Redox reactions electron transfer process

Of special significance are the catalytic properties of small metal clusters. At their surface such clusters have a large number of atoms with a low coordination number to which substrates bind. Catalytic reactions are being studied in hydrogenation, hydrosilylation, hydration, and the Heck reaction. Metal clusters are also of importance with regard to redox and electron transfer processes such as the photochemical decomposition of water (fuel cells) and photocatalytic hydrogenation. 

In redox initiation, the free radicals that initiate the polymerization are generated as transient intermediates in the course of redox reaction. Essentially this involves an electron transfer process followed by scission to give free radicals. A wide variety of redox reactions, involving both organic and inorganic components either wholly... 

The latter reaction must involve a large number of molecular steps and may be a much slower process. The mechanisms of a few inorganic electron transfer processes have been summarized by Taube (1968). The presence of very slow reactions when several redox couples are possible means that the Eh value measured with an instrument may not be related in a simple way to the concentrations of species present, and different redox couples may not be in equilibrium with one another. Lindberg and Runnells (1984) have presented data on the extent of disequilibrium... 

The description of redox reactions may afresh be carried out by introducing oxidation and reduction as an electron-transfer process. For this purpose, a process involving burning of magnesium in oxygen is considered, the reaction being written chemically as ... 

The aim in solution studies on metalloprotein is to be able to say more about intermolecular electron transfer processes, first of all by studying outer-sphere reactions with simple inorganic complexes as redox partners. With the information (and experience) gained it is then possible to turn to protein-protein reactions, where each reactant has its own complexities... 

The reduction of dioxygen to its fully reduced form, H20, requires the transfer of 4 electrons, and the transfer may proceed via a series of intermediate oxidation states, such as 02 /H00, HOO /HOOH, 0 /OH. These reduced forms of oxygen exhibit different redox properties and in the presence of substrate(s) and/or catalyst(s) may open different reaction paths for the electron transfer process. Fast proton transfer reactions between the corresponding acid-base pairs can introduce composite pH dependencies into the kinetic and stoichiometric characteristics of these systems. 

In order to probe these effects, a number of studies on the kinetics of electron transfer between small molecule redox reagents and proteins, as well as protein-protein electron transfer reactions, have been carried out (38-41). The studies on reactions of small molecules with electron transfer proteins have pointed to some specificity in the electron transfer process as a function of the nature of the ligands around the small molecule redox reagents, especially the hydrophobicity of these... 

In this picture, the electron transfer processes mediated by metallic electrodes (redox reactions in a heterogeneous phase) can also be classified to proceed according to outer-sphere or inner-sphere mechanisms (obviously, considering the electrode surface as a reagent). 

A preliminary electrochemical overview of the redox aptitude of a species can easily be obtained by varying with time the potential applied to an electrode immersed in a solution of the species under study and recording the relevant current-potential curves. These curves first reveal the potential at which redox processes occur. In addition, the size of the currents generated by the relative faradaic processes is normally proportional to the concentration of the active species. Finally, the shape of the response as a function of the potential scan rate allows one to determine whether there are chemical complications (adsorption or homogeneous reactions) which accompany the electron transfer processes. 

In general, the study of the variation of the formal electrode potential of a redox process with temperature has thermodynamic implications. Hence, one is interested in the measurement of AG°, AS° and AH° for the electron transfer process. It is recalled from thermodynamics that, under standard conditions, AE° is directly proportional to the free energy of the redox reaction according to the equation ... 

As pointed out in Ref. [4], no entropy variation appears in the description given by the harmonic model, apart from the weak contribution arising from the frequency shifts of the oscillators. The applications of this model are then a priori restricted to redox reactions in which entropic contributions can be neglected. We shall see in Sect. 3 that the current interpretations of most electron transfer processes which take place in bacterial reaction centers are based on this assumption. 

In most of the discussions so far, we have been concerned with reactants undergoing one-electron transfer processes. When one or both of the participants of a redox reaction has to undergo a change of two in the oxidation state, the point arises as to whether the two-electron transfer is simultaneous or nearly simultaneous, a question that has been much discussed. The T1(I)-T1(III) second-order exchange ( exch) proceeds by a two-electron transfer. One would need to postulate the equilibrium (5.61) if Tl(II) was involved in the... 

Most compounds oxidized by the electron transport chain donate hydrogen to NAD+, and then NADH is reoxidized in a reaction coupled to reduction of a flavoprotein. During this transformation, sufficient energy is released to enable synthesis of ATP from ADP. The reduced flavoprotein is reoxidized via reduction of coenzyme Q subsequent redox reactions then involve cytochromes and electron transfer processes rather than hydrogen transfer. In two of these cytochrome redox reactions, there is sufficient energy release to allow ATP synthesis. In... 

The net result of a photochemical redox reaction often gives very little information on the quantum yield of the primary electron transfer reaction since this is in many cases compensated by reverse electron transfer between the primary reaction products. This is equally so in homogeneous as well as in heterogeneous reactions. While the reverse process in homogeneous reactions can only by suppressed by consecutive irreversible chemical steps, one has a chance of preventing the reverse reaction in heterogeneous electron transfer processes by applying suitable electric fields. We shall see that this can best be done with semiconductor or insulator electrodes and that there it is possible to study photochemical primary processes with the help of such electrochemical techniques 5-G>7>. 

When the above factors are put under control, the possibility of changing the ligand L in the pentacyano(L)ferrate complexes adds a further dimension for studying systematic reactivity changes, brought out by the controlled modification of the redox potentials of the Fe(II)-Fe(III) redox couples. In this way, the rates of electron transfer reactions between a series of [Fen(CN)5L]re complexes toward a common oxidant like [Coin(NH3)5(dmso)]3+ showed a variation in agreement with Marcus predictions for outer-sphere electron transfer processes, as demonstrated by linear plots of the rate constants versus the redox potentials (123). 

Photoinduced electron transfer processes involving electron donor (D) and acceptor (A) components can be tuned via redox reactions. Namely, the excited-state properties of fluorophores can be manipulated by either oxidation of electron donors or reduction of electron acceptors. Also, the oxidized and the reduced species show different properties compared to the respective electron donors and acceptors. By making use of these properties of electron donors and acceptors, a number of molecular switches and logic gates have been described in recent years. In the following, we will introduce these redox-controlled molecular switches according to the redox centers. 

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