the physical and chemical nature of the catalytic substance. · In heterogeneous catalysis, the catalyzed reaction occurs at the interface between a metal, metal oxide, or other solid and either a gaseous or liquid mixture of the reactants. · In homogeneous catalysis, reaction occurs in either gas or liquid phase in which both catalyst and reactants are dissolved. · In electrocatalysis, reaction occurs at an electrode surface in contact with a solution but assisted by a flow of current. Electrocatalysis includes the advan- tages of catalytic rate control, including specificity, and adds the opportunity to inject or extract electrical energy. · In photocatalysis, reaction may take place at an interface (including electrode surfaces) or in homogeneous solution, but in these reactions energy encouragement is provided by absorbed light. · In enzyme catalysis, some characteristics of both heterogeneous and homo- geneous catalysis appear. Whether natural or artificial enzymes are considered, large molecular structures are involved that can be seen to provide an "interface" upon which a dissolved reactant molecule can be immobilized, awaiting reaction (as in heterogeneous catalysis). In addition, the structure incorporates at the site of immobilization a suitable chemical environment that facilitates the desired reaction when a suitable reaction partner arrives (as in homogeneous catalysis). We discuss below the aspects of each of these catalytic situations that are relevant to the development of new chemical processes. Then they will be revisited in Section IlI-B because of their importance in the development of new energy sources. Heterogeneous Catalysis A heterogeneous catalyst is a solid material prepared with large surface area (1-500 m2/g) upon which a chemical reaction can occur at extremely high rate and selectivity. Some major new commercial processes based on heterogeneous catalyst developments in recent years are shown in Table IlI-1. The potential

III-A. NEW PROCESSES TABLE III-1 New Processes Based on Heterogeneous Catalysis Feedstocks Catalyst Product Used to Manufacture 1982 U.S. Production (metric tons)a Ethylene Silver, cesium chlo- ride salts Bismuth molybdates Propylene, NH3, O2 Ethylene Propylene Chromium titanium Titanium, magne- . . . SlUm OX1C .es Ethylene oxide Polyesters, textiles, lubricants Acrylonitrile High-density polyethylene Polypropylene 2,300,000 Plastics, fibers, resins 925,000 Molded products 2,200,000 Plastics, fibers, films 1,600,000 a Production by all processes, including the innovative process; U.S. Tariff Commission Report. economic significance is displayed in the last column, the total U.S. production by all processes. Surface science is developing rapidly and now gives us experimental access to this two-dimensional reaction domain. Because of the unused bonding capabil- ity of the atoms at the surface, chemistry here can be qualitatively different from that of the same reactants brought together in solution or in the gas phase. However, when chemists are able to "see" what molecular structures are on the sur- face, our knowledge of reactions in conventional settings becomes applicable and opens the door to understanding and control of chemistry in this surface domain. There are five areas of heteroge- neous catalysis where this understanding will have major impact on new chemical technologies. Molecular Sieve Synthesis and Catalysis HOW DOES CARBON MONOXIDE BIND TO A METAL SURFACE? Molecular sieves are natural or synthetic crys- talline aluminosilicates containing pores or channels within which chemical reactions can be initiated. They offer unparalleled efficiency both for cracking of petroleum and for conversions such as methanol to gasoline. We need to know better how to synthesize molecular sieves with controlled molecular pore size, and how to determine the elementary reaction steps and intermediates that account for their efficacy. Metal Catalysis Finely dispersed transition metals are increasingly coming into use to catalyze hydrocarbon conversions and ammonia synthesis for fertilizers. Other such applications and improved performance will follow from intensive research into the control of surface structures, oxidation states, residence time of reaction intermediates, and resistance to catalyst "poisons" (such as lead and sulfur). 23

24 HIGH H: 1'H HI H Para-xylene CONTROL OF CHEMICAL REACTIONS Substitutes for Precious Metal Catalysts Many of the most effective catalysts are rare metals of limited availability in the United States, including cobalt, manganese, nickel, rhodium, platinum, palladium, and ruthenium. Their strategic value requires a concerted research effort to find more accessible substitutes, such as other transition-metal oxides, carbides, sulfides, and nitrides. Conversion Catalysts We must find catalysts to convert abundant substances to useful fuels and industrial feedstocks. These reactions include conversion of atmospheric nitro- gen to nitrates, methane to methanol, carbon dioxide to formate, and depoly- merization of coal and biomass to useful hydrocarbons. Catalysts to Improve the Quality of Air and Water We have many environmental pollution problems for which we need solutions that will match the success of the catalytic converter used in cleaning automo- bile exhaust gases. To begin, we need catalysts that remove sulfur oxides from smoke plumes, that purify water, and that prevent acid rain. As we learn more about molecular structures at the solid-gas interface (reactants, intermediates, and products), a better understanding of surface chemical bonding will follow. We can look forward to understanding additives that modify catalyst performance ("promoters" and "poisons". Then, the chal- lenge of synthesis of the designed catalyst can be addressed. All this fundamen- tal knowledge will underlie and facilitate the development of new and more selective heterogeneous catalysts. Homogeneous Catalysis Homogeneous catalysts are soluble and active in a liquid reaction medium. Often they are complex, metal-containing molecules whose structures can be modified to tune reactivity in desired directions to achieve high selectivities. In this respect, homogeneous catalysts can be supe- rior to heterogeneous ones. The largest industri- al-scale process using homogeneous catalysis is the partial oxidation of para-xylene to tereph- thalic acid with U.S. production of 6.2 billion pounds in 1981. The process uses salts of cobalt and manganese dissolved in acetic acid at 215°C as the catalyst system. Most of the product ends up copolymerized with ethylene glyco] to give us polyester clothing, tire cord, soda bottles, and a host of other useful articles. The strength of the chemical industry in the United States has been repeatedly enhanced by the introduction of new processes based upon homoge- l catal yet Co, Mn salts AH O~C`o Terephthalic Acid U.S. PRODUCTION (19SI), $2.3 BILLION!

III-A. NEW PROCESSES TABLE III-2 New Processes Based on Homogeneous Catalysis 25 Start- 1982 U.S. up Production Feedstocks Catalysta Product Used to Manufacture Date (metric tons) Propylene, oxi- MoV' complexes Propylene oxide Polyurethanes (foams) 1969 303,000 dizer Polyesters (plastics) Methanol, CO [Rh(CO)2I2] Acetic acid Vinyl acetate (coatings) 1970 495,000 Polyvinyl alcohol Butadiene, HCN Ni(L~)4 Adiponitrile Nylon (fibers, plastics) 1971 220,000 a-Olefins PhH(CO)(L2)3 Aldehydes Plasticizers 1976 300,000-350,000 Lubricants Ethylene Ni(L3)2 cr-Olefins Detergents 1977 150,000-200,000 CO, H2 (from [Rh(CO)2I2] Acetic anhy- Cellulose acetate 1983 [225,000, Capacity] coal) dride (films) a L = Ligand; Lo = Triaryl Phosphite, L2 = PPh3, L3 = 0OCCH2PPh2, Ph = C6H5 neons catalysts. Table IlI-2 lists six such processes, whose 1982 production figures were valued at over $1 billion. An important branch of homogeneous catalysis has developed from research in organometallic chemistry. For example, in the second reaction in Table IlI-2, rhodium dicarbony] diiodide catalyzes the commercial production of acetic acid from methanol and carbon monoxide. With this catalyst present, the reaction economically gives more than 99 percent preference for acetic acid over other products. Almost a billion pounds of acetic acid is so produced, a large part of which is used to manufacture such polymeric materials as vinyl acetate coatings and polyvinyl alcohol polymers. There are three areas of homogeneous catalysis where increased understand- ing has potential for major impact on new chemical technologies. Activat~on of Inert MoZecules Several relatively inert molecules are enticing as reaction feedstocks because of their abundance, including nitrogen, carbon monoxide, carbon dioxide, and methane. One way to facilitate their use might be through homogeneous organometallic catalysis, and quite promising examples are beginning to appear. For examnIe. soluble comcounds of tun~sten and molybdenum with ~ , , molecular nitrogen have been prepared and induced to produce ammonia under mild conditions. The carbon-hydrogen bonds in normally unreactive hydrocar- bons have been split by organorhodium, organorhenium, and organoiridium complexes. Hope for build-up of complex molecules from one-carbon molecules, such as carbon monoxide and carbon dioxide, is stimulated by recent demon- strations of carbon-carbon bond formation at metal centers bound in soluble metal-organic molecules. Synthesis of compounds with multiple bonds between carbon and metal atoms has had major impact in cIarifying the catalytic interconversion (metathesis) of olefins. While there is much to learn, the stakes are high and the odds favor success.

26 Cys-S CONTROL OF CHEMICAL REACTIONS Metal Cluster Chemistry An adventurous frontier of catalysis involves the expanding capability of chemists to synthesize molecules built around several metal atoms in proxim- ity. Many of these "metal cluster" compounds consist of several metal atoms bound to each other in the "core" of the molecule with carbon monoxide molecules chemically attached on the periphery. These metal carbonyIs have formulas MX(CO)Y' and x can be made very large. The worId's record as of this writing is a platinum compound with x = 3S, Pt3~(CO)24. At the same time, very low temperature techniques are revealing the structures and chemistry of small aggregates containing only metal ions or atoms ("naked clusters". In still another direction, cubical units of four metal atoms and four sulfur atoms are now known for iron, nickel, tungsten, zinc, cobalt, manganese, and chromium. This "cubane" structure, which involves three metal-sulfur bonds to each metal atom, has its own characteristic chemistry. This is demonstrated by the iron example, which proves to be a functional unit in the ferrodoxin iron proteins that catalyze electron transfer reac- tions in biological systems. These cluster compounds, bound or "naked," furnish a natural bridge between homogeneous catalysis and bulk metal, heterogeneous cataly- sis. What makes it intriguing is that many of the metals that are most active as heterogeneous catalysts also form cluster compounds (e.g., rhodium, platinum, osmium, ruthenium, and iridium). Now the chemistry of these elements can be studied as a function of cluster size. There is much to be gained from better understanding because all the elements mentioned above are derived from imported strategic mineral ores. ll --- Fe—s-cvs Cys-S' S-Cys THE BIOLOGICAL ENZYME FERRODOXIN: AN I RON-SULFUR "CUBANE " STRUCTURE Stereoselective Catalysts Another frontier full of promise involves the development of homogeneous stereoselective catalysts. Many biological molecules can have either of two geometric structures connected by mirror-image (chiral) relationships, and generally only one of these structures is functionally useful in the biological system. If a complex molecule has seven such chiral carbon atoms and a synthetic process produces all the mirror-image structures in equal amounts, there would be 27 = 128 structures, 127 of which might have no activity or, worse, might have some undesired effect. Thus, the ability to synthesize preferentially at every chiral center the desired structure with the desired geometry is essential.

28 CONTROL OF CHEMICAL REACTIONS PhotocataZysis An electrochemical cell can be built with one or both electrodes made of semiconductor materials that absorb incident light. In such a cell, the light absorbed by the electrode can be used to promote catalytic oxidation-reduction chemistry at the electrode-solution interface. The same sort of chemistry can be induced in solutions containing suspensions of semiconductor materials but now at the particle-solution interface. Such oxidation-reduction chemistry has significant scientific interest and, without doubt, practical importance as well. For example, photodestruction of toxic waste material, such as cyanide, has been demonstrated at titanium dioxide surfaces. A more popularized and conceivably feasible concept is that such photocatalytic chemistry, driven by solar energy, could give a process for producing massive amounts of hydrogen and oxygen from water. It is an intriguing prospect: to convert from diminish- ing, polluting petroleum fuels to a renewable fuel hydrogen that burns to water and that is made from water using energy from the Sun. Electrocatalysis Apart from light-initiated processes, electrode surfaces with catalytic activity offer a new domain for chemical synthesis. In a field with a long heritage, recent developments have shown that electrode surfaces can be chemically tailored to promote particular reactions. For example, electrocatalytic control of electron flow opens new synthetic pathways that require one-electron transfer in preference to two. Furthermore, this research area has benefitted from tech- niques used by the semiconductor industry, such as chemical-vapor deposition, by coupling them with imaginative synthetic chemical techniques for surface modification. An example is the electrocatalyst family developed for use in chlorine generation in chIor-alkali cells. A successful case is based upon a thin layer of ruthenium dioxide—the catalys~eposited on a base-metal electrode. This electrocatalyst has dramatically improved energy efficiency and reduced cell maintenance in the chIor-alkali industry, an industry representing billions of dollars in sales. The cumulative savings are enormous because this crucial industry consumes up to 3 percent of all electric power produced in the United States. Chemistry at the Solid/Liquid Interface Before the technological potentialities of any of the above can be realized, we must have a much better understanding of the nature of chemistry at the semiconductor/liquid interface. Most of the extraordinary instrumentation so far developed for surface science studies is applicable only at solid/vacuum interfaces. We need comparable capability at the solid/liquid boundary, and we will gain it from fundamental research in solid state chemistry, electro- chemistry, surface analysis, and surface spectroscopy. There is already reason for optimism. The surprising discovery of the million-fold intensification in-

III-A. NEW PROCESSES volved in the surface-enhanced Raman effect provides a technique with appli- cability yet to be determined. Perhaps even more promising is the demonstra- tion of surface-enhanced second harmonic generation. This method, which depends upon the intrinsic asymmetry at a phase boundary, has become possible because of the high intensity of laser light sources. It may have general applicability and, because pulsed lasers are used, temporal (kinetic) measure- ments can be expected. The potential gains from these areas are considerable. We need to know how to catalyze multielectron transfer reactions at an electrode surface. That is the chemistry required, for example, to photogenerate a liquid fuel like methanol from carbon dioxide and water. Multi-electron transfer catalytic electrodes for oxygen reduction in electrochemical cells would find a welcoming home in the fuel cell industry. It is also likely that research on semiconductor electrode surface modification will reflect beneficially on the field of electronics. Thus, the integrated circuit technology based upon gallium-arsenide may depend upon control of its surface chemistry. Already scientists concerned with photoresist/chemical etching tech- niques are recognizing the importance of the chemistry involved in surface modification, as shown by the active pursuit of "anisotropic chemical etching." In summary, our evolving understanding of the electrode/solution interface, buttressed by concepts based on semiconductor electrodes and the development of new methods for modifying electrode surfaces, has opened novel approaches to both photocatalysis and electrocatalysis. Future advances will benefit syner- gistically from progress in heterogeneous and homogeneous catalysis, increased understanding of mass and charge transport within the electrode surface layers, and continued development of experimental methods and theoretical models for the interface. Artificial-Enzyme Catalysis The most striking benefit of our expanding knowledge of reaction pathways and the analytical capacity of modern instrumentation has horn the develon- ment of our ability to deal with molecular systems ot extreme complexity. With the synthetic chemist's prowess and such diagnostic instruments as nuclear magnetic resonance, X-ray spectroscopy, and mass spectroscopy, we can now synthesize and control the structure of molecules that approach biological complexity. This control includes the ability to fix the molecular shape, even extending to the mirror-image properties so crucial to biological function. There is no application of these capabilities more intriguing than that of coupling them with our growing knowledge of catalysis in an attempt to synthesize artificial enzymes. In Nature, enzymes are the biological catalysts that accelerate a wide variety of chemical reactions at the modest temperatures at which living organisms can survive. An appropriate enzyme selects from a system with many components a single reactant molecule and transforms it to a single product with prescribed chiral geometry. Without catalysts, many simple reactions are extremely slow under ambient 29

30 CONTROL OF CHEMICAL REACTIONS conditions. Raising the temperature speeds things up, but at risk of a variety of possible undesired outcomes, such as acceleration of unwanted reactions, destruction of delicate products, and waste of energy. Hence, there are compel- ling reasons to develop synthetic catalysts that work like enzymes. First, natural enzymes do not exist for most of the chemical reactions in which we have interest. In the manufacture of polymers, synthetic fibers, medicinal compounds, and many industrial chemicals, very few of the reactions used could be catalyzed by naturally occurring enzymes; even where there are natural enzymes, their properties are not ideal for chemical manufacture. Enzymes are proteins, sensitive substances that are easily denatured and destroyed. In industries that do use enzymes, major effort is devoted to modifying them to make them more stable. Controlled Molecular Topography ancl Designec! Catalysts We have a pretty good idea of how enzymes work. Nature contrives a molecular surface suited to a specific reactant. This surface attracts from a mixture the unique molecular type desired and immobilizes it in a distinct shape that facilitates reaction. When the reaction partner arrives, the scene is set for the desired reaction to take place in the desired geometry. Organic chemists who have taken up this challenge are making notable progress. Without special control, large molecules usually have exclusively convex surfaces (ball-like shapes). So a first step has been to learn to synthesize large molecules with concave surfaces, after which the concave surfaces could be shaped to accommodate a desired reactant. Cyclodextrins, which are toroidal in shape, provide examples. The crown ethers, developed over the last 15 years, have a quite different surface topography. For instance, 18-crown-6 consists oftweIve carbon atoms and six oxygen atoms evenly spaced in a cyclic arrangement. In the presence of potassium ions, the ether assumes a crown-like structure in Top View which the six oxygen atoms point toward and bind a potassium ion. Lithium and sodium ions are too small, and rubidium ions too large, to fit in the crown-shaped cavity; so this ether preferentially extracts potassium ions from a mixture. More ornate examples now exist. Chiral binaphthy] units can be coupled into cylindrical or egg-shaped Side View cavities. With benzene rings, enforced cavities have been made with the shapes of bowls, pots, saucers, and vases. We are clearly moving toward the next step, which is to build into these shaped cavities a catalytic binding site, such as a transition metal complex that is already known to have catalytic activity in Br Br p:Br Br air CAVITANDS WHAT SHAPE DO YOU NEED?

III-A. NEW PROCESSES solution. The earliest successes are likely to be patterned after natural enzymes, but there is no doubt that, in time, artificial, enzyme-like catalysts will not be limited to what we find already known in nature. Biomimetic Enzymes A short-cut approach to improved catalysis is to pattern artificial enzymes closely after natural enzymes sometimes called "biomimetic chemistry." For example, mimics have been prepared for the enzymes that biologically synthe- size amino acids. Artificial enzymes that incorporate the important catalytic groups of a natural enzyme, such as vitamin B-6, can show good selectivity and even the correct chirality in the product. Mimics have been prepared for several of the common enzymes involved in the digestion of proteins, and substances that catalyze the cleavage of RNA have been synthesized based upon the catalytic groups found in the enzyme ribonuclease. Mimics have also been synthesized that imitate the class of enzymes called cytochromes P-450, which are involved in many biological oxidations, and the oxygen carrier hemoglobin. In addition, mimics have been prepared for biological membranes and for those molecules that carry substances through membranes. These have potential applications in the construction of organized sys- tems to perform selective absorption and detec- _ Ho CH=CH2 tion, as in living cells. It is important for the HC^CH United States to build on its early lead in this H3C ~ I JU'CH3 field. Although most of the work mentioned above O '~N--Ff+-N~ has been done in the United States, the Japanese Ho>CCH2CH2 Hi - N. - l H CH=CH2 have also become quite active and have specifi- O>CCH2C/ - CH3 c'- cally targeted b~om~met~c chemistry as an area HO of special opportunity. Research on synthetic or- HEMIN: THE ACTIVE PART OF HEMOGLOBIN ganic chemistry develops novel methods to con- struct the required molecule and elaborates new kinds of structures. The study of detailed reaction mechanisms in organic and biological chemistry permits a rational approach to catalyst design. It is an area ripe for develop- ment, and it deserves encouragement as a part of this program in chemical catalysis. Conclusion A significant share of our economy is built upon the chemical industry. The long-range health of this critical industry will depend upon our ability to innovate, developing new processes that increase energy and cost efficiency, and creating new products for new markets, all the while enhancing our protection of the environment. Today's basic research in all facets of catalysis will provide the source of such innovation. It will also produce the cadre of young scientists working at the frontiers of knowledge with the state-of-the art instrumental skills needed to recognize and implement the potentialities. Research in catalysis, viewed in the broad sense presented here, is one of the research fields of chemistry that deserves high priority. 31

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~ ~— , :~ ~ ~ . A_ a , ::::::~::::~:~ :~ ~: A. .lthlUIn-. ~owerecl. eart :: ~ : ~- . ~ ~ . , ~ ~ . ~ · . , ~ ~ I ~ r~ '1'. le: care .1ac pacema. ~er :1S a ~mof ~ern mlrac e OI :~sclence ~ lal many ol: us raEe~ lor ~ ~ ~ ~ ~ r ~ ~ ~ ~ ~ J : grabtec ~—3Ut not ~a person w lo owns one! lese pacemakers opera ;e on: JarTery ~ ~ ~ · , · , ~ , :: :::::::~power':anc .~the~c .eman£ts put: on:::tne:::tmy ~Datterles~ tnat generate~:~lt;~are~ awesome! ~ ; ~ ~ ~ ~ ~ ~ ~ ~ ~ . · · ~ ~ · ~ ~ :: ~ ~ - t: ~ ~ ::: ::: Th~r;~:must~art the~human macn~:ne~: every ~mor~lng~wl:{nou~t::~a;t~, :anc tne~numan : :: ~ : :: ~::~ :I:~ghts~and radio~:~are~::n~nning~ all :the ~time. Yet many, many people:~are~:adding h~e~althy ~: ::: :: ~: ::::: :; :;:: ::::::::years~:~to ~life:~:by:betting:~:;on: t he:: chemi:cal: reactions that ;occur::in:;~these battenes :to: ~ ; :~: ~ : : : : ::;:: generate~ay~ln~c ayou~ le:e ectriccurren ;:t late .rlv~e:s::t. lelrpacema~ ~ers. ~ ;: ::: : ~ _ - ~ ~ ~ ~ . . . . .. J ~ . . . . . :::~ : ~ ::: ~ '1' lese Jatteries . lave :specl:a. . requlrem~ents Decause tney musr ~ne~ l~mplamea~ ln a : ~ :: ~ ~ ~ ~ ~ :~: ~ ],~,,~ ~ ~ ~ ~ ~ ~% ~ ~ ~ . ~ ~ ~ ~ ~ . ~ ~ ~ :~: : : human~ no~7. l~ney ~must be ruggeCL: a~ ieaKprool, nave 1ong llle ancl~mmlmal we~gnt, : ~ :: : : : ~:and,::~ol: co~se,~they must be nontox~c. lhe~ Ilrst bat ter~es; used 1n~ pacem~ers ~hact ; a l~fespan ;~of onl~r 2 years,~and the~periodic operations ;re~lre~d 1:or repl~acement ; ; meant a~itional~:riskandstress~f~thep~ient.: ~ ~ ~ :: ~ ;; ~ ;;~; ~ ~ ~ . . . ~ ~ ~ ~ . ~ . . . . . . . ~ . . ^^ . . . . . . . Unemlsts: Degan to tackle ~thls pro:Ulem, ann researcn eIlorts ln ~electrocnem~stry ~ . · . . - ~ : . ~ ~ . : · i ~ . ~ · · · ~ · · t : t ~ ~ uncovered~llthlum metal, a new sunstance WltI1 tne:poremlal:ro:~g]*ve long llIe : ~i ~ ~ . ~ ~ . ~ . . ~ . ~ ~ ~ ~ . . ~ to battenes. Unto~nately, llthlum ~ls; nlghly ~reactlve—1t ourns m:~alr ~ ~ ~ ~ . , . ,,, . . . ~ ~ :. ~ .. . . ~ ~ and reacts with water to proctuee flammable nyc~ogen gas. ;:ll ~l~th~um ~ ~ , . ~ ~ ~ were~ to be used, it~ ~would ~ be necessary to ctlscover= nevv, nonaqueous ele~ctiol`~e :s~rs~ms ~ ~ ~ ~ ~ : : ~ ~ J ~ J ~ ~ ~ ~ ~ _ , _ ~ ~ : :: :: :;~E~lec~o~tes;are~: substances: that di:ssolve:::in:::water::to:form con- /:~: ~ ~: :~,`' : : \ ~ ~__ _ ~ ~ · ~ ~ ~ ~ ~— : rrl~ ~ · ~ ' ~ ~ · t L~ ~ ~—_~ ~ ouctmg solutions. lney ~ulssolve tO procuce 1ons; particles ear- . ~ . . . ~. . ~ ~ . . ~ . . , ~ ~ . 3 ng~electrlcal charge. Ine movement o! tnese cnarge~s~carrles 1 : ~ :~: ~ I ~ ~ ~ ~_ _ 1 the current~ as the nattery~ s chemistry releases lts sto;red ener~. ~\ ~ ~~ ~ ~ _ . ~_ ~ ~ 1 ~ . , ql · ·, , . . · ~ ~ · , \ ~ . , ~ Y, conventlonal~ :0atterles :tnat ~aw on:tne cnemlca~::ene~gy ot z~nc \ ~ '> ~ ~ ~ ~ ~ ~ . ~ ~ ~ ~ ~ _. , and:~:mercuriG:::~oxide::depend upon aqueous::electrolytes.~;;~tne~: :\ : :: ,+: :~ problemforthe~chemiststo solve was: defined~to design a:bat:-:~ \x ~ ~ ~`l~/ / , _ ~ , . ~ , ~ ~ , .,, ~ ~ ~ ~_ _^ ~ tery~atwoulOoperate~wltnou~waler. ~ ; ~ ~ ~ ; ~ L~ MS~~ , : . . : . :: ~ : ~ ~ ~ J: · ~ ~ ~ ~ I _ ~ _ ~ lixtens~ve ~nvestagations into new solvents anct new~marerla~s ~ ~ ~ ~ ~~ ~ ~ ~ ~ %\ ~ , ,, ~ . . . . ~ . · . . ~ i ~ ~ ~ ~ %\ ·, tor~use~1n high energy, long-llle batteries eventually led ~;o tne ; ~ \ ~ ~~ , ~,, , , ,, :: :: · ~ ~ · ~ · ~ : ~: ~~ t: ': :~ ~ ~ ~ ~ ~ ~~ di:scoven?:ola~;~so~llclelect~olyte:toruse:wltnllmlummelal.:lnesolla: ~ ~ ~ :: 1:/ :~ : : :electrol~e~is:iodine,~;dthelithium-iod~ne:~batterywasborntor:blomed- : : ::ical ~applications. These revoIu1;ionary:~batteries: are c~refflly :in: use,: ~and they ;: ~ ~ · · ~ · ~ ~ ~ ~ ~ m~ ~ ~ ~ 1 · · · · · ;hWave ~arT lmpress~ve ll1:espan ot 1V years! lne Denerl~s to tnose wno~must oepencr . . ~ . . . . . upon~ caml~ac ~pacemaRers~are 1nealculaDle.: ~ ~ ~; ~ ~f ~ · . · · . ~ . · · · ~ · ~ · - : ~ ~ Tne lltulum-loctlne nattery: ls not tee:: eno::ot: tne story. lt lS :a:Vast::lmprovement :::: ~ : :over: its: predecessors and extremely:~useful;::~ln pacem~ers, :~tit;::lt~;has a~Iower power ::: : ~:: :than:wo:uld:;:beoptimum:fUrotheruse:s.Onthehorizon::i~s:the:need:fornewthi~er- : : : : power: batteri~es fUr~use in other implantable organs like:artificialL kidneys and hearts. : : ~ , ~ , ~ T I t but turtner electrocnem~ca1 researcn will unoounlealy provlcle tne answer. lr nas . . . . · .. · m the past, and lt Wlll agam. "c~ a

34 100 _ ~ 60 _ o cr _ m z 40 _ o J _ _ _ 80 20 n HOW MUCH Wl LL NEW SOURCES d NUCLEAR + HYDROELECTRIC ma. to ? :s,1 he'd. ', I ~ ? ~ ~1# 1 1900 1925 1950 1 97 5 2000 YEAR U.S. ENERGY USE: NEW SOURCES ARE NEEDED CONTROL OF CHEMICAL REACTIONS III-B. More Energy This country's economic development is tied to the growth of its use of energy. For six decades, the Industrial Revolution was fueled primarily by coal. Then, petroleum energy use caught up with coal in 1948. Meanwhile, throughout the 20th century, the 3-fold in- crease in population has been accompanied by a 10-fold growth in energy use in all its forms. As we look ahead, there can be no doubt that the nation's wealth and quality of life wit] be strongly linked to continued access to energy in large amounts. Every proposed scenario for energy consumption over the next four decades projects the need to optimize access to ev- ery energy source at our dis- posal. This need is made more urgent as we face the inevita- ble depletion of liquid fossil fuels because there will also be stiffening constraints im- posed by the desire to protect our environment. In this set- ting, chemistry will continue to be the central scientific dis- cip1tine in meeting the nation's energy requirements. How chemistry has served us in the recent past and its potential role over the next 30 years will be considered for the following important energy sources and emerging alternatives: · Petroleum · Natural Gas · Coal, Lignite, Peat Petroleum Lou years. me s~gn~cance ~1~ ~ am: ~ _ 1 ~ _ 1 ~ · Shale Oil, Tar Sands · Biomass · Solar · Nuclear Fission · Nuclear Fusion · Conservation Petroleum use has increased steeply, worldwide: as much petroleum was taken from the ground between 1968 and 1978 as was produced in the preceding Ale ~ ^ of the increase is accentuated by the complex chemical processing requ~rect to convert the raw natural product into chemical forms that meet the demands of modern, high-compression engines. Refinement

IlI-B. MORE ENERGY of the crude of] begins with distillation for separation by boiling range. Hydro-treating may be needed to remove sulfur and to upgrade feedstock product quality. Then, by catalytic cracking, the large molecules are frag- mented into lower boiling molecules. Alternatively, catalytic reforming can convert the molecular structures to higher octane forms. All this is carried out on a gargantuan scale with perilously inflammable substances. Chemical catalysis has made this miracle of process engineering possible. Table IlI-3 lists four important catalytic processes recently introduced during a TABLE III-3 Heterogeneous Catalysis in the Petroleum Industry Feedstocks Catalyst Product Used for Zeolite C~6-C24 oils molecular sieves C7-Cg alkanes, alkenes "Cracking" to high- (aluminosilicates) octane fuels C7-Cg unbranched Platinum-rhenium/ Aromatics, other hydra- "Reforming" to hydrocarbons platinum-iridium carbons high-octane fuels CO, NO, NO2 Platinum/palladium CO2, N2 Auto exhaust rhodium cleanup Zeolite CH3OH molecular sieves C7-Cg branched Gasoline produc- (aluminosilicates) hydrocarbons, aromatics tion period when environmental concerns dictated the reduction of noxious byproducts and development of high-octane, lead-free gasoline. The need for new discoveries is even greater today as we turn to lower grade petroleum feedstocks with higher sulfur content, with higher molecular weights (Alaskan oils), and with catalyst-poisoning constituents E.. vanadium and nickel in California off-shore oils). ~ , Challenging research opportunities for chemists and chemical engineers lie in such key areas as recovery (getting more of] from the known deposits), refining (converting the crude of] into the most useful chemical form), and combustion (getting the most energy from the finished fuel). Recovery About 4000 billions of barrels of oil have been discovered, worldwide, with about 12 percent of it in the United States. Most of that oil, however, is not recoverable by presently known extraction methods. Primary recovery, based upon natural pressure, typically can recover no more than 10 to 30 percent of the of] from its natural reservoir, a complex structure of porous rock. Secondary recovery, in which water, gas, or steam injection is used to revitalize the deposit, can raise the recovery efficiency; but even then, only about 35 percent of the known U.S. oil deposits are classified as recoverable, and of that, more than 80 percent has already been extracted and consumed. Tertiary recovery requires new chemistry and new methods to gain access to the rest of this valuable resource. Surfactants (detergents) and solution poly- 35

36 ~//~-V~l/ \c/ \c, ° ° c ~ ~ Hi ^~ ~10 MA ~ ~ _ A - /5 ~ - ///// act/ ~ -—-I==- ~AyW&~' ~01/~ ~////////10' ' - _ _ In_ 01~ ~//////////,\0/ .~ l~ DETERGENT NACELLES AROUND OIL a//, X/// DROPLETS CARRY THEM TO THE SURFACE //////////// /////////////////,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, ////////////// CONTROL OF CHEMICAL REACTIONS mers can be used to lower interfacial tension between oil and water and reduce cap- ilIary pressure. MicelIar-poly- mer, caustic, and micro- emuIsion flooding are some of the enhanced" oil recovery methods under development or on the drawing boards. Fundamental questions of transport phenomena, phase behavior, viscosity, interfa- cial tension, and influence of electrolytes on surfactants must be better understood. There are difficult problems to be faced; but if recovery could be made feasible, it Would have enormous eco- nomic significance because it would permit us to tap the remaining 350 billions of bar- rels of U.S. of] already discovered but currently beyond economic reach. Refining The of] most easily removed by primary and secondary recovery also has the most desirable composition. As these best fractions become depleted, we must learn to refine heavier crude oils (higher molecular weights) with lower hydrogen content and more undesirable contaminants, such as sulfur, nitrogen, and organometallic compounds. A new generation of catalysts may be needed to escape the poisoning effects of some of these contaminants. Thus, vanadium seems to be carried by porphyrin complexes into molecular sieve-zeolite- catalysts where it clogs catalyst pores and blocks catalytic sites. In contrast, nickel contaminants have their own undesired dehydrogenation catalytic activ- ity, which increases the amount of coke, again clogging catalyst pores. It is likely that future refining techniques will differ markedly from those currently used. Petroleum refining technology is already undergoing an evolu- tion as refineries are being adapted to lower quality feedstocks. Some of the heavier components are being converted through catalytic hydroprocessing and coke-forming operations. Future developments may be based upon combustion of the low-hydrogen and coke components to fuel energy-consuming processes. The least desirable crude components may be gasified to generate hydrogen, a useful reactant in catalytic hydroprocessing. More dramatic departures are to be expected, though their development will follow from new research discover-

III-B. MORE ENERGY ies in separation techniques, molecular characterization of heavy crudes, high temperature chemistry, and catalysis. Combustion The United States annually spends about $30 billion (10 percent of its GNP) on combustion materials. It seems ironic that there is much remaining to be learned about this chemistry, one of the oldest technologies of mankind dating back to the discovery of fire. The need for more knowledge stems from ever increasing dependence on combustion, from changing fuel compositions, and, most important, from the sudden awareness and concern about the environmen- tal impacts of combustion. In the last 30 years, society has recognized and begun to grapple with the undesired side effects of profligate and careless combustion of fossil fuels. These side effects include smog from nitrogen oxides, acid rain from sulfur impurities, dioxins from inefficient burning of chlorinated com- pounds, and the almost imponderable long-range problem of the effect of accumulating CO2 on the global climate. The combustion process is a tightly coupled system involving fluid flow, transport processes, energy transfer, and chemical kinetics. This complexity is epitomized in the methane-oxygen flame. After 60 years of intensive study, only in the last 3 years has this flame been quantitatively modelled with a satisfactory molecular/free radical mechanism. Fortunately there is no subarea of chemistry offering greater promise than that of chemical kinetics. Such optimism derives from an array of new, sophisticated instrumental techniques that permits us to address and clarify the fundamental behaviors at work. These im- pending advances of molecular dynamics will be treated in Section ITI-D; such advances, as they occur, will be quickly taken up by chemical engi- neers and translated into higher combustion effi- ciencies and decreased environmental pollution. As one index of its importance, an increase of only 5 percent in the efficiency with which we combust coal, oil, and gas would be worth $15 billion per year to the U.S. economy and an immeasurable additional value if it is accompanied by reduction in the growing problems of smog and acid rain. Natural Gas Natural gas is a mixture of Tow-molecular- weight hydrocarbons, mostly methane (in North America, typically 60 to 80 percent methane, the rest, ethane, propane, and butane in varying percentages). While it contains some sulfur- and nitrogen- containing impurities, they are removable to give a clean-burning fuel and a SOME ACETYLENE REACTIONS IMPORTANT IN GASOLINE COMBUSTION THERMAL REACTIONS C2H2 +M ~ C2H+H+M + C2 H2 C4 H3 + H +H+M — C2H3 +M +C2H3 ~ C4H4 +H +C2H ~ C4H2+H COMBUSTION C2 H2 + O2 HCCO + OH HCO + HCO +O ~ CH2 +CO ~ HCCO + H +OH+M ~ C2H2OH+M 37

38 CONTROL OF CHEMICAL REACTIONS versatile chemical feedstock. The ethane and propane can be catalytically converted to ethylene, propylene, and acetylene, all valuable precursors to products needed by our society. Its ease of transport via pipelines and its desirable qualities for application make natural gas an important resource; its contribution to U.S. energy use has almost doubled since 1960. The U.S. natural gas reserves are comparable to our petroleum reserves, perhaps somewhat larger. However, again like petroleum, natural gas is limited in amount both worldwide and domestically and its nrod~.inn will undoubtedly peak one or two decades hence. Coal v, 7 —— ~— ~ _ ~ _ ~ ~ `, ^, ,, ,, ~ Coal is the most abundant of the fossil fuel energy sources. Estimates of recoverable supplies worldwide indicate 20 to 40 times more coal than crude oil. The contrast is even more dramatic here in the United States where the estimates indicate 50 to 100 times more coal than crude oil. There can be no doubt that dependence on coal must increase during the next two or three decades as petroleum reserves are depleted. Fortunately, this predictable chronology gives us time for the basic research needed to use this valuable resource efficiently and cleanly. It must be noted, too, that petroleum is not only a fuel, it also provides us with many important fine chemicals and chemical feedstocks. In fact, some people contend that petroleum as a source of other chemicals ought to be classified as "too valuable to burn." Inso- far as coal can be economi- c ~ cally converted on a massive ' scale into combustible fuels, we gain the option to "save" petroleum for more sonhisti- ,. ::::::::::::::::::::::::::::::: ~ .............................. ......... CREW ~ \Ru~ . . fathom .................... .......... ...... . ............... ..... syn gas CATALYB" ~ ...'2 CO ~ H2'2222 ' _ ....... ~ [DIET - ~0 .~ .................................... ............................... COAL: CHEM ICAL CORNUCOPI A STEAM HEAT ~ . _ ~ ............... CORL 1 tUtILEADEDi REM - J l ~ catect uses. Then, further ahead, we can foresee that with creative advances in chemistry, coal itself can pro- vide its own array of valuable feedstocks, including some now derived from petroleum. Coal is a carbonaceous rock containing chemically bound oxygen, sulfur, and nitrogen as well as varying amounts of mineral matter and moisture. As a fuel, it has too low a hydrogen-to-carbon ratio (its H/C ratio is near unity, about

III-B. MORE ENERGY half that of gasoline). For any use of coal more sophisticated than simple combustion, its molecular weight must be reduced, sulfur, nitrogen and mineral matter must be removed, and hydrogen content must be increased. These ends can be approached, either through processes that convert coal to liquid products susceptible to refining (hydroliquefaction), or through conversion to the gaseous form called "syn gas" ("synthesis gas," a mixture of carbon monoxide and hydrogen). The potentialities of synthesis gas are tantalizingly clear but not as yet economically competitive. Table ITI-4 lists some of the catalysts that have been found to display product specificity. TABLE III-4 Catalyst Specificity for Syn Gas Conversion to Useful Products Catalyst CO + H2 ~ Product Catalyst Product Useful for Nickel Copper/zinc oxide/alumium ox- ide Irona/cobalt Molybdenum/cobalt Ruthenium complexes (in solu- tion) Thorium oxide Rhodium complexes (in solu- tion) Methane, CH4 Methanol, CH3OH Straight chain hydrocarbons, CH3 (CH2)NCH3, N = 0 to 30 Mixed alcohol Cal to C3 oxygenated compounds Low-molecular-weight branched chains, hydrocarbons Ethylene glycol Fuel Fuels, via zeolite catalysts, chemical feedstock Feedstock for petroleum refin- er~es Octane booster Chemical feedstocks High-octane fuel Polyester feedstock a The catalyst developed by Hans Fischer and Franz Tropsch in the early 1920s. The details of catalytic conversion of CO and H2 to particular desired products is an active research area. Equally promising are the potentialities of liquefac- tion processes, for which more research would clearly be fruitful. The importance of the processes that can be expected to flow from research on both types of coal conversion was strikingly displayed during World War IT. Germany, denied easy access to petroleum, was able to produce 585,000 tons of ~ ~ ~ ~ ~ ~ ~ IT'S · ~ ~ ~ ~ · ~ ~ ~ . ~ ~ . tuel nyclrocaroons trom coal. Wnlle a good traction came tnrougn gaslncatlon combined with cobalt catalysts in Fischer-Tropsch chemistry, the larger share was produced through catalytic liquefaction. In a current situation, the Repub- lic of South Africa now produces 40 percent of its gasoline requirements by similarly converting coal into 1,750,000 tons of hydrocarbons annually (using iron catalysts). Its plants and refineries are literally built on top of extensive coal deposits, and the coal enters the chemical reactors via conveyor belts rising from the mines. These examples do not, however, furnish a general model except under the stress of denied access to petroleum. To impart economic competitiveness to the vast energy resource furnished by coal will surely require advances in catalysis, with specific emphasis on organometallic chem- istry and surface chemical science. 39

40 CONTROL OF CHEMICAL REACTIONS Shale Oil and Tar Sands Shale is a major potential source of liquid hydrocarbons in Colorado, Utah, and Wyoming. In these three states alone, the hydrocarbon content of the sedimentary rock argillaceous dolomite is estimated at 4000 billion barrels. If as much as a third of this enormous reserve could be recovered, it would give us almost 10 times as much fuel as has been removed to date from U.S. of] wells. Complicated new problems of chemistry, geochemistry, and petroleum engi- neering must be surmounted to reach this end, and a wealth of pertinent research problems must be addressed. These ancient marine deposits contain varying amounts of kerogen, a mixture of insoluble organic polymers, and smaller amounts of bitumin, a benzene-soluble mixture of organic compounds. Formidable environmental questions (water sources, land recIamation) are raised in the exploitation of shale deposits because a ton of shale may yield only 10 to 40 gallons of crude oil. The kerogen must be heated to about 600°C to be decomposed and produce of] (65 to 70 percent), gas (10 to 15 percent), and coke (15 to 20 percent). To avoid environmental damage, underground thermal processing has been tried. Shale of] has a favorably high H/C ratio—about 1.5 but it also contains undesired organic nitrogen and sulfur compounds that must be removed. Arsenic compounds can also present a special problem. Crude shale of] obtained by destructive distillation at 500°C has to be subsequently upgraded through centrifugation, filtration, and hydrogenation (to reduce nitrogen and sulfur content) before entering existing petroleum refineries. New extraction and upgrading processes are needed. Chemical kinetics, catalysis research, selective extraction, and selective absorption present directions worthy of study. In Utah, sands are found impregnated with dense, viscous petroleum. Such tar sane] deposits are now known in amounts equivalent to about 25 billion barrels of petroleum. Problems analogous to those discussed for of] shales must be confronted, particularly the environmental aspects. Because of the latter, practical use of this potential energy reserve may depend upon whether the rather complicated chemical conversions needed can be handled in situ, i.e., underground. Kinetics of combustion, heat and mass transfer, and multiphase flow in porous media are relevant research areas. Biomass An estimated 500 to 800 million tons of methane (equivalent to about 4 to 7 million barrels of of] and with H/C = 4!) are released annually into the atmosphere through bacterial action that takes place in the absence of oxygen. The obvious possibility of applying such anaerobic digestion to methane production from garbage, agricultural by-products, or other wastes is obstructed by the slowness of the process and by its great sensitivity to solution acidity. A detailed understanding of the chemical mechanism of methane production and of the biochemistry of the organisms involved could suggest strategies for

III-B. MORE ENERGY overcoming the problems. Concerning the former, the reduction of carbon dioxide is now believed to occur in a succession of enzyme-catalyzed, two- electron steps. Nickel plays a key role in the active enzyme, but neither its coordination state nor its specific action are known. Research on both synthesis and catalytic activity of metal organic compounds, artificial enzymes, and natural enzymes should help us assess the potentiality of biomass as a source of hydrocarbon fuels or chemical feedstocks. The appeal of this possibility is considerably enhanced by the prospect of extracting useful energy from garbage and sewage disposal. A particularly appealing aspect of biomass as a major fuel source relates to the atmospheric carbon dioxide content. Because carbon dioxide, CO2, is transparent to visible light but absorbs infrared light, it lets most of the normal solar radiation reach the ground while intercepting infrared light which the cooler earth's surface radiates. Thus CO2 "traps" the solar energy, tending to warm up the atmosphere (the "greenhouse" effect). The problem we face is that measurements throughout the century indicate that the amount of CO2 in the atmosphere is rising, which raises concern that in time the atmospheric temperature might rise enough to melt the polar ice caps (an average rise of only 5° might be sufficient). It is likely that most of the atmospheric carbon dioxide increase over the last 60 years has resulted from combustion of fossil fuels. To arrest this trend, we should be seeking new energy sources that do not release CO2. Solar energy is such an alternative. Less widely recognized, however, is that new biomass is an ongoing solar energy use that does not exacerbate the CO2 problem. While combustion of new biomass does produce CO2, its carbon content was all recently extracted from the atmospheric CO2 reservoir during growth of the biomass. Hence there is no net change in the CO2 balance. As mentioned above, this desirable concept can be put into practice only after research progress points to economically viable chemical processes for massive conversions of biomass to combustible substances. Further, there are trade-offs to be considered, such as partial diversion of agricultural land use from food to biomass production. With the prospects aborted by genetic engineering, even this conflict may be diminished or eliminated. Food and energy-producing biomass may be optimized in the same plant. Perhaps, as well, plants can be genetically "engineered" that tend to stabilize carbon dioxide by growing more efficiently when carbon dioxide availability goes up. We should be pursuing the necessary research avenues more aggressively than we are at present. Solar Energy By far the most important natural process for use of solar energy is photosynthesis the process by which green plants use the energy of sunlight to synthesize organic (carbon) compounds from carbon dioxide and water, with the concomitant evolution of molecular oxygen. To be able to replicate this process in the laboratory would clearly be a major triumph with dramatic implications. 41

42 CONTROL OF CHEMICAL REACTIONS Despite much progress in understanding photosynthesis, we are still far from this goal. The basic path from carbon dioxide to carbohydrate was traced in the 1950s. Since then, further insight has been gained, particularly into the initial events in photosynthesis, by means of new developments in magnetic resonance spectrometry and laser techniques. The solar spectrum that drives photosynthesis places about two-thirds of the radiant energy in the red and near-infrared spectral regions. Understanding the way nature manages to carry out photochemistry with these low-energy photons is one of the keys to understand- ing (and mimicking) photo- synthesis. Current-day expla- nations are generally based upon the so called "Z scheme" in which the energy of one near-infrared photon initiates a series of electron transfer reactions (oxidation-reduc- tion steps). While each of these steps necessarily ex- pends some of the absorbed energy, a fraction is stored through the energy-consuming production of adenosine triphosphate (ATP). Further, the chemistry is set up for the absorp- tion of a second infrared photon to produce still more ATP and to initiate reduction of atmospheric CO2. This sequence of events gives the "feedstocks" from which the cellular factory manufactures its high energy carbohydrate products. This factory is run by the energy stored in ATP. Thus natural photosynthesis is energized by near-infrared light through production of energy-storing intermediate substances with long enough life- times to await arrival of a second near-infrared photon. The second photon "stands on the shoulders" of the first so that their combined energy is adequate for making and breaking the conventional chemical bonds in the organic molecules of which plants are made. Several of the steps in this sequence take place in much less than a millionth of a second, at rates too fast to measure only 15 years ago. Now we have picosecond laser and nanosecond electron-spin-resonance spectroscopic tech- niques with which to probe each successive reaction on its own characteristic time scale. Hence we are in a period of rapid progress in clarifying the chemistry of the photosynthetic process. This type of spectroscopic study reveals photosynthesis to be a complex process involving cooperative interaction of many chlorophyll molecules. Ag- gregation of chlorophyll molecules with each other and with proteins has been 4.~ ~-~2x ~ AX ~~ ~~ _~.. ;~ . . . . . . . . . -., ~ ~ Chioroplast Membrane- Chlorophyll Pigments ~o$~ and IVY = Absorb Two Photons of Fred Lights to Power Photosynthesis

III-B. MORE ENERGY probed by infrared spectros- copy and by proton and 33C nuclear magnetic resonance (NMR). With the higher mag- netic field now becoming available for NMR, vital new information on chlorophyll behavior and the molecular architecture of in vivo chIoro- phyI] can be expected. Struc- tural studies of the photosyn- thetic membranes in which chlorophyll is imbedded will be further advanced by the application of small-angle neutron scattering. Electron paramagnetic resonance experiments have shown that rapid ejection or transfer of an electron from chlorophyll (within nanoseconds after the light absorption) leaves an unpaired electron shared by two chlorophyll molecules. This obser- vation has led to the idea that the photoreaction center consists of a pair of parallel chlorophyll rings held in close proximity by hydrogen bonding between · · ~ amino acid groups. Another promising approach to using solar energy is the direct conversion of sunlight to electrical or chemical energy with the aid of electrochemical devices. Recent advances in electrochemistry have brought us closer to practical real- ization of this possibility, and there is much opportunity for further research. In a photoelectrochemical cell, one or both electrodes are made of light-absorbing semiconductors. The light absorption results in oxidation-reduction chemistry at the electrode-electrolyte interface and, hence, current flow in the external circuit. Alternatively, with suitable control, the ultimate products of the oxidation-reduction chemistry can be hydrogen and oxygen. Elucidation of the thermodynamics and kinetics of light-induced processes at interfaces has, over the past decade, led to an order-of-magnitude increase in efficiency of conversion from light to electrical energy (from 1 percent to better than 10 percent). Development of thin, polycrystalline semiconductor films with high conversion efficiencies to replace the expensive single crystals currently used has been another important achievement. For example, with thin cadmium-selenide- telluride films, efficiencies of nearly 10 percent have been reported. Improved understanding of those aspects of surface chemistry and structure that limit photoelectrode performance will depend on the application of the most powerful new tools of surface science. Equally important will be basic research on new materials, including surface coatings of polymers, oxides, noble metals, etc. Of particular significance will be better understanding and catalysis of multi- electron transfer reactions. C/H3 / CH3 I I \~N`` CH2 ~"2 me AH C H3~N_ ,N~°\c~2o N ~ ~ . 'C— \ ll R o O. LOCI—I CH LOROPHYLL: :~:CH3 PACKING GEOMETRY AFFECTS ITS FUNCTION 43

44 CONTROL OF CHEMICAL REACTIONS Nuclear Energy At the same time that physicists and chemists gave us the atomic bomb, they offered mankind atomic energy, a new source of energy with seemingly unlimited capacity. However, the optimum role of nuclear energy in man's energy future is clouded by long-term risks that are difficult to assess. Whatever course society ultimately chooses, the minimization of its attendant risks will rely heavily upon the ingenuity of chemists and chemical engineers. Indeed, chemical research is essential to practically all phases of nuclear energy generation and the subsequent management of radioactive waste. To begin with, geochemistry plays a lead role in locating uranium ore deposits. Then, chemical separations are centrally important in the nuclear fuel cycled from concentration steps at the uranium mill, through reactor fuel manufac- ture, to the highly automated remote-control reprocessing of fuel elements if we decide to separate uranium and plutonium from fission products. Similarly, radioactive waste management is largely based upon chemistry and geo- chemistry. If these wastes are to be stored underground, we must find appro- priately stable, leach-resistant matrices, we must develop more efficient sepa- rations of particularly hazardous radioactive elements (e.g., the actinides that pose the major health hazard after a few hundred years), and we must understand fully the geochemistry of potential waste storage sites. If tempo- rary, recoverable containers are used, the problem shifts to corrosion chemistry under intense irradiation. Next, our analytical techniques must be made more sensitive for a variety of applications that extend from exploring for new uranium deposits to environmental monitoring at trace levels that would help reveal potential problems before real hazard has developed. Finally, we must extend our understanding of the unfamiliar chemistry that would accompany a catastrophic reactor accident. We must have useful estimates of release rates for fission products from a decomposing ceramic in the presence of high pressure (~ 150 atmospheres), high temperature (~3000 K) steam, and an intense radiation field. The use of nuclear reactors to generate energy is plainly a controversial and emotionally charged issue. Nevertheless, the scientific setting must be fully understood so that political choices can be made among well-defined and well-informed options. It would be unwise to curtail the research efforts that will define these options more clearly when so much can depend upon the confidence with which costs and benefits can be assessed. Fusion Energy In fusion reactors, erosion of the internal surfaces determines to a large extent the impurities injected into the plasma. Wall materials are subjected to high temperature gradients, unipolar arcing, neutron damage, and aggressive ion/surface interactions all of which remove surface layers by chemical and physical processes. Studies of materials suitable for reactor components have

III-B. MORE ENERGY been initiated with preliminary experiments on coated refractories. These studies use testing techniques that simulate some aspect of the plasma envi- ronment but do not reproduce the total environment. Understanding high temperature physical and chemical reactions, including synergistic effects, at the interface between the plasma and the reactor components, is essential to informed selection of materials and, undoubtedly, to the practical use of fusion energy. Conclusion . Nothing is more critical to the long-term health of our technological society than continued access to abundant and clean sources of energy. Fundamental changes in these sources are clearly ahead as world petroleum production peaks near the end of this century and begins its inevitable decline. In every one of the foreseeable alternatives, developments are needed in which chemistry and chemical engineering play a dominant role. The fundamental nature of the needed advances dictates an immediate commitment to a broad research program in chemistry. The program will exploit the powerful array of new instrumental methods that permit us (1) to study chemical reactions on surfaces (heterogeneous catalysis); (2) to advance our knowledge of molecular dynamics (combustion); (3) to understand absorption of light and chemical storage of its energy (photosynthesis, photochemistry, photoelectrochemistry); and (4) to deal with the environmental and waste management dimensions of the new energy technologies that will evolve. Fortunately, chemistry is poised to respond to these challenges. Society must declare its commitment to provide the encour- agement that will attract bright young people to these tasks and to marshal! the resources needed to accomplish them. 45

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: ~ :~:Stone Age, :Iron Age:, Polymer: Age There was a time when everything from arrowheads to armchairs was made from stones. Other features of those good old days were Air-conditioned caves and char- broiled saber-toothed tiger steaks (if you caught hi instead of the other way around). Fortunately, this age~ended when someone discovered how to reduce patron oxide to metallic iron using coke (carbon) as the reducing agent. That all happened several thousand years ago, so t he caveperson chemist who got t be patent rig its to the iron age wasn't educated at OMIT or the University of Chicago. Bu This chemical discovery profoundly changed the way people lived. It flied to all sorts of new products~1ike swords and plowshares and the inner-spring mattress. Can~you imagine~how thbse stone--agers would have; reacted the first time they put on a suit of armor, or went up the: Eiffel ~Tower, or took the train to Chattanooga? ~Well, brace yourself; because chem- ists are at it againt This time, we're about to enter the Polymer Age. You may think we're already there, with your polyester shirt, polyethylene milk bottle, and polyvinylchloride~suitease. We walk on polypropylene carpets, sit on polystyrene furniture, ride on poly- isoprene tires, and feed our personal computers a steady diet of polyvinylacetate floppy disks. In just the last 40 years, the volume of polymers produced in the United States has grown I00-fold and, since 1980, actually exceeds the volume of iron we produce. But the best Is yet to come. The structural materials with which we have been building bridges since even before the one to Brooklyn, and automobiles since the Model T. would seem to be the last stronghold of the iron age (pun intended). Would anyone dare to suggest that polymers could com- pete on this sacred gro:und?~ Well, no one perhaps except chemists. Right now, there's talk of an a~-p~asr~c auromoo-~e, and you re u~- ready flying in commercial airliners with substantial structural elements made of composite polymers. One of these, poly~para-phenylene terephthalamide3, has a tensile strength slightly higher than that of steel. But where this polymer really scores is in applications where the strength-to-weight ratio matters a lot, as it does in airplanes. Even with its cumbersome name, this polymer has a strength-to-weight ratio six-fold higher than steel! To appreciate this advantage, you should know that a 1 pound reduction in the structural weight of an airplane reduces its take-off weight by 10 pounds (counting the fuel to lift the pound~and the fuel to lift the extra fuel). No: wonder this polymer, under the trade name Kevlar~, is used to build tail sections for the biggest airliners. Oh, and bullet-proof vests, too. So what about this all-plastic automobile? Of course, weight reduction is the name of the game in trying to build fuel-e~cient cars. Already there are automobile driveshafts made of polymers strengthened with stiff fibers, and~similar composites are used for leaf springs (oops, there goes the inner-spring mattresses. Right now, U.S. cars contain about 500 pounds of plastics if you count, as well, the rubber and paint and sealants and lubricants and upholstery. But what about the engine and the electrical system? What will we do about these in this allegedly all-polymer car? Gee, I'm glad you asked ~ ~ ~ ~ ~ · ~ ~ 1 · 1 ~ ~ ~ I__ _ ~ 1 - NEXT TRAIN _ ~ . . .. .. ~ . . B ~ C~t~lOOGA 47

48 CONTROL OF CHEMICAL REACTIONS Webster: Material. noun IT]:-C. New Products and Materials The substance or substances out of which a thing is constructed. Chemistry. noun The science that deals with the composition, prop- erties, and changes of properties of substances. Expectations are universally high for advances in the materials sciences. What is a material? Webster's definition includes all the substances from which one might construct autos and airplanes, bridges and buildings, dishes and doors, hoses and hose, raincoats and radios, spacecraft and sewer pipe, tires and transistors, windows and walls, shirts, sheets, and shoes. That sweep of application is reason enough for the high hopes scientists have for finding new substances and new ways to tailor their properties to better fit our changing needs. Chemists clearly have a role here because chemistry is the central science for understanding and controlling the composition, structure, and properties of substances. When tailoring a substance to a need, the chemist's particular talent for synthesis and control of composition helps define this role. By no PHYS I CS means does that exclude other disciplines. To make this point, we need only men- tion the remarkable advances made in solid state physics over the last three decades characterizing and develop- ing semiconductor materials. The fields of ceramics and metallurgy, too, have pro- vided substances to meet spe- cial needs, from heat shields to tank armor. Equally important are the contributions of engineers in the processing and fabrication of the products we wish to use. There is probably no scientific frontier that is more interdisciplinary. The following analysis will focus on the rich opportunities for chemists to advance materials science to benefit mankind. However, realization of these opportunities will often depend upon synergistic interaction with other scien- tists in the materials science community. Synthesis CHEMISTRY ,~ | MATERIaLS | Composition ~ Physical Ad/ Characterization Processing Fabrication ENGI NEERI NG MaTERIaLS SCIENCE IS INTERDISCIPLINARY Plastics and Polymers We find natural polymeric materials all around us in proteins and cellulose, for example. But chemists probably learned most about how to make polymers through their attempts to imitate nature in synthesizing natural rubber. Today,

III-C. NEW PRODUCTS AND MATERIALS chemists have designed so many polymers for so many purposes that it is difficult to picture a modern society without their benefits. Their importance is dramatically displayed in the 100-fold growth of U.S. pro- duction of plastics over the last 40 years. Its production, expressed on a volume basis, now exceeds that of steel, whose growth has barely dou- bled over the same period. The economic implications of these comparisons are self- evident. Furthermore, pro- duction of plastics continues o coo upward. New uses are al- ready on the horizon to guar- antee a rich future for poly- mers, a future that will derive from continued research in chemistry. Polymer chemistry has many dimensions, and chem- ists are increasingly able to manipulate and control them. Judicious choice of reaction conditions (such as tempera- ture, pressure, polymeriza- tion initiator, concentration, solvent, and emuIsifiers) and reactant (monomer) struc- tures can determine the average and spread of chain length (molecular weight), extent of chain branching, cross-linking between polymer strands, and, through emplanted functional units, the physical and chemical properties of the final polymer. Sophisticated analytical methods have been developed to relate molecular structure to macroscopic properties. Recent additions to this arsenal include field flow fractionation, size exclusion chromatography, and high- molecular-weight mass spectrometry. By clever orchestration of these factors, chemists can design a polymer with tailored properties to build in plasticity or hardness, tensile strength, flexibility, or elasticity, thermal softening or thermal stability, chemical inertness or solubility, attraction or repulsion of solvents (wetting properties), permeability to water, responsiveness to light (photodegradability), responsiveness to orga- nisms (biodegradability), viscosity variability under flow (thixotropy), or optical anisotropy. All these possibilities account for the continuing growth of plastics a o In j 49 \! 107 _ lo4 _ STEEL UP LAST I CS f . ---.-'-~-- {...----- 4...--~- ,.. A;UMINUM ~ )9f^!,' . 1 1 1 1 1 1 1 1910 1920 1930 1940 1950 1960 1970 1980 - YEAR , U.S. PRODUCTION OF PLASTICS: 1 940 -1 980 1 00 - FOLD GROWTH

50 A FEW SIMPLE POLYMERS SHOW THAT POLYMERS CAN BE TAI LORED TO NEED \ - Rl R2 \ / ~C\ ~C\ ~C/ ' \C/ /\ /\ R3 R4 R3 R4_ R1,R2,R3,R4 Name H,H,H,H F,F,F,F H,H,H,CH3 H,H,H,C1 H,H,H,C6H5 H,H,H,CN Polyethylene Polytetrafluoro- ethylene Polystyrene Polyacrylonitrile H,H,H,OCOCH3 Polyvinyl acetate H,H,Cl,C1 H,H,CH3,COOCH3 Polymethyl neth- acrylate CONTROL OF CHEMICAL REACTIONS production and their increasing ubiquity in the things we use, wear, sit upon, ride in, eat from, and otherwise find in our everyday environment. Polymers as Structural Materials The potentiality of polymers as structural materials is expressed vividly in the hundreds of commercial airplanes flying today that have substantial structural elements made of a composite material made up in part by the light-weight, ultrastrong organic polymer with the trade name, KevIar, poly~para-phenyTene terephthalamide). The Lear jet, built largely of polymer composites, is a more publicized case. More down to earth, the efforts directed toward an all-plastic/ceramic automo- bile show the high expecta- tions for polymers' capacity to reduce weight, eliminate cor- rosion, and lower cost. In the past, differences in mechanical properties of poly- mers were discussed in empir- ical and heuristic ways. Now- adays, theoretical approaches Production are based upon the primary (tons/year, molecular data and funda- _ 1g82) mental principles of chemical 5,700,000 bonding and conformational structure. The elasticity in the polymer chain direction can now be calculated from 1,600,000 bond lengths, bond angles, 2430,000 and the experimental defor- mation force constants de- rived from infrared spectro- scopic measurements. The 2,326,000 resulting progress Is ev~- g20,000 denced in Table ITI-5 which compares demonstrated ten- sile strengths of two organic 500,000 polymer fibers to both alumi- num alloy and drawn steel. In what really matters, strength per unit weight, the two polymers significantly out- perform both of the conven- tional structural metals. Further developments will R\ /R2 _ R. R2 \ / ~C\ Product Plastic bags; bot- tles; toys Cooking utensils; insulation (e.g. Teflon) Polypropylene Carpeting (indoor, outdoor); bottles Polyvinyl chloride Plastic wrap; pho- nograph records; garden hose; in- door plumbing Insulation; furni- ture Yarns; fabrics; wigs (e.g. Orlon'~, Acrimony) Adhesives; paints; textile coatings; floppy disks Polyvinylidine Food wrap (e.g. chloride Saran~) Glass substitute; bowling balls; paint (e.g. Lucite~, Plexiglass)

llI-C. NEW PRODUCTS AND MATERIALS surely flow from continued re- search. It is already known, for example, that the elastic- ity of a zig-zag polymer chain can be far higher than that of a helical structure. Calcula- tions predict that fully ori- ented polyethylene will be inherently stiffer than poly- propylene, if its properties can be preserved in the proc- essing steps. In fact, the strength-to-weight ratio shown for polyethylene in Table ITI-5, 10-fold better than steel, is 5 times less than is theoretically possible. Research is needed to tell us how to exploit these possibilities. The impact of such research advances will be significantly felt in our international economic and strategic posture. TABLE III-5 Polymer Fibers Compete as Structural Materials Tensile Strengtha Tensile Strength per Unit Weighta (1.0) 1.7 10.0 15.0 Alumium alloy Steel (drawn) Poly(p-phenylene terephthalamide)b Polyethylene a Relative to aluminum alloy. b Kevlar~ (1.0) 5.0 5.4 5.8 Liquid Crystals and Polymer Liquid Crystals Though known for over a century, liquid crystals flared into prominence only a decade ago. Today, liquid-crystal display (LCD) devices provide the basis for an industry second in dollar volume only to cathode ray tubes in the world market of display technology. No rival matches LCD's in low power consump- tion for small area displays. I~iquid crystals are organic substances synthesized to possess geometric and/or polar characteristics that will encourage one- or two-dimensional order. Because at least one dimension remains disordered, the substance remains fluid and appears to be a liquid. However, the optical properties reflect the order on the molecular level. Long slender molecules with skeletal rigidity tend to become aligned, like logs floating down a river (such one-dimensional order is called a "nematic phased. More complex shapes, such as large but flat molecules, can form layered structures, like the successive sheets in a piece of plywood (such two-dimension order is called a "smectic phased. The actual behavior is determined by a subtle balance between the effects of molecular shape and electrical charge distribution as the molecule interacts with its local environment. The balance can often be significantly affected by a small electric field, which provides a ready means of switching from one optical behavior to another (e.g., from transparent to opaque). Plainly, design of liquid crystals is grist for a chemist's mill. Whether to promote basic understanding or to meet an envisaged use, the chemist's ability to synthesize new molecules, of spherical, rod-like, or disc-like shape with prescribed functional groups placed as desired, figures importantly in the advances already seen and those sure to appear in the future. In fact, one of the most promising frontiers of liquid-crystal chemistry is the application of this . ~ 51

~2 CONTROL OF CHEMICAL REACTIONS knowledge to polymerization processes. Coupling the molecular ordering of a nematic liquid with polymerization chemistry permits the order to be built into the polymer, with dramatic effects on physical (and optical) properties. It is just this control that lies behind the production of fibers of exceptionally high tensile strength, which, because of their better strength-to-weight ratio, can replace steel in products ranging from airframe construction to bulletproof vests. Block Polymers anct Self-Organized Solids Another fascinating area of research that is destined to lead to entirely new types of materials is connected with block polymers. Their concept is based upon recently expanded theories that amorphous polymers of suitable structure will "self-organize" into a continuous medium in which are lodged "microdomains." The microdomains might be spheres or alternating layers or rods in a contin- uous matrix. A "triblock" polymer has a segment of one polymer B sandwiched between a segment of a different polymer, A. The resulting material, A-B-A, has the properties of A at its extremities and the properties of B at the middle. R—B—R = \ ~ \AT \B/ NAB/ tB A>iM A If B and A are chemically designed to be incompatible, the dominant polymer will try to reject the other. The chemical paranoia thus set up can cause the A molecule ends of a polymer A-B-A to curl up into a ball to minimize contact with B. The result is a polymer in which spheres of A molecules are found dispersed fairly regularly in a continuous matrix of B molecules. B ..... B I ....... ........ R . ~ The potentialities of such molecular design are vividly shown by comparing the tensile strengths of the two types of triblock polymers that can be made from butadiene (B) and styrene (A). With B chains containing 1400 B molecules and

III-C. NEW PRODUCTS AND MATERIALS A chains with 250 A molecules, the triblock polymer A-B-A has a useful tensile strength (about 30 MPa). If the polymers are hooked together in the reverse triblock arrangement, B-A-B, the polymer is a viscous liquid, showing no real tensile strength at all. The first of these two, A-B-A, can be shaped to desired form at high temperature. On cooling to room temperature, it becomes rigid and behaves like a cross-linked rubber. However, unlike conventional rubber, the A-B-A block polymer can be warmed again and reshaped. Such "thermoplastic" behavior has many functional applications. This is only the beginning, however. The ability of block polymers to self-organize into microdomains of 10 to 100 A size and of different shapes (spheres, rods, planes) is sure to provide new materials with hitherto unknown combinations of properties. The microdomains can impart orientational pref- erences (anisotropic behavior) in mechanical, optical, electrical, magnetic, and flow properties. As research advances give us control of these various dimensions, new applications, new devices, and possibly new industries will be seen. Photores~sts and Chemical Etching Microelectronics is increasingly dependent upon compressing intricate elec- trical circuitry ("integrated circuits") onto extremely small semiconducting wafers ("silicon chips". Demagnification of a carefully drawn circuit diagram coupled with some sort of photographic transfer is a favored technique. Conse- quently, the fabrication of these complex circuit devices depends critically on the use of thin films (less than .01 microns) of radiation-sensitive polymers called photoresists. In a typical use, the photoresist is placed on top of an equally thin layer of insulating silica (SiO2), which, in turn, covers the silicon whose conductive properties will determine the electrical functions of the chip. The photoresist film is exposed to the sharply focussed image of a desired circuit using a wavelength of light that will either cross-link (to strengthen) or degrade (to weaken) the polymers. Then a suitable solvent dissolves away the uncross- linked (or degraded) parts of the polymer film. This step exposes the SiO2 layer ~ ~ · ~ ~ ~ ~ ~ ~ ~ · · ~ `* ~ ~ ~ ~ ~ · ~ · ~ ~ ~ ~ below In the pattern of the circuit. Next, the exposed 5102 IS etched away by more aggressive chemical agents, to which the photoresist polymer must be impervious. Finally, the silicon substrate lies exposed in a faithful reproduction of the original circuit with a density of electrical elements approaching a million per square millimeter. Older processes used visible light for the photo-cross-linking, drawing upon familiar photographic experience. However, the insatiable desire to place ever more circuit information on a single chip brought the process to a point where diffraction effects at the mask features became limiting. The diffraction effects can be reduced by using shorter wavelength radiation, and chemists are now at work developing new organic polymers sensitive to exposure to electron beams, X-rays, and short wavelength ultraviolet light. 53

54 CONTROL OF CHEMICAL REACTIONS Of course, the photoresist image must still survive the succession of etching steps. Fluid etchants must not swell the mask, and the photoresist mask must remain intact during the subsequent etching of the SiO2. The current trend is to use chemically reactive gas plasmas (the emitting gas in a fluorescent tube is a "gas-plasma") to etch the exposed SiO2 surface. It is difficult to design polymers with the necessary combination of chemical and physical properties for use in this technology. Yet they are essential for the manufacture of the next generation of memory chips that will appear in the latter half of this decade and that are expected to carry one-half billion elements (information "bits") per chip. New types of polymers are needed to obtain the optimum high thermal stability, dimensional stability, and easy processing. Hence, economic compet- itiveness in the microcircuit industry will depend heavily on continued ad- vances in polymer chemistry. Novel Optical Materials Optical Fibers Just as the vacuum tube has been replaced by the transistor in modern electronics, copper wires are being replaced by hair-like silica fibers to transmit telephone conversations and digital data from one place to another. Instead of a pulse of electrons in a copper wire, a pulse of light is sent through the transparent fiber to convey a bit of information. The critical development that made this optical technology possible was the production of highly transparent silica fibers through a new process known as chemical vapor deposition (CVD). Essentially, a silicon compound is burned in an oxygen stream to create a "soot" of pure silica that is deposited inside a glass tube. The tube and its silica deposit are drawn out to produce a glass-coated silica fiber about one-tenth the diameter of a human hair. The CVD process made it possible in less than a decade to reduce fiber light-Iosses from 20 Db/km to .2 Db/km. Fibers that are even more transparent may result from a new class of materials, the fluoride glasses. In contrast to traditional glasses, which are mixtures of metal oxides, fluoride glasses are mixtures of metal fluorides, such as ternary glasses derived from ZrF4, I~aF3, and BaF2. Although many practical problems remain to be resolved, the new glasses would, in principle, permit transmission of an optical signal across the Pacific Ocean without any relay stations. Optical Switches In addition to chemistry's role in development of new materials and processes for optical fibers, it also plays a major part in synthesis of materials for "active" optical devices. These are devices to switch, amplify, and store light signals just as silicon-based devices manipulate electrical signals. Current optical devices are based on lithium niobate and gallium aluminum arsenide, which are spin-offs from the electronics industry, but there is great potential for new

III-C. NEW PRODUCTS AND MATERIALS materials with unusual optical properties. Chiral organic molecules, liquid crystals, and polyacetylenes can display desirable optical ejects greater than those of lithium niobate. The potentials for discovery and practical applications in this field are especially high. Novel Electrical Conductors Semiconductors The modern age of solids was launched during the 1950s by brilliant advances of solid state physicists as they developed deep understanding of pure semicon- ductor materials. There were early challenges to chemists, too, as it became clear that elemental silicon and germanium were needed in single crystal form with impurity levels as low as one part per 100 million. Thereafter, comparably interesting behaviors were found in compounds of two elements, one from the third group of the Periodic Table (gallium, indium) and one from the fifth group (phosphorus, arsenic, antimony). These "TIT-V" compounds are typified by the mixed semiconductors, indium antimonide, which has for 15 years provided one of the most sensitive detectors known for near-infrared light. Lately, much attention has been given to single crystals of the TIT-V compound gallium- arsenide grown on single crystal substrates of indium phosphide, another IlI-V compound. The resulting structures may have as many as half-a-dozen layers of controlled Ga/As ratio, impurity composition ("doping"), and thickness (micron or less). This class of materials forms the basis for lasers and laser display devices (LED's) for long-wavelength optical communications. As the range of materials used in semiconductor technologies has broadened, more and more chemists have joined the physicists in such work. This upswing of the chemist's participation was encouraged by the startling discovery that amorphous silicon can also demonstrate semiconductor behavior. Because the prevailing and extremely successful textbook theory of semiconductor behavior is based upon the properties of perfectly ordered solids, such amorphous semiconductors were neither predicted nor comfortably described by theory. Language more familiar to chemists than to physicists has come into use (e.g., "dangling bonds"~. We are on the verge of a new era in the solid state field, one in which physicists will continue their important role in characterizing new solids, but in which chemists wit] now play an increasingly important role. The reason is that entirely new families of electrically conducting solids are now being discov- ered—families susceptible to a chemist's ability to control local structures and molecular properties. As will be seen, some of these new families are inorganic solids and some are organics. Conducting Stacks The field of organic conductors had its beginning in the late 1960s to early 1970s with the synthesis of charge-transfer complexes formed by the reaction of 55

56 l of I of POLYACETYLENE AN I NSU LATOR B ECOM ES A M ETAL 1 o2 1 2 0 0 -8 0 -10 0 10 -12 -14 -16 0 -18 0 CONVENTIONAL NOVEL MATERIALS CONDUCTORS SILVER, COPPER BISMUTH MERCURY GERMAN IUM SILICON SODI UM CHLORIDE IODINE GLASS DIAMOND SULFUR QUARTZ PARAFFI N INDIUM iTTF TCNQ ANTIMONIDE ~ Act' IF CONTROL OF CHEMICAL REACTIONS compounds such as tetra- thiafuIvalene (TTF) with tetracyanoquinodimethane (TCNQ). Both of these mole- cules are flat, and in their mixed crystal they are found alternately stacked like poker chips. The interaction be- tween two neighbor mole- cules is a familiar one to chemists- a "charge-trans- fer" complex is formed. Such an interaction always in- y eludes an electron donor, a molecule from which elec- trons are readily removed, and an electron acceptor, a molecule that has a high elec- tron affinity. These two roles are filled, respectively, by TTF and TCNQ. The surprise is that this charge transfer between two neighbors in the crystalline stack provides a mechanism for current flow up and down the stack, the length of the crystal. When detailed study showed that the charge-transfer crystal, when cooled to 60 K, conducted electricity as well as copper at room temperature the excitement grew. The bright future for conducting stacks has recently been assured by the imaginative synthesis of polymeric conductors based on charge transfer inter- actions. Large, flat molecules again furnish the elements of the conducting stack (metallomacrocycles). However, the clever innovation lies in lacing them together with a string of covalently bound oxygen atoms. The fact that this chemically designed molecule is, indeed, an electrical conductor is quite a breakthrough. Plainly, the metal atom and the peripheral groups in the flat metallomacrocycle can be mod- ified in great variety. These units can then be connected by an intervening atom chosen to give the desired spacing. The result is a polymer in which carefully chosen macrocycles are held in a molecular stacking that is rigidly enforced and designed to fit the desired function. / .- 7 ,, , .7 r ~ r7 PHTHALOCYAN INKS LINKED IN A "CONDUCTING STACK"

III-C. NEW PRODUCTS AND MATERIALS Organic Conductors Polyacetylene is one of the simplest organic polymers. It has a carbon skeleton of alternating single and double bonds. Chemists call this bonding situation "conjugation," which means that electric charge has special mobility along the skeletal chain. Nevertheless, it came as a surprise when, half-a-dozen years ago, the unusual electrical properties of these polymers were discovered. When exposed to suitable chemical agents such as bromine, iodine, and arsenic pentafluoride (which physicists call "dopants"), such polymers assume metallic lustre and display electrical conductivi- ties higher than those of many metals (though not yet as high as copper). Structural studies have since shown that the electrical conductivity requires molecular structure control (the trans- form has electrical behavior different from that of the cis-form). The most amazing and perhaps the most promising aspect of the polyacetylene polymers is that they can display electrical conductivity over a range of 14 orders of magnitude, depending upon the skeletal structure His or bans) and the chemical exposure selected. C C<"""' 57 CI_ , .. . ... ~ CIS ,C C" ,C TIC Ad \ C// A/. .. C C i ........... TRANS ~ . . POLYACETYLENE DIFFERENT STRUCTURES => DIFFERENT PROPERTIES plainly, the gates are open now, ana orner conducting polymers are already appearing. Thus, the polymer poly(para- phenylene) has been shown to become a conductor on suitable chemical exposure. Next came poly~paraphenylene sulfide) and poly-pyrrole. Now, the artistry of the organic chemist can be brought to bear in combining electrical conductivity with the other manifold benefits of polymers, such as structural strength, thermoplasticity, and flexibility. Semiconductors and transistors became possible when it was shown that polyacetylene could be electrochemi- cally "doped" to either a p- or e-semiconductor. Because response to light can be designed to match the solar spectrum, these polymers give us hope for cheap organic photovoltaic cells with which to convert solar energy to electricity. Extensive research is in progress to develop lightweight, high power density, rechargeable batteries with polymeric electrodes. Superconductors Another discovery as significant as polyacetylene was the synthesis of oure' single crystals of the inorganic polymer, polyLsulfur nitride), (SN)x. This material not only showed metallic conductivity, it was found to become superconducting at about .3 K. It was the first covalent polymer with metallic conductivity (preceding polyacetylene by 4 or 5 years) and also the first covalent polymer composed of nonmetals to show superconductivity. It liberated the _ _ _, — — 1

58 CONTROL OF CHEMICAL REACTIONS thinking of solid state scientists about candidates for electrical behavior as the search proceeded for new superconductors with high transition temperatures. The potential power of chemistry in furthering developments here is sug- gested by the conducting stack compound mentioned earlier involving tetrathiafuIvaTene (TTF). Chemists have now synthesized the analogous com- pound in which the sulfur atom in each TTF molecule is simply replaced by a selenium atom. Like TTF, this selenium analogue also forms conducting salts but now displaying superconductivity at much higher temperatures than poly (sulfur nitride), (SN)X. It is suggestive and significant that this substitution of selenium for sulfur can have such profound effect on conduction. Inorganic compounds involving three elements are also under systematic study, and materials with relatively high superconducting temperatures have been discovered among the family of ternary compounds known as Cheverel phases. An example is PbMO6Ss, which can remain superconducting in the presence of magnetic fields of several thousand gauss. This is a crucial property because construction of compact, high-field magnets is one of the most impor- tant applications of superconductors. The cagestructure nature of the MO6SS unit in the Cheverel phases might be a key structural factor in their electrical and magnetic properties. Such systematic investigations will undoubtedly lead to new directions for research in superconductors. In pursuing these new directions, synthetic inorganic chemistry will be in a central position. Solid State ionic Conductors Solid materials with ionic structures are now known with ionic charge mobilities approaching those in liquids. Investigations of such materials over the last decade have already led to their use in memory devices, display devices, chemical sensors, and as electrolytes and electrodes in batteries. Thus, sodium beta-alumina provides the conducting solid electrolyte in the sodium/sulfur battery. Normally an ionic solid-like sodium chloride has fixed composition and is an electrical insulator. The new solid electrolytes are produced through deliberate manipulations of crystal defects and deviations from integer chemical formulas (nonstoichiometry). The mobile charge carriers might be small ions like lithium ion or hydrogen ion in a crystal lattice that facilitates charge migration. Thus, substances with layered molecular structures graphite is a familiar exam- pIc provide versatile crystal hosts for such behavior. Charge insertion between the weakly bound layers, called intercalation, places them in a two-dimensional zone where mobility can be exceptionally high. Many such layered structures are known, so significant opportunities for new discoveries lie ahead. In a practical application of ionic conduction, zirconium dioxide is used as a sensing element in the oxygen analyzer of an automobile emission control system. The electrical conductivity of this solid changes with the oxygen content of the exhaust gases.

III-C. NEW PRODUCTS AND MATERIALS And Other Things By no means has our discussion of novel conductors been all-inclusive. Here are additional research areas of promise. Acentric materials (materials with directional properties, such as ferroelec- trics and pyroelectrics) are under active development; they include a wide variety of ionic crystals, semiconductors, and organic molecular crystals. Both electrical and optical applications are foreseen: optical memory devices, display devices, capacitors for use over wide temperature ranges, piezoelectric trans- ducers, pyroelectric detectors (for fire alarm systems and infrared imaging), nonlinear optics (second harmonic generation, optical mixing, and paramag- netic oscillations, and integrated optics. To cite an example, the polymer of vinylidene chloride—(CH2CCI2)n—is piezoelectric and has found use in sonar detectors and microphones. Filamentary composites are solids containing metallic filaments of small and controlled diameter (.1 to .005 microns in diameter). The physical characteris- tics of such compounds, including electrical properties, inhibit striking depen- dence on the size and uniformity of dispersion of the filaments. Conducting glasses, both metallic and semiconducting, can be prepared by rapidly freezing a liquid, condensing gases on a very cold surface, or ion- implanting in ordinary solids. Thus, amorphous, semiconducting silicon can be prepared by rapidly condensing the products from a glow discharge through gaseous silane, SiH4. The performance of the Tow-cost solar cells made of such material depends critically upon hydrogen impurities chemically bound to the interstitial "dangling bonds" of the silicon atoms randomly lodged in the amorphous solid. Also, inorganic nonmetallic glasses are important for optical fiber communication and for packaging solid state circuits. Materials for Extreme Conditions Many areas of modern technology are limited in performance by the available materials of construction. Jet engines, automobile engines, nuclear reactors, magnetohydro-dynamic generators, and spacecraft heat shields provide contem- porary examples. The hoped-for fusion reactor lies ahead. Engine performance provides a convincing case. Any thermal engine, be it steam, conventional internal combustion, jet, or rocket engine, becomes more powerful and more .. efficient if the working temperature can be increased. Hence, new materials that extend working temperatures to higher ranges have real economic impor- tance. New Synthetic Techniques There are a number of promising synthetic techniques for producing new refractory materials. Among them are ion implantation, combustion synthesis, levitation melting, molecular-beam epitaxy, and plasma-assisted chemical vapor deposition. Most recently, laser technology has provident unusual syn- 59

60 CONTROL OF CHEMICAL REACTIONS thetic approaches. A high-power, pulsed laser beam focused on a solid surface can locally create a very high temperature (up to 10,000 K) for a very short time (less than 100 nanoseconds). Such a transient high temperature pulse can cause significant chemical and physical changes, modifying the surface, forming surface alloys, and promoting specific chemical reactions when coupled with vapor deposition. All these techniques share the ability to form thermodynam- ically metastable compounds with special properties "frozen in." (Diamond is an example. This expensive gemstone, valued for its sparkling beauty and extreme hardness, is thermodynamically unstable with respect to graphite under normal conditions.) Some Examples Real and Projected Two examples of"exotic" high temperature materials recently developed are silicon nitride, Si3N4, and tungsten silicide, WSi2, both of technological importance in the semiconductor industry. The first, Si3N4, can be an effective insulating layer even at thicknesses below .2 microns. The second, WSi2, is a low-resistance connecting link in microcircuits. Plasma deposition synthetic techniques allow sufficient control to permit these high temperature materials to be deposited upon a less refractory substrate held at much lower temperatures (usually below 700 K). Thus the temperature-resistant material can be deposited without detracting from the desired electrical properties of the substrate. Polymer precursors offer another promising route to new, "high-tech" ceram- ics. Preformed silicon-containing polymers, on pyrolysis, form silicon carbide and silicon nitride solids with the predetermined complex shapes. These and other recent advances in the synthesis and fabrication of ceramics make it reasonable to anticipate the future construction of an all-ceramic internal combustion engine. Conclusion The next two decades will bring many changes in the materials we ~~ TV materials In which we are clothed, housed, transported, the materials of our daily lives. New industries will be founded just as polymers led to synthetic fabrics, as phosphors led to television, and as semiconductors led to computers. Metals will be used more deliberately and sparingly as tailored materials outperform them in many of their traditional functions. It is the potentiality for tailoring that points to the increasing role for chemists in this interdisciplinary field. Ultimately the control of the properties of any material depends upon understanding its composition, bonding, and geometry at the atomic/molecular level the chemist's home territory. What we can do with this understanding then depends on what we can make- and synthesis is again the chemist's bag. That is why industries dependent upon use of new materials are looking for bright young chemists to add to their scientific staffs. That is why more chemists are being attracted to research in the materials sciences. The chemist's talents are not the only ones needed, but they are among the essential ones.

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: : :~ ::: ::: ~ :: : ::: ~ : : :: :: : ::~ : ::: ::::::: : : :: :: ::: ~ I: : ~ :: : ::: : ~ : :: ::::: I: I: ~ : : : ::: ~ :: : : : : : : ~ : : :: : :: ~ : : Hi: ~ ::: :: :::: : : :: : : ~ : ~ : : :: ~ ~ I: ~ ~ : ~ :: : ~ ~ ~ :: : ~ ~ ~ ~ ~ I:: :: ~ :: : :~ ::: ~ : : ~ : ~ I:: ~ ~~ ~~:~::~: :; 1 ~e~:~Tim:e:~: ~It~:~T~akes~to:~:~Wag ~~a~:~Tai1 ~ ~ ~~:~::~ ~ ~~:~ :: ~:~ ~ ::::: ~~ ~~:::~ ::::: ~~ ::: :: :~: :~ ~ I: ~ ~ :: ~ If: :: : :: :: Hi: ~ I: I: ~::~: ~ ::: ~ ~ I: : : ~ When~your~pet~g ~~sniffs~a~bone, instantly his tail begins~to ~~1vag.~ But it~must Take some time~for the~northerrunost~canine~extremity~to send~the news~all~the way ~south~where~enthusiasm~can~be~registeredl~lIow~long~does~it~takeforthatdelicious~ ~aroma~to~lead~to the~happ,? response at the other~end?~ Chemists~are now~asking~the~ questions Much Bali emit this a He Pet me le u1 ~s~!~If one end Excite do how ~~1ong~does~ t~take~r~ the other e don't Share i t e ~~;m~det r items wheat ~ the~excitatio~ Law Ill ~ result ~~ Ad chemical :: ~~ ~~ ~~ ~ ~ ~ ~~ ~ ~ ~ ~~ ~~ the~part~ ofthe~molecule~v~here the~energy was~inJeCted,:somewhere:else,~or~nowhere ~~ ~~ ~~: stalls ~~ ~ ~~ ~ ~~ ~~ ~ ~~;~ ~~ ~~ ~ ~ ~ ~~ ~ ~ ~ ~ ~ ~ . . ~ :: ~:~ ~~ ~ Yore ,. le~can~ne~experiment,~ ~we~need~a hung~y~dog,~a~quick~hand~with~the~ bone, And a ~q~ul~ck~eye~to read~the stopwatch~.~For~molecules, it's~much~harcler.~Only~w~ithi~n~ The ash ~~ftw~years~h~as~ ~it~b~een~possi~b~to~ measure ~~the~rate~of~ ener~r~moveme~; w~th~n~a~mo~lecule.~But ch~em~sts~now have pulse lasers giving bursts of light filth Durations ~~a~s~sho~ as~a~m~lionth~ of a~mill~ionth~of a~second~(a~"picosecond'~'~.~C'om-~ ~~ ~~ ~~;~ ~par~ng ~~a~chem~cal~changp~that~takes~ place~in~1~ picosecond to~a~l~-se£orld~tai1-wag delay 7nvolves~the~ same sp*ed-up~ as ~ a 10-second instant replay= ~~of~alI~historical ;~ ~ ~ ~ ~ ~ Vow iamb A miss Were ouiit. ~~ ~ ~~ ~ ~ ~ aft; ~~ ~~ ~ ~ ~ ~ ~~ ~ ~~ ; The Al ~benzenes~provide An example.~Each~of t7nese~ molecules has fat rigid benzene ring at One en and aft flexible All ~group~at~thie~ ~other.~At:~room I, temperature, ~~ thistle flexible "Gil" Vibrates flank bends ~ under Thermal Eli- ~~ ~~ ~~ ~ ~~ , : :: ::: ~ :~ :~ ~~ :: I: :~ :; ~~ : I:: ~~ ~ ~ : I: :::: ~~ :::: ~~:~:~::~:~::: ~ ~ ~ :: ~~ ~ : ::::~:::~ :::: ~ I: :~ I:: ~~ ~ ~ I: I: ::: : :: ~ ::: ~ If: :~ : I: :: : :: :: :: ~ : :::::: :::: ::::: : ~ : :: : :::: :::: :: :: tation~.~nut ~~to~act~l~ke Our hungry am,: thief molecules~must ~be~coolLed to cryogenic ;~ ~ ~~ ~~ ~;~;~ ~~ ~~ ~~ ~~temperatures,~while~avoidI~ Condensation ~~;~Supeisonic~j~et~ possible~.~When~a~gasS~mixtbre~flows~througtNa~jet~nozzle~into~a~high~vaeuum,the~ ~moleculescan~becooled~almost~toa70solute~zero~.AnalkyI~benzene~molecule~carried ~;~ ~~ ~~ ~~along~n such ~a~stream~Ioses~all~its~v.ibrationa~energy, Thus relaxing t~e~molecular ~tail.~Then,~thecold7no7leculesintersect~a~'0rief~pllIse~oflight=~withcolorthatisab6,orbed ~by~the~benzene~ring.~Withu~eareful~"coltor-tuning,"~e~xtra~vib7ational~e~nergy canoe ;~ Placed in~e~he~wi~o~ than Vibration 1 Axe tatio~n~ in;; ~vvatch~the~mpleculeto~see~howlongit~takes~fo~r~the~tailtowag.~Fluoreseencelets~ ~ ~ at; : ;:~ ~~::US:( A . 31S.:~::~: :::::::::: :::::;: ~~;: ~ ~~ ~~;~ ~;~ ~~ ~~ ~~;~;~ ~ ~~ ~~ ~~ ~~ ~~ ~~ ~ ~When~a~molecu~le ~in~a~vacuum~abso~'s :: ~ ~ cog lo, :: I ~~lig~t,~the Only ~ way it~can~get~ roof The ~:~:~ener~is~:to ~~re~-emit;~light~.~Suth~fluorescence~:can:~be~::~recorded:: with:~a :~f~st-respo~nse ~detection~system to give Ma Spectrum that tarries a ~~tell-tale~ pattern showing~where t. lee extta~energy~wa~s~at the instant the ~~ight~wa~s~ emitted. ~Those~mo~lecul~es~ that ~happen~to~emit~ri~ht~aw~after~excItatio~n~sho~w~the~mole~cul~ar~head~vibratingand ~lThe~tail~;stil~col~.~l~hose~that~emit~later~have~an emission~spectrum that~shows~th~at~ one brat Swagging. ~ This Sways, Tweet have;~learned thatch the time itch takes Or the~alkyl; Benzene ~tai To Begin ~to~wag~depen~on~how~ ~lo~g~the~tail~ ~is. ~Supprisingly,~the~ longer ~e~alkyT,~the~sterthe~movement~out~ofthe~ring.~'rhe~result~showswhat~determines~ ~ener~flow~withirl~mol~ecuTes~(the~"aensi~ty~Qfstates").~Such~infiormation~might~one~ ~clari~combustion~and~help~us~make~nn~e~c em~icals~out~ofcoal. : : : ~ :: : : :: ~ : ~ : :::: : : : : : ~ : :::: : : :: ~ ~ : :: :: :

III-D. INTELLECTUAL FRONTIERS IlI-D. Intellectual Frontiers A substantial array of benefits has been shown to flow from and depend upon chemistry. Subsequent chapters will amplify this theme. All these rewards derive ultimately from our ability to control chemical change, a control made possible by our understanding of chemical reactivity. That is the foundation upon which chemistry is built. Fortunately, this is a time of special opportunity for intellectual advances that will greatly strengthen this foundation. The opportunity derives from our developing ability to probe the elemental steps of chemical change and to deal with extreme molecular complexity. Powerful new instrumental techniques are a crucial dimension. They account for the recent acceleration of progress that gives chemistry unusual promise. Three of these intellectual frontiers of particular promise will be explored here: molecular dynamics, some aspects of chemical catalysis, and reaction pathways. Molecular Dynamics Molecular Dynamics encompasses the theoretical and experimental explora- tion of: cles i. the energetics, structures, and reactivities of equilibrium molecular spe- ii. the detailed mechanisms by which such equilibrium species change from one set of structures to another. The theoretical foundation for all chemical behavior resides in quantum mechanics. Though this has been known for half a century, most of the predictive benefit to be derived has been out of reach because of mathematical intractability. Experimental progress on equilibrium molecular species has been rapid and counts as one of the successful enterprises in chemistry of the last three decades. It undergirds the remarkable advances in chemists' ability to control molecular compositions and configurations and, hence, chemical prop- erties. There is no more convincing evidence of this prowess than the single datum that chemists have prepared and characterized more than eight million compounds, 95 percent of them since 1965. In contrast, our advance on the temporal aspects of chemical change has been sporadic and limited by experi- mental bounds set by transient events too fast to be observed. Now a new era has begun. Chemical theory, in the hands of a brilliant cadre of chemical theorists and supported by the power of modern computers, has emerged from heuristic empiricism. At the same time, we have at last a battery of new experimental techniques that has opened the way to detailed mapping of the temporal dimension of chemical change. Over the next three decades we will see advances in our understandings of chemical kinetics that will match the advances in molecular structures over the last three decades. 63

64 CONTROL OF CHEMICAL REACTIONS Fast Chemical Processes in Real Time ~XT~ STIR 1~— +~ +~1_.~ ~ ~~ ~~ ~ ~ . ~ Most crucial to these advances is the capability to identify and characterize intervening molecular species that participate in the train of events between the mixing of reactants and the formation of final products. Fifteen years ago, Y,~ tV=1~ 11111;~" ~~ ~' "~511g aim uranslen~ molecules tnat persisted as long as a millionth of a second. The many interesting studies on this time scale only whetted chemical appetites because it became clear that a whole world of processes took place too rapidly to sense at that limit. Nowhere was that more apparent than in the centuries-old desire to understand combustion, perhaps the most important reaction type known to man. Lasers have spectacularly expanded these experimental horizons over the last decade. One of their novel capabilities is to provide short duration light pulses with which to probe chemical processes that occur in less than a millionth of a second down to another million-fold (from a microsecond, 10-6 see, to a picosecond, 10-~2 see). At the state-of-the-art, physicists are learning how to shorten these pulses even more; pulses as short as .01 picoseconds (10 femtoseconds) have been measured and kinetic studies are just beginning in the .1 picosecond range. At one-tenth of a ninny fr~l,Pnr~r ~~ril'~' it! 1;~;~ ~ _ _~ ~ ~ ., ~ ~ ~~ air c~~ ~1 any 1~ 111111 ~~Q to about 50 cm- l by a fundamental physical principle the "Uncertainty Principle." This implies that chemists can now interrogate a reacting mixture on a time scale short compared to the lifetime of any transient molecular species that can be said to have an identifiable vibrational signature. The exploitation of this remarkable capability has only just begun. At this time there are probably fewer than 500 pulsed and tunable dye lasers being used for nanosecond spectroscopy throughout the world. There may be fewer than two dozen laboratories studying chemical reactions in the 1()-1 non picosecond time domain. Already a host of exciting types of investigation have been demonstrated, a number of them opening entirely new fields. Study of transient species free radicals, electronically excited molecules, intermediates in photosynthetic processes, vibrationally excited molecules goes beyond de- tection and characterization. It now includes quantitative measurements of time behavior, such as reaction rates, energy relaxation rates, reactivity, and mobility. Such capabilities foretell a decade of striking advances in our understanding of the factors that control rates of chemical reactions. For example, when benzophenone in ethanol solution absorbs light at 316-nm wavelength, it reemits at two longer wavelengths, near 410 and 450 nm. The active emission processes are definitively clarified when 10-picosecond light pulses are used for excitation (at 366 nm) and the emission is collected with attention to time scale. "Prompt" emission is seen at 410 nm with intensity that diminishes with a 50-picosecond half-life. This corresponds to prompt reemis- sion of the light absorbed in the singlet excitation of the n, IT* state, So. It occurs at slightly longer wavelength because of vibrational deactivation that takes place too rapidly to measure, i.e., in less than 10 picoseconds. Weaker emission

III-D. INTELLECTUAL FRONTIERS at 410 nm continues, however, with a half-life four orders of magnitude slower, i.e., in about 1 microsecond. This "delayed" emission can be attributed to the transfer of some of the So excitation to the Tower-lying triplet state TO followed by thermal reexcitation to the SO state. Then the emission at still longer wavelengths, 450 nm, is due to direct emission from TO to the ground state, SO. All this is verified by its temperature dependence. At Towered temperatures, the delayed fluorescence disappears, and the 450-nm phosphorescence becomes much brighter and diminishes with a much longer half-life, about a millisecond. Thus the temporal measurements verify the interpretation and give us quantitative information about the speed of the various processes. The vibra- tional deactivation with SO occurs in less than 10 picoseconds. Then the singlet-triplet transfer occurs with about a SO-picosecond lifetime since it regulates the intensity of the "prompt" emission. The delayed emis- sion reveals the SO SO radia- tive lifetime, in agreement with expectation based on ab- sorption coefficient. Finally, the phosphorescence decay at low temperature measures the lifetime of the triplet state, whether by TO SO "for- bidden" radiation or by radi- ationIess relaxation pro- cesses. Here we see clarified a set of processes that have characteristic times from 50 picoseconds to a millisecond, a range of 100 million. All these types of energy movement are active in natural photosynthesis. There are many other types of laser-based, real-time studies of rapid chemical reactions now being made, including chemical isomerizations, proton-transfers, and photodissociations. Some of the phenomena to follow also depend upon use of short-puIse laser excitation, molecular beams, fast-response electronic cir- cuits, and computer-controlled data acquisition and interpretation. Such instru- mentation has opened new frontiers of chemistry. he o FLUORESCENCE it> ~ PHOSPHORESCENCE 'PROMPT' he' ho, 50 psec , 1 so so— 50.l0~l2 sec 1. lo-6 sec . 'DELAYED' hit 1 Em so 1~10-3 sec EXCITED BENZOPHENONE EM ITS L IGHT W ITH TWO COLORS AND THREE CLOCKS me - ' Energy Transfer and Movement In all chemical changes, the pathways for energy movement are determining factors. Competition among these pathways, including energy dissipation, determines the product yields, the product state distributions, and the rate at which reaction proceeds. This competition is influential in stable flame fronts (as in Bunsen burners, jet engines, and rocket engines), explosions, shock waves, and photochemical processes. T

. 66 HOT RING COLD TAIL OCOLD RING COLD TAIL CONTROL OF CHEMICAL REACTIONS The collisional transfers of vibrational ener~v h.?t~ween an`] within mn1 Ill . . . have long been recognized as key processes in determining kinetic behavior. Yet progress has been slow because of limited experimental access to the phe- nomena. Now a variety of techniques—almost all based on laser methods has opened the way to providing critical data relevant to the pathways and rates of energy flow. These data, in turn, furnish a basis for the testing and development of useful theory. As much has been learned about vibrational energy movement in the last 15 years as was learned in the preceding half-century. As tuned lasers became available, they were used to excite selectively particular modes in a molecule. Then, a variety of detection schemes revealed subsequent transfers of that energy into other degrees of freedom, either of the same molecule in absence of collisions or of the molecule and its collision partner, if collisions occur. Propensity rules are emerging that govern the probability of particular pathways. Nanosecond and picosecond excitation, often combined with fluorescence detection, are giving quantitative rate constants for these extremely fast processes. A clear-cut example is provided by recent studies of the alkyl benzenes, C6H5-(CH2)nCH3 with n from 1 to 6. This molecule has a structure like that of a pollywog, where n determines the length of its tail. First, the molecule is cooled almost to O K by gas expansion through a supersonic jet. Then tuned- laser excitation permits deposition of prescribed amounts of vibrational energy in one end of the cold molecule (in the head of the polywog) under conditions in which the molecule has no collisions with other molecules. When this energy is reradiated (fluorescence), it has a spectral signature that characterizes its vibrational excitation at the instant of radiation. Since this light emission is a time-dependent process, it permits us to mon- itor the movement of energy from the locus of excitation into the rest of the molecule. This movement in absence of collisions is called Intra- molecule Vibrational R e- distribution (IVR i. Light emitted in the first few pi- cosecon(ls shows that the en- ergy has not yet left the benzene unit where it was absorbed. The time scale for appearance of vibrational ex- citation in the alkyl tail (le- FL U ORES CEN CE REVEALS I NTRAMOLECULAR VIBRATIONAL RED ISTRIBUTION 6~>>~7 COLD RING HOT TAIL . ~ _

III-D. INTELLECTUAL FRONTIERS pends upon the tad] length. For n = 4, vibrational energy moves out into the tail in 2 to 100 picoseconds. In contrast, for n = 1, (ethy~benzene), it takes much longer, probably 100 nanoseconds or more. Thus, we have direct evidence about the factors that determine IVR energy movement in an isolated molecule. This technique is called laser-induced fluorescence. Of course, a reaction between two molecules requires that they come close together, so chemists must also know about energy transfer caused when molecules collide. Some experiments successfully explore these processes with ·1 ~ · 1 · ~ ~ · · ~ ~ ~ ~ ~ ~ · ~ ~ · ~ ~ vibrational excitation using a short-pulse laser and time-resolved infrared fluorescence measurements. In another type of experiment, the highly reactive formyl radical HCO can be produced from formaldehyde with a 12-nanosecond pulse of ultraviolet radiation at 308 nm. Then a second laser tuned to a suitable HCO absorption reveals the presence of HCO, its vibrational state, and its lifetime. In the presence of various collision partners, such lifetime data show collision efficiencies for both vibrational deactivation and reaction. The detailed understandings to be gained are shown by the different behaviors observed with the two diatomic collision partners N2 and NO. Vibrational deactivation of the bending motion of HCO required about 1500 collisions with N2 but only about 10 collisions with NO! This two-order- of-magnitude increase in collision efficiency is made even more intriguing because reaction be- 67 .., ~ ABSORBANCE OF _ )~ HCO (010) - b , . . . . . . 0 2 4 6 8 10 T I nE (mi croseconds) MCO (010) ~ ~ _ HCO (0003 + ~ ~ ~ . VIBRATIONAL DEACTIVATION tween HCO and NO takes place as well Surprise- MEASURED IN REAL TIME ingly, the results show that the reaction is slower if HCO is vibrationally excited than if it is not! The unexpected outcome is reasonably interpreted in terms of a long-lived collision complex. These elegant experiments display the power of modern spectroscopic techniques in clarifying rapid and competing chemical processes. State-to-State Chemistry When two gaseous reactants A and B are mixed and observed to react and form products, C and D, the outcome is a statistical one. The encounters between A and B include all the possible energy contents, specific types of excitation, and orientational geometries at the moment of collision. Not all of these collisions are favorable for reaction—most collisions have too little energy, or the energy is in the wrong place, or the collision is at an awkward geometry. If we are to understand in all detail the factors that permit chemical reactions to occur, we would like to control the energy content of each reactant, i.e., control the "state" of each reactant. Then we could systematically vary the amount and type of energy available to determine specific reaction probability. Finally, we would like to see how the available energy is lodged in the products. Such an

68 CONTROL OF CHEMICAL REACTIONS experiment is called a "state-to-state" study of reaction dynamics, and 20 years ago it was beyond all reach. Now, with modern instrumentation, chemists are realizing this goal. The earliest efforts, based upon chemiTuminescence, revealed a part of the picture: the energy distribution among the products. For example, the reaction products from a gaseous hydrogen atom and a chlorine molecule emit infrared light. Spectral analysis shows that much of the energy of this heat-releasing reaction is initially lodged in vibration of the hydrogen chloride product (39 percent). This led directly to the demonstration of the first chemical laser a laser that derived its energy from the hydro- CHEMICAL LASERS REVEAL THE PRODUCT ENERGY DISTRIBUTION F+ H2 IN ko\k_ o V = 0 HE+ H BERCTIOR COORDIDBTE tailed shape of the reaction surfaces. c, %, gen/chIorine explosion. Chemical lasers then joined chemiluminescence as a means for deter- mining in a detailed way the energy distribution among the products in quite a variety of gaseous reactions. Through the principle of microscopic reversibility, these findings are equally informa- five about the important degrees of freedom the ones that need to be energetically excited to cause the reverse energy-consuming reaction. These beginnings led to the discovery of dozens of chemical lasers, including two sufficiently pow- erfuT to be considered for initiation of nuclear fusion and for military use (the I~ and HE lasers). New concepts based on information theoretic con- siderations were developed, giving us a new basis for describing these nonstatistical behaviors the "surprisal" method. The data encouraged ab initio and trajectory calculations to investigate the de- "Molecular beams" move even closer toward "state-to-state" investigations. In such experiments, reactants meet at such low pressures—10- if atmo- spheres that each reactant molecule has at most one collisional opportunity to react, and the products have none. These sophisticated instruments depend upon ultrahigh vacuum equipment, high-intensity supersonic beam sources, sensitive mass spectrometers for detectors, and electronic timing circuitry for time-of-flight measurements. It has become possible to select the energy state of each reactant molecule and to measure both reaction probability and energy distribution in the products. We are nearing complete state-to-state measure- ments. For example, a current study has elucidated a key reaction in the combustion of ethylene. The beam experiments show that the initial reaction of oxygen atoms with ethylene produces the unexpected transient molecule CH2CHO. With this starting point, theoretical calculations have confirmed that a hydro- gen atom can be knocked out of an ethylene molecule by an oxygen atom more

III-D. INTELLECTUAL FRONTIERS readily than it can be moved about within the molecule. This combustion example illustrates the intimate detail with which we can now hope to understand chemical reactions. Now tunable lasers at ultraviolet frequencies (including vacuum ultraviolet) are being coupled with these molecular beam (collision-free) reactors. They are used to produce exotic molecules usually only found in flames, furnaces, and explosions, such as radicals, refractory atoms, and short-lived excited states. Supersonic jets are used to cool large molecules to almost zero Kelvin, removing the spectral complexity of such a molecule at normal temperatures. Despite the difficulty and cost of molecular beam experiments, the "state-to- state" information is of crucial relevance to many of the fundamental questions of molecular dynamics and to practical problems of combustion. The molecular beam technique will increase in importance as the sophisticated equipment becomes more generally available. Multiphoton and Multiple Photon Excitation Photochemistry has traditionally been concerned with the consequences of absorption of a single photon by an atom or a molecule. This fruitful field accounts for the energy storage in photosynthesis, the ultimate source of all life on this planet. Photochemistry furnishes new routes in organic synthesis and, through photodissociation, provides a variety of transient molecules that play critical roles in flames and as intermediates in reactions. Now lasers give us optical powers 10,000 times higher at a given frequency than even the largest flashIamps built in the prelaser era. Clearly these devices do not simply extend the properties of conventional light sources, they open doors to new processes as molecules interact with such intense photon fields. As an obvious example, the simultaneous absorption of two photons depends on the square of the photon flux; if we intensify the light source by a factor of 10,000, the two-photon absorption will be enhanced relative to one-photon absorption by four orders of magnitude. When two visible or ultraviolet photons are absorbed, molecular states can be prepared that cannot be reached with a single photon (e.g., states whose wave functions have a center of symmetry and Ry~berg states placed just below the ionization limit). Furthermore, the total energy absorbed can be enough to produce ions. This opens a new avenue to the chemistry of ions, a field of rapidly rising interest because of the discovery of interstellar ion-molecule reactions and because ions are major species in the plasmas (glow discharges) of nuclear fusion. Applications are multiplying as more chemists are able to purchase the high power lasers and as their ease of operation is improved. Multiphoton ionization has been used to detect specific molecules in difficult environments, such as in molecular beams and in flames. Thus the smog constituent nitric oxide, NO, can be easily measured in a flame by counting the ions produced by a laser probe so carefully tuned that only the desired molecule, NO, can absorb. Atomic species can be detected at almost the one-atom level. A multiphoton ionization source in 69

70 SF6 + hV -- ' RESONANT ABSORPTION -1-_ 34SF6 + hY 394SF6 h y 34SF ,, by 34SF *$$ + nay 3. SF CONTROL OF CHEMICAL REACTIONS a mass spectrometer enhances selectivity, again because the laser can be tuned to resonance with only a single molecule in a mixture. In quite a different application, a laser focussed on even such refractory metals as tungsten can produce gaseous metal atoms. When this is done in the throat of a supersonic jet expanding into a vacuum, the atoms form clusters of controlled size in the range of 2 to 200 atoms. Immediate interest lies in the spectroscopy and chemistry of these clusters because they bridge the gap between the gaseous and the metallic state. The major impact will be in heterogeneous catalysis because the chem- istry of such clusters is dominated by their surface reactivity. Coherent Raman spectroscopy depends upon the cube of the optical power. In conventional Raman effect, photons of one frequency produce scattered light at a different frequency and leave behind the energy difference in vibrational excitation of the scattering molecule. With extremely high intensities, so many molecules are left excited that they begin to participate in the scattering in a coherent way. This causes a huge enhancement of intensity (as large as a million-fold) and the same sort of collimated beam emission that characterizes the initial laser beam. Equally important, the effect preferentially highlights vibrations localized in the molecular group responsible for interaction with the radiation. Hence the coherent Raman effect can be used to study complex biological molecules like rhodopsin, which plays an essential role in human . . vision. However, the most spectacular instance of multiphoton excitation came with the development of extremely high power CO2 infrared lasers. One of the most surprising scientific discoveries of the 1970s was that an isolated molecule whose vibrational adsorptions are in near resonance with the laser frequency could absorb not two or three, but dozens and dozens of photons. In a time short compared to collision times, so many photons can be ab- sorbed that chemical bonds can be ruptured solely with vibrational excitation. This unprecedented and unpre- dicted behavior is usually called multiple photon excita- tion to distinguish it from two-photon (multiphoton) ex- citation. QUASI CONTINUUM DISSOCIATIVE CONTINUUM ISOTOPE SEPARATION THROUGH MULTIPHOTON EXCITATION n* SO ~ F 5 This behavior has already added substantially to our un- derstanding of infrared radia- tion, and it helped trigger a host of studies on internal en- ergy flow within excited poly- atomic molecules. Many uni-

III-D. INTELLECTUAL FRONTIERS molecular decompositions and rearrangements have been initiated without involvement of higher electronic states using multiple photon excitation fol- Jowed by laser-induced fluorescence for product detection. However, the practi- cal importance of this new phenomenon may transcend in importance the new fundamental insights it provides. Infrared absorption depends upon vibrational movements whose resonant frequencies are quite sensitive to atomic mass. Consequently, the tuned laser can be used to dissociate only those molecules containing particular isotopes, leaving behind the others a new method for isotope separation. For example, deuterium is present at .02 percent abundance in natural hydrogen. Yet, by multiple photon excitation, this tiny percentage can be extracted using the freon, trifluoromethane, CF3H. The process has been shown to have a 10,000-fold preference for exciting CF3D over CF3H. This could be of considerable importance because "heavy water," D2O, is used in large quantity in some nuclear reactors. Even more significant is sulfur isotope separation through excitation of sulfur hexafluoride, SF6. This gaseous compound gave the first convincing evidence that multiple photon excitation really occurred so rapidly that collisional energy transfer could be avoided. The successful use of SF6 for sulfur isotope separation could have heavy implications for human history. The gaseous substance that has always been used in the laborious uranium isotope separa- tion processes is uranium hexafluoride, UFO. Because UFO and SF6 have identical molecular structures, they have similar vibrational patterns. Thus multiple photon excitation might over a new and simpler approach to isolation of the uranium isotopes that undergo nuclear fission. It depends, of course, upon finding a sufficiently powerful and efficient laser at the lower frequencies absorbed by UFO. It will bring more general access to the critical ingredients of nuclear energy and nuclear bombs. It is sobering that Soviet chemists were among the leaders in this work with SF6 at a time when few U.S. chemists had the resources to purchase the large lasers needed for such fundamental research experimentation. Mocie-Selective Chemistry With high power, sharply tunable lasers, it is possible to excite one particular degree of freedom for many molecules in a bulk sample. As long as this situation persists, such molecules react as if this degree of freedom is at a very high temperature while all the rest of the molecular degrees of freedom are cold. The chemistry of such molecules has the potential to reveal the importance of that particular degree of freedom in facilitating reaction. To extract this valuable information, energy redistribution and relaxation must be brought under control, and this frontier is being explored. Because molecular collisions are needed for bimolecular reactions, one of the crucial aspects of mode-selective chemistry is the competition between reaction when a collision occurs and the collision-induced randomization of the mode- specific excitation. For that reason, energy transfer under collisions has already 71

CONTROL OF CHEMICAL REACTIONS been extensively investigated, and it remains one of the most active research questions in chemistry today. Propensity rules are now coming into focus, and understanding of vibrational energy transfer among polyatomic molecules- both experimentally and theoretically is advancing rapidly. Both unimolecular reactions and molecular beam studies of bimolecular reactions escape this problem. Unimolecular reactions involve only one mole- cuTe, so collisions are not required. At sufficiently low pressures, reactivity enhancement under selective excitation can be studied. The beam experiments sidestep the problem by giving each excited molecule only one chance for collision and by noticing only reactive collisions. Nevertheless, mode-selective reactions are not readily com- ing from such experiments. Apparently the problem is that intramolecular vibra- tional redistribution takes place even without collisions. This problem is of such basic importance to molecular dy- namics that it will be one of the most important study top- ics for the next decade. Pico- second lasers will play a key role. There is some evidence for mode-selective bimolecular chemistry in solid inert-gas environments, where the cry- ogenic situation holds the re- active molecules immobilized in a "sustained, cold collision" with rotational movement quenched. In gas phase exper- iments, search for conditions under which nonstatistical behavior can be perceived (non-RRKM) is an active research area. It is already known that the density of states close in energy to that of the excited mode is important and that rotational degrees of freedom play a role (through Coriolis coupling). These clues may indicate that super- sonic jet reactant cooling in molecular beam experiments will be productive. In any event, as intramolecular vibrational redistribution becomes more well understood, the potential for mode-selective chemistry will become clear. ,C=C~ + ,C—Cat Quantum Jo.+ Yield . .oolo - /\ _ / all- ~ ~~ ~H'c=c' H' ~ ,C=C~w who .0005 - / ai-1U ~- 1 600 1 800 2000 Photon Energy (cm~~) H ~ ~ D he H ,D D' AH 2 ~ F ,C Cx F The Reaction Rate Depends SELECT I VELY On The Mode Excited Ab Initio Calculations of Reaction Surfaces With today's computers, the structure and stability of any molecular com- pound with up to three first-row atoms (carbon, nitrogen, oxygen, fluorine) plus

III-D. INTELLECTUAL FRONTIERS various numbers of hydrogen atoms can be calculated al- most to the best accuracy available through experi- ment. This capability opens to the chemist many situations not readily accessible to ex- perimental measurement. Short-lived reaction interme- diates, excited states, and even saddle points of reaction can now be understood, at least for small polyatomic molecules. In a major ad- vance, we can now calculate the forces on all the atoms during their reorganization from reactant to product mol- ecule geometries. 1 F+H2 ~HF(v)+H TH EOR ETI CALLY CA LOU LATED PRODUCT SCATTERING ANGLES AND VIBRATIONAL D I STR I BUTI ON One example so studied is the reaction of fluorine atoms with molecular hydrogen. This reaction is of great practical importance because it drives one of the most powerful of all chemical lasers (the HE chemical laser). It is of theoretical importance because so much is known experimentally about the product energy distribution that the F + H2 reaction has become a prototype to test and develop our under- standings. The entire energy surface on which this reaction proceeds has been mapped with the best available math- ematical techniques. Conse- quently, we know the height of the energy hill to be sur- mounted, its early placement as reactants approach each other, and the precise energy contours thereafter as the re- actants move to closest ap- proach and then separate as excited products. This surface contains the explanation for the efficiency of this laser. A second case is the key acetylene combustion reac- tion involving hydroxyl radi- 73 - - o 10 ~ -10 ,~ -20 - -30 _. LO -50 ~ -60 —..~.o R—O ,C—Can C=C,~ —~—~ BU-'C~O B'C=C'. ~ ~~ ~ If_ B' 's ~—~—1 , BCC_, ~ ~ 1 2 OR ·· \t / 1 ......... ~t- R l 1- - o ~ ~ ~—B 2'2'-.-'.--2.2''.'.2--2.-''.'' '''.2' - -'-' R 0. \_ Jo_ ~— ~C= C O - - : .:-: :-: :.: J:--:~ .- -- ::- ~ ~ ._—C ~ C—O-:- ~—ARC—C . . it, . ... ....... ~ ., ~ ,:.2'.2 2' 22,' .'' .'-'.:2 :.:.:.:.:.:.:.:::.::.:::.:.:.:.:.:::::::::::::::::: · . :.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.: :.:.:.:.:.:: . . H2CCO ~ H _ ... _ :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: _ .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . _ ~ CH3 + CO ::::::::::::::::::::::: :::::::::::::::::::::::::::: ................................................ ....... .. . noun t an... ~ THEORETICALLY CALCULATED COURSE OF AN ACETYLENE COMBUSTION REACTION

74 CONTROL OF CHEMICAL REACTIONS cals and acetylene. Experimental evidence is found for ketone and hydrogen atoms one set of products and also for methyl and carbon monoxide a second plausible set. Ab initio calculations fill in substantially the intervening sequence of events. After addition across the double bond with an 11 kcal/mole activation energy, hydrogen migration takes place. The calculations show that, at this point, branching takes place. The detailed information advances our understanding of this important combustion reaction and shows how significant ah initio calculations have become in chemistry. Theorists now aspire to extend their calculations to compounds involving elements deeper in the Periodic Table. These are important chemical species in catalysis, isotope separation, organic conductors, and biological mechanisms. For tractability, it is still essential to separate within the wave function the part due to the inner (core) electrons from that due to the outermost (valence) electrons. Thus, in mercury, with 80 electrons, the quantum mechanical description of the outermost 12 electrons is carried out in the presence of a "pseudopotential" that aims to reproduce the effect of the inner 68 electrons. Agreement between such "effective potential" calculations and tour de force all-electron calculations provide encouraging evidence of accuracy. The problem is complicated even further for elements beyond the lanthanides in the Periodic Table because relativistic effects for interatomic potential curves become increasingly important. It is now believed that the chemical properties of goIcI, mercury, and thallium are all markedly influenced by these relativistic terms. By no means is this an esoteric nicety. Thus the common Hg2+ 2 aqueous ion and the dominance of UO2+ 2 in the solution chemistry of uranium both depend significantly on relativistic effects. Currently, relativistic calculations for molecules with only one or two heavy atoms are feasible, but for larger molecules, such as metalcontaining biological molecules, only the largest existing computers are adequate. The field will be inhibited until theoretical chemists gain access to such machines. Theory of Reactions The rapid accumulation of detailed experimental information about chemical reactions presages a corresponding blossoming of the applicable theory. Theo- retical understanding of radiationless transitions, curve crossing, predissocia- tion behavior, energy redistribution, and intermolecular energy transfer is beginning to develop. In addition, recent theoretical developments are provid- ing new insight into the dynamics of reactive collisions. There has been renewed interest in deducing reaction path HamiTtonians for molecular dissociations, isomerizations, and reactive collisions. Gradient techniques in electronic struc- ture calculations can now reveal reaction paths and provide force constants for vibrational motion normal to the reaction path. State-to-state isomerization rates can be computed from action integrals drawn from classical trajectories. Semiclassical quantization of classical trajectories has led to excellent predic- tions of threshold and resonance energies.

III-D. INTELLECTUAL FRONTIERS The most active advances in theories of solution reactions have been con- nected with electron transfer reactions. These important reactions are relevant to biological oxidation-reduction reactions, photosynthesis, electrochemical pro- cesses, and solar energy conversion at electrodes. There has been fruitful interaction between theory and experiment that has now been-extended to the transfer of other particles, such as H-, CH3, and H+. Rotational isomerizations in simple liquids can now be treated because of advances in the statistical mechanical theory of the liquid state. Further developments in nonlinear dynamics now permit theoretical understanding of the fascinating "oscillating" chemical reactions. Hence, we can mimic biological oscillating (cyclic) reactions and understand steady-state behaviors in engineering-scare processes. These theoretical advances will have impact on and support a number of neighboring disciplines. For example, atmospheric and planetary chemistry, hydrocarbon combustion, the development of high-power gas lasers (including excimer lasers), chemical processing, and photochemical reactions will all benefit from better theoretical models. Bulk reactions can be understood and controlled only with microscopic understanding of the dynamical details, including energy transfer processes. Chemical theory is moving rapidly on all these fronts. New Reaction Pathways A manifestation of our increasing understanding and control of chemical reactivity is the rapid advance now taking place in devising new reaction pathways in synthetic chemistry. The progress presents a high leverage opportunity because it provides the foundation for future development of new products and new processes. Again powerful instrumental techniques play a central role. The rapid and definitive identification of reaction products, in both composition and structure, accounts for the speed with which synthetic chemists are able to test and develop their synthetic strategies. The use of X-ray structure determination has become an essential technique. The nuclear magnetic resonance and the mass spectra reveal the presence and the local chemical environment of every atom. The X-ray crystal structure reveals the complete molecular structure: the interatomic distances, bond angles, and even mirror-image relationships when present. Spectroscopic techniques, as well, have been essential to progress in the rapidly (leveloping area of organometallic chemistry. The visible and infrared spectra of transition metal complexes reveal electronic configurations and bonding, the foundation for clarifying mechanisms for ligand substitution and electron transfer processes. Organic Chemistry Organic chemistry today involves three areas of concern. First there is the study of isolation, characterization, and structural determination of substances from nature. New natural products are thus identified alkaloids and terpenes 75

76 CONTROL OF CHEMICAL REACTIONS from plants, antibiotics from microorganisms and fungi, peptides and polynucleotides from animal and human sources. Success here is strongly coupled to major developments in other disciplines. For example, chromatogra- phy permits purification and characterization of a substance present in only trace amounts from a complex mixture of similar compounds. Thus workers in pheromone chemistry regularly fractionate microgram amounts of these biolog- ically potent molecules. The next challenge lies in determination of composi- tion, gross structure, and three-dimensional stereostructure. Here nuclear magnetic resonance, mass spectroscopy, and X-ray crystallography fill essential roles. Using proton NMR, only 100 nanograms of a substance will provide crucial information about the number and types of molecular linkages. With only 100-picogram amounts, mass spectrometry supplements by furnishing precise molecular weights up to 13,000 and, through the fragmentation pat- terns, providing revealing clues to substructures. Then, if 10 micrograms or more of a crystalline material become available, every sterochemical nuance of structure is displayed through X-ray spectroscopy. Physical organic chemistry is the second major area; it seeks to relate changes in physical, chemical, and spectroscopic behavior of organic compounds to changes in molecular structure. It rant with t.h~ H~t.~il~H n~thw~v.~ he ~ =- --¢ ~ ~ rid Jo ~ Jo ___1_ _ _ 1 ~ _ _ _ 1 _ 1 1 ~ 1 · ~ · ~ ~ . . ~ wn1cn reactants become products it inters what transient species or structures intervene and determines how the reaction path is influenced by solvent environment, catalysts, temperature, pH, etc. It provides a theoretical frame- work with which to predict behavior and probable viable synthetic routes toward materials not yet known. Physical organic understandings intellectu- ally undergird the logic and practice of both structure determination and synthesis. Synthesis, the third area, is a process of inventive strategy. Two contempo- rary challenges it faces are to supplement availability of useful natural products and to synthesize new and useful substances not found in nature. Thus thousands of pounds of ascorbic acid (vitamin C) are synthesized annually at purities suitable for human consumption so that society can have an ample supply of this healthful substance. Smaller amounts of 5-fluorouracil, an artificial drug extremely elective in curbing certain skin cancers, are synthe- sized for prescription medicinal use. Meeting such challenges has required a creative evolution of the process and philosophy of organic synthesis. Only a few decades ago, synthetic strategies were based on clever choices from an array of already known reactions. Like moves in a chess match, the range of feasible reactions was defined in advance. With the development of mechanistic reasoning, it has now become possible to invent new reactions for applicability to specific synthetic goals. Because of the success of this reasoning process, which combines analogy, deduction, and a priori conception, organic synthesis has unprecedented power. At the same time, there has been an imaginative and fruitful expansion in the settings in which reactions are conducted. An example is solid phase peptide

III-D. INTELLECTUAL FRONTIERS synthesis in which amino ac- ids are added sequentially to produce a desired peptide, all carried out under covalent at- tachment to an insoluble poly- mer substrate. Such polymer- bound peptide synthesis is already being applied to syn- thesis of important hormones and bioregulatory peptide substances (see p. 1721. A quite different dimension now being exploited is pressure. Equilibrium can be shifted to favor products with specially compact structures, and acti- vation barriers can some- times be affected to speed up selectively a desired process. A step in the synthesis of alkavinone, used in the syn- thesis of certain drugs, provides an example. At 15,000 atmospheres and room temperature, quinone will react with the conjugated butadiene ester of the correct structure to form the desired bicyclic ester. This process completely avoids undesired alternative structures that would be obtained if high temper- ature were used instead of high pressure as the control variable. A third dimension discussed later in this chapter is the use of "tuned photochemistry" in which selectivity can be increased by use of lasers to induce photochemical reactions. Striking developments have occurred in the use of catalysis to reach new synthetic Coals. In a major advance over traditionalist views, synthetic chemists °Nc~c~ OPTIC/ ITCH ACHE 0 it_ 1 5,000 + 1; No Reaction ! o o JO 1 5,000 C:~` + ¢) atm. o a Drug Precursor OPTIC/ OUCH ¢~^ o L Catalyst Zn(BH4)2 O C~ Alkavinone ~~ H RAISING PRESSURE CAN SELECTIVELY SPEED UP A DESIRED REACTION _ ~ ~ ~ 1 1 ~ 1 ~ ~ 11 _ lo__ _ :1: ~ rl~1~1 ~ now search tor new reactants over one entire sweep C)I tne re~-lunl'; tools. Notable successes have been registered in the discovery of metal catalysts that achieve quite exact transformations. Palladium and molybdenum catalysts, for example, are able to activate with surgical precision one particular bond in a complex structure. Nowhere has progress had more far-reaching significance than in our growing ability to control molecular complexities in the third dimension. This frontier, stereochemistry, can be divided into issues of topology and "handedness" (i.e., "relative" and "absolute" stereochemistry). The production of a particular molecular topology already requires artful control of spatial relationships as reaction proceeds. However, this control does not usually differentiate between spatial relationships that diner only in a mirror image sense (i.e., in chirality). Thus an elegant pair of Italian gloves uses the same sort of patterns for the left 77

78 CONTROL OF CHEMICAL REACTIONS as for the right glove. When right- and left-handed molecular structures are possible, most chemical reactions will produce a mixture of the two. Of course, a left-handed glove will not fit a right hand, so it cannot serve the function of a right-handed glove. It is the same in the function of biolo~in~llv _ A ~ ~ 1 ~ I 1 ~ ~ J Important natural proctucts where this "handedness" aspect of molecular structure assumes critical importance. Biological molecules must, of course, have proper topological conformation (relative stereochemistry); but, for them to be functional, nature also insists upon a particular handedness (absolute stereochemistry). A molecular "right-han(led" glove can play a crucial role in a biological reaction while its "left-handed" counterpart will be totally impotent or, worse, may introduce undesired chemistry. Though stereochemistry has been recognized for almost a century, major advances have been made within the last decade. In one technique, an auxiliary molecular fragment of defined handedness is attached to a reactant. This "chiral auxiliary," properly placed, can govern the handedness of products derived from that reactant. The auxiliary is then removed from the product and reused in another cycle. Synthesis of stereospecific propionates for biological precursor reactants provides an example. Even more exciting is the use of asymmetric (chiral) catalysts to direct the handedness of the products. This obviates need for embedding and then later disengaging the chiral guid- ing element within one of the reactants. Thus highly e~ec- tive asymmetric oxidation- reduction catalysts have re- ALLYLIC ALCOHOL EPOXIDATION EFFECTED BY LEFT-HANDED / CATA LY ST ~ COW EPOZI DATI ON EFFECTED BY RIGHT-HANDED CATA LY ST o<' Em, cently appeared. Asymmetric LEFT -HANDED O H O H R. I GHT -HANDED · · - EPOZIDE ~ EPOXIDE reduction IS now a key step in the industrial synthesis of the NOW THE CATALYST CAN FIX THE important anti-Parkinsonian DESIRED HANDEDNESS OF THE PRODU CT agent ~-dopa. More generally applicable has been the devel- opment of asymmetric epoxidation through asymmetric catalysis. When an oxygen atom is inserted equally into either face of a carbon-carbon double bond to produce an epoxide, two mirror image-related products result. With inexpen- sive and recyclable chiral catalysts, it is now possible to prepare uniquely whichever one of these two steroisomers is needed. The resulting stereospecific epoxide can be used in many synthetic pathways, carrvin~ Ricing And nrP~=rvin~ ~ 1_ _ ~ _ (` / · 1 ~ · ~ · ~ T · ~ . . . -—D ~ -—D ~— ~ ~— ~~ ' ~ ~ Ills l~l-/~-lg~lv By. In a mayor application, all the naturally occurring six-carbon sugars have been synthesized with nature's preferred handedness. Many specific examples could be cited to demonstrate the pi o~nifir~nrP of the ~ . . ~ . .. . revs_. vex ~~_~u~ Ivy vet At_ ~~11111~C~11~ V1 Ally new Ironers or organic synthesis. Thus the prostoglandins have biological activities that have such extensive applicability in medicine that they may play a central role in therapeutic strategies a decade hence. The biosyntheti- cally related leukotrienes may have a similar role, specifically including a new

III-D. INTELLECTUAL FRONTIERS approach to the control of asthma. The availability of modified prostoglandins and leukotrienes for biological scrutiny and testing is an eas- ily identifiable triumph of synthetic organic chemistry. Equally far-reaching ac- complishments are connected with the synthesis of safe compounds for population control, such as the 19-nor- steroids and 18-homosteroids. The modified cephalosporin and thienamycin antibiotics help meet the challenge of in- creasingly sophisticated mi- croorganisms. The roles of Aldomet~ in antihyperten- sion, procardia in anti- erythmia, and Tagamet~ as an anti-uIcer medication dis- play the creative confluence 79 i~ POOH _~4 ~~=~~ AA H~OH H 5-HPETE -H. 2 O ,COOH ~ LTA ENZ7~IC HID Hi 5 H H$H AH H H OH H POOH H LEUKOTRIENE B H POOH 7~ POOH \=A=~~ 5-METE H OH ,COOH C H HS 5 11 1 R R =GluLathione LTC R=Cys-gly LID R =Cysteine LIE BIOSYNTHESIS OF LEUKOTRIENES of organic synthesis and biology. An impressive example from the world of large-volume commodity chemicals is provided by the sophisticated monomer needed to manufacture the promising structural plastic Ultem~. The continuance and acceleration of these advances can be foreseen. Within the next few years we shall see efficient, asymmetric catalysis of carbon-carbon bond formation. Catalysts will be developed for cleaving unactivated carbon- carbon bonds. New strategies for synthesis will be devised. Such advances will be incorporated into the production of new medicinals (e.g., an anticholestimic or antitumor drug), new agrochemicals (e.g., fully safe and biodegradable fungicides), new structural plastics (e.g., an inexpensive structural polymer stable to 400°C). They will result from creative collaboration of the three branches of organic chemistry: isolation-structure determination, physical- organic chemistry, and synthesis, all working in tandem with the cognate disciplines physical, inorganic and, computational chemistry, biochemistry, and biology. Inorganic Chemistry There is great intellectual ferment now in inorganic chemistry, much of it at the interfaces with sister disciplines: organometallic chemistry, bioinorganic chemistry, solid state chemistry, biogeochemistry, and other overlapping fields as yet unnamed. The same instrumental array that is so effective in organic

CONTROL OF CHEMICAL REACTIONS chemistry is applicable, making new problems amenable to study in every area of science that involves the inorganic elements. Deeper insights have been gained concerning the structures of molecules and materials, the reactivities of new species, and the possible uses of new compounds and new materials. Geochemists are exploring the role of organic substances in sedimentary ore deposition. Thus inorganic chemistry is entering a revolutionary period; some of the important discoveries of the next decade will be found at the boundary areas just being touched today. For example, there is uncommon interest in bioinorganic chemistry, stimu- lated by the growing awareness of the crucual roles played by inorganic elements in biological systems. Living things, far from being totally organic, depend sensitively on metal ions distributed across the Periodic Table in such essential life processes as transport and consumption of oxygen, absorption and conversion of solar energy, communication through electrical signals between cells, establishment of osmotic discontinuities across membranes, and subtle roles in enzyme action. This has led to a surge of research activity in the inorganic chemistry of biological systems. Many problems, both structural and dynamic, are being attacked. How does an organism or plant build its compo- nents so as to carry out the chemical reactions necessary to life? What structures surround the metal atoms, and how do these structures enable the metal atoms to react with such exquisite sensitivity to changes in pH and oxygen pressure, and to electron donors and acceptors? Can we understand the mechanisms of the reactions and recreate the steps by which the transforma- tions take place? Can we create simple mode] systems that mimic the compli- cated biological systems while preserving the functionality of the metal? What is the minimal model system that will accomplish in vitro what the biological system does in vivo? Answers to these significant structural questions are being sought by spec- troscopists, synthetic chemists, and crystallographers around the world who want to learn how Nature has solved these extremely complicated chemical problems. These scientists are joined by those studying reaction dynamics in these systems, seeking to describe the details of electron transfer through protein systems, energy transfer and utilization in cells, electrical signal transfer through nerves and across gaps, and molecular recognition at all levels. Every one of these processes involves inorganic chemistry and materials, most often metal atoms with organic ligands, and the tools of the physical inorganic chemist have worked well in opening up this new field of chemistry. Significant major challenges await tomorrow's chemists in their attempts to reach a complete understanding of the processes and structures involved in respiration and energy use by plants and animals. Thus bioinorganic chemistry is a major frontier of inorganic chemistry. A second new frontier is clearly visible at the intersection of solid state chemistry and inorganic chemistry. Using the tools of both the inorganic chemist and the solid state scientist, we are making progress in understanding

III-D. INTELLECTUAL FRONTIERS the reaction chemistry of surfaces, specifically inorganic surfaces. New, more accurate descriptions of the structure of a surface are available, and diffraction techniques plus ultrasensitive spectroscopic measurements now tell us how and where a chemical species is attached to such a surface. This knowledge is vital to catalyst design but is also applicable in semiconductor and electronic chip fabrication. Our abilities to understand the nature of a surface species and a substrate-surface bond will be reflected in improved catalytic processes of all kinds and in smaller, cheaper, more efficient electronic devices. The synthesis of new materials and the development of processes for the control of microstruc- ture are at the heart of much of the scientific and technological progress of the last 40 years; the full understanding of the structure and bonding in these materials, at the atomic level, will be a significant challenge of the next 40. There are a number of specific needs in this area that can only be met if we achieve such an understanding. Composite structures are needed in several fields: multilayer ceramic substrates for interconnections between semiconduc- tor chips and new compositionally modulated layered structures are now being fabricated, their designs based on experience and guesswork. The rational .. . .. . . · ~ ~ . ~ . · . . ·.. . synthesis of these materials will be a significant challenge to the inorganic chemist of the next decade. Another new class of materials of considerable interest comprises the ultrafine filamentary composites. The unique microstruc- ture of these metallic composites, which contain an extremely dense uniform dispersion of very small filaments (50-lo00-A thick) leads to dramatic changes in material properties as compared to conventional composites. The physical properties of ultrafine filamentary composites exhibit striking size effects not predictable by the rules of mixtures, some related to the composite microgeometry and others due to extremely high interracial and dislocation density. Some of these anomalies are synergistic in nature, the mere presence of one phase affecting the properties of the adjacent phase. The challenge for the future will be to obtain a full understanding of all these material interactions so as to be able to design and synthesize new materials with properties to order. Bridging the gap now between solid state chemistry and the chemistry of simple substances in solution is the field of metal cluster chemistry. The field has grown from a subfield of organo-metallic chemistry itself an interface between traditional areas to a disci- pline of its own. In the process of developing, a variety of questions have been left unanswered that will become some of the frontier areas of inorganic chemistry in the decades ahead. First, is metal cluster chemistry really a bridge between single atoms and solid metals? How big must a cluster be before it exhibits some of the bulk properties of the metal? Perhaps the clusters— now known up to one containing 38 platinum 81 L L L An' ' elf MEL Lit MEL L A GOLD CLUSTER COMPOUND L- LIGAND

CONTROL OF CHEMICAL REACTIONS atoms in cubic close packing are simply a different phase in themselves and should be studied as such. Second, how can the detailed electronic structures of these metal cluster complexes be described? Until these questions are answered the highly creative synthetic chemistry in the field will remain semiempirical. As we gain real insight into the electronic structures of clusters, progress will accelerate toward understanding and controlling their reaction mechanisms. Because the outcome may have significant bearing on the advance of the field of catalysis, a heavy investment in fundamental research in the area is called for. Many cluster complexes can be made and studied in the gas phase as well as in solution or as solids. They can be prepared and studied snectrosconicalIv in 3.20 D ;" ASH 2.25 D H ,, O C 0 ~350° 1.9l~ F 1 .85D o 11 C C 11 2.84 ~ NH o AH IF ,~ ~ 64° N—N 0----- o 11 C ~ Ar 11 3~49 o N N. ~ .. 11 3 47A o VAN DER WAALS MOLECULES- THE WEAK lNTERACTlONS THAT GOVERN SOLUBILlTY, GAS lMPERFECTlON, LIQU EFACT1ON ~ Ar ~ - ~ ~ cryogenic matrices and under molecular beam conditions using supersonic jet cooling. The latter techniques have provided a wealth of informa- tion about the so-called "van der Waals" molecules, includ- ing molecular geometry, vi- brational amplitudes, dipole moments, and ease of energy movement from one part of the complex to another. Such studies, and studies of even simpler molecules, lead to two kinds of information: the energetics of bonding and bond-breaking within molecules, and the nature of weak interactions between molecules and ions. Here there should be progress in the coming years on both the experimental and theoretical sides. Better techniques for laser excitation of complex inorganic ions can lead to more precise definition of the energy input to a system; the subsequent reaction can be studied to identify the bond that broke, for example, and precisely what its energy content was. Information like this is crucial to the development of detailed theories of reaction rates and prediction of reaction pathways. Further studies should be made of the anisotropy of nonbonding interactions, in both gas and condensed phases. These forces need to be understood to explain such phenomena as condensation, critical behavior, solvation, and surface attraction. Perhaps the most explored interface area, one of the richest in reward and still rich in potential, is organometallic chemistry. Here is a discipline that has developed its own literature and that promises, in the decades ahead, more surprises and more satisfaction as workers attempt to correlate facts and to apply rational design principles. The molecule-makers of the field have had a highly synergistic interaction with structural chemists, who, by spectroscopic and diffraction techniques, are unraveling many unexpected bonding patterns and structural motifs. The key to further progress is to understand the reaction

III-D. INTELLECTUAL FRONTIERS mechanisms of these molecules. Through the design of new ligand environ- ments and unusual oxidation states for the metal, organometallic chemists have prepared some remarkable compounds, which exhibit selective reactivity toward molecules previously thought too inert to participate in useful chemical transformations. For example, the activation of the carbon-hv~ro~en bonds of ~~..~ ~ ~~ 1 1~ AL- _ 1___ lo 1 ~ . ~ ~ O~'vUl-~lv~1 Llilpil~lvlC nyarocaroons nas recently been achieved by several re- search groups. Low-valent rhodium or iridium compounds with tertiary phosphine or carbony] and pentamethy~cyclopentadieny] ligands oxidatively add the C-H bonds of hydrocarbons such as methane and cyclopropane, whose C-H bond energies exceed 100 kcal/mol-~. The challenge now is to couple this important new reaction with other well established transformations such as olefin or CO insertion and hydrogenation, so that new catalytic reactions may be developed that use saturated hydrocarbons directly. The direct conversion of methane to ethanol could have a tremendous impact on the world energy situation, and the realization of such a catalytic process seems closer than ever before. A key to understanding these new types of chemical reactivity and their application, for example, in the design of new, more efficient catalytic processes, or in the synthesis of fine chemicals and new types of polymers, is the elucidation of reaction mechanisms. The primary steps of a chemical reaction (bond formation, bond breaking, atom or electron transfer, etc.) can be studied by examining intermediates, if they can be isolated, or by applying one or more of the many powerful new techniques that are available to the chemist. In particular, two-dimensional NMR can reveal some of the most intimate details of a reaction; fast-scanning spectrophotometers aid the study of moderately fast reactions, and variable-temperature NMR studies are indispensable for probing equilibrium situations. In fact, the entire array of tools of the physical organic chemist is now being used in the stub of the reactions of or~nom~t~llir `;umpounas Elan results as surprising and as useful as when they were first applied in organic chemistry. The chemistry of the main group elements those to the right of the Periodic Table has in the past largely been explored abroad. This situation is rapidly changing as exciting new developments are now appearing in the United States. 1 ~ 1 1 1 ~ For example, the discoveries of compounds involving double bonds between silicon atoms, phosphorus atoms, and arsenic atoms open an essentially new area of main group chemistry. There has also been success with inorganic polymers involving phosphorus-nitrogen link- ages. There is even a polystyrene analog with a silicon-silicon polymeric backbone. A new subfield is being developed in oxidation chemistry. Classical oxidation systems are hard to study, but new metal-based oxidizing systems that allow careful control of the reaction promise to provide chemoselective, regioselective, and ~ , ~ . ~ . 83 si _ si ~ \ \ P—P \ \ MAIN GROUP ELEMENTS FORM DOUBLE BONDS— AND NEW COMPOUNDS As As

CONTROL OF CHEMICAL REACTIONS stereospecific oxidations. Ultimately, new methods for activating molecular oxygen to perform these oxidations must be sought; it is likely that these will involve metal species. New tools for characterization of these compounds are being developed—t70 and metal nuclide NMR, chiral supports for chromato- graphic analysis- that will help the experimentalist establish the conceptual framework of the field. Already a metal-containing, highly enantioselective system for the epoxidation of prochiral allylic alcohols has been discovered and assimilated into the organic chemist's battery of synthetic techniques. Electrochemistry of inorganic materials has recently increased in impor- tance. Electrochemists and inorganic chemists working together have designed inorganic molecules that can catalyze the four-electron reduction of dioxygen to water at nearly reversible potentials. The new catalysts are absorbed on the surface of electrodes rather than dissolved in solution, a significant advantage in reducing the quantity of catalyst needed and facilitating its separation from the product. They should lead to the development of more efficient fuel cells. Further advances in sophisticated analytical techniques, such as hydrodynam- ically modulated rotating disk voTtammetry and digital simulation, promise more applications in the future. Many of the new experimental developments will eventually come from more complex systems. The major future opportunities appear to lie in the chemistry of metal clusters, reactions that take place on solid surfaces, and reactions in ordered media, such as matrices and micelles. An important need exists for the development of stereoselective synthesis techniques. Despite several demon- strations of this effect, the basic factors producing stereochemical control are still not well understood. Similarly, organometallic photochemistry offers great possibilities for future growth. The photochemical generation of reactive species and catalysts is in its infancy, but it shows great promise. Finally, one can note that most organometallic compounds studied to date have been diamagnetic, but the presence of unpaired electrons in outer orbitals should have a marked influence on reactivity and should open new catalytic pathways for exploration. Selective Pathways in Organic Synthesis Selectivity is the key challenge to the organic chemist—to make a precise structural change in a single desired product molecule. The different intrinsic reactivity in each bond type must be recognized (chemoseZectivity), reactants must be brought together in proper orientation (regioselectivity), and the desired three-dimensional spatial relations must be obtained (stereoselectivity). The degree to which this type of control can be achieved is shown in the synthesis of the substance adamantane COHN. This novel molecule resembles in structure a 10-atom "chip" off a diamond crystal. In a tour de force, it was finally produced in a many-step process in only 2.4 percent yield. Recent research in polycyclic hydrocarbon synthesis now gives adamantane in one step in 75 percent yield. Then a surprise practical payoff came when it was discovered that adding a

III-D. INTELLECTUAL FRONTIERS single amine substituent to _ - adamantane gave adaman- tine (l-amino-adamantane)7 1 ~ r ~ ~ which proved to be an antivi- ral agent, a prophylactic drug for influenza, and a drug to | combat Parkinson s disease. i_ ~ i,_ / Cycloadd~t~on to make five- membered rings becomes ~m- portant for a diverse array of ADAMANTANE ADAMANTADINE applications ranging from novel electrical conductors to LAB CURIOSITY ANTIVIRAL AGENT pharmaceuticals (e.g., antibi- otics and anticancer compounds). The catalytic rhodium catalyst ring closure to form a critical precursor to thienamycine is an example. In this case, the five-membered ring contains a nitrogen atom. The final product proves to be a relative to penicillin and an important drug in the battle against infectious diseases. At another extreme, large ring compounds have been exceptionally difficult to synthesize. Their structures are complicated by function- ally crucial left/right handed structural geometries ("chi- ral" centers). Their wide- ranging biological proper- tie~from pleasant fragrances for perfumes to anti-fungal, anti-tumor, and antibiotic activities make large ring synthesis an interesting challenge. An example is erythromycin, CON, which, even after the desired atomic hookup is found, can be shaped into 262,144 different structures derived from the many possible ways to couple the right- and left-handedness at chiral centers (2~8 = 262,1441. Twenty-five years ago, this compound was judged to be "hopelessly" complex by R. B. Woodward, who won the Nobel Prize for synthesizing molecules as complex as quinine and vita- min Big. Today we can aspire to such a goal, in part because of the development of spe- cially designed templates that bring together the termi- nal atoms of a 14-atom chain to form a 14-membered ring. This provides the structural framework of erythromycin, OH OH Rhodium ~ ~S~NH3 COOR COO THIENAMYCINE CATALYTIC CLOSURE OF FIYE-MEMBERED RINGS H OH We Met O ,/~O ~NMe2 HO ~4:0~O::Me Me Me MeO~H Me Memo: ERYTHROMYCIN ONCE CONSIDERED "HOPELESSLY COMPLEX"

86 CONTROL OF CHEMICAL REACTIONS and it has already resulted in the synthesis of a number of constituents of musk and contributed to our understanding of smell. Crossing InorganiclOrganic Boundaries The traditional line of demarcation between organic and inorganic chemists has virtually disappeared as the list of fascinating metal organic compounds continues to grow. The ubiquitous appearance of these compounds in biological systems underscores the importance of encouraging cross-boundary research. Furthermore, research in developing new inorganic substances has provided a surprising dividend in their frequent applicability in organic synthesis. The latter situation is illustrated by the borohydrides. The cohesive picture we have, at last, for this strange boron/hydrogen family was not possible before their study by X-ray crystallography, infrared CH2OH 1 c=o CH ~ OH Cortisone CH2OH C=0 O~OH Prednisone LESS ARTHRITI C PA IN, SMALLER DOSES and NMR spectroscopy, and molecular orbital theory. Now borohydrides are widely used as selective, mild reducing agents in organic synthe- sis. Silicon and transition metal organometallic compounds give other examples. Silicon com- pounds, for example, are used to fold an extended molecular reactant precisely as needed to synthe- size the molecule cortisone. Now this valuable therapeutic agent can be made in fewer than 20 steps at a yield 1000 times higher than was achieved in the earlier, 50-step process. Cortisone is well known in the treatment of arthritis. Unfortunately, experience showed that relief could be temporary and that continued use had undesired side effects. These developments made the new silicon-mediated synthetic routes all the more valuable. A variety of cortisone derivatives were prepared and tested for therapeutic effectiveness. One such product, prednisone, is more effective than cortisone, even when used in much smaller doses, with the result that side effects are much diminished. Because organometallic species are vital intermediates in most metal- mediated organic reactions, it has been important to establish how they make and break carbon-to-metal bonds rapidly, selectively, and with stereospecificity. Oxidation-reduction forms an important basis for such understanding. Organometallics are electron-rich, hence susceptible to oxidation by both inorganic oxidants and organic electron acceptors in solution and at electrode surfaces. Important theoretical developments have accelerated progress here. Electron transfer theories from inorganic chemistry and charge-transfer views on or- ganic systems have been unified to provide a basis for predicting oxidation-

Ill-D. INTELLECTUAL FRONTIERS reduction reaction rates. Fine tuning of the steric and electronic properties of organic ligands allows electron transfer mechanisms to be placed within a spectrum of mechanisms connected with proximity of the reactant approach at the time of electron transfer. The extremes are called "outer-sphere" and "inner-sphere" mechanisms. Transition states with little penetration of coordination spheres of uncharged oxidant (A) and reductant (D) are then insensitive to steric effects, and electron transfer theory sucessfully describes reaction rates. Configurational changes of the electron donor (D) as it releases its electron to become D+ may then contribute importantly to the energy barrier to electron transfer. An example is the change that occurs as the tetrahedral tetralky] tin reagent releases an electron and changes to trigonal pyramidal geometry. ~.~ `~ "Outer Sphere" Transition State for Electron Transfer A much larger number of reactions involve significant interpenetration of the coordination spheres, such as additions to alkenes, substitution in aromatics, Diels-Alder ring closures, and bond cleavages. While calculations of ion pair interactions are less feasible at such close approach, charge-transfer spectral absorption bands provide a valuable diagnostic too! for recognizing inner-sphere electron transfer reactions and then for predicting reliably their reaction rates. This unified understanding wit] continue to be extremely fruitful because it bears directly on important catalytic processes, including a num- ber with commercial importance. Striking exam- ples include the cobalt-catalyzed oxidation of para-xyTene to terephthalic acid (see Section TTI- A, p. 24), the catalytic oxidation of cyclohexane to adipic acid for nylon production, and the radical chain processes for ligand substitution of metal carbonyIs to aid us in synthetic use of carbon monoxide ("syn-gas" from coal). Pathways Using Light as a Reagent "caner Sphere" Transition State for Electron Transfer Another promising chemical pathway is connected with the use of photons in chemical synthesis. Many natural products and complex molecules of medicinal importance involve high energy ("strained") molecular structures. In tradi- tional synthetic procedures, the aggressive reagents needed to force molecular reagents into these uncomfortable geometries tend to threaten the fragile product. Photochemistry has been remarkably successful in circumventing this difficulty. 87

88 CONTROL OF CHEMICAL REACTIONS The reason for this success is that absorption of light can change the chemistry of a molecule dramatically. After excitation, familiar atoms can have unexpected ideas about what constitutes a comfortable bond angle; functional groups can have drastically different reactivities; acid dissociation constants can change by 5 to 10 orders of magnitude; ease of oxidation-reduction can be drastically altered; and stable structures can be made reactive. The energy absorbed by the molecule puts its chemistry on a high energy "hypersurface" whose reactive terrain can be nothing like the ground state surface below, the one that chemists know so well. Though much imaginative synthetic photo- chemistry has been performed in the last 10 or 15 years, the field is still opening. Many examples can be given to illustrate the potentialities. Most dramatic are those that involve cyclic structures that require unusual ("strained") bond angles around carbon. Thus, rings containing three or four carbon atoms are relatively unstable and, hence, difficult to synthesize; they were long sought just because they were chemical oddities. Now we know that many biologically active mol- ecules or their synthetic precursors contain such "strained" rings as essential structural elements, so their synthesis has assumed great practical importance. These unusual, energy-rich struc- tures are natural targets for photon-assisted syn- thesis. The photon provides extra energy, and it places the reaction on a "hypersurface" where unconventional bond angles can be the preferred geometry. Using these principles, chemists have made many molecules of bizarre structure. Aptly named cubane is an example: eight carbon atoms are placed symmetrically at the corners of a perfect cube. Once formed, the molecule is sur- prisingly unreactive. Propellane also involves eight carbon atoms, now in a structure made up of three squares sharing a side. Even more amazing is the family of tetrahedranes whose central struc- tural element looks like a three-sided pyramid. Each corner carbon atom is simultaneously bound to three others at 60° angles to form four interlinked three-membered rings. As mentioned above, these photochemical syntheses have proved to be much more than an intellectual chemical chess game. All these syntheses store energy in chemical bonds (the reactions are endothermic). The energy can be recovered later for its own use or to facilitate subsequent synthetic steps to form Normal Carbon C C Bac'b AQUA Ugly I I Carbon 109°, 120° 180° C C Has :: : CY CLOBU TA N. E l. one 9~,,,~, Yet all of these "strained" compounds have been synthesized! C C C CYCLOPROPANE |, ohe.~60°~, . C C CmC~I I~->c CUBANE C/ 1,~C~ C~C`C~ PROPELLANE .thr~( Z~C's it| TETRA HED RA NE | three (iiC<!~ ~4 C's

III-D. INTELLECTUAL FRONTIERS other desired, energy-rich molecules. Among the important biological molecules already prepared photochemically are the alkaloid atisine, several mycine antibiotics, and precursors of vitamin D3. To exploit the new domain offered by light-assisted pathways, chemists need to become as familiar with the energy topography of the multidimensional reaction "hypersurfaces" as they are with the "ground" reaction surfaces upon which stable molecules react. Lasers will be a powerful aid in this exploration. Already it is known that a 1 percent change in the wavelength for the exciting light (from 3025 to 3000 ~) 89 can double the yield in syn- thesis of provitamin D3. In <~~~ bV Ho the formation of the steroid ,~ ~ ' Am I/ hormone mentioned above, HO'U~ Ho~J the combination of wave- length control (3000 A' and low temperature (-21°C) can quadruple the product yield. As tunable and intense ultra- violet lasers become available to synthetic chemists, the reactive surface topography can be systematically mapped and then exploited to move selec lively on that surface toward the desired products. 1 '7 - Hydroxyprevitamin D' 1 n - Hydrosyprovitamin Do TUNED LASER IRRADIATION DOUBLES THE EFFICIENCY OF THIS STEP TOWARD YITAMIN-D3 Novel Solicis The synthesis of solid materials tailored to need has traditionally been a fairly empirical branch of chemistry. Symptomatically, this has led to specialist communities, each with highly developed expertise in connection with a particular class of materials such as glasses, ceramics, alloys, polymers, or refractory materials. Now we are seeing a broader resurgence of interest in the chemistry and properties of novel solids. Advances in both experimental techniques and chemical understandings are permitting more systematic strat- egies that actively draw upon the established principles of chemical thermody- namics, chemical kinetics, chemical bonding, and molecular structure. Increas- ingly, chemists are joining and expanding the specialist communities mentioned above to address a variety of important societal needs. A number of examples that have been discussed in a practical context in Section IlI-C are appropriately identified with emerging intellectual frontiers. For example, there is an intrinsic spatial (stereo) control associated with chemistry in the solid state. Spectacular use of this control is embodied in the zeolite catalyst structures. These aTumino-silicate solids can now be synthesized with cavities and channels deliberately contrived to accommodate reactants and products of particular shapes. Guest molecules that slip comfortably into these channels can be held in favored conformations as reactants bring about desired

so CONTROL OF CHEMICAL REACTIONS

III-E. INSTRUMENTATION :~IT-E. Instrumentation All our knowledge, both factual and interpretive, is rooted ultimately in our observational and measuring skill. Hence, the advance of science inevitably accelerates when more incisive diagnostic and measuring techniques come on the scene. This is the situation in chemistry today. A ubiquitous theme in our discussions of the impressive opportunities before us has been the crucial role of sophisticated, complex instrumentation in opening new questions to investiga- tion and extending the boundaries of our scientific frontiers. The advance of chemistry and the realization of these opportunities depend upon access to this instrumentation and the support structure needed to operate and maintain it. Current support levels for research in the chemical sciences do not provide such access. The discussions that follow identify a number of instrumental methods that serve in the everyday arsenal of research chemists. First we focus on capabili- ties: today's levels, how much they have changed over the last decade or two, and the capabilities that might develop in the next decade. Then costs are examined and compared to available resources. Lasers An array of complementary experimental methods has come into existence that makes molecular dynamics one of the most active frontiers of chemistry research. On this frontier, perhaps the highest potential for gain lies in understanding, at last, the factors that control temporal aspects of chemical change. Nothing is more fundamental to our ability to control chemical change. Lasers, the most popular of the new techniques, are finding a wide variety of applications that depend upon one or more of the qualities of a laser to deliver light of extremely high intensity, extremely high power, extremely high spectral purity, and/or extremely short duration. Generally, laser design is dictated by the one feature among these that is of greatest value to the experiment at hand, PULSE DURATION < ONE MICROSECOND = .000 OOI SECOND ONE NANOSECOND = .000 OOO DO I SECOND > SPECTRAL PURITY < .ooooo5 cm l ~ ~ .005 cml ONE PICOSECOND = .000 O00000001 SECOND < > 5 cm l ONE FE"TOSECOND - .000 OOO OOO OOO DO I SECOND < > soooeml PULSE DURATION LIMITS FREQUENCY ACCURACY AND V I CE VERSA 91

92 CONTROL OF CHEMICAL REACTIONS and usually at some sacrifice to the others. Some of the trade-off is imposed in a fundamental way by the uncertainty principle, which relates the duration of a light pulse to its spectral purity. Thus, a 1-picosecond light pulse must spread over a frequency range of 5 cam. At this band width, most information is lost about molecular rotations of gaseous molecules. Conversely, if a line width of .005 cm-t is needed to discern individual rotational states, then the molecule of interest must be examined by a light pulse at least as Tong as 1 nanosecond. This deprives us of temporal information about species with shorter lifetimes or about events of shorter duration than the nanosecond probe. Developments in the Last Decade There were three crucially important developments in laser technology that took place during the 1970s and which are having great impact on chemistry. First, a wide range of tunable lasers (in particular the dye laser) evolved and became commercially available. Second, the invention of efficient short wave- length lasers (excimers) gave access to ultraviolet wavelengths at high power. The third development was the ability to produce shorter and shorter pulse durations, reaching 1 picosecond and beyond. In 1970 the tunable dye laser did not exist except as a laboratory curiosity. As of the early l980s, almost every chemistry research laboratory has more than one tunable laser source. Tunable lasers can now be conveniently operated over the wavelength range from 1600 in the vacuum ultraviolet to 4 microns in the infrared. Already in the state-of-the-art stage are lasers that extend the wavelength range to beyond 20 microns in the infrared and to less than 1000 A in the vacuum ultraviolet. The tunable laser source has opened the fields of nonlinear spectroscopy, including coherent antistokes Raman spectroscopy (CARS), picosecond spectros- copy, saturation absorption spectroscopy, two-photon Doppler-free spectroscopy, multiphoton-ionization spectroscopy, laser magnetic resonance spectroscopy, and many other important variations of known spectroscopic techniques. These spectroscopic techniques in turn have led to a broader understanding of all aspects of chemical reactions. Developments to Come In the next generation of tunable source development, we shall see computer- controlled coherent sources with narrow linewidth, higher peak powers, and subpicosecond duration. The wavelength range will extend over the 1000 ~ to 50 microns spectral range. The basic tunable laser system cost may rise to the range $100,000 to $200,000 as the capability continues to evolve. A new frontier is soft X-ray (XUV) sources for spectroscopy, surface studies, microscopy, and lithography. Here the laser may play an important future role as a source of incoherent soft X-rays via the laser-produced plasma and coherent soft X-rays via the free electron laser (FEL) and nonlinear mixing techniques. The laser-produced plasma source should provide about the same average power output in a laboratory as is available from current synchrotron sources,

III-E. INSTRUMENTATION TABLE III-6 List of Laser Types and Their Cost Type 1. Continuous wave ion laser 2. Continuous wave pumped solid state 3. Pulsed solid state Wavelength Range (microns) Discrete visible lines 1.064 (most common) Cost ($) 4. Excimer (pulsed) 5. Color center laser 6. Continuous wave dye laser 7. Pulsed dye laser 8. TEA CO2 9. Molecular gas, cw or pulsed 10. Semiconductor diode 11. Flashlamp excited dye 12. Pulsed metal vapor 13. High-resolution ring dye laser 14. Mode locking accessory for short pulse applications 1.064, 532, .355, .266, .73-.79, .37-.40 .193, .222, .248, .308, .351 1.4-1.6 and 2.3-3.3 .4 to .1 .35 to .95 or second harmonic down to .26 Near 10 Regions from 2 to 11, depend- ~ng on gas. .8 to .30 .34 to .78 .51, .58, .628 .4 to .8 45K depending on power 45K depending on accessories 45K 75K 40K 25K pumped with 1 or 2 lOK pumped with 1 30K pumped with 3, 4, or 12 25K 30K 1-20K depending on line-width and tuning 20K 45K 50K pumped with 1 20K addition to 1, 2, 3, or 8 although it may not compete with undulator-equipped synchrotrons. However, the laser plasma source will be priced at one-tenth the synchrotron beam line cost. The free electron laser is an outstanding new development in tunable lasers. As discussed later in this section, these devices currently operate in the TR but will soon be extended into the UV and vacuum ultraviolet regions. Finally, a combination of excimer lasers, dye lasers, and nonlinear mixing techniques will be used to generate coherent soft X-rays in an intensity range intermediate to the free electron laser and the synchrotron, but at a cost considerably less than either. The mixing techniques have already been demonstrated in research laboratories. Optical coatings, filters, mirrors, and spectrometers are all evolving for use in the 10-30 eV soft X-ray spectral region. Although affordable and portable commercial systems may be several years away, there is a significant application for them in X-ray lithography for ultra small integrated circuit fabrication. Costs anct Applications Table IlI-6 lists many different types of lasers, their wavelength ranges, and rough costs at present. It is important to note that most of the more powerful lasers have only discrete output wavelengths and are most useful in the study of solid materials, which usually have broad absorption features. For most chemical applications, tunable sources are critically important, and these lasers are often excited with a powerful monochromatic laser. The cost listed in Table IlI-6 is for the laser only and does not include detector, measurement electron- 93

94 CONTROL OF CHEMICAL REACTIONS ics, optics, specialized support equipment (e.g., data acquisition), and other items that may be necessary to perform an experiment. These ancillary pieces of equipment will add at least $20K to $40K to the laser cost. Consequently, the cost of equipping a laboratory with tunable sources over a wide spectral range with both pulsed and continuous capability can easily exceed $250K. These are often delicate instruments that are not readily shared' so dedicated use is required. Their impact on chemistry already extends over a wide range of research applications (see Table ITT-71. Ready access to the optimum laser system is essential for work at many of today's most exciting research frontiers. Table IlI-7 lists the most common uses of these different types of lasers in chemical research and applied problems. The last column refers to the type of laser used in these experiments (as numbered in Table IlI-61. Computers The use of computers by chemists has paralleled the revolutionary computer development of the last three decades. The magnitude of the growth is typified by the number of industrial 2000 _ installations of the largest IBM computers, for which MAINFRAME COMPUTERS ~ data are available over this - ~ 500 He - 1000 lo: in Lo o ~ 500 _ ~111111111111 1 960 1 970 1 980 YEAR period. In the mid- 1 950s, there were 20 or 30 such ma- chines (IBM 701s). By the mid-1960s, the much more powerful 7094 and 360 sys- tems numbered about 350. Today, there are perhaps 1700 industrial installations of IBM 3033s. This numerical growth has been accompanied by an increase in computer power that has taxed the field's supply of superlatives. We shall use only a bit of this jargon, as approximately de- fined in Table III-~. The extent to which chem- lNDUSTRIAL USE OF LARGE COMPUTERS istry has benefitted by the growth of computing speeds is well illustrated by comparing two landmark calculations. The first ah initio self-consistent field methods for polyatomic molecules appeared in the early 1960s, and study of the internal rotation barrier in ethane was of special importance. In that work, a minimum basis set of 16 functions was employed.

III-E. INSTRUMENTATION TABLE III-7 Research Areas Utilizing the Laser Relative Number Laser Used Area of Users Application (see Table III-6) 95 A. Subpicosecond kinetics Very few Vibrational relaxation in Passively mode-locked dye condensed phase laser 1, 6 Fast semiconductor de- Amplifier 3, 7 cay B. Several picosecond ki- Few Fast electronic state re- High ~ Mode-locked netics taxation rep. ~ pump—1 or 2 Decay of coherent pro- rate J Synchronously pumped cesses in the condensed dye laser 6, 14 phase High l Mode-locked power J Solid state 3, 14 C. Nanosecond Many D. Microsecond kinetics Many E. Photochemistry F. Isotope separation G. Materials science H. Microprobe analysis I. Raman spectroscopy J. Atomic absorption/ Many Few Many Few Few Lifetimes of excited states Characterization of fast reactions Gas phase decay processes; non- thermal chemical reac- tions UV or visible photolysis Pure element isolation Controlled melting and crystallization Source for mass spec- trometer or atomic emission Very many Routine sample charac- terization and analysis Trace element analysis Visible 3 or 4, 7 IR 8 Visible—11 IR- 8 or 9 4 or 11 4 or 12, 7 1, 3, or 8 3 1, 6 1 optionally 6 fluorescence K. Combustion diagnostics Few Probing reaction cham- 3, 7 hers L. Atmospheric gas Sam- Many Monitoring industrial 10 pling processes M. Ultrahigh resolution Few Linewidth measurement; Visible 1, 6, 13 spectroscopy excitation of single IR—10 quantum states in the gas phase N. Tunable cw spectroscopy Very many Condensed phase absorp- 1, 6 tion and fluorescence excitation 0. High power experiments Many Saturation of transitions Visible 3 or 4, 7 IR 8 or 9 P. Generation of extreme Few Frequency conversion Visible 3 or 4, 7, 8 frequencies stimulated Raman IR range .03 to 300 m scattering or frequency sum/difference Q. Cells sorting Many Discrimination based on 1 fluorescent tags R. Cellular bleaching Few Photochemistry within 1, 6 microscopic structures S. Nonlinear spectroscopy Few Saturation spectroscopy; 3 or 4, 7 CARS and related methods

96 TABLE III-8 Relative Computing Speeds of Computer Levels Computing Superminicomputers Mainframes Supercomputers Example DEC VAX 11/780 IBM 3033 CRAY IS (1) 10-15 80-120 CONTROL OF CHEMICAL REACTIONS This can be contrasted with a recent study of decamethyl Relative ferrocene, which involved 501 Speed basis functions. Because such studies require computing ef- fort proportional to the fourth power of the number of basis functions, the decamethyl fer- rocene computation involves (501/1614 or 1 million times more computation than the prototype ethane problem! Superminicomputers This level of computer has become a workhorse in chemistry. Instruments like the DEC VAX 11/780 are comparable to the largest mainframe computers available in the late sixties. They have revolutionized computing in chemistry because of their substantial capacity, high speed, and lower cost, which is now in the range of $300K to $600K. The last 20 years have also seen three important development phases in the use of computers in chemical experiments. In the first, the computerization phase, advances in both hardware and software greatly enhanced data acqui- sition. Then an automation phase increased the possibilities for experiment control through real-time monitoring of critical parameters. Finally, a "knowI- edge engineering" phase ushered in an era in which computers perform high-level tasks in interpreting collected information An excellent example is the Fast Fourier Transform algorithm, which permits us to record spectral data in the time domain and then to transform the results to the frequency domain. Because this allows detection of quite weak signals, the algorithm is now routinely used to record i3C NMR signals and to transform infrared interferograms. This is accomplished by building into the instrument a dedicated computer of adequate capability and speed. Because of the success of these instruments, the Fourier transform algorithm is now being ; ~ ~~ ~~ ~ ~ ~ 1 : ~ ~ ~ ~ 1 ~ ~1~ ~ ~1~ ~ _ 1 ~ _ · ~ 1 . lil~Vlp~l=~U lilLU ~l~-~il~lnl~l, microwave, ion cyclotron resonance, dielec- tric, and solid state NMR equipment. Now, microprocessors have come down sufficiently in cost to motivate chem- ists and instrument designers to integrate them into their scientific instru- ments. Such microprocessor-based equipment can monitor data from several sensors simultaneously to provide a multidimensional perspective that is difficult to obtain otherwise. Research examples include excitation-emission fluorescence and mass spectrometry, and practical applications include Com- puterized Axial Tomography (CAT) and NMR imaging as diagnostic tools in . . mec 1c1ne. Despite their capability, superminicomputers are barely practical for the

Ill-E. lNSTRUMENTATlON larger chemical computations because of Tong turn-around times (e.g., many days of continuous computing). However, addition of an attached processor greatly enhances computational speed, while retaining the convenience of the local superminicomputer but with a price/performance ratio better than those of many large mainframe computers by several-fold. Mainframe and Supercomputers Even so, the potential for some computation in chemistry can be met only with the greater capacity and capability of the largest scientific computers (Cray/M and X-MP or CYBER 205) coupled with specialized resources, such as software libraries and graphics systems. This is true most notably for ab initio electronic structure studies. The entire development of ab initio electronic structure studies has been closely linked with parallel developments of computer hardware and software to facilitate manipulation of massive amounts of high-precision numbers. In some cases, computational quantum chemists have interacted closely with hardware manufacturers in the area of design and performance standards. For example, the Hitachi vector processor was designed in consultation with quantum chemists at the Institute for Molecular Science in Okazaki, Japan. In its April 1984 report, the Task Force on Large Scale Computing of the Committee on Science (ComSci) of the American Chemical Society recommended that ". . . the ACS take initiative to establish interaction between the scientists who are users of large-scale computation and the designers of new supercomputers." Another area that would be stimulated by increased access to supercomputers is computational biochemistry. Most dynamical simulation procedures applica- ble to biological molecules require energy solutions for the simultaneous motions of many atoms. A conventional 100-picosecond molecular dynamics simulation of a small protein in water would require about 100 hours on a DEC VAX 11/780 or 10 hours on an IBM 3033. Calculations of the rate constant for a simple activated process require a sequence of dynamical simulations to determine the free energy barrier and additional simulations to determine the nonequilibrium contributions; times required can now reach 1000 hours on a DEC VAX 11/780. More complicated processes or longer simulations become impossible without extensive access to supercomputers. Costs It is plain that computers have already exerted a strong and beneficial impact on chemistry and that this will continue. At present, the progress that can be made using computers in chemistry is limited exclusively by lack of resources. Existing computers at all levels could increase enormously the research produc- tivity of chemists if they had access to the computing equipment of the ap- propriate capability (whether dedicated, locally shared, or networked) and to the support infrastructure needed for cost-effective use of that capability. For chemistry, the problem is less the need for a next-generation development than 97

98 CONTROL OF CHEMICAL REACTIONS 35M it is for ability to use, when- LARGE COMPUTERS ever applicable, the computa- ~ tional capabilities that al- / ready exist. S4M CD ~ S3M z ~ S2M z S1M .' _ O / ~~ . a' CRAY O CDC ~ IBM 1 950 1 960 1 970 1 980 1 990 YEAR Molecular Beams When the NRC report Chemistry: Opportunities and Needs was written in 1965, the powerful crossed molecu- lar beam studies on reaction dynamics relied entirely on the surface ionization method for the detection of scattered products, and systems inves- tigated were limited to those containing alkali metals. The situation has changed drasti- cally over the past 17 years. AS COMPUTING POWER GROWS, COSTS ESCALATE The development of univer- sally applicable crossed mo- lecular beam systems and high-intensity beam sources has made this method a powerful experimental too] for the investigation of elementary chemical reac- tions, energy transfer processes, and intermolecular potentials. Capabilities A typical, crossed molecular beam apparatus can contain as many as eight differentially pumped regions provided by various high-speed and ultrahigh vacuum pumping equipment. It may be necessary to maintain a pressure differential of 14 orders of magnitude, from 1 atm of pressure behind the nozzle of the molecular beam source to 10- torr at the innermost ionization chamber of the detector. What is glibly called the detector is likely to be an extremely sensitive, ultrahigh-vacuum electron-bombardment mass spectrometer detector with which to measure the velocity and angular distributions of products. With optimum design, an angular resolution better than 1° and velocity resolution better than 3 percent can be achieved for scattering processes that provide a steady state concentration of scattered molecules of less than 100 molecuTes/sec (~lo-~5 torr) in the ionization region of the detector chamber. With such sensitivity and versatility, many new experiements become possible. For example, by replacing one of the beams by a high power laser, molecular beam systems are now giving new kinds of information on the dynamics and mechanism of primary photochemical processes. In the past 5 years, molecular beam experiments have played a crucial role in

III-E. INSTRUMENTATION advancing our fundamental understandings of elementary chemical reactions at the microscopic level. The advances provide deeper insights with which to build our explanations of macroscopic chemical phenomena from the informa- tion gathered in microscopic experiments. In view of this success, more than 10 new advanced crossed molecular beam systems have been constructed abroad in the last ~ years, in Germany, France, England, ~Japan, Italy, Australia, and other countries. China and Taiwan are contemplating building comparable molecular beam laboratories in the coming years. But the high cost and the complexity of setting up and operating a crossed molecular beam apparatus have limited the general availability of this powerful too] in chemistry commu- . . nines. Costs A state-of-the-art crossed molecular beam apparatus equipped with high- intensity beam sources and data acquisition electronics will cost $350K to construct and $40K/year to maintain. If a tunable laser is also used for the excitation of reagent atoms or molecules to specific quantum states, an addi- tional $100K will be needed. In addition, auxiliary general supporting equip- ment costing in the neighborhood of $100K will generally be needed in a molecular beam laboratory. Thus about half-a-million dollars in equipment funds is needed to establish a new, state-of-the-art, molecular beam laboratory. Such an amount has become almost beyond reach in the United States, particularly for scientists in aca- demic institutions. Only two new crossed molecular beam systems equipped to measure angular and velocity distributions of products have been constructed in the United States during the past 5 years. If the current trend were to continue for another decade, we would certainly fall significantly behind other countries in this area of research. As the experimental sophistication of crossed molecular beams methods continues to increase, a broader range of investigations come within reach. :Laser technology is playing a more important role. Thus we can expect crossed molecular beam techniques to have great impact on chemistry provided re- sources are made available to exploit the potentialities. Synchrotron Light Sources Existing Characteristics of Synchrotron Sources The most intense, currently available source of tunable radiation in the extreme ultraviolet and X-ray region is synchrotron radiation, which is pro- duced when energetic electrons are deflected in a magnetic field. Current capabilities and future needs for synchrotron radiation were recently reviewed by the NAS/NRC Major Materials Facilities Committee. As described in detail in their 1984 report, the principal current use of tunable synchrotron radiation 99

100 CONTROL OF CHEMICAL REACTIONS TABLE III-9 U.S. Synchrotron Light Sources Beam Critical Energy Energy Spectral Facility Name (Location) Yeara (GeV) (keV) Brilliancef Dedicated Tantalus (U. Wisconsin) 1968 0.24 0.05 4. 10~° SurfII (NBS) 1974 0.28 0.06 5 10~i NSLS (Brookhaven, N.Y.) 1981 0.75 0.5 3 10~3 NSLS (Brookhaven, N.Y.) b 2.5 5.0 1 ~ 1014 Aladdin (U. Wisconsin) b,C 0.75 0.45 (2 10~3) "Hard X-ray" ring ~ 6 10 (1 ~ 1Oi8)e "Soft X-ray" ring ~ 1-2 0.010 (3 1Oi8)e Parasitic SPEAR (Stanford U., Calif.) 1974 3-4 5 6 10~2 CESR (Cornell U., N.Y.) 1980 5.5 11.5 5 10~2 Year first experiments were conducted. b Not yet operational. c Construction interrupted. Proposed, NRC Report "Major Facilities for Materials Research and Related Disciplines," D. E. East- man and F. Seitz (1984). e Proposed brilliance using undulator insertion devices. f Spectral brilliance is the number of photons per square millimeter per square radian per unit band- width. falls in the photon energy range of 1 to 100 keV as provided at several, dedicated facilities and as a parasitic activity at a few high energy particle-physics accelerator facilities here and abroad. Table IlI-9 shows that in the United States, sychrotron radiation is currently in research use at three dedicated synchrotron laboratories and parasitically at two particle-physics accelerator laboratories. By comparison, there are in foreign countries seven dedicated synchrotrons operating (in France, Germany, Great Britain, Japan (three), and the Soviet Union) and five under design or construction. Over the past decade, there have been important advances as attention has turned from synchrotrons as accelerators (with radiation seen as an undesired energy dissipation) toward synchrotrons as sources of light. Insertion devices were designed to place sharp bends in the electron trajectories to increase the radiative properties (bends, "wigglers," or "undulators"~. These devices show potential for intensity increases by several powers of 10. Such a quantum jump in brightness will surely lead to new types of experiments. For many chemical applications, the intensity will be of particular importance if the design specifications attempt to optimize radiation in the vacuum ultraviolet spectral region rather than regard it as a parasitic use. It is important to note that the pulsed nature of synchrotron radiation (tens of picosecond pulse durations) and its high repetition rate (108-109 pulses per second) can be of particular value in chemical kinetic investigations if the wavelength range is appropriate.

III-E. INSTRUMENTATION Applications of Synchrotron Sources in Chemistry Scientific use of synchrotron radiation in the United States has been building. There were some 210 individual users in 1976, and the number had grown to about 600 by 1982. Statistics are not available on the fraction of these users who would identify themselves as chemists, but the number is not negligible. A recent estimate at the NSES facility indicated that less than 20 percent of the research was placed in chemistry. The range of problems under study at our synchrotron light sources is illustrated in the examples below. Extended X-ray Absorption Fine Structure (EXAFS) has been one of the more fruitful applications of synchrotron radiation to solid substances. When one of an atom's inner-shell electrons is excited above its X-ray edge energy, the atom emits light that is diffracted by neighboring atoms. The result is a diffraction pattern that contains information about the interatomic spacings of these neighbors. Much attention has been directed toward crystal structures of inorganic solids, some of it seeking information on oxidation state when other methods are not definitive. Because heavy atoms are most readily detected, EXAFS has been usefully employed to learn the immediate chemical environ- ment of transition metal atoms as they occur in biologically important mole- cules, including manganese in chlorophyll. The method is also applicable to dilute species (as Tow as one part in 104-105) using fluorescence intensity as a function of incident photon energy. Applicability to surface corrosion studies can be achieved through measurement of photon-induced Resorption. Time- resolved EXAFS is also feasible through the use of dispersive techniques in which the entire absorption spectrum is measured simultaneously. As synchrotron intensities are increased, photoelectron emission intensity can be measured instead of absorption coefficient. A promising development in this area is called Angle Resolved Photoemission Extended Fine Structure (ARPEFS). The diffraction patterns so obtained are sensitive to bond distances and bond angles of atoms located beyond the closest neighbors. Still another possible application of brighter synchrotron X-ray sources would be time- resolved X-ray scattering. Time dependence of puIsed-laser surface damage (annealing) could be measured in real time. Time-resolved, small angle scat- tering experiments on muscles have already shown changes in repeat distances as muscles undergo contraction and relaxation. Synchrotron Radiation Costs The costs of synchrotron radiation are highly varied because such facilities range from relatively small, in-house facilities to large, user facilities. However, the recently proposed hard X-ray (optimum, 1.2 A wavelength) 6-GeV synchro- tron light source (which would find some limited use in chemical applications) has been priced at about $160M for construction, including its proposed 10 insertion devices and associated beam lines. A European counterpart proposal 101

102 1022 - - ._ 20 0 - N L 1016 C o ~ 1 0 L) 15 Z 10 - I ol 4 13 ~ 10 a: in, I ol 2 CD CONTROL OF CHEMICAL REACTIONS considering a 5-GeV storage ring has an estimated construction cost of $200M. Softer X-ray synchrotron light sources, for which there may be more general chemical applicability, might cost in the range $60M to $80M. At the storage rings at the University of Wisconsin ancI at the Brookhaven NSL:both clesignec! specifically as radiation sources once the synchrotron is in place, the acTditional cost for a new, cleclicatec! beam line falls in the range $.8M to $1.5M. Of course, once built, such an installation requires anc! warrants a substantial, continued annual investment for operation anc! maintenance. For example, the 1985 operating budget at NSL`S is about $15M. ~ I, — ~ ~ . ~ Free Electron Lasers When a beam of electrons with velocities near the speec! of light moves through a static, periodically alternating magnetic fielcI (a "wiggler"), light is emitter! in the direction of electron beam propagation. The wavelength of the light is cleterminecI by the period of the wiggler field ancl the energy of the electrons. This provides a "gain medium" that, if placed between the mirrors of a conventional laser, can emit coherent laser light. Such a crevice is called a free electron laser (FEL). Potential Capabilities Experience to date indicates that high-e~ciency, wavelength tunability anc3 high average ancl peak power will all be forthcoming over a wavelength range extending from microwave fre- ....................... . . ~ . quenches turoug n t. He ~ntrarec . BW = BAND WIDTH ancI visible to the vacuum ul- traviolet spectral ranges. If the hopes and expectations of the most enthusiastic FEL propo- nents are realizecI, average brightnesses several powers of 10 greater than those proviclecI by conventional tunable lasers or synchrotron sources may be possible, particularly in the ultraviolet. Furthermore, short wavelength perform- ance may be extenclec! beyond the limit of present mirror reflectivity, e.g., to the 10- to 30-eV range, either with multilayer mirrors now under development, or with "single pass" high-gain FEL's that do not use mirrors. //// LASERS ,,,,,,,,,,,,,,,, _ ~(,.,,~,,,,4,X,,B,,W,?9 1-2 GeV SYNCHROTRON (.1XBW) x~'~\= BENDS . 1 , . 1 , . 1 1 0 1 00 1 000 1 0,000 PHOTON ENERGY (eV) DESIGN GOALS ARE AMBITIOUS - AND PROMISING

III-E. INSTRUMENTATION Possible Costs The optimum design (and cost) may vary greatly, depending upon the wavelength to be produced. Theoretical estimates of energy efficiency range up to 20 percent for linear accelerator-driven FEL'S, a performance most readily achieved in the microwave and infrared spectral regions. Such devices, which might cost from $10K to $30M, have proposed performance characteristics of 30- picosecond pulse duration and peak powers comparable to those of laboratory CO2 lasers (tens of megawatts peak power). An FEI~ with approximately these characteristics has already been operated at Los Alamos National Laboratory. It is based on a linear accelerator 2 or 3 meters long to produce electron energies of 10 to 25 MeV. The device provides a train of pulses of tunable infrared radiation currently in the 9- to 11-micron range with 30-picosecond pulses, peak power of 5 megawatts, 50-nanosecond spacing between pulses, and a pulse train duration of 80 microseconds once a second. Such performance extended over the mid-infrared spectral region (4 to 50 microns) would open the way to many novel applications in chemistry. Examples are vibrational relaxation, multiphoton excitation, nonlinear processes in the infrared region, fast chemi- cal kinetics, infrared study of adsorbed molecules, and light-catalyzed chemical reactions. As the wavelength is moved through the visible and toward the ultraviolet, not only the electron energy, but also the current density of the electron beam must be raised. Some researchers (but not all) fee] that the current density needs are best met by the larger (and more costly) storage rings like those proposed for use as synchrotron light sources. In fact, free electron laser emission has been demonstrated at visible wavelengths making use of existing storage rings not initially designed for FEL use. It seems likely that a synchrotron-type storage ring with electron energies in the range of .5-1.5 GeV could provide an effective electron beam gain medium for FEL use. At this energy range, the synchrotron could be suitable both as a soft X-ray source and as a tunable FEL source. If designed with the FEL use as a primary application, extremely high brightness might be achieved in the ultraviolet and vacuum ultraviolet spectral ranges. Such a synchrotron might cost between $30 and $80M. Again, a variety of novel chemical applications could be explored in photochemistry and fast chemical kinetics, as well as multiple photon and other nonlinear processes. Instrument developers should be aware that most of these applications could be productively pursued in the visible and normal ultraviolet spectral ranges. 103

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W. . . .... ~ . - .. hipping a wlcRecl weed . The plant Striga asiatica is one of the most devastating destroyers of grain crops in the world. This wicked weed~restricts the food supply of more than ;400 million people in Asia and Attica. It is a parasite that~no~shes itself by latching onto and draining the vitality~of a nearby gain plant. The results are stunted grain, a meager harvest, and hungry people. ~ Basic ~research~on Striga asiatica by chemists~and biologists has revealed one: of the plant world's incredible host-parasite adaptations. The:~parasite~seed lies in wait until it detects the proximity ofthe host plant by using an uncanny chemical _ rat ar. '1' He give-away is provided by specific chemical compounds exuded by the host. Striga asiatica can recognize the exuded compounds and use~them to trigger its own growth c role. Then the parasite has an independent growth period of 4 days, Wring which it must locate~the nearby host. ~ ~ ~ ~ ~ ; _ . · ~ ~ . ~ ~ ~ · ~ ~ . ~ · nesearcners trying to solve tne mystery ot tnls recognition system tacea:tormldable . . . ~ ~ . . . O. Stan .es; t Hey were see mug un gnome comp ex molecu es proc ucec on y ln tmy amounts. But, by extending~the sensitivity ofthe most~modern~instruments, chemists have been able to Educe the chemical structures of these host-recogr~ition sub- stances, even though the agricultural~scientist could~accumulate~the~active chem- icals in amounts no larger than a few bits of trust (a few micrograms). One method used, nuclear magnetic resonance (NMR), dopers upon the fact thank the nuclei of madams have magnetic fields that respond measurably to Tithe Presence of other such nuclei nearby.:Thus precise NMR~measurements reveal molecular go- : ~ _ : : : Got _ ..£~ ~ ~ ~ ::::: ~ : ~ : ~ ~~, I, ~~:~ ~ ::~: ~ —~^ /~ P~} · ~ ~ ~ : the ~ ~ ~ : ~ ~ : t ~ · ~ · ~ ~ ometrles,::evel~ or ornate mo-~lecu es. ~ secono' equally sop ~~sUcatea~approach~s~h~gh- reso: utlon mass ppectrome~y.~ n a 1lg ,~vacuum, mo .ecu es~are~g~ven angled ectrlc : ~ ~ c ~arge,:then;;accelerated with~a known energy. By Leasing the velocities ~ which these molecules ar~ffagments from them are traveling (or heir curved paths in magnetic~fields), chemist can~measure~;the masses and decide~the ~atomic~o~ings present.These~are~critical~cluestothe~molecul~identities. ~~ ~ ~~ ;~ ~~ ~~ :~: ~ ~ ~ AT ~ :i ::: if: I: :i: : I ~ ~ · o : : · · ~ , : ~ :~: ~ ~ · ~ · ~ ~ ~ ~ l~OW, me complex nosr-reco~Itlon xenoglstlc} ;sunst~ces cam neen 1uentlilea , . . . . . . ~ ~ ~ . ~ . ~ ~ , . hi .: ~ . . ~ . . . ~ . t1 1 ant I. 1elr c etallea~structures are Known. :Wlth thls:~lr~ormatlon In hand, we moor ]] be able to beat this wicked weed at its own game. Chemists can; now synthesize the: V substances Andes give agricultural scientists enough material for f eld tests designed to trick the parasite into beginning its 4-day growth cycle. It will die on never having found its host. A few days later, grain can be planted safely. ~ AT' ~ ~ l al · ^ ~ · ~ - · ~ ~ Olin Miss success Ior gulc~ance, similar host~araslte r~ionships are being sought— . ~ . . . . and round—nere In tne;United~States. Tn. addition to grains, bean crops have similar parasite enemies. Thus in collaboration with agricultural and biological scientists, chemists play a crucial role indoor efforts to increase the world's food supply and 1 net h ~ —~ ~ he 1ml e lunge, 1~ ~ : : :: ~~: ~ :~ _ ~ _ ~ —~ ~ ,,H ~ ,_ ~ : 1 1 l : HE - U:GH A:

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Opportunities in Chemistry is based on the contributions of hundreds of American chemists in academia and industry and should be taken as the best available consensus of the chemical community regarding its intellectual frontiers and the economic opportunities that lie beyond them," says Science. This volume addresses the direction in which today's chemical research is heading, including recent developments, technological applications, and the ways advances in chemistry can be used to improve the human condition. In addition, the book examines economic and political implications of chemical research and lists resources for basic research and education in the chemical sciences.

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