Crystal structure and melting points

The use of the thermodynamic data for a discussion of stability of crystals is demonstrated in Fig. 4.52. The differences in crystal structure and melting temperatures of successive n-alkanes have been linked to the symmetries of odd and even CHj-sequences. Note, that planar zig-zag chains of odd-numbered sequences of carbon atoms in a molecular backbone point with their final bonds into the same direction, while even ones point into opposite directions. Odd paraffins have an orthorhombic, rectangular layer-stmcture, while the even ones up to C24H50 are triclinic with oblique layers. From structural analyses, orthorhombic crystals can accommodate even and odd chains without difference in packing density. The triclinic... 

Two carbenes that are stable at room temperature have been reported. ° These are 44 and 45. In the absence of oxygen and moisture, 44 exists as stable crystals with a melting point of 240-241°C. ° Its structure was proved by X-ray crystallography. 

In a manner similar to that just described for differential thermal analysis, DSC can be used to obtain useful and characteristic thermal and melting point data for crystal polymorphs or solvate species. This information is of great importance to the pharmaceutical industry since many compounds can crystallize in more than one structural modification, and the FDA is vitally concerned with this possibility. Although the primary means of polymorph or solvate characterization s centered around x-ray diffraction methodology, in suitable situations thermal analysis can be used to advantage. 

In order to achieve efficient build-up to heavy depths when dyeing cellulose acetate at 80 °C it is customary, particularly for navy blues, to use a mixture of two or more components of similar hue. If these behave independently, each will give its saturation solubility in the fibre. In practice, certain mixtures of dyes with closely related structures are 20-50% less soluble in cellulose acetate than predicted from the sum of their individual solubilities [87]. Dyes of this kind form mixed crystals in which the components are able to replace one another in the crystal lattice. The melting point depends on composition, varying gradually between those of the components, and the mixed crystals exhibit lower solubility than the sum of solubilities of the component dyes [88]. Dyes of dissimilar molecular shape do not form mixed crystals, the melting point curve of the mixture shows a eutectic point and they behave additively in mixtures with respect to solubility in water and in the fibre. 

The polymerization proceeds under photo- [49,50],X-ray [51], and y-ray [52] irradiation in the dark in vacuo, in air, or even in water or organic solvent as the dispersant (nonsolvent) for the crystals, similar to the solid-state polymerization of diacetylene compounds [ 12]. The process of topochemical polymerization of 1,3-diene monomers is also independent of the environment surrounding the crystals. Recently, the thermally induced topochemical polymerization of several monomers with a high decomposition and melting point was confirmed [53]. The polymer yield increases as the reaction temperature increases during the thermal polymerization. IR and NMR spectroscopies certified that the polymers obtained from the thermally induced polymerization in the dark have a stereoregular repeating structure identical to those of the photopolymers produced by UV or y-ray irradiation. 

Compoimds of the elements are also presented in similar format. This includes CAS Registry Numbers, formulas, molecular weights and the hydrates they form (if any). This is followed by occurrence (for naturally occurring compounds) and industrial applications. The section on Physical Properties covers the color, crystal structure, density, melting and boiling points and solubihties of the compounds in water, acids, alkalies and organic solvents. 

In contrast to benzoisotellurazole (Section III,B) no shortened intermo-lecular Te. . . N contacts were found in the crystal structure of the 2-phenylbenzotellurazole. As a consequence of this, no anomalies in solubility and melting point were revealed for 10a when compared with its sulfur and selenium analogs. 

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