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CHAPTER IV Intellectual Frontiers in Chemistry A remarkable bounty of benefits has been shown to flow from chemistry. This chapter will provide abundant evidence that these benefits will increase greatly in the years to come. The basis for this optimistic expectation is that this is a time of special opportunity for intellectual ad- vances in chemistry. The opportunity comes from our developing ability to in- vestigate the elemental steps of chemical change and the ability to deal with ex- treme molecular complexity. His

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The Time It Takes to Wag a Tail When your pet dog sniffs a bone, instantly his tail begins to wag. But it must take some tune for the northernmost canine extremity to send the news all the way south where enthusiasm can be registered! How long does it take for that delicious aroma to lead to the happy response at the other end? Chemists are now asking questions much like this about their pet molecules! If one end of a molecule is excited, how long does it take for the other end to share in the excitement? That time may determine whether the excitation will result in a chemical reaction in the part of the molecule where the energy was Projected, somewhere else, or nowhere at all. For the canine expenment, we need a hungry dog, a quick hand with the bone, and a quick eye to read the stopwatch. For molecules, it's much harder. Only within the last few years has it been possible to measure the rate of energy movement within a molecule. But chemists now have pulsed lasers gong bursts of light with durations as short as a millionth Of a millionth of a second (a "picosecond''). Comparing a chemical change that takes place one picosecond to a one-second tail-wag delay involves the same speed-up as a l~second instant replay of all historical events since the pyramids were built. The aLa~yl benzenes provide an example. Each of these molecules has a nod benzene Iing at one end and a flexible alkyl group at the other. At room temperature, this flexible "tail" vibrates and bends under thermal excitation. But to act like our hungry dog, the molecules must be cooled to cryogenic temperatures, while avoiding condensation. Supersonic jet expansion makes this possible. When a gas mixture cows through a jet nozzle into a high vacuum, the molecules can be cooled almost to absolute zero. An alkyl C' ~ :~/C~1 1 benzene molecule camed along in such a stream loses all its vibrational energy, thus relaxing the molecular tail. Then, the cold molecules intersect a brief pulse of light with color that is absorbed by the benzene ring. With careful "color-tun~ng,~' extra vibrational energy can be placed in the head without any v~b~onal excitation In the tail. Then we must watch the molecule to see how long it takes for the tail to wag. Fluorescence lets us do this. When a molecule In a vacuum absorbs light, the only way it can get rid of the energy is to reelect light; Such fluorescence can be recorded with a fast-response detection system to give a spectrum that carries a tell-tale pattern showing where We extra energy was at the instant the light was emitted. Those molecules that happen to emit right away after excitation show the molecule head vibrating and the tail still cold. Those Mat emit later have an emission spectrum Mat shows that the tail is wagging. In this way, we have learned that the time it takes for the aLky! benzene ~ to begin to wag depends on how long the tad] is. Su~pnsingly, the longer the aLkyl, the faster the movement out of the nng. The result shows what detentes energy flow within molecules (the "density of sagest. Such information might one day charm combustion and help us make fine chemicals out of coal. ~6 r

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IV-A. CONTROL OF CHEMICAL REACTIONS IV-A. Control of Chemical Reactions Ultimately, success in responding to society's needs depends upon the ability to control chemical change, a control made possible by our understanding of chemical reactivity. Today, this understanding is being broadened and deepened at an astonishing pace because of an array of powerful new instrumental techniques. These instruments permit us to pose and answer fundamental questions about how reactions take place, questions that were beyond reach only a decade ago. They account for the recent acceleration of progress in the most basic aspects of chemical change. MOLECULAR DYNAMICS Chemistry is the science concerned with the changes that occur around us when one set of chemicals turns into another set of chemicals. Such a change, a chemical reaction, is understood at the atomic level in terms of one set of molecules rearranging into another set of molecules. The study of these rearrangements is called molecular dynamics and it encompasses: molecular structure, the stable geometries of the reactant and product mole- cules; chemical thermodynamics, the energy effects that accompany the change; and chemical kinetics, the time it takes for the reaction to occur. The theory behind all chemical behavior rests in quantum mechanics. Quantum mechanics is the mathematical description of atoms and molecules devised by Erwin Schroedinger in 1926. It is based upon a wave-picture of the atom that has the potential for explaining all of the chemistry of that atom. Though this has been known for over 50 years, most of the predictive power of quantum mechanics has been out of reach because the mathematics has been too difficult to solve. In contrast, experimental progress on stable molecules has been extremely rapid. This is evident in the fact that chemists have prepared more than ~ million compounds, 95 percent of them since 1965. On the other hand, our understanding of the speed aspects of chemical change has been limited by reaction steps too fast to be observed. Now a new era has begun. Chemical theory, supported by the power of modern computers, has emerged from empirical modeling. At the same time, we have expenmental techniques that open the way to understanding the time 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 decacles. Fast Chemical Processes A chemical reaction begins with mixing reactants and ends with formation of final products. In between, there may be a succession of steps, some extremely rapid. To understand the reaction completely, we must cIanfy all the steps between beginning and end, including identification of all of the intermediate molecules that are involved in the steps. ~7

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118 INTELLECTUAL FRONTIERS IN CHEMISTRY Fifteen years ago, we could track intermediate molecules only if they hung around at least as long as a millionth of a second. The many interesting studies on this time scale only increased the chemist's curiosities because it became clear that a whole world of processes took place too rapidly to be detected at that limit. Nowhere was that more apparent than in the centuries-old desire to understand combustion, perhaps the most important type of reaction known. Laser light sources have spectacularly expanded these experimental horizons over the last decade. One of their unique capabilities is to provide short-duration light pulses with which to investigate chemical processes that occur in less than a millionth of a second all the way down to a millionth of a millionth of a second (i.e., down 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 0.01 picoseconds (10 femtoseconds) have been measured, and kinetic studies are beginning in the 0.1-picosecond range. At one-tenth of a picosecond frequency accuracy is limited ~ ~ . ~ . . . . , . .. .. . , ~ . to about 50 cm-l by a fundamental physical principle the Uncertainty Principle (See Section V-A). These developments imply that chemists can now investigate a reacting mixture on a time scale that is short compared with the lifetime of any intermediate molecular species involved. The exploitation of this remarkable capability has only just begun. The absorption of visible or ultraviolet light by a molecule adds enough energy to redistribute the bonding electrons, to weaken chemical bonds, and to produce new molecular geometries. The outcome might be a high-energy molecular structure difficult to reach by chemical reactions stimulated by heat. So the excited electronic states reached by absorption of light furnish a new chemical world that we have only begun to understand and put to practical use. When a molecule absorbs light, it gains energy. One of the ways it can dispose of the energy is to reemit light, generally of a different color than the absorbed light. If this emission occurs quickly, it is called fluorescence. "Quickly" can mean anywhere from within a microsecond to a picosecond. The blue light emitted by a Bunsen burner flame and the spectacular display of the Northern Lights are examples of fluorescence. If the light emission occurs more slowly, it is called phosphorescence. "Slowly" can mean anywhere from a millisecond to several seconds or even minutes. Some clock dials that glow in the dark and the blue glow of evening ocean tides are examples of phosphorescence. We have some basic understandings about the differences that cause these two behaviors. When two electrons are shared in a chemical bond, they must have opposite magnetic spins (as expressed in the Pauli Principle). But if absorption of light adds enough energy to move one of these electrons to another part of the molecule, the Pauli Principle no longer limits the electron spins. Then they can be oriented opposed to each other, like two magnets whose fields cancel each other, to give a "singlet" state. But they can also be oriented parallel so that the two magnetic fields add together. This is called a "triplet" state. We have learned to associate fluorescence with light emission processes that begin and end in singlet states. Phosphorescence, however, requires moving from a triplet to a singlet state (or the reverse). Apparently, the need to change the electron spin makes the emission much more difficult, so it occurs more slowly.

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lV-A. CONTROL OF CHEMICAL REACTIONS There has been a spectacular increase in our ability to clarify what is going on in these excited states since lasers have come into the chemistry laboratory. We can now excite particular states (by control of the laser color, or wavelength), and we can measure the time it takes for reemission to occur (by use of laser pulses of very short duration). Even for the fastest fluorescent processes, we can measure the radiative lifetimes, and by measuring the wavelength of the light emitted (spectral analysis) we can see how rapidly energy moves within the molecule and where it goes. Thus, we are beginning to map and understand the high-energy electronic states of molecules so that they can be used to open new reaction pathways. Benzophenone is a substance that demonstrates how lasers are being used to probe these high energy states. When benzophenone in ethanol solution absorbs ultraviolet light at a 316-nm wavelength, it reemits light at two different colors, at wavelengths of 410 and 450 nm. If the exciting light (316 nm) is delivered in a laser pulse of 10 picoseconds duration, "prompt" emission is seen at 410 nm, with intensity that decreases with a 50-picosecond half-life. This fluorescence is fol- lowed, however, by weaker emission, still at 410 nm, but with a longer half-life (a microsecond). This slower fluorescence disappears at lowered temperatures and is replaced by longer-wavelength phosphorescence at 450 nm with an even longer lifetime (a millisecond). Photochemists have been able to interpret these clues about the excited states of ben- zophenone. Absorption at 316 nm reaches a singlet state (S~) but with extra energy placed in the vibrational motions of the benzophenone. This vibra- tional excitation is lost so quickly in liquids (warming the solvent) that even the "prompt" fluorescence back to the ground state (S0) occurs at longer wavelengths (410 nary). On the other hand, the low-temperature behavior shows that benzophenone also has an excited triplet state (T~),that can be reached via So. Once occupied, To emits phosphorescent light with the characteristic long lifetime of a triplet-singlet transition (T~ > Ski. The temperature dependence of the delayed fluorescence shows that To is lower in energy than So and by how much. The set of processes clarified here have lifetimes that range from 50 picoseconds to a millisecond, a difference of 20 million. The observations reveal the excited states of benzophenone and the rates of movement between them. These under- standings are of extreme significance because they can all be applied to natural photosynthesis, a process scientists would like very much to master. There are many other types of laser-based, real-time studies of rapid chemical reactions now o FLUORESCENCE (3 _ _ ~ - 'A _ _ S 1 S I _ ~ T I 'PROMPT' 'DELAYED' he he' he he/' 50 psec 1 User _ _ _ _ _ ~ _ _ so sow _ 50-10-12 sec 1. 10-6 sac PHOSPHORESCEN CE ~3 _ ~ . ~ T1 ~ LOW T he ~ he" _ / I msec _ ~ _ _ Hi= so- 1~10-3 sec EXCITED BENZOPHENONE EM ITS LIGHT WITH TWO COLORS AND THREE CLOCKS 119

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120 HOT RING COLD TAIL _ COLD RING COLD TAIL INTELLECTUAL FRONTIERS IN CHEMISTRY being made, including chemical isomerizations, proton transfers, and photodisso- ciations. Some of the phenomena to follow also depend upon use of short-puIse laser excitation instrumentation. Energy Transfer and Movement In all chemical changes, the pathways for energy movement are determining factors. Competition among these pathways determir~es the product yields, the product state distributions, and the rate at which reaction proceeds. This competition is highly important in stable flame fronts (as in Bunsen burners, jet engines, and rocket engines) and in explosions, shock waves, and photochemical processes. When two gas phase molecules collide, vibrational energy can be transferred from one molecule to another. Thus, a vibrationally "cold" molecule might be heater! up and caused to react or a vibrationally "hot'' molecule might be cooled off so it cannot react. These transfers of vibrational energy between and within molecules as a result of collisions between them have long been recognized as central to determining reaction behavior ire flames. But progress has been slow because the processes have been too fast to measure. Now a variety of tech- niques almost all based on laser methodshas opened the way to providing critical data related to the pathways and rates of energy flow. These data, in turn, furnish a basis for the develop- ment 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 ex- cite particular vibrations in a molecule. Then, experiments were devised to permit us to watch this carefully placed en- ergy move into other parts of the molecule or into another molecule if collisions occurred. Fluorescence provides one way to follow this energy movement. The light reemitted during fluorescence carries a spectral signature that shows what part of the molecule is vibrating at the moment of emission. A clear-cut example is provided by recent studies of the alkyd benzenes, C6Hs-(CH2)nCH3 with n from 1 to 6. This molecule has a structure like that of a tadpole, where n determines the length of its tail. Tuned-laser excitation allows - FLUORESCEN CE REVEALS INTRAMOLECULAR VIBRATIONAL REDISTRIBUTION _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ~ ~ a ~ c= l- ~~' COLD RING HOT TAIL

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IV-A. CONTROL OF CHEMICAL REACTIONS us to deposit prescribed amounts of vibrational energy in the benzene end of the cold molecule (in the head of the tadpole). When this energy is reradiated, its spectral signature displays its vibrational excitation at the instant of radiation. Since this light emission is a time-dependent process, we can monitor the movement of energy from the original location of excitation into the rest of the molecule. This movement in absence of collisions is called Intramolecular Vibrational Redistribution (IVR). Light emitted in the first picoseconds shows that the energy has not yet left the benzene unit where it was absorbed. The time scale for appearance of vibrational excitation in the alky! tail depends 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 is a thousand times slower, it takes 100 nanoseconds or more. Thus, we have direct evidence about the . . .. . . .. . factors that determine IVR energy movement in an isolated molecule. State-to-State Chemistry When two gaseous reactants A and B are mixed and react to form products C and D, the outcome is determined by statistical probabilities. The different encounters that may happen between A and B include all the possible energy contents, specific different types of excitation, and all the ways molecules may be oriented in space at the moment of collision. Not ad of these collisions are favorable for reaction most collisions have too little energy, or the energy is in the wrong place, or the collisions are at an awkward geometry. If we are to understand finely the factors that permit chemical reactions to occur, we should 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 for reaction. Finally, we would like to see how the available energy is lodged in the products. Such an experiment is ceded 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 chemiluminescence, revealed a part of the picture: the energy distribution among the products. For example, when a gaseous hydrogen atom and a chlorine molecule react they form hydrogen chloride and a chlorine atom. These reaction products emit infrared light. Analysis of the spectra from that light shows that the energy released in the reaction is not randomly distributed between the final products. Instead, a large fraction of it (39 percent) is initially located in the vibration of the hydrogen chloride product. Discoveries like this won John Polanyi (University of Toronto) a share of the 1986 Nobel Prize in Chemistry. This measurement led directly to the demonstration of the first chemical laser a laser that derived its energy from the hydrogen/chlorine explosion. Chemical lasers differ from conventional lasers in that the energy to produce their light comes from a chemical reaction instead of an electrical source. These beginnings led to the discovery of dozens of chemical lasers, including two sufficiently powerful to be considered for possible initiation of nuclear fusion (the iodine laser) and for possible military use in the "Star Wars" program (the hydrogen fluoride laser). "Molecular beams" move even closer toward "state-to-state" investigations. A molecular beam is a stream of molecules produced by a suitably hot oven. A 121

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122 Cot c Cat INTELLECTUAL FRONTIERS lN CHEMISTRY CHEMICAL LASERS REVEAL THE PRODUCT ENERGY DISTRIBUT ION F+ H2 ~~k ~ ko: ~ = 0 HF~ H RERCTIQn COORDInRTE substance is placed in this oven, and when it melts and vaporizes the vapor is directed out a tiny hole to form a unidirectional beam of molecules. Out- side the oven the pressure is kept extremely low- so low that no molecular collisions occur. The molecular beam can then be directed toward reac- tants. In such experiments, the reactants collide at such low pressures10-~ atmospheres that each reactant molecule has at most one collisional opportunity to react, and the products have none. These sophisticated instruments depend upon ul- tra-high vacuum equipment, high-intensity super- sonic beam sources, sensitive mass spectrometers for detectors, and electronic timing circuitry for time-of-flight measurements. With this incredible control it has become possible to predetermine the energy state of each reactant molecule and then to measure both the probability of a certain reaction and the energy distribution in the products. For bringing such elegant experi- ments into chemistry, Yuan-Tseh Lee (University of California, Berkeley) and DudIey Herschbach (Harvard University) shared in the 1986 Nobel Prize in Chemistry. For example, a current study has explained a key reaction in the combustion of ethylene. These molecular beam experiments show that the initial reaction of oxygen atoms with ethylene produces the unexpected short-lived molecule CRECHE. With this starting point, calculations have confirmed that a hydrogen atom can be knocked out of an ethylene molecule by a reacting oxygen atom more easily than that atom can be moved about within the molecule. This combustion example illustrates the intimate detail with which we can now hope to understand chemical reactions. Multiphoton and Multiple Photon Excitation Photochemistry has traditionally been concerned with what happens when a single photon is absorbed by an atom or a molecule. This productive field accounts for the energy storage in photosynthesis, the ultimate source of all life on this planet. Photochemistry also provides us with new ways to synthesize organic compounds and, through photodissociation, to produce a variety of short-lived molecules that play critical roles in flames and as intermediates in reactions. Now lasers give us optical powers lO,OOO times higher at a given frequency than even the largest flashiamps ever built. Clearly, these devices do not simply extend the boundaries of conventional light sources, they open doors to new processes as molecules interact with such intense photon fields. For example, at normal light intensities, the simultaneous absorption of two photons by a single molecule takes place so rarely that it cannot be detected. However, the probability of this happening increases with the square of the light intensity. Thus, if a laser increases

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lV-A. CONTROL OF CHEMICAL REACTIONS light intensity by a factor of 10,000, then the chance of two-photon absorption increases by four orders of magnitude over the chance of one-photon absorption. This lets us do experiments in which we can prepare molecular states that cannot be reached with a single photon. 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. Two-photon ionization has been used to detect specific molecules in difficult environments, like those found in explosions and in flames. Thus, nitric oxide, NO, which is an ingredient in smog, can be easily measured in a flame by counting the ions produced by a finely tuned laser probe. The probe is tuned so carefully that only the desired molecule, NO, can absorb light energy. 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 close vibrational harmony (near resonance) with the laser frequency could absorb not two or three but dozens and dozens of photons. In a time short compared with collision times, so many pho- ~ ~ ~ tons can be absorbed that ~ E _ chemical bonds can be broken QUASI ~ I entirely with vibrational exci- CONTINUUM ~ Ml is: S _~ ration. This unpredicted be- ~ --Gus''' _ ~ ~ RESONANT ~_L'~!~~r;~. ~ havlor 1S commonly called mul- ABSORPTION_ ~ --,.~; is. DISSOCIATIVE tiple photon excitation to dis- - .~. - - CONTINUUM tinguish it from two-photon ' T (multiphoton) excitation. ~ ~ -'- ~ '' - . - This behavior stunulated a 32 large group of studies on energy 34s~6 hV 3~` flow within excited polyatomic SF6 ~ hV ~ SF6 molecules. Many un~molecu- ~ hV 34sF ,, tar breakdowns and rearrange- ments have been triggered using multiple photon excitation. Yet, the understanding gained from this phenomena may be over- shadowed by the importance of its practical uses. Infrared absorption depends upon vibrational movements whose frequencies are quite sensitive to atomic mass. As a result, the tuned laser can be used to break up just those molecules containing particular isotopes, leaving behind the othersa new method for isotope separation. For example, deuterium is present at 0.02 percent in natural hydrogen. Yet, by multiple photon excitation, this tiny percentage can be extracted using 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 as a source of deutenum since '`heavy water," D2O, is used in large quantities in some nuclear reactors. ISOTOPE SEPARATION THROUGH MUTT I PHOTON ho 34SF $$* EXCITATION nhy 3.SF`,nt 1' 3 SF5 ~ F 123

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124 INTELLECTUAL FRONTIERS lN CHEMISTRY Even restore 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 significance in human history. The gaseous substance that has always been used in the difficult processes used to separate uranium isotopes is uranium hexafluoride, UFO. Because SF6 and UFO have identical molecular structures, they have similar vibrational patterns. Thus, multiple photon excitation might offer 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, unfortunately, nuclear bombs as well. The dangers of increased proliferation of nuclear weaponry can only be increased by such access. Mode-Selective Chemistry When two molecules collide with each other, the violence of the collision may cause their atoms to rearrange to form two new molecules (i.e., a reaction may occur). Such an outcome al- most always requires that the molecular collision involve some minimum energy- enough to break some of the bonds in the reactants in order to form the new bonds in the products. This minimum en- ergy, the activation energy, de- termines the rate of the reac- tion and it accounts for the dramatic effect of temperature on reaction rates. However, the question of whether a reaction will result from a molecular collision turns out to involve more than just whether there was enough energy. There is also a ques- tion of whether the collisional energy is in the right form. To understand what this means, consider a bedspring thrown against a wall. As it bounces off, it has energy of several types. It will be moving through space, which is energy of the old-fashioned kinetic en- a! ~ VIBRATION STRETCH I NO $ BEND I GIG H TRANSLATION ~ C H H C \ ' H H ROTATI ON . - ~i,~ .. ' ,,. ' ,,. ' ,,.. ,, ,,.,,~, ~ . ,, 7 .. 7 ,^~ ~ i< ~ ~~ ~ ,. .. ~= ~ i' ,. ~ ,,. ~ ~ ,. /",~,,r,~t " Z 4, ~ ,. ~ ,. ~~~, ~ ,. " , " ,. ~ , ,"' ," ' j'C ~,~ H7-C ~ H_ ~ " C `. r ~ an H ~ C _ _ \~ H MOLECULES TRANSLATE ROTATE AND VIBRATE LIKE A BEDSPRING THROWN AGAINST A WALL

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~-A. CONTROL OF CHEMICAL REACTIONS ergy type. This is called translational energy. In addition, the bedspring will be tumbling in space. This, too, is a form of kinetic energy called rotational energy. Then, the spring will be twisting and vibrating to and fro. This vibrational energy consists of both potential and kinetic energy. Molecules carry energy in exactly the same ways. Whether we are talking of bedsprings or molecules, the directions of translational motion, the axes of rotation, and the spring connections (in molecules, the bonds) are called degrees of freedom. The total energy in a collision is the sum of all of these forms of energy translational, rotational, and vibrational from both molecules. Chemists have long wondered whether it matters which degree of freedom carries the energy in a reactive collision. If all of the energy is in translational energy, the molecules are near each other only a short time. If the same amount of energy is brought to the collision mostly as vibration, the molecules move toward each other slowly, but now the bonds that must be broken are vibrating rapidly. Is this more or less effective? Only since chemists have acquired lasers has it been possible to seek an answer to this fundamental question. With high-power, sharply tunable lasers, we can 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 particular 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 show us the impomnce of that particular degree of freedom in causing reaction. This is called mod~e-selective chemistry. Both unimolecular reactions and molecular beam studies of bimolecular (two- molecule) reactions escape this problem. Unimolecular reactions involve only one molecule, so collisions are not required. At sufficiently low pressures, the effects of selective excitation on reactivity can be studied. The beam experiments sidestep the problem by giving each excited molecule only one chance for collision and by noticing only those collisions which result in a reaction. Nevertheless, mode- selective reactions are not readily coming from such experiments. Apparently the problem is that vibrational redistribution takes place within molecules even without collisions (IVR). This problem is of such basic importance to molecular dynamics that it will be one of the most important study topics for the next decade. There is, however, evidence for two-molecule mode-selective chemistry in certain solid inert-gas environments. In this situation the environment is so cold (IOK) that the reactive molecules are held immobilized. They are "frozen" in a prolonged, cold collision and rotational movement has been halted. For example, fluonne, F2, and ethylene, C2H4, suspended in solid argon at lOK do not react until one of the vibrational motions of ethylene is excited with a resonantly tuned laser. Then it is found that the most efficient vibrational motions are those that distort the molecular plananty. This is plausible because this type of distortion changes the molecular shape "toward'' the nonplanar, ethane-like structure of the final product. Theoretical Calculations of Reaction Surfaces Schroedinger~s wave equations of quantum mechanics have long been known to describe all chemical events. Yet quantum mechanics has been used in chemistry 125

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156 INTEr~r ACTUAL FRONTIERS IN CHEMISTRY Molecules in liquids can be highly efficient agents for storing or transferring energy. The very structure of liquid water determines our planetary environment and influences the course and nature of all biochemical processes essential to life. The structure and dynamics of a wide range of fluids, from liquid hydrogen to molten sili- cates, can be investigated by a number of spectroscopic tech- niques, such as X-ray and neutron diffraction, nuclear magnetic resonance, and laser Raman and light scattering. Among the newer experimen- tal approaches, pulsed laser excitation techniques are par- ticularly powerful. On a pico- second time scale (10- ~2 seconds), we can sense the freedom of movement of a sol- ute molecule held in its solvent cage. Now we can watch fun- damental chemical events as they take place: how two io- dine atoms combine in a liquid to produce an iodine molecule; how electrons released in liquid water become trapped, or solvated; how energy placed in a solute molecule like nitrogen or benzene is transferred to its solvent environment. Quite a different opportunity area is connected with the melting of small clusters of metal atoms. We have a variety of new experimental methods for producing and studying small metal clusters, as well as the theoretical tools with which to interpret the results. We can look ahead to an understanding of how the change from the fluid liquid state to the rigid solid state emerges as cluster size increases toward bulk amounts. Furthermore, the computer can keep track of the energy and randomness associated with each arrangement, so thermodynamic data can be calculated for comparison with experiments, and then for predictions under conditions out of reach expenmentally. ~ fOI03 ~ ~ & To LASERS LET US MEASURE FAST CHANGES IN THE SOLVENT CAGE Critical Phenomena For any fluid, there is a characteristic temperature and pressure above which the liquid and gaseous states are identical. Fluid behavior under these "critical conditions" can differ markedly from normal behavior and give rise to new phenomena. The past 20 years have seen a revolution in our understanding of such critical phenomena. Undoubtedly, the most important single theoretical advance in our understanding in the last 15 years has been the development of the new mathematical technique called the "renormalization group" approach. It has

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IV<. NATIONAL WELL-BEING shown promise for quantitative description of fluid properties and their dependence upon molecular shapes and forces. The past 15 years have seen the beneficial use of critical phenomena in a variety of applications. Critical point drying is now a standard sample preparation method in electron microscopy. Further, there are remarkable changes in the solvent power of a liquid near its critical point. These are at work, for example, in the removal of caffeine from coffee for cadeine-free instant coffee and in the extraction of perfume essences. In addition, there are valuable research applications in liquid chroma- tography. Chemistry of the Terrestrial and Extraterrestrial Materials The Earth's geochemical phenomena involve complex mixtures, frequently with a number of crystalline and glassy (amorphous) phases, and they may take place at extremely high pressures and temperatures. Recent advances in high-pressure technology have made studies possible that duplicate conditions near the earth's core. In recent years many earth scientists have studied the "geochemical cycles" of elements that is, the changing chemical and physical environment of a given element during such natural processes as crystallization, partial dissolving, change of mineral structure (metamorphism), and weathering. These processes may lead to concentration (e.g., ore deposits) or dispersion of an element. The geochemical cycle of carbon has provided a focus for the reawakened field of organic geochemistry. Research on the stability, conformation, and decomposition reac- tions of fossil organic molecules has led to greater understanding of the origin and composition of coal and other organic deposits. Such knowledge has obvious value that extends from guiding our exploration for new fossil fuel deposits to helping us decide how to use the ones we have. Meteontes are of considerable chemical interest because they include the oldest solar system materials available for research and they provide samples of a wide range of parent bodies some primitive, some highly evolved. Meteorites carry records of certain solar and galactic events and yield data otherwise unobtainable about the genesis, evolution, and composition of the Earth and other planets, satellites, asteroids, and the Sun. Unusual isotopic percentages of many metals and gaseous elements, and compositional data particularly trace elementshave shed light on stages of the formation, evolution, and destruction of the original parent body or asteroid where the meteorite originated. Within the last decade, the study of meteorites has been dramatically advanced by the recognition that if these projectiles from outer space land on the Antarctic ice sheet, they are immediately entombed in an inert environment and permanently refrigerated, stopping chemical changes. The question, of course, is how does one find these meteorites in the wide and forbidding spaces of this hostile region? Nature provides an astonishingly convenient answer. The Antarctic ice sheet is a vast glacier, so it gradually flows northward, carrying the meteorites with it. Over thousands of years, snow that fell near the South Pole finally reaches the end of the glacier where the ice begins to evaporate. Here at the glacier edges, the meteorites are dropped in great numbers, essentially never having been exposed to terrestrial life forms, erosion, or weathenng. Since this discovery, more meteorites have been 157

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158 . METEORITES: AN ANTARCTIC TREASURE TROVE I7vTEr r ~.cTuAL FRONTIERS IN CHEMISTRY collected (in the last decade) than over all of history before. The chemical and physical analysis of this meteorite trea- sure trove has only just begun. ANALYTICAL CHEMISTRY Characterization of atomic and molecular species their structures, compositions, etc., is called qualitative analytical chemistry. The measurement of the relative amount of each atomic and molecular species is called quantitative analytical chemistry. Both areas contribute to and benefit from the current rapid progress in science. Basic discoveries from physics, chemistry, and biology are providing new methods of analysis. In return, these new abilities are central to research progress in chemis- try, other sciences, and medicine, as well as to a wide range of applications in environmental monitoring, industrial control, health, geology, agriculture, defense, and law enforcement. Further, the lO-fold growth of the analytical instrumentation industry to $3 billion in sales worldwide has been led by the United States with its nearly $1 billion positive balance of trade in this area. A key factor in this growth has been the incorporation of computers into analytical instrumentation. The benefits here are circular; modern computers have evolved through advances in solid-state technology. In turn, these advances have critically depended upon the ability to analyze quantitatively the concentrations of trace impurities in silicon, the key element in current computer technology. Now microprobe analyzers using computer imaging techniques are answering questions critical to making microcircuitry even smaller, which will produce computers that are faster, more reliable, and cheaper. Analytical Separations Analyses of some complex mixtures are possible only after separation of the mixture into its components. Then, a variety of identification and quantitative measurement schemes become effective that would be confusing or impossible if applied to the unseparated mixture. Hence, devising new separations for use in an analytical context is an active field of research. There is no single technique more effective and generally applicable than the chromatographic method. The basic principle depends upon the fact that each molecular species, whether gaseous or in solution, has its own characteristic strength of attachment to, and ease of detachment from, any surface it encoun- ters. The differences in these attachment strengths can furnish a basis for separation. The differences can depend upon heat of adsorption, volatility, interaction with the solvent, molecular shape (including stereogeometry),

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Rae. NATIONAL WEr r-BEING charge, charge distribution, and even functional chemistry. Great ingenuity has made it possible to use the whole range of molecular properties for analytical separations that can require only tiny amounts of material. The different instrumental methods of chromatography will be discussed in Section V-C. For this discussion, a few illustrative examples will show the potential. In liquid chromatography, a solution of the mixture of interest passes through a column loaded with a suitable particulate material. For example, if an aqueous solution of pigments (such as those contained in carrot juice) is slowly passed through a tube containing small lumps of a suitable resin, the various pigments pass through the tube at different rates. The pigments that attach most weakly to the resin wash through fastest, and the ones that attach most strongly come out last. This provides a vivid example because we can actually see the different colors of the carrot juice pigments once they are separated. Of course, the method works to separate all sorts of compounds, whether colored or not. Under the best conditions, liquid chromatography can separate and reveal the presence of as little as lo-~2 grams of a substance in a mixture. For gaseous samples, the technique can separate literally thousands of components such as are found in flavors, insect communication chemicals (pheromones), and petroleum samples. It is even possible to separate compounds that differ only in isotopic composition (e.g., deuterium instead of hydrogen!) by this method. Two-~unensional chromatography can give additional specificity, resolution, and sensitivity by coupling with techniques such as electrophoresis, which involves the movement of substances in the presence of a high electric field. For example, two~unensional electrophore- sis can sort 2,000 blood proteins at once by separating a mixture spot of the sample linearly under one set of conditions, and then using another set of conditions to separate further the initial line of spots at right angles. Spot locations and amounts can be measured quantitatively with computenzed scanning based album] . . . ' ~'~ ~CT;;T~ _a01~ps~n tQnsf~rrin ' ] ~ ,~ ,, ,,,,, ., ,, >~ : *I :t :~ - ~ ;~- Alga ;~=, ... - ;- ~ _ on National Aeronautics and Space Administration computer programs developed for satellite pictures. Optical Spectroscopy The intellectual opportunities in this field, which introduce a variety of valuable analytical techniques, can be illustrated by two notable achievements of the last decade: the incorporation of computers as an essential part of most instrumentation, and the detection of single atoms and molecules. "Smart" commercial instruments now include microcomputers preprogrammed to carry out a wide variety off expen- mental procedures and sophisticated data analyses. The more powerful computers of :~ ... .~ . ~ .~.~- i; ~ ; ; ~: Am.. ~ - ~~ - 4 ~ ~ . a ~ , ; ~ : ;~ ~ ;., PROTEIN GEL PATTERN HU MAN MYALOMA SERUM 5~Q . . . 159

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160 INTELl~CTUAL PRONTlERS lN CHEMISTRY ~.Llp,l w7 '(~L cC14 CFC13 700 750 800 850 CO2 . ~~r~CO2_ )~-~-'''''"1 ~ ~ !r CF2C12 950 looo lose ~ loo ~ (cm~l) ~N2O . art_ 1150 1200 1250 1300 THE INFRARED SPECTRUM SHOWS ATMOSPHERIC POLLUTANTS EVEN AT NIGHT the future will digest huge vol- umes of data from spectroscopic methods (especially Fourier transform and two-dimensional methods) much more efficiently. This will further improve resolu- tion, detection limits, interpreta- tion, spectral file searching, and immediate presentation of the results with three-dimensional color graphics to permit direct human interaction with the ex- periment. Intense laser light sources are revolutionizing analytical opti- cal spectroscopy. An immediate benefit is increased sensitivity. In special cases, resonance-en- hanced two-photon ionization using tuned lasers has achieved the ultimate sensitivity: detec- tion of a single atom (cesium) or molecule (naphthalene). Achievements in laser-induced fluorescence are approaching this same incredible limit. Laser remote sensing, such as for atmospheric pollutants, is effective at distances of over one mile; fluorescence excitation and pulsed laser Raman are particularly promising. In these latter methods, a laser pulse is emitted in the direction of the sample, which might be a smokestack plume. Then the time that it takes the fluorescence or Raman signal to return (at the speed of light) is measured to determine how far away the sample is. Thus, the signal not only tells us what substances (pollutants) are in the sample but also permits us to track them as they move away from the source. The ability of a laser to emit a precise wavelength means there is the potentiality for the identification of one component in a mixture (without need for separation). Yet this selectivity is sometimes defeated because atomic and molecular absolutions can be much broader in wavelength than the laser line width. However, the resulting overlap can be eliminated by the wavelength narrowing that occurs at extremely cold, cryogenic temperatures. This cooling can be achieved for gaseous molecules by passing them through a nozzle to bring them to supersonic velocities. In an alternate approach, molecules can be embedded in a cryogenic solid, such as solid argon, at temperatures near that of liquid helium (a process ceded matrix isolation). These two complementary techniques me ze interference by rotational and vibrational abso~p- tions and improve detection sensitivity and diagnostic capability. Mass Spectrometry This method involves separation of gaseous charged species according to their mass (see Section V-B), and it offers unusual analytical advantages of sensitivity,

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IV-C. NATIONAL WELL-BEING specificity, and speed (10-2-second response). All of these attributes make for an ideal marriage to the computer. In the celebrated Viking Mars Probe, mass spectrometry was the basis for both the upper atmosphere analysis and the search for organic material in the planetary soil 30 million miles from home. Such sensitive soil sniffing to detect hydrocarbons might become a fast method for of! exploration. A special tandem-accelerator/mass spectrometer can detect three atoms of ~4C in low atoms of TIC, which corresponds to a radiocarbon age of 70,000 years. The broad applications of mass spectrometry include the analysis of elements, isotopes, and molecules for the semiconductor, metallurgical, nuclear, chemical, petroleum, and pharmaceutical industnes. In tandem mass spectrometry, one mass spectrometer (DISC) feeds ions of a selected mass into a collisional zone where impacts cause fragmentation into a new set of fragment ions for analysis in a second mass spectrometer (MS-~. This technique, abbreviated MS/MS, offers a particularly promising frontier for analysis of mixtures of large molecules. "Soft" ionization that avoids extensive fragmen- tation is used first to produce a mixture of molecular ions. From this mixture, one mass at a time is selected by MS-l, and it is more vigorously fragmented to produce an MS-~l spectrum that characterizes the structure of that one component. High speed and molecular specificity are important features of MS/MS. It is a powerful tool for analysis of groups of compounds sharing common structural features. It is particularly effective in removing any background signal caused by the contaminant species usually present in biological samples. It is now possible to determine the sequence of peptides with up to 20 amino acids and, in some instances, with sample sizes as small as a few micrograms. Combined ("Hyphenated") Techniques There is a growing appreciation for the extra benefits of using these computerized instruments in combination, such as the mass spectrometer coupled to a chroma- tograph (gas or liquid, GC/MS or LCtMS) or to another mass spectrometer (MS/MS), or these coupled with the Fourier transform infrared spectrometer (GC/IR, GC/IRtMS). High-resolution MS gives one part per trillion (1/10~2) analyses for the many forms of dioxin (TCDD) to see if the toxic form is present in human milk and the fatty tissue of Vietnam war veterans. GC/MS is necessary for the specific detection of 2,3,7,8-TCDD, the most toxic dioxin isomer. GC/MS is used routinely for detecting halocarbons in drinking water at concentrations far below the toxic level, polychIorobiphenyIs (PCBs), viny} chloride, nitrosamines, and for detecting most of the Environmental Protection Agency's list of other priority pollutants. MS/MS with atmospheric pressure ionization can monitor many of these contaminants continuously at the parts per billion level, even from a mobile van or helicopter. The high specificity as well as sensitivity of these methods make them especially promising for detecting nerve gases, '~yellow rain," and natural toxins in foodstuffs (10-~ g of vomitoxin in wheat) and plants (Astragalus or "Ioco weedy. Metabolites found by GC/MS have led to the identification of more than 50 metabolic birth defects in newborn infants where early identification is critical in preventing severe mental retardation or death. One of the most exciting intellectual 161

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162 lN~TE.r I EcTu~ FRONTIERS IN CHEMISTRY frontiers is the possibility that routine profiling of human body fluids can detect disease states well before external symptoms of those illnesses appear. Electroanalytical Chemistry Electrochemistry has a long history of analytical applications, beginning with pH meters. Today, pulse voltammetric techniques permit detection of picomole quantities (10~ i2 moles). Solid-state circuitry, microprocessors, miniaturization, and improved sensitivity have made possible continuous analysis in living single cells (with electrode areas of a few square microns). Electroanalytical methods are also useful in such difficult environments as flowing rivers, nonaqueous chemical process streams, molten salts, and nuclear reactor core fluids. SEPARATIONS SCIENCES 1.8 1.6 1.2 Separations Chemistry Separations chemistry is the application of chemical principles, properties, and techniques to the separation of specific elements and compounds from mixtures (including mineral ores). It takes advantage of the differences in such properties as solubility, volatility, adsorbability, extract- ability, stereochemistry, and ion properties of elements and mol- ecules. As an example, the rare earth elements neodymium (Nd) and praseodymium (Pr), impor- tant in laser manufacture, must be separated from a mineral called monazite. A difficult part of this extraction is the separa- tion from cenum, which is chem~caDy similar. Photochem~- cal studies show that this sepa- ration cart be greatly enhanced by selective excitation to take advantage of the different chem- istnes of the elements under photoexcitation . The availability of cntical and strategic matenals to U.S. industry and the military is dependent in many instances on the development of practical, econom- ical chemical separations methods. Table IV-C-] shows our dependence on imports for some critical metals and minerals. For example, almost 90 percent of our use of platinum, in great demand as a catalyst, comes from imports. Mining of the major platinum source in the United States, in Stillwater, Montana, has not yet begun. A second important example concerns our access to uranium. About 13 percent of the nation's electncal energy is denved from nuclear energy, and a much larger OPTIMUM ~ \ , _ / O _ ~ J ~5 1.4 _ 2 _ ~ _/ 1.0 INdl/ /1~1 1 | UNDER SELECTIVE ~ / RADIATIVE EXCITATION \ ,,,, I,,,, 1,,,, 1,,,, 1,, .5 2.0 2.5 3.0 HCI CONCENTRATION AT ALL MC' CONCENTRATIONS SELECTIVE EXCITATION FAVORS NEODYMIUM

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IV'. NATIONAL WELL-BEING percentage than that is utilized in the industrialized Northeast. Chemical sepa- rations are vitally important in the nu- clear fuel cycle, beginning at the uranium mill where low-grade uranium ores (typi- cally only 0.1 to 0.3 percent U3Os) are treated in selective chemical processes to produce a concentrate of more than 80 percent U3Os. Then, further refinement, based on transfer from one solvent to another (solvent extraction), or formation of the volatile fluoride, UFO, produces a uranium product pure enough for use in nuclear fuel manufacture. Then, after re- TABLE IV-C-1 U.S. Import Dependence, Selected Elements (Imports as Percentage of Apparent Consumption) 1950 1980 Manganese Aluminum (bauxite) Cobalt Chromium Platinum Nickel Zinc Tungsten Iron (ore) Copper Lead 77 71 92 100 91 99 37 80 35 59 97 94 93 91 87 73 58 ~4 22 14 <10 moval from the reactor, the highly radio- active fuel is subjected to a selective chemical process to separate uranium and plutonium from the fission products for recycling or for weapons use. This step is a remarkable feat of chemistry and chemical engineering because the aim is to separate two similar elements, uranium and plutonium, from each other and also from the highly radioactive fission products, which include about half of the Periodic Table. All of this must be done in a remotely operated plant which, by robotics, handles tons of materials so radioactive that they cannot be approached by a human being. These are only a few examples of the many ways we depend upon separations chemistry. Future availability of many of the critical elements listed in Table IV-C-l will depend, sooner or later, upon developing new chemical mining or separations processes that permit us to use low-grade domestic ores and the salt solutions (brines) that are found in geothermal wells. These developments will require research advances across a wide front, mainly focusing on the action of solvents and all of the properties of the liquid state that affect solvent power. NUCLEAR CHEMISTRY Since the days of the Curies, chemists have played a key role in the fundamental exploration of radioactivity and nuclear properties, as well as in nuclear applica- tions to other fields. Thus, the 1944 Nobel Prize for the discovery of nuclear fission went to a chemist, Otto Hahn. Then, the 1951 Nobel Prize for the discovery of the first elements beyond uranium in the Periodic Table, neptunium and plutonium, went jointly to a chemist, Glenn Seaborg, and a physicist collaborator, Edward McMilian. Most of the advances in our understanding of the atomic nucleus have depended strongly on the complementary skills and approaches of physicists and chemists. Furthermore, the applications of nuclear techniques and nuclear phe- nomena to such diverse fields as biology, astronomy, geology, archaeology, and medicine, as well as various areas of chemistry, have often been, and continue to be, pioneered by people educated as nuclear chemists. Thus, the impact of nuclear chemistry is broadly interdisciplinary. 163

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164 INTEL~iCTUAL FRONTIERS IN CHEMISTRY Studies of Nuclei and Their Properties Particularly exciting advances have been made in extending our knowledge of nuclear and chemical species at the upper end of the Periodic Table. In the last 15 years, elements 104 to 109 have been synthesized and identified, often by ingenious chemical techniques geared to deal with the very short half-lives of these species (down to milliseconds). In addition to these new-element discovenes, many new isotopes of other elements beyond uranium have been found, and the study of their nuclear properties has played a vital role in advancing our understanding of alpha decay, nuclear fission, and the factors that govern nuclear stability. Fission research in particular has been quite fruitful. For example, the "nuclear Periodic Table" identifies particular stable pro/on-neutron combinations ("closed shells"; one of these is the tin isotope ~32Sn (50 protons, 82 neutrons). Changing this nucleus by only one nucleon gives a dramatic change in the nuclear fission behavior, both in the distnbution of fission products obtained and in their kinetic energies. Furthermore, the study of spontaneously fissioning isomers among the heaviest elements has led to the important realization that the potential energy surfaces of these nuclei have two specially stable regions. This, in turn, opened the way to a new approach to calculating such surfaces the so-called shell correction method. Further exploration of the limits of nuclear stability is clearly in order, both at the upper end of the presently known nuclei and on the neutron-nch and neutron-poor sides of the region of stability defined by the stable nuclei found in nature. Newly discovered nuclear reaction mechanisms, based upon accelerating heavy nuclei as bombarding particles, promise to give access to more neutron-nch, and therefore much longer lived (minutes to hours), isotopes of elements with Z > 100 than have been available. This should open the way to more detailed investigations of the chemistry of these interesting elements at the upper end of the actinide series and beyond. The quest for so-called "superheavy'' elements, i.e., nuclear species in or near the predicted "island of stability" around atomic number 114 and neutron number 182, has not been successful so far, but this exciting goal is still being pursued. Space Exploration The wide range of applicability of nuclear techniques is demonstrated in the exploration of the Moon and our companion planets during the past two decades. For example, the unmanned Surveyor missions to the Moon provided the first chemical analyses of the Moon. They employed a newly developed analytical technique that utilized the synthetic transuranium isotope 242Cm. The analyses identified and determined the amounts of more than 90 percent of the atoms at three locations on the lunar surface. These analyses, verified later by work on returned samples, provided answers to fundamental questions about the composition and geochemical history of the Moon. Nuclear techniques also played an important role in the chemical analyses performed by Soviet unmanned missions to the Moon, and in experiments designed to seek life on the surface of Mars by the U.S. Viking missions. Similarly, isotopic distributions were important results in the analyses of

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ILIAC. NATIONAL WELL-BEING returned lunar samples and of meteorites, making possible clarification of the history of the Moon and meteorites. Isotopic Composition Ever since the discovery of the isotopic composition of the chemical elements, it has been assumed that this isotopic composition is essentially constant in all samples, an assumption that provides the basis for assigning atomic weights. The only exceptions involved elements with long-lived radioactive isotopes. Since 1945, however, humans have affected the atomic weights of several elements (e.g., Li, B. U) under some circumstances. More fundamentally, it has been discovered that the solar system is not composed of an isotopically homogeneous mixture of chemical elements. Even for an element as abundant as oxygen, variations of the isotopic abundance have been noted for different parts of the solar system. Such isotopic variations have now been established for several chemical elements and provide clues to the processes that gave rise to the chemical elements, as well as to the conditions that existed at the birth of the solar system. A startlingly large isotopic variation was discovered in the uranium of ore samples from the Oklo Mine in Gabon (West Africa) in 1972. Unusually low isotopic abundances of uranium-235 in these ores led to the astonishing conclusion that, 1.8 billion years before the first man-made nuclear reactor, nature had accidentally assembled a uranium fission reactor in Africa! This reactor was made possible by the higher 23su concentration (~3 percent instead of the present-day 0.7 percent) at the time. Mass spectrometric analyses of various elements in the Oklo ore proved that isotopic compositions labeled them unmistakably as fission products. It also made it possible to deduce such characteristics of the reactor as total neutron flow (] .5 x 102' neutrons cm-2), power level (~20 kW), and duration of the self-sustaining chain reaction (~106 years). An important practical result of the Oklo studies is the fact that most fission products, as well as the transuranium elements produced in the reactor, did not migrate very far in 1.8 billion years. This has a clear relationship to the possibility of long-term confinement of radioactive waste products in geologic formations. Nuclear Chemistry in Medicine Nearly 20 million nuclear medicine procedures are performed annually in the United States (radioactive iodine thyroid treatment is one example). Advances in nuclear medicine depend crucially on research in nuclear and radiochemistry. For example, great progress in our knowledge of the chemistry of the element technetium in the past decade will clearly lead to much more elective applications of radioactive technetium, 99Tc. This is the most widely used radionuclide, because the chemical properties of technetium compounds give them therapeutic activity. For example, technetium tends to concentrate in bone and particularly in cancer- ous bones, providing important diagnostic power. Another important example is the development of especially rapid ways to incorporate into molecular structures short-lived isotopes that emit positrons. Two examples are the carbon isotope ~C, with a 20-minute half-life, and the fluorine isotope, OFF, with a 110-minute half-life. Both are produced through cyclotron 165

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166 INTEL r.F`CTUAL FRONTIERS IN CHEMISTRY bombardment. These nuclei are then placed in such compounds as '~F-2-deoxy-2- fluoro-D-glucose and 1-~C-palmitic acid in a time short enough to permit their use in positron emission tomography (PET), which is analagous to X-ray tomography (CAT scan). The positron technique is finding new clinical applications in studies of the nervous system and the heart, known as neurology and cardiology. Stable isotopes, in conjunction with NMR spectroscopy, also have important applications in medicine. With tic, 2H, '5N, and ~70 tracers, NMR spectroscopy of humans will allow new insights into the molecular nature of diseases, provide a noninvasive method for their early detection, and make possible studies of metabolic processes in living subjects. This has led to one of the most exciting developments of the last few vears. large object imagine. In this technioue. a ~ , ~ ~ ~ computer stores the NMR signals that result when an object as large as a human is slowly moved through the magnetic field of the NMR sample space. Then the computer reconstructs a three-dimensional image of the object, showing the location and local concentration of the atoms whose NMR is being measured. Thus, the presence and chemical form of key elements can be mapped in entire human organs in living patients. These powerful, noninvasive techniques were literally undreamt of 15 years ago. They have arisen in response to demands for ability to study via NMR ever larger biomolecules and working biological systems. SUPPLEMENTARY READING Chemical & Engineering News "Vibrational Optical Activity Expands Bounds of Spectroscopy" by S.C. Stinson (C.& E.N. staff), vol. 63, pp. 21-33, Nov. 11, 1985. "Progress Reported in Coupling LC and MS" (C.& E.N. staff), vol. 63, pp. 38-40, May 20, 1985. 4'New Chromatography Columns Cut Need for Sample Preparation" by W. Worthy (C.& E.N. staff), vol. 63, pp. 47-48, Apr. 29, 1985. "New Methods for Trace Analysis of Man- ganese" (C.& E.N. staff), vol. 63, pp. 56-57, Jan. 14, 1985. "Microsensors Developed for Chemical Analysis" (C.& E.N. staff), vol. 63, pp. 61-62, Jan. 14, 1985. "New Laser System Far Surpasses Mass Spec for Surface Analyses'' by W. Worthy (C.& E.N. stair, vol. 62, pp. 20-22, Oct. 8, 1984. "New Detectors for Microcolumn HPLC" (C.& E.N. staff), vol. 62, pp. 39-42, Sept. 17, 1984. "New Methods Shed Light on Surface Chemistry" (C.& E.N. staff), vol. 61, pp. 30-32, Sept. 12, 1983. "Archeological Chemistry" by P.S. Zurer, vol. 61, pp. 26 44, Feb. 21, 1983.