Ultraviolet photochemistry of the 2-buten-2-yl radical

The ultraviolet (UV) photodissociation dynamics of the 2-buten-2-yl (C₄H₇) radical were studied using the high-n Rydberg atom time-of-flight (HRTOF) technique in the photolysis region of 226-246 nm. 2-Buten-2-yl radicals were generated by 193 nm photodissociation of the precursor 2-chloro-2-butene. The H-atom photofragment yield (PFY) spectrum of 2-buten-2-yl is broad, peaking at 234 nm. Quantum chemistry calculations show that the UV absorption is due to the 3p and 3p Rydberg states (parallel to the plane of CC double bond). The translational energy distributions of the H-atom loss product channel, P(E_T)'s, of 2-buten-2-yl show a bimodal distribution indicating two dissociation pathways. The major pathway peaks at E_T ∼ 7 kcal mol^-1 with a nearly constant fraction of average E_T in the total excess energy, 〈f_T〉, at ∼0.11-0.12. This main pathway has an isotropic product angular distribution with β ∼ 0, consistent with the unimolecular dissociation of a hot 2-buten-2-yl radical following internal conversion from the electronically excited state, resulting in the formation of 2-butyne + H (∼84%) and 1,2-butadiene + H (∼16%). Additionally, there is a minor non-statistical pathway with an isotropic angular distribution. The minor pathway peaks at E_T ∼ 35 kcal mol^-1 in the P(E_T) distributions and exhibits a large 〈f_T〉 of ∼0.40-0.46. This fast pathway suggests a direct dissociation of the methyl H-atom on a repulsive excited state surface or on the repulsive part of the ground state surface, forming 1,2-butadiene + H. The fast/slow pathway branching ratio is in the range of 0.03-0.08.