TIME-RESOLVED RESONANCE RAMAN STUDIES OF THE STRUCTURES OF THE LOWEST TRIPLET-STATE AND THE RADICAL-ANION OF BENZIL

Time-resolved resonance Raman spectra of the T1 state and the radical anion of benzil and its isotopically substituted analogues have revealed that the C=O symmetric stretch exhibits dramatically large downshifts in the T1 state and in the radical anion, while the downshifts of the phenyl vibrations are small. This indicates that the spin density is predominantly localized on the C=O groups, which drastically weakens the C=O bonds of the two transients. The much larger weakening of the C=O bond in the T1 state than in the radical anion is interpreted in terms of the nature of HOMO and LUMO. On the other hand, the central C-C stretch is markedly upshifted in the radical anion and in the T1 state. In particular, the remarkably high frequency of the C-C stretch of the radical anion may indicate that the radical anion takes a more planar structure than the S(o) state. The C-C stretch of the radical anion exhibits unusually large high-frequency shifts in strong hydro-gen-bond-donor solvents. This may imply that the planarity of the molecule increases as the hydrogen bond between the radical anion and the solvent becomes stronger.