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

Integral cross sections and product velocity distributions were measured fo r reaction of acetaldehyde cation with methanol over a center-of-mass colli sion energy range from 0.1 to 2.2 eV. Reactivity is dominated by exoergic p roton 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 energie s. Of the two possible PT channels, it appears that transfer of the aldehyd e proton dominates. Hydrogen abstraction (HA) is a minor channel at low col lision energies, also complex-mediated. Abstraction is observed from both h ydroxyl and methyl sites on methanol, and the two channels have different, and counterintuitive collision energy dependence. Despite being exoergic, w ith no barriers, the HA channel shows apparent threshold behavior, attribut ed to competition with the dominant PT channel. The competition indicates t hat different intermediate complexes must interconvert efficiently, at leas t for low collision energies. At low energies, HA is strongly enhanced by c ollision 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 chan nel shows substantial recoil energy release and an asymmetric velocity dist ribution. A series of ab initio and RRKM calculations were performed to hel p interpret the results. (C) 2001 American Institute of Physics.