Polymorphs, concomitant

Obtaining the thermodynamically metastable form in an enantiotropic system the information for obtaining and maintaining this form is essentially found in the energy-temperature diagram. 

In situations where there is overlap between the occurrence domains of two or more polymorphs the moditications may appear simultaneously or in overlapping stages so that a particular procedure or process yields more than one form under identical conditions. This phenomenon is termed concomitant polymorphism and has been treated in considerable detail in a recent review (Bernstein et al. 1999). 

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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)... 

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. 

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... 

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 ... 

Among the sugars, mannitol was mentioned earlier. It was one of the first concomitant polymorphs noted by Groth (1906a), citing work by Zepharovich (1888). In fact... 

Two-component (i.e. two molecules) systems also exhibit concomitant polymorphism, implying a balance for the equilibrium situations governing the formation of the isomeric complexes as well as the kinetic and thermodynamic factors associated with the crystallization processes. The often serendipitous nature of the discovery of concomitant polymorphs is also illustrated by an example of a hydrogen-bonded two-component system, pyromellitic acid 3-XIV and 2,4,6-trimethylpyridine 3-XV (Biradha and Zaworotko 1998). The first polymorph (A) was obtained by reacting 3-XIV with four equivalents of 3-XV in a methanolic solution. Using 3-XV as the solvent yielded a second polymorph (B) in 15 min. Modification A was found to have crystallized as well in the same reaction vessel after about 24 h. These two polymorphs are not readily distinguishable by their morphology. However, the authors point out that the experimental evidence indicates that Form B is the kinefically controlled one, while Form A is the thermodynamically preferred one. 

Fig. 3.2 (See also plate section.) Three of the concomitant polymorphs of the cyanine oxonol dyes 3-XVI and 3-XVII. Gold (by reflection otherwise red by transmission) and red forms mentioned in the text are easily distinguishable. The third form is a purple one, normally diamond shaped as on the middle right, but many of these crystals are undergoing conversion, as indicated by varying degrees of mottled surfaces. (From Bernstein et al. 1999, with permission.)...

A number of cases of concomitant polymorphs in which the variable of time or the equilibrium composition of the solution might be factors demonstrate the principles discussed in Section 1.2. 

To this point this account of instances of concomitant polymorphs has been phenomenological. We have discussed the thermodynamic and kinetic crystallization of polymorphs. There is still the question if any insight concerning controlling the polymorph obtained can be gained from the study of the crystal structure of concomitantly crystallized polymorphs. A qualitative attempt was made to see if details of the... 

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