Energetics of Fullerenes with Octagonal Rings

Abstract

The energetic costs of widening the classical fullerene definition to include carbon cages with octagonal as well as pentagonal and hexagonal faces are investigated theoretically. Relative energies of all 16 C₄₀ and 620 C₄₈ cages that can be assembled with one face octagonal, 14 pentagonal and all others hexagonal are calculated within two independent semi-empirical models and compared with the 295 C₄₀ and 2664 C₄₈ one-square, one-heptagon and classical-fullerene cages. All isomers are lo-cal minima on the potential surface, and many non-classical structures fall within the energy range spanned by the classical fullerenes. Penalties for introduction of a single non-classical face increase in the order heptagon < square < octagon, estimated for C₄₈ as 58–123, 108–236, and 329–450 kJ mo⁻¹, respectively, depending on model. The energy variation across the range of classical and non-classical structures is rationalised by extension of the isolated-pentagon rule: when 1