The crystal structure of a polymorph of N′-[(4-hy­droxy­phen­yl)methyl­idene]pyridine-4-carbohydrazide reveals a distinct supra­molecular arrangement arising from variations in inter­molecular hydrogen bonding relative to the known polymorph.

The crystal structure of Na4(As2O5)(H2O)0.5 consists of a half-eclipsed (As2O5)4– anion embedded in a framework of [NaO4] and [NaO6] polyhedra. O—H⋯O hydrogen-bonding consolidates the packing.

In the title compound, the 4H-pyran ring adopts a boat conformation. The dihedral angle between the phenyl and pyran rings is 87.8 (18)°. In the crystal, mol­ecules are linked by N—H⋯O and N—-H⋯N hydrogen bonds.

The monoclinic and ortho­rhom­bic polymorphs of 2,6-di­methyl­phenol are constituted of very similar hy­dro­gen-bonded chains, but the packing of the chains differs significantly.

Single crystals of 2-amino-4-nitro­imidazole hydro­chloride were synthesized by slow evaporation and characterized by X-ray diffraction. The crystal structure features a planar imidazolium cation and a chloride anion, forming a three-dimensional supra­molecular network via N—H⋯Cl and N—H⋯O hydrogen bonds.

In the complex anion of the title salt, the Hg2+ ion is coordinated by four Cl− ligands, forming a distorted tetra­hedral geometry. In the extended structure, the cations and anions are linked via N—H⋯Cl and C—H⋯Cl hydrogen bonds.

The crystal structures of a solvated and an unsolvated di­thio­carbazate imine derivatives are compared and contrasted.

The title compound crystallizes with two independent mol­ecules in the asymmetric unit, which are connected into chains by N—H⋯O hydrogen bonds.

The supra­molecular assemblies in two benzamide-based derivatives are stabilized by inter­molecular hy­dro­gen bonding between the amide groups. Strong directional connectivity provided by hy­dro­gen bonds joins mol­ecules into one-dimensional chains. Thermal analysis allowed us to propose a mechanism for the transformations of one of the derivatives on heating, including exothermic polymerization, amide-group abstraction and partial benzene abstraction.

The extended structure of the title compound features strong N—H⋯N hydrogen bonds forming infinite C(4) chains propagating along the c-axis direction.

The title mol­ecular salt, C23H21N2+·C6H2N3O7−, featuring discrete ion pairs, crystallizes from ethanol through slow solvent evaporation. In the crystal, the imidazolium cation and the picrate anion are linked by N—H⋯O hydrogen bonds as a result of proton transfer from picric acid to the pyrimidine-type nitro­gen atom of the imidazole ring.

The crystal structures of the two title hydrated phases of K3(VO4) consist of isolated vanadate tetra­hedra linked via K+ cations. The difference in water content is noticeable in the hydrogen-bonding inter­actions, which define a finite network for the 0.56-hydrate phase and an infinite network for the 4-hydrate phase.

The title hydrated salt, a new monoclinic polymorph, crystallizes in space group P21/c. The components pack in sheets in the ab plane and these sheets are hydrogen bonded to their neighbours, generating a three-dimensional network.

The packing of the title solvate features hetero-halogen⋯halogen (Cl⋯F), and chalcogen (S⋯O) bonding, as well as non-standard offset π–π stacking.

The short C—NH2 bond length of 1.3469 (12) Å in the title mol­ecule is indicative of substantial involvement of the aniline N-atom in the aromatic π bonding system of the ring. In the extended structure, N—H⋯O hydrogen bonds link the mol­ecules into [001] tapes, which aggregate into zipper-like folded ribbons. Layers of parallel ribbons stack along a to complete the structure.

In the title compound, the dihedral angle between the coumarin ring system and the phenyl ring is 63.46 (5)°. In the crystal, the mol­ecules are linked by weak C—H⋯O hydrogen bonds and aromatic π–π stacking inter­actions

(Z)-N-(4-Fluoro­phen­yl)-2-oxo­propane­hydrazonoyl chloride forms concomitant colour polymorphs, namely, block-shaped pale-orange crystals of form I (space group P21/n, Z = 4) and needle-shaped pale-yellow crystals of form II (space group P21/c, Z = 4).

The structure of O-ethyl N-phenyl­thio­carbamate has been redetermined. Layers of hy­dro­gen-bonded dimers lie parallel to (031). A refinement using nonspherical scattering factors greatly reduced the number and severity of badly-fitting reflections.

Crystal structure determination and quantum chemical calculations have given a deeper insight into the structural features of 3-[5-(chloro­benzyl­idene)rhodanine]propionic acid derivatives, showing that the Z isomers are significantly more stable than the corresponding E isomers.

In the title salt, the cation is protonated at the thia­zole N atom and in the anion, the sulfonate group is deprotonated and an intra­molecular O—H⋯O hydrogen bond occurs. In the crystal, cation-to-anion N—H⋯O and anion-to-anion O—H⋯O hydrogen bonds link the component ions into (101) sheets.