Crystal Preparation

The mechanism of ion polymerization in formaldehyde crystals proposed by Basilevskii et al. [1982] rests on Semenov s [1960] assumption that solid-phase chain reactions are possible when the arrangement of the reactants in the crystal prepares the configuration of the future chain. The monomer crystals capable of low-temperature polymerization fulfill this condition. In the initial equilibrium state the monomer molecules are located in the lattice sites and the creation of a chemical bond requires surmounting a high barrier. However, upon creation of the primary dimer cation, the active center shifts to the intersite, and the barrier for the addition of the next link... 

C(N=PCl3)i +[SbCL - Colourless crystals prepared from [C(N3)i + - -PCl3 >... 

Apart from TiO and the lower halides already mentioned, the chemistry of these metals in oxidation states lower than 3 is not well established. Addition compounds of the type [TiCl2L2] can be formed with difficulty with ligands such as dimethylformamide and acetonitrile, but their magnetic properties suggest that they also are polymeric with appreciable metal-metal bonding. However, the electronic spectra of Ti in TiCl2/AlCl3 melts and also of Ti incorporated in NaCl crystals (prepared by... 

On the other hand, in the single crystals prepared from equivalent amounts of heterochiral 1 1 complexes, a pair of two heterochiral 1 1 complexes are incorporated in a unit cell to form a layered structure with alternate layer distances of 7.33 and 7.6 A. Two perchlorate ions stay in the narrower gap, and two additional acetone molecules as crystallization solvent occupy the wider gap. The perchlorate ions interact with two axial water ligands by hydrogen bonds (3.71 and 3.77 A) to construct a layered structure. The adjacent two molecules of heterochiral 1 1 com-... 

In terms of photoelectrode material quality, single crystals comprise a rational choice since their bulk properties can be controlled better and their influence on cell performance may be evaluated in a rather accurate manner, as being microstruc-turally well-defined solids. However, the cost and convenience of single-crystal preparation are not suited to the practical requirement of cheap device components. Polycrystalline photoelectrodes are advantageous in terms of fabrication cost, ability to prepare large areas in one operation, and material economy. 

The surface properties of these nano-objects match those of metal nano crystals prepared in ultrahigh vacuum, for example the C - O stretch of adsorbed carbon monoxide or the magnetic properties of cobalt particles embedded in PVP. This demonstrates the clean character of the surface of these particles and its availabihty for reactivity studies. 

The transition-state model of this reaction has been proposed as (1), based on X-ray analyses of single crystals prepared from Ti(OPr )4, (R,R)-diethyl tartrate (DET), and PhCON(OH)Ph and from Ti(OPr )4, and (R,R) N,/V -dibenzyltartramide.30-32 The Z-substituent (R2), located close to the metal center, destabilizes the desired transition state and decreases enantioselectivity (vide supra). When the Z-substituent is chiral, face selection induced by the substituent strongly affects the stereochemistry of the epoxidation, and sometimes reversed face selectivity is observed.4 In contrast, the. E-substituent (R1) protrudes into an open space and E -allylic alcohols are generally good substrates for the epoxidation. 

Table 1 shows the dimensions of the zeolite L crystals prepared by varying the water content in the preparation. As the water content in the reactant gel is increased the aspect ratio of crystals was found to increase. Figure 1 shows SEM images of the three types of crystals, from which this increase in aspect ratio with water content can be observed. 

JCPDS card 25-0297 Single crystal prepared from the elements... 

Since polymer single crystals prepared these days are too small for x-ray diffraction experiments, the Crystal Structure of a polymer is generally determined from x-ray patterns of a fibre drawn from the polymer. Due to the alignment of the crystalline regions with the long axes of the molecules parallel to the fibre axis, the pattern is essentially identical to a rotation pattern from... 

Single-crystal preparation by means of chemical transport reactions. (Ni-Sn-S compounds as an example)... 

Clavilier method for crystal preparation and flame annealing has been improved since 1980 and is employed as a standard procedure for singlecrystal electrode experiments using Pt, Pt metals, and Au. 

For a compound semiconductor to be useful as a substrate in studies of electrodeposition, it is desirable that clean, unreconstructed, stoichiometric surfaces be formed in solution prior to electrodeposition. For CdTe, the logical starting point is the standard wet chemical etch used in industry, a 1-5% Brj methanol solution. A CdTe(lll) crystal prepared in this way was transferred directly into the UHV-EC instrument (Fig. 39) and examined [391]. Figure 66B is an Auger spectrum of the CdTe surface after a 3-minute etch in a 1% Br2 methanol solution. Transitions for Cd and Te are clearly visible at 380 and 480 eV, respectively, as well as a small feature due to Br at 100 eV. No FEED pattern was visible, however. As described previously, a layer of solution is generally withdrawn with the crystal as it is dragged (emersed) from solution (the emersion layer). After all the solvent has evaporated, the surface is left with a coating composed of the... 

For synthetic drug substances, purification is most often accomplished through crystallization. Preparative chromatography is currently more widely used for biotechnology-derived products than for synthetic drug substances... 

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