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Eyring, Henry

EYRING, HENRY

(b. Colonia Juárez, Mexico, 20 February 1901; d. Salt Lake City, Utah, 26 December 1981)

physical chemistry.

Eyring’s father, Edward Christian Eyring, was a cattle rancher whose father had emigrated in 1853 from Germany to the United States. His mother, Caroline Cottam Romney, was a descendant of the talented English families of Romney and Cottam, members of which had emigrated from England to Utah in the 1830’s and 1840’s. Eyring was a Mexican citizen until 1935, when he took out U.S. citizenship.

Following his ancestors on both sides, Eyring was an active Mormon throughout his life. He attended school first in Colonia Juárez and later in Pima, Arizona, where the family moved in 1914. He attended high school in Thatcher, Arizona. During his school years Eyring did much work on the family farm, yet managed to excel academically.

In 1919 Eyring entered the University of Arizona to study mining engineering; his courses were mainly in engineering and mathematics, and he studied little chemistry. Having obtained his bachelor’s degree in mining engineering, he transferred to metallurgy, obtaining his master’s degree in 1924. Then, after working as a metallurgist in a copper smelter, he decided to study chemistry and in 1925 went on a fellowship to the chemistry department of the University of California at Berkeley, where he came under the influence of such men as G. N. Lewis and Wendell Latimer. His research director was George E. Gibson, and his work was on the radiation chemistry of hydrogen. He received his Ph.D. degree in 1927.

Eyring’s first appointment was as an instructor at the University of Wisconsin, where, under the influence of Farrington Daniels, he developed his lifelong interest in chemical kinetics. He carried out some experimental work with Daniels, but because of some friction with the chairman of the department he left in 1929 to work with Michael Polanyi in Berlin on a National Research Foundation fellowship. In 1930 he took a one-year lectureship in chemistry at the University of California at Berkeley. On the recommendation of Hugh S. Taylor, he was then appointed an assistant professor at Princeton University, where he remained until 1946, having become a full professor in 1938. In 1946 he went to the University of Utah in Salt Lake City as the first dean of its graduate school, with the responsibility of establishing a program of graduate study and research. He remained at Utah to the end of his life, continuing an active research program until shortly before he died.

In 1928 Eyring married Mildred Bennion, and they had three sons, Edward Marcus, Henry Bennion, and Harden Romney, all of whom had highly successful careers. Edward followed in his father’s footsteps and had a distinguished career at the University of Utah as a physical chemist, with a special interest in chemical kinetics. Mildred Eyring died in 1969, and two years later Eyring married Winifred Brennan Clark, who had been born in Scotland and was a convert to Mormonism.

After his experimental work with Farrington Daniels on the decomposition of nitrogen pentoxide, Eyring’s work was largely theoretical, although he always had a deep interest in experimental matters and designed his theoretical work so as to interpret experimental results. There is no doubt that his association with Michael Polanyi in Berlin strongly influenced the direction of his research. He and Polanyi developed for the first time a potentialenergy surface for a chemical reaction; they chose the simple process H + H2 → H2 + H, and for the energy of a triatomic species they made use of a semiempirical and very approximate formula that had been given by Fritz London in 1928. This surface contained two valleys meeting at a col or saddle point. In the final part of their published paper Eyring and Polanyi considered the course of the reaction in terms of the motion of a representative point over the surface. This pioneering and highly influential paper was undoubtedly one of the most important contributions made by Eyring and by Polanyi.

At Princeton, Eyring continued to work on a variety of problems, mainly in kinetics, One early investigation that attracted much attention, and that won him an award from the American Association for the Advancement of Science in 1932, was on reactions involving conjugate double bonds, to which he applied quantum-mechanical methods. He also collaborated with H. S. Taylor on some research, including the investigation of reactions brought about by high-energy radiations. This work was important for being the first to demonstrate the mechanistic similarity between radiation-chemical reactions and photochemical reactions. In the 1930’s Eyring also became particularly interested in the separation of isotopes, and worked on the theory of the electrolytic separation of heavy water. In 1937 he published a significant paper, with E. U. Condon and W. Altar, on optical activity, in terms of a one-electron model. This work was extended three years later in a paper with W. J. Kauzmann and J. E. Walter. Also during the 1930’s Erying and his students investigated the theory of liquids, and introduced the useful idea of treating a liquid in terms of “holes”; just as a gas is assumed to consist of molecules moving about in empty space, so a liquid may be regarded as made up of “holes” moving around in matter. On this basis Eyring was able to explain in a simple way the near constancy of the sum of the densities of coexisting vapors and liquids. This treatment of liquids became the subject of some controversy, although it was generally conceded to have some usefulness as a simple approach to the problem.

Probably the most important work done by Eyring and his students during the 1930’s was on the fundamental theory of the rates of chemical reactions. Following his work with Polanyi on the construction of a potential-energy surface for the H3 system, Eyring and his associates constructed potentialenergy surfaces for a number of other systems by the application of quantum-mechanical and empirical procedures. At that time, in the absence of computers, such calculations entailed considerable labor, even for the H3 system, and were much more difficult for systems involving more electrons unless considerable empiricism was involved. In spite of this, J. O. Hirschfelder, Eyring, and N. Rosen carried out in 1936 a purely variational calculation for the H3 system. To avoid an impossible amount of labor, however, they had to use a rather simple variational eigenfunction, and the calculated activation energy was far from the experimental value. It was not until over forty years later that it became possible to treat this problem in a manner that provided a reliable value for the activation energy of the H + H2 reaction.

At the same time that this work was being done on the construction of potential-energy surfaces, Eyring and his students were beginning to make dynamical calculations of the movement of a representative point on the surfaces. The construction of such trajectories had to be done by solving Newton’s equations numerically by use of mechanical calculators. The trajectory was produced point by point, and the work was extremely tedious. In spite of the fact that a wide range of conditions could not be employed, the results illustrated important principles as to how chemical processes proceed.

During the early 1930’s Eyring gave considerable thought to the formulation of a general treatment of reaction rates, one that would avoid the tedious calculation of trajectories. An important paper in 1932 with H. Pelzer and E. Wigner included a treatment of the rate with which systems pass through the col or saddle point of a potential-energy surface. This work helped Eyring to realize the usefulness of focusing attention on the saddle point, and he came to the conclusion that systems at the saddle point, which he called “activated complexes,” are in a state of quasi-equilibrium with the reactants. Also, he concluded, once systems have reached this region of the potential-energy surface, they have reached a point of no return and must continue on their way to become products. This led him to formulate what he called the theory of absolute reaction rates but which is now called transition-state theory. In the first version of this theory the concentration X‡ of activated complexes is calculated on the basis of statistical mechanics, and the rate of reaction is this concentration multiplied by the frequency with which the activated complexes pass over the barrier. The resulting expression for the rate is Where T is the temperature, k̠ the Boltzmann constant, and h the Planck constant. The calculation of X‡ requires some knowledge of the structure of the activated complexes, and if this is known X‡, and hence the rate, can be calculated satisfactorily.

In November 1934 Eyring submitted his important paper dealing with this theory to the Journal of Chemical Physics; the editor, Harold C. Urey, at first rejected it outright on the basis of a referee’s report. However, H. S. Taylor and E. Wigner intervened, and as a result the paper, one of the most important ever written in chemical kinetics, was published in the February 1935 issue of the journal. A paper published later that year by Eyring and W. F. K. Wynne-Jones presented an equivalent treatment, but in the language of thermodynamics rather than statistical mechanics. Another 1935 paper, by M. G. Evans and M. Polanyi, presented a very similar treatment, arrived at quite independently.

The original version of transition-state theory, now often referred to as “conventional transitionstate theory,” involved four basic assumptions, all of which were successfully brought together in the 1935 formulation. The assumptions are: (1) Systems that have reached the activated state are bound to form product molecules; (2) the various types of motion in the activated state can be regarded as separate motions; (3) the motion through the activated state can be treated as classical; and (4) the activated complexes can be treated as being in quasiequilibrium with the reactant molecules. At first the last of these assumptions was the most controversial, but later work has established that in many cases it is close to the truth. In the original version of the theory, assumption (3) was dealt with in terms of a “transmission coefficient” for reaction, but later treatments have dealt with quantummechanical “tunneling” through the energy barrier. Assumption (2) rarely leads to much error, while inaccuracies arising from assumption (1) have been minimized by more recent treatments, such as variational transition-state theory.

Transition-state theory had considerable impact and is still widely used by those who work on the rates of chemical, physical, or biological processes. Its value is not so much in providing a way of making exact calculations of rates, for there the conventional theory has severe limitations. Its value is rather in providing a conceptual framework with the aid of which one can gain insight into how processes occur. It provides both a statisticalmechanical and a thermodynamic insight, and it leads to useful qualitative predictions, with no need for calculations, of such matters as solvent effects, kinetic-isotope ratios, and pressure influences. Later refinements of the theory, some made by Eyring himself, have allowed it to calculate rates more quantitatively.

Subsequent to the formulation in 1935 of transitionstate theory, much of Eyring’s effort went into the application of the theory to a variety of problems, both chemical and physical. In an important paper published in 1935, H. Gershinowitz and Eyring applied the theory to trimolecular reactions and showed that it was successful in interpreting them. This was a significant result; no previous theory had dealt satisfactorily with collisions between three molecules, but transition-state theory, by focusing attention on the equilibrium between reactants and activated complexes, was able to treat the problem in a very simple manner. A little later Eyring dealt successfully with physical processes such as viscosity, diffusion, and plasticity in terms of transition-state theory. The arrival in Princeton in 1939 of the electrochemist Samuel Glasstone aroused in Eyring a further interest in electrochemical problems, and in that year, with Glasstone and Laidler, he formulated a treatment of overvoltage. At about the same time he and his co-workers for the first time applied transition-state theory to reactions on surfaces, and were able to obtain reasonable agreement with experiment.

In about 1942 Eyring developed a particular interest in biological systems, largely as a result of a problem in bioluminescence to which he had been introduced by F. H. Johnston of Princeton’s department of biology, with whom he was to collaborate for the next three decades. Bioluminescent materials, such as luminescent bacteria, exhibit some curious temperature and pressure effects, and Eyring was able to interpret these on the basis of his transitionstate theory of action of such drugs as sulfanilamide and formulated a treatment of the synergism and antagonism effects.

In 1944 Eyring, while retaining his position at search program of the Textile Research Foundation, which in that year had established laboratories at Princeton. Eyring had little previous knowledge of textiles, but his work on physical processes and his insight into reaction rates allowed him in a very short time to develop an active research program. With a small group of associates he studied the physical properties of fibers and fabrics, and the effects brought about by such processes as spinning, stretching, and weaving. On the basis of transitionstate theory, mathematical models were formulated in order to explain the observed effects. In 1945 and 1946 a series of eleven papers appeared, with George Halsey as coauthor of most of them, under the general title “Mechanical Properties of Textiles.”

Eyring’s decision to leave Princeton in 1946 for the University of Utah was based on his and his wife’s belief that the family’s religious and social needs would be better served if they lived in a Mormon community. He was dean of the graduate school at Utah from 1946 until 1966, after which he was appointed distinguished professor of chemistry and metallurgy. During the entire period that he was in Salt Lake City, aside from his own research with his students and colleagues, he exerted a profound influence on the university, greatly strengthening its teaching and research functions.

Eyring’s research in Salt Lake City to some extent continued his work at Princeton, but it opened up exciting new avenues and continued to be diverse and original. It included work on processes occurring in the mass spectrometer, on the mechanical properties of fibers, and on deformation kinetics. In addition, a good deal of his effort went into continuing his research on biological systems.

In the 1950’s Eyring and his students made an important contribution to the field of mass spectrometry by developing the theory of the interaction of electrons with atoms and molecules. They constructed potential-energy surfaces, applied transitionstate theory, and predicted the fragmentation patterns that occurred with electron beams of various velocities. Little had previously been known of the processes that occur in the mass spectrometer, and a 1950 paper by H. M. Rosenstock, M. B. Wallenstein, A. L. Wahrhaftig, and Eyring greatly clarified the problem and suggested further approaches to it.

In 1957 Eyring resumed his work on the theory of liquids, and his most important work on that problem was done during the next few years. This later work was based on three general idea: (1) that liquids contain some solidlike regions; (2) that liquids contain holes of molecular size into which molecules can move; and (3) that molecules in these holes show some of the characteristics of gas molecules. On the basis of this model and certain experimental data. Eyring and his students constructed partition functions for a number of substances, including argon, xenon, krypton, hydrogen, chlorine, water, some organic substances, metals, and fused salts. These partition functions were able to interpret the behavior of the solid, liquid, and gaseous phases and to lead to calculated properties that were in good agreement with experimental ones. The properties considered included melting points, critical points, heat capacities, and viscosities. One of the most important accomplishments of this approach was the satisfactory treatment of liquid water, into which two solidlike structures, resembling ice-I and ice-III, were incorporated.

In the 1960’s Eyring and his associates worked on the theory of optical rotation and circular dichroism, developing what has been called the octant theory of optical activity. They explored many practical aspects of the problem, applying the theory to determine the conformations of many compounds, some of them important in cancer chemotherapy.

Eyring’s work at the University of Utah on biological systems included further work on luminescence and studies of nerve conduction and of diffusion through biological membranes. He worked with T. F. Dougherty in the 1950’s on the mechanism of stress and inflammation. They proposed a theory in which stress sets off a chain reaction among body cells, with histamine acting as the destructive agent leading to inflammation. During the next few years these physiological studies were extended to such matters as sodium transport. the functioning of the heart, and the nature of nerve action.

In about 1969 Eyring began to work, particularly with Betsy Jones Stover, on other biological problems including the nature of mutations. They developed a theory of the probability of survival of an individual in homogeneous population. The basic equation they obtained was ds/dt = — ks (1 — s), where s is the probability of survival, t is the time and k is the appropriate rate constant. This equation integrates to s = 1 + exp{ —k (t½ — t)}, where t½ is the half-life of the process. This treatment was applied to mutation rates and death rates, and the constant k was interpreted by the use of transitionstate theory. This work with Stover was described in a series of papers that appeared in 1970 with the general title “The Dynamics of Life.” A little later Eyring and his students worked on the molecular and kinetic basis of anesthesia.

At the University of Utah, Eyring continued theoretical work started at Princeton on the mechanical properties of fibers and other materials. He considered natural fibers such as cotton, wool, keratin, and collagen, and synthetic fibers like nylon and saran. This work, besides being of great theoretical interest, proved to be of considerable practical value, for example, in the wool industry. These studies were later extended to the physiological problem of the stretching of tendons.

One matter to which Eyring and his associates gave considerable attention was deformation kinetics. When a solid is placed under stress, plastic flow occurs, and this is associated with the making and breaking of intermolecular bonds. Transition-state theory provides a powerful tool for investigating such processes. The matter is complicated, since plastic flow has to be treated in terms of parallel and sequential processes, but Eyring was successful in arriving at useful treatments. After 1970, with Alexander S. Krausz, he gave special attention to the thery of deformation processes, which are important in metallurgical engineering. This collaboration led to a book on deformation kinetics that summarizes the scientific principles relating to the constitutive laws of plastic deformation. An extension of these concepts proved important to the understanding of time-dependent fractures.

Eyring’s research style was unusual. He was always brimming with ideas, which he enjoyed sharing with anyone willing to listen. An important function of his students and colleagues was to sift the good ideas from the bad, of which there were many. His work was always stimulating and often controversial. His transition-state theory had its severe critics in its earlier years, and for a period was not taken seriously. Since about 1960, however, it has been realized that in spite of some weaknesses on the quantitative side it does provide, at the cost of very little labor, important insights. Eyring’s treatments of the liquid state and of optical rotation have also had their critics. However, even the most controversial aspects of his work have been valuable in stimulating the research of others.

Eyring’s devotion to science was matched by his deep devotion to his Mormon faith and to his family. He played an active role in his church, for many years as a member of its General Sunday School Board. He published a number of articles on his faith and on educational topics. The Mormon church is somewhat fundamentalist in its beliefs, and during the 1950’s Eyring had a mild clash with church authorities because of his opinions, expressed in a scientific paper, on the origin of life in the universe. The problem was resolved, however, in a dignified manner.

Eyring received many distinctions and awards, including fifteen honorary doctorates and membership in the National Academy of Sciences. He served as president of the American Chemical Society and received its Peter Debye Award, Irving Langmuir Award, and Joseph Priestley Medal. He also received the National Medal of Science and the Berzelius Gold Medal of the Swedish Academy of Sciences.

BIBLIOGRAPHY

1. Original Works. A Complete list of Eyring’s Publications is in S. H. Heath. “Henry Eyring, Mormon Scientist” (M.A. thesis, University of Utah, 1980), and in Journal of Physical Chemistry, 87 (1983), 2642–2656. Listed are ten books and over six hundred research articles. Heath also lists a number of articles on religion, education, and other topics.

Eyring’s first book, with S. Glasstone and K. J. Laidler, was The Theory of Rate Processes (New York, 1941); this was the first detailed presentation of transition-state theory. This work was later brought up to date in Eyring, S. H. Lin, and S. M. Lin, Basic Chemical Kinetics (New York, 1980). Other influential books are Eyring. G. E. Kimball, and J. Walter, Quantum Chemistry (New York, 1944); F. H. Johnston, Eyring, and B. J. Stover, The Theory of Rate Processes in Biology and Medicine (New York, 1974); and A. S. Krausz and Eyring, Deformation Kinetics (New York, 1975).

An early paper of great importance was Eyring and M. Polanyi, “Über einfache Gasreaktionen,” in Zeitschrift für physikalische Chemie. Abt. B, 12 (1931), 279–311, which presented the first construction of a potential-energy surface. Eyring’s most important publication was probably “The Activated Complex in Chemical Reactions.” in Journal of Chemical Physics, 3 (1935), 107–115; this was the first paper on what is now known as conventional transition-state theory.

The mass spectrometry work is presented in H. M. Rosenstock, M. B. Wallenstein, A. L. Wahrhaftig, and Eyring, “Absolute Rate Theory for Isolated Systems and the Mass Spectra of Polyatomic Molecules,” in Proceedings of the National Academy of Sciences, 38 (1952), 667–678. Optical activity is covered in L. L. Jones and Eyring, “A Model for Optical Rotation,” in Journal of Chemical Education, 38 (1961), 601–606, and in more detail in D. Caldwell and Eyring, The Theory of Optical Activity (New York, 1971). The work on liquid structure is reviewed in Eyring and M. S. Jhon, Significant Liquid Structures (New York, 1969).

II. Secondary Literature. A detailed account of Eyring’s life and work is to be found in Heath’s thesis, cited above. Other valuable sources are S. H. Heath. “The Making of Physical Chemist: The Education and Early Researches of Henry Eyring,” in Journal of Chemical Education, 62 (1985), 93–98; J. O. Hirschfelder, “Henry Eyring, 1901–1981,” in American Philosophical Society Year Book 1982 (Philadelphia, 1983), 482–489; and D. W. Urry, “Henry Eyring (1901–1981); A 20thCentury Architect of Cathedrals of Science,” in Proceedings of the International Symposium on Quantum Biology and Quantum Pharmacology, 9th (1982), 1–3, and “Henry Eyring (1901–1981); A 20th-Century Physical Chemist and His Models, “in Mathematical Modelling, 3 (1982), 503–522. A volume in honor of Eyring’s seventieth birthday, J. O. Hirschfelder and D. Henderson, eds., Chemical Dynamics (New York, 1971), contains appreciations of Eyring’s contributions and many articles by his former students and other associates.

Interesting reminiscences on Eyring’s early work on the calculation of potential-energy surfaces are included in two articles by J. O. Hirschfelder, “A Forecast for The oretical Chemistry. “in Journal of Chemical Education. 43 (1966), 457–463, and “My Fifty Years of The oretical Chemistry: I, Chemical Kinetics,” in Berichte der BunsenGesellschaft für physikalische Chemie, 86 (1982), 349–355; the first of these articles is reprinted in Chemical Dynamics, cited above. See also D. Henderson. “My Friend, Henry Eyring,” in Journal of Physical Chemistry, 87 (1983), 2638–2640.

An account of some of the work that led Eyring to formulate transition-state theory, with some discussion of the reception of the theory, is in K. J. Laidler and M. Christine King, “The Development of Transition-State Theory,” ibid., 2657–2664. On recent extensions of the theory, see D. G. Truhlar, W. L. Hase, and J. T. Hynes. “Current Status of Transition-State Theory.” ibid., 2664–2682.

Keith J. Laidler

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