Endohedral complexes: Atoms and ions inside the C60 cage

The results of ab initio electronic structure calculations on the C60 cage and its endohedral (''inside-the-cage'') complexes with F-, Ne, Na+, Mg2+, and Al3+ are presented. Placing the ions at the center of the cage results in a net stabilization and screening of the charges. The ionic guests either decrease (F-) or increase (Na+, Mg2+, and Al3+ ) the cage radii. The complexes with the ions at the cage center are local maxima with respect to the displacement of the guests. The C60·Ne complex, which is destabilized by ca. 0.4 kcal/mol relative to the separated components, is an energy minimum. In the C60·Na+ complex, the energy minimum (which lies only 0.8 kcal/mol below the maximum) corresponds to the Na atom displaced by 0.66 A from the cage center. The calculated properties of the endohedral complexes are easily rationalized with a model involving a double-layer polarizable C60 cage affected by the electrostatic potential produced by the enclosed guests