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

The ultraviolet photodissociation dynamics of 2-methyl-1-propenyl radical, (CH₃)₂CCH, were explored within the wavelength range of 226-248 nm utilizing the high-n Rydberg atom time-of-flight (HRTOF) technique. Generation of 2-methyl-1-propenyl radicals was achieved through 193 nm photolysis of the precursors of 1-chloro-2-methylpropene and 1-bromo-2-methylpropene. The H-atom photofragment yield (PFY) spectrum exhibited a broad profile, peaking at around 240 nm. Translational energy distributions of the H-atom product channel, P(E_T)’s, were observed to be modest, with a peak around ~7 kcal/mol. The fraction of the average products' translational energy in the total available energy, <f_T>, remained constant within the range of 0.13-0.15. Additionally, the angular distribution appeared isotropic. Theoretical calculations were also carried out to characterize the excited states involved in photodissociation. The simulated absorption spectrum agreed with the observed PFY spectrum, indicating the involvement of an excited state with π* character in the photodissociation in the 226-248 nm region. The photodissociation mechanism of 2-methyl-1-propenyl was found to agree with statistical unimolecular decomposition of the highly vibrationally excited ground electronic state into methylenecyclopropane + H products, subsequent to internal conversion from the excited electronic state.