Basis of crystal structure

The development of apparatus and Techniques, such as x-ray diffraction, contributed greatly to research on clay minerals. Crystalline clay minerals are identified and classified primarily on live basis of crystal structure and the amount and locations of charge (deficit or excess) wiih respect to the... 

Fig. 10. View of the region around the zinc ion and the SH-group in the complex of human carbonic anhydrase C and acetoxy-mercurisulfanilamide. The model was built on the basis of crystal structure data to 5.5 A resolution. Two inhibitor molecules are bound via the Hg atom to the SH-group and via the SC>2NH2-group to the zinc ion. From Fridborg et at. (61)...

Because of the great diversity of bonding types exhibited by minerals, a variety of criteria has been used to classify minerals within a Class. Some minerals, like the borates and silicates, can be classified mainly on the basis of crystal structure, ie the degree of polymerization of the anionic polyhedra. Other minerals, such as the sulfides and oxides. 

Electron donor characteristics determined on the basis of crystal structure data (Table 3) correspond to ligands with a certain composition and coordination type in both crystals and aqueous solutions. 

The above systematic trend of the catalytic activity of various spinel compositions may be explained on the basis of crystal structure, electronic activation energy and distribution of metal ions between the tetrahedral and octahedral sites. A radical mechanism was suggested by Deren etal for the decomposition of hydrogen peroxide on semiconductor surfaces. According to this mechanism, for the surface of a compound to be active, both donor and acceptor centres should be present. 

Although Fc304 is an inverse spinel it will be recalled that Mn304 (pp. 1048-9) is normal. This contrast can be explained on the basis of crystal field stabilization. Manganese(II) and Fe" are both d ions and, when high-spin, have zero CFSE whether octahedral or tetrahedral. On the other hand, Mn" is a d and Fe" a d ion, both of which have greater CFSEs in the octahedral rather than the tetrahedral case. The preference of Mn" for the octahedral sites therefore favours the spinel structure, whereas the preference of Fe" for these octahedral sites favours the inverse structure. 

The importance of the proximity effect in cyclodextrin catalysis has been discussed on the basis of the structural data. Harata et al. 31,35> have determined the crystal structures of a-cyclodextrin complexes with m- and p-nitrophenols by the X-ray method. Upon the assumption that m- and p-nitrophenyl acetates form inclusion complexes in the same manner as the corresponding nitrophenols, they estimated the distances between the carbonyl carbon atoms of the acetates and the adjacent second-... 

On the basis of the structures of each of the following molecules, predict which ones would be most likely to have a residual entropy in their crystal forms at T = 0 (a) CO, ... 

The previous example shows that, as a group of crystal structures is better understood, structural interference may be tamed to the extent that it forms the basis of a new family which may be... 

This is not a trivial problem, and has important implications for the mechanism of the reaction. However, the bulk of the evidence is for centrosymmetric rings, which would be in keeping with our experience in small-molecule systems. For the present purposes we assume this to be the case. On this basis DSP is one of a class of monomers of crystal structural type 100 that polymerize to polymers 101. Note that, as is typical of topochemical reactions, there are cases of polymorphism of the monomers, in which only those of structure 100 are reactive. Also small changes in the substitution of this molecule frequently result in changes in crystal structure and reactivity. 

The report of a crystal structure for PtBra (oA-OEt)(oA) (9) has been the basis of much structural interpretation. At the time, however, it was thought by the original workers Kneen and Nyholm that the compound submitted was PtBra (oS-OEt) (oS), and this is now tacitly admitted to be the case (74), although a rather ambiguous "corrected structure is described in the footnote. The fuU structure has been determined 13) the product has a six-membered ring, as shown in Fig. 8. 

Zachariasen rounded off this work by calculating the three principal refractive indices of the crystal on the basis of his structure, accepting Bragg s theory (1924). The calculated values are close to the known indices of the crystal. 

For instance, head-head photodimers are predicted from the crystal structures of 9-cyanoanthracene and 9-anthraldehyde, but the head-tail isomer is produced. Craig and Sarti-Fantoni and later others found that photoreactions of 9-cyanoanthracene and 9-anthraldehyde take place at defect sites [96,215], Systematic photochemical and crystallographic studies by Schmidt and co-workers uncovered many cases of substituted anthracenes which behave in an unexpected fashion (Scheme 40) [216,217]. Examples shown in Scheme 40 clearly illustrate that, unlike cinnamic acid derivatives, the stereochemistry of the product dimer from anthracenes cannot be predicted on the basis of crystal packing. An example from the laboratories of Venkatesan is noteworthy in this context [218], Irradiation of crystals of 7-... 

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