Electron transfer by tunnelling

All through this chapter, we have avoided the terminology electron transfer by tunneling which is rather confusing though it often appears in the literature [14]. The nature of the different tunneling effects involved in electron transfer processes is discussed in the previously cited reviews [4, 22, 23]. 

The rate of cathodic electron transfer by tunneling, at an electron level c, from the electrode to the oxidant particles is proportional to the product of the state density occupied by electrons Ai(.)(c) in the electrode and the state density... 

Equation 8-19 indicates that the rate constant of electron transfer by tunneling is the same in the forward and backward directions at constant electron energy. The same tunneling rate constant of electrons in the forward and backward directions is valid not only in the equilibrium state but also in the nonequlibrium state of electron transfer [Gerischer, I960]. 

This is called a chemical, radical or stepwise mechanism. Or was it (ii) by the action of the bridging group to increase the probability of electron transfer by tunneling, termed resonance transfer 56,9i... 

Because the dependence on the distance of separation of donor and acceptor is similar for both energy transfer by the exchange effect and for electron transfer by tunnelling, this section can be abbreviated, the more so since the subject has been reviewed recently by Rice and Pilling [39]). 

Electron Transfer by Tunneling Through Blocking Films... 

A large Franck-Condon factor means that by exciting the reactants to the vibrational state uj there is a particularly high probability for the electron transfer (by tunneling) with the products in vibrational state V2-... 

In Chapter 7 general kinetics of electrode reactions is presented with kinetic parameters such as stoichiometric number, reaction order, and activation energy. In most cases the affinity of reactions is distributed in multiple steps rather than in a single particular rate step. Chapter 8 discusses the kinetics of electron transfer reactions across the electrode interfaces. Electron transfer proceeds through a quantum mechanical tunneling from an occupied electron level to a vacant electron level. Complexation and adsorption of redox particles influence the rate of electron transfer by shifting the electron level of redox particles. Chapter 9 discusses the kinetics of ion transfer reactions which are based upon activation processes of Boltzmann particles. 

In electrode kinetics, interface reactions have been extensively modeled by electrochemists [K.J. Vetter (1967)]. Adsorption, chemisorption, dissociation, electron transfer, and tunneling may all be rate determining steps. At crystal/crystal interfaces, one expects the kinetic parameters of these steps to depend on the energy levels of the electrons (Fig. 7-4) and the particular conformation of the interface, and thus on the crystal s relative orientation. It follows then that a polycrystalline, that is, a (structurally) inhomogeneous thin film, cannot be characterized by a single rate law. 

The rate of weakly coupled electron transfer by a tunneling mechanism is expected to decrease exponentially with increasing thickness of the spacer between donor and acceptor (due to the exponential decrease in electronic coupling with increasing separation [7]). Ideally, the thickness of the spacer in an organized assembly is the fully extended length of the spacer molecule. Any imperfections in a film will affect the spacer thickness and, thus, the electron-transfer rate. In properly designed and... 

Amide bonds connecting electron donor-acceptor moieties also support electron transfer by a-tunneling superexchange (Tsai, 1998). Both forward and back-transfer of electrons proceed through bonds of oligoamide chains rather than through space (Slate, 1998). 

The theme here is electron transfer, in inner- and outer-sphere reactions and, to a lesser degree, in related processes like optically induced charge transfer or excited state decay. Three books have been written on electron transfer, by Reynolds and Luniry, Cannon and Ulstrup, the last of which emphasizes theoretical aspects. In addition, a series of theoretical and experimental articles appear in the book Tunneling in Biological Systems , edited by Chance et and in volume 74 (1982) of the Faraday Discussions of the Chemical Society. A number of reviews have appeared, dealing both with general aspects - and more specialized themes, and many will be referred to in the sections that follow. 

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