Conformational polymorphism isomorphism

Fig. 5.1 Schematic illustration of three possibilities for arrangement of molecules of the same chemical composition but different conformations in the crystal. For each case two conformations represent symbolically cisoid and transoid dispositions around a single bond. The rectangle defined by a broken line represents one possible choice of the unit cell, (a) and (b) conformational polymorphs (c) conformational isomorphism (d) conformational syn-morphism. (From Bernstein 1987, with permission, as adopted from Corradini 1974, with permission.)...

Polymorphs are classified according to the following terminologies. Concomitant polymorphs crystallize simultaneously from the same solvent and crystallization flask under identical crystal growth conditions. They may be viewed as supramolec-ular isomers in a chemical reaction. Conformational polymorphs occur for flexible molecules, i.e. these molecules can adopt more than one conformation under ambient conditions. When different conformers of the same molecule are present in the same crystal structure the situation represents conformational isomorphs. Conformational isomorphism, the existence of multiple conformations in the same crystal structure, is closely related to the presence of more than one molecule in the asymmetric unit, i.e. Z >1. The exact reasons why some crystals have Z > 1 are still not properly understood even as several research groups are working to seek answers to this enigma [9]. Pseudopolymorphism, [10] the occurrence of the same molecule with different solvent molecules in the crystal lattice or the same solvent in a different stoichiometry, is closely related to polymorphism. 

Diphenyl-2,5-cyclohexadienone 3 exists as four polymorphs, labeled A-D, whose crystallographic details are summarized in Table 3-2 [14]. Polymorphs A-D are conformational polymorphs since they have different molecular conformers in their crystal structures. They are also concomitant polymorphs because they crystallized simultaneously from the same flask and under identical crystal nucleation and growth conditions. Forms B-D with multiple molecules in the asymmetric unit Z > ) may also be classified as conformational isomorphs. Such polymorph clusters with different molecular conformations, crystal packing and C—H - O... 

Mocilac P, Donnelly K, Gallagher JF (2012) Structural systematics and conformational analyses of a 3x3 isomer grid of fluoro-N-(pyridyl)benzamides physicochemical correlations, polymorphism and isomorphous relationships. Acta Cryst B68 189... 

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