Conformational differences and intermolecular C-H...N interactions in three polymorphs of a...

The structural characterization of several polymorphic forms of a compound allow the interplay between molecular conformation and intermolecular interactions to be studied, which can contribute to the development of strategies for the rational preparation of materials with desirable properties and the tailoring of intermolecular interactions to produce solids with predictable characteristics of interest in crystal engineering. The crystal structures of two new polymorphs of 5,6-dimethyl-2-(pyridin-2-yl)-1-[(pyridin-2-yl)methyl]-1H-benzimidazole, C₂₀H₁₈N₄, are reported. The previously reported polymorph, (1) [Geiger & DeStefano (2014). Acta Cryst. E70, o365], exhibits the space group C2/c, whereas polymorphs (2) and (3) presented here are in the Pnma and P-1 space groups, respectively. The molecular structures of the three forms differ in their orientations of the 2-(pyridin-2-yl)- and 1-[(pyridin-2-yl)methyl]- substituents. Density functional theory (DFT) calculations show that the relative energies of the molecule in the three conformations follows the order (1) < (2) < (3), with a spread of 10.6 kJ mol^-1. An analysis of the Hirshfeld surfaces shows that the three polymorphs exhibit intermolecular C-H...N interactions, which can be classified into six types. Based on DFT calculations involving pairs of molecules having the observed interactions, the C-H...N energy in the systems explored is approximately -11.2 to -14.4 kJ mol^-1.