Diatomic metal encapsulates in fullerene cages: A Raman and infrared analysis of C₈₄ and Sc₂@C₈₄ with D_2d symmetry

Raman scattering and infrared absorption of the C₈₄ and Sc₂@C₈₄ isomers 23:D_2d were studied at room temperature and 95 K. The results are compared to the response of pristine and doped C₆₀. According to the lower symmetry and the higher number of atoms C₈₄ exhibits much more vibrational modes than C₆₀, in particular at wave numbers above 500 cm^-1. For lower energies the vibrational structure of C₈₄ resembles a downshifted and split C₆₀ spectrum. After the encapsulation of two scandium atoms the overall vibrational structure and the number of C₈₄ modes was preserved as a result of the similar geometric structure. From the very good correlation of the C₈₄ and Sc₂@C₈₄ cage modes metal to fullerene charge transfer induced shifts could be analyzed. The lines were found less shifted compared to the C₆₀ modes in exohedral doped A₆C₆₀ (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 Sc₂@C₈₄ below 200, at 246 and at 259 cm^-1, and attributed to Sc-C₈₄ vibrations. These vibrations were further identified as Sc-C₈₄ stretching and Sc-C₈₄ deformation modes. The Sc-C₈₄ valence force constant of 1.19 N/cm was derived with a linear three-mass oscillator model for Sc₂@C₈₄. Both, the charge transfer induced line shifts and the Sc-C₈₄ 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 (Sc²²⁺)₂@C₈₄^4.4- 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-C₈₄ vibrations is attributed to crystal field and factor group splitting.