Computations of metal-covered C-60 and C-70

Semiempirical quantum-chemical PM3 calculations are reported for a new clas s of exohedral metallo-fullerenes metal-coated or metal-covered fullerenes: C60Mn and C70Mn. The exohedral species have been observed in gas phase, ho wever, their geometrical and electronic structures are not known yet. Relat ively-even metal-atom distributions over the fullerene rings are considered - such regular forms are computed for M = Be, Mg, Al. Three selected types of stoichiometries are treated in particular: C60M12/C70M12 (metal atoms a bove all pentagons), C60M20/C70M25 (metal atoms above all hexagons), and C6 0M32/C70M37 (metal atoms above all rings). If an odd number of electrons sh ould result (Al), the related cation is computed, or one metal atom added o r removed. This interesting arrangement above the rings is possible only fo r some types of atoms, while other elements are localized above bonds or at oms, or inside the cage, or even react and destroy the cage. Other limitati on comes from the parametrization of the computational technique used - the PM3 semiempirical method is parametrized only for some selected metals. Me tal-layer atomization heats are suggested as a stability measure. Structura l characteristics are also presented and interesting reductions of the cage symmetry are found. Their relationship to Jahn-Teller effect is discussed. The metal covered fullerenes can represent models for metal catalysis in t he nanotube synthesis and could eventually lead to new interesting material s, (C) 2000 Elsevier Science B.V. All rights reserved.