Electron transfer-chemical process

A diagnostic approach for the analysis of various pre-electron-transfer chemical reactions32 is summarized in Table 4.3. Thus, the variation of the first and second derivatives of the quantity ir1/2, as a function of current, indicates the type of pre-electron-transfer chemical process. Two experimental plots will provide sufficient data to use the criteria of this table ir1 2 versus i at constant C and 71/2 versus C at constant i. 

A systematic description of all possible combinations of homogeneous chemical processes coupled to electron transfer at an electrode surface is impossible because an infinite range of theoretically possible reaction schemes can be constructed. Unfortunately, a consistent form of nomenclature for defining the possible web of reaction pathways has not yet been invented. However, the lUPAC nomenclature [89] is of assistance with respect to simple reaction schemes. In this article, the commonly employed descriptors for electron transfer (E) and chemical (C) sequences of reaction steps, e.g. ECEC, will be used for a sequence of reactions involving electron transfer-chemical process-electron transfer-chemical process. Reaction schemes involving branching of a reaction pathway will be considered later. 

First-order chemical reaction preceding a reversible electron transfer. The process in which a homogeneous chemical reaction precedes a reversible electron transfer is schematized as follows ... 

In the last two decades, studies on the kinetics of electron transfer (ET) processes have made considerable progress in many chemical and biological fields. Of special interest to us is that the dynamical properties of solvents have remarkable influences on the ET processes that occur either heterogeneously at the electrode or homogeneously in the solution. The theoretical and experimental details of the dynamical solvent effects on ET processes have been reviewed in the literature [6], The following is an outline of the important role of dynamical solvent properties in ET processes. 

The photosynthetic process involves photochemical reactions followed by sequential dark chemical transformations (Fig. 3). The photochemical processes occur in two photoactive sites, photosystem I and photosystem II (PS-I and PS-II, respectively), where chlorophyll a and chlorophyll b act as light-active compounds [6, 8]. Photoinduced excitation of photosystem I results in an electron transfer (ET) process to ferredoxin, acting as primary electron acceptor. This ET process converts light energy to chemical potential stored in the reduced ferredoxin and oxidized chlorophyll. Photoexcitation of PS-II results in a similar ET process where plastoquinone acts as electron acceptor. The reduced photoproduct generated in PS-II transfers the electron across a chain of acceptors to the oxidized chlorophyll of PS-I and, consequently, the light harnessing component of PS-I is recycled. Reduced ferredoxin formed in PS-I induces a series of ET processes,... 

Radical ion pairs play an important role in photoinduced electron transfer (PET) processes. Several types of them may exist and it will be shown in tins article that contact ion pairs (CIPs) and solvent-separated ion pairs (SSIPs) can be differentiated by various experimental methods. Moreover, their controlled formation facilitates the control of chemical reactions. Although this review is by no means exhaustive, it is hoped that interest in the fundamental mechanistic aspects of PET processes in homogeneous media as well as in their synthetic applications will arise. 

An electrochemical process that is followed by a chemical reaction represents an EC (electron-transfer-chemical) mechanism... 

The term ion pump, synonymous with active ion-transport system, is used to refer to a protein that translocates ions across a membrane, uphill against an electrochemical potential gradient. The primary pumps do so by utilization of energy derived from various types of chemical reactions such as ATP hydrolysis, electron transfers (redox processes), and decarboxylations, or from the absorption of light (Table 1). Secondary pumps are symport and antiport systems that derive the energy for uphill movement of one species from a coupled downhill movement of another species. The electrochemical gradient driving the latter movement is often created by a primary pump. 

ECE Electron-transfer-chemical reaction-electron-transfer process... 

Beyond a general recognition of their importance there is no consistent theory to model the participation of electrons in chemical processes. The well-known empirical procedures to describe charge transfer within and between molecules are commonly introduced with the disclaimer that a full quantum-mechanical treatment would be required to describe the mechanisms more rigorously. However, attempts to formulate such a treatment are rare. 

No example has so far been reported of a shuttling process controlled by electron transfer chemical reactions. There are, however, very interesting examples of shuttling processes controlled by acid/base reactions. One case is that of the previously discussed compound 13 " (see Figure 14), in which the shuttling of the macrocycle component can be controlled not only electrochemically, but also by protonation/ deprotonation of the benzidine unit [43]. 

Note that this definition does not imply that the products obtained upon electron transfer must be stable, but only that the electron transfer activation process does not imply any important molecular rearrangement. Indeed, one or both of the products may chemically evolve through fast follow-up reactions as in the reaction sequences (1) or (9) and (10). 

Compared with alcohols, which possess a high standard potential of oxidation, moderate nucleophilicity and weak basicity, amines very often serve as good electron donors and relatively strong bases and nucleophiles in chemical reactions. Photoinduced electron transfer (PET) processes, in which an amine donates an electron to the reaction partner in either its ground or excited electronic state, result in the formation of an amine substrate exciplex (Scheme 6.196).670 1224 The driving force for electron transfer is related to the standard potential of oxidation of the donor, the standard potential of reduction of the acceptor and the excited state energy of the absorbing partner (see Chapter 4). 

In the course of its conversion to chemical energy, light reduces the plant acceptor by electron transfer. This process results at the same time in an oxidised chlorophyll molecule, which must be reduced before it can function again (Fig. 6.1) (Corbett, 1974). 

Electrochemistry on Thiol-based SAMs Modified at Electrodes Electrode reactions consist of the combination of elementary processes, that is, electron-transfer reaction processes between redox-active species and electrodes coupled to the preceding and/or succeeding chemical processes [479]. If these chemical processes are fast relative to the electron-transfer reactions, the chemical processes are thermodynamically reflected in the overall electrode reaction. If these chemical processes are slow, the overall electrode reactions are kinetically characterized. Therefore, the electrode reaction can express the specificity depending on the characteristics of the chemical processes spatially and temporally coupled to the electron-transfer reaction. We can take advantage of this fundamental principle of electrochemical reactions to provide a wide variety of specificity into the nanoscale electrochemistry on the exposed... 

In the absence of chemical complications (e.g., fast decomposition of the oxidized and/or reduced species), photoin-duced electron transfer (PET) processes, that is, (8) and (9), are followed by spontaneous back-electron-transfer reactions that regenerate the starting ground state system (8a) and (9a), and photoinduced energy transfer (10) is followed by radiative and/or nonradiative deactivation of the excited acceptor (10a) ... 

This chapter is concerned with measurements of kinetic parameters of heterogeneous electron transfer (ET) processes (i.e., standard heterogeneous rate constant k° and transfer coefficient a) and homogeneous rate constants of coupled chemical reactions. A typical electrochemical process comprises at least three consecutive steps diffusion of the reactant to the electrode surface, heterogeneous ET, and diffusion of the product into the bulk solution. The overall kinetics of such a multi-step process is determined by its slow step whose rate can be measured experimentally. The principles of such measurements can be seen from the simplified equivalence circuit of an electrochemical cell (Figure 15.1). 

The electrochemical reduction proceeds at the carbene ligand as a two-electron irreversible process. Since a single reduction wave is observed, the first reduction step must be followed by a very fast chemical reaction yielding the intermediate (probably of radicalic nature), which is reduced even more easily than the starting compound to a final product (the so-called electron transfer, chemical reaction, electron transfer (ECE) mechanism). To understand the mechanism more deeply, the products of electrochemical reduction were separated during the preparative electrolysis from the solution using a continuous extraction to hexane, isolated, and analyzed. 

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