Dissociation Channels of the 1-Propenyl Radical and Its Photolytic Precursor cis-1-Bromopropene

Abstract

The primary photodissociation dynamics of cis-1-bromopropene upon excitation at 193 nm and the unimolecular dissociation dynamics of the nascent 1-propenyl radical are investigated in a crossed laser−molecular beam apparatus. The lowest-energy dissociation barrier of the 1-propenyl radical is experimentally determined for the first time and is found to be 31.5 ± 2.0 kcal/mol, in substantial agreement with the theoretical calculations of Davis et al. There are three dissociation channels of the 1-propenyl radical seen in these experiments: C−C bond fission to give C2H2 + CH3, C−H bond fission to give propyne + H, and isomerization to the allyl radical followed by H atom loss to give allene + H. The data show that the C−C fission channel dominates the product branching for the dissociative radicals with the lowest internal energies. The branching to the H + allene channel at higher energies suggests that the calculated isomerization barrier is too high with respect to the H + propyne barrier. The data for the precursor molecule, cis-1-bromopropene, indicate that there are at least three competing primary product channels, HBr elimination and two C−Br bond fission pathways leading to 1-propenyl + Br formation. In addition, there is a product channel described by the total reaction C3H5Br → Br + H2 + C3H3. We also show evidence that the Br atom is formed in the excited (2P1/2) spin−orbit state from the C−Br bond fission product channels.