Morphotropism: link between the isostructurality, polymorphism and (stereo)isomerism of organic crystals.

An ongoing analysis of the supramolecular self-assembly of disubstituted cycloalkanes has led to the discovery of seven packing patterns built up from hydrogen-bonded homo- and heterochiral chains of racemic molecules, associated in either antiparallel or parallel arrays [Kálmán et al. (2001). Acta Cryst. B57, 539-550]. Two further patterns have been revealed in the close packing of analogous alicyclic beta-amino acids [Fábián et al. (2005). Cryst. Growth Des. 5, 773-782]. Since each pattern is represented by at least one crystal structure, the chemical similarity and crystallographic forms of these crystals have facilitated the recognition that these patterns differ by one or two rotation(s) of the common motifs (e.g. dimers, tetramers, helices etc.), or the whole pattern may rotate through 180 degrees in an oblique unit cell. Such non-crystallographic--with the exception of polymorphism--virtual rotations as a whole may be denoted by the expression morphotropism. According to Kitaigorodskii [(1961), Organic Chemical Crystallography, pp. 222-231. New York: Consultants Bureau], morphotropism is an attempt to keep the packing coefficient above 0.6 whenever there are alternative possibilities for the structures of closely related molecules. It has been found that crystals of stereoisomers are also frequently related by such virtual rotations. Similarly, non-crystallographic rotations effect bridges between homostructural crystals [Kálmán et al. (1993b). Acta Cryst. B49, 1039-1049] and occasionally hallmark the polymorphism of organic compounds [Kálmán et al. (2003) J. Am. Chem. Soc. 125, 34-35]. In polymorphs, however, such rotations really transform one molecule into another in order to achieve a better packing mediated by solvents, temperature etc.