Competing isomeric product channels in the 193 nm photodissociation of 2-chloropropene and in the unimolecular dissociation of the 2-propenyl radical

This paper presents product translational energy spectroscopy measurements of the primary photofragmentation channels of 2-chloropropene excited at 19 3 nm and of the unimolecular dissociation of the 2-propenyl radical. Tunabl e vacuum ultraviolet (VUV) photoionization of the products allows us to dis tinguish between the various product isomers formed in these processes. The data show evidence for three significant primary reaction channels in the dissociation of 2-chloropropene: An excited-state C-Cl fission channel prod ucing fast Cl atoms, a C-Cl fission channel producing slow Cl atoms, and HC l elimination. A minor C-CH3 fission channel contributes as well. The measu red branching of the major primary product channels is: [fast C-Cl]:[slow C -Cl]:[HCl elimination] = 62%:23%:15%. The experiments also allow us to reso lve selectively the product branching between the unimolecular dissociation channels of the 2-propenyl radical, a high energy C3H5 isomer; we measure how the branching ratio between the two competing C-H fission channels chan ges as a function of the radical's internal energy. The data resolve the co mpetition between the unimolecular H + allene and H+propyne product channel s from the radical with internal energies from 0 to 18 kcal/mol above the H + propyne barrier. We find that the barrier to H + allene formation from t his high-energy C3H5 radical is higher than the barrier to H+propyne format ion, in agreement with recent theoretical calculations but in sharp contras t to that predicted for the most stable C3H5 isomer, the allyl radical. The experiments demonstrate a general technique for selectively forming a part icular CnHm isomer dispersed by internal energy due to the primary photolys is, thus allowing us to determine the branching between unimolecular dissoc iation channels as a function of the selected radical isomer's internal ene rgy. (C) 2001 American Institute of Physics.