Uric Acid Dihydrate Revisited

Abstract

At 120 K, crystals of pure laboratory-grown uric acid dihydrate [7,9-dihydro-1 H -purine-2,6,8(3 H )-trione dihydrate], C 5 H 4 N 4 O 3 .2H 2 O, are monoclinic, P 2 1 / c , with a = 7.237 (3), b = 6.363 (4), c = 17.449 (11) Å, β = 90.51 (1)°, Z = 4, wR 2 (all data) = 0.1094, R 1 = 0.0406 for data with I > 2σ( I ). The crystals exhibit pseudo-orthorhombic twinning with refined twin fractions of 0.89 and 0.11. Within each twin, disorder about a noncrystallographic twofold exists with refined occupancies of 0.83 and 0.17. Packing consists of layers of hydrogen-bonded uric acid separated by layers of hydrogen-bonded water. The epitaxy of uric acid dihydrate with its anhydrous counterpart is readily explicable from the refined model. Treatment of the structure as twinned and disordered in space group P 2 1 / c fully accounts for well documented violations of systematic absences in orthorhombic space groups.