Charge delocalisation in hydrofullerenes and substituted hydrofullerenes: effect of deprotonation

Abstract

The delocalisation that occurs upon deprotonation of a series of hydrofullerenes and substituted hydrofullerenes is analysed. The charges and bond lengths of the acidic and basic forms of each system are considered. We find approximately one electron delocalising from the deprotonated C atom to the rest of the system on deprotonation. The softness of the cage and the bond alternation permit delocalisation of the resulting negative charge over the main part of the fullerene cage, following the pattern of conjugated bonds, showing a clear ‘delocalisation path’. The same paths were found for hydrofullerenes and substituted hydrofullerenes, so that the properties of different functional groups are seen not to influence the delocalisation pattern in a significant way. This delocalisation greatly stabilises the basic form and explains the high acidities of these hydrocarbon systems.

The deprotonation energies for the two most stable C₆₀H₂ and the two most stable C₇₀H₂ isomers have been calculated at the ab initio HF/6-31G* level of theory. 5,6-C₇₀H₂ was found to be more acidic than 1,2-C₆₀H₂. This inversion as compared to ¹H NMR predictions results from the difference in cage structure of C₆₀ and C₇₀. The delocalising charge reaches the equatorial band of six rings present in the C₇₀ structure, giving rise to a better stabilisation of the basic form than for 1,2-C₇₀H₂.