Ab initio studies on acidity and tautomeric equilibrium constants of some benzoxa-, benzothia-, benzoselena-zolinone derivatives

This paper presents ab initio procedure to calculate aqueous-phase acid dissociation constants (pKa) values, accurately. In this work, dissociation and tautomeric equilibrium constants (KT) of some benzoxa-, benzothia-, benzoselena-zolinone derivatives were studied theoretically. For this purpose, Gibbs free energy values for neutral and deprotonated forms of investigated compounds were calculated at gas and aqueous phase by using Hartree-Fock (HF) and density functional theory (DFT) calculation methods. For both calculation methods it was found that 6-311G(d) basis set is the most convenient method. Solvent effects are taken into account by means of conductor-like polarizable continuum model (CPCM). It was shown that, theoretically calculated pKa values are in good agreement with the existing experimental pKa values, which are determined from capillary electrophoresis, potentiometric titration and UV-visible spectrophotometric measurements. Calculated pKa values also revealed that the DFT method (R2 = 0.92) gave much closed results to experimental values in comparison to Hartree-Fock method (R2 = 0.86). Due to the all investigated compounds have keto-enol tautomerism, KT and pKT values were calculated without need to form model compounds. Theoretically calculated KT values indicated that, keto forms are predominant for all the investigated compounds.

Highlights► Thermodynamic properties of compounds under investigation were examined. ► All thermodynamic properties were computed with HF and DFT method. ► pKa values of the compounds were calculated and compared with experimental data. ► Tautomeric equilibrium constants of the compounds were also calculated.