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A low-temperature redetermination of the title compound, C12H16N2O3, more commonly known as hexobarbitone, is reported, with significantly improved precision. The crystal packing reveals an infinite hydrogen-bonded hexobarbitone chain linked by a single N—H...O interaction, an extremely rare motif in barbiturate crystal packing. Unlike some other barbiturate crystal structures, there is no phase transition on cooling to 150 K.

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In the title complex, [CdCl2(C4H6N2S)2], the Cd atom has a distorted tetra­hedral coordination geometry, with two Cl and two monodentate neutral thione ligands bonded through S. There are intra­molecular N—H...Cl and inter­molecular N—H...S hydrogen bonds, generating centrosymmetric dimers.

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The title compound, 2-p-tolyl-5,6,7,8-tetra­hydro­benzo­[d]­imidazo­[2,1-b]­thia­zole, C16H16N2S, is a condensation product of pifithrin-α, which has been previously reported as an inhibitor of the tumour suppressor protein p53. The mol­ecule contains a planar fused pair of heterocyclic five-membered rings and the attached p-tolyl substituent is also essentially coplanar.

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In the title complex, (C16H36N)[WCuO2S2(C3H6N2S)], the W and Cu atoms have tetra­hedral and trigonal-planar coordination, respectively. Two sulfide ligands bridge the two metal centres; tungsten is additionally coordinated by two terminal oxo ligands and copper by the exocyclic S atom of imidazolidine-2-thione. There is an intra­molecular N—H...S hydrogen bond in the anion, and anions are linked into chains by N—H...O inter­molecular hydrogen bonds. The more loosely held cation is disordered.

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The asymmetric unit of the title compound, C6H16N22+·2(HO)2(O)PCH2P(O)2(OH)-, contains two singly charged diphospho­nate anions and two half-cations, each doubly charged cation lying on an inversion centre. Single deprotonation of methyl­enediphospho­nic acid to give a salt with an organic amine is unprecedented, double deprotonation being normal. All N-H and O-H groups act as hydrogen-bond donors in the crystal structure, with unproton­ated O atoms as the acceptors, giving a three-dimensional network.

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A tetra­gonal polymorph of malonamide, C3H6N2O2, is reported. The unit-cell dimensions, crystallographic symmetry and some aspects of the mol­ecular geometry are significantly different from those of the known monoclinic form [Chieh et al. (1970). J. Chem. Soc. A, pp. 179-184]. An R33(12) hydrogen-bonding motif links molecules together into a three-dimensional network.

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An ortho­rhom­bic polymorph of malonamide, C3H6N2O2, is reported. The unit-cell dimensions, crystallographic symmetry and some aspects of the mol­ecular geometry are significantly different from those of the known monoclinic form [Chieh et al. (1970). J. Chem. Soc. A, pp. 179-184] and of the tetra­gonal form described in the previous paper [Nichol & Clegg (2005). Acta Cryst. E61, o3424-o3426]. A simple R42(8) motif links the mol­ecules together and the symmetry of the mol­ecule means that this extends into a three-dimensional network.

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A second monoclinic polymorph of the title compound, neocuproine dihydrate, C14H12N2·2H2O, is reported. Unlike the first polymorph [Baggio, Baggio & Mombrú (1998). Acta Cryst. C54, 1900–1902], in which the phenanthroline ring system was constrained to lie in a crystallographic mirror plane, here there is no such imposed symmetry. Consequently, the mol­ecule shows small deviations from planarity, the outer rings being twisted slightly in opposite directions from the plane of the central ring. The hydrogen-bonding motifs remain essentially the same as in the first polymorph, involving small rings of four water mol­ecules and large rings containing four water mol­ecules and two neocuproine mol­ecules, but with no H-atom disorder for the water mol­ecules in this case. There are also aromatic π–π stacking inter­actions.

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A redetermination at 150 K of the structure of violuric acid monohydrate, C4H3N3O4·H2O, confirms that the space group is non-centrosymmetric Cmc21, despite indications from the intensity statistics and possible mol­ecular symmetry that it could be centrosymmetric Cmcm. Issues raised in the original reports [Craven & Mascarenhas (1964). Acta Cryst. 17, 407-414; Craven & Takei (1964). Acta Cryst. 17, 415-420] suggested either a disordered model or an ordered one with high thermal motion. The redetermination shows that an ordered model is correct, and the low-temperature data collection leads to normal displacement parameters. The precision of the structure is significantly improved in this new study. The violuric acid mol­ecule is entirely planar, and every atom in the structure lies on a crystallographic mirror plane. Violuric acid and water mol­ecules form hydrogen-bonded sheets.
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