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. [Britannica, 2002 Deluxe Edition CD-ROM.] The theory was first developed by R. Marcelin in 1915, then continued by Henry Eyring and Michael Polanyi (Eyring equation) in 1931, with their construction of a potential energy surface for a chemical reaction, and later, in 1935, by H. Pelzer and Eugene Wigner. [cite book | last = Laidler | first = Keith, J. | title = The World of Physical Chemistry | publisher = Oxford University Press | year = 2001 | id = ISBN 0198559194] Meredith Evans, working in coordination with Polanyi, also contributed significantly to this theory. [Evans, M.G.; Polanyi, M., (1938). "Trans. Faraday Soc"., 34, 11]

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

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. [Yarnell, Amanda. (2003). ” [http://pubs.acs.org/cen/science/8120/8120sci2.html Hammond Postulate] - 1955 paper used transition-state theory to explain structure-reactivity relationships.” May 19,Volume 81, Number 20CENEAR 81 20 p. 42ISSN 0009-2347] In 1955 Hammond stated:

cquote|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 [JACS history [77, 334 (1955)] ]

ee also

*RRKM theory
*Transition state

References

External links

* [http://www.engin.umich.edu/~CRE/03chap/html/transition/ Transition State Theory] - University of Michigan
* [http://www.rpi.edu/dept/chem-eng/Biotech-Environ/Projects00/enzkin/transition.htm Transition State Theory] - Rensselaer Polytechnic Institute