Tunneling redox process

Another type of tunneling redox process is related to the long-distance electron transfer from donor to acceptor in the solid y-irradiated polar media. This type of reaction, in which the rate constant low-temperature plateau was also observed, plays an important role in radiation chemistry. Research into... 

Thus, the discovery of the low-temperature limit of the electron transfer rate in the redox processes became the first convincing proof of the existence of electron-nuclear tunneling, and further research confirmed the universal character of this phenomenon. 

While Gurney referred in his treatment of electrochemical charge transfer to the Fermi distribution function for electronic states in the metal, he did not, however, pursue the consequences of using this function in preference to a Boltzmann distribution. In Gerischer s treatment of redox reactions, also referred to by Schultze and Vetter in their treatments of the role of electron tunneling in O2 evolution and other redox process at oxide-covered (Pt) electrodes, the Fermi distribution was explicitly used in the current-potential function which is written (cf. Gurney and Geris-cher ) as... 

In anaerobic environments, fhe biological redox-processing of carbon monoxide and carbon dioxide is accomplished by carbon monoxide dehydrogenases (CODH), which are often found in combination with a second enzyme, acetyl CoA synthase (ACS, see above) [124]. In the CODH/ACS bifunctional enzymes, the two substratebinding prosthetic groups, the A cluster (site of acetyl CoA synthesis and decarbonylation, see previous section) and the C cluster (site of CO/CO2 interconversion) are located in the a and P subunits, respectively. CODH from R. ruhrum is not associated with ACS. The CO produced at the C-cluster travels via a tunnel through the enzyme to the a subunit where it is processed by acetyl CoA synthase (the A-cluster) [125]. CODH catalyzes the interconversion of CO2 and CO (Eq. 12.11). 

Oxidation—Reduction. Redox or oxidation—reduction reactions are often governed by the hard—soft base rule. For example, a metal in a low oxidation state (relatively soft) can be oxidized more easily if surrounded by hard ligands or a hard solvent. Metals tend toward hard-acid behavior on oxidation. Redox rates are often limited by substitution rates of the reactant so that direct electron transfer can occur (16). If substitution is very slow, an outer sphere or tunneling reaction may occur. One-electron transfers are normally favored over multielectron processes, especially when three or more species must aggregate prior to reaction. However, oxidative addition... 

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. 

The key requirement for a SET step in the photocatalytic process seems to be the surface complexation of the substrate, according to an exponential dependence of the probability of electronic tunneling from the distance between the two redox centers [66]. However, as was pointed out in the preceding section on the key role of back reactions, the presence of a SET mechanism could be a disadvantage from an applicative point of view. If the formed SET intermediate (e.g., a radical cation) strongly adsorbs and/or does not transform irreversibly [e.g., by loss of CO from a carboxylic acid or fast reaction with other species (e.g., superoxide or oxygen)], it can act as a recombination center, lowering the overall photon efficiency of the photocatalytic process. 

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