H atom Product Channels in the Ultraviolet Photodissociation of the 2-Propenyl Radical.

Abstract

The H atom product channels in the ultraviolet photodissociation of 2-propenyl (CH2CCH3) radical were investigated in the wavelength region 224-248 nm using photofragment translational spectroscopy. The CH2CCH3 radicals were generated by 193 nm photodissociation of 2-chloropropene and 2-bromopropene precursors. The H atom photofragment yield spectra from both precursors revealed a broad feature peaking near 232 nm. The translational energy distributions of the H + C3H4 products peaked around 7-8 kcal/mol and extended close to the maximum excess energy. The fraction of the total available energy released as products' translation was nearly a constant (∼0.16 using the 2-chloropropene precursor and ∼0.18 using the 2-bromopropene precursor) in the wavelength range 224-248 nm. The angular distribution of the H atom product was isotropic. Quasi-classical trajectory (QCT) calculations were performed on the ground-state potential energy surface of CH2CCH3 for its decomposition at a 124 kcal/mol excitation energy (equivalent to 230 nm photolysis photon energy). The calculations yielded branching ratios for different dissociation product channels, 32% H + allene, 35% H + propyne, 0.5% H + cyclopropene, and 32% methyl + acetylene. The experimental and QCT translational energy distributions of the H atom loss channels qualitatively agreed, consistent with the main H atom product channels being the H + allene and H + propyne dissociations. The time scale of the 2-propenyl dissociation on the ground electronic state was calculated to be ∼2 ps, smaller compared to that of the overall UV photodissociation (≥10 ps, implied on the basis of the isotropic H atom product angular distribution). The mechanism of the UV photodissociation of 2-propenyl is consistent with unimolecular dissociation proceeding on the ground electronic state after internal conversion of the electronic excited states.