Low-energy vibrations in Sc2@C84 and Tm@C82 metallofullerenes with different carbon cages

Raman scattering and infrared absorption is reported for various empty and filled fullerene isomers in the low-energy range below 600 cm(-1) to clarify the influence of different carbon cage structures on the bonding strength between encapsulated metal ion and fullerene cage and to check the potential of vibrational spectroscopy for isomer identification. The spectra of three isomers of Tm@C-82 and three isomers of SC2@C-84 were measured at room temperature. The results are compared to the response of the most abundant isomers of empty C-84 as well as to data for C-82 and C-60. The vibrational structure of the higher empty fullerene cages C-82 and C-84 resembles a downshifted and split C-60 spectrum. Moreover the spectra of the two C-84 isomers exhibited only small differences due to the very similar molecular structure, i.e. identical hexagon indices and direct neighbourhood in the Stone-Wales conversion map. Larger differences of the low-energy cage modes were found for the Tm@C-82 isomers and in particular for the Sc-2@C-84 isomers. This goes along with an increasing difference in hexagon indices and a larger distance on the Stone-Wales conversion map. Due to the charge transfer from the endohedral metal to the fullerene the low-energy cage modes an shifted in the same direction as the modes of C-60 during the exohedral doping process with alkali metals. New lines induced by the endohedral scandium and thulium ions with almost complementary Raman and infrared intensities were found for Sc-2@C-84 below 200, around 250 and 260 cm(-1) and for Tm@C-82 at 116 to 118 cm(-1) and at 42 cm(-1). These vibrations were further identified as M-C-2n stretching and M-C-2n deformation modes. Only a minor influence of the cage isomerism on these modes was observed. This is consistent with a simple ionic picture for the interaction between carbon cage and encapsulated metal ion. It is the amount of metal to fullerene charge transfer and the distance of the oppositely charge centres which determine the carbon cage-metal ion bond strength. (C) 2000 Elsevier Science B.V. All rights reserved.