It has not been possible to obtain pure Si fullerene-like structures.
However, it is an interesting experimental finding that up to 3 Si atoms seem to readily incorporate into the carbon cages. Nevertheless, the detailed structures, stability, and properties of these heterofullerenes (silafullerenes) are still far from being well understood. In this direction, we have performed PM3 semiempirical calculations (full geometry optimizations and no symmetry restrictions) on heterofullerenes with the form C 2n-m Si m (2n = 28, 60; m = 1, 2, 3). We study the effect of substitution of m C atoms by Si atoms in different positions. From our total energy calculations, we can conclude that some favored silafullerene isomers (with 2 and 3 Si-substituted atoms) are those in which Si atoms occupy alternate hexagon positions (1,3 and 1,4). Other electronic properties are computed to assess the effect of gradual substitution of Si atoms in the cages. A discussion on the nature of the chemical bonds involved in these heterofullerenes, in terms of hybridization, is presented. It is predicted that C 58 Si 2 should be specially stable and relatively abundant, due to its large hardness and low electron affinity. These results may shed some light on the nature of silafullerenes and their resulting stabilities and observed photofragmentation patterns.