Synthesis, crystal structure at 219 K and Hirshfeld surface analyses of 1,4,6-tri­methyl­quinoxaline-2,3(1H,4H)-dione monohydrate

The asymmetric unit of the title compound, C(11)H(12)N(2)O(2)·H(2)O, contains a mol­ecule of 1,4,6-trimethyl-1,4-di­hydro­quinoxaline-2,3-dione and a solvent water mol­ecule. Four atoms of the benzene ring are disordered over two sets of sites in a 0.706 (7):0.294 (7) ratio while the N-bound methyl groups are rotationally disordered with occupancy ratios of 0.78 (4):0.22 (4) and 0.76 (5):0.24 (5). In the crystal, mol­ecules are linked by O—H⋯O and C—H⋯O hydrogen bonds into layers lying parallel to (10[Image: see text]). The Hirshfeld surface analysis indicates that the most important contributions to the packing arrangement are due to H⋯H (51.3%) and O⋯H/H⋯O (28.6%) inter­actions. The mol­ecular structure calculated by density functional theory is compared with the experimentally determined mol­ecular structure, and the HOMO–LUMO energy gap has been calculated.