H atom product channels in the ultraviolet photodissociation of the 2-methyl-1-propenyl radical

Abstract

The ultraviolet (UV) photodissociation dynamics of 2-methyl-1-propenyl radicals are investigated using the high-n Rydberg atom time-of-flight (HRTOF) technique in the 226–248 nm photolysis region. The radicals are generated from 193 nm photolysis of two precursors, 1-chloro-2-methylpropene and 1-bromo-2-methylpropene. The H-atom photofragment yield (PFY) spectrum of 2-methyl-1-propenyl in the 226–248 nm region exhibits a broad absorption feature peaking around 240 nm and an increased intensity at shorter wavelengths <228 nm, which quantum chemistry calculations attribute primarily to the 3s Rydberg state and/or the π* state. The product translational energy distributions, P(E_T)'s, of the H-atom loss channels are modest, peaking at ∼7 kcal mol⁻¹, with the average fraction of translational energy release 〈f_T〉 remaining constant at 0.13–0.15 across the studied wavelength range. The angular distribution of the H-atom photofragments is isotropic and indicates a dissociation timescale longer than the rotational period of the radical. The photodissociation mechanism of 2-methyl-1-propenyl is found to be consistent with statistical unimolecular decomposition of the radical in a highly vibrationally excited ground electronic state to the methylenecyclopropane + H products, following internal conversion from the excited electronic states.