Two 1H-3-nitro-5-methylpyridine-2-one polymorphs crystallized in centro- and noncentrosymmetric orthorhombic space groups Pna21 and Pnma with similar unit cell parameters are prepared and characterized by single crystal XRD. The analysis of the crystal packing sequentially constructed from a single molecule to the crystal showed that both polymorphs are characterized by the same supramolecular motif: a layer constructed from H-bonded chains arranged along second-order helical axes. In one case, the chains are nonplanar, so corrugated chiral layers are formed; in the other case, all molecules of a planar layer lie on a mirror symmetry plane. Different conformations of generally similar layers result in their different combinations, so that the polymorphs are crystallized in different space groups. crystal lattice and cohesion energies calculated by the PBE0-D3/POB-TZVP quantum chemical method show that the packing of the less dense polymorph with planar layers is 1.0 kcal/mol more profitable, and the main energy gain is due to the planar chain formation.
I. V. Fedyanin (Sun,) studied this question.