Concomitant polymorphism

Concomitant polymorphs concomitant polymorphs are polymorphic forms of a substance that crystallise out at the same time from the same crystallisation mixture. Here at the same time does not necessarily mean that the crystals form at exactly the same instant, just that they can be observed co-existing at some point during the crystallisation process. We have already come across concomitant polymorphs in the early example of benzamide where forms I and II are concomitant. Many more interesting and beautiful examples are covered in a recent review.62... 

Some other studies showed that the combination of the three polymorphs with reduced crystallite size and high surface area can lead to the best photocatalysts for 4-chlorophenol degradation [37], or that particles in the dimension range 25-40 nm give the best performances [38]. Therefore, many elements contribute to the final photocatalytic activity and sometimes the increased contribution of one parameter can compensate for the decrease of another one. For example, better photocatalytic activity can be obtained even if the surface area decreases, with a concomitant increase in the crystallinity of the sample, which finally results in a higher number of electron-hole pairs formed on the surface by UV illumination and in their increased lifetime (slower recombination) [39]. Better crystallinity can be obtained with the use of ionic liquids during the synthesis [39], with a consequent increase of activity. 

Crystallization remains the primary means of controlling the polymorphic or solva-tomorphic state of a compound, and various groups have examined the influences of processing parameters on the identity and quality of the isolated form. Seeding was used to reduce the size of the metastable zone of eflucimibe, and thereby control the identity of the desired polymorphic identity of the product through a reduction in concomitant crystallization [16], Process improvements have been developed that were found to improve the filterability and enhance the bulk density of ranitidine Form-1 [17], while the variation of process parameters used in an oscillatory baffled crystallizer enabled better selection to be made between the metastable a- and /i-forms of (z.)-glutamic acid [18]. 

The isolation of crystalline products having mixed polymorphic compositions (often referred to as concomitant polymorphism) remains a topic of interest, even though the phase rule predicts that a system at equilibrium consisting two components (solvent + solute) and three phases (solution + Form I + Form II) is uni variant. Hence, for crystallizations performed at a fixed pressure (typically atmospheric) the system becomes nonvariant and genuine equilibrium can exist at only one temperature. Therefore, concomitant products must be obtained under nonequilibrium conditions. Flexibility in molecular conformation was attributed to the concomitant polymorphs of a spirobicyclic dione [34] and of 3-acetylcoumarin [35],... 

Given the complementary nature of molecular recognition, it would appear that, when a compound crystallizes, the crystallization pathway, and hence the crystal structure obtained, should be quite specific to the molecule in question. However, the very existence of the phenomenon of polymorphism indicates that, under certain conditions, alternative crystallization pathways are feasible. In the special circumstance of concomitant polymorphism, or the simultaneous appearance of polymorphic forms in the same crystallization batch, these pathways even co-exist.1321 So, in general, the study of polymorphic systems has a bearing on a better... 

Concentration standards, 75 750-751 Concentrators, sulfuric acid, 23 787 Concentric annular reactors, 23 544 Concentric cylinder viscometer, 27 733 Concentric hemispherical analyzer (CHA), 24 103-104, 105 energy resolution of, 24 106 Conching, milk chocolate, 6 363-364 Concomitant polymorphism, 8 69 CONCORD program, 6 10 76 752 Concrete(s)... 

Where drug are shown to form reactive intermediates by specific enzyme systems, it might be expected that the polymorphism or absence of these systems would track idiosyncratic reactions but to date no such link has been established. In fact, the most confounding element of idiosyncratic reactions is their unpredictable nature and whilst reactive intermediates may present a risk factor the true nature of the response is likely to be multifactorial. Changes in dmg metabolizing capacity, affecting the balance of bioactivation and bioinactivation processes which may be mediated through concomitant infection may also be a contributory factor. 

It is important that the solid-state chemists keep a watchful eye on the possibility of disappearing polymorphs [92] and also of the well-known phenomenon of concomitant polymorphs which has been discussed recently [93], The generation of new polymophs via the addition of a meaningful choice of impurities in controlled amounts [93,94] is worthy of experimentation. 

A (molecular) crystal polymorph is a solid crystalline phase of a given compound resulting from the possibility of at least two different arrangements of the molecules of that compound in the solid state Conformational polymorphs are formed by molecules that can adopt different conformations in different crystal structures formed by coordination complexes where ligands bound in delocalized bonding modes adopt different relative orientations Concomitant polymorphs are polymorphic modifications of the same substance obtained from the same crystallisation process Pseudo-polymorphs are ... 

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