On the formation of the fullerenes

Summary

The chemistry by which the closed-cage carbon clusters, C₆₀ and C₇₀, can be formed in high yield out of the chaos of condensing carbon vapour is considered. Several mechanisms for this process that have been proposed are critically discussed. The two most attractive are the ‘pentagon road’ where open sheets grow following the alternating pentagon rule and the ‘fullerene road’ where smaller fullerenes grow in small steps in a process which finds the buckminsterfullerene (C₆₀) local deep energy minimum and to a lesser extent the C₇₀ (D_5h) minimum. A clear choice between the two does not seem possible with available information.

Introduction

The observation (Kroto et al. 1985) that the truncated icosahedron molecule, C^BF₆₀ (buckminsterfullerene), is formed spontaneously in condensing carbon vapour was greeted by some in the chemical community with some doubt. It seemed incredible that this highly symmetrical, closed, low entropy molecule was forming spontaneously out of the chaos of condensing high-temperature carbon vapour. We still believe that the formation of C^BF₆₀ in supersonic cluster beam sources must be a relatively minor channel, probably accounting for less than 1% of the total carbon. Thus when C^BF₆₀F was finally isolated from graphitic soot (Krätschmer et al. 1990), it came as a surprise that the C^BF₆₀ plus C₇₀ yields were as large as 5%. Later yields have improved substantially, for example Parker et al. (1991) obtained a total yield of C^BF₆₀ of about 20% with total extractable fullerene yields totalling 44% from a carbon arc soot. Thus conditions can be found where C^BF₆₀ and fullerene formation in carbon condensation can hardly be called a minor channel.