Impact of α-substitution on the photodissociation dynamics of nitromethane: The CH3 and NO release channels

Photolysis of 1-nitropropane, 2-methyl-2-nitropropane, and (nitromethyl)benzene at 213 and 225 nm reveals distinct mechanistic features in CH3 and NO elimination channels. The CH3 fragments exhibit anisotropic, non-statistical translational energy distributions, consistent with barrier-impulsive dissociation on the T1 surface occurring on femtosecond timescales with restrictive intramolecular vibrational redistribution (IVR). In contrast, NO fragments display bimodal translational energy distributions with large slow/fast branching ratios, indicating preferential dissociation on the S0 surface following efficient IVR. Two-dimensional potential energy surface calculations along C-N and C-O coordinates show that NO2 roaming constitutes the minimum energy pathway on the T1 surface, while semi-tight transition states dominate on the S0 surface. Despite structural variation, α-substitution does not significantly alter the fundamental NO release mechanism; however, variations in triplet yield modulate the contribution of the fast component. Interestingly, the CH3 and NO release follow contrasting behavior in terms of IVR dynamics.