Carbon hexagons

There is an infinite number of possible atomic structures of graphene tubules. Each structure is characterized by its diameter and the helical arrangement of the carbon hexagons. Presumably, only single-shell tubes with small diameters of about 10 A are formed and tubes with larger diameters are multishell tubes. 

Its structure resembles that of graphite, but the latter s flat planes of carbon hexagons are replaced in boron nitride by planes of hexagons of alternating B and N atoms (Fig. 14.27). Unlike graphite, boron nitride is white and does not conduct... 

The vibrational entropy is related to a disorder originated from the Li vibrational motion about the equilibrium position at the center between two adjacent carbon hexagons. Such a motion can be decomposed in two... 

The structure of carbon nanotubes depends upon the orientation of the hexagons in the cylinder with respect to the tubule axis. The limiting orientations are zigzag and arm chair forms, Fig. 8B. In between there are a number of chiral forms in which the carbon hexagons are oriented along a screw axis, Fig. 8B. The formal topology of these nanotube structures has been described [89]. Carbon nanotubes have attracted a lot of interest because they are essentially onedimensional periodic structures with electronic properties (metallic or semiconducting) that depend upon their diameter and chirality [90,91]. (Note. After this section was written a book devoted to carbon nanotubes has been published [92], see also [58].)... 

Its structure resembles that of graphite, but the latter s flat planes of carbon hexagons are replaced in boron nitride by planes of hexagons of alternating B and N atoms (Fig. 14.31). Unlike graphite, it is white and does not conduct electricity. Under high pressure, boron nitride is converted to a very hard, diamondlike crystalline form called Borazon. In recent years, boron nitride nanotubes similar to those formed by carbon have been synthesized (Section 14.18), and they have been found to be semiconducting (see Box 14.2). 

Elastic neutron diffraction was first performed (analyzer in Fig. 1 set to zero energy transfer) to establish the structure of the monolayer at low temperature. Three Bragg reflections were observed which could be indexed by a triangular lattice having a nearest-neighbor distance about 10% smaller than required for a 3 X /3 R30° commensurate structure (every third carbon hexagon in the graphite basal plane occupied). 

The component circles, by definition, have the same radius as the inscribed circle of the carbon hexagon, i.e. r = 1.25A- somewhat smaller than the previously calculated o-a-m circle of ethylene. Using r = 1.25A, the recalculated absorption wavelength, A = 146nm, is within the range (145 - 180 nm) of the first intense absorption band of ethylene in the vacuum UV [85]. 

We used to believe that there are three allotropic forms of carbon graphite, diamond, and amorphous carbon. However, an important new carbon al-lotrope, the fullerenes, was discovered as recently as the 1980s. The most famous fullerene is buckminsterfullerene, Ceo, which is depicted in Fig. 3.1. The structure of this soccer ball-shaped molecule consists of a sphere of sixty carbon atoms arranged in pentagons and hexagons each carbon pentagon is surrounded by five carbon hexagons. 

Fig. 9 Structure of C. AsPs. Requirement of staggering of carbon iayers imposed by the nestiing of AsF anions. Soiid- and dashed-line fluorine atoms, respectively, are nestled in solid- and dashed-line carbon hexagonal nets...

The network of carbon hexagons remains, with a practically unaltered C—C distance, as in graphite. There is, however, a difference from the graphite structure in that the carbon planes are in identical positions one above the other. Their distance apart is 5.34 A for preparations with... 

Introduction of the metal layer leads, for all the stages, to the same increase in the distance between the adjacent carbon planes, as may be seen from the identity periods for potassium-graphite (see Table I). In addition the carbon planes next to the metal layers always have identical positions. Thus entry of the alkali metal into the lattice is linked not only with an expansion lattice but also with a lateral displacement of the carbon planes. Metal atoms are then able to arrange themselves so that they lie over and under the carbon hexagons and are surrounded symmetrically by twelve carbon atoms. Figure 8 shows the sequence of... 

The majority of the reactions of benzene are substitution reactions and not. as might be expected, addition reactions. The reason is that the continuous cloud of electrons above and below the carbon hexagon is very stable and it takes energy to break it. The preferred reaction is to replace a hydrogen atom so that the delocalised ring structure is kept intact. This is best achieved by substitution reactions. Addition across the double bonds would destroy the delocalised electron cloud of the ring. These addition reactions are not very common for benzene and similar compounds, although they are possible. 

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