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.