Study On The Electron Transfer Of P-benzoquinone By IR Cyclic Voltabsorptometry

In this report, experimental technique and achievements of electrochemical studies of quinone compounds were reviewed firstly. Secondly, Cyclic voltammetry (CV), IR spectroelectrochemistry cyclic voltabsorptometry (CVA) and derivative cyclic voltabsorptometry (DCVA) technique were used to investigate the electron transfer mechanism of a representative quinone, p-benzoquinone, in aprotic media, organic-water mixed solvent, unbuffered neutral water, unbuffered acidic aqueous ([H+]<[BQ]), and well-buffered aqueous solution([H+]>[BQ]). 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. This paper mainly covered the following research works:1,The electron transfer mechanisms and electron pass pathways of BQ were studied in aprotic media, organic-water mixed solvent and unbuffered neutral water. BQ was studied in aprotic media in this section is to assign the IR absorption peaks during electrochemical process. Several well-separated IR absorption peak pairs,1656,1316; 1502,1340; and 1230,1473 cm-1, are observed and can be used to trace the concentration of BQ, BQ-·and BQ2- independently during CV scan. The information obtained in this section can help us to understand the change of IR absorption peaks of BQ in different medias. In 3D spectra, red shift and blue shift of the absorption peaks at 1502 and 1473 cm-1, respectively, occur when we add H2O to CH3CN solution. As a result, an overlapping IR peak at 1496 cm-1 is observed in mixed solution containing high concentration of H2O. IR peak displacement is due to H-bonding forming between reduction product and H2O. Only one nearly reversible coupled redox peak was obtained in unbuffered neutral water, but though IR spectroelectrochemistry, CVA and DCVA, we can reconstruct i-E curves and get more information about each electron transfer. In the neutral water, IR peak at 1496 cm-1 which assigned to vC=C from BQ-…H2O and BQ2-…H2O is observed. A broad and strong IR absorption from 3200 to 2200cm-1 which peak is about 2814 cm-1 assigned to vO-H coming from BQ2-…(H2O)m can also be observed in 3D spectra. The evidences indicate that H-bonding between reduction product and H2O. So the results indicate that this process is H-bonding coupled-electron transfer reactions.2,In well-buffered aqueous solution, the electron transfer mechanisms and electron pass pathways of BQ were is universally interpreted as 2e,2H+ reaction. But we found some complex electrochemical process existing when we compare the DCVA at 1656 cm-1 with that at 1512 cm-1 carefully, so it is not simple 2e,2H+ reaction. The CV of BQ shows two coupled redox waves in unbuffered aqueous. The results suggest that the electron transfer mechanism may be 2e + 1H+ process for the first reduction step and the e+e process for the second reduction step In unbuffered acidic aqueous solution and well-buffered acidic aqueous solution, IR peak at 1512cm-1 assigned to vC=C coming from H2BQ can be observed. At the same times, the sunken region in the range of ca.3000-1700 cm-1 assigned to vO-H coming from BQ…(H3O+)m in 3D spectra are found. The results indicate that this process is proton-coupled electron transfer (PCET) reactions.