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 C70 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 C70 cylinder, the low strain of their arrangement apparently outweighing the total loss of π delocalization.