Photoinduced Intramolecular Hydrogen Transfer Reaction of Ortho Nitrobenzyl Compounds

Time-resolved resonance Raman and absorption spectra have revealed that the photoinduced intramolecular hydrogen transfer reaction of ortho nitrobenzyl compounds is initiated by the abstraction of methylene hydrogen by the ortho nitro group to generate the ortho aci-nitro acid isomer. In polar solvents the ortho aci-nitro acid is dissociated into the aci-nitro anion and a proton, and the proton is captured by other hydrogen accepting sites, such as 2-pyridyl, 4-pyridyl and 4-nitro groups to generate the ortho N—H quinoid, para N—H quinoid and para aci-nitro acid isomers, respectively. For 2-nitroethylbenzene and 2- and 4-(2′-nitrobenzyl)pyridines the structure of the aci-nitro anion is very similar to that of their respective ortho aci-nitro acid, while for 2,4-dinitroethylbenzene the structure of the aci-nitro anion quite resembles that of the para aci-nitro acid.