This article deals with a combined experimental and computational study of a 6-substituted coumarin derivative, namely, 2-oxo-2H-chromen-6-yl 4-methoxybenzoate (I). The compound was synthetized by O-acylation of 6-hydroxycoumarin with 4-methoxybenzoyl chloride in the presence of tetrahydrofuran as a solvent and triethylamine as a base. The crystal structure has a P21 space group and crystallizes in the monoclinic system with dimensions of 𝑎 = 3.8956 (4) Å , 𝑏 = 10.1366 (6) Å , and 𝑐 = 17.3178 (13) Å . The angles between the crystal axes are 𝛼 = 𝛾 = 90° and 𝛽 = 90.580 (7)°, with a 𝑍 value of 2. The compound was structurally characterized by both spectroscopy and single-crystal X-ray diffraction (XRD).
In the latter, the structure of (I) was solved by direct methods and refined as a 2-component twin to a final R value of 0.0495 for 2966 independent reflections. The structure is stabilized by H-π and π-π stacking interactions between neighboring aromatic rings, as well as intra- and intermolecular C—H…O hydrogen bonds that extend along the [001] direction. The analysis of intermolecular interactions was conducted using dnorm and shape-index mappings. The results of this analysis highlighted the same interactions as those found by XRD analysis, namely C-H...O hydrogen bonds, H-π and π-π stacking interactions. As well, the two-dimensional fingerprint plots (FP) show specific close contacts between atom pairs and the contributions from different contacts. The largest contributions to the Hirshfeld surface, 34.9% and 33.3%, come from O•••H/H…O and H•••H contacts, respectively. In additionally, the molecular geometry of (I) was optimized using both ab initio method namely restricted Hartree-Fock (RHF) method and density functional theory (DFT/RB3LYP) with the
6-311++G(d, p) basis set in ground state. The outcomes of these quantum chemical calculations are consistent with the observed structure. The only reported difference concerns the Hartree-Fock calculations where the computed torsion angle between the coumarin ring system and the benzoate ring, C10—O3—C8—C9, of -85.6° is slightly larger than the observed value (−51.3 (8)°). Also, frequency calculations were carried out with the optimizing structures to perform vibrational analysis, check compound stability and obtain some thermodynamic parameters. Molecular orbital calculations providing electron-density plots of the HOMO and LUMO molecular orbitals were also performed with the frequency calculation methods using the same basis sets. The theoretical values of the HOMO-LUMO energy gap yielding from these calculations are 4.40 eV for (DFT/B3LYP/6–311++G(d,p)) and 9.77 eV for (RHF/6–311++G(d)) methods.

6-substituted coumarin derivative, Spectroscopic analysis, Crystal structure, Conformational analysis, Hirshfeld surface analysis