Carbon atoms in fullerenes

Carbon atoms in fullerenes possess an sp--type electronic configuration, but, in contrast with graphitic carbons, addition reactions at the double bond junction between six-membered rings are allowed. This is due to the pyramidalization of s T carbons as a result of the curvature of the structure (Bard et al., 2008). Accordingly, a variety of functionalized fullerenes can be prepared (Hirsch, 1993). In addition, fullerenes can encapsulate anions and neutral molecules, thus forming the so-called endohe-dral fullerenes (Guha and Nakamoto, 2005) and form different types of polymeric chains. 

The replacement of carbon atoms in fullerenes by boron and/or nitrogen atoms has been repeatedly discussed in the literature, both theoretically j79-81j and experimentally. Smalley s group at the Rice University in Houston has successfully obtained polyaza-fullerenes from a mixture of graphite and boron nitride under the same conditions in which graphite yields fullerenes [82j by adding azides to buckminsterfullerene, Prato et al. j83j synthesized azafullerenes. 

The study of the fullerenes by NMR spectroscopy is not straightforward. has a low natural isotopic abundance (1.11%) and a low relative receptivity. In addition, the carbon atoms in fullerenes have relatively long relaxation times and spectra for the higher, less symmetric fullerenes can only be obtained within a reasonable time by using paramagnetic relaxation reagents, typically Cr(acac)3. 

D. Plavsid, D. Vukicevid, and M. Randid, On canonical numbering of carbon atoms in fullerenes C q buckminsterfullerene, Croat. Chem. Acta 78 (2005) 493-502. 

Fig. 4.31. Schematic diagrams comparing the magnetic equivalence of carbon atoms in fullerenes Cgo and C70. In (a) all sixty atoms are equivalent while in (b) there are five different types of carbon atom...

Some examples for 3D spherical aromaticity can be fonnd in different fullerenes. It is well known that the Ji orbitals of the carbon atoms in fullerenes are perpendicular to the surface and these electrons are itinerant, similarly to the benzene. Hence, the electronic structure of these compounds can be modeled using a Hiickel-type model, and the condition to obtain a closed electronic structure and aromaticity can also be formulated as previously done by Hirsch et al. Their rule is valid for icosahe-dral fullerenes. We will see that spherical aromaticity is a more common feature of metal clusters. 

Figure 17. Plot of the relative energy E per carbon atom in fullerenes Cn (relative to Ceo) versus the endohedral chemical shift dendo (top) and versus dendo scaled for fullerenes size (bottom), see text. Data are computed at the ab initio Hartree—Fock/DZP level.

Fullerenes can be considered as a molecular full stop to organic synthesis highly complex and possibly very useful molecules are formed by self-organization of carbon atoms in the vapor phase. Sometimes synthetic chemists are not needed. 

A second doping method is the substitution of an impurity atom with a different valence state for a carbon atom on the surface of a fullerene molecule. Because of the small carbon-carbon distance in fullerenes (1.44A), the only species that can be expected to substitute for a carbon atom in the cage is boron. There has also been some discussion of the possibility of nitrogen doping, which might be facilitated by the curvature of the fullerene shell. However, substitutional doping has not been widely used in practice [21]. 

The higher mass fullerenes (C76, Cs4), with multiple isomers of different shapes, also crystallize in the fee structure at room temperature, with an fee lattice constant which is approximately proportional to where n is the number of carbon atoms in the fullerene [53]. 

All of the carbon atoms in buckminster-fullerene are equivalent and are sp -hybridized each one simultaneously belongs to one five-membered ring and two benzene-like six-membered rings. The strain caused by distortion of the rings from coplanarity is equally distributed among all of the carbons. 

Until 1985, the only known elemental forms of carbon were diamond, graphite, and amorphous carbon. Then Kroto et al. announced the discovery of C6o, a spherical arrangement of carbon atoms in hexagons and pentagons, as shown in Figure 17.7. They called this form Buckminsterfullerene after the architect Buckminster Fuller, who developed the geodesic dome. The name for this type of carbon molecule has since been shortened to fullerene, but it is commonly called a buckyball. Since this first discovery, it has been found that fullerenes can be made in quantity from electrical arcs between graphite electrodes. About 75% of... 

The unfolded deltahedral nets provide a convenient way to illustrate the environments of the different carbon atoms in the asymmetric unit of higher fullerenes, in particular those of relatively low symmetry. The interrelation among fullerenes of different size and among various isomers can also be displayed by such... 

The discovery of the fullerenes was received with a mixture of enthusiasm and hostility. The molecules existed only in the mass spectrometer and some researchers were dubious. Final proof came in 1990 with the isolation of macroscopic amounts of C60. This allowed the 13C NMR spectrum of the compound to be recorded. As predicted, it proved to be a single line (at 6 143 ppm) - all of the carbon atoms in C60 are equivalent. 

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