Electron transfer free enthalpy

As it has already been pointed out, electronic excitation modifies the redox potentials of chemical compounds [32]. Generally, electron transfer is facilitated when electronic excitation is involved. The Rehm-Weller equation (Eq. 29.1) [33] enables an estimation of the exothermicity of a photochemical electron transfer (free enthalpy of electron transfer AG ). Even when the electron transfer at the ground state ( (D+/D)- (A/A )) is endothermic, this may be compensated by the excitation energy E. The attraction of the resulting ions is given by the term w, which is derived from Coulomb s law. It is reduced when reactions are carried out in a polar reaction medium, thus stabilizing the ions with respect to back election transfer. Most frequently, in the case of organic... 

For a redox reaction in an electrochemical cell the decrease in free enthalpy (- AG) is in accordance with the energy delivered by the transfer of electrons through an external circuit if this takes place in a reversible way, i.e., at a rate slow enough to allow complete attainment of equilibrium, the conversion of 1 gram mole will deliver an electrical energy of - AG = z FE. In total cell reaction mred, + n ox2 m ox, + nred2, where m81 = nS2 electrons are transfered (<5, and S2 represent the respective valence differences of the two redox systems), we have... 

A. Z. Weller, Photoinduced electron transfer in solution Exciplex and radical ion pair formation free enthalpies and their solvent dependence, Z. Phys. Chem. Neue Folge, Wiesbaden 133, 93-98 (1982). 

The electrode potential in the equilibrium of redox electron transfer may also be defined by the free enthalpy change in the reaction of the hydrated redox particles with the standard gaseous electron eisro) as shown in Eqn. 4—20 ... 

It, thus, follows that the electrode potential in equilibrium of metal ion transfer is given by the free enthalpy for the formation of a solid metal from both hydrated metal ions and standard gaseous electrons as shown in Eqn. 4—25 ... 

Similar results were obtained [139] with the three dimethoxybenzenes and acrylonitrile, methacrylonitrile, and crotonitrile. The amounts of substitution products decrease in the order acrylonitrile (49%) > methacrylonitrile (45%) > crotonitrile (6%), which agrees with the electron affinities of these compounds. Simultaneously, the amount of addition product increases acrylonitrile, 0% methacrylonitrile, 38% crotonitrile, 67%. In the series of anisole and the dimethoxybenzenes with crotonitrile, the amount of substitution products decrease in the order ortho- and para-dim ethoxy benzene > meta-dimethoxyben-zene > anisole, which is just the reverse of the order of their oxidation potentials. Ohashi et al. [139] have attempted to relate the photochemical behavior of these systems to the free enthalpy of electron transfer in the excited state as calculated with the Rehm-Weller equation, AG = E(D/D+) - E(A /A) - el/eR - AE00. 

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... 

Table 8 Free Enthalpies of Electron Transfer and Mode of Reaction of a,a,a-Trifluorotoluene With Various Olefins...

A correlation between free enthalpy of electron transfer and mode of the photoreaction was also constructed for addition of alkenes to benzonitrile. Four areas could be differentiated Full electron transfer, leading to substitution, is only observed if AG < 0 eV cycloaddition to the cyano group occurs if 0 < AG < 0.4 eV. All olefins for which AG > 0.4 eV preferentially undergo cycloaddition to the aromatic ring, ortho cycloaddition if AG < 1.7 eV and meta cycloaddition if AG > 1.7 eV. 

The correlations between free enthalpies of electron transfer and the mode of the reactions are empirical and therefore may be restricted to one series of molecules owing to similar structural features of intermediates (i.e., absolute values of AG may change in some series) [183],... 

Fig. 6. Fluorescence quenching rate constants as a function of the free enthalpy, AG°, for the electron transfer process from a donor to an excited acceptor ( ) or from an excited acceptor to a donor (o) [136]...

An electron transfer mechanism also has been reported for the interaction between excited ketones and vinyl monomers [57-61]. The olefin acts either as donor or as acceptor for the transferred electron (see Scheme 5). But, the positive free enthalpy of these reactions (AGet), expressed by the Rehm-Weller-Eq. (5) excludes such reactions in the most cases. Furthermore, in those... 

Table 5. Free reaction enthalpies (AGct), rate constants for electron transfer between excited singlet (4k ) and triplet state (3k ) of donor and onium salts, efficiencies of isc-process of the donor in presence of onium salts (ilisc), quantum yields of onium salt decomposition (in) and of polymerization (methyl methacrylate (measured in acetonitrile/water 90 vol%)... 

The proximity of the diffusion limit also inhibits a detailed discussion of the data in Table 7, but a significant difference to the substituent effects discussed in Section III.D.4 is obvious. Whereas the reactivities of terminal alkenes, dienes, and styrenes toward AnPhCH correlate with the stabilities of the new carbenium ions and not with the ionization potentials of the 7r-nucleophiles [69], the situation is different for the reactions of enol ethers with (p-ClC6H4)2CH+ [136]. In this reaction series, methyl groups at the position of electrophilic attack activate the enol ether double bonds more than methyl groups at the new carbocationic center, i.e., the relative activation free enthalpies are not controlled any longer by the stabilities of the intermediate carbocations but by the ionization potentials of the enol ethers (Fig. 20). An interpretation of the correlation in Fig. 20 has not yet been given, but one can alternatively discuss early transition states which are controlled by frontier orbital interactions or the involvement of outer sphere electron transfer processes [220]. 

Figure 5.26. Dependence of the rate constant log k for electron-transfer processes on the free reaction enthalpy AG, Rehm-Weller plot (—) and Marcus plot (—) for A = 10 kcal/mol (adapted from Eberson, 1982).

From a thermodynamic point of view, the variation of standard free enthalpy associated to the electron transfer process represented by Equation (2.1), tAG °, can be related with the variation of such tliermodynamic quantity for the electron transfer process for species in solution phase, lAG °. and for the transfer of the oxidized, IaG, and reduced, forms of the electroactive species and the electrolyte cations, from the solution phase to the porous solid. The corresponding Bom-Haber-type cycle is shown in... 

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