Directed electron transfer definition

The model shown in Scheme 2 indicates that a change in the formal oxidation state of the metal is not necessarily required during the catalytic reaction. This raises a fundamental question. Does the metal ion have to possess specific redox properties in order to be an efficient catalyst A definite answer to this question cannot be given. Nevertheless, catalytic autoxidation reactions have been reported almost exclusively with metal ions which are susceptible to redox reactions under ambient conditions. This is a strong indication that intramolecular electron transfer occurs within the MS"+ and/or MS-O2 precursor complexes. Partial oxidation or reduction of the metal center obviously alters the electronic structure of the substrate and/or dioxygen. In a few cases, direct spectroscopic or other evidence was reported to prove such an internal charge transfer process. This electronic distortion is most likely necessary to activate the substrate and/or dioxygen before the actual electron transfer takes place. For a few systems where deviations from this pattern were found, the presence of trace amounts of catalytically active impurities are suspected to be the cause. In other words, the catalytic effect is due to the impurity and not to the bulk metal ion in these cases. 

Anomalous response to the NADH oxidase in animal cells, such as amiloride-insensitive proton transport, may be based on activation of the H+-ATPase or direct electron transport-linked proton transfer. Further definition of the components of the NADH oxidase and the characteristics of electron transport are needed. In addition, the presence of a poorly characterized glutathione oxidase in the plasma membrane opens an alternative for oxidation-reduction control of proton transport. At this stage no evidence has been found for control of HCOj/Cl" exchange or organic acid transport by the plasma membrane oxidase. 

A significant problem in studies on photocatalysis is the definition of positive hole. Positive hole is defined as a defect of an electron (i.e., a positive hole must be included in a substance, while an electron is a real substance). Therefore, not only h produced by photoinduced band-to-band transition in solid materials but also a hydroxyl radical, which is a one-electron deficient hydroxyl anion, can be a positive hole. If this definition is accepted, there should be no difference in the photocatalsrtic oxidation mechanisms between direct hole transfer and surface-adsorbed hydroxyl radical reaction, since it is well known that the surface of a metal oxide is covered with chemically or physically adsorbed water and a positive hole passing through this water layer into a solution may be a hydroxyl radical or its protonated or deprotonated species (Fig. 4). Actually, hydroxyl... 

The silver ion, then, does not exhibit the same degree of back-bonding that the more familiar transition elements do. Since back-bonding is an essential factor in the forbidden-to-aUowed process and, in particidar, in direct oxidative addition, silver s function in this chemistry could differ. It may be that the silver ion (and other similar metallic species) stands apart from the other transition elements (W, Mo, Cr, Fe, Co, Ni, Rh, etc.) in its mode of catalysis. In the valence isomerization of quadricyclene, some oxidation occurs as evidenced by the deposition of metallic silver 45). Certainly, irreversible redox cannot be a feature of the actual catalytic path, since silver s role is definitely catalytic and the isomerization itself precludes it i.e., the oxidation state of the system remains fixed). Some electron transfer, however, clearly proceeds and may be a critical feature of the catalysis. One could speculate on the possibility of intermediate ion radicals generated through electron transfer from a reactant to Ag(I) followed by electron recapture by the rearranged species in the catal5dic system. 

To clarify the mechanism of reaction of P-450, it is crucial to characterize the reactive intermediates in the rate-determining step. Definitive evidence for an electron-transfer mechanism (C in Scheme 2) for the 7V-demethylation of N,N-dimethylanilines has been obtained by direct observation of the reduction of the high-valent species responsible for P-450 catalysis [96]. For peroxidase, an oxoferryl porphyrin 7r-radical cation, compound I ([(P)Fe =0] "), has been well characterized as the species equivalent to the proposed active intermediate of P-450 [97-103]. Compound I of horseradish peroxidase (HRP) can be readily generated by chemical oxidation of HRP [100-103]. The involvement of the electron-transfer process of compound I in the oxidation of several amines catalyzed by HRP was... 

In principle, if the X values for a set of like reactions are similar to one another, and W r) is small or constant, a plot of AG versus AG° will be linear and have a slope of 0.5. As noted above, it is rarely possible to measure AG values directly. An alternative option for plotting data using a linear relationships is to use Equation 1.15 and the definition of the equilibrium constant, K = A exp( AG° // 7), in which A is a constant. If the equilibrium constants for a series of reactions can be measured or calculated, one can plot In A versus In AT (Equation 1.15). A linear result with a slope of 0.5 is indicative of a common outer-sphere electron transfer mechanism. 

An electron-transfer in the first step would give cationic intermediate 168. Formation of R3 SiX may then arise via a direct nucleophilic displacement of the silyl ligand, or through the neutral intermediate 169 by a reductive-elimination process. Cleavage reaction may also occur by competitive pathways. In the present state of knowledge, since few experiments have been done, it is difficult to pinpoint definitive mechanisms. 

While DFT does not require orbitals, it is not inconsistent to consider molecular orbitals in the form of frontier orbital theory. This means the electrons must come from definite occupied orbitals in D, and go into definite empty orbitals in C. Usually there is some electron transfer in both directions, as in a+7r-bonding. The nature of these interacting orbitals is of great importance in determining the interaction between C and D. 

We can now state a broader definition of oxidation and reduction. Oxidation is an increase in oxidation number reduction is a decrease in oxidation number. These definitions are more useful in identifying the elements oxidized and reduced when the electron transfer is not apparent. All you must do is find the elements that change their oxidation numbers and determine die direction of each change. One element must increase, and the other must decrease. (This corresponds with one species losing electrons while another gains.)... 

This introductory section includes basic definitions related to chemical and electrochemical reactions in the forward (f) and reverse (r) directions. The word Corrosion stands for material or metal deterioration or surface damage in an aggressive environment. Corrosion is a chemical or electrochemical oxidation process, in which the metal transfers electrons to the environment and undergoes a valence change from zero to a positive value z. The environment may be a liquid, gas or hybrid soil-liquid. These environments are called electrolytes since they have their own conductivity for electron transfer. 

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