Orientation-dependent conformational polymorphs in two similar pyridine/pyrazine phenolic esters
On the basis of crystal engineering landscape, two similar esters of phenyl pyridine-2-carboxylate (I) and phenyl pyrazine-2-carboxylate (II) were designed and synthesized. The compounds were characterized using FT-IR, mass spectrometry, CHN-elemental analyses, NMR and PXRD. For each compound, two polymorphs were obtained (Ia, Ib and IIa, IIb) and identified by TGA, DSC and SCXRD. Comparison of crystal structures for polymorphs revealed that different torsion angle of pyrazine (τ1) and phenyl ring (τ2) to the ester backbone make conformers of I and II, respectively. Theoretical criteria (max(Δθ), rmsd[r]-crystal, energy profile) confirmed that the structural differences of conformer are in the range of acceptable value for the detection of conformational changes. Phase stability of polymorphs were investigated by slurry and grinding methods as well as HSM technique. Since the orientation of pyrazine and phenyl moieties altered in polymorphs, hydrogen bond donor and acceptors including C−H∙∙∙N, C−H∙∙∙O and C−H∙∙∙π interactions, take meaningful supramolecular architectures in the crystal packing. The energetic study of the noncovalent interactions in the molecular pairs (dimer) of polymorph crystal structures was performed by DFT-D calculations.