Free-energy correlation for catalysis of outer sphere electron-transfer reactions by noncoordinated pyridine derivatives

Recent experimental data concerning the rate constants and their free energy of the outer sphere electron-transfer reactions, as catalyzed by noncoordinated pyridine derivatives in aqueous solution, are examined for possible correlation. For the electron transfer reactions between V or Eu and the bipyridyl derivatives, such as N,N'-dimethyl-4,4′-bipyridyl (paraquat) or diquat, the data are correlated quite well by the Marcus equation. The electron exchange rate constant, 5 × 10⁷M^?1·s^?1, for an organic radical ion and its parent molecule obtained from semiquinones or their related compounds can be applied to these pyridine derivatives. However, in some cases such as electron transfer from paraquat or diquat cation radical to Co(en), positive departures from the Marcus model are observed. These positive departures are interpreted in terms of interaction between the molecular orbitals of electron donors and electron acceptors in the transition state.