Volumetric measure of isostructurality.

The numerical descriptors of isostructurality are critically reviewed and to overcome their limitations a new volumetric measure is proposed. It is defined as the percentage ratio of the overlapping volume of molecules in the analyzed structures to the average of the corresponding molecular volumes. For the calculation of this index a numerical approach is presented, which is also capable of treating disordered structures. The use of isostructurality calculations is demonstrated using a series of examples. Homologous 9-alkylthiophenanthrenes are used as simple illustrations of the necessity and applicability of the new descriptor. The structural changes in the inclusion compounds of 5-methoxysulfadiazine with chloroform, dioxane and tetrahydrofuran are analyzed and rationalized with the aid of isostructurality indices. The diverse relationships among a series of helical tubuland diols cocrystallized with simple phenols are also characterized. The modifications in molecular and crystal structures are correlated with the calculated degree of isostructurality.