Mode Specificity in the OH + HO2 → H2O + O2 Reaction: Enhancement of Reactivity by Exciting a Spectator Mode
Abstract
A chemical reaction typically involves a few active modes in the reactant(s), while the other modes are largely preserved throughout the reaction as spectators. Excitation of an active mode is expected to promote the reaction, but depositing ener-gy in a spectator mode typically has no effect, because of the differing ability for energy flow to the reaction coordinate. In this work, we report a surprising case of mode specificity, in which such canonical expectations do not hold, as illustrated by quasi-classical and quantum dynamics of a prototypical radical-radical reaction OH + HO2 → H2O + O2 on a globally accurate full-dimensional potential energy surface. Despite its spectator nature, the stretching vibration of the OH reactant is shown at low collision energies to enhance the reactivity significantly when excited. This unique effect can be attributed to the in-creased attraction with HO2 due to the larger dipole of the stretched OH. At low collision energies, the stronger attraction increases the chance of capturing the reactants to form a hy-drogen-bonded complex, thus of passing through the sub-merged barrier. The novel mechanism differs from the conven-tional vibrational enhancement via coupling to the reaction coordinate at the transition state, enriching our understanding of mode specificity in chemical reactions.
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