Consecutive Electron Transfer Processes

In an aqueous medium the reduction of inorganic ions (for example, Cu2+, Zn2+, Cd2+) to their respective metallic states takes place by a single two-electron process. In effect, however, the process only apparently involves a two-electron step, in that it is assumed that multi-electron processes proceed by a sequence of elementary one-electron steps. Every elementary step is characterized by its own rate constant and its own standard potential. 

The high current intensity associated with the return peak relative to the second reduction process is characteristic of the so-called anodic stripping, which originates from the fast reoxidation of the metallic copper deposited on the electrode surface during the Cu+/Cu° reduction. 

Therefore, given that a multi-electron process can be described as a series of one-electron transfers, more or less separated from each other, the shape of the cyclic voltammogram depends on the following factors 14 

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One of the favourite generic arrangement is the molecular triad, consisting of a photoactive centre (PC), an electron donor (D) and an electron acceptor (A). In systems such as D-PC-A, the charge separated state D+-PC-A is obtained in two consecutive-electron transfer processes after excitation of PC. Of course, several variants exist, depending on the electron transfer properties of PC and its excited state, PC, as well as on the precise arrangement of the various components (PC-Ai-A2 or D2-Di-PC, in particular, if PC is an electron donor or an electron acceptor, respectively). 

Two one-electron transfers with different extents of reversibility. In the case where not all the processes of a consecutive electron transfer sequence are reversible, the irreversibility of a particular step becomes evident by the absence of the reverse peak in its pertinent response. For all other aspects the preceding considerations remain valid. 

As mentioned, DPV is particularly useful to determine accurately the formal electrode potentials of partially overlapping consecutive electron transfers. For instance, Figure 40 compares the cyclic voltammogram of a species which undergoes two closely spaced one-electron oxidations with the relative differential-pulse voltammogram. As seen in DPV the two processes are well separated. 

Let us now pass to multiferrocene compounds. Many compounds containing two, three, four or more ferrocene groups have been prepared and characterized. For these derivatives one can confidently expect that the number of one-electron oxidations equal the number of ferrocene groups. However, one must still determine whether these processes occur at separate or at identical potential values (according to that discussed in Chapter 2, Section 1.5, for consecutive electron transfers). 

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>. 

It should be noticed that, unlike consecutive electron transfer reactions whose kinetics are determined by the slowest process, mixed potentials are determined by the fastest of several possible occurring electrode reactions. 

More complex electrode processes than those described above involve consecutive electron transfer or coupled homogeneous reactions. The theory of these reactions is also more complicated, but they correspond to a class of real, important reactions, particularly involving organic and biological compounds. 

There are a considerable number of reactions in which the products contain two electrons, more than the starting compounds, and the consecutive two-step one-electron electron transfer process proves to be energetically unfavorable. In such cases, it is presumed that the two-electron process occurs in one elementary two-electron step. An example of a two-electron process is the hydride transfer, when two electrons are transported together with a proton. BH4, hydroquinones and reduced nicotinamides are typical hydrid donors. A specific feature of quinones is the capacity to accept and then to reversibly release electrons one by one or two electrons as a hydride. Therefore, quinones can serve as a molecular device, which can switch consecutive one-electron process to single two-electron process. 

Solutions of indium (I) can be prepared by treatment of indium amalgam with silver triflate in dry acetonitrile in the absence of oxygen, and then diluted with water to give the low-concentration aqueous solution, which plays a sizable role in the study of the details of intermolecular electron transfer processes in solution. Aqueous In(I) solution has been used to examine the behavior of this hypovalent center in inorganic redox transformations. Reactions with complexes of the type [(NH3)5Co (Lig)] and [(NH3)5Ru (Lig)] (Tig = Cl, Br , I or HC2O4 ) show two consecutive one-electron reactions initiated by the formation of the metastable state In , which is then rapidly oxidized to In , and the first of which is predominating an inner-sphere mechanism. ... 

The direct characterization of an eT mechanism requires a much more complicated technique time-resolved spectroscopy. The solution containing the system under investigation is irradiated by a laser pulse, and the absorption spectra of the solution are consecutively recorded at chosen and very short time intervals (e.g. every 10 ns). If, in the envisaged two-component system F1 M, an M-to-Fl eT process takes place upon illumination, one should be able to measure the absorption spectra of Fl and M" ", as well as their decay, which allows the determination of the lifetime of the transient species F1 M. It goes without saying that very sophisticated and expensive instrumentation is required to carry out this type of experiment. Moreover, the smaller the fluorophore lifetime and the faster the back-electron transfer process, the more rapid and expensive the data acquisition equipment required. In particular, narrow laser pulses and especially fast data collections are needed for systems such as 1, where a short-living polyaromatic fluorophore (anthracene, r = 5 ns) is linked to the electron donor (or acceptor) group by a rather short carbon chain. 

When a second electron transfer process exists, the reaction scheme for the overall electrode process becomes considerably more complicated. An example of an ECE reaction scheme, the ErevCirrevErev reaction for two consecutive one-electron oxidation processes coupled via a chemical reaction step, is given in Eq. (II. 1.26), and voltammetric responses simulated for this type of process are shown in Fig. II. 1.23a. 

In aqueous solutions, the EC mechanism proposed by Ruiz [13] for the oxidation of AA at low pH is widely accepted. It involves two consecutive one-electron transfer processes to form dehydroascorbic acid immediately followed by irreversible hydration to give the final product 2-3 diketogluco-nic acid. Although the electrochemical reaction is reversible at Hg electrodes [13], the large overpotential needed at carbon electrodes renders the oxidation of AA to be irreversible and the anodic potential (--300 mV at pH 3.9) is considerably higher than its standard value [14, 27], Figure 1. 

Electron transfer reactions are central to many of the metabolic processes necessary for the survival of all living organisms. These reactions depend upon the approach of an electron donor and an electron acceptor. In biology, intermolecular electron transfer is common and occurs between sites on different proteins. This leads to electron-transfer chains that function as a series of consecutive electron-transfer reactions between metal sites within a protein or group of proteins (Chapter 9). Electron-transfer chains are important in photosynthesis and respiration (see Chapter 23 on the accompanying website). 

Around 1980, some branching mechanisms were proposed with the intention of describing the processes occurring in the active, transition, and prepassive ranges of the overall active state, and explaining the different values of the experimental kinetic data obtained by their authors. In addition to this, the supporters of the consecutive electron-transfer concept offered an explanation for the disagreement between the experimental low steady-state Tafel slope and the inductive behavior of the electrode, on the one hand, and the theoretical predictions, on the other, as demonstrated by Plonski " since 1969. 

At this point, special mention37 should be made of the behaviour of highly conjugated ethylenic sulphones in weakly acidic media. For example, in the case when R1 =Ph (Z isomer), a fairly stable anion radical was obtained in dry DMF. However, either in aprotic (consecutive two one-electron transfer) or in protic media (ECE process, occurrence of the protonation step on anion radical), C—S bond cleavage is observed. The formation of the corresponding olefins by C—S bond cleavage may occur in high yield, and is nearly quantitative when R1 = H and R2 = Ph for an electrolysis conducted in... 

Let us consider the case that underpotential deposition takes place at potentials where specifically adsorbed anions depart from the surface, or the removal of underpotential deposition species induces specific adsorption of anions. Then the underpotential deposition process is taken to consist of three consecutive steps (1) desorption of specifically adsorbed anions from substrate M, (2) adsorption and electron transfer of metal ions M"" to form an underpotential deposition metal layer on the substrate metal M, and (3) readsorption of the anions on the underpotential deposition metal M on M, i.e.. 

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