Stabilizing the unstable: In textbooks barbituric acid is always drawn in its keto tautomeric form, which is indeed preferred in solution and in most polymorphic phases. However, phase IV, obtained by grinding, consists of molecules in the enol form, as shown by neutron powder diffraction. This phase is found to be the most stable one at room temperature; the “unstable” enol tautomer is stabilized by a higher number of hydrogen bonds.

No abstract

The crystal structures of two industrially produced laked yellow pigments, Pigment Yellow 183 [P.Y. 183, Ca(C16H10Cl2N4O7S2), alpha phase] and Pigment Yellow 191 [P.Y. 191, Ca(C17H13ClN4O7S2), alpha and beta phases], were determined from laboratory X-ray powder diffraction data. The coordinates of the molecular fragments of the crystal structures were found by means of real-space methods (simulated annealing) with the program DASH. The coordinates of the calcium ions and the water molecules were determined by combining real-space methods (DASH and MRIA) and repeated Rietveld refinements (TOPAS) of the partially finished crystal structures. TOPAS was also used for the final Rietveld refinements. The crystal structure of beta-P.Y. 183 was determined from single-crystal data. The alpha phases of the two pigments are isostructural, whereas the beta phases are not. All four phases exhibit a double-layer structure, built from nonpolar layers containing the C/N backbone and polar layers containing the calcium ions, sulfonate groups and water molecules. Furthermore, the crystal structures of an N,N-dimethylformamide solvate of P.Y. 183, and of P.Y. 191 solvates with N,N-dimethylformamide and N,N-dimethylacetamide were determined by single-crystal X-ray analysis.

Crystal structures of API can be determined from X-ray powder diffraction data with good reliability. Fexofenadine exhibits only one polymorphic form, which is stabilised in the crystal by strong hydrogen bonds of the type (+)N-H···Cl(-), O-H···Cl(-), and between COOH groups.

The structure of the ζ phase of Pigment Red 53:2 was solved using electron diffraction data. The unit cell determination appeared to be more complicated due to twinning of the crystals. A series of electron diffraction patterns derived by sequential crystal tilt around a main axis consisted of two complementary sequences originating from different twin domains. Subsequent assignment of the obtained zones to the two individuals allowed zone indexing and lattice parameter determination. The structure was solved by intensity distribution analysis in selected experimental electron diffraction patterns. Finally, the derived structure was refined against X-ray powder data.