Internal Energy Dependence of the H + Allene/H + Propyne Product Branching from the Unimolecular Dissociation of 2-Propenyl Radicals

Citations Over TimeTop 20% of 2000 papers

Abstract

This Letter introduces a new technique to probe the competing unimolecular dissociation channels of isomerically-selected hydrocarbon radicals as a function of internal energy in the radical. The crossed laser−molecular beam scattering experiments produce 2-propenyl radicals by photolysis of 2-chloropropene 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 the 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 unimolecular H + allene and H + propyne product channels 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 this high-energy C3H5 radical is slightly higher than the barrier to H + propyne formation, in agreement 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 this isomer persists at the higher internal energies in agreement with RRKM predictions 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 that channel.

Related Papers

• → Thermal decomposition of propyne and allene in shock waves(1989)142 cited
• → Isomer-selected ion–molecule reactions of acetylene cations with propyne and allene(2020)20 cited
• → The ignition and oxidation of allene and propyne: Experiments and kinetic modeling(1996)35 cited
• → Thermal decomposition of 2-bromopropene, and 2-chloropropene(2003)14 cited
• → Kinetics of Reactions of 1- and 2‑Naphthyl with Propyne and Allene(2020)1 cited