Selective crystallization

Research. A significant impact on research at high pressure has come about with the use of gem quaHty diamonds as Bridgman-type anvils in a smaU compact high pressure device (40—42). With this type of apparatus, pressures greater than those at the center of the earth (360 GPa = 3.6 Mbars) have been reached, and phase transformations of many materials have been studied. Because of the x-ray transparency of diamond, it is possible to determine the stmcture of the phases under pressure. Because of the strenuous environment, crystals selected for this appHcation have to be of very high quaHty. 

Platellke a-form crystals easily separate from the solvent In commercial operation but halrllke 3-form crystals are troublesome because their suspension Is extremely viscous. So the main purpose of this work Is to develop a modified crystallization process to obtain a-form crystal selectively. 

Fig. 1.3.4 Structure of the sulfonimidoyl-substituted bis(allyl)titanium complex 28 in the crystal. Selected bond lengths Ti—C 244 and 229 pm, Ti—N 209 and 221 pm.

Figure 10.2. Various forms of rock-crystal. Selected from Atlas der Kristallformen (ref. [1], Chapter 9).

Crushing has been recommended as a primary method because it is safe and will lead to the determination of the principal refractive indices of any crystalline substance, provided a sufficient number of randomly oriented fragments is observed it is a beginner s method. But the more experienced worker may often dispense with it, when the crystals being examined have a well-defined polyhedral shape. If the relation between crystal shape and optical properties is properly understood, it is possible to determine the principal indices by a limited number of observations on crystals selected because they lie in such positions that they necessarily show their principal indices. 

If there is alignment, contrast in TEM images is strong, because of the periodic strain field in the crystal. Selected-area diffraction shows evidence of such alignment by the location of satellite intensities around the Bragg peaks arising from the modulation of atomic scattering factors, lattice constant, or both [19]. In Fig. 18.10, the electron diffraction effects, expected from an f.c.c. crystal with (100) composition waves, are depicted with a [001] beam direction. 

Single crystal X-ray studies on representative examples of the platinum bis(acetylides) were carried out to gain information on both the molecular geometry and nature of the packing of the acetylide moieties in the crystal. Selected crystal data for the various compounds are given in Table I. Full details of the structural analyses will be reported elsewhere. 

Theory of infra-red and Raman processes in crystals selection mles in diamond and zinc blende. Phys. Rev. 131, 1489-96. 

Crystallization, experimental methods overview, 1, 207 Crystal mounting, experimental methods overview, 1, 209 Crystal selection, experimental methods overview, 1, 209 CSA, and apparent dynamic NMR, 1, 423 CSD, see Cambridge Structural Database C2-symmetric metallocenes, for polypropylene polymerization, 4, 1058 CT transitions, see Charge-transfer transitions Cubanes... 

The crystal selectively adsorbs and occludes onlyLa-amino acids. The solution is enriched inLct-amino acids... 

With liquid crystals, the differences in the longitudinal dimension of molecufeand in their planarity play a major role in determining the strengh of interaction with guest molecules. Sterespecific effects are, in general, less pronounced than those observed with cyclodex-trins. Therefore, in order to be able to use liquid crystals for separations, the modest liquid crystal selectivity has been combined with the high efficiency of capillary columns. 

Triple-crystal Spectroscopy (Fig. 2) — A collimated beam of thermal neutrons impinges on a monochromating crystal, iif- This crystal selects monochromatic neutrons of wavelength, by Bragg reflection... 

Under these conditions the first two lithium silylcuprates were isolated and structurally characterized. The complexes were found to be extremely air-and moisture-sensitive. During crystal selection and mounting the flask and the flushing argon were cooled down to ca. -50°C. At a temperature slightly above -30°C both compounds decompose instantaneously. 

Fig. 1. Molecular structure of 2 in the crystal. Selected bond lengths (pm) and angles (°) C5-01 123.5(4).

Whether a vessel is called an evaporator or a crystallizer depends primarily on the criteria used in arriving at its sizing. In an evaporator of the salting-out type, sizing is done on the basis of vapor release. In a crystallizer, sizing is normally done on the basis of the volume required for crystallization or for special features required to obtain the proper product size. In external appearance, the vessels could be identical. Evaporators are discussed in Sec. 11. Genck (loc. cit., 2004) provides a detailed discussion of guidelines for crystallizer selection and operation. 

The first crystal selects a wavelength from the polychromatic source, which is reflected along the initial direction by the second crystal. The lattice planes of the latter must be perfectly aligned with the first crystal for efficient transmission of the beam. 

Sample Selection and Mounting. The sample should now be safe for a few minutes, and one can proceed to selection and mounting of a crystal. Selection of a suitable crystal is similar to procedures commonly used for chemically stable samples. A sowing needle held with a pin vice, and a sharp razor blade are useful tools. The binocular microscope should have a polarizing attachment to assist in judging quality of crystallinity. 

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