Electronic Structure, Biological Activity, Spectral Analysis, NBO, NLO, and Thermodynamic Properties of 3-Chloro-4-Hydroxyquinolin-2(1H)-One. DFT Approach

Chlorination of 3-acetyl-4-methylthioquinolin-2(1H)-one (1) with sulfuryl chloride may lead to 3-(2,2-dichloroacetyl)-4-methylthioquinolin-2(1H)-one (2). Whereas analytical and spectral results  for the product of this reaction evoked the proposal of obtaining 3-chloro-4-hydroxyquinolin-2(1H)-one (3), which was found by reduction of 3,3-dichloroquinolin-2,4-dione.  Density Functional Theory (DFT) and time-dependent density functional theory (TD-DFT) calculations of the electronic structure at the B3LYP/6-311++G (d,p) level of theory were used to investigate the geometries, linear polarizability ⟨Δ𝛼⟩, first order hyperpolarizability ⟨𝛽⟩, natural bonding orbital (NBO), molecular electrostatic potential contours (MEP ESP), electrophilicity (ω), and UV-Vis spectra, in both ethanol and dioxane solvents for compound 3. The geometrical and energetic characteristics have been thoroughly studied to determine why compound 3 was formed instead of another expected result compound 2. At the same time, the thermo-chemical parameters, NMR, harmonic vibration frequencies, and equilibrium geometries were computed. The calculated acidity constant (pKa) for the protonated and deprotonated forms in ethanol for the present compound 3. Band maxima (λmax) and spectra intensities are reflected as blue and red shifts in the solvent dependence. The excited state was identified and contributed to the electronic configurations. Finally, DFT calculations were used to connect the structure-activity relationship (SAR) with real antibacterial results for compound 3.