Thermodynamic Parameters and Electrochemical Behavior of Tetrafluoro-p-Quinone in Aqueous Solution

Abstract

The electrochemical behavior of tetrafluoro-p-quinone, TFQ, at bare and activated glassy carbon electrodes (BGCE and AGCE) was studied in aqueous solution by cyclic voltammetry. The effects of scan rate, pH and concentration on the electrochemical behavior of TFQ indicate that the reduced form of TFQ (tetrafluoro-p-hydroquinone, TFHQ) is adsorbed weakly at the AGCE surface whereas the oxidized form does not show any adsorption at this surface. The temperature dependency of the TFQ redox potential has been used for determination of the changes of entropy, enthalpy and Gibbs free energy of the electrochemical reaction of TFQ. Theoretical calculations have been also carried out by means of standard ab initio modern composite methods (G3B3 and G4 theories). Theoretical and experimental values for the standard redox potential of TFQ are in good agreement with each other.Quinone-hydroquinone redox systems have been studied widely because they play significant roles in different areas such as antitumor therapeutics and storage of energy. 1-7 Quinone redox reactions are sensitive to conditions of electrode surface, molecular structure and nature of substituents. 5-8 Electronegative substituents such as fluorine are known to enhance reactivity of molecules in different reactions. 9 Compounds containing fluorine are widely used as intermediate products for synthesizing pesticides, drugs, dyes and monomers for polymeric materials. 10 Tetrafluoro-p-quinone, TFQ, (Scheme 1), is an important molecule and has many applications in chemistry. 11 It is used as a photo-excited electron acceptor 12 and as a π-acceptor for the determination of some anti-inflammatory drugs such as mesalazine and antiemetic drugs such as metoclopramide. 13-15 TFQ, as a π-electron acceptor, usually forms colored charge-transfer complexes with some donor drugs which provides the possibility of their quantitative determination by spectrophotometric methods. [13][14][15] The chemical and electrochemical properties of TFQ are affected by four strongly electronegative fluorine atoms and one can expect significant changes for this species compared to the compound without fluorine atoms (phydroquinone). Although many papers have been published on the electrochemical behavior of various quinone derivatives, there are only few reports on the electrochemical behavior of TFQ in aqueous solution. 8,16,17 In continuing our studies of quinones, 4-6 we now report the electrochemical behavior of tetrafluoro-p-quinone, TFQ, in an aqueous solution at bare and activated glassy carbon electrodes (BGCE and AGCE). Thermodynamic parameters of the electrochemical reaction of TFQ have been evaluated experimentally and theoretically in this work. ExperimentalTFQ and all other chemical reagents used for the preparation of the buffer solutions were reagent grade and were purchased from Merck Company and used without purification. Solutions were prepared with doubly distilled water. Buffer solutions were made from 0.1 M H 3 PO 4 and the pH was adjusted with 2.0 M NaOH. The pH measu...

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