Thermochemistry and conformational polymorphism of a hexamorphic crystal system

5-Methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile has been crystallized as six solvent-free polymorphs, which differ in the mode of packing and in molecular conformation. The conformational difference results principally from the thiophene torsion relative to the o-nitroaniline fragment, which leads to different crystal colors (red, orange, and yellow). Thermodynamic stability relationships between polymorphs' have been determined from solid-state conversions and calorimetric data of melting and eutectic melting. Vibrational spectroscopy and ab initio calculations showed that most conformers in solution feature perpendicularly arranged thiophene and o-nitroaniline fragments, although a minor population of more planar conformers also exist. Crystallization has a stabilizing effect for more planar and higher dipole conformers over perpendicular ones by 3-6 kJ/mol. The only exception to this pattern is the one polymorph containing weak intermolecular hydrogen bonds.