Study On The Redox Mechanism Of Hydroxyquinones By In Situ FT-IR Spectroelectrochemistry

Quinone compounds are of good electrochemical activity. They are elecellent carrier for the electron transfer. Study on the electrochemical reaction mechanism of quinones can make us a better understanding of the metabolic processes in the living systems.In the paper the redox mechanisms of2-hydroxy-1,4-naphthoquinone (Q-OH),5-hydroxy-1,4-naphthoquinone (HNQ),5-hydroxy-2-methyl-1,4-naphthoquinone (HMeNQ) and emodin (Q) were studied by the techniques of cyclic voltammetry (CV), in situ FT-IR spectroelectrochemistry (FT-IR), cyclic voltabsorptometry (CVA) and derivative cyclic voltabsorptometry (DCVA). The main achievement was summarized as follows:1. The electron transfer mechanism of Q-OH in acetonitrile and in proton donors mixed media.(1) When scan range is from0.2V to-1.8V, the cyclic voltammetry(CV) of Q-OH shows two couples of cathodic and anodic peaks in acetonitrile. When the scan range is1.0V～-2.0V, there is another anodic peak at more positive potential in CV. Tracing the change of several IR absorption peaks, we found that there are two kinds of intermediate. Radical dianion formed by the reduction process of deprotonated quinone (Q-O-) continues to reduce. So Q-O-reduces by an EE(electrochemical reaction-electrochemical reaction) pathway. In addition, it involves the formation of strong hydrogen bonding.(2) In proton donors mixed media, addition of proton donors results in positive shift of all reduction steps. For given concentration, the shift is much larger for the second one. We conclude that the second couple is easily influenced by proton donors owing to hydrogen-bonding.2. The electrochemical reductions of HNQ and HMeNQ in acetonitrile and proton donors mixed media.(1) In acetonitrile both HNQ and HMeNQ are reduced in two steps. The first couple of peaks corresponds to neutral/anion radical couples, while the second couple of peaks corresponds to anion radical/dianion couples. However, in the CV of HNQ not only two couples of anodic and cathodic peaks but also two small anodic peaks can be observed. To study the reaction mechanism of HNQ in a thin-layer cell, in situ FT-IR spectroelectrochemistry, CVA and DCVA spectroelectrochemical techniques are used to track the electrochemical reduction process. Through the analysis of absorbance at1618cm-1, we conclude that HNQ-reacts to produce dimer in a thin-layer cell. To study the structure of dimer, theoretical computation was applied. The relative energy of anion radical and possible dimer of both HMeNQ and HNQ was calculated at B3LYP/6-311++G**level. Although there is merely a small difference in structure between HMeNQ and HNQ, having/not having a methyl, the reduction of HNQ involves dimerization, while HMeNQ not. Among the dimmers, dimer2is the most stable one. In the forming process of dimer2, electron transfer (ET) occurs when framing intermediate HNQ-, then proton transfer (PT) occurs when forming dimer2. So reactions occur by stepwise pathways in which electron and proton both transfer but are separated. Dimer2of HNQ is further oxidized to radical anion in the first step and quinone in the second step, corresponding to A3and A4in CV.(2) In proton donors mixed media, addition of proton donors results in positive shift of all reduction steps of HMeNQ and HNQ. For given concentration, the shift is much larger for the second one. We conclude that the second couple is easily influenced by proton donors owing to hydrogen-bonding. In addition, variation related to dimer of HNQ in CVA and DCVA also vanishes when a certain proportion proton donor is added.3. The electrochemical reductions of Q in acetonitrile and proton donors mixed media.(1) The electrochemical reduction of emodin have been investigated in acetonitrile. Anion radical undergoes interaction with neutral Q to form dimer Q2-When the scan is between-0.2V and-2.0V, the substance in solution is Q22-instead of Q when the potential reaches the final potential. C1and C2peaks disappear in the second and third scans in CV under consecutive scan between-0.2V and-2.0V. When the scan is between1.0V and-2.0V, there are two new anodic peaks (A1, A2) at more positive potential in CV. C1and C2peak still appear in the second and third scans in CV under consecutive scan from1.0to-2.0V, indicating the regainer of Q after each potential cycle. When the scan range is0.3～-1.4V, the current value of A2increases with the added scan circles, indicating that Q2-accumulates in solution. And Q2-will be oxidized to Q at E=0.577V.(2) In proton donors mixed media, addition of proton donors results in positive shift of all reduction steps of Q. For given concentration, the shift is much larger for the couple of peaks C4/A4. C3/A3and C4/A4merge into one, while two reduction steps can be observed in DCVA.