Fullerenes

Fullerenes

Fullerenes are made by heating graphite in an electric arc under an inert atmosphere (helium or argon). The resulting sooty condensate, formed from vaporised carbon clusters, consists mainly of $C_{60}$, with a smaller amount of $C_{70}$, plus traces of even larger fullerenes (up to 350 carbon atoms or more, always an even number).

Because their cage-like structure has no "dangling" (unsatisfied) bonds, fullerenes are considered the only pure form of elemental carbon. The $C_{60}$ molecule — shaped like a soccer ball and named Buckminsterfullerene (nicknamed a "bucky ball"; Fig. 11.5) — is built from twenty six-membered rings and twelve five-membered rings. A six-membered ring can fuse to either another six-membered ring or a five-membered ring, but two five-membered rings never fuse directly to each other.

All 60 carbon atoms are equivalent: each is $sp^2$ hybridised, forms three $\sigma$ bonds to its neighbours, and contributes its leftover electron to a delocalised system spread over the whole cage, giving the molecule an aromatic character. The C–C bond distances within the cage are of two lengths, 143.5 pm and 138.3 pm, reflecting a mix of single and double bonds.

Relative stability. Graphite is the most stable form of carbon (its standard enthalpy of formation is taken as zero by convention); diamond's is +1.90 kJ/mol and fullerene $C_{60}$'s is +38.1 kJ/mol — both less stable than graphite. …