Mechanistic insights into nitric oxide (NO) release following UV photolysis of nitromethane

The dynamics of nitric oxide (NO) release following 225/213 nm excitation of nitromethane is investigated and compared with the corresponding investigations on nitrobenzene to elucidate the mechanistic details of NO release and involvement of various electronic states in the NO release. The translational energy distribution profiles of various product states of the NO fragment exhibit bimodal behavior for nitrobenzene with competing slow and fast components, which are attributed to the NO release on the S0 and T1 surfaces, respectively. In contrast, the NO released from nitromethane exhibited a dominant slow component across all product states measured, indicating the predominance of dissociation on the S0 surface. However, a small fraction of translationally fast NO originates from the dissociation of nitromethane in the T1 state primarily via the NO2 roaming pathway (loose transition state), in contrast to nitrobenzene, wherein the fast component is due to the oxaziridine pathway (tight transition state). Interestingly, the dissociation of nitromethane to yield NO in the S0 state follows multiple pathways involving tight, semi-tight, and loose transition states.