Resonance Raman and Molecular Orbital Studies of the Effects of Deuteration on the Vibrational Structure of the p-Benzosemiquinone Radical Anion

Article abstract of DOI:10.1021/j150644a010

Deuterium substitution of the p-benzosemiquinone radical anion results in decreases of 23 and 317 cm^-1 in the vibrational frequencies of its totally symmetric ring stretching (Wilson 8a) and C-H bending (9a) modes as observed by resonance-enhanced Raman methods.Ab initio molecular orbital calculations corroborate the vibrational assignments and predict decreases of a similar magnitude.Both the experimental and theoretical studies show that the bands associated with the CO stretching and C-C-C bending vibrations are affected to only a very minor extent by deuteration, i.e., that there is very little contribution from local modes involving hydrogen motion.The symmetrical ring breathing (1) and C-H stretching (2) modes are not resonance enhanced and not observed in the Raman experiments.Of particular note is the Raman band at the ring stretching frequency of the deuterated radical which is considerably broader than the other lines in H2O and partially resolved into two lines in D2O.This feature is tentatively interpreted as being due to an accidental near degeneracy with the combination of the 9a and 1 modes.Deuteration of the radical somewhat simplifies its optical absorption spectrum in the region of 400-410 nm indicating that the frequency of the 9a vibration in the excited state is decreased in much the same way as in ground state.