Mode specificity in the HCl + OH → Cl + H2O reaction: Polanyi's rules vs sudden vector projection model

The dynamics and mode specificity of the HCl + OH → Cl + H2O reaction are investigated using a full-dimensional quantum dynamics method on an accurate global potential energy surface. It is shown that the vibrational excitation of the HCl reactant greatly enhances the reactivity while the OH vibrational excitation has little effect. The surprising HCl vibrational enhancement of this early barrier reaction contradicts a naive extension of Polanyi's rules, but can be explained by the sudden vector projection model, which attributes the promotional effect of the HCl vibration to its strong coupling with the reaction coordinate at the transition state. In addition, it is found that the fundamental and overtone excitations of the HCl reactant change the reaction mechanism from a direct barrier crossing process to a capture-like process.