Activation free energy inverted region

Another, and even more striking aspect of the variation of the symmetry factor with the driving force is the prediction, from (9) and (10), that an inverted region should exist at large driving forces, i.e. when the inequality (40) applies. The activation free energy is then predicted to increase with the... 

Figure 4. Schematic diagram to show the reorganization energy X for nonisotopic reactions for harmonic free energy profiles. This figure shows a normal region activation barrier when-AG° < an activationless situation when -AC =. l.and an inverted region activation barrier when-AG° > A for the harmonic potential inii andGfin represent the initial (reactant) and the final (product) system free energy, respectively.

Fig. 18 Free energy surfaces illustrating the activation energy AG for charge recombination in the Marcus inverted region and the coupling term //,y between the initial state, i, and the final state, y...

This quadratic dependence of the activation energy on the reaction free energy leads to the prediction of an inverted region in which the reaction rate constant (which depends on AG ) falls when the overall reaction free energy becomes more favourable. This is readily seen from the simple picture shown in Figure 4.14. When the intersection point of the wells leads to A G = 0 the reaction becomes free of an activation barrier, but as the products well sinks deeper the point of intersection rises again. 

The Marcus Inverted Region (MIR) is that part of the function of rate constant versus free energy where a chemical reaction becomes slower as it becomes more exothermic. It has been observed in many thermal electron transfer processes such as neutralization of ion pairs, but not for photoinduced charge separation between neutral molecules. The reasons for this discrepancy have been the object of much controversy in recent years, and the present article gives a critical summary of the theoretical basis of the MIR as well as of the explanations proposed for its absence in photoinduced electron transfer. The role of the solvent receives special attention, notably in view of the possible effects of dielectric saturation in the field of ions. The relationship between the MIR and the theories of radiationless transitions is a topic of current development, although in the Marcus-Hush Model electron transfer is treated as a thermally activated process. 

The Inverted Region.— The expression (11) for the activation energy as a function of the standard free-energy change is well known. It predicts that when —... 

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