Reorganization barrier

The reactivity of RhL32+ or RhL33+ toward outer-sphere oxidation or reduction is a function of the intrinsic reorganization barrier for the couple as well as the driving... 

So far the attention has been on the nuclear reorganization barrier. Nevertheless, other important factors previously hidden in the pre-exponential factor (and ultimately in the standard rate constant) have to be considered, namely, the fundamental question of the magnitude of the electronic interaction between electroactive molecules and energy levels in the electrode (i.e., the degree of adiabaticity) and its variation with the tunneling medium (electrode-solution interface), the tunneling distance, and the electrode material. Thus, within the transition-state formalism, the rate constant for electron transfer can be expressed as the product of three factors [39—42] ... 

Fig. 12. Schematic representation of the relation between the spectroscopic Stokes shift and the difference in the reorganization barriers for ground and excited state electron transfer reactions. AB = absorption CD = emission B C + D A = Ahv (Stokes shift) 2Ea = reorganization barrier for reaction (35) 2 Ea = reorganization barrier for reaction (36)...

To illustrate the approach used to calculate the iimer-shell contribution to the reorganization barrier we consider the symmetrical stretching vibrations of the two reactants in the Fe(H20)6 " -Fe(H20)6 + self-exchange reaction (Eq. la). The inner-shell reorganization term is the sum of the reorganization parameters of the individual reactants, i.e. ... 

The type 1 copper site thus provides a set of ligand donor atoms and a stereochemistry that is a compromise between the preferences of Cu (soft ligands and tetrahedral geometry) and Cu" (hard ligands and square planar geometry). The structural restriction placed upon the copper by the protein results in a diminishing of the Franck-Condon reorganization barrier to electron transfer. 

The inner-shell reorganization barrier may be smaller for the inner-sphere reaction. This is likely if the formation of the precursor complex involves the elimination of a ligand (e.g., a coordinated HjO molecule) for which the distance to the metal center is different in the oxidized and reduced forms. 

The solvent-reorganization barrier is lowered as a consequence of the closer approach of the metal centers in the inner-sphere reaction. 

The electron-transfer rates in the bridged systems [(HjO)(NH3) Ru(II)-X-Co(III)(NH3)5]3 and [(EDTA)Ru(II)-X-Co(III)(NH3)5) increase in the order bis(4-pyridyl)methane < 4,4 -bipyridine < pyrazine, which is also the order of decreasing separation of the metal centers. The effect of decreasing the separation is to decrease the solvent-reorganization barrier and to increase the strength of the electronic coupling of the metal centers [the former varies as (l/2a2 + l/2aj — 1/r) and the lat-... 

Note that the influence of work terms upon k, will only be eliminated if w = Wp. On the other hand, k, is more directly related to the reorganization barrier for the elementary electron-transfer step. 

The driving forces for oxidative quenching by [EU( J and [MV] " are similar, but the rate constant for quenching by [MV] is lO times greater than for quenching by [EU(aq)] - These rate differences reflect the varying reorganization barriers of the oxi-... 

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