Direct evidence of hydrogen-bonding and/or protonation effect on p-benzoquinone electrochemical reduction by in situ IR spectroelectrochemical study

Cyclic voltammetry (CV), IR spectroelectrochemistry cyclic voltabsorptometry (CVA) and derivative cyclic voltabsorptometry (DCVA) technique were used to investigate the electrochemical character of a representative quinone, p-benzoquinone (BQ), in aprotic media, organic-water mixed solvent, unbuffered acidic aqueous ([H+] < [BQ]), and well-buffered aqueous solution ([H+] > [BQ]). Several well-separated IR absorption peak pairs, 1656, 1316; 1502, 1340; and 1230, 1473 cm−1, corresponding to redox transitions (BQ, BQ−, BQ2−), are observed and can be used to trace the concentration of BQ, BQ− and BQ2− independently during CV scan. Based on CVA and DCVA techniques, it is clearly distinguished that each step of the two consecutive electron transfer process of BQ in unbuffered neutral aqueous takes place, although only one CV wave is experimentally observed during CV scan. Direct evidence of hydrogen-bonding (H-bonding) forming between BQ2− (BQ−) and H2O in unbuffered neutral water and that between BQ and H3O+ in strong acidic solution are observed. The results of IR CVA and DCVA not only help us understand the fundamental factors of H-bonding and protonation on redox mechanism of BQ, but also draw the entire map of the electrochemical reduction of BQ in different solution.