Electron transfer free energies

Mitchell based his concept on the suggestion that as electron is transported along the respiratory chain, H+ ions are ejected to cytoplasm (the mitochondrion environment). As a consequence, a gradient of H+ ion concentration occurs in external and internal mitochondrial spaces. Of course, this H+ ion concentration gradient is supported by electron transfer free energy decrease and in the case of membrane impermeability for H+ ions. 

Fig. 2. Fluorescent quenching rate constants of excited /J-naphtholate anion correlated with the electron transfer free energy [96]...

This is an unimportant contribution when solvents of similar polarities are concerned (as DMF and acetonitrile) but very significant corrections are calculated if one changes from the polar acetonitrile to benzene (from 1 to 1.5 V). This type of correction has been applied in the case of intra ion-pair electron transfer between borate anions and excited cyanine in order to make a correct estimation of the electron transfer free energy [93]. 

The rate constants for the reactions of both BPHT lowest excited singlet (Si) and triplet (T i) states with bromo- and chloro-substituted compounds were found to be well correlated to the electron-transfer free energy. 

Such reactions are possible at interaction of transition metal oxide surface in the redox reactant/surface site pairs possessing low electron transfer free energy and lead to formation of radical ions from the reactant molecules [64]... 

Simonson T. Free Energy Calculations. In Computational Biochemistry and Biophysics Becker OM, MacKerellm AD, Roux B, Watanabe M, eds. 2001. Marcel Dekker Inc., New York. Leontyev FV, Basilevsky MV, Newton MD. Theory and computation of electron transfer reorganization energies with continuum and molecular solvent models. Theor. Chem. Acc. 2004 111 110-121. 

Figure 1. Relation between electron-transfer and energy-transfer processes, illustrating the operation of the restriction on nuclear motion and kinetic energy change. The electron transfer (or energy transfer) is concerted, and free electrons (or photons) are not normally produced. The parallelism between electron and energy transfer is even more striking if a hole (h ) formalism is used for the electron (e ) transfer (Ox -> Red + h, rather than Ox -I- e" — Red).

Intramolecular proton transfer in electronically excited molecules has been reviewed in salicyclic add esters it is said to lead to deexdtation of the exdted electron the free energy change involved in that proton transfer is reported as about 0.13-0.22 eV. ... 

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... 

Within this framework, by considering the physical situation of the electrode double layer, the free energy of activation of an electron transfer reaction can be identified with the reorganization energy of the solvation sheath around the ion. This idea will be carried through in detail for the simple case of the strongly solvated... 

Similarly, changes must take place in the outer solvation shell diirmg electron transfer, all of which implies that the solvation shells themselves inliibit electron transfer. This inliibition by the surrounding solvent molecules in the iimer and outer solvation shells can be characterized by an activation free energy AG. ... 

Sekiguchi S, Kobori Y, Akiyama K and Tero-Kubota S 1998 Marcus free energy dependence of the sign of exchange interactions in radical ion pairs generated by photoinduced electron transfer reactions J. Am. Chem. Soc. 120 1325-6... 

Figure C3.2.10.(a) Dependence of electron transfer rate upon reaction free energy for ET between biphenyl radical anions and various organic acceptors. Experiments were perfonned with the donors and acceptors frozen into...

When the e.m.f. of a cell is measured by balancing it against an external voltage, so that no current flows, the maximum e.m.f. is obtained since the cell is at equilibrium. The maximum work obtainable from the cell is then nFE J, where n is the number of electrons transferred, F is the Faraday unit and E is the maximum cell e.m.f. We saw in Chapter 3 that the maximum amount of work obtainable from a reaction is given by the free energy change, i.e. - AG. Hence... 

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