Theoretical vibrational mode-specific dynamics studies for the HBr + C₂H₅ reaction

A quasi-classical trajectory (QCT) study is performed for the HBr + C₂H₅ multi-channel reaction using a recently-developed high-level ab initio full-dimensional spin-orbit-corrected potential energy surface (PES) by exciting five different vibrational modes of reactants at five collision energies. The effect of the normal-mode excitations on the reactivity, the mechanism, and the post-reaction energy flow is followed. A significant decrease of the reactivity caused by the longer initial distances of the reactants for the v_HBr = 1 reaction at low collision energy (E_coll) is observed due to the intramolecular vibrational-energy redistribution and the classical nature of the QCT method. All of the three reaction pathways (H-abstraction, Br-abstraction, and H-exchange) are intensely promoted when the HBr-stretching mode is excited. No clear promotion is observed when excitation is imposed to C₂H₅ except that asymmetric CH-stretching helps the H-exchange process. The enhancement effect of the excitation in the HBr vibrational mode is found to be much more effective than increasing the translational energy, in contrast to the HBr + CH₃ reaction. The forward scattering mechanism can be clearly promoted by the excitation of the HBr-stretching mode, or by the high collision energy, indicating the dominance of the direct stripping mechanism in these cases. At low collision energy with no excitation or excitation of any vibrational mode of C₂H₅, the forward scattering feature is less obvious. At E_coll = 1 kcal mol^-1, when HBr-stretching is excited, the product clearly gains more relative translational energy. However, it is interesting to see that when the excitation is in C₂H₅, the effect is the opposite, i.e., the product gains less relative translational energy compared to the ground-state reaction.