Examples of Electron Transfer Reactions

As we have seen, the rate of electron transfer depends on both nuclear and electronic factors. The most experimentally accessible approaches to character- 

The overall kinetics of bimolecular electron transfer reactions are complicated by the reactant association and product dissociation steps. To eliminate the contributions of these events to the reaction kinetics, many recent studies have concentrated on intramolecular electron transfer processes, where both the donor and acceptor groups are present in the same molecule. These systems provide the opportunity to measure directly the rate of electron transfer in the absence of competing bimolecular processes. For this reason, the examples in this section are concerned only with intramolecular processes. 

In many ways, analysis of the factors influencing biological electron transfer is analogous to the study of electric conduction in macroscopic systems. The properties of conducting systems may be described by Ohm s law  

Two intramolecular electron transfer systems of current interest, ruthenated proteins and photosynthetic reaction centers, are now discussed. These examples serve to illustrate the types of experimental approaches and information that may be obtained concerning the details of electron transfer processes in biological systems. 

Covalent attachment of ruthenium ions to proteins (ruthenation) provides a powerful approach for introducing a second redox active group into an electron transfer protein (77, 78). Under suitable conditions, intramolecular electron transfer may be monitored between Ru and the intrinsic redox group. Ruthenated proteins display a number of advantages for the study of intramolecular electron transfer reactions (77, 78)  

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Table 4. Examples of electron transfer reactions which show the Marcus Inverted Region...

One of the best known examples of electron transfer reactions involving hemi-carceplexes is the one investigated independently by Pina, Balzani and colleagues [99] and by Deshayes and coworkers [100], The latter study is focused mainly on triplet energy transfer, a process which can be mechanistically related to electron transfer [101],... 

Of course, there are also dendritic structures that belong to more than one of these categories. The most significant examples of electron transfer reactions involving dendrimers containing electroactive units will be presented in Section 9.4, arranged on the basis of the electroactive unit involved. Moreover, there are examples where the dendrimer itself does not contain any electroactive unit, but is in-... 

Figure 8 Some examples of electron-transfer reactions of pesticide compounds in relation to dominant terminal electron accepting processes (TEAPs) in natural waters. TEAP sequence based on Christensen et al. (2000). Manganese, iron, sulfur, or carbon may be present in either the dissolved or soUd phase. Location of each compound indicates the condition(s) under which it has been found to be relatively stable in natural waters. Incomplete lists of transformation products denoted by ellipsis (...) compounds shown are those inferred to have been derived directly from the parent compound. One-way arrows denote essentially irreversible reactions two-way arrow denotes a reversible reaction. The various references cited are a, Egli et al. 1988 b, Picardal et al., 1995 c, Milligan and Haggblom, 1999 d, Tesoriero et al., 2001 e, Klecka et al., 1990 f, Gibson and Suflita, 1986 g, Adrian and Suflita, 1990 h. Miles and Delfino, 1985 i, Lightfoot era/., 1987 j, Nairand Schnoor, 1992 k, Papiemik and Spalding, 1998 ...

Transition metal ions are commonly involved in electron transfer reactions. Iron, copper, zinc and manganese are important in these reactions. Redox potentials are important when evaluating the viability of these reactions. Examples of electron transfer reactions discussed in diis volume include Fenton reaction and transformation of oxymyoglobin to metmyoglobin with die release of superoxide. 

These processes are quite rapid with an apparent rate constant which exceeds lO" cm s" [5,6] The only example of electron transfer reaction which has been observed was between the hydrophobic ferro-cinium - ferrocene redox couple in nitrobenzene and the hydrophilic hexacyanoferrate redox couple in water [9]. A more complex mechanism is involved in the case of ion transfer facilitated by an iono-phore[10]. This is the case, for example in the transfer of the alkali and alkaline earth metal cations across a water/nitrobenzene interface facilitated by synthetic neutral cyclic or acyclic iono-phores derived from 3,6-dioxaoctanedicarboxylic acid [11]. 

Other examples of electron transfer reactions in surfactant assemblies are those between pyrene and dimethylaniline in micelles, between viologen derivative and zinc porphyrin as an electron relay, and between chlorophyll a and methylviologen in microemulsions the photoinduced reduction of duroquinone by zinc porphyrin in micellar solution the photoinduced redox reaction of proflavine in aqueous and micellar solutions retardation of back reactions in micellar systems light-driven electron transfer from tetrathiafulvalene to porphyrin and tris a, a -bipyridine)... 

Examples include luminescence from anthracene crystals subjected to alternating electric current (159), luminescence from electron recombination with the carbazole free radical produced by photolysis of potassium carba2ole in a fro2en glass matrix (160), reactions of free radicals with solvated electrons (155), and reduction of mtheiiium(III)tris(bipyridyl) with the hydrated electron (161). Other examples include the oxidation of aromatic radical anions with such oxidants as chlorine or ben2oyl peroxide (162,163), and the reduction of 9,10-dichloro-9,10-diphenyl-9,10-dihydroanthracene with the 9,10-diphenylanthracene radical anion (162,164). Many other examples of electron-transfer chemiluminescence have been reported (156,165). 

How deeply one wishes to query the mechanism depends on the detail sought. In one sense, the quest is never done a finer and finer resolution of the mechanism may be obtained with further study. For example, the rates and mechanisms of electron transfer reactions have been studied experimentally and theoretically since the 1950s. but the research continues unabated as issues of ever finer detail and broader import are examined. The same can be said of other reactions—nucleophilic substitution, hydrolysis, etc. 

The field of electrochemical ion transfer reactions (EITRs) is relatively recent compared with that of electron transfer reactions, and the application of molecular dynamics simulations to study this phenomenon dates from the 1990s. The simulations may shed light on various aspects of the EITR. One of the key questions on this problem is if EITR can be interpreted in the same grounds as those employed to understand electron transfer reactions (ETRs). Eor example, let us consider the electrochemical oxidation reaction of iodine ... 

Another interesting situation which demonstrates the role of solvation on cluster reactions, with particular relevance to identifying mechanisms of possible importance in the condensed phase, concerns the initiation of electron transfer reactions. Interesting examples include those initiated through Penning ionization.161... 

The discussion above provides the necessary elements to answer the question posed in the heading. If the intermediate does not exist (i.e., its lifetime is shorter than one vibration), the concerted mechanism is necessarily followed. Conversely, however, if the intermediate exists, the reaction pathway does not necessarily go through it, depending on the molecular structure and the driving force. Dichotomy and competition between the two mechanisms is a general problem of chemical reactivity. The example of electron transfer/bond reactions has allowed a detailed analysis of the problem, thanks to the use of electrochemical techniques on the experimental side and of semiempirical models on the theoretical side. 

This book deals only with the chemistry of the mineral-water interface, and so at first glance, the book might appear to have a relatively narrow focus. However, the range of chemical and physical processes considered is actually quite broad, and the general and comprehensive nature of the topics makes this volume unique. The technical papers are organized into physical properties of the mineral-water interface adsorption ion exchange surface spectroscopy dissolution, precipitation, and solid solution formation and transformation reactions at the mineral-water interface. The introductory chapter presents an overview of recent research advances in each of these six areas and discusses important features of each technical paper. Several papers address the complex ways in which some processes are interrelated, for example, the effect of adsorption reactions on the catalysis of electron transfer reactions by mineral surfaces. 

The electron formed as a product of equation (2.5) will usually be received (or collected ) by an electrode. It is quite common to see the electrode described as a sink of electrons. We need to note, though, that there are two classes of electron-transfer reaction we could have considered. We say that a reaction is heterogeneous when the electroactive material is in solution and is electro-modified at an electrode which exists as a separate phase (it is usually a solid). Conversely, if the electron-transfer reaction occurs between two species, both of which are in solution, as occurs during a potentiometric titration (see Chapter 4), then we say that the electron-transfer reaction is homogeneous. It is not possible to measure the current during a homogeneous reaction since no electrode is involved. The vast majority of examples studied here will, by necessity, involve a heterogeneous electron transfer, usually at a solid electrode. 

Figure 2.1 Simplified schematic plots showing the exponential relationship between the current density i and the potential of the electrode, E. (The latter is represented here as being relative to the standard electrode potential of the couple undergoing electromodification for now, the abscissa ( — ) can be thought of as deviation from equilibrium.) Three examples of electron-transfer rate (/feei) are shown (a) (coincident with the y-axis) representing a very fast rate of electron transfer of 10 A cm" (b) representing an average rate of electron transfer of 10 A cm (c) representing a slow rate of electron transfer of 10 A cm . For each trace, T = 298 K and the reaction was symmetrical , i.e. a = 0.5, as defined later in Section 7.5.

Various substituted cyclopropanes have been shown to undergo nucleophilic addition of alcoholic solvents. For example, the electron transfer reaction of phenylcyclopropane (43, R = H) with p-dicyanobenzene resulted in a ring-opened ether 44. This reaction also produced an aromatic substitution product (45, R = H) formed by coupling with the sensitizer anion. This reaction is the cyclopropane analog of the photo-NOCAS reaction, but preceded it by almost a decade. 

Early examples of electron transfer processes are shown in equations (2), (12), and (13). Birch in 1944 followed up the findings of Wooster, and demonstrated that Na metal and ethanol in ammonia reduce benzene, anisole, and other aromatics to 1,4-cyclohexadienes. Birch speculated about the mechanism of this reaction, but did not explicitly describe a radical pathway involving 55 (equation 87) until later, as described in his autobiography. Electron transfer from arenes was found by Weiss in 1941, who obtained crystalline salts of Ci4H]o from oxidation of anthracene. ... 

This reaction profile of the polymer complex has some similarities with the phenomenon of the polyelectrolyte-catalyzed reactions. It has been reported that the reactions between two positively charged species in aqueous solution are drastically accelerated in the presence of polyanionsS2 84 For example, the electron-transfer reaction between [Co(IIIXen)2(Py)Cl]2+ and [Fe(IIXOH2)6]2+ is very slow because the reaction occurs between two cations however, the addition of a small amount of poly(styrenesulfonate) accelerates the reaction by a factor of 103 84). This result is also interpreted as indicating that the two positively charged reactants are both concentrated in the polyanion domain, so that they encounter each other more frequently [Fig. 17(b)]. 

An example of electron transfer at ITIES can be obtained if NB contains ferrocene (Fc) and water contains [Fe(CN)6]3. The reaction at the interface is... 

In the case of electron transfer reactions, besides data on the dynamic Stokes shift and ultrafast laser spectroscopy, data on the dielectric dispersion (w) of the solvent can provide invaluable supplementary information. In the case of other reactions, such as isomerizations, it appears that the analogous data, for example, on a solvent viscosity frequency dependence 17 ( ), or on a dynamic Stokes fluorescence shift may presently be absent. Its absence probably provides one main source of the differences in opinion [5, 40-43] on solvent dynamics treatments of isomerization. 

Many of the simplest chemical reactions involve only an interchange of atoms or ions between reactants, or perhaps only the dissociation of one reactant into two parts. In such reactions, there is no change in the electrical charge of any of the atoms involved. This chapter deals with another type of reaction, in which one or more electrons are transferred between atoms, with the result that some of the atoms involved do have their electrical charges changed. These reactions are known as electron-transfer reactions. You can appreciate their importance when you realize that every battery used in electronic devices and machines, every impulse involved in nerve transmission, every metabolic reaction that produces energy in biological systems, photosynthesis, and combustion processes (to mention but a few examples) requires electron-transfer reactions. 

Examples of electron tunneling reactions on the surface of heterogeneous catalysts have been discussed in Chap. 7. These reactions provide electron transfer between spatially separated donor and acceptor centres on the surface of heterogeneous catalysts as well as between the centres one of which is on the surface of the catalyst and the other is in the subsurface layer. Such processes are expected to be important for photocatalytic reactions, as well as for thermal catalytic reactions proceeding at low temperatures by heterolytic mechanisms. 

A single oxo bridge may subtend an angle between 140° and 180°, this angle being determined by steric or electronic factors (Table 3).95 103 Almost all these examples refer to the solid state, but there are also several homo- and hetero-nuclear M—O—M and M—O—M—O—M species known in solution. Often these are intermediates in, or products of, electron transfer reactions with oxide-bridging inner-sphere mechanisms. Examples include V—O—V in V(aq)2+ reduction of VO(aq)2+, and Act—O—Cr in Cr(aq)2+ reduction of UOj+ or PuOj+ a useful and extensive list of such species has been compiled. Tlie most recent examples are another V—O—V unit, this time from VO(aq)2+ and VOJ,105 and an all-actinide species containing neptunium(VI) and uranium-(VI).106 An example of a trinuclear anion of this type, with the metal in two oxidation states, is provided by (31).107... 

In addition to the type of electron transfer reaction, shown in Equations 6.142-6.145, there are examples where pure MLCT excited states induce ligand substitutions by associative or dissociative mechanisms. A well-established example of a MLCT excited state-mediated ligand labilization reaction is shown in Equation 6.149.136... 

Unfortunately, the experimental data concerning the distances at which electron exchange reactions in the membranes take place are very scarce. Tsuchida et al. have shown [147], that even when the photoexcited Zn porphyrin embedded in the membrane cannot approach the membrane // water interface closer than 12 A, the electron transfer is still possible to MV2+ located in the water phase outside the membrane. However, when the distance of the closest approach of these reactants is increased up to 17 A, the electron transfer is totally stopped. Examples of electron transfer proceeding presumably via electron tunneling across molecular layers about 20 A thick, which separate electron donor and acceptor molecules, can be found in papers by Mobius [230, 231] and Kuhn [232, 233]. Note, that in... 

It remains, however, as a very interesting example of selective oxidation using mixed transition metal ions in a zeolite matrix. The extent to which the zeolite matrix directly or indirectly facilitates the electron transfer step between Pd° and Cu2+ has not been examined. This would seem worthy of study within the general concept of electron transfer reactions in heterogeneous catalysis. 

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