The photodissociation of nitromethane at 193 nm

Abstract

The dissociation of nitromethane following the excitation of the π* ← π transition at 193 nm has been investigated by two independent and complementary techniques, product emission spectroscopy and molecular beam photofragment translational energy spectroscopy. The primary process is shown to be cleavage of the C–N bond to yield CH3 and NO2 radicals. The translational energy distribution for this chemical process indicates that there are two distinct mechanisms by which CH3 and NO2 radicals are produced. The dominant mechanism releasing a relatively large fraction of the total available energy to translation probably gives NO2 radicals in a vibrationally excited 2B2 state. When dissociated, other nitroalkanes exhibit the same emission spectrum as CH3NO2, suggesting little transfer of energy from the excited NO2 group to the alkyl group during dissociation for the dominant mechanism. This conclusion is supported by the apparent loss of the slow NO2 product in the molecular beam studies to unimolecular dissociation to NO+O, which will occur for NO2 with 72 kcal/mol or more internal energy. Evidence is presented which suggests that the NO2 produced via the minor mechanism, which releases a smaller fraction of the available energy to translation, has a large cross section for absorbing an additional photon via a parallel transition and dissociating to NO+O.