Quantum Chemical Study on the Configurations of Encapsulated Metal Ions and the Molecular Vibration Modes in Endohedral Dimetallofullerene La ₂@C₈₀

The configuration of La ions of La₂@C₈₀ in the [80]fullerene cage was investigated by use of quantum chemical calculations. We found that the D_3d configuration is the global minimum in total energy, being more stable by 1.9 kcal/mol than the D_2h configuration, which has been considered to be the most stable. The potential energy surface calculation clarified that La ions travel between 10 equivalent D_3d positions through D_2h positions and consequently form pentagonal dodecahedral trajectory, which is in good agreement with the previous synchrotron radiation structural study. The experimental and theoretical investigation of the Raman spectrum revealed that the symmetry of molecular vibration is dramatically reduced simply by encapsulation of two La ions, and resulting vibrational modes were successfully assigned. The Raman peak at 163 cm^-1 was interpreted as the in-phase synchronously coupled mode of the [80]fullerene cage elongation and the La-La stretching, rather than a conventional and naive assignment as a metal-to-cage vibration mode.