The isomer problem for fullerene derivatives: Structural proposals for C70H36

Calculations are performed on the optimal structures and relative stabilities of the experimentally identified but as yet uncharacterized C70H36 molecule. A model in which addition preserves nominal aromaticity is tested against semi-empirical MNDO calculations and found to be inadequate: large aromatic regions on the surface of the C-70 cage would demand planarity and, hence, imply large steric strain. A structure with isolated double bonds in the polar caps and conjugation in a cyclopentaphenyl belt is predicted to be more stable by some 300 kJ mol(-1) than the nominally aromatic isomers, but the most stable structure found so far, improving by about 85 kJ mol(-1) on all previously published proposals, has 17 localized double bonds, one in each polar cap and the others in three bands of five around the C-70 cylinder, the low strain of their arrangement apparently outweighing the total loss of pi delocalization.