Pulse radiolysis and electron spin resonance studies of nitroaromatic radical anions. Optical absorption spectra, kinetics, and one-electron redox potentials

Abstract

Nitrobenzoic acids, dinitrobenzenes, and di- and trinitrobenzoic acids were reduced by e/sup -//sub aq/ and by the CO/sub 2//sup -/ and (CH/sub 3/)/sub 2/C(dot)OH radicals in irradiated aqueous solutions. The rate constants for reduction by the latter radicals were found to be close to 10/sup 9/ M/sup -1/ s/sup -1/ in most cases, with some substituent effects noted. The radical anions produced exhibit intense (epsilon approximately 10/sup 4/ M/sup -1/ cm/sup -1/) absorption spectra peaking around 300 nm, except for p-dinitro derivatives which are shifted to the 400-nm region. They protonate with pK/sub a/2 to 3 to produce radicals which absorb at lower wavelengths and which decay considerably more rapidly. One-electron redox potentials for the nitro compounds were determined using the electron transfer reactions between their radical anions and duroquinone and observing the kinetics of these reactions and the equilibrium concentrations of all components. The potentials were found to vary from -0.433 V for m-nitrobenzoic acid to -0.257 V for p-dinitrobenzene. The radical anions have also been observed by ESR and the assignment of hyperfine constants was usually straightforward. The nitro group nitrogen hyperfine constants were correlated with the redox potentials. The mononitro derivatives were found to follow a previousmore » correlation of various nitroaromatic and nitroheterocyclic compounds, while the dinitro derivatives yield a similar linear correlation but with a different slope. The reason for this difference is discussed. Considerations of the present results and previous studies on intramolecular electron transfer in nitrobenzoatopentaamminecobalt(III) complexes show that the redox potential is of lesser importance in determining the rate of this transfer. The important factor appears to be the spin density on the carbon bearing the carboxyl group, which when higher provides a better channel for the electrons to be transferred from the nitro anion group to the cobalt center.« less