Double-sheet structure

Double-Sheet Structures Xerogels, d-Vanadium Oxides, and Nanotubes... 

A double-sheet structure in which the molecules arrange themselves in a head-to-head and tail-to-tail fashion. The alkyl groups and the OH groups form alternating hydrophobic and hydrophilic double sheets respectively. This type of arrangement is observed in BuSi(OH)3 [23] and (c-C6Hii)Si(OH)3 [14]. 

The different steric situation at these silanetriols is clearly seen in the X-ray structures. Based on hydrogen-bonded networks silanetriols are arranged in the solid state in a variety of supramolecular structures [1]. Whereas 1 crystallizes as a hemihydrate in a double-sheet structure [2] (Fig. 3), 2 forms a tube structure [3] (Fig. 4). 

The increasing tendency of silanols to condense as the number of OH groups on a particular silicon increases means that relatively few silanetriols have been isolated and even fewer structurally characterized. A wide range of substituted arylsilanetriols have been prepared (102), but none have been studied by x-ray crystallography. The first silanetriol to be structurally characterized was c-C6HuSi(OH)3, which has a sheet structure comprising double layers of molecules joined in a... 

Fig. 27. A view parallel to the double-sheet hydrogen-bonded structure formed by BulSi(OH)3, showing the hydrophilic interior and the hydrophobic exterior of the sheet. Hydrogen atoms are omitted for clarity. Drawn using coordinates taken from the Cambridge Crystallographic Database.

The outer most levels in C60 are due to rc orbitals . These are formed by 2p electrons which have their orbitals oriented along the radius of the molecule. The different environment inside and outside the spherical molecule causes the double-peaked structure in the momentum densities. In graphite the n band is formed by 2p orbitals oriented perpendicular to the sheets of carbon atoms. Using single-crystal graphite films we have a unique opportunity to study the effects of the orientation of these 2p orbitals in detail. 

PXX groups in complexes 87-90 can also slipped-stack to form columns located between the Pc columns and sheets. Since the crystal structures and supramolecular assembles of this type of phthalocyanine were well summarized in 2001 [12] and 2005 [79], we will present briefly only one typical Pc %-% supramolecular structure that has not been included in the previous section. As shown in Fig. 23, the Pc units in 89 form a 2D sheet along the ac plane. The overlap mode is type G Pc %-% interactions along both the a- and c-axes. This sheet further interacts with another sheet by type C overlap, forming a double sheet. 

Double absorption sulfur burning plant, flow sheet for, 23 775 Double-arm kneading mixers, 16 722 Double-barrier structure, in RTDs, 22 170-171... 

Figure 4.4 Infinite chain silicates (single, double, and sheet) (a) infinite single chain silicate with two corners shared per tetrahedron (pyroxene structure) (b) infinite double chain, with alternate two and three corners shared (am-phibole structure) (c) infinite sheet structure, with each tetrahedron sharing three corners (sheet silicates). (From Putnis, 1992 Figure 6.3, by permission of Cambridge University Press.)...

Fig. 2.1 Configurations of the tetrahedral units and chain, double chain, and sheet structures in the silicate and aluminosilicate minerals. (A) Two-dimensional representation of a single silicate tetrahedron. (A ) Two-dimensional representation of an extended silicate chain. (B) Three-dimensional representations of single tetra-hedra in two orientations. The apexes of the tetrahedra point above or below the plane of the paper. (B ) Three-dimensional representations of extended silicate chains showing different orientations of the tetrahedra in two of the many possible configurations. Single chain pyroxenes (C), wollastonite (D), rhodonite (E). Double chains amphiboles (F). Sheets as found in the serpentines, micas, and clays (G).

The structure of boehmite contains double sheets of octahedra with A1 ions at their centers, and the sheets themselves are composed of chains of octahedra. In diaspor the oxygens are in a hexagonal close packed layer those within the double octahedral layers in boehmite are in a cubic pacldng relationship. Luminescence center Cr characterizes steady-state spectra (Solomonov et al. 1994 Shoval et al. 1999). The natural boehmite and diaspor in our study consisted of twelve samples. The laser-induced time-resolved technique enables us to detect Cr emission centers (Figs. 4.62-4.63). 

For an octapeptide sequence taken from the C-terminal residues of the Alzheimer s Ap-peptide Lansbury et al. identified a large intensity enhancement for 13C-labeled modes that was sequence-dependent and assessed as largely due to interstrand dipole-coupling.1207,2351 Mendelsohn et al. found a similar effect by double labeling on alternate sites a peptide that formed a P-sheet-like structure in methanol.12501 Subsequent theoretical modeling showed this latter intensity enhancement to be a function of forming extended, flat, anti-parallel P-sheet structures,12511 and the overall effect to be highly position sensitive.12451... 

Fig. 14. The formation of a double-strand /(-sheet structure, induced by the binding of Cu(TI). (Reproduced with the permission of Ref. 28)...

The crystal structure of CaAl2Si2Og, omisteinbergite, is hexagonal, D, P63/mcm, a0 = 5.10, and c0 = 14.72 A with two molecules per cell. Figure 10.35 shows that there are double sheets of (Al,Si)04 tetrahedra joined by the apical oxygen atoms so that all oxygens are shared. These are TT layers. Between the doubled TT sheets, Ca2+ ions are in octahedral sites giving a TTO structure. The octahedra, formed... 

The structure of mica consists of two Si2052 sheets bonded together by alkali ions. There are layers of positive Al+3 and Mg+2 ions and negative O 2 and OFT1 ions on the outside of the double sheet. 

---