2-Nitro-N-(4-pyridinio)benzenesulfonamidate monohydrate

The title compound, C11H9N3O4S·H2O, contains both an acid and a base centre and displays a zwitterionic structure. There are two independent molecules and two water molecules in the asymmetric unit. The dihedral angles between the benzene ring and the pyridinium ring are 109.7 (1) and 110.7 (1)°. The dihedral angles between the nitro group and the benzene ring are 116.1 (2) and 116.7 (1)°. The crystal structure is stabilized by N—H⋯O, O—H⋯N and O—H⋯O hydrogen bonds.

The title compound, C 11 H 9 N 3 O 4 SÁH 2 O, contains both an acid and a base centre and displays a zwitterionic structure. There are two independent molecules and two water molecules in the asymmetric unit. The dihedral angles between the benzene ring and the pyridinium ring are 109.7 (1) and 110.7 (1) . The dihedral angles between the nitro group and the benzene ring are 116.1 (2) and 116.7 (1) . The crystal structure is stabilized by N-HÁ Á ÁO, O-HÁ Á ÁN and O-HÁ Á ÁO hydrogen bonds.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: HG2431). = 1.365 (3) Å] indicate the slight conjugation of the sulfonamide N with the pyridinium ring (Allen et al., 1987).
The dihedral angles between the benzene ring and the pyridinium ring are 109.7 (1)° and 110.7 (1)° respectively. And the dihedral angles between the nitro group and the benzene ring are 116.1 (2)° and 116.7 (1)° respectively. The crystal structure is stabilized by N-H···O hydrogen bonds.

Experimental
A solution of 2-nitrobenzenesulfonyl chloride (2.2 g, 10 mmol) in CH 2 Cl 2 (10 ml) was added dropwise to a suspension of 4-aminopyridine (0.9 g, 10 mmol) in CH 2 Cl 2 (10 ml) at room temperature with stirring. The reaction mixture was stirred overnight. The yellow solid obtained was washed with warm water to obtain the title compound in a yield of 55.7%. A colorless single crystal suitable for X-ray analysis was obtained by slow evaporation of an NaOH (10%) solution at room temperature over a period of a week.

Refinement
The N-bound H atoms were located in a difference map and their coordinates were refined with U iso (H) = 1.2U eq (N). The C-bound H atoms were positioned geometrically (C-H = 0.93 Å) and refined as riding with U iso (H) = 1.2U eq (C). The Flack test results are ambiguous because of the presence of merohedral twin. The water O-bound H atoms were refined freely, but the O-H distances were restrained to 0.85 (1) Å, and the water HA···HB distance to 1.45 (1) Å.   as those based on F, and R-factors based on ALL data will be even larger.