Beyond the Isolated-Pentagon Rule

The most stable IPR isomers are expected to be those with the optimal hexagon neighbor index signatures defined by 

The HNR applies a further filter to the set of isolated-pentagon isomers. It selects at most 44 optimal isomers of any one fullerene in the range 70 n 140(Table 3). It has not been proved by exhaustive calculation that these do in fact always include the most stable isomer in all cases, but the HNR does give a qualitativeiustification of the empirical observation that leapfrog and carbon-cylinder isomers beyond C70 are not those found separated from the Kratschmer-Huffman soot. 

This competition of steric and electronic effects rationalizes the known experimental isomer distributions for C76, C78, and C84. The conclusion from comparisons of total energies, Hiickel properties, and hexagon-neighbor signatures is that, while TT-electronic effects are dominant for C76, the balance is more even for C78, and for C84 the steric effects have taken over the major role. The indications are that the relative importance of the 71-electronic stability will continue to decline as n increases. 

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