Reaction of acetaldehyde cations with water: The effects of CH3CHO+ vibrational mode and impact parameter on reactivity and product branching

Scattering of mode-selectively excited acetaldehyde cations from D2O was st udied in a guided ion beam instrument. The effects of reactant vibrational state and collision energy on reactivity, product branching, and product io n recoil velocity distributions were measured. Ab initio calculations were performed to help understand the reaction coordinate. The dominant reaction is H/D exchange, which occurs in about 40% of low energy collisions, dropp ing to just a few percent at high energies. H/D exchange is also inhibited by CH3CHO+ vibration, but with a smaller effect than the equivalent amount of collision energy. H/D exchange is mediated by a long-lived complex, and several candidates are identified. The other low energy channel corresponds to methyl elimination from the collision complex. This channel is the most energetically favorable, but is only a few percent efficient, even at low energies, and is negligible at high energies. Methyl elimination is strongl y suppressed by both collision energy and vibration, and the vibrational ef fects are nonmode specific. The most interesting channel is proton transfer (PT), which occurs by a direct mechanism at all collision energies. At low energies, PT occurs only in small impact parameter collisions, while at hi gh energies, PT occurs primarily for large impact parameters, and is suppre ssed for small impact parameters. PT also shows strongly mode-specific depe ndence on CH3CHO+ vibrational state. (C) 2001 American Institute of Physics .