Radical-anions of dinitrophenols in aqueous solution: intramolecular electron exchange and acid–base equilibria

Abstract

The radical-anions of three symmetrical dinitrophenols (3,5-dinitrophenol, 2,6-dinitrophenol and 4,6-dinitrobenzene-1,3-diol) in aqueous solution have been studied by EPR spectroscopy in the range of pH values from 6.5 to 13. EPR spectra of radical-anions of 3,5- and 2,6-dinitrophenol show line-broadening effects due to intramolecular electron exchange induced by asymmetric solvation of the two nitro groups. The position of the OH group is a decisive factor in the variation of the rate constant with the deprotonation of the radical-anion. This variation is larger when the OH group is in the ortho position to one nitro group. The activation parameters for the intramolecular electron exchange reaction have been determined by using the Marcus theory and the Rips–Jortner equation for the rate constant of a uniform and adiabatic reaction. ΔG* values range from 12.6 kJ mol^–1 for radical-dianion of 2,6-dinitrophenol to 46.0 kJ mol^–1 for radical-dianion of 3,5-dinitrophenol. The EPR spectra of the radicals derived from 4,6-dinitrobenzene-1,3-diol show no line-broadening effects in the experimental range of temperatures. In the radical-anion and radical-dianion, the electron is localised mainly in one nitro group, while in the radical-trianion the spin density is evenly distributed over the two nitro groups. The pK_a values of the radical-anions are in the range 8–11.