A theory of chemical reaction rates that postulates a quasi-equilibrium between reactants and an activated (transition) state on the potential energy surface, with the rate determined by the flux of activated complexes converting to products.

Principle

Assume a separable activated complex in quasi-equilibrium with reactants and no recrossing of the dividing surface; rate constants are related to the equilibrium constant for formation of the transition state and its crossing frequency (statistical mechanical connection between thermodynamics and kinetics).

Demonstration

For a unimolecular isomerization in the gas phase, TST yields the Eyring expression for the rate in terms of activation free energy (ΔG‡), from which temperature dependence and activation enthalpy/entropy can be extracted to rationalize experimental rate changes.

Misapplication

Assuming TST without testing for dynamical recrossing, neglecting tunneling in light-atom transfers, or applying TST blindly to diffusion-controlled, highly non-equilibrium, or strongly frictional condensed-phase reactions where quasi-equilibrium does not hold.

Consequence

Provides a practical framework to estimate rate constants from potential energy surfaces and partition functions, enabling extraction of activation parameters, comparing mechanisms, and guiding catalyst design; connects PES features to observable kinetics.

Reversal

If one rejects the quasi-equilibrium and no-recrossing assumptions, reaction dynamics approaches (variational TST, RRKM, molecular dynamics) become necessary and can predict rates modified by recrossing, energy redistribution, or tunneling effects.

Boundary

Valid when a well-defined transition region (saddle point) separates reactants and products and statistical sampling of states near the saddle is meaningful; limited for strongly nonstatistical dynamics, extreme quantum tunneling, or reactions lacking a single identifiable barrier.

Semantic Tension

Competes with collision theory for elementary gas-phase reactions and with RRKM or dynamical trajectory methods for unimolecular and complex processes; tension centers on whether rates are governed by a quasi-equilibrium activated complex or by collision/frequency and dynamic recrossing effects.

Synthesis

Transition state theory formulates a statistical‑mechanical bridge between the potential energy surface and macroscopic rate constants by treating an activated complex in quasi‑equilibrium with reactants; it is powerful and practical but must be corrected for recrossing, quantum tunneling, and nonstatistical dynamics when observed.