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

We report and compare two sets of thermal rate constants for the collinear reaction Cl + HBr → HCl + Br, which is dominated by the n = 0 vibrational state of reactants and the n = 2 vibrational state of products. One set is based on converged multichannel quantal calculations carried out using hyperspherical coordinates. The other set is based on variational transition state theory (VTST) with a least-action (LA) ground-state (G) transmission coefficient (VTST/LAG). The two sets of rate constants agree within a factor of 2.2 over the whole factor-of-12 temperature range (200-2400 K) studied. The error is comparable to what was found previously for the symmetric reaction Cl + HCl′ → HCl + Cl′, which is dominated by n = 0 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 ground-state-to-ground-state tunneling reactions. We also show that the VTST/LA approximation provides useful accuracy for the rate constant of the collinear excited-state reaction Cl + HBr(n = 1) → HCl + Br.