From the molecular behaviors of fullerene derivatives C50X2 (X = H, F, Cl, Br, OH) to the general parallels among isostructural derivatives of fullerenes and carbon nanotubes

Abstract

A systematic investigation of all possible isomers of fullerene derivatives C₅₀X₂ (X = H, F, Cl, Br, OH) has been performed using the semiempirical AM1 method. The equilibrium geometrical structures, heats of formation, HOMO–LUMO energy gaps, ionization potentials, electronic affinities, strain and aromaticity have been studied. The results indicate that the selection rule for two groups adding to fullerene C₅₀ is independent of the type of functional group. The isomer-78, which corresponds to a 1,4-addition at the six-membered ring located on the equator, is the most stable isomer for C₅₀X₂ (X = H, F, Cl, Br, OH). The driving force governing the stabilities of the presently studied C₅₀X₂ isomers is the strain inherent in the C₅₀ cage. The contribution of the conjugation effect to the stabilization is not able to compete with that of the strain. The more stable C₅₀X₂ isomers have larger ionization potentials and smaller electronic affinities compared with C₅₀, which suggests that it is more difficult to oxidize and reduce C₅₀X₂ than to oxidize and reduce C₅₀. Energies as well as HOMO–LUMO gaps of isostructural C₅₀X₂ (X = H, F, Cl, Br, OH) isomers are almost parallel, i.e., energy differences between isostructural isomers of any two kinds of C₅₀X₂ derivatives are constant. This phenomenon can be called H/F/Cl/Br/OH parallels, which may result from the same degree of perturbation for addition of different functional groups to the structure of the parent carbon cage. H/F parallels are generalized characteristics among not only isostructural isomers of fullerenes but also isostructural isomers of carbon nanotubes. Furthermore, it is predictable that general H/F/Cl/Br/OH… parallels may exist among various derivatives of other fullerenes and carbon nanotubes.