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

The rate constants of the intramolecular electron exchange reaction in the 1,4-dinitrobenzene radical anion in linear alcohols were determined from al ternating line-broadening effects in EPR spectra. Rate constants at 293 K r ange from 2.9 x 10(9) s(-1) in methanol to 2.7 x 10(8) s(-1) in octan-1-ol. The rate constant was found to depend on the reverse of the longitudinal c orrelation time of the solvent, tau(L), as expected for a solvent-controlle d adiabatic reaction. Applying Marcus theory and the ellipsoidal cavity mod el for evaluating lambda(0), the amount of transferred charge between nitro groups, z, and the outer-sphere reorganisation energy were determined. z c hanges from 0.56 at 273 K to 0.52 at 313 K. lambda(0) at 293 K changes from 61 kJ mol(-1) in octan-1-ol to 80 kJ mol(-1) in methanol. The activation e nergies 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. T he variation of this energy clearly reflects the increase in the exponentia l term of z, for the more viscous alcohols. The results for the 1,4-dinitro benzene radical anion were compared with those previously reported for the radical anion of 1,3-dinitrobenzene. Rate constants are smaller in the form er, by three orders of magnitude, due to conjugation of the nitro groups.