Degeneracy loss contributions to the stabilisation of the eccentric position of lithium in Li@C60

The eccentric position adopted by the lithium in endohedral LiC60, approximately 1.5 Å from the centre of the carbon cage at equilibrium, has been variously explained in terms of polarisation and dispersion interactions. We investigate a novel stabilisation effect for such complexes based upon electronic degeneracy loss in the orbitals of Li+@C60− as the charged lithium is displaced from the centre of the cage. This effect is small compared to polarisation effects and the eccentric stabilisation calculated by ab initio methods, amounting to approximately 20% of the polarisation contribution; however, the concomitant degeneracy splitting is expected to be observable through electronic Raman spectroscopy.