Rehm-Weller relationship

With regard to the simplified version of the Rehm-Weller relationship (1) follows that in the presence of an electron donor with an oxidation potential less than ca. 2.1V an exergonic electron transfer can occur [25b]. 

Fig. 2. Plot of the fluorescence quenching rate constants k, vs the free energy changes AG for the electron transfer process (a) Marcus relationship, (b) Rehm-Weller relationship...

The fluorescence quenching dynamics of excited state electron donors by various pyrimidine and 5,6-dihydropyrimidine substrates have been examined and found to obey the Rehm-Weller relationship." In addition, an unexpected difference was observed between the reduction potentials for the trans-syn and cis-syn diastereoisomers of dimethylthymine cyclobutane dimers, and this has been ascribed to a stereoelectronic effect in the cis-syn dimer anion radical resulting from an unfavourable charge-dipole interaction between the added electron and the O carbonyl group of the pyrimidine ring... 

The Rehm-Weller formulation of the free energy relationship in eq 17 can be rewritten as... 

Figure 14. Dependence of the Rehm-Weller free energy relationship on the intrinsic barrier AG. Key left, activation free energy change and right, transfer...

Determined by adaptation of the Rehm-Weller free energy relationship for photoinduced electron transfer. bRate constants for PhCH2SiMe3. The values in parentheses denote those for CH2=CHCH2SiMe3. Calculated using the free energy relationship for photoinduced electron transfer. 

In view of the occurrence of electron transfer and also in view of the observations made by Bryce-Smith and Gilbert (on which the ionization potential rule is based), Mattay et al. [15,143,144] have proposed a relationship between the mode of reaction (ortho addition, meta addition, substitution) and the free enthalpy of electron transfer between the reaction partners. The free enthalpy was calculated using the Rehm-Weller equation... 

In the above sections, nothing was said about the type of reaction between M and Q. This is because the Stem-Volmer equation is model independent, as explained above and also because eqs. (20)-(22) are for a diffusion-controlled reaction. Some information can be obtained regarding an electron transfer from various quenchers of similar chemical structures towards M. In this case, one may derive a relationship between ksv (as obtained from eq. (17)) and the ionization potential of these inhibitors. This is the Rehm-Weller equation, which is schematically depicted in fig. 4. In this plot, the plateau value corresponds to fcdin. For a general overview of problems related to electron transfers, see Pouliquen and Wintgens (1988) (in French). 

At the singlet excited state, ortho and meta photocycloadditions are often competitive processes and physicochemical investigations were carried out to rationalize the modes of cycloaddition of arenes with alkenes. In the context of the study of photochemical electron transfer reactions, it has been proposed that the difference of the redox potentials of the reaction partners might play an important role in this competition [10]. Such a discussion involves the intervention of an exciplex as intermediate. The Rehm-Weller equation [11] was used to quantify the relationship. When an electron transfer process is strongly endergonic (AG>1.5eV), the meta cycloaddition should be favored. When such a process is less endergonic (1 < AG< 1.5 eY), the ortho addition dominate [12]. This means that the... 

The relationship between driving force and proton transfer has been much more elusive despite considerable evidence that the vast photo synthetic electron transfer machinery mainly exists to set up a charge gradient to drive proton transfer. This is due to a combination of two factors. First, there is a vast reservoir of readily available materials with which to examine electron transfer. Second, the relationship between rates and driving force for electron transfer, based upon the excitation energies and the relevant redox potentials (the Rehm-Weller equation [1]) is reasonably straightforward. 

To alleviate the apparent problem, Rehm and Weller proposed an empirical relationship for electron transfer between excited donor/acceptor pairs (7), eq. 7. 

Although the occurrence of photoinduced electron transfer between organic redox participants can be roughly predicted by Rehm and Weller s empirical relationship, detailed study of such processes will provide a stimulating and highly productive research area for many years to come. 

In view of this difficulty, Rehm and Weller proposed an empirical relationship (73) between AG and AG° for the fcel step which gives virtually the... 

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