The Theoretical Calculations and Experimental Measurements of Acid Dissociation Constants and Thermodynamic Properties of Betanin in Aqueous Solutions at Different Temperatures

In this study, we experimentally determined the protonation constants, enthalpy change (ΔH^o), change in Gibbs energy (ΔG^o), and entropy change (ΔS^o) for the deprotonation process of betanin over the temperature range of 298.15 to 318.15 K and at an ionic strength of 0.1 mol·dm⁻³ (NaClO₄) with a combination of spectrophotometric and potentiometric methods. In addition, we theoretically calculated the values of the acidic dissociation constants, the optimized structure, and the thermodynamic properties of betanin in aqueous solutions at various temperatures by ab initio and DFT methods. Density function theory (DFT) methods with the B3LYP/6-31 + G(d) level of theory were applied for calculating the acidic dissociation constants of betanin and also interactions among the solvent and solvated cation, anion, and neutral species of betanin. Thomasi's method was used to analyze the formation of intermolecular hydrogen bonding between water molecules and various species of betanin. There is a good correlation between the theoretically calculated and experimentally determined values of pK_a of betanin.