Spectroscopic, structural, and intermolecular interactions of 4-(2-hydroxy-3-methoxybenzylideneamino)-N-(5-methylisoxazol-3-yl)benzenesulfonamide enol-imine and keto-amine isomers

The spectroscopic (IR vibrational snd UV–vis analysis), structural, electronic properties, NBO, QTAIM studies of enol-imine (A) and keto-amine (B) isomers of 4-(2‑hydroxy-3-methoxybenzylideneamino)-N-(5-methylisoxazol-3-yl)benzenesulfonamide was investigated using ωB97XD, PBE0, M06–2X, B3LYP-GD3(BJ), and TPSS functionals with the 6–311++G(d, p) basis set within the framework of density functional theory (DFT). The calculations were carried out in the gas phase and in solvents (benzene, chloroform, DMSO, and ethanol), and compared with experimental data. Global quantum descriptors, dipole moment, and thermodynamic properties were further calculated to predict the charge distribution, reactivity, and stability of the isomer forms of the titled compound. Results show that the PBE0 theoretical method gave a more reliable estimation of the experimental IR vibrational frequencies for isomers A and B, compared with other functionals. Structural analysis shows that geometrical parameters obtained for ωB97XD functional for both isomers A and B were consistent with the experimental results. For UV–vis analysis, the B3LYP-GD3(BJ) with the smallest MAPE for isomer A and ωB97XD method with the least MAPE for isomer B have the UV–vis values in close agreement with experimental data. Dipole moment calculations show that electron charge distribution is higher in isomer B, evident from the donor-acceptor interaction pattern observed from the NBO calculations. Energy, HOMO-LUMO energy gap, and thermodynamic calculations indicate that isomer A has higher stability while isomer B has higher reactivity.