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The results of ab initio electronic structure calculations on the C 60 cage and its endohedral ("inside-the-cage") complexes with F -, Ne, Na + , Mg2 + , and Al 3 + 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 (N a + , Mg2 + , and Al 3 + ) the cage radii. The complexes with the ions at the cage center are local maxima with respect to the displacement of the guests. The C 60 ' Ne complex, which is destabilized by ca. 0.4 kcal/mol relative to the separated components, is an energy minimum. In the C 60 • 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 C 60 cage affected by the electrostatic potential produced by the enclosed guests.

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Accurate standard enthalpies of formation of 115 isolated pentagon rule (IPR) fullerenes with 60-180 carbon atoms have been derived from energies of isodesmic interconversion reactions computed at the B3LYP/6-31G* level of theory. The calculated values of ∆H°f, which may serve as benchmarks for both calorimetric measurements and less sophisticated theoretical studies, are reproduced within 3 kcal/mol by a simple scheme based upon counts of 30 distinct structural motifs composed of hexagons together with their first and second neighborhoods. The extremely low computational cost of such a scheme makes it ideally suited for a rapid prescreening for thermodynamically viable IPR fullerenes with cages composed of hundreds of carbon atoms. With the inclusion of a global curvature term, this scheme is expected to be equally successful for small and large carbon clusters.