Reorientation energy

However, the activation energies are invariably small and generally fall in the range 6-30 kJ moH with the majority around 15 kJ moHh The latter observation led Hart and Anbar [67] to suggest that reaction (31) has an activation energy associated with reorientation of the solvent shell to facilitate transfer of the electron, and that this reorientation energy is the same as that required for e q to diffuse in water. The corollary to this argument is that the... 

Thus, the model incorporating the direct hole trapping by adsorbed dichloroacetate molecules, which has been proposed by Bahnemann and co-workers, appears to be probable [7]. Moreover, calculations using the Marcus electron transfer theory for adiabatic processes which result in a reorientation energy of 0.64 eV suggest that also in the case of SCN- the hole transfer occurs in the adsorbed state [7]. 

Since in most cases the reorientation energy is not known very well, the theoretical predictions are rather uncertain. Lewis recently made some calculations assuming much higher 2-values for the same redox couples (e.g. = 1.6 eV for [Fe(CN)6] ) [129]. His calculation is based on a model recently derived... 

The energy level, E , and energy level, E,, can be related to the redox energy level, Sredox. by a quantity called the reorientation energy X = l/2( ox - J red), which is determined by the relaxation process involved in the regrouping and reorientation of the solvation shell after electron transfer between the oxidized and reduced states. The value of X can be experimentally determined and is on the order of 0.5 to 1 The... 

With respect to the electron transfer, we are interested in the rather slow orientation change of the solvent molecules. The corresponding energy involved in such a process can be obtained by performing a thought experiment as follows. In the first step the electric field is switched on very slowly so that all processes can follow. In the second step the field is switched off very rapidly so that only the distortion of electrons can follow. The outer sphere reorientation energy is thus given by... 

The adsorption system is in many respects similar to biological systems. Especially the smallness of the solvent reorientation energy, with Ag < and -AGg = A, of a donor-acceptor system with ion-paired product state in a diffusionless, semi-rigid environment, opens many possibilities to study certain aspects of electron transfer which are of importance to biological systems. 

Accordingly, not only the energy distance of the maxima of the distribution curves from p.redox but also the half-width depends on the reorientation energy, A. Typical values of A are in the range from a few tenths of an eV up to 2 eV (see Section 4.5). It is assumed here that the reorientation energy A after electron transfer is equal for the oxidation and reduction processes. 

Evaluations of Exchange Currents and Determination of Reorientation Energies... 

At first sight, it should be possible to determine the reorientation energy, A, from Eqs. (36a) and (36b) if the exchange current C or il is measured. Difficulties arise, however, since assumptions have to be made concerning the collision frequency Z and the transition factor c. Z has been calculated by Marcus, who suggests a value of lO cmsec It is impossible, however. 

According to Table 4, the reorientation energies differ considerably from one redox system to another. The unusually high A value obtained for... 

In Section 3.2 we have characterized a redox system by its Fermi level EF,redox (redox potential), in terms of occupied and empty energy levels ( ed and Elx) and their distribution, and by the reorientation energy A (Figure 7). Now it is well known that an excited molecule is more easily reduced or oxidized because the excitation energy AE is stored in the molecule. Possible reactions are... 

In the previous sections the expressions for the admittance of materials were developed on the assumption that they had no dc conductivity. The real part of the admittance arose from the dissipative process of dipole reorientation. Energy was absorbed by the system when the orientation of dipoles was changed with respect to the electric field vector. 

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