Marcus theory predicted inverted region

The Marcus theory predicts that the k, values should increase up to a maximum and then decrease with increasing the oxidation potentials of D, or with decreasing the reduction potentials of A [19], The region where k, decreases with increasing the AG values is called the Marcus inverted region . The bell-shaped curve a in Fig. 2 illustrates this theory. 

The second article deals with probably the most fascinating predictions of modem electron transfer theories i.e. the Marcus Inverted Region (M.I.R.) . It was shown only one decade ago, nearly 20 years after the first formulation of the Marcus theory, that the M.I.R. does indeed exist First for thermal charge shifts and later for charge recombination. Even a charge separation reaction was recently found to behave according to the Marcus theory. Nevertheless, many reactions do not follow the Marcus model and therefore the second contribution of this issue is mainly concerned with this question. 

Using the Marcus theory, the a value (see -> charge-transfer coefficient) can be predicted, and its dependence on the potential applied. For low - over potentials, and when neither Ox nor Red are specifically adsorbed on the electrode surface, a should be approximately equal to 0.5. Further, the theory describes the relation between homogeneous and heterogeneous rate constants characteristic of the same redox system. An interesting prediction from Marcus theory is the existence of a so-called inverted region for the homogeneous electron transfer reactions, of importance to the phenomenon of... 

Picosecond infrared studies (144-146) of the dynamics of [(NC)5-RuIICNRu11I(NH3)5]1 following MMCT photolysis permitted observation of the formation and decay of the MMCT excited state and the evaluation of vibronic coupling and energy-transfer dynamics. The experimental results were in agreement with recent electron-transfer theories that have been used to predict excited vibrational populations resulting from back electron transfer in the Marcus inverted region (146). 

The results obtained clearly demonstrate that the Marcus model for ECL processes may be used for qualitative as well as for quantitative descriptions of this kind of electron transfer reactions. The more sophisticated approach, taking into account the vibronic excitation in the reaction products (important in the inverted Marcus region), solvent molecular dynamics (important in the case of large values of the electronic coupling elements), as well as the changes in the electron transfer distance, should be used. The results indicate that the Marcus theory may also be used for predicting the ECL efficiency, provided that some conditions are fulfilled. Especially, during the ECL process, only the annihilation of ions should occur, without any competitive reactions. The necessary rate constants can be evaluated from pertinent electrochemical and spectroscopic data. 

The temperature dependence of electron-transfer rate constants is interesting. In the normal region, it shows an activation energy as predicted from simple Marcus theory. In the inverted region, the activation energy is very small or zero. This agrees with the quantum mechanical version of the theory (Kestner et al., 1974 Fischer and Van Duyne, 1977), which makes it clear that the transition from the upper to the lower surface behaves just like ordinary internal conversion. 

V. Like the classical Marcus theory, equation 1 predicts an inverted region, although the decrease of rates for highly exoergic reactions may be less pronounced than in the Marcus theory. The quantum mechanical theory also predicts modifications of the effects of temperature and polarity. Some principal features of these predictions have been verified by experiments using both pulse radiolysis and laser photoexcitation. 

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