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 $C84$ and $Sc2@C84$ isomers $23:D2d$ 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 $(Sc2.2+)2@C844.4−$ 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.