Carbon clusters five-membered rings

The fullerenes, parttCulabiy Qq, have been well characterized (1-3), but few reports about their chemical reactivity appeared 4-6). It has been proposed that the relatively high electronegativity of this carbon cluster was due to its pyracylene character (2). Further, the inter-five-membered ring bonds are fulvenoid and are potent electrophilic as well as dienophilic and dipolarophilic sites 5, 7). 

Figure 6.24. Mass spectrum of carbon clusters in a supersonic beam, originally observed by Nobel Laureates Smalley and Curl. Reproduced with permission from J. Chem. Phys. 1984, 81, 3322. Also shown is the molecular structure of BuckminsterfiiUerene, Cgo, containing alternating six- and five-membered rings of sp hybridized carbon atoms. This is only one isomer for Cso, out of a staggering total of 1,812 possible stractures. ...

As shown by the molecular structure of the compound C6o(Os04)-(4-tert-butylpyridine)2 reproduced in Fig. 4.49, the fulleride part of the molecule actually corresponds to a soccerball-like arrangement of carbon atoms. There, 20 six-member rings are fused with 12 five-member rings to give a carbon cluster with 32 faces. These features totally agree with the structure of the buckminster-fullerene discussed above. 

Around 1985, Kroto, Smalley, and Curl (Rice University) isolated a molecule of formula Cf,cj from the soot produced by using a laser (or an electric arc) to vaporize graphite. Molecular spectra showed that G , is unusually symmetrical It has only one type of carbon atom by 13C NMR (5143 ppm), and there are only two types of bonds (1.39 A and 1.45 A). Figure 16-16 shows the structure of C(1, which was named buckminsterfiillerene in honor of the American architect R. Buckminster Fuller, whose geodesic domes used similar five- and six-membered rings to form a curved roof. The Cg0 molecules are sometimes called buckyballs, and these types of compounds (Cgo and similar carbon clusters) are called fullerenes. 

An interesting correlation is found between the kinetic stabilities of the fullerenes ] to C70. as determined by simple Hiickel theory HOMO-LUMO energy separations, and the intensities of photoionisation signals from carbon clusters produced by laser vaporisation of graphite. This correlation provides further circumstantial evidence that the observed C34 to C70 clusters are indeed fullerenes, closed carbon cages containing only five- and six-membered rings. 

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