Resonance Raman scattering from solutions of C-60

The resonance Raman (RR) spectrum of C-60 has been studied in benzene and carbon disulfide using eight excitation wavelengths between 406.7 and 647.1 nm. Raman excitation profile calculations have been performed on the five most intense RR bands; the H-g(1), A(g)(1), G(g)(4), H-g(7), and A(g)(2) vibrational modes. Two main scattering mechanisms predominate, Herzberg-Teller (HT) B-term scattering and nonadiabatic D-term scattering. This is the first observation of rare D-term scattering in a system without a metal. The requirement of a Jahn-Teller distortion of the excited state, produced upon population of the degenerate T-1u LUMO, is essentially negated by solvent distortion of the symmetry of C-60 While the I-h point group is a good descriptor for the 10 fundamental Raman modes of C-60 the slight reduction in the high symmetry of the molecule, due to solvent and C-13 effects, activates at least 6 of the remaining 36 Raman-silent modes. At resonance with the HOMO-LUMO transition of C-60, or its vibronic sideband, the solution resonance Raman spectra of C-60 display almost the full gamut of RR scattering phenomena with no fewer than 13 distinct classes of first- and second-order vibrational features. The intensity behavior and the depolarization ratio of the band due to the I-h-Raman-silent G(g)(4) mode at 1140 cm(-1) suggest that the distorted excited state is best approximated as having D-5d symmetry. The presence of overtones and combinations of Raman-active and Raman-silent vibrational modes is explained in terms of a second-order HT scattering. These studies of the RR spectroscopy of C-60 in solution have implications for the electron-pairing mechanism in the superconductivity of fullerides and the nature of solute-solvent associations such as between water-soluble derivatives of C-60 and HIV-1 protease.