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transition state theory

A theory of the rates of elementary reactions which assumes a special type of equilibrium, having an equilibrium constantK ‡, to exist between reactants and activated complexes. According to this theory the rate constant is given by:
k = k B T h K ‡
where k B is the Boltzmann constant and h is the Planck constant. The rate constant can also be expressed as:
k = k B T h exp ( Δ ‡ S ° R ) exp ( − Δ ‡ H ° R T )
where Δ ‡ S °, the entropy of activation, is the standard molar change of entropy when the activated complex is formed from reactants and Δ ‡ H °, the enthalpy of activation, is the corresponding standard molar change of enthalpy. The quantities E a (the energy of activation) and Δ ‡ H ° are not quite the same, the relationship between them depending on the type of reaction. Also:
k = k B T h exp ( − Δ ‡ G ° R T )
where Δ ‡ G °, known as the Gibbs energy of activation, is the standard molar Gibbs energy change for the conversion of reactants into activated complex. A plot of standard molar Gibbs energy against a reaction coordinate is known as a Gibbs-energy profile; such plots, unlike potential-energy profiles, are temperature-dependent. In principle the equations above must be multiplied by a transmission coefficient, κ, which is the probability that an activated complex forms a particular set of products rather than reverting to reactants or forming alternative products. It is to be emphasized that Δ ‡ S °, Δ ‡ H ° and Δ ‡ G ° occurring in the former three equations are not ordinary thermodynamic quantities, since one degree of freedom in the activated complex is ignored. Transition-state theory has also been known as absolute rate theory, and as activated-complex theory, but these terms are no longer recommended.
PAC, 1996, 68, 149 (A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)) on page 190
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IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8.
Last update: 2014-02-24; version: 2.3.3.
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