Test of Variational Transition State Theory and the Least-Action Approximation for Multidimensional Tunneling Probabilities against Accurate Quantal Rate Constants for a Collinear Reaction Involving Tunneling into an Excited State

摘要

Rate constants are calculated using vibrational transition state theory (VTST) with a least-action (LA) ground-state (G) transmission coefficient (VTST/LAG) and are compared with accurate quantum mechanical ones for the collinear Cl + HBr reaction. This reaction is highly exoergic and also has large reaction-path curvature in the interaction region near the saddle point. The quantum mechanical results show the reaction is highly state specific proceeding from the ground vibrational state in reactants almost exclusively to the second excited vibrational state of products. The LAG method has been shown to work well for nearly thermoneutral systems with large reaction path curvature but this is the first test of the method for a system involving tunneling from a ground vibrational state to an excited one. The VTST/LAG results agree with the quantum mechanical ones to within a factor of 2.2 over the whole factor of 12 temperature range studied. The error is comparable to that found previously for the symmetric Cl + HCl reaction, which is dominated by the ground vibrational states of reactants and products. This indicates that the VTST/LAG method is as applicable to reactions that proceed by tunneling into excited states as it is for reactions dominated by tunneling from ground state to ground state. The VTST/LA approximation is also shown to provide useful accuracy for the rate constant of the collinear excited-state reaction Cl + HBr(n=1) yielding HCl + Br.