A versatile procedure for the identification, description and quantification of structural similarity in molecular crystals

A method is described for the identification of similar supramolecular constructs'' ( sub-components of complete crystal structures) of 0-, 1- or 2-dimensionality in different crystal structures of specific molecules ( polymorphs) or crystal structures of similar molecules ( families) and of 3-dimensionality in isostructural assemblies. A computer program, XPac, has been developed to enable a convenient application of the method, and procedures for the quantification of similarity relationships are proposed. The method is demonstrated and discussed on the basis of a number of case studies. It has been found that supramolecular constructs of different dimensionality are frequent and, therefore, likely to be important features in the consideration of nucleation, crystal growth and polymorphism.