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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