Transition state theory

Transition state theory estimates chemical reaction rates by treating reactants as crossing a dividing surface through a high-energy activated complex on the way to products.

Eyring equation: A rate equation that relates a chemical reaction’s rate constant to its activation free energy, temperature, and transmission coefficient. It turns the theory’s activated-complex picture into a quantitative rate constant.
Canonical ensemble: A statistical-mechanical ensemble describing systems at fixed temperature that exchange energy with a heat reservoir. The usual derivation counts activated complexes using a thermal distribution.
Henry Eyring: An American theoretical chemist who developed transition state theory and its rate equation. His 1935 formulation gave the theory its widely used quantitative form.
Kramers theory: A theory of escape rates that accounts for friction and fluctuations as a system crosses an energy barrier. It treats solvent friction explicitly, which conventional transition state theory omits.
Activated complex: A short-lived, high-energy molecular configuration near the top of a reaction barrier. The theory treats this configuration as the population poised to cross into products.
Partition function: A statistical-mechanical quantity that sums the thermal weights of a system’s accessible states. Reactant and activated-complex partition functions determine the theory’s rate prefactor.
Meredith Gwynne Evans: A British physical chemist who independently developed an activated-complex approach to reaction rates. His contemporary work established a parallel formulation of the theory.
Variational transition state theory: A refinement that selects a dividing surface to minimize the predicted reaction rate and reduce recrossing errors. It improves the theory by optimizing the surface instead of fixing it at a saddle point.
Potential energy surface: A function describing a molecular system’s potential energy as its nuclear coordinates vary. The dividing surface is placed across a pathway on this energy landscape.
Transition state: A first-order saddle point on a potential energy surface, with one unstable direction along a reaction pathway. It supplies a common structural approximation to the activated complex.

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