Crystallization additives

Material crystallized Additive(s) Effect Concentration References... 

In addition to induction time measurements, several other methods have been proposed for determination of bulk crystallization kinetics since they are often considered appropriate for design purposes, either growth and nucleation separately or simultaneously, from both batch and continuous crystallization. Additionally, Mullin (2001) also describes methods for single crystal growth rate determination. 

All diaziridines which contain at least one NH-group give, with chloral, w ell crystallized addition products in molar ratio l ." The adducts liberate tw-o equivalents of iodine and thus retain the CNN three-membered ring [Eq. (38)]. By treatment with alkali the chloro-... 

When l,2,3,4-tetrakis(4-pyridinyl)cyclobutane is treated with ethyl chloroformate in the presence of ethyldiisopropylamine, radialene 74a is formed and can be isolated as red crystals. Addition of AgBEt to a red solution of 74a results in an immediate color change to deep blue caused by the formation of the dicationic species 75a (equation 5)43. Undoubtedly, the tetrapyridiniocyclobutane 73a is an intermediate in the formation of 74a. By way of contrast, the fourfold deprotonation of the analogous tetrakis(7V-methyl-4-pyridinyl)cyclobutane (73b) did not succeed in the presence of oxygen. Treatment with NaH/EtOH in the presence of oxygen produced the dication 75b, which could be reduced to the [4]radialene only electrochemically44. Deprotonation of 73b with sodium hydride... 

In general, the limitations of color centers as lasers arise from their poor thermal stability and the degradation of their optical bands nnder illnmination. In fact, F2 center based solid state lasers must be stabilized by incorporating additional impurities into the crystals. Additionally, color center lasers shonld operate at low temperature, a problem that strongly limits their mannfactnre. 

Problems arising from the crystallization of lactose in milk and whey powders may also be avoided or controlled by pre-crystallizing the lactose. Essentially, this involves adding finely divided lactose powder which acts as nuclei on which the supersaturated lactose crystallizes. Addition of 0.5 kg of finely ground lactose to the amount of concentrated product (whole milk, skim milk or whey) containing 1 tonne of lactose will induce the formation of c. 106 crystals ml-1, about 95% of which will have dimensions less than 10/an and 100% less than 15 /an, i.e. too small to cause textural defects. 

Figure 6 provides a series of 29 Si MAS NMRs taken of PMS samples heated to selected temperatures. The Kumada rearrangement occurs at 400 °C and essentially phase pure SiC (2-4 nm crystallites by TEM) crystallizes at 1000 °C. Holding the polymer for periods of 5-10 h at 800 or 900 °C also results in crystallization. Additional MAS29Si NMR studies on samples pyrolyzed to 1400 °C showed no changes in peak position (from —15.7 ppm), suggesting the formation of well-defined /1-SiC crystalline phases at 1000 °C. 

With values for the fractional filling of each cavity type, hydrate density may be determined based upon a unit crystal. Additional required input data are the dimensions of a unit crystal, the number of water molecules per crystal and the number of small and large cavities per unit crystal, as specified in Table 2.2. Based upon a single unit cavity, the hydrate density (p) may be calculated by the formula ... 

Remarkable changes in the product profiles were recognized when the reaction was carried out at low (5%) conversion or at low temperature (-78°C). In the former case, a good ee value (77% ee) was obtained with exclusive product formation (>95% as observed by H-NMR of the crude mixture). Under these conditions, the molecules were supposed to be still under topochemical control leading to the better stereoselective reaction in the crystals. Additionally, the reaction at -78°C gave similar results (50% yield, 65% ee) for the same reason, where the molecules were strongly frozen in the crystal lattices. Obviously, the topochemical control was much more effective at lower temperature since 65 %ee was obtained even at 50 % conversion, which value corresponded to about 39 %ee for 0°C on estimation of the curve. Apart... 

Crystallization additives are a common method to improve crystal quality in proteins (McPherson and Cudney, 2006). With RNA, it is common to screen a series of different cations, as their role in RNA structure and catalysis is well documented for many RNA and RNA-protein systems (Pyle, 2002). Since various cations will interact with the RNA differently, our lab uses a cation screen comprised simple metal cations and polyamines (Table 6.1). Each solution in this table is a lOx stock that is added to an optimized condition (or one found in a sparse matrix) and the set of 24 conditions assessed for potential improvement in crystal quality. 

To a slurry of (CsMes C Ihq (0.62 g, 0.64 mmol) in ethanol (40 mL) was added 1,5-cyclo-octadiene (0.82 mL, 6.7 mmol). The solution was refluxed for 90 min, and the solution color changed to a clear orange and an off-white precipitate formed. The solvent was removed under vacuum, the residue was extracted with diethyl ether (4-30 mL), and the extracts were filtered. The filtrates were combined, concentrated to ca. 50 mL, and cooled to —20°C to afford orange crystals. Additional crops of crystals were obtained by further concentrating and cooling the supernatant. Yield 0.48 g (74%). 

The coincidence of substrate and adsorbate symmetries is crucial for the ordering, i.e., for the formation of 2D crystals. Additionally, the HBC 22 molecule, which itself resembles a graphite segment, is oriented along the HOPG lattice in the same way as an (second) imposed graphite layer.230 This concludes that the substrate rules the orientation of the molecules. Ordered monolayers of HBC could also be used as a ground floor for deposition of a second layer... 

After the identification of aprepitant as a clinical candidate, Merck invested considerable process research toward an improved synthesis of aprepitant, which culminated in the elegant manufacturing process shown in Scheme 6.21,22 The key step relies on displacement of a trifluoroacetate from intermediate 48 by the optically active alcohol intermediate 49. The synthesis of 49 was accomplished via an oxazaborolidine-catalyzed borane reduction of the corresponding acetophenone. Although the displacement resulted in an almost equal mixture of the two diastereomers 50 and 51, the desired diastereomer 50 could be recovered in high yield by base-catalyzed equilibration of the mixture and crystallization. Addition of p-fluorophenyl magnesium bromide followed by hydrogenolysis afforded the key intermediate 40, which can be readily converted to 1 as detailed in the previous synthesis. 

For a Newtonian liquid in contact with the quartz crystal, additional... 

Crystallization of o-acetotoluidide sometimes occurs at this stage and is allowed to go to completion. This is indicated by a decrease in the evolution of heat of crystallization. Addition of nitrous acid is not begun until the reaction mixture has reached 3°. 

On exposure to air and moisture, sodium metabisulfite is slowly oxidized to sodium sulfate with disintegration of the crystals. Addition of strong acids to the solid liberates sulfur dioxide. 

Such an inductor generates a temperature field of almost square symmetry. The main slit in the inductor, which forms the current loop, however, is not completely equivalent to the other three slits as seen in Fig. 3.9b. Furthermore, the surface tension surrounds the corners by minimizing the melt surface, which is not desired because the goal are quadratic wafers directly cut from the crystals. Additionally, on the straight sides, where the horizontal curvature is almost zero, growth instabilities can occur. Regarding the application for the solar industry, which would save material loss and the costs of cutting the round crystal into a square one, it is essential to get a stable... 

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