SUBSTITUENT EFFECTS ON THE INTRAMOLECULAR ELECTRON-EXCHANGE REACTION IN 5-SUBSTITUTED 1,3-DINITROBENZENE RADICAL-ANIONS

Nine new radical anions of 1,3-dinitrobenzenes with various substituen ts on position 5 (2, 3, 5 - 7, 9 - 11 and 13) were prepared by electro chemical 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 o f data. Their EPR spectra showed alternating linebroadening effects du e to intramolecular electron exchange induced by asymmetric solvation of the two nitro groups. Rate constants at 293 K range from 1.4 x 10(9 ) s-1 (3,5-dinitrophenolate) to 1.2 x 10(10) s-1 (3,5-dinitrobenzonitr ile). Good Hammett correlations between the rate constants of intramol ecular electron exchange and the parameter sigma(m) of the substituent on position 5 were obtained. The reaction is accelerated by electron withdrawing groups. Rho 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. DELTA G was found to increase with the electron donating ability of the sub stituent, mainly due to an increase of the interaction between the rea ctants and the solvent. The pre-exponential factor A is constant withi n the experimental error, as predicted by theory.