Toward a quantitative description of crystal packing in terms of molecular pairs: application to the hexamorphic crystal system, 5-methyl-2-[ (2-nitrophenyl)amino]-3-thiophenecarbonitrile

Elements of a theory of crystal packing are presented in the form of a systematic analysis of crystal packing molecular pairs, i.e., neighboring molecule-molecule pairs in the crystal, rather than in terms of selected intermolecular atom-atom contacts. Intermolecular energies, based on the molecular electron density distribution and partitioned over Coulombic, polarization, dispersion, and repulsion contributions, are calculated for such pairs by the Pixel-semiclassical density sums (SCDS) method, recently updated for a better treatment of the dependence between electron density overlap and repulsion energy. The advantages of the pairs treatment are illustrated by a study of the six known polymorphs of 5-methyl-2-[(2-nitrophenyl)amino]-3- thiophenecarbonitrile, in which diffuse interactions between aromatic rings over a wide range of stacking modes are shown to contribute more to the cohesive energy than do lateral interactions between polar moieties, often described as weak hydrogen bonds.