Electron transfer thermodynamic driving force

In general, it has been found that under the correct combination of electronic coupling, thermodynamic driving force, solvent and temperature, P-Q systems readily undergo photoinitiated electron transfer as shown in Fig. 2. Singlet excitation localizes on the porphyrin, which has the energetically lowest-lying... 

The fact that total stereospecificity is observed rules out the HHA mechanism. Indeed, the same loss of stereospecificity as in the outer-sphere case should be observed since the reaction goes through the same interconverting radicals in both cases. One is thus left with an EL mechanism in which the elimination of the second bromine as a bromide ion from the carbanion should be concerted with the first step or at least faster than the interconversion of the two carbanions. Estimation of the thermodynamic driving forces for the outer-sphere electron transfer, HAA, and EL mechanisms confirms the advantage of the latter.lb... 

Coordinative Environment. The coordinative environment of transition metal ions affects the thermodynamic driving force and reaction rate of ligand substitution and electron transfer reactions. FeIIIoH2+(aq) and hematite (a-Fe203) surface structures are shown in Figure 3 for the sake of comparison. Within the lattice of oxide/hydroxide minerals, the inner coordination spheres of metal centers are fully occupied by a regular array of O3- and/or 0H donor groups. At the mineral surface, however, one or more coordinative positions of each metal center are vacant (15). When oxide surfaces are introduced into aqueous solution, H2O and 0H molecules... 

Apply the Marcus theory to photoinduced electron transfer and show how experimental evidence gives a relationship between the kinetics of the process and the thermodynamic driving force. 

This equation, based on the Marcus model, therefore gives us a relationship between the kinetics (kEr) and the thermodynamic driving force (AG°) of the electron-transfer process. Analysis of the equation predicts that one of three distinct kinetic regions will exist, as shown in Figure 6.24, depending on the driving force of the process. 

In the normal region, thermodynamic driving forces are small. The electron-transfer process is thermally activated, with its rate increasing as the driving force increases. 

In the activationless region (so called because AG = 0), a change in the thermodynamic driving force has a negligible effect on the rate of electron transfer. 

The inverted region is so-called because the rate of the electron-transfer process decreases with increasing thermodynamic driving force. 

Figure 6.24 Marcus theory predictions of the dependence of the electron-transfer rate on the thermodynamic driving force...

There is one experimental parameter that does serve to distinguish between the semiclassical model and the quantum model for nonadiabatic proton transfer. In the semiclassical model, if one assumes that the magnitude of the electronic barrier directly correlates with the thermodynamic driving force, a statement of the Hammond postulate, then as the driving force increases the rate of reaction increases, eventually reaching a maximum rate. The quantum model has a... 

The above model has been further explored to account for reaction efficiencies in terms of a scheme where nucleophilicities and leaving group abilities can be rationalized by a structure-reactivity pattern. Pellerite and Brau-man (1980, 1983) have proposed that the central energy barrier for an exothermic reaction (see Fig. 3) can be analysed in terms of a thermodynamic driving force, due to the exothermicity of the reaction, and an intrinsic energy barrier. The separation between these two components has been carried out by extending to SN2 reactions the theory developed by Marcus for electron transfer reactions in solutions (Marcus, 1964). While the validity of the Marcus theory to atom and group transfer is open to criticism, the basic assumption of the proposed model is that the intrinsic barrier of reaction (38)... 

Using mutant proteins as well as a variety of redox pairs and electron-transfer distances the validity of the Marcus equation with respect to the thermodynamic driving force and distance dependence has been verified.153 This is even true for cytochrome c mutants functioning in living yeast cells.146... 

Y, this bond is at first going to be a two-center, three-electron bond (H5C6.. Y). This process of nucleophile addition can and must be viewed as an inner-sphere electron transfer. In other words, formation of the new bond is connected with a transfer of the electron to phenyl radical. Therefore, the good electron donor character of Y is necessary for this step eventually to lead to the efficient formation of (H5C6Y), i.e., the product anion radical. Galli and Gentili (1998) have evaluated the thermodynamic driving force (TDDF) of the nucleophile/radical addition step. With respect to this step, they classified a nucleophile... 

The fundamental theories behind electron transfer were discussed above in Section 2.1. Indeed, some of the most important empirical proofs for these theories have originated from photoinduced electron transfer in supramolecular donor-acceptor complexes. The difference between thermally and photochemi-cally induced electron transfer lies in both the orbitals participating in the reaction and in the additional thermodynamic driving force provided by the excited state. It is therefore important to consider the redox properties of excited-state species. 

According to Eq. (1), the thermodynamic driving force of a PET process increases with the solvent polarity and therefore, photoreactions can be simply switched from energy transfer to electron transfer by changing the solvent [8]. However, back electron transfer (BET) often diminishes the yields of radical ions formed and therefore various efforts have been undertaken to circumvent this energy loss process [14]. Among these approaches two processes have been widely used and will thus be described in more detail. 

Application of Hush theory to the observed IPCT bands yielded information about the relationship between optical and thermal ET in these systems. The redox potentials of both the metal dithiolene donors and the viologen acceptors can be systematically varied, which, in turn, tunes the thermodynamic driving force for electron transfer. The researchers found that the IPCT band energy increases linearly with more positive free energy AG for ET, and that the reorganization energy (x) remains constant with variation in the metal or cation redox potentials (66, 67). 

The rate of electron transfer within the complex is a function of the thermodynamic driving force of the reaction, the conformation of the... 

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