Fullerene-diamond compounds

The state of research on the two classes of acetylenic compounds described in this article, the cyclo[ ]carbons and tetraethynylethene derivatives, differs drastically. The synthesis of bulk quantities of a cyclocarbon remains a fascinating challenge in view of the expected instability of these compounds. These compounds would represent a fourth allotropic form of carbon, in addition to diamond, graphite, and the fullerenes. The full spectral characterization of macroscopic quantities of cyclo-C should provide a unique experimental calibration for the power of theoretical predictions dealing with the electronic and structural properties of conjugated n-chromophores of substantial size and number of heavy atoms. We believe that access to bulk cyclocarbon quantities will eventually be accomplished by controlled thermal or photochemical cycloreversion reactions of structurally defined, stable precursor molecules similar to those described in this review. 

Not too long ago, graphite and diamond were the only two known modifications of carbon. That changed dramatically with the discovery of in 1985 and all the higher fullerenes soon thereafter. Nevertheless, this breakthrough did not stand alone in paving the way to the new era of chemical and physical research into carbon rich compounds that we are now enjoying. 

Even veteran elements can still present surprises. Many elements exist as different allotropes. This means that the atoms are arranged differently. In the case of carbon, the amorphous (soot), the dull gray graphite, and the brilliant diamond forms were known. It was therefore a great surprise when a new form was discovered in 1982 the fullerenes opened up a completely new area of chemistry. Hence it is not too far-fetching to deduce that further secrets lie buried in the elements, not to mention their compounds. 

Graphite and diamond are network solids that are insoluble in all liquid solvents except some liquid metals. Flowever, the fullerenes, which are molecular, can be dissolved by suitable solvents (such as benzene) buckminsterfullerene itself forms a red-brown solution. Fullerite currently has few uses, but some of the compounds of the fullerenes have great promise. For example, K3C60 is a superconductor below 18 K, and other compounds appear to be active against cancer and diseases such as AIDS. 

From the perspective of structural chemistry, the modes of bonding, coordination, and the bond parameters of a particular element in its allotropic modifications may be further extended to its compounds. Thus organic compounds can be conveniently divided into three families that originate from their prototypes aliphatic compounds from diamond, aromatic compounds from graphite, and fullerenic compounds from fullerenes. 

A characteristic of the DOS of a metallic element is its large magnitude in the vicinity of the Fermi level. This feature, above and below the Fermi level, is associated with the highly delocalized character of the metallic bonding. However, most solid-state compounds are not metallic. Hence, we consider now the examples of graphite and diamond, two allotropic forms of elemental C. The chemically bonded network of the former is two-dimensional and that of the latter is three-dimensional. In Section 6.2.4 we presented the band stmcture of a hypothetical one-dimensional allotropic form of C. Zero-dimensional (molecular) forms do exist also these are the fullerenes such as C6o which was mentioned in Chapter 2 and will again be discussed in Chapter 7. C nanotubes are intermediate between molecules and macroscopic solids and also will be considered further in Chapter 7. 

Carbon is the basis of all life on earth, and without a doubt, one of the most versatile elements known to man. More than ten million carbon compounds are known today, many times more than that of any other element. Carbon itself exists in several allotropes. Its flexible electron configuration allows carbon to form three hybridization states which lead to different types of covalent bonding. The most representative macroscopic forms of carbon are graphite and diamond. In 1985, Kroto et al. discovered a third carbon allotrope, the fullerenes. While their experiments aimed at understanding the mechanisms by which long chained carbon molecules are formed in interstellar space, their results opened a new era in science - the beginning of nanotechnology. 

Carbon atoms crystallize in several forms. Graphite and diamond are well known carbon polymorphs. Fullerenes, which were discovered in the 1980 s, have also been well characterized. Carbon materials show a variety of different physical and chemical properties. Because of this the electronic structure of carbon materials has been investigated using a number of different experimental techniques, for example, XPS, UPS and XANES. Theoretical studies of carbon materials have been also performed. However, experimentally observed spectra are not always consistent with theoretical predictions. Recently, in order to understand the various kinds of observed electronic spectra, DV-Xa calculations have been performed on a small cluster model. [1] In the present paper, we report results of DV-Xa calculations performed on the carbon materials graphite, alkali graphite intercalation compounds (GIC), fullerene, and fluorinated fullerenes. 

Why are there so many organic compounds There are several reasons. First, carbon can form stable, covalent bonds with other carbon atoms. Consider three of the allotropic forms of elemental carbon graphite, diamond, and buckminster-fullerene. Models of these allotropes are shown in Figure 11.1. 

A larger number of compounds with linked and discrete homoatomic clusters are known for the group-14 elements. The structures of the different modifieations of the elements themselves are indicative of a tendency to form clusters. For carbon, several crystalline modifications are known the polymeric diamond structure, the two-dimensional structure of graphite, and the isolated units of the fullerenes. There are two modifications of Si-Sn - the a or the diamond structure in which all atoms are four-connected and the metallic fi-iin structure with coordination number six (four shorter and two longer distances). A eoordination number greater than... 

The MO model of C2 predicts a doubly bonded molecule, with all electrons paired, but with both highest occupied molecular orhitals (HOMOs) having tt symmetry. C2 is unusual because it has two tt bonds and no cr bond. Although C2 is a rarely encountered allotrope of carbon (carbon is significantly more stable as diamond, graphite, fullerenes and other polyatomic forms described in Chapter 8), the acetylide ion, C2 , is well known, particularly in compounds with alkali metals, alkaline earths, and lanthanides. According to the molecular orbital model, 2 should have a bond order of 3 (configuration TT TT a-g ). This is supported by the similar C—C distances in acetylene and calcium carbide (acetylide) . ... 

Graphite paint n. A painting compound consisting of powdered graphite and oil used to coat metallic structures to inhibit corrosion. Pierson HO (1994) Handbook of carbon, graphite, diamond and fullerenes. Noyes Data Corporation/Noyes Corporation, New York. 

Carbon is the atom of the periodic table that forms the greatest number of compounds. Pure carbon may also form large molecules and crystallize into a number of allotropic forms, such as 35, C o, and C70 from the large fullerene family, nano tubes (a tube-like form of a fullerene), graphite, and diamond. These substances have widely different conductivity properties. 

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