The title organic salt, C4H6N3O+·C4H3O4 −·C4H5N3O, was synthesized from cytosine base and maleic acid. An intra­molecular O—H⋯O hydrogen bond occurs in the hydrogen maleate anion. The crystal packing is stabilized by inter­molecular N—H⋯O, N—H⋯N and C—H⋯O hydrogen bonds, giving rise to a nearly planar two-dimensional network parallel to (101).

The title compound, C4H8N3O+·ClO4 −, is built up from creatininium cations and perchlorate anions. Crystal cohesion and perchlorate stability are ensured by N—H⋯O hydrogen bonds that together with weak C—H⋯O inter­actions build up a three-dimensional network.

The asymmetric unit of the title compound, C6H8N+·HSO4 −, contains two cations and two anions which are linked to each other through N—H⋯O hydrogen bonds, formed by all H atoms covalently bonded to the N atoms. In addition, strong O—H⋯O anion–anion hydrogen-bond inter­actions are also observed.

Three new proton transfer compounds, [2-ammonio-5-methylcarboxybenzene perchlorate (1), (C8H10NO2(+)·ClO4(-)), 4-(ammoniomethyl)carboxybenzene nitrate (2), (C8H10NO2(+)·NO3(-)), and 4-(ammoniomethyl)carboxybenzene perchlorate (3), (C8H10NO2(+)·ClO4(-))], have been synthesized, their IR modes of vibrations have been assigned and their crystal structures studied by means of single-crystal X-ray diffraction. Their asymmetric units consist of one cation and one anion for both compounds (1) and (2). However, the crystal structure of compound (3) is based on a pair of cations and a pair of anions in its asymmetric unit. The three-dimensional Hirshfeld surface analysis and the two-dimensional fingerprint maps revealed that the three structures are dominated by H...O/O...H and H...H contacts. The strongest hydrogen-bonding interactions are associated with O-H...O and N-H...O constituting the highest fraction of approximately 50%, followed by those of the H...H type contributing 20%. Other close contacts are also present, including weak C...H/H...C contacts (with about 10%).

Key indicators: single-crystal X-ray study; T = 100 K; mean (C-C) = 0.002 Å; R factor = 0.033; wR factor = 0.093; data-to-parameter ratio = 20.3.The title compound, C 7 H 8 NO 2 + ÁH 2 PO 3 À , is formed from alternating layers of organic cations and inorganic anions stacked along the a-axis direction. They are associated via O-HÁ Á ÁO, N-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonding, giving rise to two different R 2 2 (8) graph-set motifs and generating a three-dimensional network. Related literature ExperimentalCrystal data