Crystallization processes complexity

On the other hand, the crystallization process of diolefin compounds often plays a significant role in determining their topochemical behaviour, by changing their crystal structure or by forming solvent inclusion complexes. Furthermore, topochemical photoreactions of crystals with )8-type packing are accompanied by thermal processes under moderate control by the reacting crystal lattice (see p. 140). These factors seriously complicate the whole reaction scheme. 

Other detection modes in bright CL or BL reactions are multichannel detectors, which provide simultaneous detection of the dispersed radiation and produce a permanent image of a wide area. Photographic films or plates are emulsions that contain silver halide crystals in which incident photons produce stable clusters of silver atoms within the crystals. Internal amplification is provided in the development process by an electron donor that reduces the remaining silver ions to silver atoms within the exposed crystals. A complexing agent is used to remove the... 

The limited results obtained are not inconsistent with a 1.25 dependence on supersaturation as proposed by Shiau and Berglund (10). The dependence on solvent composition and temperature appears complex and is probably related to the solution viscosity which varies considerably over this range. It thus appears that mass transfer plays a significant part in the crystallization process. Further work needs to be done to clarify these effects. 

In other studies, changes in the structural arrangement of the trinudear units was not only due to the change in substituents. In fact, another unusual structural finding, that also led to a difference in the optical properties of the complex [Au3(CH3N=COCH3)3], appeared as a consequence of the discovery of polymorphic structures in the crystallization process [45]. 

Somewhat similar are the so-called adductive crystallization processes, often (wrongly) called extractive crystallization, where reactions of complex/ adduct formation are used to separate compounds that are otherwise difficult to separate. Examples of adductive crystallization include separation of p- and m-cresols (137), separation of o- and p - n i troch I oro ben zcn cs (138), separation of quinaldine and isoquinoline (139), separation of nonaromatic compounds from naphtha-cracking raffinate (140), and separation of p-cresol from 2,6-xylenol (141). Other examples of reactive crystallization/precipitation reported in the literature are listed in Table 5. 

Fractions of higher molecular mass, which are mostly insoluble, can withhold large amounts of metals, especially in alkaline environments. Metals are thus subtracted from precipitation and subsequent crystallization, processes that would decrease their availability (Schwertmann, 1966), and a reserve of micro-nutrients is created which is in equilibrium with complexing molecules. On the other hand, under conditions of high metal concentrations, complexation by humified organic matter may limit the amount of metals in solution under these conditions, interchain bonds may form, with possible precipitation of humic molecules. This process can be important for toxic elements, the activity of which can thus be reduced to nontoxic levels (Gerke, 1992). 

The steric course of the photodimerization reactions of coumarin (37a) and thiocoumarin (37b) succeeded in being controlled almost perfectly by carrying out the reaction in inclusion complexes using various host compounds. Furthermore, enantioselective dimerization reactions of 37a and 37b were found to proceed through a single-crystal to single-crystal process. 

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