Evaluation of Dynamical Approximations for Calculating the Effect of Vibrational Excitation on Reaction Rates. O + H2(n=0,1) yields OH(n=0,1) + H

摘要

A theory is proposed to incorporate the effect of vibrational nonadiabaticity in the calculation of excited state rate constants within the framework of variational transition state theory (VTST). In a previously proposed generalization of VTST to treat excited state reactions it was assumed that the excited degree of freedom is adiabatic from the beginning of the collision until the free energy of activation bottleneck is reached. A modification is proposed in which the reaction is assumed to behave vibrationally adiabatically only up to the first occurrence in proceeding from reactants to products of an appreciable local maximum in the reaction path curvature; at this point the reaction is treated as if all flux is suddenly diverted to the ground vibrational channel. This partial reaction path (PRP) adiabatic model is applied to the collinear O + H2 reaction for three quite different potential energy surfaces and it is tested against collinear exact quantum mechanical rate constants in each case. Although the approximations of the PRP adiabatic model are less stringent than those of the fully adiabatic model they are still quite severe; however the PRP adiabatic model yields quite accurate results. For the excited state reaction on all three surfaces the PRP adiabatic results are within 60% of the accurate quantel ones at temperatures from 200 to 400 K and the errors are less than factor of 3.7 up to 2000 K.