THE CARBON ALLOTROPES

We can understand the differences in properties between the carbon allotropes by comparing their structures. Graphite consists of planar sheets of sp2 hybridized carbon atoms in a hexagonal network (Fig. 14.29). Electrons are free to move from one carbon atom to another through a delocalized Tr-network formed by the overlap of unhybridized p-orbitals on each carbon atom. This network spreads across the entire plane. Because of the electron delocalization, graphite is a black, lustrous, electrically conducting solid indeed, graphite is used as an electrical conductor in industry and as electrodes in electrochemical cells and batteries. Its... 

Table 2 Restricted Hartree-Fock energies (Hartrees) for Ceo and C70 and their muon adducts. AE is the difference in energy between the carbon allotrope and its adduct. In all cases, except where indicated by f, only the six carbon atoms in the immediate vicinity of the muon have had there positions optimised, f means that a full geometry optimisation has been carried out. The type specifies the defect and for C70 is identified in Table 1. is the spin density at the muon in atomic units (and the hyperfine coupling constant in MHz). JMuon constrained to lie in equatorial plane. indicates geometry not fully optimized.

J. Osterodt, F. Vogtle, C6iBr2 A New Synthesis of Dibro-momethanofullerene and Mass Spectrometric Evidence of the Carbon Allotropes C121 and C122 , J. Chem. Soc., Chem Commun. 1996, 547-548. 

The different structures of the carbon allotropes lead to widely different properties. Because of its three-dimensional network of strong single bonds that tie all atoms in a crystal together, diamond is the hardest known substance. In addition to its use in jewelry, diamond is widely used industrially for the tips of saw blades... 

One may expect that future work on the electrochemistry of diamond should take two paths, namely, an extensive investigation (search for new processes and applications of the carbon allotropes in the electrochemical science and engineering) and intensive one (elucidation of the reaction mechanisms, revealing the effects of crystal structure and semiconductor properties on the electrochemical behavior of diamond and related materials). It is expected that better insight into these effects will result in the development of standard procedures for thin-film-electrodes growth, their characterization, and surface preparation. 

Single-Walled Carbon Nanotubes The carbon allotropes known as single-walled carbon nanotubes (SWNTs) are essentially one-atom-thick cylindrical rolls of graphite. SWNTs fluoresce in the range 900 to 1600 nm, a transparent region... 

It is conceivable that a parallel bundle of linear polyyne chains (the carbon allotrope named chaoite or carbyne) may form, via thermally-allowed (4 + 2)-n-electron cycloadditions, six-membered rings which lead to graphene sheets and/or nanotubes. However, the mechanism of nanotube formation in a plasma produced by arc discharge appears to require the presence of a transition metal atom at the rim of the growing nanotube [77 j. 

High Pressure Synthesis of the Carbon Allotrope Hexagonite with Carbon Nanotubes in a Diamond Anvil Cell... 

High Pressure Synthesis of the Carbon Allotrope Hexagonite... 

Diamond De (Fig. 11.1), the classical, beautiful and useful diamond has kept its leading interest among the carbon allotropes, even as the newer nano varieties. Along with electronic properties, the mechanical characteristics appear of great... 

The structural analogy with the carbon allotropes can be extended to the properties of the two borrm-nitride allotropes. For instance, hexagonal boron nitride (h-BN) is soft and lubricious like grsqihite, and cubic boron nitride (c-BN), like diamond, is extremely hard. 

Divalent carbon does indeed exist and is found in some highly reactive transient-organic intermediates such as the carbenes (for instance methylene). However, the carbon allotropes and the stable carbon compounds are not divalent but tetravalent, which means that four valence electrons are present.l l Howthis increase in valence electrons occurs is reviewed in Sec. 3.0. 

Diamond is one of the more recent of the carbon allotropes that has been examined as an electrode material. It exhibits several superior properties, including low background current, wide potential window, long-term stability, relative insensitivity towards the presence of dissolved oxygen in the solution, and biocompatibility [18-20]. Thus, diamond is becoming an interesting material to consider for electroanalysis. 

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