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SCIENCE William O. Baker, Bell Laboratories
The science of chemistry is becoming worthy of its goals: to understand the properties of matter—and how they change—and its preparation. Now chemistry is recognizing that it has a part in all of modern science—from nucleus to nebula, from biophysics to botany. Once often just an accessory to the concepts and techniques of physics, biology, astronomy, and geology, chemistry is moving into a partnership with all of these in the new understanding of the nucleus, the crystal, and the cell. It is, as well, bringing even more combinations of knowledge into its traditional study of atoms, bonds, and molecules. The next decade will see a more rational and systematic understanding of the origin and distribution of the chemical elements, whose interrelationships were originally organized by Dmitri Ivanovich Mendeleev and Nikolai A. Menshutkin 110 years ago. Already we see a fascinating extension of the transuranium systems of elements as a result of nuclear reactions induced by both accelerators and natural radioactivity. Thus, Glenn T. Seaborg, in his Priestley Medal address to the American Chemical Society last spring, could describe, as the William 0. Baker's career combines significant achievements as an industrial scientist and research administrator with distinguished service in a host of consulting and advisory posts with the federal government and national scientific and academic organizations. Last spring, he was named chairman of Bell Telephone Laboratories after having been president of Bell Labs since 1973. Baker, 64, joined Bell Labs in 1939, following his graduation with a Ph.D. in physical chemistry from Princeton, to do research on macromolecules, the field to which he has devoted most of his scientific work. Later, he teamed with Field H. Winslow to study the movement of electrons in and through organic substances. He has served on most of the committees that have shaped U.S. science and technology policy during the past two decades. His long list of awards includes the ACS Priestley Medal and Charles Lathrop Parsons Award and the Society of Chemical Industry's Perkin Medal. 30
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discoverer or synthesizer of a host of new elements, the way in which nuclear structure interacts with the outside shell of electrons to yield a new area of modern chemistry. He also points out that many new superheavy transuranium elements may occupy what he calls "islands of stability" in the outer reaches of the periodic table. Thus, new chemistry may arise as a result of fundamental shifts in the structure of the synthetic nucleus. In much the same way, we shall see in the coming decade, as well, a better recognition of the extraterrestrial conditions which led to the creation of our universe and the elements which comprise it. Although there has been much discussion of how the elements were created, based on modern nuclear and particle physics, only recently has there been good evidence for the conditions under which synthesis might have taken place, which seems to bring together all the many astrophysical, astrochemical, and cosmic concepts about the nature and occurrence of the elements. First, better understanding of the reactions taking place within stars and of the conditions that preceded their birth opened up fascinating possibilities for the state of the universe that created the variety of elements now existing. Then George Gamow and others considered ways to characterize the conditions which could account for the origin of the elements and explain the reactions they undergo. These studies led to work during the past two decades to establish the residual properties, such as relict radiation, remaining from the formation of the universe. Future work is likely to map the probable sequence of events which could have created the elements and, in doing so, the essence of chemistry. These developments were covered by Arno A. Penzias in the 1978 Nobel Prize lecture on the origin of the elements. They provide a basis for understanding the elements that we have now on earth and for searching for modifications of elemental matter in other parts of the universe. Already, we have strong reasons to expect that many new elements and isotopes will be synthesized and their chemical functions explored. Obviously, the way to extend the various modes of chemistry—whether detection, reaction, property, or principle—is by synthesizing new forms of the elements. It is reasonable to expect that several thousand more isotopes—perhaps more than
5000, including such exotic isotopes as 70C2—will be added to the 300 naturally occurring stable isotopes and the 1600 that have been created by reactions of neutrons and other particles. In any case, the frontiers of chemistry that will be opened up by synthesis and modification of elements are vast and will be explored vigorously, using heavy ion reactions and other means. Fascinating chemical influences derived from the details of new nuclear structures also will be found. For instance, Nicholas J. Turro and his associates at Columbia University, using dibenzylketone photolysis, have applied the slight magnetic moment that is characteristic of isotopes with odd atomic weight (such as 13 C, 1 7 0, and 235 U) to show that the products of free radical reactions are altered according to the quantities of magnetic nuclei involved. New understanding of the behavior of synthetic elements in biosystems will greatly extend the use of radioactive pharmaceuticals for in-vivo and in-vitro assays. Technetium-99, made by neutron bombardment of molybdenum, already is the most widely used isotope in nuclear medicine but is probably only a forerunner of a variety of other synthetic tracers and emitters that will get wide attention in the years ahead. The delicate techniques for radiation detection and analysis used by Penzias and Robert Wilson to discover the cosmic background radiation are also widely used now to find chemical compounds in outer space. The increasing discovery of interstellar molecules is rapidly dispelling the notion that organic chemical structures are uniquely terrestrial. Even the earlier generalization that interstellar molecules can contain only hydrogen, carbon, nitrogen, and oxygen has been superseded with the discovery of many sulfur analogs. Recent studies with Bell Telephone Laboratories' millimeter wavelength radio telescope have established, for instance, the presence of methylmercaptan and isothiocyanic acid in the nebula SGRB2. This new insight into both the origin of and interrelationships among the chemical elements and certain of their compounds throughout the universe points toward new frontiers of learning about chemical synthesis. In the emptiness of the cosmos, the mean free paths of ions, atoms, and molecules are longer by many orders of magnitude than anything studied yet on Earth. Thus, we may be able to study reaction kinetics involving energy and, especially, entropy conditions, such as the longevity and geometry of an activated complex, impossible at present. Here on Earth, vacuums of about 10 - 1 1 torr have been assured by techniques recently developed by Bell Labs' Homer D. Hagstrum. Furthermore, we can expect to see not only more studies of cosmic chemistry but also its extension to heterogeneous systems and the influences of surfaces in interstellar space. Such new studies, for example, will involve chemical changes associated with carbon and also with ice particles steadily bombarded, often in the presence of minute concentrations of other elements, by electrons and cosmic rays. The significance of these studies of the formation and nature of elementary and cosmic matter is for each chemist to decide in the light of his or her own interest. But some of the implications of understanding the origins of the elements and of their distribution are intrinsic to compelling issues which will involve chemistry during the next decade. One or two such matters should be noted. Obviously, the issue of scarce minerals and other resources on this and perhaps neighboring planets is of global import. Harold Urey has already done much chemical work in this regard. One recent idea suggested by him and explicated by Thomas Gold of Cornell is that our major hydrocarbon resources may have been formed from reactions of elementary carbon with hydrogen in the
Earth's interior to form methane, much of which was then converted to other hydrocarbons as it was forced to the Earth's surface by the high temperatures and pressures of the core. Probing the basic chemistry of this postulate may open the gate to a new era of hydrocarbon discovery and modification. Our expectation that new effort in the use of the findings of astrophysics, cosmochemistry, radio spectroscopy, and related fields to expand the horizons of chemical reactions and structure applies also to ions. These include plasmas, whose containment and reactions are already important in respect to plans for energy conversion in the century ahead. Another track likely to advance chemical science is the culmination of centuries of work founded on classical studies of analysis—of how the elements differ and can be identified in the presence of each other. We need to know how to measure as few atoms as possible in the presence of all the rest, and ultimately how to detect just one atom, molecule, or ion among any others. Here the movement is led by solid-state science, particularly the chemistry and metallurgy of solids and especially of the crystals comprising semiconductors and photon (hole plus electron) generators. Ironically, it was not in a highly dilute gas but in the perfect crystals produced in the zone refining of germanium and silicon that it became possible to detect parts per billion or less of certain atoms, such as dopants. Now, the recent discovery of ion impact spectroscopy by Bell Labs' David Joy permits light elements to be determined in a sample with a total mass of 10~ 18 gram.
New schemes are likely to extend... analyses ever further toward the detection and scaling of single atoms
Already, instruments derived from solid-state electronics permit us to advance well beyond even the elegant sensitivity of a very short time ago, when measuring the presence or absence of a few parts per million (corresponding to about 1017 molecules per cc) was a respectable chemical achievement. Yet already in the new silica-based lightguide glasses, 40 ppb of nickel or 20 ppb of iron can double the light absorption at 800 nm in the very pure fibers that now can be formed by modified chemical vapor deposition. Indeed, in many cases the presence of as few as 1012 molecules per cc greatly changes the properties of the system. At that concentration in the atmosphere, for example, sulfur dioxide is thought to be unacceptable. And in the stratospheric ozone layer at a density of about 1012 molecules per cc, nitrogen oxides NO and NO2 keep a precarious balance at about 109 molecules per cc. So, studies at this range of sensitivity become a new need for chemistry. In the challenging domain of heterogeneous reactions and adsorbed monolayers, the monolayer contains generally less than 10 15 molecules per sq cm. Clearly the detection of new species in cosmochemistry also is likely to demand measurement of exceedingly low concentrations. Accordingly, it is encouraging that nuclear detection by neutron activation can respond to 1012 molecules per cc, electron spectroscopy in the range of 1011 to 1014, mass spectroscopy 103, and emission spectroscopy 106 to 1010. Intermediate and hybrid techniques should be able to fill in a wide range of concentrations. In the future, moreover, new schemes are likely to exNov. 26, 1979 C&EN
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tend such analyses ever further toward the detection and scaling of single atoms. Thus, the adsorption of an atom on metal affects polarizability, producing a surfaceenhanced Raman spectrum that is perhaps a million times more sensitive than the 10 15 molecules per cc Raman response usual in the gas phase. Single atoms have long been observable in terms of field ion microscopes. It is also likely now that laser activation, such as done at Oak Ridge National Laboratory, will permit detection of various kinds of single atoms. For example, cesium can be observed, by its ionization from an excited state as one atom, in a quantity of 1019 others. Similarly, single sodium atoms have been observed by laser fluorescence. This latter application of the laser has been applied to examine a variety of complex chemical processes, such as combustion in a gas flame. The years ahead will demand many such new probes of combustion dynamics, probes that eventually will be used to regulate energy conversion. Similarly, fluorescence by laser exposure has been applied in liquid jets from high-pressure liquid chromatography for the detection of l ( r or less aflatoxin molecules per cc. Further progress in lasers, such as a free-electron device being developed by C. K. N. Patel and his associates at Bell Laboratories, will permit tuning in the 10,000-cm -1 range of the far-infrared, opening the way to enhanced detection of the behavior (such as rotation and vibration) of molecules. Already improved diode lasers and color center devices are showing reasonable power at ever widening tuning ranges. Likewise, the steadily growing capability of synchrotron radiation, with its very-high-power fluxes and sharp beams, will bring spectroscopy readily into the vacuum ultraviolet and x-ray region of the spectrum. This will be specific and highly sensitive when fluxes of greater than 10 13 photons per second are available. For those without synchrotrons, a laser-pulsed plasma yielding 4.45-A xrays (from Cl + 1 5 ions) gives a useful scattering diagram from a nanosecond exposure, which promises to permit tracking of changes in structures during reactions. Extraordinary progress in the use of pulsed lasers also brings the sensitivity of photon spectroscopy to a point where it can be used to pinpoint or take a "snapshot" of the gamut of chemical change—excitation, relaxation, reacting species, and so on—at a particular instant. Thus, continuously operated dye lasers recently have been arranged by E. P. Ippen and C. V. Shank at Bell Laboratories to produce pulses of 0.3 X 10~ 12 second that are reproducible and precisely measurable. They are being used to measure directly the photochemistry of bacterial rhodopsin in studying its special role in energy conversion in connection with vision. Using a pulsed laser to measure redox reactions in bacterio chlorophyll directly, Peter M. Rentzepis of Bell Laboratories has found that electron donation by Mg-porphyrin takes place in 1 0 - 1 4 second and that the subsequent acceptance (within 5 X 10~ 12 second) by pheophytin is controlled by a separation of the molecules around 6A. In all the new ways of measuring molecules, independence of state of aggregation will be prominent. Accordingly, at last simultaneous and sensitive insights can be gained in both the primary bonding or classic valency and the secondary interactions of chemical systems. As in the case of surface-perturbed Raman spectroscopy, we shall be able to study continuously through phase barriers and states of aggregation. The frontier, inhabited by metals with their continuous band structure on the one hand and polymers with their dominance of primary valence on the other, now also will contain hosts of other interesting and biologically important systems. Thus, Patel and his coworkers are applying the opto-acoustic effect (conceived by Alexander Graham Bell about a century ago) and laser 32
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sources to determine fractional absorptions of 10~5 in films of solutions. Solutions of rare-earth chlorides in water have been examined at 200-A thickness. All sorts of swollen and solid films can be similarly investigated. The significance of these new laser-based capabilities is illustrated by their use in making the first precise measurement of the absorption spectrum of water at visible wavelengths. The transmission of light by water is a central event of human experience, as well as being basic to the ecology of three quarters of the surface of the earth. Thus, with pulsed dye lasers and immersed piezoelectric transducers, Patel and A. C. Tarn have shown the whole range of absorption coefficients at 21.5° C (characterized by the one fine structure of the harmonics of the hydroxy 1 stretching frequency). The breadth of application of these new techniques is demonstrated by the uses of the spin-flip Raman laser. Because this tunable spectroscopic system measures absorption coefficients as small as 1 0 - 1 cm, it has been effective in following the most subtle chemical changes involving nitrogen oxides and other components of the stratosphere. Hence, the future holds great promise for filling in the big blanks that still remain in our understanding of the origins and reactions of molecules and atoms. Of course, filling in these blanks will be imperfect, but outlines will be seen which up to now could hardly be imagined. A more coherent view will be developed about the continuum in bonding, from liquid helium to rock salt and diamond. In addition, present notions of electronic configuration for reactions and bonding will become more generalized, along the lines pioneered by William M. Lipscomb of Harvard University with the boranes. Already those notions challenge us to seek widening usage, as in the scheme suggested by Robert S. Mulliken of the University of Chicago that if BH 3 equals 0 , then H 3 B CO2 should resemble a carbonato group or form a borano carbonato complex M(H3B CO2) leading to carbonates of M 2 (0 CO2). Such systems have been found and can be viewed as symbolic of large arenas yet to be described.
The future holds great promise for filling in the big
blanks...
in our understanding of the origins and reactions of molecules and atoms
Thus, a guiding theme for what lies ahead stands out strongly. It is, of course, chemical theory and the principles of quantum and statistical mechanics that seemed, earlier in the 20th century, to hold together a galaxy of discoveries. We are struck by the relative constancy of concept and method. Ideas containing the many formalisms of quantum mechanics, and also its anomalies, seem to have flourished in an age of massive digital computation. This number-crunching has often been applied for testing quantum mechanical interpretations of complex atomic, as well as subatomic, phenomena, yet no drastic alternatives to the "new quantum theory" have been established yet. Thus, as computation and data handling in general are made dramatically easier by new operations systems, and eventually by programs stored in software-shaped microprocessors, we can expect to see more successes in theoretical chemistry. The ceaseless progress in digital machines now has gone far beyond a mere increase in capacity or decrease in the cost of com-
puting operations. It extends also to large advances in the algorithms on which can be developed the software ap propriate for a realization of chemical theory. Such the oretical progress will apply to a whole range of new fronts in chemistry. These extend from estimates of whether strange molecules such as HNC occur in outer space to speculations about the origins of life and the identifica tion of the biological functions of specific groups on molecules like morphine and nalorphine. Electron correlation methods and their derivatives are allowing quite complex molecules to be characterized, especially as to charge distributions and bonding states affecting both the kinetics and overall directions in re activity. Perhaps these capabilities will stimulate a search for entirely new structures and undiscovered molecules, like cyclobutyne. And the age-old intuition of the syn thetic chemist will be augmented by new concepts of molecules. These concepts often may be built on conventional and familiar ones, such as shapes and charges already dis cerned and especially the energy contours and transitions which have been so useful thus far. But it seems that new ideas must be heeded more widely, as we search for ex planations of ever-growing complexity. The dimensions of our problems are sometimes overlooked, such as the numbers of new compounds now being added to the in ventory of chemistry. It was recently estimated, based on Chemical Abstracts listings as of last year, that the base count of 4,039,907 recognized compounds is growing at an average rate of 6000 per week. New ideas to cope with this domain should relate to the principles of energy surfaces and curves that we believe govern the formation and stability of atomic and molec ular systems. But we believe the action of tunneling, rather than strict confinement within these curves and surfaces, is much more widespread than expected. Re actions in many biosystems depend on electron tunneling, such as has been observed in the laboratories of Britton Chance at the University of Pennsylvania and discussed generally by Willard F. Libby, thus showing temperature insensitivity and energy anomaly. For example, in quantum mechanical tunneling, even though the heights of the barriers exceed the kinetic energy of the particles, in the nuclear phenomenon of spontaneous fission, thermonuclear reactions, and «-particle decay, tunneling seems to be dominant. Chance's finding, from the laser flash-initiated oxi dation of cytochrome C by chlorophyll, has opened wide pathways to the explanation of many remarkable events. For instance, in this kind of chemical tunneling there is also a range of variations of nuclear separation. These lead to, among other things, a wide distribution of acti vation energies and barrier shapes, in contrast to the fa miliar presumptions of single values for a particular set of reactions. These "polychromatic kinetics" seem to betoken a new stage in interpreting chemical change. Already the implications for a variety of bioprocesses, such as reactions of iron with carbon monoxide in he moglobin, have been underscored. Vitalii I. Goldanskii of the Institute of Chemical Physics of the Soviet Acad emy of Sciences has reported the remarkable temperature independence, below 12° K, of chain growth rate of ra diation-induced polymerization of solid formaldehyde. He points out the possibility of many related phenomena occurring in the low temperatures of interstellar space. The implications for the formation of complex species, particularly in association with the particles so prevalent in outer space, are dramatic. Overall, new concepts of valency and reactive state that underlie the essence of chemistry—such as the nature of nuclear effects and their use in tracking and chemical characterization, electronic properties determined with
the new techniques of spectroscopy, the structures de duced from observation of intimate wave scattering such as is possible with synchrotron radiation—are to be ex pected. These concepts will be formulated as chemistry continues to interact with astronomy, solid-state science, and particle physics on one side, and biological and be havioral phenomena on the other. In connection with the last, we are already finding striking analogies between the coding of chemical structure, as to isomers, including geometrical configuration, and the information recording of digital states macroscopically. Thus the DNA codes may be further elucidated by simpler models. The current popularity of "structure-function" to categorize the bridging between chemical units and their assembly in genes and other organic elaborations, including the anomalies of "introns" and "exons," may offer timely challenges to produce new ideas.
The age-old intuition of the synthetic chemist will be augmented by new concepts of molecules
Efforts to model enzymes by Jean-Marie Lehn and his coworkers at the University of Strasbourg through the synthesis of a macrotricyclic molecule, named a "cryptate," are encouraging. They support a belief that the modern chemistry of solids, with its knowledge of charge distributions and long-range interactions, will increas ingly be joined with advances in stereochemistry and organic synthesis in probing quasiorganisms that are simpler and lower in function than viruses but still might have the ability to replicate themselves. Concurrently we can expect also that many other conventional positions about chemistry will be revised. After centuries of specifying acid function and ion dis sociation, we now see that carbocations can be prepared and kept in solution by "super acids" whose hydrogen ion activity may be 10 12 that of sulfuric acid. Similarly, the ability to implant charges, including electrons, ions, and positrons, into thin films and surfaces, as a result of re search and development on semiconductor ion implan tation, could lead to "artificial" ion arrays at surfaces, and probably in solutions, through which novel reactions could be effected. This control of the charge environ will complement the growing recognition of isoelectronic species throughout the periodic table. Thus we can expect linear systems such as H 2 C = S , F 2 C = P 4 , and H 2 C=PC1, much as the III-V element compounds (such as GaP, GaAs) are found to provide crystals whose bonding is like silicon or carbon. Compelling problems in describing even the simplest organic reactions, such as those involving methylene chloride solutions, have been discussed by Frank Westheimer of Harvard. These problems are posed by the effect of the environs, such as solvents, in which the re actions take place. A macroscopic generalization char acterizing the influence of solvents on reactions seems likely, similar to solid-state science's knowing that crystal and glass volumes often involve solid solutions. One of the factors will be the ability to work at enormously higher dilutions than has been done yet, which wjll permit kinetics and structure to be followed through the highly sensitive "single molecule" approaches that were dis cussed earlier. ο Nov. 26, 1979 C&EN
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