Oxidation potentials electron donors

O2 ( h202.2) to the positive side and of the first one-equivalent oxidation of electron donors to the negative side, as shown in Figure 1. The Em of these molecules in the bound state on the oxidase will be the same as in the free state provided the aflSnities of the oxidase for the reduced and oxidized forms are equal. The one-order-of-magnitude difference between these affinities will make 0.03 volt difference in the normal potentials (Em) between the free and bound state. It may be said that hydrogen donors and O2 are activated by the oxidase when it stabilizes the intermediates and increases the semiquinone formation constant, K. An increase in Ks results in a rise of Eo and a drop of Ei. 

Early investigatioiis of the photochemistry of a-heteroatom substituted carboxylates 40 (Scheme 19) concentrated on the operation of SET processes. These substances possess two potential electron donor sites. SET from the carboxylate moiety leads to formation of an acyloxy radical 41 while SET from the heteroatom center gives rise to a zwitterionic radical 42. Subsequent loss of carbon dioxide from either of these intermediates yields the same carbon-centered, heteroatom-stabilized radical 43. The critical factors determining which of these pathways is responsible for the SET-promoted decarboxylation reactions of substrates in this family is (1) the relative oxidation potentials of the two potential donor moieties and (2) the relative rates of decarboxylation of intermediates 41 and 42. Redox potential data indicate that electron transfer from sulfur (E(-t) = ca. 0.4 V) and nitrogen (E(-l-) = ca. 0.7 V) centers to acceptors should be thermodynamically more favorable than from carboxylate groups (E(-t) = ca. 1.4 V). 

At the point when adsorption ceases, the difference between the bulk chemical potential of the solid and the surface is balanced by a potential difference between the bulk and the surface. The surface is effectively at the chemical potential of the adsorbate. The amount of adsorption depends Intimately on the electronic properties of the solid. For example the term "depletive" chemisorption is used to describe the adsorption oxygen (an electron acceptor) on "n" type zinc oxide (an electron donor). Equilibrium is reached when no further electrons are available at the surface and the electrical conductivity has dropped. The similarity to contact charging is obvious. 

Electron donor molecules are oxidized in solution easily. Eor example, for TTE is 0.33V vs SCE in acetonitrile. Similarly, electron acceptors such as TCNQ are reduced easily. TCNQ exhibits a reduction wave at — 0.06V vs SCE in acetonitrile. The redox potentials can be adjusted by derivatizing the donor and acceptor molecules, and this tuning of HOMO and LUMO levels can be used to tailor charge-transfer and conductivity properties of the material. Knowledge of HOMO and LUMO levels can also be used to choose materials for efficient charge injection from metallic electrodes. 

Scheme 1 Structures of the Sa and Sd hairpin linkers, electron donor nucleobases and their oxidation potentials, and the hairpins 3G and 3GAGG shown in Fig. 1...

---