Violuric acid monohydrate: a second polymorph with more extensive hydrogen bonding.

Crystals of a second polymorph of violuric acid monohydrate [systematic name: pyrimidine-2,4,5,6(1H,3H)-tetrone monohydrate], C(4)H(3)N(3)O(4).H(2)O, have higher density and a more extensive hydrogen-bonding arrangement than the previously reported polymorph. Violuric acid and water molecules form essentially planar hydrogen-bonded sheets, which are stacked in an offset ...ABCABC... repeat pattern involving no ring-stacking interactions.