Activation parameters of electron-transfer reactions with synchronous cleavage of a bond

The temperature dependence of the rate constants of the disproportionation reactions of verdazyl radicals (R') in their complexes with zinc ions and of the inner-sphere oxidation of copper(I) by tetraethylthiuram disulfide (tds) in acetonitrile and mixtures of the latter with benzene has been measured. The activation parameters of the reactions studied have been determined according to the Arrhenius equation. In these reactions electron transfer is accompanied by the synchronous cleavage of a chemical bond Zn-R in the complex Zn(R')₂ ²⁺ or S-S in tds], which is manifested by a decrease in the value of the preexponential factor. It has been established that the dependence of the rate constants on the polarity of the medium does not conform to the framework of Marcus' theory, but the activation energy and the preexponential factor decrease symbatically and approximately exponentially as the mole fraction of the polar molecules of acetonitrile in its mixtures with benzene is increased.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 4, pp. 501–504, July–August, 1988.