Phototautomerism of o-nitrobenzyl compounds:: o-quinonoid aci-nitro species studied by matrix isolation and DFT calculations

Photolyses of 2-nitrobenzyl methyl ether and 2-nitrotoluene with 254 nm light have been investigated in Ar and N-2 matrices at 12 K, and have been found to give o-quinonoid aci-nitro species as the primary photoproducts, along with other products. The o-quinonoid species have UV absorptions at relatively long wavelengths (lambda (max) at 385-430 nm) and undergo facile secondary photolysis when irradiated in these absorption bands. By means of this selective photolysis, fairly complete IR spectra of the o-quinonoids have been obtained. Comparison of the matrix IR spectra of these species with simulated spectra computed using density functional theory (DFT) has confirmed the identity of these reactive intermediates. Moreover, detailed analysis of the fit between the computed and experimental IR spectra has allowed the specific stereoisomers generated to be identified with reasonable confidence. Computationshave also been made of the relative energies of the starting compounds, intermediate o-quinonoid isomers and the possible secondary products, together with the transition states connecting them. The results of these computations indicate that the observed stereoisomer of each of the o-quinonoid species cannot arise by photoinduced H-atom transfer followed by isomerizations on the electronic ground-state surfaces, since the energy barriers for reversion to starting compounds are substantially lower than those for the necessary isomerizations. It is therefore concluded that H-atom transfer and conformational interconversion occur in an electronic excited state.