Dissociation of the ground state vinoxy radical and its photolytic precursor chloroacetaldehyde: electronic nonadiabaticity and the suppression of the H+ketene channel

This work is a study of the competition between the two unimolecular reaction channels available to the vinoxy radical (CH(2)CHO), C-H fission to form H+ketene, and isomerization to the acetyl radical (CH(3)CO) followed by C-C fission to form CH(3) + CO. Chloroacetaldehyde (CH(2)ClCHO) was used as a photolytic precursor to the vinoxy radical in its ground state; photodissociation of chloroacetaldehyde at 193 nm produces vinoxy radicals with internal energies spanning the G3//B3LYP calculated barriers to the two available unimolecular reaction channels. The onset of the CH(3) + CO channel, via isomerization to the acetyl radical, was found to occur at an internal energy of 41 +/- 2 kcal/mol, agreeing well with our calculated isomerization barrier of 40.8 kcal/mol. Branching to the H+ketene channel was too small to be detected; we conclude that the branching to the H+ketene channel must be at least a factor of 200 lower than what is predicted by a RRKM analysis based on our electronic structure calculations. This dramatic result may be explained in part by the presence of a conical intersection at planar geometries along the reaction coordinate leading to H+ketene, which results in electronically nonadiabatic recrossing of the transition state.