Activation reorganisation energy

The potential energy surfaces on which the electron-transfer process occurs can be represented by simple two-dimensional intersecting parabolic curves (Figure 6.23). These quantitatively relate the rate of electron transfer to the reorganisation energy (A.) and the free-energy changes for the electron-transfer process (AG°) and activation (AG ). 

Figure 4.8 Classical Marcus theory section across the reaction coordinate X through the free energy hypersurface of the reaction complex R and product complex P for an ET reaction, showing the activation barrier AG, the reorganisation energy A and the free energy of reaction AG°.

Figure 11.25. Photocurrent dependence on the Gibbs free energy of electron transfer for the photo-oxidation of ferrocene derivatives (a) and photoreduction of quinone-type molecules (h) at the water/DCE interface. AG ( is evaluated from Equation (11.47), employing the formal redox potentials summarised in Table 11.1 and the applied Galvani potential difference. A deconvolution of the photocurrent relaxation in the presence of the electron acceptors was performed in order to estimate the flux of election injection g. The second-order rate constant for the photoninduced heterogeneous electron transfer is also calculated assuming values of 1 nm for dec and 5 x 10 s for A ,. The trends observed in both set of data were rationahsed in terms of a single solvent reorganisation energy and activation-less limit for the rate constant. Reprinted with permission from refs.[101] and [60]. Copyright (2002/2003) American Chemical Society.

V(PB) /V(PB)g=i2.0 0.4. Therefore, provided that this asymmetry also holds for Rb.sphaero-ides, the branching ratio k /kg > 25 points to an additional asymmetry in the Franck-Condon-factors FC /FCg > 6. With the assumption that kj(A) is activationless, kj(B) has to be activated with at least 120cm i (taking the full quantum expression [11] with hw 100 cm i). This corresponds to A(AGj) > 440cm" if we adopt a reorganisation energy... 

AG is the activation energy required to transfer an electron from the oxidized form of reactant A to its reduced form the same is valid for AG and reactant B. These reactions are called homonuclear electron transfer or electron exchange reactions here AG and the differences in the Coulomb interactions between reactants and products are zero. The physical interpretation of equ.IV arises from the fact that AG is the reorganisation energy required to reach the transition state (A(ox) A(red)), AG is the... 

This quantity A, is known as the reorganisation energy . It will later be seen that the activation free energy of the electron-transfer step (AG j) depends only on A and AG j, and takes the value A/4 if AG = 0 (see Section 9.1.2.5, Equation (9.15)). 

The free activation energy is determined by the driving force — AG°, which is related to the standard redox potential dilference (Aii°), and by the so-called reorganisation energy X (eqn (3.4))... 

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