ELECTROCHEMICAL AND ESR STUDIES OF ORGANIC COMPOUNDS OF BIOLOGICAL SIGNIFICANCE

Abstract

In the present work cyclic voltammetric and electron spin resonance (ESR) spectroscopic investigations of fifteen quinones have been carried out. Quinones belong to a class of organic compounds, which find potential applications in biology and chemistry. Five compounds from each of the three series of quinones, namely benzoquinones (BQs), naphthoquinones (NQs) and anthraquinones (AQs), were selected for the present study. Systematic cyclic voltammetric measurements were made on all compounds in solvents dichloromethane (DCM), acetonitrile (AN) and propylene carbonate (PC) at 25 o C. Analysis of the voltammograms provided fundamental electrochemical parameters (redox potentials, peak separation, peak currents, half-wave potential, peak width) which helped in the interpretation of role of solvents, role of structure and the effect of substituents. The redox behaviour of the compounds was examined first within a series and then a comparative analysis of the three series was made. It was found that substituents affect strongly the redox bahaviour of the compounds. The quinones with electron withdrawing groups were easily reduced than those with electron releasing groups. Among the three series the ease of reduction followed the order BQs>NQs>AQs. 2-Hydroxy-1, 4-naphthoquinone behaved differently due to self protonation. The heterogeneous electron transfer rate constants (k o ) of the first and the second reduction steps were determined in the three solvents employing Nicholson and Kochi methods. The experimental results were compared with those calculated theoretically from the modified form of the Marcus theory. The solvent reorganization energy (λ o ) in the Marcus equation was calculated using the conventional spherical as well as multisphere models. Experimental rate constants in acetonitrile from the Kochi’s method for benzoquinones, naphthoquinones and anthraquinones were found in the range 1.65 x 10 -3 – 8.47 x 10 -3 cm s -1 , 7.09 x 10 -3 – 11.16 x 10 -3 cm s -1 , and 0.19 x 10 -3 – 7.10 x 10 -3 cm s -1 respectively. The experimentally determined rate constants for quinones were found in close agreement with the theoretically calculated rate constants from the Kochi’s method. The effect of medium on electron transfer rates was rationalized in terms of solvent properties such as polarity, viscosity, density relaxation time. An increase in the solvent polarity and a decrease in viscosity favoured the heterogeneous iiielectron transfer rate. Electron transfer rates were found inversely proportional to solvent longitudinal relaxation time. Electrochemical behaviour of quinones was also investigated in the presence of tert-butanol, 2-propanol, ethanol and methanol (monoalcohols), ethylene glycol (a diol) and glycerol (a triol) as proton donors. The quinone-alcohol interaction was analyzed from changes observed in the shape and peak position of the redox waves in voltammograms as a result of change in concentration of the added alcohol. An estimate of the strength of the quinone-alcohol interaction which is hydrogen bonding in nature was obtained by calculating the thermodynamic association constants and the number of alcohol molecules attached to anion or dianion of quinones. The interaction with dianion was found much stronger than with the anion. The strength of the hydrogen bond depended upon the basicity of the quinone and acidity of the alcohol. The presence of α- hydrogens in the quinone structure strengthened the interaction. A comparison of the results for monoalcohols, diol and triol shows that the polyalcohols formed stronger hydrogen bonds. The strength of interaction increased with the increase in the number of OH groups. Homogeneous electron transfer rate constants of quinones in acetonitrile were determined from ESR spectroscopic measurements at 298K. The anion radical of the quinone was generated in situ in a locally fabricated esr-electrochemical cell and the hyperfine spectrum recorded. The hyperfine coupling constants and the line widths were determined from the experimental and simulated ESR spectra. The electron self exchange rate constant was determined from the concentration depended chemical line broadening produced by the addition of neutral quinone. The rate constants were found in the range of 5.2 x 10 8 - 4.4 x 10 9 M -1 s -1 . The strength of precursor complex is observed in terms of association constants (K A ) calculated from the experimental electron transfer rate constants are in the range 0.028 – 1.023 M -1 . Theoretical values of K A were calculated using Eigen-Fuoss and reaction zone models. The values calculated with the reaction zone model agreed with the experimentally determined values. The ESR results support the observed trends in electrochemical behaviour of quinones.