Abstract
A redetermination at 150 K of the structure of violuric acid monohydrate, C4H3N3O4·H2O, confirms that the space group is non-centrosymmetric Cmc21, despite indications from the intensity statistics and possible molecular symmetry that it could be centrosymmetric Cmcm. Issues raised in the original reports [Craven & Mascarenhas (1964). Acta Cryst. 17, 407–414; Craven & Takei (1964). Acta Cryst. 17, 415–420] suggested either a disordered model or an ordered one with high thermal motion. The redetermination shows that an ordered model is correct, and the low-temperature data collection leads to normal displacement parameters. The precision of the structure is significantly improved in this new study. The violuric acid molecule is entirely planar, and every atom in the structure lies on a crystallographic mirror plane. Violuric acid and water molecules form hydrogen-bonded sheets.
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