Evaluation of dynamical approximations for calculating the effect of vibrational excitation on reaction rates. O + H₂(n=0,1) ↔OH(n=0,1) + H

We previously proposed a generalization of variational transition-state theory (VTST) to allow the calculation of excited-state rate constants, summed over final states, by assuming that the excited degree of freedom is adiabatic from the beginning of the collision until the free energy of activation bottleneck is reached. Here we propose a modification in which the reaction, is assumed to be vibrationally adiabatic only up to the first occurrence in proceeding from reactants to products of an appreciable local maximum in the reaction-path curvature; at that point the reaction is treated as if all flux is suddenly diverted to the ground vibrational channel. The new theory is applied to calculate both forward and reverse excited-state rate constants for collinear O + H₂ → OH + H for three quite different potential energy surfaces, and it is tested against collinear exact quantal rate constants in each case. We draw conclusions about the conditions for the validity of variational transition-state theory for excited-state reaction rates as well as about the possible accuracy of the potential energy surfaces that have been proposed for this reaction.