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, C11H12N2O2·H2O, contains a disordered molecule of 1,4,6-trimethyl-1,4-dihydro-quinoxaline-2,3-dione and a solvent water molecule. In the crystal, molecules are linked by O—H⋯O and C—H⋯O hydrogen bonds into layers lying parallel to (10). The Hirshfeld surface analysis is carried out.

The asymmetric unit of the title compound, C 11 H 12 N 2 O 2 ÁH 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-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds into layers lying parallel to (101). The Hirshfeld surface analysis indicates that the most important contributions to the packing arrangement are due to HÁ Á ÁH (51.3%) and OÁ Á ÁH/HÁ Á ÁO (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.

Structural commentary
The title compound crystallizes in space group P2 1 /n with one quinoxaline and one water molecule per asymmetric unit. The organic molecule is disordered over two sets of sites with an occupancy ratio of 0.706 (7):0.294 (7). The disorder involves not only the orientation of the methyl group attached to the benzene ring, but also the positions of four carbon atoms of this ring, which are split (Fig. 1). Only the predominant orientation of the 1,4,6-trimethylquinoxaline-2,3(1H,4H)dione molecule is discussed below. Besides this, the methyl groups attached to N1 and N2 nitrogen atoms are also rotationally disordered with occupancy ratios of 0.78 (4):0.22 (4) and 0.76 (5):0.24 (5), respectively. The quinoxaline ring system is essentially planar, the largest deviation from the mean plane being 0.015 (3) Å for the N2 atom. The C O and Csp 2 -N bond lengths are typical of such type of compounds and indicate strong conjugation in the amide fragments.

Supramolecular features
In the crystal, molecules are linked by O-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds (Table 1) into double layers lying parallel to (101). The smallest element of the hydrogenbonding motif, where the R 2 4 (8) rings are formed, is shown in Fig. 2, whereas the whole packing diagram is presented in Fig. 3. The water molecule behaves both as a donor and an acceptor of hydrogen atoms in the hydrogen bonds. As seen in Fig. 3, in centrosymmetric pairs of organic molecules, the aromatic and heterocyclic rings overlap with each other with an intercentroid distance of 3.522 (4) Å , indicating that some interactions occur. The asymmetric unit of the title compound, showing the atom labelling and displacement ellipsoids drawn at the 40% probability level. O-HÁ Á ÁO hydrogen bonds are indicated by dashed lines. The benzene fragment of the organic molecule, C2/C3/C4/C5/C7, is disordered over two sets of sites. Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) Àx þ 1; Ày þ 3; Àz; (ii) Àx þ 1 2 ; y À 1 2 ; Àz À 1 2 ; (iii) x À 1 2 ; Ày þ 5 2 ; z À 1 2 .

Figure 2
A view along the a axis of a hydrogen-bonded fragment. The O-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds (shown as dashed lines) form an R 2 4 (8) ring motif.

DFT calculations
The structure of the title organic molecule was optimized in the gas-phase approximation at the level of density functional theory (DFT) using the B3LYP functional (Becke, 1993) and 6-311 G(d,p) basis set as implemented in GAUSSIAN 09 (Frisch et al., 2009). The theoretical and experimental bond lengths and angles are in good agreement ( Table 2). The energetic and spatial characteristics of the highest occupied molecular orbital (HOMO), acting as an electron donor, and the lowest unoccupied molecular orbital (LUMO), acting as an electron acceptor, are very important parameters for quantum chemistry. When the energy gap is small, the molecule is highly polarizable and has high chemical reactivity (Fukui, 1982 Views of the three-dimensional Hirshfeld surface for the title compound plotted over (a, b) d norm and (c) shape-index. some important information on the reactivity and site selectivity of the molecular framework, E HOMO and E LUMO , electronegativity (), hardness (), electrophilicity (!), softness () and fraction of electrons transferred (ÁN). These data are given in Table 3. The parameters and are significant for evaluation of both the reactivity and stability. The electron transition from HOMO to LUMO is shown in Fig. 6. The HOMO and LUMO are localized in the plane of the whole 1,4,6-trimethylquinoxaline-2,3(1H,4H)-dione bicyclic ring system. The energy gap [ÁE = E LUMO À E HOMO ] of the molecule is 4.6907 eV, the frontier molecular orbital energies E HOMO and E LUMO being À6.1139 eV and À1.4232 eV, respectively. The dipole moment of (I) is estimated to be 5.56 Debye.

Synthesis and crystallization
To a solution of 6-methyl-1,4-dihydroquinoxaline-2,3-dione (0.3 g, 1.73 mmol) in DMF (15 ml) potassium carbonate (0.47 g, 3.61 mmol) and tetra-n-butylammonium (0.07g, 0.23 mmol) were added. After 10 min of stirring, 0.27 ml (4.32 mmol) of iodomethane were added, and the mixture was stirred at room temperature for 6 h. The inorganic salts were filtered off, DMF was evaporated under reduced pressure and the residue was dissolved in dichloromethane. The organic phase was dried over Na 2 SO 4 and then concentrated. The crude product was purified by chromatography on a silica gel column [eluent: hexane/ethylacetate (2/1)].  Table 2 Comparison of observed (X-ray data) and calculated (DFT) geometric parameters (Å , ).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. Water molecules were refined as rigid groups with U iso (H) = 1.5U eq (O). Other H atoms were positioned geometrically, with C-H = 0.94 and 0.97 Å for aromatic and aliphatic H atoms, respectively, and constrained to ride on their parent atoms, with U iso (H) = 1.2U eq (C) or U iso (H) = 1.5U eq (C-methyl). The disorder of the organic molecule was taken into account using free variables.