Substituent effects on the intramolecular electron exchange reaction in 5‐substituted 1,3‐dinitrobenzene radical anions

Abstract

AbstractNine new radical anions of 1,3‐dinitrobenzenes with various substituents on position 5 (2, 3, 5–7, 9–11 and 13) were prepared by electrochemical reduction in acetonitrile and studied by EPR at temperatures between 240 K and 320 K. Another eight known radicals of the same type were reexamined in the same conditions in order to complete the set of data. Their EPR spectra showed alternating linebroadening effects due to intramolecular electron exchange induced by asymmetric solvation of the two nitro groups. Rate constants at 293 K range from 1.4×109s−1 (3,5‐dinitrophenolate) to 1.2×1010s−1 (3,5‐dinitrobenzonitrile). Good Hammett correlations between the rate constants of intramolecular electron exchange and the parameter σm of the substituent on position 5 were obtained. The reaction is accelerated by electron withdrawing groups. ρ values range from 0.95 at 240 K to 0.74 at 320 K. Activation parameters were estimated by using the Rips and Jortner equation for the rate constant of a uniform adiabatic reaction. ΔG* was found to increase with the electron donating ability of the substituent, mainly due to an increase of the interaction between the reactants and the solvent. The pre‐exponential factor A is constant within the experimental error, as predicted by theory.