Reaction mechanisms nonadiabatic processes

An important factor is the electron coupling between the electrode metal and the redox species or between the two members of the redox couple. If this coupling is strong the reaction is called adiabatic, i.e., no thermal activation is involved. For instance, electrons are already delocalized between the metal and the redox molecule before the electron transfer therefore, in this case no discrete electron transfer occurs [see also -> adiabatic process (quantum mechanics), - nonadiabatic (diabatic) process]. 

While studies of specific acid catalysis of redox cofactors shed light on the intricacies of the electron transfer process [54], the conditions required for preprotonation of the cofactor are highly acidic (pH < 0), and would not generally be found in biological systems. There are, however, systems such as Qb reduction in the Rhodobacter sphaeroides reaction center [55], where kinetic data indicate proton transfer prior to or simultaneous with electron transfer. This would seem to indicate that a general acid process is operative. At first glance, this sort of mechanism would seem to be contrary to the Born-Oppenheimer approximation. This apparent paradox can be avoided, however, if quantum chemical (nonadiabatic) processes are considered. 

If the interaction between the reactants leading to the reaction is weak enough (nonadiabatic processes), the transition probability per unit time may be calculated using the formula of the first order in quantum mechanical... 

The study of molecular systems using quantum mechanics is based on the Born-Oppenheimer approximation. This approximation relies on the fact that the electrons, because of their smaller mass, move much faster than the heavier nuclei, so they follow the motion of the nuclei adiabatically, whereas the latter move on the average potential of the former. The Born-Oppenheimer approximation is sufficient to describe most chemical processes. In fact, our notion of molecular structure is based on the Born-Oppenheimer approximation, because the molecular structure is formed by nuclei being placed in fixed positions. There are, however, essential nonadiabatic processes in nature that cannot be described within this approximation. Nonadiabatic processes are ubiquitous in photophysics and photochemistry, and they govern such important phenomena as photosynthesis, vision, and charge-transfer reactions. 

Understanding the mechanism of this nonadiabatic radiationless decay is central to explaining excited state processes. There are two possible mechanisms (see nonadiabatic reactions in Figure 1). When real surface crossings exist (conical intersection, see left side of Figure 1) and are accessible, the Landau-... 

Most of the discussion in this chapter is based on a classical mechanics description of chemical reactions. Such classical pictures are relevant to many condensed phase reactions at and above room temperature and, as we shall see, can be generalized when needed to take into account the discrete nature of molecular states. In some situations quantum effects dominate and need to be treated explicitly. This is the case, for example, when tunneling is a rate determining process. Another important class is nonadiabatic reactions, where the rate detennining process is hopping (curve crossing) between two electronic states. Such reactions are discussed in Chapter 16 (see also Section 14.3.5). 

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