Some Examples of Crystal Structures

SOME EXAMPLES OF CRYSTAL STRUCTURE DETERMINATION BY TRIAL... 

Good evidence for the formation of charge-transfer complexes comes from the isolation and structure determination of solids. Chlorine and Br2, as well as the interhalogens IBr and Id, have been found to behave similarly to iodine. Some examples of crystal structures are ... 

Some salts, such as sodium chloride, copper carbonate and sodium nitrate, crystallise in their anhydrous forms (without water). However, many salts produce hydrates when they crystallise from solution. A hydrate is a salt which incorporates water into its crystal structure. This water is referred to as water of crystallisation. The shape of the crystal hydrate is very much dependent on the presence of water of crystallisation. Some examples of crystal hydrates are given in Table 8.6 and shown in Figure 8.20. 

The crystal stmctures of several sodium- and potassium-activated enzymes have now been determined and the position of the monovalent cation estabhshed. Structural data are listed in the Protein Data Bank (PDB). They provide a clue (but not an absolute proof) to their location within the enzyme on activation. Further biochemical studies may be necessary to verify this finding. Some examples of these structures are presented here. The first example (thrombin) is a sodium-activated enzyme, but the others are potassium-activated enzymes. 

Besides the structures already discussed, more complex morphologies may be obtained from the growth of polymer crystals from solutions. The structure that emerges from the crystallization of a polymer is a function of a complex interaction of factors that include the type of solvent, solution temperature, concentration, and polymer molecular weight. Some examples of these structures include spiral growth, dendrites, and hedrites. 

The Ozin group pioneered the fabrication of planarized microphotonic structures and the growth of ordered crystals using evaporation techniques derived from convective assembly. Figure 15.17 illustrates the methods used in these experiments and some examples of PBG structures. " Opened microchannel templates were immersed vertically in a suspension of silica microspheres. The assembly was then driven by the evaporation of the colloidal suspension. Whereas the assembly of 100-500 nm... 

Introduction of bulky lateral substituents on monomer units to increase interchain distance and prevent close packing in polymer crystal. The use of unsymmetrically substituted monomers, resulting in a random distribution of head-to-head and head-to-tail structures in polymer chains, further helps in disrupting regularity. Some examples of substituent effects are given in Table 2.16. 

If it were not for a number of crystal structures that unambiguously confirmed the structures of the products, it would be easy to believe that some of the structural assignments were incorrect. At present no completely satisfactory explanation for these results is available. Examples of the reactions with esters are given in Scheme 21. 

Figure 30 illustrates nine examples of the structures obtained in these simulations. It is clear that the chains group into crystallized kebabs on the shish surface. There are very few areas where the chains are partially or completely stretched under the influence of the shish template. The dominant mode of crystal nucleation on the shish is the growth of folded chains grouped into lamellar nuclei. Also, some of the chains do not join the central structure but drift away from it leaving a large gap on the shish between them. These simulations show clearly that the presence of the ordered template (the shish) influences the nucleation of lamellae and the formation of kebabs. 

We first describe the crystal structure and magnetic properties of some examples of SCO oligomers that can be considered as model systems for the polymers. 

Although it has often proved useful as a mnemonic, this approach has led to a number of misconceptions about relative atomic sizes and the origin of close-packing geometry, to some of which we allude below. More relevant in the present context is the observation that one natural and simple description of crystal structures has been. overlooked, and an unnecessarily complicated and opaque one used instead. We will provide many examples throughout this article. 

Table 2 records some examples of this phenomenon in which oxygen arrays in oxides are the same as metal atom arrays in alloys. Recognition of this fact has been exploited to simplify the description of complex alloys (see especially Andersson ), which is essentially the reverse of what we propose to do here, namely to simplify the description of oxide structures by giving them in terms of known alloy structures. Nevertheless Tables 1 and 2 provide striking evidence of Nature s parsimony in the use of patterns in crystal structures. 

---