URIC-ACID DIHYDRATE REVISITED

At 120 K, crystals of pure laboratory-grown uric acid dihydrate [7,9-d ihydro-lH-purine-2,6,8(3H)-trione dihydrate], C5H4N4O3.2H(2)O, are mon oclinic, P2(1)/c, with a = 7.237(3), b = 6.363(4), c = 17.449(11) Angs trom, beta = 90.51(1)degrees, Z = 4, wR(2) (all data) = 0.1094, R-1 = 0.0406 for data with I > 2 sigma(I). The crystals exhibit pseudo-ortho rhombic twinning with refined twin fractions of 0.89 and 0.11. Within each twin, disorder about a noncrystallographic twofold exists with re fined occupancies of 0.83 and 0.17. Packing consists of layers of hydr ogen-bonded uric acid separated by layers of hydrogen-bonded water. Th e epitaxy of uric acid dihydrate with its anhydrous counterpart is rea dily explicable from the refined model. Treatment of the structure as twinned and disordered in space group P2(1)/c fully accounts for well documented violations of systematic absences in orthorhombic space gro ups.