Photoinduced Intramolecular Hydrogen Transfer Reaction of Ortho Nitrobenzyl Compounds

Time-resolved resonance Raman and absorption spectra have revealed that thephotoinduced intramolecular hydrogen transfer reaction of ortho nitrobenzyl compoundsis initiated by the abstraction of methylene hydrogen by the ortho nitro group togenerate the ortho aci-nitro acid isomer. In polar solvents the ortho aci-nitro acid isdissociated into the aci-nitro anion and a proton, and the proton is captured by otherhydrogen accepting sites, such as 2-pyridyl, 4-pyridyl and 4-nitro groups to generate theortho 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 theaci-nitro anion is very similar to that of their respective ortho aci-nitro acid, while for2,4-dinitroethylbenzene the structure of the aci-nitro anion quite resembles that of thepara aci-nitro acid.