ABSTRACT:The discovery and phase transition studies of four polymorphs of 2-(phenylamino)nicotinic acid are described in this report. The four polymorphs were crystallized from solution under different conditions. All polymorphs differ in the degree of conjugation of the two aromatic rings and thus are of conformational polymorphism. Three of the four modifications have more than one conformer in the asymmetric unit. Due to the difference in conformation, all polymorphs display distinctive colors as well as different hydrogen-bonding arrangements. Phase behaviors of the four forms were studied by differential scanning calorimetry and hot-stage microscopy, indicating that metastable forms underwent phase transition to the stable one initiated by mechanical perturbation in solution or assisted by heating in the solid state. As such, this polymorphic system exhibits polychromism, conformational isomorphism, thermochromism, and mechanochromism. It may be valuable for further solid-state structure-property relationship studies.
Four polymorphs of 2-[methyl(phenyl)amino]nicotinic acid were discovered, and their single crystal structures were obtained at 90 K. In addition, one monohydrate and one salt were also crystallized and measured. Among the crystal structures, there are 17 crystallographically independent conformations. More interestingly, the four anhydrate forms are tessellated similarly by stacking of one-dimension hydrogen-bonding chains and, yet, the directionalities of the hydrogen-bonding chains are unique in each polymorph.
The structure of N''-cyano-N,N-diisopropylguanidine, a push-pull nitrile that includes the H(2)N-C=N-CN fragment, has been determined. Although the number of molecules in the asymmetric unit is exceptionally high (Z' = 10), there were no special crystallographic difficulties associated with data collection, structure solution or structure refinement because the individual molecules are small (12 non-H atoms) and because there is no important crystallographic pseudosymmetry. A complete set of N-H...N bonds is formed. Pairs of molecules form dimers, which associate to form ribbons that twist into a helix having five dimers per turn. The helices lie parallel to [101] and fit together with close contacts in three different directions perpendicular to that axis. The problems with optimizing so many different intermolecular contacts lead to a loosely packed structure.
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