ELECTRONIC STATES OF GROUP-IV ENDOHEDRAL ATOMS IN C-28

We have systematically studied the interaction between the C28 fullerene molecule and the group-IVA endohedral atoms C, Si, Ge, and Sn, using state-of-the-art electronic structure and total-energy calculations based on density-functional theory. We find no covalent bonding between these atoms and the fullerene cage. The nature of the cage-atom interaction involves charge transfer from the endohedral atom to the cage which produces weak, mostly ionic binding. The degree of charge transfer increases with the atomic number of the endohedral atom. By comparing the electronic structure of the group-IVA endohedral cage compounds to Zr@C28 (a group-IVB endohedral), we find that the latter bonds strongly to the cage due to a covalent interaction between the Zr 4d states and the C28 states. We extrapolate the trends found for the group-IV endohedrals to suggest other possibly stable systems and to provide insight to the stability of the experimentally observed U@C28 complex.