Carbon polyhedron

CNTs can be chemically functionalized to achieve good dispersion in polymer/ CNT composites and strong interface adhesion (Gao et al., 2004). CNTs can be assembled as ropes or bundles, and there are some catalyst residuals, bucky onions, spheroidal fullerenes, amorphous carbon, polyhedron graphite nanoparticles, and other forms of impurities during the growth process of CNTs. 

The first publication of which I am aware that postulates the existence of a truncated icosahedral carbon cluster is a book by Yoshida and Osawa (70) that was published in 1971. The first Hiickel calculation for BMF-60 was carried out by Bochvar and Gal pern (71) and reported in 1973. This work is amplified by additional examples of carbon polyhedrons that were given in a review (72) published in 1984 on existing and hypothetical crystalline modifications of carbon. Hiickel calculations employing a graphical theoretical approach were performed by Davidson (73) for several polyhedrons, including BMF-60, and were reported in 1981. 

Carbon tends to adopt the position of lowest coordination number on the polyhedron and to keep as far from other C atoms as possible (i.e. the most stable isomer has the greatest number of B-C connections). 

Metal-metal bonds and metal-carbon bonds in the chemistry of molybdenum and tungsten alkox-ides. M. H. Chisholm, Polyhedron, 1983, 2, 681-721 (90). 

Furthermore, protonation results in a significant distortion of the coordination polyhedron, i.e., the metal ion is displaced from the plane formed by the four cyano ligand carbon atoms toward the oxo along the M = 0 axis by as much as 0.34 A, which represents about 20% of the total metal-oxo bond length. In spite of this distortion stronger metal-cyano bonds are observed crystallographically, suggesting a better n back-donation by the metal center to the cyano carbons since d-ff overlap is increased. This observation is in line with both the 13C and 15N chemical shift and kinetic data (Section V) for the protonated complexes (8). 

Stable carbon clusters look like polyhedrons where each carbon atom is three-coordinated. 

A conglomerate of three hexagons contains one central atom and 12 atoms around it. A conglomerate of seven hexahedrons comprises 12 external and 12 internal (common) atoms. In these two cases geometric centers of hybridized molecular orbitals of each hexahedron are equidistant from such nearest centers of a conglomerate. This, apparently, explains the experimental fact that polyhedrons of carbon clusters represent an icosahedron - 12-apex crystalline structure each apex of which is connected with five other apexes. 

Wade electron counting rules borane-like cluster nomenclature. On initially studying compounds such as boranes (boron hydrides) and carboranes (or carbaboranes boron—carbon hydrides), Wade (1976) proposed a number of rules which have then been extended to several compounds and which relate the number of skeletal electrons with the structure of deltahedral clusters. A polyhedron which has only A-shaped, that is triangular, faces is also called a deltahedron. 

Buckminsterfullerene, an allotrope of carbon, is topologically equivalent to a truncated icosahedron, an Archimedean solid that possesses 12 pentagons and 20 hexagons (Fig. 9-16). [17] Each carbon atom of this fullerene corresponds to a vertex of the polyhedron. As a result, C6o is held together by 90 covalent bonds, the number of edges of the solid. 

The Yb(III) ion in [Yb(acac)3 H2O 0.5CeH6] is seven coordinated and the coordination polyhedron for this complex is identical with the polyhedron found for Yb(l) in the unsolvated complex, i.e. a capped trigonal prism. Like the unsolvated complex, the hemibenzene solvated ones are also linked in pairs by hydrogen bonds. The benzene molecule occupies a centre of S3unmetry in the complex. All intermolecular contacts involving benzene carbon atoms and acac carbons or oxygens are above 3.5 A (84). 

Shinn and Eick 212) found that the orthorhombic crystals of La2(C03)3-8H2O contains two distinctive decacoordinated La(III) ions (space group Peon, Z—A, a =8.934, 6 = 9.580, c = 17.00 A). The coordination polyhedron of La is made up of ten oxygen atoms from water molecules, bidentate carbonates and xmidentate carbonates. One fourth of the water molecules in this compound are not bonded to the metal ion but these are situated between layers. The coordination polyhedron is assumed to be distorted Civ The oxygens of the bidentate carbonate ion form longest La—O bond [La(l)—0(bident. CO3) =2.73 A, La(2)— O (bident. C03)=2.74A], whereas the water molecules are nearer to the lan-thanium ion (La—OH2 =2.63 A). 

Each skeletal boron or carbon atom has a hydrogen atom (or some other singly bonded ligand) terminally attached to it, by a bond radiating outward (exo) away from the center of the polyhedron, to which a pair of electrons is allocated. 

No. of vertices on fundamental polyhedron Structure type Coordination No. of carbon atoms C—C distance (pm) Compound Ref. 

Reduction of cZoso-dicarbaboranes, CaBn-zHn, to the dianionic nido species, CaB 2Hn , which are more susceptible to rearrangement reactions, also provides a route to isomers of the starting materials in which the carbon atoms have moved to different polyhedron vertices, e.g. 185,186),... 

The approach adopted is to view the molecule in three dimensions, imagining each atom or group to be placed at a vertex of hn appropriate polyhedron. In organic chemistry this is usually the tetrahedron with carbon at the centre. Table 3.3 (p. 18) shows the polyhedra normally encountered in organic and inorganic chemistry. It also includes for each polyhedron the polyhedral symbols to denote shape and coordination number. It is to be noted that these polyhedra are often presented in a highly formalised fashion. An octahedron is often represented with the apices rather than the octahedral faces depicted, thus ... 

Four examples of catalytic or regulatory zinc proteins are reviewed here, and the discussion of metalloprotein function is set within the context of the metal ion and its coordination polyhedron. In the zinc enzymes carbonic anhydrase (carbonate dehydratase) II and carboxypeptidase A, the coordination polyhedron of the metal ion changes as the... 

Another contrast between the zinc proteases and the carbonic an-hydrases concerns the zinc coordination polyhedron. The carbonic an-hydrases ligate zinc via three histidine residues, whereas the zinc proteases ligate the metal ion through two histidine residues and a glutamate (bidentate in carboxypeptidase A, unidentate in thermolysin). Hence, the fourth ligand on each catalytic zinc ion, a solvent molecule, experiences enhanced electrostatic polarization in carbonic anhydrase II relative to carboxypeptidase A. Indeed, the zinc-bound solvent of carbonic anhydrase II is actually the hydroxide anion [via a proton transfer step mediated by His-64 (for a review see Silverman and Lindskog, 1988)]. 

The engineering of zinc-binding sites in a-helical peptides, where metal binding stabilizes protein tertiary structure, has been reported by Handel and DeGrado (1990). In these experiments zinc-binding sites are incorporated into a dimeric helix-loop—helix peptide (H3 2) and a protein composed of four helices connected by three short loop sequences (H3 4). a model of one subunit of the H3 2 dimer is found in Fig. 47. In addition to metal complexation by two histidine residues at positions n and n+4 of one a helix, the metal is coordinated by a third histidine residue of an adjacent a helix. The composition of the zinc coordination polyhedron is like that of carbonic anhydrase (i.e., Hiss), and spectroscopic results suggest that all three histidine residues are involved in zinc complexation. This work sets an important foundation... 

Makarova TL (2004) Magnetic properties of carbon structures. Semiconductors 38 615-638 Enoki T, Kawatsu N, Shibayama Y et al (2001) Magnetism of nano-graphite and its assembly. Polyhedron 20 1311-1315... 

The p3-bridged carbonato complex posseses a pseudo-3-fold molecular symmetry. Each of the Cu atoms is five coordinate with the four nitrogen atoms of tren and one oxygen atom of the carbonate ligand (C). The coordination polyhedron of the Cu atom can be described as almost (TBP), the copper ions being slightly out of the plane (0.15 A) of the three primary amine groups (Fig. 5). 

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