Bonds in fullerenes

Figure 13.1 The three [6,6]-bonds in fullerenes connecting the C atoms with the highest degree of pyramidalization.

Stone and Wales examined rotation of C-C bonds in various fullerene structures using approximate Huckel calculations. The 90° rotation of C-C bond in fullerene is called Stone-Wales (SW) or pyracylene rearrangement (O Fig. 22-15) (Stone and Wales 1986). Austin et al. reported that 94% of all fullerene Ceo isomers can rearrange to Buckminsterfullerene by SW transformation (Austin et al. 1995). The C78 cage represents the smallest fullerene in which SW rearrangement can give stable IPR isomers C7s 5 (D3/,) <- C7s 3 (C2v) Cjr.l (C2v) C7s 4 (D3 ,) where... 

Figure 15.12.1. Synthesis of ferrocene/fullerene hybrid molecule (2). The single and double bonds in fullerene are only formal, as for fullerene itself. (Reproduced by permission from the principal author E. Nakamura, and from the American Chemical Society.)...

In other materials synthesis applications, the utilization of the strong bonding of fullerenes to clean silicon surfaces, has led to the application of a monolayer... 

Chapter 1 contains a review of carbon materials, and emphasizes the stmeture and chemical bonding in the various forms of carbon, including the foui" allotropes diamond, graphite, carbynes, and the fullerenes. In addition, amorphous carbon and diamond fihns, carbon nanoparticles, and engineered carbons are discussed. The most recently discovered allotrope of carbon, i.e., the fullerenes, along with carbon nanotubes, are more fully discussed in Chapter 2, where their structure-property relations are reviewed in the context of advanced technologies for carbon based materials. The synthesis, structure, and properties of the fullerenes and... 

The elimination of the energetic dangling bonds present at the edges of a tiny graphite sheet is supposed to be the driving force to induce curvature and closure in fullerenes this phenomenon is also associated with the formation of larger systems, such as nanotubes and graphitic particles. 

Recently, there has been considerable interest in fullerene derivatives. Finding the lowest energy isomer among a variety of choices of attachment is always an interesting and important question. In C50, all carbons are equivalent, but there are two types of C-C bonds ... 

Golden MS, Pichler T, Rudolf P (2004) Charge Transfer and Bonding in Endohedral Fullerenes from High-Energy Spectroscopy 109 201-229 Goodin DB, see Contakes SM (2007) 123 177-203 Gorelesky SI, Lever ABP (2004) 107 77-114... 

Figure 3.3 Bonding structures for different carbon materials (a) diamond, (b) graphite, (c) carbon nanotubes and (d) fullerenes. Scheme of the pyramidalization angle (0p) in deformed sp bonding in comparison with a trigonal structure.

Since there are 30 double bonds to react in fullerene Cgo, the [2-1-2] cycloaddition of Cgo molecules at these bonds results in the formation of so-called fullerene polymers [19]. Although it seemed important to clarify the structure and properties of the most basic unit of these polymers such as dimers and trimers, the method for preparation of these polymers such as high-pres-sure/high-temperature treatment or photoirradiation was not suitable to stop the [2-1-2] reaction at the stage of dimerization or trimerization. 

Figure 1.17 Schematic representation of [60-/h]fullerene with the lengths of the two different bonds in the moiecule and Schlegel diagram of the lowest energy Kekul structure.

The number of possible fullerene isomers without any constraints reduces considerably if only I PR structures are allowed. Taking into account that open-shell structures are avoided [233, 248,249] and that the number of double bonds in pentagons is minimized [250], which favors a meta over a para relationship of the pentagons (Figure 1.19) [251], the number of allowed isomers is further reduced. 

The NMR spectra of C qHR exhibits 37 resonances of the fuUerene carbons, with two of them in the sp region, proving Q-symmetry for the C oHR adducts. This is consistent with an addition to a double bond of a pole corannulene imit (1,2-addition), leading to l,2-dihydro[70]fullerene derivatives. These particular [6,6] bonds of Cyg, located between the carbons of the sets A and B, have almost the same bond length as the [6,6] bonds in Cjq [30], and the pole corannulene unit also exhibits bond alternation with longer [5,6] bonds. NMR data imply that the initial attack of the nucleophile occurred on C-1 and the protonation on C-2. 

The chiral fullerene C75 was also asymmetrically osmylated using the chiral ligands and (Scheme 8.8) [63]. In this way an optically active allotrope of a pure element was prepared. C75 contains 15 different types of [6,6] bonds. The pronounced regioselectivity of C7Q towards osmylation [58] suggests that specific bonds in C75 may be favored for an attack by OSO4. An analysis of the ab initio calculated curvature of 75 shows that two of the five pyracylene-type bonds are particularly distorted, which could enhance their reactivity [64]. Indeed HPLC analysis of C75[Os04L ] shows that two regioisomers are predominantly formed upon osmylation of C75 [63]. 

Cycloadditions to [6,6]-double bonds of Cjq are among the most important reactions in fullerene chemistry. For a second attack to a [6,6]-bond of a C q monoadduct nine different sites are available (Figure 10.1). For bisadducts with different but symmetrical addends nine regioisomeric bisadducts are, in principle, possible. If only one type of symmetrical addends is allowed, eight different regioisomers can be considered, since attack to both e - and e"-positions leads to the same product. Two successive cycloadditions mostly represent the fundamental case and form the basis for the regioselectivity of multiple additions. In a comprehensive study of bisadduct formations with two identical as well as with two different addends, nucleophilic cyclopropanations, Bamford-Stevens reactions with dimethoxybenzo-phenone-tosylhydrazone and nitrene additions have been analyzed in detail (Scheme 10.1) [3, 9, 10]. 

The possible opening and closure of transannular cis-1 bonds in bisimino-bridged fullerenes such as 6 and 7 is governed by the following factors [17, 18] ... 

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