Internal energy dependence of the H+allene H+propyne product branching from the unimolecular dissociation of 2-propenyl radicals

This Letter introduces a new technique to probe the competing unimolecular dissociation channels of isomerically-selected hydrocarbon radicals as a fu nction of internal energy in the radical. The crossed laser-molecular beam scattering experiments produce 2-propenyl radicals by photolysis of 2-chlor opropene and disperse the radicals by the neutral velocity imparted in the photolysis; thus dispersing them by internal energy in the neutral time-of- flight spectrum. For the unstable radicals, the experiments then measure th e branching between the two competing C-H bond fission product channels via tunable vacuum-UV photoionization of the products. Dispersing the neutral products by arrival time at the detector allows us to measure the branching between isomeric product channels as a function of internal energy in the dissociating radical isomer. The data resolve the competition between the u nimolecular H + allene and H + propyne product channels from the radical wi th internal energies from 0 to 18 kcal/mol above the H + propyne barrier. W e find that the barrier to H + allene formation from this high-energy C3H5 radical is slightly higher than the barrier to H + propyne formation, in ag reement with recent theoretical calculations but in sharp contrast to that predicted for the most stable C3H5 isomer, the allyl radical. The dominance of the branching to H + propyne formation over H + allene formation for th is isomer persists at the higher internal energies in agreement with RRKM p redictions that take into account the freezing of the methyl rotor in the H + allene channel and predict a concomitant reduction in the A factor for t hat channel.