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The resonance Raman (RR) spectrum of C60 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 Hg(1), Ag(1), Gg(4), Hg(7), and Ag(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 T1u LUMO, is essentially negated by solvent distortion of the symmetry of C60. While the I h point group is a good descriptor for the 10 fundamental Raman modes of C60, the slight reduction in the high symmetry of the molecule, due to solvent and 13C effects, activates at least 6 of the remaining 36 Raman-silent modes. At resonance with the HOMO−LUMO transition of C60, or its vibronic sideband, the solution resonance Raman spectra of C60 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 Gg(4) mode at 1140 cm-1 suggest that the distorted excited state is best approximated as having D 5 d 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 C60 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 C60 and HIV-1 protease.

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The resonance Raman (RR) spectrum of C70 has been studied in benzene using 11 laser excitation energies across the main visible absorption band (MVAB) of C70 between 514.5 and 406.7 nm. Raman excitation profiles (REPs) were constructed for the 15 most intense RR bands of C70, and symmetry assignments have been made partly on the basis of polarization work. Contrast is made to work performed on thin films where problems have arisen from the symmetry-lowering effect of the surface and from neglect of resonance. Assignments for nine other less intense RR bands are suggested. Three electronic transitions under the MVAB are identified and assigned definitively:  the HOMO − 4 (e2‘‘) → LUMO + 1 (e1‘‘) transition in the 514/501-nm excitation region, the HOMO − 5 (e1‘) → LUMO + 1 (e1‘‘) transition in the 476/472-nm excitation region, and the HOMO (a2‘‘) → LUMO + 2 (a1‘‘) in the 457/452-nm excitation region. The REPs reveal that these three electronic transitions are vibronically coupled to the strong electronic transition at 382 nm which is assigned to the HOMO − 2 (a2‘) → LUMO + 3 (e1‘) transition. RR B-term scattering mechanisms are the major source of intensity enhancement for bands of the totally-symmetric A1‘ and the non-totally-symmetric E1‘‘ and E2‘ Raman modes. The REPs of the 15 bands are grouped into four types that provide insight into the change in the electronic distribution upon excitation for each transition. Unlike C60, whose extraordinarily high symmetry makes it very sensitive to solvent-induced symmetry lowering and whose RR spectrum is rich in forbidden, overtone, and combination bands, C70 displays a restricted subset of RR scattering phenomena. The lower symmetry and more localized molecular orbitals of C70 make it a better model for the RR scattering mechanisms and vibronic coupling expected in the higher fullerenes.

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