Unified statistical model for ’’complex’’ and ’’direct’’ reaction mechanisms

A unified statistical theory for bimolecular chemical reactions is developed. In the limit of a ’’direct’’ mechanism it becomes the usual transition state theory, which is correct for this situation, and if the reaction proceeds via a long-lived collision complex it reduces to the statistical model of Light and Nikitin. A general criterion for locating the ’’dividing surfaces’’ that are central to statistical theory is also discussed. This prescription (Keck’s variational principle) is shown not only to locate the usual dividing surfaces that pass through saddle points and minima of the potential energy surface, but it also selects the critical surfaces relevant to the ’’orbiting’’ and ’’nonadiabatic trapping’’ models of complex formation.