Outer-sphere electron transfer, nonadiabatic

The height of the potential barrier is lower than that for nonadiabatic reactions and depends on the interaction between the acceptor and the metal. However, at not too large values of the effective eiectrochemical Landau-Zener parameter the difference in the activation barriers is insignihcant. Taking into account the fact that the effective eiectron transmission coefficient is 1 here, one concludes that the rate of the adiabatic outer-sphere electron transfer reaction is practically independent of the electronic properties of the metal electrode. 

Unlike the simplest outer-sphere electron transfer reactions where the electrons are the only quantum subsystem and only two types of transitions are possible (adiabatic and nonadiabatic ones), the situation for proton transfer reactions is more complicated. Three types of transitions may be considered here5 ... 

An important conclusion that can be drawn from the above discussion is that most outer-sphere electron transfer reactions of metal complexes are, at best, marginally adiabatic and that the reaction will rapidly become nonadiabatic with increasing separation of the reactants. In view of these considerations, eq 11 can be integrated to give (50)... 

The importance of a nonadiabatic path for outer-sphere electron transfer reactions of Eu(III)/Eu(Il) was again examined by Yee et al. (1983) via a study of a series of reactions with Eu(III)/(II) cryptates (table 12). The cryptate (polyoxadiazamacrobi-cyclic) ligands form thermodynamically stable and substitution inert complexes with both Eu(lll) and Eu(ll), markedly changing the primary coordination spheres of the Eu ions. The dramatic variation in the values for the Eu exchange reactions with such a change is demonstrated by the respective calculated values for EUavalues calculated from the cross reactions are consistent with the values of the Franck-Condon barriers estimated from structural data. 

The nonadiabatic pathways for outer-sphere electron transfer would be characterized by small values of activation enthalpies and large negative entropies of activation. 

In practice, the weak electronic coupling limit includes most conventional outer-sphere electron transfer reactions, but it also includes some reactions in which the reactants are covalently linked. The limit in which the electronic mixing between the reactants and products approaches zero is the diabatic (or nonadiabatic) limit in which in Figure 1, where Hnj =... 

A recently proposed semiclassical model, in which an electronic transmission coefficient and a nuclear tunneling factor are introduced as corrections to the classical activated-complex expression, is described. The nuclear tunneling corrections are shown to be important only at low temperatures or when the electron transfer is very exothermic. By contrast, corrections for nonadiabaticity may be significant for most outer-sphere reactions of metal complexes. The rate constants for the Fe(H20)6 +-Fe(H20)6 +> Ru(NH3)62+-Ru(NH3)63+ and Ru(bpy)32+-Ru(bpy)33+ electron exchange reactions predicted by the semiclassical model are in very good agreement with the observed values. The implications of the model for optically-induced electron transfer in mixed-valence systems are noted. 

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