Diatomic metal encapsulates in fullerene cages:: A Raman and infrared analysis of C84 and Sc2@C84 with D2d symmetry

Raman scattering and infrared absorption of the C-84 and Sc-2@C-84 isomers 23:D-2d were studied at room temperature and 95 K. The results are compared to the response of pristine and doped C-60. According to the lower symmetry and the higher number of atoms C-84 exhibits much more vibrational modes than C-60, in particular at wave numbers above 500 cm(-1). For lower energies the vibrational structure of C-84 resembles a downshifted and split C-60 spectrum. After the encapsulation of two scandium atoms the overall vibrational structure and the number of C-84 modes was preserved as a result of the similar geometric structure. From the very good correlation of the C-84 and Sc-2@C-84 cage modes metal to fullerene charge transfer induced shifts could be analyzed. The lines were found less shifted compared to the C-60 modes in exohedral doped A(6)C(60) (A=K,Rb,Cs). Increased line widths of low energy cage modes were attributed to an additional intramolecular relaxation channel related to the dynamics of the encapsulated scandium ions. A set of nine new lines with almost complementary Raman and infrared intensities was found for Sc-2@C-84 below 200, at 246 and at 259 cm(-1), and attributed to Sc-C-84 vibrations. These vibrations were further identified as Sc-C-84 stretching and Sc-C-84 deformation modes. The Sc-C-84 valence force constant of 1.19 N/cm was derived with a linear three-mass oscillator model for Sc-2@C-84. Both, the charge transfer induced line shifts and the Sc-C-84 valence force constant indicate an effective transfer of approximately two electrons per scandium to the carbon cage. This is in agreement with an electronic state (Sc2.2+)(2)@C-84(4.4-) previously proposed on the basis of x-ray powder diffraction, x-ray photoemission spectroscopy (XPS), and quantum chemical calculations. The unexpected high number of Sc-C-84 vibrations is attributed to crystal field and factor group splitting. (C) 1999 American Institute of Physics. [S0021-9606(99)00841-7].