Crystallization of molecules

In the pharmaceutical industry most solid compounds are crystallized in batch operations, i.e., the crystalline product is isolated at the end of the operating cycle. Many bulk chemicals, such as table sugar, are prepared through continuous processes, in which the product is collected throughout the crystallization cycle. Batch crystallization produces a narrower range of particle size and may afford better control for the efficient crystallization of molecules from complex mixture [14]. 

Electrocrystallisation is the usual method appHed to grow single crystals of molecule-based conductive materials. Confined electrocrystallisa-... 

The structure of potassium hydrogenmaleate, Fig. 7.14, is closely related to the other dicarboxylates but here the H-bond is intramolecular. Growing single crystals of molecules that were deuterated at the carbon positions uniquely isolated the INS of this symmetric O-H-0 bond [56]. The resulting INS spectrum provided conventional values for the y- and < modes but was unexpectedly rich in features below 1000 cm, associated with the Ks mode. 

The crosslinking between PTFE polymer chains and fluorinated-pitch should prevent crystallization of molecules from the molten state. When the crosslinking density is low, the molecules with rather long chain between crosslinking points might ciystallize, where the crystal size would be small and also the ratio of crystalline against amorphous parts would decrease. The DSC results are well explained by above model. 

Bimodal distribution of branches, similar to the above two, except that the melting shows two endotherms. Usually a broad lower temperature melting and a sharp higher temperature melting, the latter for crystals of molecules with few branches. This is caused by uneven reactivity giving two phases in the reactor. 

Crystals of Molecules with Charge Transfer, Radical-ion Salts... 

While in the previous sections of this chapter we treated organic semiconductors which consist of single crystals of molecules with conjugated tt-electron chains, in... 

Vectra A is a commercially available polymer from Hoechst-Celanese. It is a random co-polyester of hydroxy benzoic acid (HBA) and hydroxy napthoic acid (HNA), which is a well-known class of thermotropic liquid crystalline polymer (TLCP) [92-95]. Crystallization of molecules of TLCPs is considerably different from that of polymers like polyethylene or polyethylene terphthalate [1], TLCPs have reduced flexibility compared to the latter, which implies that large translations of their molecules are required for recrystallization [96-99],... 

The first step in the generation of the database of molecular properties in cocrystals was the retrieval of co-crystal structures from the CSD. Co-crystals were defined as organic structures containing at least two eharge neutral nonsolvent molecules with different structural formulas. To avoid possible statistical bias by the frequent occurrence of some eommonly used eo-formers e.g., urea, 4,4 -bipyridine, see Table 1 in ref. 7), struetures with these moleeules were excluded. Initial survey of the data showed that inelusion eompounds, which consist of a large and a small molecule, form a distinet group in the data set. Co-crystals of molecules with more than 30 and with less than six nonhydrogen atoms were thus removed and the analysis foeused on eo-erystals other than inclusion compounds. The final data set eontained 710 struetures. 

It is possible to apply the plane-wave scheme to nonperiodic systems, such as isolated molecules, by using the supercell method, In the supercell method, identical copies of the molecule in question are used to build up a periodic crystal of molecules. If the distance between neighboring copies is large enough, then a plane-wave treatment of the supercell will give an adequate description of the isolated molecule. Recent results show that the supercell method can reproduce the accuracy of large Gaussian basis calculations at a competitive cost. 

---