Mechanistic variances in NO release: isomers of nitrophenol and nitroaniline

The NO release following 266 nm photolysis of ortho and meta isomers of nitrophenol and nitroaniline shows a bimodal translational energy distribution, wherein the slow and fast components originate from dynamics in the S 0 and T 1 states, respectively. The translational energy distribution profiles for any NO product state show a higher slow-to-fast (s/f) branching ratio for the ortho isomer in comparison with the meta isomer. The observed variation in the s/f branching ratio vis-à-vis the ortho and meta isomers is attributed to the presence of intramolecular hydrogen bonding between the ortho substituent and NO 2 moiety, which favours the roaming mechanism. A switch in the NO release mechanism is influenced by the hydrogen bonding ability of the substituent.