Limitations of Variational Transition State Theory for Barrierless Radical–Radical Recombination Reactions

Abstract Variational transition state theory (VTST) is widely used for the modelling of barrierless radical-radical recombination reactions. In this application, VTST suffers from a number of limitations some of which are of more technical, others of more fundamental nature. The former are caused by inappropriate averaging over individual adiabatic channel potentials or by the neglect of quantum effects, the latter are due to deviations from adiabatic dynamics. It is shown that most radical-radical recombination reactions are characterized by Massey parameters which are smaller than unity such that the dynamics is nonadiabatic. VTST treatments which generally assume adiabatic dynamics, therefore, have a fundamental problem. Calculations of rate constants by VTST often exceed classical trajectory results by about 10 to 20percent. This is normally attributed to “recrossing trajectories”. In the present work it is shown, however, that deviations of this magnitude also have to be expected for nonadiabatic dynamics in comparison to adiabatic dynamics. It is, therefore, suggested that “recrossing” at least in part has to be attributed to nonadiabatic dynamics. A way out of the dilemma is the use of a combination of statistical adiabatic channel and classical trajectory concepts.