Covalent graphite

Like the covalent graphite compounds, the intercalation compounds are formed by the insertion of a foreign material into the host lattice. The structure however is different as the bond, instead of being covalent, is a charge-transfer interaction. This electronic interaction results in a considerable increase in electrical conductivity in the ab directions. 

The wave function T i oo ( = 11 / = 0, w = 0) corresponds to a spherical electronic distribution around the nucleus and is an example of an s orbital. Solutions of other wave functions may be described in terms of p and d orbitals, atomic radii Half the closest distance of approach of atoms in the structure of the elements. This is easily defined for regular structures, e.g. close-packed metals, but is less easy to define in elements with irregular structures, e.g. As. The values may differ between allo-tropes (e.g. C-C 1 -54 A in diamond and 1 -42 A in planes of graphite). Atomic radii are very different from ionic and covalent radii. 

Boron nitride is chemically unreactive, and can be melted at 3000 K by heating under pressure. It is a covalent compound, but the lack of volatility is due to the formation of giant molecules as in graphite or diamond (p. 163). The bond B—N is isoelectronic with C—C. 

Pure carbon occurs naturally in two modifications, diamond and graphite. In both these forms the carbon atoms are linked by covalent bonds to give giant molecules (Figure S.2). 

The covalent compounds of graphite differ markedly from the crystal compounds. They are white or lightly colored electrical insulators, have Hi-defined formulas and occur in but one form, unlike the series typical of the crystal compounds. In the covalent compounds, the carbon network is deformed and the carbon atoms rearrange tetrahedraHy as in diamond. Often they are formed with explosive violence. 

Nitrophenyl groups covalently bonded to classy carbon and graphite surfaces have been detected and characterized by unenhanced Raman spectroscopy in combination with voltammetry and XPS [4.292]. Difference spectra from glassy carbon with and without nitrophenyl modification contained several Raman bands from the nitrophenyl group with a comparatively large signal-to-noise ratio (Fig. 4.58). Electrochemical modification of the adsorbed monolayer was observed spectrally, because this led to clear changes in the Raman spectrum. 

Several nonmetallic elements and metalloids have a network covalent structure. The most important of these is carbon, which has two different crystalline forms of the network covalent type. Both graphite and diamond have high melting points, above 3500°C. However, the bonding patterns in the two solids are quite different... 

Graphite, the most important component of the lead of pencils, is a black, lustrous, electrically conducting solid that vaporizes at 1700°C. It consists of flat sheets of sp2 hybridized carbon atoms bonded covalently into hexagons like chicken wire (Fig. 5.22). There are also weak bonds between the sheets. In the commercially available forms of graphite, there are many impurity atoms trapped between the sheets these atoms weaken the already weak intersheet bonds and let... 

In 1975, the fabrication of a chiral electrode by permanent attachment of amino acid residues to pendant groups on a graphite surface was reported At the same time, stimulated by the development of bonded phases on silica and aluminia surfaces the first example of derivatized metal surfaces for use as chemically modified electrodes was presented. A silanization technique was used for covalently binding redox species to hydroxy groups of SnOj or Pt surfaces. Before that time, some successful attemps to create electrode surfaces with deliberate chemical properties made use of specific adsorption techniques... 

Covalent compounds, arising from the attack of strong oxidizing systems, such as fluorine or Mn(VII), on graphite. The aromatic planarity of the graphite sheet is destroyed, and a buckled, sp -hybridized sheet is created. 

Although the covalent compounds of graphite are thus important in their own right, they represent the extreme form of oxidative intercalation. The use of fluoride compounds to achieve highly conductive materials may ultimately lead to new forms of graphite fluoride SI). 

Infrared spectra and F-NMR spectroscopy showed the presence of IF5 and covalently bonded fluorine. Grafoil turns white upon intercalation with IF, this is reminiscent of graphite fluoride, CFi.ij (1,6). The IF, intercalate also evolves IF5 upon heating, but at much higher temperatures than C/IF5 this has been attributed to the lowered mobility of IF5 in the fluorinated matrix, which may no longer be planar. At 450°C, considerable amounts of fluorocarbons are evolved. 

---