Solvent effects on intramolecular electron exchange in the 1,4-dinitrobenzene radical anion

Abstract

The rate constants of the intramolecular electron exchange reaction in the 1,4-dinitrobenzene radical anion in linear alcohols were determined from alternating line-broadening effects in EPR spectra. Rate constants at 293 K range from 2.9×10⁹ s^-1 in methanol to 2.7×10⁸ s^-1 in octan-1-ol. The rate constant was found to depend on the reverse of the longitudinal correlation time of the solvent, τ_L, as expected for a solvent-controlled adiabatic reaction. Applying Marcus theory and the ellipsoidal cavity model for evaluating λ₀, the amount of transferred charge between nitro groups, z, and the outer-sphere reorganisation energy were determined. z changes from 0.56 at 273 K to 0.52 at 313 K. λ₀ at 293 K changes from 61 kJ mol^-1 in octan-1-ol to 80 kJ mol^-1 in methanol. The activation energies were obtained from the temperature dependence of the rate constants, changing from 26 kJ mol^-1 in methanol to 36 kJ mol^-1 in octan-1-ol. The variation of this energy clearly reflects the increase in the exponential term of τ_L for the more viscous alcohols. The results for the 1,4-dinitrobenzene radical anion were compared with those previously reported for the radical anion of 1,3-dinitrobenzene. Rate constants are smaller in the former, by three orders of magnitude, due to conjugation of the nitro groups.