transition state theory 

https://doi.org/10.1351/goldbook.T06470
A theory of the rates of elementary reactions which assumes a special type of equilibrium, having an equilibrium constant K‡, to exist between reactants and activated complexes. According to this theory the rate constant is given by: k=kB Th K‡ where kB is the Boltzmann constant and h is the Planck constant. The rate constant can also be expressed as: k=kB Th 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 Ea (the energy of activation) and Δ‡H⚬ are not quite the same, the relationship between them depending on the type of reaction. Also: k=kB Th 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.
Source:
PAC, 1996, 68, 149. (A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)) on page 190 [Terms] [Paper]
Citation: 'transition state theory' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.T06470 RIS BibTex EndNote
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Last revised: February 24, 2014