Transition state theory

In chemistry, transition state theory is a conception of chemical reactions or other processes involving rearrangement of matter as proceeding through a continuous change or "transition state" in the relative positions and potential energies of the constituent atoms and molecules.^[1] The theory was developed by Henry Eyring and Michael Polanyi in 1931, with their construction of a potential energy surface for a chemical reaction, and later, in 1935, by H. Pelzer and Eugene Wigner.^[2] Meredith Evans, working in coordination with Polanyi, also contributed significantly to this theory.^[3]

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Transition state theory is also known as activated-complex theory or theory of absolute reaction rates.

Transition state theory assumes once a reaction passes through its reaction barrier it cannot go back again. Whilst transition state theory is a large improvement on simple collision theory, it requires knowledge of the partition function of intermediate species which, due to their high energy and short lifespan, can be hard to determine spectroscopically.

Organic chemistry

Main article: Hammond's Postulate

In 1940s and 50s, chemists were struggling to understand how rates of chemical reactions vary with structure change. To solve this problem, George S. Hammond, an organic chemist at Iowa State College, suggested that transition state theory, which had previously been the domain of physical chemists, could be used in a qualitative manner to explain structure-reactivity relationships in a variety of organic reactions.^[4] In 1955 Hammond stated:

“

If two states, as for example, a transition state and an unstable intermediate, occur consecutively during a reaction process and have nearly the same energy content, their interconversion will involve only a small reorganization of the molecular structures.

”

This postulate was published in the Journal of the American Chemical Society, a paper that ranks among the 125 most cited in JACS history^[5]

References

International Union of Pure and Applied Chemistry. "transition state theory". Compendium of Chemical Terminology Internet edition.

^ Britannica, 2002 Deluxe Edition CD-ROM.
^ Laidler, Keith, J. (2001). The World of Physical Chemistry. Oxford University Press. ISBN 0198559194.
^ Evans, M.G.; Polanyi, M., (1938). Trans. Faraday Soc., 34, 11
^ Yarnell, Amanda. (2003). ”Hammond Postulate - 1955 paper used transition-state theory to explain structure-reactivity relationships.” May 19, Volume 81, Number 20 CENEAR 81 20 p. 42 ISSN 0009-2347
^ JACS history [77, 334 (1955)]