Crystal formation seeding strategy

Although the crystallization process for strategies 1-4 inherently affords homochiral single crystals, only the use of homochiral tectons guarantees that all crystals will be of the same enantiomorph. Indeed, batches of crystals will often be heterochiral as both enantiomers tend to be formed equally during crystallization. Fortunately, it has been demonstrated58 that formation of homochiral bulk materials can be afforded by seeding with the desired enantiomer. 

Crystallization is generally preceded by two types of nucleation. The primary nucleation occurs with the formation of clusters of molecules at the submicron level. When the concentration exceeds saturation to afford supersaturation, the clusters become nuclei. The secondary nucleation is caused by particles due to primary nucleation or seeds. There are many strategies to achieve supersaturation to initiate crystallization such as cooling, evaporation, and antisolvent addition. 

The general recommendation is to keep supersaturation levels in the low to medium range to prevent additional nucleation events. This strategy has an additional beneficial effect on purity owing to slow crystal growth, the purity of the final crystals is usually higher compared to that in the case of fast growth rates. Finally, the addition of seeds efficiently avoids the formation of amorphous material, which can later recrystallize in an uncontrollable fashion. 

Most of the mentioned problems can, however, be overcome by addition of small amount of seed crystals (ZSM-5, silicalite-1) in the TPA+-free reaction mixture [6, 27, 35, 38, 40, 45, 50], Seed induction synthesis is a well developed strategy which could not only shorten the duration of synthesis, but also control the product properties [51] addition of seed crystals results in the formation of zeolite ZSM-5 with high degree of crystallinity and a narrow size distribution at short synthesis times [38,40]. Such method has been used for the synthesis of zeolites with various framework topologies [52]. Recently, small sized zeolites were obtained fastly, using this approach [53]. This method, although old, is still under developing. 

Concerning myth 1, it is conunon knowledge that some compounds crystallize easily, some have to be induced to crystallize, and some have never been crystallized. Useful, generally applicable hints concerning crystallization have been described elsewhere, and there is only little to be added to that. Empirical evidence indicates that the most favorable temperature for the spontaneous formation of the first seed crystals is lower than the temperature at which the bulk of the material crystallizes in the highest yield. Therefore, when more conunon strategies fail, it can sometimes be useful to concentrate a solution of a substance to thick syrup, close the flask, and cool it in dry ice. After some time (-30 min), remove it from cooling, and allow the... 

The next stage in the development of a systematic asymmetric synthesis strategy addressed the question of whether or not there was any feedback mechanism whereby the seeding of the initial growth solution with product of one chirality (say D, and formed from an earlier solid state reaction) would modify product formation by induced crystallization such that even more D isomer may be formed on further reaction. Green and Heller proposed a cyclic process as shown in Scheme 6.12a. It transpired, however, that in such experiments the product obtained from a crystal grown in the presence of right-handed product repeatedly caused preferential crystallization into a crystal of the left-handed form — the so-called inversion or reversal rule. ... 

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