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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
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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
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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
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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!
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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.
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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.
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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-
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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
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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?
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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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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
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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
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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
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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
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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
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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.
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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
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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
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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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— : : : : ~ (AH: iMISTRY:isacentral~science~ i: ~ ~ i: : : ~ : : : : : A. : :: : : If: ~ ~~:~:~:tnat~re~sponusto~socl~tal~needs. ~ ~ ~ ~~ . : : ~ : : :: : :~ :~ ~~ ~ ~~ :~It:is:critical~:in:Man:'sattemptto.~:~.:. ~ ~ :: ::~:::~:~ : : : :~:~:::~f~ed~th~ewo:rld's 0 ul t If: ~ :: :~ : : , ~ p p a ton :: : ~ : ~ : ~ ~ : ~
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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:
Representative terms from entire chapter: