Sheets hexagonal

We are investigating the template formation of 2-D and 3-D metal-di-cyanamide anionic networks, for instance of type Mn(dca)3, by use of [M(N,N)3]n+ cations such as [M(2,2 -bipy)3]2+. A hexagonal sheet network was formed in [Fen(2,2 -bipy)3][Fen(dca)3]2 in which the cations fitted beautifully within the hexagonal windows. The cation remained LS between 4-300 K [66], Attempts to make the Con(2,2 -bipy)32+ analogue unfortunately led to dissociation of dca and bipy and formation of a zig-zag chain structure in the weakly-coupled HS complex [Con(dca)2(2,2 -bipy)2]n. The complex [Fen(propyl-tetrazole)6]2+, which has a very sharp SCO transition [42], unfortunately did not yield a network product. 

Everyone knew that there are precisely three forms for the element carbon amorphous carbon, as in charcoal crystalline graphite, which is packed in hexagonal sheets and crystalline diamond, which is packed in three-dimensional tetrahedral networks. 

A nanotube can be thought of as a hexagonal sheet of carbon atoms (graphene sheet), rolled up to make a cylinder and capped at the ends by a half of a buckyball, as illustrated in Figure 17.8. Tubes typically have diameters of about 1 nm. The diameter of the smallest nanotube corresponds to the diameter of the smallest buckyball (Ceo-) The length-to-diameter ratio is typically about 104. 

By decoration of these various infinite two-dimensional manifolds (just as the sphere has been decorated with closed networks) several related structures have been proposed for graphite nets. These are mostly based on the P, D and G surfaces (the first two due to Schwarz (1890) and the last, the gyroid, discovered by Schoen (1970). However, many other surfaces (perhaps 50) are available for consideration. Some fit naturally with hexagonal sheets and others with sheets of square or lower symmetry. In general, the P, D and G surfaces are the least curved from planarity. Surfaces parallel to the surfaces of zero mean curvature have lower symmetry than those with H = 0. When decorated with graphite nets the symmetry may be further lowered to that of a sub-group of the symmetry group of the surface itself. 

Figure 12.9. Hexagonal sheets in the crystal structure of the acetone solvate of 16,...

Fig. 13a—d Solid state structural types formed by biimidazolate complexes a dimers b linear tapes c zigzag tapes d hexagonal sheets [111]... 

Figure 2. Schematic diagram of the hexagonal sheet of graphite. Carbon atoms are at the vertices. The parameters that define the nanotube structure when the sheet is rolled (chiral angle, chiral vector, basis vectors ai and a2) are indicated in the figure. From reference 12.

The second polymorph, polymorph B, can be considered as homeotypic with CaCuP (AlB2-type) structure. The structure consists of undulating hexagonal sheets. Five such nets interpenetrate to file the void space in a single net. see Venkataraman, D. Lee, S. Moore, J. S. Zhang, P. Hirsch, K. A. Gardner, G. B. Covey, A. C. Prentice, C. L. Submitted for publication to Chemistry of Materials. 

Figure 1.12.4. Graphite is covaiently bonded carbon in hexagonal sheets.

FIGURE 1.8. The crystal structure of graphite. This consists of hexagonal sheets of carbon atoms with weak forces between the sheets. Note that the arrangement of the carbon atoms in benzene (black bonds) is a portion of this structure. 

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