Clusters of molecular type the fullerenes

Clusters of molecular type can come in many forms, but if we confine our attention to clusters formed from nearly identical atoms, a good example of a cluster of molecular type is afforded by the Ceo molecule, known as buckminster-fullerene (because its shape is reminiscent of the domes built 

Although less stackable than the other systems considered here, fullerenes, in other respects, also qualify as clusters, for several reasons. First, they do so because of their large size and symmetrical shapes. Second, there can exist similarly formed groupings of C atoms with diverse numbers (70, 120, etc) and related properties. Third the p bonds of C give rise to a delocalised electronic shell structure similar in many respects to the closed shells of metallic clusters each carbon atom donates four electrons to a shell whose precise shape is determined by the fullerene structure, but which is closed, has a thickness of about one atom, and is very similar to the closed shells of metallic clusters. 

The hollow cage structure of the fullerenes allows a spherical standing wave to be formed inside the cluster, and this has been advanced as the explanation for marked oscillations of the photoionisation cross section of Ceo which persist in both the gaseous and the solid phase [666]. 

Clusters with hollow cage structures and more than 60 atoms are known as higher fullerenes. Their structures and modes of growth are a subject of study [667, 668, 669]. Experimentally, six different sizes (Ceo, C70, C76, C78, C82 and C84) are well characterised as possessing hollow closed cages, 

2 It is in fact stronger than diamond, but does not fill space to form a uniform lattice, and so does not give rise to such a hard solid. The intercallation of foreign ions in solids formed from fullerenes is discussed in the next section. 

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