Graphite-like chains

An interesting variant of the formation of nanometer-sized conductive strands is pyrolysis of intrazeohte polymers such as polyacrylonitrile in zeolite NaY,the color of which turns from white to grey-black [136]. The electronic spectriun of the extracted material is practically structureless with a feature at about 350 nm and resembles that of graphite. In this way graphite-like chains with conductivity in the order of 10 S cm can be prepared. 

Another, simple form of elemental carbon would be chains formed from carbon atoms. As a prototype model a single>stranded chain is most suitable. If branching were to be considered, all intermediate forms up to and including the diamond and graphite like clusters would be included. For non branched chains, the two variants to choose from are a system of alternating singly and triply bonded carbon atoms (poly-ynes), and a system with all double bonds (cumulenes). Cumulene structures are assumed to be the preferred ones for odd membered chains, whereas the even ones may have some poly-yne character. Recent studies on linear Cg show that a cumulene-like structure is preferred, both at the SCF level and when correlation is accounted for(50). 

Boron carbide (B4C) is also an extremely hard, infusible, and inert substance, made by reduction of B203 with carbon in an electric furnace at 2500°C, and has a very unusual structure. The C atoms occur in linear chains of 3, and the boron atoms in icosahedral groups of 12 (as in crystalline boron itself). These two units are then packed together in a sodium chloride-like array. There are, of course, covalent bonds between C and B atoms as well as between B atoms in different icosahedra. A graphite-like boron carbide (BQ) has been made by interaction of benzene and BC13 at 800°C. 

The modifications of boron and boron compounds are characterized by nets or chains of boron atoms bound to each other by covalent bonds. The coordination number of the boron atoms varies between 1 and 7. The nets have the form of linked icosahedra, cuboctahedra, octahedra, graphite-like planes, or linear branched boron chains. Two-electron three-center bonds in boron-cornered triangles are the rule in the lattice structure. Boron can only form ions in combination with hydrogen (BH4 ) and oxygen (BOj ). The borates (boron oxide derivatives) form covalent nets like the silicates. They are often amorphous. 

The first nanofiber with a graphite-like-assembled structure was reported by Mullen and coworkers. HBC 14 with six dodecyl side chains, on drop-casting of its toluene solution (10 M), self-assembles via tt-stacking into several hundreds of micrometer long, birefringent microfibers with an average diameter on the order of submicrometers. By... 

A plausible nucleation mechanism could be the gradual formation of carbon structures For instance by formation of linear carbon chains which via the addition of other linear chains and the addition of small carbon radicals can grow to bear graphite-like polycyclic networks. 

Bamboo-shaped tubes. A carbon tube with a peculiar shape looking like bamboo, produced by the arc evaporation of nickel-loaded graphite, is shown in Fig. 8. The tube consists of a linear chain of hollow compartments that are spaced at nearly equal separation from 50 to 100 nm. The outer diameter of the bamboo tubes is about 40 nm, and the length typically several /im. One end of the tube is capped with a needle-shaped nickel particle which is in the normal fee phase, and the other end is empty. Walls of each compartment are made up by about 20 graphitic layers[34]. The shape of each compartment is quite similar to the needle-shape of the Ni particle at the tip, suggesting that the Ni particle was once at the cavities. 

The alternation of double and single bonds means that each C atom has an unhybridized /7-orbital that can overlap with the /7-orbital on either side. This arrangement allows electrons to be delocalized along the entire chain like a onedimensional version of graphite. 

The cohesive energy per carbon atom in a poly-yne ring is only 99.1 kcal/mol, clearly lower than the value in Cc. Anticipating a long and complicated route of formation when starting from graphite, in does not seem likely that any of the larger clusters observed experimentally would have a linear or cyclic chain structure. 

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