Nearest-neighbor arrangement

Each H2O molecule has four nearest neighbors arranged near the vertices of a regular tetrahedron centered about the molecule of interest. The oxygen atom of each molecule is strongly covalently bonded to two hydrogen atoms, while the... 

Polytypism is a special one-dimensional case of polymorphism. All polytypes consist of identical close-packed Si-C double layers, whose stacking sequences differ along a certain direction. The nearest neighbor arrangement of atoms is identical in all crystal structures. 

Place the equatorial bonds so as to approximate a tetrahedral arrangement of the bonds to each carbon The equatorial bond of each carbon should be parallel to the ring bonds of its two nearest neighbor carbons... 

The primary structure of a peptide is its ammo acid sequence We also speak of the secondary structure of a peptide that is the conformational relationship of nearest neighbor ammo acids with respect to each other On the basis of X ray crystallographic studies and careful examination of molecular models Linus Pauling and Robert B Corey of the California Institute of Technology showed that certain peptide conformations were more stable than others Two arrangements the a helix and the (5 sheet, stand out as... 

In water at room temperature some molecules have more than four nearest neighbors, but at any moment the majority have four neighbors, as in ice. If the molecules tend to have a tetrahedral arrangement like that of ice, the average distance between next-nearest neighbors would be... 

Figure 16-19. A Tt-stacked arrangement of Ooci-OPV5-CN" molecules in the crystal lattice (view at a slight angle with respect to the long molecular axis). B lop-view (lop) and side-view (bottom) of two nearest-neighbor Oocl-OPV5-CN" molecules in the crystal. related by a translation along the ci-axis. The distance of 3.5 A corresponds to the shortest distance between two atoms on different molecules (sec Fig. 16-18 for details).

Sometimes the atomic arrangement of a crystal is such as not to permit the formulation of a covalent structure. This is the case for the sodium chloride arrangement, as the alkali halides do not contain enough electrons to form bonds between each atom and its six equivalent nearest neighbors. This criterion must be applied with caution, however, for in some cases electron pairs may jump around in the crystal, giving more bonds than there are electron pairs, each bond being of an intermediate type. It must also be mentioned that determinations of the atomic arrangement are sometimes not sufficiently accurate to provide evidence on this point an atom reported equidistant from six others may be somewhat closer to three, say, than to the other three. 

In either of these close-packed structures, each sphere has 12 nearest neighbors 6 in the same plane, 3 in the dimples above, and 3 in the dimples below. The expanded views in Figure 11-30 show the different arrangements of the hexagonal and cubic close-packed crystalline types, hi the hexagonal close-packed structure, notice that the third layer lies directly above the first, the fourth above the second, and so on. The layers can be labeled ABAB. 

The low solubility of AuCN(s) is thought to be a consequence of a polymeric structure based on linear chains, —Au-CN-Au-CN—, which lie parallel to one another with a close-packed arrangement of gold atoms in which each is in contact with six nearest neighbors.36 The addition of cyanide and dissolution of [Au(CN)2] is believed to take place at the chain ends. This process is enhanced by the presence of Ag2+ or other ions such as Pb2+ (see below) normally present in the gold-bearing ores. 

A surface atom has, by definition, a smaller number of nearest neighbors than an atom in the interior of the crystal. An interior atom in the metals with which we are concerned here, and which are all fee, has twelve nearest neighbors. On every metal crystal there will be found various types of surface atoms that differ in the number j and in the arrangement of their... 

Let us compare the energy of linear (Tim) and square (Tsq) arrangements, assuming the same nearest-neighbor distance R for each ... 

All BaCvN nanotubes are made of a hexagonal network of sp2 bonded atoms, with three nearest neighbors to each atom (15, 55, 69, 78, 79). In the case of BC2N nanotubes, two different arrangements of the sheet are possible, leading to two isomers with different structures and with distinct electrical properties (78b). 

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