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Transition State Theory and Experiments in Chemical Reactions

Marcus, R. A. (1997) Transition State Theory and Experiments in Chemical Reactions. In: Femtochemistry and femtobiology : ultrafast reaction dynamics at atomic-scale resolution. Imperial College Press , London, pp. 54-79. ISBN 9781860940392.

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The transition state theory of chemical reactions has proven to be a formidable tool for analyzing chemical reaction rates of a wide variety of reactions. In the present article some of its developments, extensions, and applications are described. Transition state theory was developed in its current form by Eyring and Evans and Polanyi in 1935, only about a half a dozen years after the quantum mechanical description of potential energy surfaces by London for chemical reactions. The advances in the understanding of the dynamics in the 1930s were rapid, particularly by Hirschfelder, Wigner, and O.K. Rice, among others. A number of the problems, concepts, and themes that are currently discussed were considered at that time, and even some classical trajectory calculations were made then by Hirschfelder. There was another thread in chemical reaction rate theory, unimolecular reaction rate theory, which developed earlier in the 1920s, the initial work being that of Hinshelwood, followed by the more detailed theory of Rice and Ramsperger and of Kassel, which became well known as the RRK theory. That tl1eory was applied to and stimulated extensive experiments on the unimolecular decomposition of organic molecules during the 1920s and early 1930s. However, such experiments revealed that those reactions had complex mechanisms: They were found to proceed via a series of elementary steps involving free radicals as intermediates, rather than occurring in a single step. A new field of study, reaction rates and mechanisms, and later the direct spectral observation, of gas phase free radicals was born. The use of unimolecular theory went into decline-there were practically no systems to which it could be applied and which could stimulate its further growth. During 1939-45 there was a general lull, however, in all these activities because of the Second World War. Interest in unimolecular theory resumed in the late 1940s with N. B. Slater's sophisticated and exciting extension of earlier ideas of Polanyi and Wigner and Peltzer. His work was based on treating the decomposing or isomerizing molecule as a collection of harmonic oscillators. The neglect of the effects of anharmonic coupling, and the results of various incisive experiments, led later to its abandonment as a method for treating the emerging experimental data on these reactions. Nevertheless, it introduced new and refreshing ideas into the field. In 1949, as a postdoctoral with O.K. Rice, I blended ideas drawn from transition state and RRK theories and formulated a molecular structure-based statistical theory for these reactions. The time was ripe for this development. New molecular ideas, such as potential energy surfaces, had developed only after RRK theory had been formulated, and post-war research was beginning to flourish. My own interest stemmed from the experimental work I had done on the unimolecular decomposition of the CH_3OCH_2 free radical and on the recombination of methyl radicals in the laboratory of E.W.R. Steacie in Canada. This unimolecular theory, known since the 1960s as RRKM theory, is in current use today. To treat reactions for which the reverse reaction of bimolecular association has no energy barrier, the defining the position of the transition state (TS) becomes a problem. A variational form of the theory was subsequently developed to determine the TS, and has been extensively applied. In the present paper I comment on a number of types of reactions investigated with transition state theory and on some of the features which arise when specific aspects of the reaction or of the potential energy surfaces are introduced. They include electron transfers, energy dependence of unimolecular reaction rates in the gas phase, unimolecular reactions in clusters, including solvent dynamics, global contour plots for reactions, S_N2 reactions, and extensions of transition state theory which include, in addition to reaction rates, the distribution of quantum states of the reaction products of unimolecular dissociations. In the latter we again consider dissociations in which there is no energy barrier for the reverse reaction, the recombination of the fragments. A striking feature in this history is that some sixty years after the development of transition state theory it continues to be a useful tool for chemists, to which additional concepts can be added. The field of chemical reaction rates itself has been stimulated by the new types of data which are becoming available with ultrafast techniques, as well as by the earlier classical techniques applied to interesting systems.

Item Type:Book Section
Marcus, R. A.0000-0001-6547-1469
Additional Information:© 1997 Imperial College Press. It is a real pleasure to acknowledge the support of this research by the National Science Foundation and the Office of Naval Research. It is also a pleasure to acknowledge many stimulating conversations with my colleague, Ahmed Zewail. The references given below are intended to be illustrative rather than comprehensive.
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Office of Naval Research (ONR)UNSPECIFIED
Record Number:CaltechAUTHORS:20150617-094941310
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:58314
Deposited By: Tony Diaz
Deposited On:17 Jun 2015 18:51
Last Modified:22 Nov 2019 09:58

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