Breakdown of the vibrationally adiabatic approximation in the early-barrier CH3 + HBr → CH4 + Br reaction

Abstract

The prototypical hydrogen abstraction reaction between hydrogen bromide and methyl was investigated using both the eight-dimensional quantum dynamics and quasi-classical trajectory methods. The dynamics calculations showed that the individual excitation of the CH3 symmetric stretching mode, the umbrella mode and the HBr stretching mode visibly enhances the reaction while they are all less efficient than the translational energy in promoting the reaction, as expected for an early-barrier reaction. The vibrationally adiabatic approximation fails in predicting the promotional effect of the excitation energy in the CH3 umbrella mode. However, the promotional effect can be partially rationalized by the sudden vector projection model as there exists non-negligible coupling between the vector of the CH3 umbrella mode and the reaction coordinate vector at the transition state.