Fullerenes of Buckminster Fuller - Essay Example

FULLENERES Fullerenes are large, all-carbon, cage-like molecules, d after the American architect of the geodesic dome structure, BuckminsterFuller (Andreoni, 2000). The molecules containing 60 and 70 carbon atoms are the most abundant among the fullerenes. Because of their extraordinary properties, fullerenes promise to launch an array of new compounds, with possible uses in electronics, polymers, catalysts, diamond film production, optical sensors and pharmaceuticals (Da Ros et al, 1999). Fullerenes are presently produced commercially in two steps, both of which lead to high production costs. The first step consists of the production of fullerene-bearing soot by the arc-vaporization of graphite and the second step involves the purification of fullerenes by column chromatography. When they were first discovered in 1985, fullerenes were described as the third allotrope of carbon (Andreoni, 2000), the other two well-known forms of carbon being graphite and diamond. From the chemical perspective, each carbon form has a characteristic molecular geometry which is responsible for the unique properties of each allotrope. Graphite, is an all-carbon material consisting of a two-dimensional network of fused six-membered rings, similar to benzene rings, as illustrated in Figure 1 (see Appendix). The individual carbon atoms in graphite are held together by rigid sigma, a, bonds, giving graphite its overall flat structure. However, graphite, like other aromatic compounds, also possesses pi, π, electrons, which are shared by neighbouring carbon atoms (Hirsch and Brettreich, 2005). This type of bonding configuration is known as sp bonding. The layers in graphite are held together by Van der Waals forces, thus the energy required to slide the layers over one another is low. This feature makes graphite a soft material which can be used as a lubricant. Graphite is also a good conductor of heat and electricity. Simultaneously, diamond constitutes a crystalline material having sp3 bonding configuration, resulting in a regular three-dimensional network of ō –bonds (Hirsch and Brettreich, 2005). The structure of diamond can be seen in Figure 2 (see Appendix). The configuration of the carbon atoms in diamond provides a rigid, stable structure resulting in a material which possesses high strength, hardness and resistance to chemical attack. Diamond also exhibits high heat conductivity and excellent electrical insulation. In 1985, a new class of carbon compounds, the fullerenes, was welcomed into the history of carbon science. Fullerenes are essentially hollow carbon caged molecules which may contain as few as 32 and as many as 600 carbon atoms (Andreoni, 2000). The first of such molecules to be discovered was carbon sixty, C60, named buckminsterfullerene, after the geodesic domes of the famous American architect and engineer R. Buckminster Fuller (Andreoni, 2000). The molecules containing 60 and 70 carbons, shown in Figure 3, have received the most attention due to their higher abundance and relative ease of isolation. Support for the shape and structure of C60 and C70 has been given mostly by mass spectroscopy and 13C nuclear magnetic resonance (Dresselhaus et al, 1996). C60 has a shape reminiscent of a soccer ball. The carbon atoms of C60 are arranged in an array of 12 pentagons and 20 hexagons; each atom existing at the juncture of two six-membered rings and one five-membered ring. C60 has a diameter of about 7A (Dresselhaus et al, 1996), while C70 has an oblong shape which is reminiscent of a rugby ball and has dimensions of 7A x 7A x 9 A (Dresselhaus et al, 1996). Fullerenes have been described as being halfway, structurally, between diamond and graphite. The bonding of fullerenes is neither sp2, like graphite, nor sp3 like diamond, but an intermediate between these two hybridizations. As a result, this characteristic would strongly affect chemical reactivity, thermal and electrical conductivity, superconductivity and optical properties of fullerenes. Larger fullerenes have also been isolated and characterized. These include C76, C, C78 and C82, C84, C90 and C96 (Diederich and Whetten, 1992) and are illustrated in Figure 4 (see Appendix). Furthermore, microtubules comprising of graphite sheets terminated with fullerene end caps have also been discovered and are also shown in Figure 4 (Diederich and Whetten, 1992). These microtubules are often referred to as buckytubes. Due to the higher relative abundance of C60 most work on characterization of fullerenes has been done on C60. Solid C60 is a molecular solid held together by van der Waals forces. Because of the weak intermolecular forces, C60 solids are highly compressible (Taylor, 1999), although the molecular cage structure itself is highly incompressible (Taylor, 1999). The bulk modulus has been measured to be 14-18 GPa (Andreoni, 2000; Hirsch and Brettreich, 2005), a factor of 40 smaller than diamond. The Vickers hardness has also been measured to be 14.5-17.5 kg/mm2 compared to 7,000-10,000 kg/mm2 for diamond (Hirsch and Brettreich, 2005). Because diamond and graphite are infinite lattices, the breaking of covalent bonds is required to release atoms or molecules. By contrast, fullerene crystals consist of molecular solids held together by van de Waals forces. As a result, dissolution and evaporation processes are simplified since they do not require the cleavage of covalent carbon-carbon bonds to release molecules. Thus, fullerenes are soluble in organic solvents and have a relatively high vapour pressure for a pure carbon material. The hybrid bonding pattern of fullerenes is partially responsible for the reactivity of C60 (Taylor, 1999). C60 has been modified to give halogenated materials, alkylated C60, C60 epoxide, C60 ethers and other interesting compounds. Researchers have also studied the oxidative behaviour of fullerenes compared with graphite and diamond. In an atmosphere of air, fullerenes decomposed at 444 °C, whereas the decomposition of diamond and graphite occurred at 629 °C and 644°C, respectively (Hirsch and Brettreich, 2005). Other researchers have found that in the presence of oxygen, C60 powder decomposes at 200 °C into an amorphous carbon-oxygen compound (Hirsch and Brettreich, 2005). In the bulk solid, fullerenes are black, but thin films are coloured. C60 and C70 thin films are a yellow and red, respectively. Solutions of fullerenes in organic solvents are also coloured, for instance magenta (C60), wine red (C70), greenish yellow (C76), brown (C78) and greenish yellow (C84) (Hirsch and Brettreich, 2005; Andreoni, 2000). REFERENCES Andreoni, W. (2000). The physics of fullerene-based and fullerene-related materials, New York: Springer Da Ros T., M Prato, Da Ros T. (1999). Medicinal chemistry with fullerenes and fullerene derivatives. Journal of the Chemical Society, Chemical Communications Diederich F., R. L. Whetten. (1992). Beyond C60; The higher fullerenes. Accounts of Chemical Research. Vol. 25, Iss. 3; 119-126 Dresselhaus M. S., G. Dresselhaus, P. C. Eklund. (1996). Science of fullerenes and carbon nanotubes, California, USA: Academic Press Hirsch, A., Brettreich, M. (2005). Fullerenes , Weinheim: Wiley-VCH Taylor, R. (1999). Lecture Notes on Fullerene Chemistry, London: Imperial College Press APPENDIX Figure 1. Structure of Graphite Figure 2. Structure of Diamond Figure 3. Structure of C60 and C70 Figure 4. Structures of higher fullerenes and buckytubes. C78 Isomers C84 and buckytubes Read More