Electron transfer normal region

Figure 2 Energy surfaces corresponding to the normal, barrierless, and inverted electron-transfer reactivity regions of Marcus theory.

The vibrational overlap integrals play a key role in electron transfer. A region of vibrational overlap defines values of the normal coordinate where a finite probability exists for finding coordinates appropriate for both reactants and products. The greater the overlap, the greater the transition rate. The vibrational overlap integrals can be evaluated explicitly for harmonic oscillator wavefunctions. An example is shown in equation (26) for the overlap between an initial level with vibrational quantum number v = 0 to a level v = v where the frequency (and force constant) are taken to be the same before and after electron transfer. 

The correct potential for a preparative electrolysis is normally chosen by inspection of a steady state current-potential (i-F) curve. Figure 1 shows a typical i-E curve for the reduction of anthracene at a mercury cathode in dimethylformamide (Peover et al., 1963) the curve shows two reduction waves. In the potential range where the current rises with variation of the potential, the rate of an electron transfer process is increasing while in the plateau regions the rate of the electron transfer... 

FIGURE 34.4 Dependence of electrochemical rate constant on the electrode potential for outer-sphere electron transfer. An exponential increase in the normal region changes for the plateau in the activationless region. 

Two situations may be distinguished according to whether the electron transfer step of the stepwise mechanism lies in the normal region or in the inverted... 

The relationship between electron transfer in the normal and inverted regions is illustrated in Figure 3 for the case of quenching of Ru(bpy) + by a nitroaromatic quencher. Excitation... 

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. 

The PET process in the dyad is located in the normal region of the Marcus curve, while the back-electron transfer from Q0 to ZnPor+ is in the Marcus inverted region (Figure 6.27). 

Figure 11 Dependence on driving force of first-order rate constant for back electron transfer from colloidal Sn02 films to covalently attached complexes. The variations indicate that the reactions occur in the Marcus normal region. The identities of the molecular redox couples, listed from highest driving force to lowest, are, Rulll/n (5-Cl-phen)2 (phos-...

The use of the terms adiabatic and non-adiabatic in this way leads to a source of confusion. Normally, in describing surface-crossing processes, a process which remains on the same potential curve is called adiabatic and in that sense every net electron transfer reaction is an adiabatic process. Processes which involve a transition between different states as between the two different potential curves in Figure lb are usually called non-adiabatic. Such processes have some special features and will be returned to in a later section dealing with the inverted region and excited state decay. 

As for electron transfer in the normal region, based on the results of time dependent perturbation theory, electron transfer in the inverted or excited state decay region is also determined by the... 

Figure 4.17 Examples of the normal and inverted region plots. The circles correspond to a photoinduced electron transfer in which an ion pair is formed, the squares to the geminate recombination of these ion pairs...

This phenomenon arises from charge-transfer reactions between the semiconductor and excited dye molecules adsorbed on its surface. If the semiconductor band gap is large compared to the dye s excitation energy, electron transfer between the dye and the electrode may involve the highest normally filled level or the excited level of the dye, but usually not both. One of the dye levels will not be electroactive because it will match some energy in the electrode s band-gap region. 

A consequence of Eq. (18) is that the maximum electron transfer rate occurs when AG° = —X. A plot of ln(ket) versus AG° is shown in Fig. 4. Electron transfer reactions with AG° more positive than —X define the normal region, where the rate of electron transfer increases with increasing exergonicity, whereas free energies negative relative to —X define the inverted region in which the rate decreases as AG° becomes more negative. For most ECL reactions, the... 

In Ref. 142, a detailed analysis of the forward and reverse electron transfer rates for capped P-L2-Q in a variety of solvents was given. The results show that forward electron transfer is in the normal region Er > — AG° and charge recombination is in the Marcus inverted region, Er < — AG°. 

---