Other molecular allotropes of carbon

Quite apart from the fullerene cluster molecules, numerous other molecular allotropes of carbon, C , have been discovered in the gases formed by the laser vaporization/supersonic expansion of graphite. The products are detected by mass 

IzuoKA, T, Tachikawa, T, Sugawara, Y, Suzuki, M, Kono, Y. Saito and H. Sinohara, J. Chem. Soc., Chem. Commun., 1472-3 (1992). 

The tremendous burst of excitement which attended the initial isolation in 1990 of weighable amounts of separated fullerenes has been followed by an unparalleled and sustained surge of activity as chemists throughout the world rushed to investigate the chemical reactivity of these novel molecular forms of carbon. 

Schwarz, Angew. Chem. Int. Edn. Engl. 32, 1412-5 (1993). R. M. Baum, Chem. Eng. News, May 17, 32-4 (1993) and references cited therein. 

Fullerene epoxide, C )0, is formed by the UV irradiation of an oxygenated benzene solution of Cfio The O atom bridges a 6 6 bond of the closed fullerene structure. The same compound is also formed as one of the products of the reaction of Cgo with dimethyldioxirane, Mc2COO (see later).  

Considerable attention has been paid to possible mechanisms of formation since a firm understanding of this aspect could lead to the development of more effective synthetic routes to the individual fullerenes. It is also known that, when thin films of Cgo and C70 are laser-vaporized into a rapid stream of an inert gas, individual molecules of Ceo or C70 can themselves coalesce to form stable larger fullerenes such as Cno or C140, and higher multiples. Even more dramatically, when a sample of C o is subjected to a pressure of 20 GPa (i.e. 200 kbar), it apparently immediately transforms into polycrystalline diamond. 

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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) . ... 

This chapter discusses the use of carbon-based electrode materials in the construction of MJs and the use of carbon-based materials in related studies (such as electrochemical experiments and in the construction of other electronic devices). The methods for making MJs are first outlined, followed by the use of the more novel allotropes of carbon. These materials have interesting electronic properties that provide additional opportunities for their application in molecular electronics relative to more conventional carbon materials. Finally, some ofthe considerations that dictate charge transport across molecular layers in MJs are discussed before we leave with some future prospects. 

Covalent-network solids consist of atoms held together in large networks or chains by covalent bonds. Because covalent bonds are much stronger than intermolecular forces, these solids are much harder and have higher melting points titan molecular solids. Diamond and graphite, two allotropes of carbon, are covalent-network solids. Other examples include quartz, Si02 silicon carbide, SiC and boron nitride, BN. 

The recent discovery of a family of large, solid cait)on molecules with great stability, the so-called "fullerenes , has considerably extended the scope and variety of carbon molecules known to exist and is opening an entirely new chapter on the physics and chemistry of carbon, with many potential applications. The fullerenes can be considered as another major allotrope of carbon and its first stable, finite, discrete molecularform. They are different, in that respect, from the other two allotropes, graphite and diamond, which eire not molecular but infinite-network solids. The other known carbon molecules, C2 to are unstable and found only in the vapor... 

Carbon is the basis of organic chemistry there are more compounds of carbon than of any other element except hydrogen and possibly fluorine. However, most of the chemistry of carbon is the province of organic chemistry and thus not covered in this encyclopedia. The inorganic chemistry of carbon discussed in this article, which is an update of an excellent article written previously by professor R. Bruce King (University of Georgia, Athens), includes the allotropic forms of elemental carbon, simple molecular carbon halides and oxides, carbon oxyacids and oxyanions, carbon snlfur derivatives, simple cyano derivatives, and carbon-based molecnlar ladders. 

The difference between the forms involves either (1) crystalline structure (2) the number of atoms in the molecule of a gas or (3) the molecular structure of a liquid. Carbon is a common example of (1), occurring in several crystal forms (diamond, carbon black, graphite) as well as several amorphous forms. Diatomic oxygen and diatomic ozone are instances of (2) and liquid sulfur and helium of (3). Uranium has three crystalline forms, manganese four, and plutonium no less than six. A number of other metals also have several allotropic forms which are often designated by Greek letters, e.g., a-, y-, and A-iron. 

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