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Violuric acid monohydrate: a definitive redetermination at 150 K

Acta Cryst. (2005). E61, o3788–o3790 [doi:10.1107/S160053680503343X]

[Violuric acid] The hydrogen-bonded sheet structure of violuric acid monohydrate.
The crystal structure of violuric acid monohydrate was originally determined over 40 years ago in 1964 from X-ray and neutron data. A number of issues concerning potential pseudo-symmetry and disorder caused by abnormally high thermal atomic motion were raised in the original reports and could not be resolved. Re-collection of data at 150 K allows confirmation that the structure is indeed in the noncentrosymmetric space group Cmc21 and is disorder-free, despite the presence of pseudo-symmetry. All atoms lie on a crystallographic mirror plane, leading to hydrogen-bonded planar sheets.
G. S. Nichol and W. Clegg