CLASSIFICATION OF THE ISOSTRUCTURALITY OF ORGANIC-MOLECULES IN THE CRYSTALLINE STATE

Since its discovery by Mitscherlich in 1819, the isomorphism of crystals has been interpreted in various, sometimes controversial ways. This can be attributed to the fact that the word isomorphous refers only to the external similarities between crystalline substances. Identical or quasi-identical packing motifs of related organic substances should therefore be distinguished by a more appropriate terminology. For organic substances, in contrast with inorganic crystals, where the recommended term is isotypic, the term isostructural is unambiguous. The present work attempts to classify the main forms as isostructural and homeostructural. Within the former class, there are two subclasses distinguished by the degree of isostructurality. Special attention is also paid to the isostructurality of molecular associates (e.g. clathrates). To estimate the degree of isostructurality two descriptors are recommended: (a) the unit-cell similarity index, and (b) the isostructurality index.