Variations from true crystallinity

Some of the experimental aspects of crystal structure analyses can now be applied to compounds that are not truly crystalline, or that are variants of the perfectly ordered crystals we have been describing so far. 

The term liquid crystal was first applied to compounds that, unlike most compounds that melt in a single step at a definite temperature, show one or more well-defined phases between the solid and the true liquid. Otto Lehmann in 1888 was contacted by Friedrich Reinitzer who had observed that crystalline cholesteryl benzoate, on heating, seemed to have two melting points.At 145.5° C a cloudy liquid forms that, on further heating, changes sharply to a clear liquid at 178.5° C. On cooling the reverse order of phases was found. The turbid liquid is doubly refracting, like the anisotropic crystals described in Chapter 5, hence the 

Dichroic dye molecules absorb light in an anisotropic way and show different colors in different directions. If such dyes are dissolved in a liquid crystal, they form a guest-host-type interaction, and are oriented by the host liquid crystal molecule. Application of an electric field will reorient the liquid crystal and dye this effect is used for liquid-crystal displays. 

Polytypismi is a special kind of one-dimensional polymorphism that is found for certain close-packed and layer structures. It occurs when an element or compound can crystallize in several different modifications, each of which may be regarded as being built up by different stacking arrangements of layers of the same structure and composition. The modifications differ only in their stacking sequence. For any successive layer, there are two or more possible positions relative to its predecessor. Periodic (ordered) as well as nonperiodic (disordered) stackings are found to occur. 

While the arrangement of atoms from unit cell to unit cell is usually consistently repeated throughout the crystal, perturbations to this exact regularity are encountered in practice. These defects can be of great technological importance because of the physical properties they impart to the crystal. The simplest of these defects are those in which atoms or ions become displaced throughout the crystal structure. Such defects have been observed by electron microscopy techniques. 

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It must be kept in mind that Intermolecular forces are different in solids as compared to solutions. This is particularly true of those functional groups which take part in hydrogen bonding. Thus solid state spectra are different from solution spectra. However, the number of resolved lines are far greater in solid state spectra. Many organic compounds exist as polymorphic variations. These different crystalline forms lead to differences in infrared spectra. It is better to measure such substances in solution, since all polymorphism is lost in that state. 

It must be apparent from the above discussion that a number of variables other than concentration affect the intensities of mineral patterns as observed on a diffractogram. Inasmuch as they cannot be monitored and compensated for mathematically, these variations must be reflected in the error of determination. This would be true regardless of the quantification procedure employed. The conclusion, therefore, is that the inherent variability in composition and/or crystallinity that exists within the major mineral components of the low temperature ashes of coal will be reflected in the statistical error of determination and that error will be of sufficient magnitude to preclude the use of the term "quantitative" to describe the procedure. Therefore, any procedure using x-ray diffraction to determine the minerals in coal must be considered semi-quantitative at best. 

X-ray and melting-point studies of these copolymers have been carried out 14S). All the copolymers studied are crystalline over the whole range of composition (the total crystallinity diminishes from 55-51 % for polybutene-1 to 30% for 1 1-1 2 copolymers, and then rises again to 61-65% for another polyolefin). The melting point variations are also small. True cocrystallization does not occur in these cases owing to the difference in monomer unit sizes. These facts led the authors (145) to conclude that there was strong compositional inhomogeneity. 

From variations in the composition of their preparations Willstatter and Waldschmidt-Leitz have been led to a bearer theory of enzyme structiue, according to which each enzyme consists of (1) a colloidal bearer, usually of protein nature, and (2) one or more active groups which enable the bearer to become affixed to the substrate. The distinction between the soluble lyo-enzymes and the insoluble desmo-enzymes is determined by the solubility of the protein bearer. Sumner and Northrop, however, maintain that the crystalline enzymes are pure proteins, and that even partial hydrolysis of the protein destroys the enzyme. While, of course, it is possible that both theories may be true in that they apply to different enzyme types, the balance of evidence supports the conclusions of the American workers, and leads to the recognition of a new class of biochemical compounds, the zymo-proteins. 

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