Promotion of crystallization

Promotion of crystallization can be achieved, e.g., by inoculation with an appropriate enantiomorphic seed crystal, by the presence of further chiral compounds or by use of solvents which already contain a guest enantiomer in small excess. The latter method allows to win the other enantiomer which is now enriched in the filtrate of recrystallization by repetition of the process... 

Membrane surfaces act as a promoter of crystallization by lowering the activation barrier to the nucleation stage, thus allowing molecules to aggregate in conditions of supersaturation that would not be adequate for the spontaneous nucleation. The relatively small elapsed time for the appearance of biomolecular crystals demonstrates the existence of the molecule-membrane interactions that favorably affect the mechanisms of nucleation. A short list of biomolecnles tested by the authors is reported in Figure 10.7. 

Solubility and Solvent Resistance. The majority of polycarbonates are prepared in methylene chloride solution. Chloroform, i7j -l,2-dichloroethylene, yy -tetrachloroethane, and methylene chloride are the preferred solvents for polycarbonates. The polymer is soluble in chlorobenzene or o-dichlorobenzene when warm, but crystallization may occur at lower temperatures. Methylene chloride is most commonly used because of the high solubiUty of the polymer (350 g/L at 25°C), and because this solvent has low flammabiUty and toxicity. Nonhalogenated solvents include tetrahydrofuran, dioxane, pyridine, and cresols. Hydrocarbons (qv) and aUphatic alcohols, esters (see Esters, organic), or ketones (qv) do not dissolve polycarbonates. Acetone (qv) promotes rapid crystallization of the normally amorphous polymer, and causes catastrophic failure of stressed polycarbonate parts. 

The effect of crystal size of these zeolites on the resulted toluene conversion can be ruled out as the crystal sizes are rather comparable, which is particularly valid for ZSM-5 vs. SSZ-35 and Beta vs. SSZ-33. The concentrations of aluminum in the framework of ZSM-5 and SSZ-35 are comparable, Si/Al = 37.5 and 39, respectively. However, the differences in toluene conversion after 15 min of time-on-stream (T-O-S) are considerable being 25 and 48.5 %, respectively. On the other hand, SSZ-35 exhibits a substantially higher concentration of strong Lewis acid sites, which can promote a higher rate of the disproportionation reaction. Two mechanisms of xylene isomerization were proposed on the literature [8] and especially the bimolecular one involving the formation of biphenyl methane intermediate was considered to operate in ZSM-5 zeolites. Molecular modeling provided the evidence that the bimolecular transition state of toluene disproportionation reaction fits in the channel intersections of ZSM-5. With respect to that formation of this transition state should be severely limited in one-dimensional (1-D) channel system of medium pore zeolites. This is in contrast to the results obtained as SSZ-35 with 1-D channels system exhibits a substantially higher... 

The effect of crystal field splitting is easily seen by studying the absorption spectrum of [Ti(H20)6]3+ because the Ti3+ ion has a single electron in the 3d orbitals. In the octahedral field produced by the six water molecules, the 3d orbitals are split in energy as shown in Figure 17.3. The only transition possible is promotion of the electron from an orbital in the t2g set to one in the eg set. This transition... 

A growing number of protein crystal structures has provided solid evidence that in many phosphoesterase enzymes, two and sometimes even three, di- or trivalent metal ions are involved in substrate transformation. Consequently, the high catalytic efficiency is, in part, the result of a perfectly coordinated catalytic cooperation of the metal ions. Dinu-clear phosphoiyl transfer enzymes have been discussed thoroughly in recent reviews [1-3]. Therefore, this chapter (Section 2) only gives a brief description of enzymes for which two-metal promotion of phos-phoester hydrolysis was proposed on the basis of detailed mechanistic or crystallographic studies (Table 1). 

Initially, a reaction of A-acetoxy-A-butoxybenzamide 25c with A-methyl aniline 61 in butyl benzoate 63(R = Bu) and acetic acid. Close examination of these highly coloured reaction mixtures indicated the presence of crystals of A,A-dimethyl-A,A-diphenyltetrazene 65 (Scheme 11, R = Bu). The reaction is promoted by polar solvents as reactants are unchanged in pure acetonitrile. A crossover experiment using a mixture of /V- a ce t o x y - A-- b u t o x y - to 1 u a m i d e 26d and A-acetoxy-A-ethoxybenz-amide 25a afforded clean yields of butyl /Moluatc and ethyl benzoate thus pointing to an intramolecular rearrangement.41... 

The F center absorption maximum for KC1 is at 565 nm and that for KF is 460 nm (Table 9.1). (a) What is the composition of a natural crystal with color centers showing an absorption peak at 500 nm (b) If the absorption peak for KF corresponds to the promotion of an electron from the F center to the conduction band, determine the energy of the color center with respect to the conduction band. (The band gap in KF is 10.7 eV.) If the relative position of the color center energy level remains the same throughout the KF-KC1 solid solution range, estimate (c) the band gap of KC1 and (d) the band gap for the natural crystal. 

As a result of these studies it became evident that mechanical and chemical treatments may alter the relative amounts of crystallized and amorphous cellulose in a sample. For example, it was estimated that a normally coagulated viscose filament was about 40 % crystalline and 60 % amorphous while filaments of the same material, after being stretched, appeared to be 70% crystalline and 30% amorphous. This effect was observed to be more pronounced in cellulose derivatives than in unsubstituted cellulose where apparently the blocking of hydroxyl groups reduces the lateral forces of cohesion between chains, facilitates slipping and consequently promotes parallelization of chains. 

The improvement of enzyme like MIP is currently another area of intense research. Beside the use of the MIP themselves as catalysts, they may also be applied as enhancer of product yield in bio-transformation processes. In an exemplary condensation of Z-L-aspartic acid with L-phenylalanine methyl ester to Z-aspartame, the enzyme thermolysin was used as catalyst. In order to shift the equilibrium towards product formation, a product imprinted MIP was added. By adsorbing specifically the freshly generated product from the reaction mixture, the MIP helped to increase product formation by 40% [130]. MIP can also be used to support a physical process. Copolymers of 6-methacrylamidohexanoic acid and DVB generated in the presence of calcite were investigated with respect to promotion of the nucleation of calcite. Figure 19 (left) shows the polymer surface with imprints from the calcite crystals. When employing these polymers in an aqueous solution of Ca2+ and CO2 the enhanced formation of rhombohedral calcite crystals was observed see Fig. 19 (right) [131]. 

In gallstone patients, the nucleation of supersaturated bile requires either an excess of promoters, or a deficiency of inhibitors, of crystallisation (or both). For the past 20 years, attempts have been made to identify these pro- and antinucleating agents - but so far without consensus. Suffice to say that the promoters and inhibitors are mainly proteins and that mucous glycoprotein is particularly important - not only as a promoter of nucleation but also because it forms a gel on the surface of the gallbladder mucosa, which is believed to trap cholesterol crystals and contribute to the stasis within the gallbladder. Table 8.1 gives a list of potential promoters and inhibitors. 

Though the core expansion leads to the appropriate fit, it may not be the proper explanation for the scale factor discrepancy. Hansen et al. (1987) note that the expansion of the core would lead to a decrease of 7.5 eV in the kinetic energy of the core electrons, at variance with the HF band structure calculations of Dovesi et al. (1982), which show the decrease to be only about 1.5 eV. An alternative interpretation by von Barth and Pedroza (1985) is based on the condition of orthogonality of the core and valence wave functions. The orthogonality requirement introduces a core-like cusp in the s-like valence states, but not in the p-states. Because of the promotion of electrons from s - p in Be metal, the high-order form factor for the crystal must be lower than that for the free atom. It is this effect that can be mimicked by the apparent core expansion. 

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