Intramolecular electron exchange in the 2,7-dinitronaphthalene radical anion : electron paramagnetic resonance and kinetic data

Abstract

The rates of intramolecular electron exchange in the radical anion of 2,7-dinitronaphthalene in protic and aprotic solvents have been determined from alternating line-broadening effects in EPR spectra. Rate constants in alcohols range from 9.0 × 105 s–1(methanol, 297 K) to 1.7 × 108 s–1(propan-2-ol, 361 K), while in aprotic solvents they range from 1.2 × 108 s–1(acetonitrile, 232 K) to 3.7 × 109 s–1(dimethylformamide, 342 K). The reactions were found to be adiabatic and uniform.Applying Marcus theory and the Rips–Jortner approach to the solvent dependence of the rates, the transferred charge ze0 was determined. It was found that z decreases with increasing temperature, changing from 1.0 at 260 K to 0.98 at 320 K in protic solvents and from 0.58 at 280 K to 0.47 at 340 K in aprotic solvents. This result is explained on the basis of the effect of temperature on the asymmetric solvation of the anions.The outer-sphere reorganization energy, λ0, was obtained by applying the ellipsoidal cavity model and was found to be strongly dependent on the solvent polarity, changing at 293 K from 57.7 kJ mol–1 in hexamethylphosphoric triamide to 235.8 kJ mol–1 in methanol. Activation energies were found to be higher in protic than in aprotic solvents.Results for the 2,7-dinitronaphthalene radical anion are compared with previous data reported for the 1,3-dinitrobenzene radical anion. Rate constants are smaller in the former while outer-sphere reorganization energies are larger. Activation parameters were found of the same order of magnitude for both radical anions.