Vibrational mode-specific quasi-classical trajectory studies for the two-channel HI + C 2 H 5 reaction.

We report a detailed dynamics study on the mode-specificity of the HI + C 2 H 5 two-channel reaction (H-abstraction and I-abstraction), through performing quasi-classical trajectory computations on a recently developed high-level ab initio full-dimensional spin-orbit-corrected potential energy surface, by exciting four 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 investigated. Both reaction pathways are intensely promoted when the HI-stretching mode is excited while the excitations imposed on C 2 H 5 somewhat surprisingly inhibit the dominant H-abstraction reaction pathway. The enhancement effect of the excitation in the HI vibrational mode is found to be much more effective than increasing the translational energy, similar to the HBr + C 2 H 5 reaction. Not like the Br-abstraction pathway, however, the I-abstraction reaction pathway could be comparable to the dominant H-abstraction reaction pathway. The dominance of the direct stripping mechanism is indicated in H-abstraction while the direct rebound mechanism is observed in I-abstraction. The H-abstraction is much pickier about the initial attack angle distributions for HI than I-abstraction is, which leads to a decrease in reactivity in the H-abstraction reaction pathway. The dominance of side-on CH 3 CH 2 attack in I-abstraction is more obvious than in the case of H-abstraction. In the case of the H-abstraction reaction pathway, the major part of the initial translational energy ends up in translational recoil, while for I-abstraction most energy excites the product C 2 H 5 I.