Calculations on all possible isomers of the substituted fullerenes C58X2 (X=N,B) using semiempirical methods

Abstract

A systematic investigation on the molecular structures of all the possible isomers of C₅₈N₂ and C₅₈B₂ has been performed using the semiempirical methods AM1, PM3 and MNDO. The equilibrium geometrical structures, heats of formation, HOMO–LUMO gap energies, heats of atomization, ionization potentials and affinity potentials of C₅₈X₂ (X=N,B) have been studied. The calculation results obtained by all these semiempirical methods show that the heterofullerenes are less stable than C₆₀, and that C₅₈N₂ should be more stable than the boron analog C₅₈B₂. All the empirical methods in this work indicate that isomer-7, which corresponds to 1,4-substitution in the cyclohexatriene unit, is the most stable isomer for C₅₈X₂ (X=N,B). The stability decreases with the increasing distance between the heteroatoms. The heterofullerenes C₅₈X₂ have smaller ionization potentials and bigger affinity potentials compared with C₆₀, thus the redox characteristics of C₆₀ can be enhanced by doping. Both C₅₈N₂ and C₅₈B₂ are expected to have significantly different chemical and physical properties from those of the fullerenes. We propose that the change of hybridization from s to s may be the underlying reason why 1,4-substitution is favored in both C₅₈N₂ and C₅₈B₂. The electronic spectra for these doped fullerenes have been calculated using the INDO/CIS method based on the optimized geometries.