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

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Abstract
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.

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