Fullerenes crystal forms

Figure 3 Photograph (by transmitted Ught) of the fullerene crystals formed by gentle evaporation of the red/brown benzene extract from soot produced by the arc vaporization of graphite. (Reprinted with permission of MacmiUan from W. Kratschmer et al. )...

Fullerene C70 does not form a complex with the same calixarene in toluene, but it does so in benzene, crystallizing as the 2 1 complex, (C7o)2(p-Bu -calix[8]arene) (structme unknown).The same fullerene also forms a 2 1 complex with / -Bu -calix[6]arene, and its precipitation from toluene solutions can be used to retrieve 87% purity C70 from Ceo-depleted fullerite. 

The POAV pyramidalization angle of the carbon on the central five-membered ring (hub) in corannulene is 8.2°, in contrast to 11.6° for Ceo. The rim carbons are predicted to be only slightly pyramidalized with pyramidalization angles in the range of 1-2°. Unlike the crystal structures of pyrene and related aromatic compounds of comparable surface area, the crystal structure of 1 is void of any aromatic face-to-face or bowl stacking. However, a mixture of corannulene and fullerene Ceo forms cocrystals [l-Ceo] [66]. The shortest distances from Ceo to the concave surface of 1 and the convex surface of another 1 were determined to be 3.75 and 3.21 A, respectively. 

Fullerene crystals can be produced at high yield. By counter diffusion from fullerene solution to pure isopropyl alcohol solvent, fullerene single crystal fibers with needle shape were formed. Needle diameters were found to be 2-100 pm and their lengths were 0.15 = 5 mm. Buckyball-based sintered carbon materials can be transformed into polycrystalline diamonds at less severe conditions using powder metallurgy methods. 

Figure 1 presents the typical geometries of the nanodimensional fillers which are commonly used to modify the elastomeric matrix [5], Nanoparticles possess many shapes and sizes (Fig. 1), but primarily they have three simple geometric forms sphere, cylinder and plate type. Three-dimensional nanofillers (3D) are relatively equiaxed particles, smaller than 100 nm (often below 50 nm [6]), e.g. nano SiOa, Ti02. These nanoparticles are described in the Sects. 2.2-2.4. Sometimes in the literature, the term 3D nanofillers (spherical) is described as a zero-dimensional (OD) system, but actually OD nanofillers are represented by POSS molecules, fullerenes, crystals or quantum dots [6]. What s more, very often the term physical form of these nanoparticles is referred to as agglomerates . The dispersion of particles from agglomerates to nanoparticles seems to be a big challenge to all... 

Figure 1.13. Shuttlecock-shaped liquid crystal formed by incorporating fullerene C60 to various liquid crystals reported. ...

Calix[5]arenes encapsulate groups of fullerenes into their cone-shaped cavity [48-59]. Atwood, Barbour, Raston and coworkers have reported that calix[5]arene 3, toluene and the [60]fullerene molecule form co-crystals one with a 1 1 1 composition with the [60]fullerene molecules arranged in a supramolecular zigzag and the other with a 4 2 5 composition, giving rise to a Z-array of the [60]fullerene... 

Fullerenes are described in detail in Chapter 2 and therefore only a brief outline of their structure is presented here to provide a comparison with the other forms of carbon. The C o molecule, Buckminsterfullerene, was discovered in the mass spectrum of laser-ablated graphite in 1985 [37] and crystals of C o were fust isolated from soot formed from graphite arc electrodes in 1990 [38]. Although these events are relatively recent, the C o molecule has become one of the most widely-recognised molecular structures in science and in 1996 the codiscoverers Curl, Kroto and Smalley were awarded the Nobel prize for chemistry. Part of the appeal of this molecule lies in its beautiful icosahedral symmetry - a truncated icosahedron, or a molecular soccer ball, Fig. 4A. 

In addition to diamond and amorphous films, nanostructural forms of carbon may also be formed from the vapour phase. Here, stabilisation is achieved by the formation of closed shell structures that obviate the need for surface heteroatoms to stabilise danghng bonds, as is the case for bulk crystals of diamond and graphite. The now-classical example of closed-shell stabilisation of carbon nanostructures is the formation of C o molecules and other Fullerenes by electric arc evaporation of graphite [38] (Section 2.4). 

Since the structure and properties of fullerene solids are strongly dependent on the structure and properties of the constituent fullerene molecules, we first review the structure of the molecules, which is followed by a review of the structure of the molecular solids formed from Ceo, C70 and higher mass fullerenes, and finally the structure of Cgo crystals. 

Let us note in addition that the layered sulfides M0S2 and WS2 have been found to form nanotubes and other fullerene-type structures, on account of their highly folded and distorted nature that favors the formation of rag and tubular structures. Such materials have been synthesized by a variety of methods [78] and exhibit morphologies, which were described as inorganic fiillerenes (IF), single sheets, folded sheets, nanocrystals, and nested IFs (also known as onion crystals or Russian dolls ). 

Similarly, other cycloadducts of nitrile oxides with C6o were synthesized. The cycloadducts were characterized by 13C NMR spectroscopy and high-resolution fast atom bombardment (FAB) mass spectrometry. It should be mentioned that X-ray structure determination of the 3-(9-anthryl)-4,5-dihydroisoxazole derivative of C6o, with CS2 included in the crystals, was achieved at 173 K (255). Cycloaddition of fullerene C60 with the stable 2-(phenylsulfonyl)benzonitrile oxide was also studied (256). Fullerene formed with 2-PhSC>2C6H4CNO 1 1 and 1 2 adducts. The IR, NMR, and mass spectra of the adducts were examined. Di(isopropoxy)phosphorylformonitrile oxide gives mono- and diadducts with C60 (257). Structures of the adducts were studied using a combination of high performance liquid chromatography (HPLC), semiempirical PM3 calculations, and the dipole moments. 

Fullerenes are the third natural form of carbon. These have been found to exist in interstellar dust and in geological formations on Earth, but only in 1985 did Smalley, Kroto and co-workers discovered this class of carbon solids and their unusual properties [447, 448]. It has been shown that Ceo, the most common fullerene, could be transformed under high pressure into the other forms of carbon, diamond, and graphite [449] or, at moderately high pressures and temperatures, into new various metastable forms [450 53]. Ceo crystals, fullerites, have/cc structure with weak van der Waals interactions. This structure is stable at ambient temperature up to 20 GPa and at ambient pressure up to 1800 K [454, 455]. 

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