Co-crystallisation

The formation of host-guest inclusion compounds in crystals (or in highly ordered membranes or other biological structures) is a phenomenon more organised than simple co-crystallisation of a pair of substances. The special characteristic of inclusion crystallisation is a constant identity of one member in a series of pairs, this member thereby being identified as host, and a constancy of structure type (but not necessarily dimensions) for the host. The higher molecular organisation involved in inclusion... 

In 1979 the bieyclic diol exo-2,ejco-6-dihydroxy-2,6-dimethylbicyclo[3.3.1]nonane (i) was prepared and observed to co-crystallise with various solvents, including ethyl acetate, chloroform, toluene, dioxane, and acetone. A crystal structure determination of the ethyl acetate compound revealed the occurrence of a helical canal host structure, containing ethyl acetate as guest (with 3 1 diol ethyl acetate stoichiometry), and that spontaneous resolution had occurred on crystallisation of the multimolecular inclusion compound 6>. 

Recent work has, however, revealed the structure of other starch polymorphs not involved in inclusion behaviour of the above type, but which are inclusion compounds in their own right by virtue of their co-crystallisation with water. 

The medium impact-sensitivity of this solid propellant component is greatly increased by co-crystallisation of certain impurities, notably nitryl perchlorate, potassium periodate and potassium permanganate [1]. The presence of certain minimum amounts of mono-, di, tri- or tetra-methylammonium perchlorates in the salt leads to a single step decomposition, at around 290°C for the monoderivative [2],... 

The topical homochirality problem is presently being investigated in several research laboratories across the world. One new object of study is systems with eutectic mixtures. The addition of chiral dicarboxylic acids that co-crystallise with chiral amino acids to aqueous mixtures of d- and L-amino acids allows tuning of the eutectic composition of the amino acids in several cases, these systems yield new eutectic compositions of 98% ee or higher. Thus, solid mixed crystals with a ratio... 

The addition of a liquid matrix, usually glycerol, improves the reproducibility of the mass spectra because inhomogeneities caused by analyte and matrix co-crystallisation are avoided. In some cases, it also enhances signal intensity and resolution. However, analytes can also be desorbed from dry surfaces [7 9]. 

Co-crystallisation of cytosine or thymine with [Cu(L2m)][BF4]2 the tetraflu-oroborate was chosen owing to the fact that [BF4] anion forms, relative to nitrate anions, considerably weaker hydrogen-bonding interactions [67] led to [Cu(L2m)] [BF4]2 2cyt and [Cu(L2m)] [BF4]2-thy-H20,both of which adopt 2-D sheet architectures. The layered structure of [Cu(L2m)][BF4]2 2cyt is shown as a typical example in Fig. 37. In both structures, the sheets are linked by anion-mediated hydrogen-bonding interactions. 

When the co-crystallised samples were irradiated with UV light for several hours, the stereospecific (100%) formation of rcff-tetrakis(4-pyridyl)cyclobutane (99) was observed. This was confirmed by spectroscopic, analytical and structural methods. The latter showed that, once irradiated, the new co-crystallized materials (99) 2(97) and (99)-2(98) are formed respectively. In both these structures the molecular components are held together by hydrogen bonding interactions. 

The examples of this section summarise the recent application of the solvent-drop grinding approach to solid-state cocrystal preparation. The approach has been shown in certain instances to provide for either acceleration of CO crystallisation kinetics or selection of a particular polymorph via solid-state grinding. The approach is attractive, as it appears to incorporate some of the beneficial aspects of solvent participation while maintaining an essentially green, eco-friendly process. 

The slight conformational modification of the cycle A (revealed by X-ray diffraction), which probably comes from an interaction between the fluorine at C-9 and the axial OH at C-1, could contribute to the change in affinity [131]. However X-ray structure of the fluorocortisol co-crystallised with the glucocorticoid receptor does not clearly explain the impact of fluorine on the increased affinity for the receptor (cortisol, Ki=0.67pM vs 9a-fluorocortisol K,=0.027 pM) [132,133],... 

The product, 5 -CIDA 14 was co-crystallised with the fluorinase but without added i-Met. This resulted in a structure with 5 -CIDA 14 bound to the active site of the enzyme as shown in Fig. 5 [15]. 

Co crystallisation is mainly done from supersaturated sugar solutions [15]. Aggregated particles (of 3-30 pm) of sugar crystals are formed which entrap guest molecules. The sugars form an oxygen barrier, thereby extending the shelf life of aroma chemicals. The procedure is simple and inexpensive, because relatively cheap encapsulation matrices can be used, such as sucrose. 

Addition of a wax modifier to crude disrupts the normal crystallisation mechanism of wax, thereby reducing wax deposition. Since the chemical reacts during the onset of crystallisation by co-crystallisation, it is only effective when introduced at temperatures above the cloud point. 

Water-quenched mixtures crystallising at very extensive degrees of supercooling displayed only one melting peak and one small-angle X-ray scattering peak, which was taken as evidence of co-crystallisation of the components. 

At low temperatures, partial co-crystallisation was indicated by transmission electron microscopy and differential scanning calorimetry [156, 157]. Both electron microscopy of stained sections and optical microscopy showed that the segregated low molar mass material was present as small domains between the stacks of dominant lamellae within the spherulites/ax-ialites [115, 157, 158],... 

Scheme 7.4 Supramolecular networks from co-crystallisation of CTV and o-carborane.39...

The observation of Z > 1 may be regarded as a special case of co-crystallisation where the two components are the same as one another. Thus research on high Z structures is closely related to co-crystals, solvates, hydrates etc. Figure 8.48 shows the X-ray crystal structure of a remarkable... 

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