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

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.