Vibrational and collision energy effects on the reaction of CH3CHO+ with methanol

Abstract

Integral cross sections and product velocity distributions were measured for reaction of acetaldehyde cation with methanol over a center-of-mass collision energy range from 0.1 to 2.2 eV. Reactivity is dominated by exoergic proton transfer (PT), which is strongly suppressed by collision energy, and mildly suppressed by CH3CHO+ vibrational excitation. PT is complex-mediated at low energies, switching to a direct stripping mechanism at high energies. Of the two possible PT channels, it appears that transfer of the aldehyde proton dominates. Hydrogen abstraction (HA) is a minor channel at low collision energies, also complex-mediated. Abstraction is observed from both hydroxyl and methyl sites on methanol, and the two channels have different, and counterintuitive collision energy dependence. Despite being exoergic, with no barriers, the HA channel shows apparent threshold behavior, attributed to competition with the dominant PT channel. The competition indicates that different intermediate complexes must interconvert efficiently, at least for low collision energies. At low energies, HA is strongly enhanced by collision energy, while vibrational excitation has no effect. Finally, there is a minor product channel corresponding to methyl elimination (ME) from a complex. Despite a relatively complicated reaction coordinate, the ME channel shows substantial recoil energy release and an asymmetric velocity distribution. A series of ab initio and RRKM calculations were performed to help interpret the results.