Crystalline arrangements

When an ionic solid like sodium chloride is melted, the molten salt conducts electric current. The conductivity is like that of an aqueous salt solution Na+ and Cl- ions are present. The extremely high melting temperature (808°C) shows that a large amount of energy is needed to tfear apart the regular NaCl crystalline arrangement to free the ions so they can move. 

Polycrystalline and well-oriented specimens of pure amylose have been trapped both in the A- and B-forms of starch, and their diffraction patterns84-85 are suitable for detailed structure analysis. Further, amylose can be regenerated in the presence of solvents or complexed with such molecules as alcohols, fatty acids, and iodine the molecular structures and crystalline arrangements in these materials are classified under V-amylose. When amylose complexes with alkali or such salts as KBr, the resulting structures86 are surprisingly far from those of V-amyloses. 

If the covalent bonds connect elements of the structure along one dimension only, as in silicon disulfide, it may be desirable to consider the substance as a polymer. This will certainly be appropriate if the valence structure supersedes whatever crystalline arrangement prevails, i.e., if the substance can be melted without seriously disrupting the continuity of the interunit connections. 

The same crystalline arrangement leads to the expression of Bragg s law applied to X-ray diffraction with incident X-ray beam of wavelength X as shown in equation 3.4 and where the terms are defined as in Figures 3.5 and 3.6. 

SAMs, in general, and thiol SAMs, in particular, are very often perceived as systems that easily form layers of high structural quality and this view is reflected in oversimplifying cartoons where a SAM is represented by a two-dimensional crystalline arrangement of molecules on a surface, similar to the one depicted in Figure 5.1b. For some systems one can get quite close to this ideal picture, as seen from Figure 5.2a, however, the more common case exemplified by Figure 5.2b is quite different. While... 

Solids lacking an ordered, crystalline arrangement are termed amorphous materials, and resemble rigid liquids in structure and properties. Glass (SiO 2) is the classic example of an amorphous solid. Such materials typically soften on heating, rather than showing a sharp melting point. 

Wolf and coworkers [30] have shown that a combination of hydrogen bonding capability with symmetry requirements can enforce similar crystalline arrangements yielding the pattern analogous to that formed by unsubstituted melamine 1 and cyanuric acid 2 (Chapter 1) independent of the steric requirements of the substituents. The pattern can be called molecular fabric. Mascal and coworkers [31] have used this term for their more loose structure 147 exhibiting voids. 

The atomic radius of an element is considered to be half the interatomic distance between identical (singly bonded) atoms. This may apply to iron, say, in its metallic state, in which case the quantity may be regarded as the metallic radius of the iron atom, or to a molecule such as Cl2. The difference between the two examples is sufficient to demonstrate that some degree of caution is necessary when comparing the atomic radii of different elements. It is best to limit such comparisons to elements with similar types of bonding, metals for example. Even that restriction is subject to the drawback that the metallic elements have at least three different crystalline arrangements with possibly different coordination numbers (the number of nearest neighbours for any one atom). 

The above remarks refer to the symmetries of molecules in crystals. It is very important to remember that the symmetry of a molecule in its crystal setting is not necessarily the full symmetry of an isolated molecule, since, as we have seen, the full symmetries of molecules are not always utilized in forming crystalline arrangements. Suppose, for example, that X-ray and other evidence leads to the definite conclusion that certain molecules in their crystal setting have no symmetry. It does not follow that these molecules in isolation are asymmetric it may be that in isolation they would have axes of symmetry or planes of... 

Lorentz1 advanced a theory of metals that accounts in a qualitative way for some of their characteristic properties and that has been extensively developed in recent years by the application of quantum mechanics. He thought of a metal as a crystalline arrangement of hard spheres (the metal cations), with free electrons moving in the interstices.. This free-electron theory provides a simple explanation of metallic luster and other optical properties, of high thermal and electric conductivity, of high values of heat capacity and entropy, and of certain other properties. 

Channel-like architectures are formed in the mesophase given by complexes of long chain crown ether derivatives [8.196a,b] and long-chain calixarene derivatives display columnar liquid-crystalline arrangements [8.196c]. Self-assembled tubular structures based on cyclic peptide components have been described [8.186]. 

If you analyze the four spontaneous endothermic processes mentioned previously, you ll see that each involves an increase in the randomness of the system. When ice melts, for example, randomness increases because the highly ordered crystalline arrangement of tightly held water molecules collapses and the molecules become free to move about in the liquid. When liquid water vaporizes, randomness further increases because the molecules can now move independently in the much larger volume of the gas. In general, processes that convert a solid to a liquid or a liquid to a gas involve an increase in randomness and thus an increase in entropy (Figure 17.3). 

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