Electrooxidation of homogentisic acid in aqueous and mixed solvent solutions: experimental and theoretical studies.

Electrochemical behavior of homogentisic acid (HGA) has been studied in both aqueous and mixed solvent solution of water-acetonitrile. Physicochemical parameters of the electrochemical reaction of HGA in these solutions are obtained experimentally by cyclic voltammetry method and are also calculated theoretically using accurate ab initio calculations (G3MP2//B3LYP). Solvation energies are calculated using the available solvation model of CPCM. The pH dependence of the redox activity of HGA in aqueous and the mixture solutions at different temperatures was used for the experimental determination of the standard reduction potential and changes of entropy, enthalpy, and Gibbs free energy for the studied reaction. The experimental standard redox potential of the compound in aqueous solution was obtained to be 0.636 V versus the standard hydrogen electrode. There is a good agreement between the theoretical and experimental values (0.702 and 0.636 V) for the standard electrode potential of HGA. The changes of thermodynamic functions of solvation are also calculated from the differences between the solution-phase experimental values and the gas-phase theoretical values. Finally, using the value of solvation energy of HGA in water and acetonitrile solvents which calculated by the CPCM model of energy, we proposed an equation for calculating the standard redox potential of HGA in mixture solution of water and acetonitrile. A good agreement between the result of electrode potential calculated by the proposed equation and the experimental value confirms the validity of the theoretical models used here and the accuracy of experimental methods.

Thermodynamic parameters of electrochemical oxidation of L-DOPA: experimental and theoretical studies.

Electrode potential and thermodynamic parameters of the electrochemical reaction of L-DOPA in aqueous solution are obtained experimentally by cyclic voltammetry method and also calculated theoretically using accurate ab initio calculations (G3MP2//B3LYP) along with the available solvation model of CPCM. The pH dependence of the redox activity of L-DOPA in aqueous solution at temperatures in the range of 10-30 °C was used for the experimental determination of the standard reduction potential, changes of entropy, enthalpy, and Gibbs free energy for the studied reaction. The experimental formal redox potential of the two-proton-two-electron reduction process was obtained to be 0.745 V versus standard hydrogen electrode (SHE). The theoretical and experimental values (0.728 and 0.745 V) for the standard electrode potential of L-DOPA are in agreement with each other. The difference between the peak potential of the L-DOPA and the products, which are produced by chemical reactions, has been measured experimentally and also calculated theoretically. There is also an agreement between experimental and theoretical potential difference. Also in this work, the changes of thermodynamic functions of solvation are calculated from the differences between the solution-phase experimental values and the gas-phase theoretical values.

Experimental and theoretical studies of redox reactions of o-chloranil in aqueous solution.

The electrochemical reduction of o-chloranil (OCA) in aqueous solution has been studied experimentally and theoretically. The effects of temperature and pH on various thermodynamic parameters were studied by means of cyclic voltammetry. The pH dependence of the redox activity of OCA in aqueous solution at temperatures in the range 25-40 degrees C was used for the experimental determination of the standard reduction potentials of both the one-proton-two-electron (0.67 V) and two-proton-two-electron (0.79 V) reduction processes. The temperature dependency of the equilibrium constants of studied reactions has been used in order to determine enthalpy, entropy, and Gibbs energy changes of the reactions. It is found that the two studied redox reactions of OCA are strongly affected by solvation effects and controlled by entropy contributions to the Gibbs free energies. High-level ab initio calculations (G3 and CBS-QB3 using the CPCM solvation model) have been employed to calculate the reduction potentials of OCA in aqueous solution for the one-proton-two-electron (0.65 and 0.69 V at the respective levels of theory) and two-proton-two-electron (0.81 and 0.83 V) reactions, which are in excellent agreement with the corresponding experimental values. The acidic strength of the reduced form of OCA in aqueous solution, pK(a), has been also calculated (5.2) and is also in excellent agreement with the experimental value (5.0). The agreement mutually verifies the accuracy of experimental method and the validity of the mathematical model.