Formation of vibrationally excited methyl radicals following state-specific excitation of methylamine.

The photochemistry of methylamine has been investigated following state-specific excitation of the S1 state. 2 + 1 resonance-enhanced multiphoton ionization was used to detect nascent methyl radical products via the 3p(2)A2″-X̃(2)A2″ electronic transition. Methyl radicals were formed at all photolysis wavelengths used over the range of 222-240 nm. The nascent products showed significant rotational excitation and several quanta of vibrational excitation in ν3, the degenerate C-H stretch. The partially deuterated methyl-d3-amine isotopologue yielded methyl-d3 fragments with vibrational distributions entirely consistent with those measured for the fully protiated species; no mixed isotopologues were detected. Energetic constraints require that the vibrationally excited methyl radicals be produced in conjunction with electronic ground-state NH2 X̃(2)B1 radicals on the S0 surface, negating the previous interpretation that dissociation occurs on the upper adiabat. New ab initio calculations characterizing the C-N bond cleavage coordinate confirm the presence of a barrier to dissociation on S1 that is insurmountable at the photolysis wavelengths used in this work. We propose a "semi-direct" mechanism in which frustrated aminyl H atom loss on the upper adiabatic potential energy surface leads to internal conversion at the exit channel conical intersection at an extended N-H distance on its return. It is proposed that C-N bond cleavage then occurs promptly and nonstatistically on the S0 surface.