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Some of these crystals also represent problem cases as the ensuing crystals are generally small and extremely prone for different kinds of twinning. Still, a first classification of such systems seems to be feasible. Results of these investigations will be reported concurrently.

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In the title compound, C7H6N2O4, an intra­molecular N—H⋯O hydrogen bond generates an S(6) ring. In the crystal, inversion dimers linked by pairs of O—H⋯O hydrogen bonds generate R 2 2(8) loops. Inter­molecular N—H⋯O and C—H⋯O hydrogen bonds then link the dimers, generating R 3 3(16)R 2 1(6) motifs. The whole mol­ecule is essentially planar, with the greatest deviation from the mean plane being 0.065 (2) Å.

In the title hydrated salt, C6H9N2O+·C14H9O3 −·H2O, the dihedral angle between the benzene rings of the 2-benzoyl­benzoate anion is 82.04 (14)°, while the angles between the aromatic ring of the pyridinium cation and each of the benzene rings of the anion are 4.42 (14) and 82.04 (14)°. In the crystal, mol­ecules are linked by N—H⋯O and O—H⋯O hydrogen bonds, generating a three-dimensional network with R 2 2(8), R 6 6(16) and R 4 4(6) motifs. The crystal packing is further stabilized by two π–π inter­actions, one between pyridinium rings, and another between the benzene benzoate and pyridinium rings of neighbouring mol­ecules, with centroid-to-centroid distances of 3.559 (2) and 3.606 (2) Å, respectively.

A 3×3 isomer grid of nine methoxyphenyl-Npyridinylcarbamates (C 13 H 12 N 2 O 3 ) as CxxOMe (x = para-/meta-/ortho-) was synthesized and studied to determine the crystal structures and correlate structural relationships from both ab initio calculations and the solid-state using conformational analysis. Eight of nine isomers crystal structures were determined using single crystal X-ray diffraction. All isomers form N-H…N hydrogen bonds as primary interaction, with one isomer (CmmOMe) forming a relatively unusual disordered hydrogen bonded trimer via the N-H…N interactions, while the CoxOMe isomers form the N-H…N hydrogen bonded dimers. In all isomers additional C-H…O interactions aid aggregation, whereas in most of isomers the methoxy group is engaged in important C-H…O hydrogen bonding patterns.The solid state and the modelled conformations are mismatched in 3 of 8 molecules in which methoxy groups and meta-methoxyphenyl rings (mOMe) of the solid state structures adopt meta-stable or unstable conformations as compared to the optimised computational models.