Electrochemical ESR spectroscopic Analysis of Reactive Intermediates in Nonaqueous and Aqueous Media

摘要

1. Flow Electrolysis-ESR spectroscopy in nonaqueous media The electrochemical reduction mechanisms of acetophenone (AP) derivatives in acetonitrile were analyzed by using a novel flow electrolysis-ESR spectroscopic method with a micro-column electrode. We found, in particular, the specific reduction behavior of p-nitroacetophenone (NAP) in acetonitrile. In the CV of the reduction process of AP, one irreversible reduction peak corresponding to the formation of the anion radical was observed at -2.35 V. In contrast, in the CV of NAP, two reduction peaks were observed at -1.17 V and -1.75 V. These peaks are corresponding to the formation of the NAP anion radical and the NAP dianion, respectively. While in the reversed oxidation scan, two additional oxidation peaks were appeared at more positive potentials than those of the normal two reversible oxidation peaks. From these results, it can be inferred that, in the case of NAP dianion, unpaired electrons localized to form two different states. As the results of the flow electrolysis-ESR measurements, we could successfully observed the ESR spectra at the first and the second reduction peak regions by controlling the applied potential on the column electrode. While the g-value of the anion radical of AP derivatives was 2.0033, the g-value of the NAP anion radical was remarkably large as g = 2.0049. This shows the unpaired electron of the NAP anion radical localized on N atom. While at the second reduction peak region, a different ESR spectrum was observed having a g-value of 2.0073, which is larger than that of the NAP anion radical From these results, it seems that the NAP dianion has paramagnetic character and the electron localizes on O atom. As a result, it can be concluded that the paramagnetic NAP dianion with biradical form is produced at the second reduction peak region and the unpaired electrons localize at two atoms; one is at the C atom bonding to the nitro group, the other is at the O atorr of the carbonyl group.