Outer activation barrier

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. 

The use of outer-sphere parameters predicts a much larger activation barrier (or smaller rate constant) than is actually observed. It appears that much of this kinetic advantage derives from differences in the equilibrium constants for outer-... 

It was of course of interest to review in this context the use of the Rehm-Weller empirical equation, Eq. (6), versus the Marcus model of Eq. (5). The method used to correlate the results for the jl-naphtholate quenchings was thus applied to the important Weller series of fluorescence quenching measurements made in acetonitrile [88] and this shows that 60 values (on a total number of 65) fit quite well with a mean intrinsic activation barrier of 5.5 Kcal. M 1 [96] (see Fig. 3). Moreover, this value is not far from the expected value for the outer contribution (AGJ), as calculated by Eq. (7) and it has often been underlined that the solvent reorganization term is effectively the dominant contribution in... 

Experimental estimates of <5r/c , are relatively difficult to obtain. While they can, in principle, be extracted from temperature-dependence studies, this approach is complicated by uncertainties in the entropic term (Sect. 4.3). An alternative method has recently been described for some Cr(III) reductions which involves comparing the work-corrected rate constants, kco , with unimolecular rate constants, ket, for structurally related reactants that reduce via ligand-bridged pathways [30]. Provided that the corresponding outer- and inner-sphere pathways involve the same activation barrier (Sect. 4.6) and the latter also follow adiabatic pathways, we can write [30]... 

Fig. 1 Conceptual energy landscapes for bound states c confined by sharp activation barriers. Oriented at an angle 9 to the molecular coordinate x, external force / adds a mechanical potential — (/cos 6)x that tilts the landscape and lowers the barrier. For sharp barriers, the energy contours local to barriers—transition states s —are highly curved and change little in shape or location under force, (a) A single barrier under force, (b) A cascade of barriers under force. The inner barrier emerges to dominate kinetics when the outer barrier is driven below it by k T.

The inner and outer sphere reorganization energies for this reaction are 50.7 and 27.7kJmol , respectively. This gives a value of equal to 314kJmol . The lowering of the activation barrier due to interaction of the energy profiles... 

From the spectroscopic methods considered so far only the electron diffraction experiment is not affected by one of the shortcomings mentioned above. The result that the amplitude of the central sulfur atom is higher than for the outer ones leaves open the possibility of a very low activation barrier or a broad U-shaped potential. 

The calculation of activation barriers of outer-sphere electron transfer processes (see Chapter 11). 

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