Supramolecular Similarity in Polymorphs: Use of Similarity Indices (I^X)

A systematic investigation to assess the degree of similarity between polymorphs was carried out. A similarity indices (I^X) approach was applied in ten series of polymorphs with different characteristics and number of molecules in the asymmetric unit. Geometric (I^D), contact area (I^C), and stabilization energy (I^G) parameters were used. It was possible to situate each comparison in different regions of similarity within the polymorphism phenomenon and determine the boundaries between quasi-isostructural polymorphs and polymorphs of low similarity. The multiparameter I^DCG index was used as a robust tool to determine the total similarity within the polymorphism phenomenon. The highest contribution of the stabilization energy parameter (45%) toward the final value of similarity (I^DCG) was observed, followed by the contact area index (32%). The geometric index contributed approximately 23% to the final value of I^DCG. This information reinforces the importance of the contact area and stabilization energy in assessing the degree of similarity between crystalline structures. A new descriptor (I^Q) based on the comparison of the energetic contribution of intermolecular interaction types present in each crystal structure is presented. I^Q can be a versatile tool and applicable even for systems that do not share any similarity.