Structural parallels in hydrogenated and fluorinated [60]- and [70]-fullerenes

Abstract

Semi-empirical molecular orbital calculations (MNDO) have been carried out on isomers of C₆₀X₃₆ and C₇₀X₃₆ (X = H, F) in order to investigate the underlying cause of the parallel behaviour of the two addends towards [60]- and [70]-fullerenes. In both series the driving force for reaction (in addition to C–X bond formation) appears, on the MNDO model, to be relief of steric strain in the fullerene cage, rather than the electronic effects on the carbon framework of the addition of hydrogen and fluorine atoms. The formation of hydrogenated and fluorinated compounds with 36 added atoms from both [60]- and [70]-fullerene appears to be coincidental, arising essentially from the fact that in both cases this number of added atoms is sufficient to maximise the relief of strain in the appropriate cage. While a T point-group structure appears significantly more stable than others for C₆₀F₃₆, the differences amongst isomers of C₆₀H₃₆ are small. The MNDO predicted structures of C₇₀, C₇₀X₄₀ (X = H, F) are analysed to determine the major contributions to strain energy.