Summary: Reaction of acetaldehyde cations with water: The effects of CH3CHOż
vibrational mode and impact parameter on reactivity and product branching
Ho-Tae Kim, Jianbo Liu, and Scott L. Anderson
Department of Chemistry, University of Utah, Salt Lake City, Utah 84112
Received 5 March 2001; accepted 19 April 2001
Scattering of mode-selectively excited acetaldehyde cations from D2O was studied in a guided ion
beam instrument. The effects of reactant vibrational state and collision energy on reactivity, product
branching, and product ion 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, dropping 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 strongly
suppressed by both collision energy and vibration, and the vibrational effects 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
high energies, PT occurs primarily for large impact parameters, and is suppressed for small impact
parameters. PT also shows strongly mode-specific dependence on CH3CHO vibrational state.