Recent Developments in the Quantum Mechanical Theory of Chemical Reaction Rates

Abstract

As one tries to construct an increasingly rigorous quantum mechanical generalization of classical transition state theory, one that is free of all ‘extraneous’ approximations (e.g., separability of a one dimensional reaction coordinate), one is ultimately driven to the dynamically exact quantum treatment. Though it seems pointless to call this a transition state ‘theory’ (it is in effect a quantum mechanical simulation), it is nevertheless possible using transition state-like ideas to cast a fully rigorous quantum approach in a form that allows one to carry out such calculations without having to solve the complete state-to-state quantum reactive scattering problem. Rigorous calculations for the reactions H + H2 → H2 + H, H + O2 → OH + O, and H + H2O → H2 + OH illustrate this approach. At the semiclassical level, there does exist a version of transition state theory — based on the locally ‘good’ action variables about the saddle point on the potential energy surface — which includes non-separable coupling between all degrees of freedom (including the reaction coordinate) in a unified manner.