Crystal structure dependence upon bonding

Most of the differences between inorgcmic, organic and protein crystals lies in the fact that ionic forces (found mostly in inorganic structures) depend upon interatomic distances but not on angle. In contrast, covalent bonds (such as those which predominate in organic and protein... 

Tadokoro and Nakasuji used the monoanionic 2,2 -biimidazoleate (HBim ) ligand in conjunction with divalent octahedral metal centres, M(II), to prepare 2D honeycomb networks based upon M(Hbim) ( building blocks linked by N-H---N hydrogen bonds in an R IO) synthon (Figure 24) [64], The overall crystal structure depends on the counter-cation used, and can be a layer structure or an interpenetrated network. The interligand N-H N hydrogen bonds can... 

In the case of polymers made up of units of the type CH2—CR2 a possible irregularity may arise as a result of the absence of symmetry about the C—C bond. Generally the polymerization proceeds such that the CRR group from one monomer unit is bonded to the CH2 unit from the next, referred to as head-to-tail addition. Occasionally however, during polymerization one CRR group becomes bonded to another to form a defect in the molecular structure which is termed a head-to-head defect. As in the copolymer case, the extent to which such defects may or may not affect the ability of a molecule to crystallize will depend upon the frequency with which head-to-head defects occur. In general, however, it is found experimentally that such defects, which tend to be present in polymers prepared by a free radical addition mechanism," " occur only infrequently and therefore do not have a major bearing on whether or not a particular system will crystallize." " ... 

The aggregation of vacancies or interstitials into dislocation loops will depend critically upon the nature of the crystal structure. Thus, ionic crystals such as sodium chloride, NaCl, or moderately ionic crystals such as corundum, AI2O3, or rutile, TiC>2, will show different propensities to form dislocation loops, and the most favorable planes will depend upon chemical bonding considerations. 

In the following chapter some special magnetic properties of the transition metal fluorides will be dealt with, which depend more clearly upon a specific bonding behaviour in and between the MeFe-octahedra than the crystal structure itself. 

Since the electron distribution function for an ion extends indefi-finitely, it is evident that no single characteristic size can be assigned to it. Instead, the apparent ionic radius will depend upon the physical property under discussion and will differ for different properties. We are interested in ionic radii such that the sum of two radii (with certain corrections when necessary) is equal to the equilibrium distance between the corresponding ions in contact in a crystal. It will be shown later that the equilibrium interionic distance for two ions is determined not only by the nature of the electron distributions for the ions, as shown in Figure 13-1, but also by the structure of the crystal and the ratio of radii of cation and anion. We take as our standard crystals those with the sodium chloride arrangement, with the ratio of radii of cation and anion about 0.75 and with the amount of ionic character of the bonds about the same as in the alkali halogenides, and calculate crystal radii of ions such that the sum of two radii gives the equilibrium interionic distance in a standard crystal. 

Comparatively, the walls of a reaction cavity of an inclusion complex are less rigid but more variegated than those of a zeolite. Depending upon the constituent molecules of the host lattice, the guest molecules may experience an environment which is tolerant or intolerant of the motions that lead from an initial ketone conformation to its Norrish II photoproducts and which either can direct those motions via selective attractive (NB, hydrogen bonding) and/or repulsive (steric) interactions. The specificity of the reaction cavity is dependent upon the structure of the host molecule, the mode of guest inclusion, and the mode of crystallization of the host. 

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