Fullerenes possible structures

Here, the topological nature of the tori will be discussed briefly. Figure 1 shows the five possible prototypes of toroidal forms that are considered to be related to fullerenes. These structures are classified by the ratios of the inner and outer diameters r, and r, and the height of the torus, h. (Note that is larger than / ,) As depicted in Fig. 1, if r, = r, and h r, and h = — r,) then the toroidal forms are of type... 

The Ba.C) N, nanotubes and fullerene-like structures have been synthesized by various laboratories in recent years. The most popular method is the plasma arc technique. The first report on the synthesis of BN nanotubes was by Zettl and coworkers (55). Since BN is an insulator, a composite anode was prepared from a tungsten rod with an empty bore in the center, which was stuffed with a pressed hexagonal BN powder. For the cathode, a water-cooled Cu rod was used. The collected gray soot contained limited amount of multi wall BN nanotubes. It is possible that in this case, the tungsten serves also as a catalyst. By perfecting this method, macroscopic amounts of double-wall BN nanotubes of a uniform diameter (2 nm)... 

In addition to the dumbbell-type structure 3, fullerene dimer structures having the two fullerene cages connected by a single bond and a methylene or an oxa bridge such as 7a and 7b are possible, and have been actually reported... 

The case where the complex isjust a closed (i.e., edgeless) surface 5 is a rather special case, involving all sites of trigonal hybridization. Significantly this case includes thefullerenes. But rather interestingly there are powerful fundamental mathematical results, applicable to a considerably richer diversity of possible structures than the usual fullerenes. 

Fig. 35. Possible structures for C59O and C59S ylide heterofullerenes or open-cage (thio)car-bonyl quasi-fullerenes...

NMR spectroscopy has been the most important method in structaral studies of newly isolated fullerenes. However, through the number of lines and their relative intensities, NMR spectrum gives at best only the symmetry of the fullerene molecule, but not the exact isomeric pattern. Therefore, it is necessary that all isomers with NMR-determined symmetry are considered as possible structural guesses. Quantum-chemical calculations and studies by other spectroscopic techniques can be then used to determine the most appropriate structure. 

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