Organic moieties crystallization

Structure of the lithio derivatives (84CC853) The crystal structure of a THF solvate of the lithium derivative of racemic bislactim ether (derived from two molecules of alanine) has been determined. In the solid state, the lithium derivative exists as a dimer in which the two lithium atoms are nonequivalent (189). The two organic moieties in each dimer are homo-chiral this means that the crystal contains equal number of enantiomers. The Li Li distance is 2.61 A. In THF solution at - 108°C, the compound seems to exist as an equilibrium mixture of monomer and dimer in the ratio 5 1. It is not clear at the moment whether the reacting species is the monomer or the dimer. 

Figure 2.26 Width of the metastable zone for an organic moiety nucleating in an organic solvent. The width is nearly the same for a cooling and evaporative crystallization. Note, that the cooling rates are given in mK/min and...

Most batch crystallization processes developed involve the crystallization of organic moieties, and, in most cases, the crystallization is from the reaction broth. This... 

Basically, two possibilities to start crystallization exist either spontaneous nucleation is initiated by increasing the supersaturation beyond the supersolubility curve or by seeding with crystals of the moiety to be crystallized within the metastable zone. Figure 10.2 shows a typical solubility curve of an organic moiety and its region of metastability. A cooling crystallization will start at point A and cross the solubility at point B. Without the addition of seed crystals, the system can be cooled to point C or D, before spontaneous nucleation occurs. The point of spontaneous nucleation will depend inter alia on the purity of the system and the rate at which the solution is cooled. 

During the nudeation phase, the amount of material crystallizing instantaneously depends on the width of the metastable zone (cf Chapter 2). This amount can be substantial for organic moieties, sometimes more than 30-40% of the entire material will crystallize upon spontaneous nudeation. This material crystallizes at high supersaturations, which can have negative effects on the purification via crystallization. 

Metal containing polyynes are multifunctional materials which combine the properties of organic polymers with those of metal centers coordinated to the organic moiety and are able to form nanotemplates, colloidal photonic crystals, multilayer capsules and hollow vesicles [127, 128], An example of a rod-like polymetallayne self-assembly in hollow nanorods has been recently reported [129] the computer simulations of the nanostructure show that the polymer chains are ordered in parallel lines that give rise to a tubular morphology rather unusual for these materials, but promising for sensor devices applications. 

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