The origin of the large bending enhancement of the reaction of C2H2+ with methane: the effects of bending momentum, ruling out the precursor mechanism, and steps toward “Polanyi rules” for polyatomic reactions

Abstract

Quasi-classical trajectories for the hydrogen abstraction (HA) reaction C₂H₂⁺+ CH₄→ C₂H₃⁺+ CH₃, were analyzed to probe the mechanistic origins of the large, mode-specific reactivity enhancement observed experimentally following excitation of the C₂H₂⁺cis-bending mode. The trajectories show the correct trend in reactivity vs. CC stretch and bending excitations, and also reproduce the experimental recoil velocity map. Analysis of the trajectories shows that at collision energy of 0.5 eV hydrogen abstraction is dominated by a direct mechanism, but ∼15% of the reaction is mediated by a precursor complex. The vibrational enhancement mostly comes from direct collisions. The bending vibration enhances the reactivity in two ways. Collisions in bent geometries (of C₂H₂⁺) are more reactive; however, the dominant vibrational effect is a consequence of the momentum associated with the bending vibration. Enhancement by vibrational momentum is reminiscent of the behavior seen by Polanyi and co-workers for late barrier A + BC reactions, and indeed, the atom transfer event in the C₂H₂⁺+ CH₄ system does occur late in the collision. We, therefore, explore the possibilities for interpreting polyatomic vibrational dynamics in a “Polanyi Rule” context, using trajectories to guide the construction of a reduced dimensionality surface.