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.