Activation electron tunneling through

Deposition of the active metal may occur by electron tunneling through partially insulating or semiconducting species. Thus, metal deposits which are not directly connected electrically with the bulk are formed. Such a type of deposition forms an isolated island of deposited metal embedded in the surface films [27]. This possibility is also illustrated in Figure 7. 

A similar situation may be obtained when alkali metals are immersed in ultrapure ethers containing benzophenone [53], The metal thus dissolves via formation of stable ketal radical anions in solution (and metal ions as well). It should be emphasized that the above processes occur even when the active metal is initially introduced into the solution covered by surface films (due to reactions with atmospheric contaminants). We assume that electron tunneling through the films enables the initiation of the dissolution process. This process breaks the film on the metal (as metal is depleted beneath the rigid surface film), thus enabling solvent molecules to reach the active surface and solvate more electrons. This increases the metal solubilization and the further breakdown of the initial surface films. Hence, an equilibrium between a bare metal and the blue solution can finally be reached, as explained above (Eq. 13). 

The IETS spectrum is caused by electrons tunneling through (or very close to) molecules that activate intramolecular vibrations, without the electric dipole selection rules that dominate the absorption of photons (thus IR and Raman lines occur together in the IETS spectrum). Specialized equipment was traditionally built to measure IETS (Fig. 11.23), but IETS software has been added to recent commercial magnetometers. 

Electron tunnelling through azurin is mediated by the active site Cu ion. Chemical Physics Letters, 376, 625-630. 

The model of Cabrera and Mott is discussed in die book on low-temperature oxidation by Fehiner [5b]. The basis of the model is die quantum mechanical concept of electron tunneling. An electron can penetrate an energy barrier without die requirement for thermal activation. As soon as a three-dimensional oxide forms on a metal, electrons tunneling through the oxide are captured by adsorbed oxygen on die oxide surface. The charge separation thus established between die oxide surfece and the metal sets up an electric field across the oxide. The proposed mechanism is illustrated in Figure 1. 

A key point that must be made is diat quantum mechanical tunneling through the Marcus-theory barrier when it is non-zero can increase the rate for electron transfer just as is true for any other activated process. Because the electron is so light a particle, tunneling can be a major contributor to die overall rate. Models for electron tunneling will not, however, be presented here. 

In the case of redox electrodes, the ease with which electrons can tunnel through a potential barrier of the type present at an electrode interface makes the use of classical activated complex theory (with the electrons as one reactant) inappropriate. In Fig. 2.11(a) an electron energy diagram of a redox electrode at equilibrium is shown. For an electron transfer between the phases to be successful, it is necessary for the acceptor or donor in solution to have an energy level exactly equal to a complementary level in the metal. In the equilibrium situation it is seen that there is an equal chance of transfer of an electron from a filled metal level to an unoccupied... 

The kinetics of electron transfer reactions at electrodes can be explained either by surmounting an activation barrier due to the chemical reorganization of the reactants or by tunnelling through the potential barrier across the electrode—solution interface. 

Although ferryl intermediates of horseradish peroxidase and microperoxidase-8 have been produced in reactions with photogenerated [Ru(bpy)3]3+ [5], analogous experiments with P450s were unsuccessful, presumably due to the inefficiency of electron transfer from the buried heme active site through the protein backbone [6]. Photoactive molecular wires (sometimes referred to as metal-diimine wires, sensitizer-tethered substrates, or electron tunneling wires) were developed to circumvent this problem by providing a direct ET pathway between [Ru(bpy)3]3+ and the heme. These molecular wires, which combine the excellent photophysical properties of metal-diimine complexes... 

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