Evidence for Jahn-Teller effects in endohedral fullerenes

Endohedral fullerenes can be formed by encapsulating one or more atom or molecule inside the cavity of the fullerene molecule. When a light molecule is encapsulated, it will remain close to the centre of the fullerene molecule without any strong interactions with its environment. This allows the quantum-mechanical behaviour of the molecule to be probed almost as if it were a free molecule. However, energy levels deduced from inelastic neutron scattering (INS) and in infrared spectroscopy show some small splittings compared to the expected results for a free molecule. This indicates that the encapsulated molecule is not totally free from the effects of its environment. More specifically, the molecule must feel an environment that has a lower symmetry than that of an undistorted fullerene cage. We will review the evidence for symmetry-lowering in different types of endohedral fullerenes, and discuss whether the symmetry-lowering could be due to the Jahn-Teller (JT) effect. We will then present some results of a model for H2O@C-60 which shows that the splittings seen in INS data could be explained in terms of JT distortions of the encapsulating fullerene cage. Nevertheless, the possibility that they could also be explained with a non-JT model can't be ruled out.