5-Amino-1H-1,2,4-triazol-4-ium-3-carboxylate hemihydrate

The asymmetric unit of the title compound, C3H4N4O2 center dot-0.5H(2)O, comprises two whole molecules of 5-amino-1H-1,2,4-triazole-3-carboxylic acid in its zwitterionic form (proton transfer occurs from the carboxylic acid group to the N heteroatom at position 1), plus one water molecule of crystallization. The organic moieties are disposed into supramolecular layers linked by N-H center dot center dot center dot O and N-H center dot center dot center dot N hydrogen bonds parallel to the bc plane. Additional O-H center dot center dot center dot O and N-H center dot center dot center dot O hydrogen bonds involving the water molecules and the organic molecules lead to the formation of double-deck supramolecular arrangements which are interconnected along the a axis via pi-pi stacking [centroid-centroid distance = 3.507 (3) angstrom].