X-ray examination of crystals

In situ X-ray examination of crystallizing polyethylene, at high temperature and pressure, then confirmed this proposal in detail, showing that the wide-angle diffraction pattern changed abruptly with the optical texture [ 10]. That corresponding to the spherulitic texture was of the usual orthorhombic form while the new intermediate phase had two-dimensional hexagonal symmetry, with an increased cross-sectional area per chain, but without... 

Bragg X ray method. The X ray examination of crystals using a single large crystal rotated through a small angle around an axis in a crystal face. 

Quinoxalin-2-ones show carbonyl stretching absorption in the region of 1660-1690 cm both in Nujol mulls and KBr discs. The ultraviolet spectrum of quinoxalin-2-one shows maxima at 343,287,254,250, and 228 nm in aqueous solution at pH 4.0 and is closely similar to that of its 1-methyl derivative. These data indicate that quinoxalin-2-one exist predominately in the cyclic amide form rather than as 2-hydroxyquinoxaline, and this conclusion is further supported by the closely similar pX values of quinoxalin-2-one and its 1-methyl derivative, which are -1.38 and -1.15, respectively. 2-Methoxyquinoxaline is by comparison an appreciably stronger base, with a pKa value of 0.28. ° X-Ray examination of crystals of quinoxalin-2-one confirms that it exists in the cyclic amide form. °° Quinoxalin-2-one fragments in the mass spectrometer, as might be expected, by the successive loss of carbon monoxide and hydrogen cyanide from the molecular ion. ... 

It is found on analysis of the problem that when only two d orbitals are available for combination with the s and p orbitals six equivalent bond orbital- of strength 2.923 (nearly as great as the maximum 3 for the best spd hybrid) can be formed, and that these six orbitals have their bond directions toward the corners of a regular octahedron. We accordingly conclude that complexes such as [CoCNHs ], [PdClc]—, and [PtCle] — should be octahedral in configuration. This conclusion is of course identical with the postulate made by Werner to account for isomerism in complexes with different substituent groups,1 and verified also by the x-ray examination of Co(NH8)el3, (NHOaPdCU, (NH4)2 PtCl , and other crystals (see Fig. 5-1). 

X-ray examination of rr-rr molecular complexes shows that the crystals of the complexes are formed from stacks of alternate donor and acceptor molecules — in agreement with theoretical analysis of MuUiken. 

In the Berg-Barrett topographs of Fig. 8-33 explain why only screw dislocations are revealed in (b), only edges in (c), and screws and edges in (d). Give the indices of the operating slip systems, i.e., the indices of each slip plane and of the slip direction in that plane. (Slip in a LiF crystal occurs on 110 planes in <110> directions, but the experimental observations in Fig. 8-33 are inconsistent with the assumption that slip occurred on all possible slip systems. Note also that only those dislocations which intersect a crystal face will be distinctly observed by x-ray examination of that face.)... 

Thus, all monomers of the ChMAA-n series fonn a monotropic liquid crystalline phase of the cholesteric type, whose temperature interval of existence depends on the rate of cooling. The liquid crystalline phase is unstable and is transformed to crystal phase so soon that X-ray examination of the mesophase structure becomes difficult. Nevertheless, polarization-optical studies have made it possible to draw certain conclusions as to the nature of the liquid crystalline phase of monomers. Cooling of isotropic melts of monomers results in a confocal texture which turns to a planar one when a mechanical field is superimposed on the sample, for example, by shifting a cover glass in the cell of the polarizing microscope (Figure 4). The observed planar texture exhibits the property of selective light reflection, which is typical of low-molecular cholesteric liquid crystals. 

Instead, x-ray examination of fine crystal structures obtained from an aqueous solution of (11) and CA revealed that two types of self-assembled complexes were... 

In his preliminary X-ray examination of amino acids and related compounds Bernal (8) determined the dimensions of the unit cells, the space groups, and the cleavage and optical properties of crystals of a-, P-, and y-glycylglycine and of diglycylglycine dihydrate. Dimensions of the unit cells of anhydrous - and /3-diglycylglycine and of n-alanyl-glycine have been published by Lenel (21). 

Both of these crucial fields of research will surface repeatedly later in this book here they are briefly discussed only as fields which by at least one of the criteria I have examined do not appear to qualify as fully blown disciplines. Both have emerged only in this century, because a knowledge of crystal structure is indispensable to both and that only emerged after 1912, when X-ray diflfraction from crystals was discovered. 

The crystallization and structural determination of the histone octamer was first reported in 1984 [34], However, the overall dimensions of the 3.3 A structure [15] did not appear to fit within the known X-ray structures of the nucleosome core particle [12,13], In an elegant analysis [16], re-examination of the original phasing of the histone octamer data revealed misplacement of the heavy atom site by 2.7 A. The structure was resolved, after which it was possible to build molecular models of the individual histones into the 3.1 A resolution electron density map of the histone core of the nucleosome [17]. Figure 2 shows the first atomic resolution model of the core histone octamer. Several additional publications followed in which the histone octamer structure formed the basis for constructing models of the NCP [17-21],... 

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