Electron transfer competition between

The study of compounds such as 1 has provided much information about the design of photochemical electron transfer systems based on biological molecules. Specifically, the roles of the linkage and the thermodynamics in making electron transfer competitive with fluorescence, intersystem crossing, and internal conversion are now qualitatively understood. However, a dyad system cannot have a branch point at an intermediate state in the electron transfer path and therefore cannot be used to study the competition between forward and recombination electron transfer. Also, the dyad system fails to mimic the long lifetimes of hundreds of milliseconds characteristic of... 

Chemical reactions can be studied at the single-molecule level by measuring the fluorescence lifetime of an excited state that can undergo reaction in competition with fluorescence. Reactions involving electron transfer (section C3.2) are among the most accessible via such teclmiques, and are particularly attractive candidates for study as a means of testing relationships between charge-transfer optical spectra and electron-transfer rates. If the physical parameters that detennine the reaction probability, such as overlap between the donor and acceptor orbitals. 

Knowledge of photoiaduced electroa-transfer dyaamics is important to technological appUcations. The quantum efficiency, ( ), ie, the number of chemical events per number of photons absorbed of the desired electron-transfer photoreaction, reflects the competition between rate of the electron-transfer process, eg, from Z7, and the radiative and radiationless decay of the excited state, reflected ia the lifetime, T, of ZA ia abseace ofM. Thus,... 

These reactions can be viewed as a competition between two kinds of atoms (or molecules) for electrons. Equilibrium is attained when this competition reaches a balance between opposing reactions. In the case of reaction (3), copper metal reacting with silver nitrate solution, the Cu(s) releases electrons and Ag+ accepts them so readily that equilibrium greatly favors the products, Cu+2 and Ag(s). Since randomness tends to favor neither reactants nor products, the equilibrium must favor products because the energy is lowered as the electrons are transferred. If we regard reaction (5) as a competition between silver and copper for electrons, stability favors silver over copper. 

Reactions of D with D20 and of 0 with 02, N20, and N02 have been studied with a magnetic sector mass spectrometer. Competition between electron transfer and ion-atom interchange has been observed in the production of 02 by reaction of 0 with 02, an endothermic reaction. The negative ion of the reacting neutral molecule is formed in 02, N2Of and N02 but not in D20. Rate constants have been estimated as a function of repeller potential. 

A complex representation of IMPS data obtained for the heterogeneous quenching of ZnTPPC -diferrocenylethane is displayed in Fig. 21(a). The semicircular response in the first quadrant corresponds to the competition between product separation and back electron transfer, while the lower quadrant is determined by the i uQi constant. The i uQi attenuation limited the frequency range to less than 1 kHz. Equation (45) describes the experimental spectra at various Galvani potential differences [solid lines in Fig. 21(a)],... 

Scheme 2 Competition between electron transfer (7 8) and water trapping (7 9+10) of sugar radical cation 7...

Both thermodynamic and kinetic factors are involved in the competition between concerted and stepwise mechanisms. The passage from the stepwise to the concerted situation is expected to arise when the ion radical cleavage becomes faster and faster. Under these conditions, the rate-determining step of the stepwise process tends to become the initial electron transfer. Then thermodynamics will favor one or the other mechanism according to equation (18). AG )eav is also the standard free energy of cleavage of the ion radical. 

It is noteworthy that both thermal and photoinduced electron-transfer activation of the [ArH, IC1] complex leads to the ion-radical triad. Consequently, iodination versus chlorination represents the competition between ion-pair and radical-pair collapse. This is confirmed by reactivity studies of dimethoxybenzene cation radical with chloride and iodine (atom), respectively,225 i.e.,... 

The electron-transfer formulation in Scheme 23 suggests that the efficiency of nitrosation is the direct result of the competition between deprotonation of the Wheland intermediate versus its breakdown to the original EDA complex via the ion-radical pair, i.e.,... 

The competition between antioxidant and prooxidant activity of flavonoids depends firstly on their chemical structure. If we suppose that the oxidation of flavonoids (Reaction (17)) takes place by one-electron transfer mechanism, then it must depend on the capacity of flavonoids to donate an electron, i.e., on their one-electron oxidation potentials. 

These expressions are designed for cyclic voltammetry. The expressions appropriate for potential step chronoamperometry or impedance measurements, for example, are obtained by replacing IZT/Fv by the measurement time, tm, and the inverse of the pulsation, 1/co, respectively. Thus, fast and slow become Af and Ah I and -C 1, respectively. The outcome of the kinetic competition between electron transfer and diffusion is treated in detail in Section 1.4.3 for the case of cyclic voltammetry, including its convolutive version and a brief comparison with other electrochemical techniques. 

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