ELECTROCHEMICAL AND INFRARED-SPECTROSCOPIC CHARACTERIZATION OF REDOX REACTIONS OF P-QUINONES

Infrared (IR) spectra of redox products of quinones in environments of different polarity and proton activity were obtained in order to get information on the molecular structure of these species. We have investigated nine p quinones in acetonitrile (MeCN), methanol, tetrahydrofuran (THF), CH2Cl2, and water with a combination of electrochemistry and ultraviolet (UV), visible (vis), and IR spectroscopy. A thin-layer electrochemical cell was used to generate the redox products and to monitor them simultaneously in the UV/vis/IR spectral range. The standard redox potentials (E0) of different redox equilibria were determined by cyclic voltammetry. In aprotic solvents such as MeCN, two individual electron-transfer reactions could be detected, corresponding to the formation of the semiquinone anion (Q.-) and the dianion (Q2-). For protic solvents, only one E0 value could be determined from cyclic voltammetry, a fact which is explained by protonation of the quinone redox products. The E0 values obtained from cyclic voltammetry were used to determine the potentials for the generation of different quinone redox products during constant potential electrolysis. UV/vis spectra were recorded as a control of the species generated. These spectra clearly demonstrate the formation of Q.-, Q2-, and QH-2. IR difference spectra (reduced minus oxidized) were recorded under the same conditions as the UV/vis spectra. Bands arising from the C=O and C=C vibrations are easy to detect for the neutral quinone; for the redox products, however, assignment of bands to C-O and C-C vibrations is more difficult. Band assignment of the C=O and C=C modes and of the C-O and C-C modes was performed (i) by monitoring band shifts of ubiquinone (UQ) 10 labeled with C-13 and O-18, (ii) by comparison of IR frequencies of different quinones, and (iii) by the influence of different solvents on these bands. The small solvent effects on the neutral quinones could be explained with the poor ability of the C=O group to form hydrogen bonds.