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″-X̃2A2″ electronic transition. Methyl radicals were formed at all photolysis wavelengths used over the range 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 X̃2B1 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 exitchannel conical intersection at extended N-H distance on its return. It is proposed that C-N bond cleavage then occurs promptly and non-statistically on the S0 surface.
3+1 Resonance-enhanced multiphoton ionization and photofragment excitation spectroscopy have been used to identify NH X(3)Σ(-) as a primary product of methylamine photodissociation after state-specific excitation to the S1 state. On the basis of standard thermochemical data, NH X(3)Σ(-) can be formed only in conjunction with closed-shell CH4 coproducts, indicating that dissociation must occur on the T1 surface. It is proposed that the mechanism for the formation of triplet NH and CH4 involves intramolecular abstraction between frustrated radical products and is an example of roaming-mediated intersystem crossing.
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