Electron transfer reactions:

An electron transfer (ET) reaction is defined here as an oxidation-reduction reaction that occurs between two coordination compounds. The compounds may be the same species, but where the metals have different oxidation states (a selfexchan reaction), or are completely different species (a cross-reaction). An example of a self-exchange reaction is given in Equation (17.41), where Co is an isotopically labeled cobalt a cross-reaction is given by Equation (17.42). 

ET reactions are typically bimolecular in the RDS and therefore display second-order kinetics. The reaction approximates a simple collision model, where the free energy of activation (AG ) involves three terms, as shown in Equation (17.43). 

The degree of the inner-sphere reorganization energy (2 ) associated with the adjustment of the metal-ligand bond lengths is given by Equation (17.44), where n 

The mechanistic steps invoived in the activation of the reactants (D = donor metai, A = acceptor metai) to reach the precursor compiex necessary for ET. Foiiowing ET, the successor compiex undergoes dissociation to form the products. 

Potential energy diagram for a self-exchange eiectron transfer reaction. 

1 Electron-transfer Reactions - Light-induced electron transfer from a donor to a suitable acceptor has been described for numerous bimolecular systems. The reagents have been dispersed in a polar solvent,at microscopic or macroscopic interfaces, in latex dispersions, in nematic liquid crystals, in reverse micelles, in vesicles, and in lipid bilayer membranes. Additional studies have been concerned with electron transfer 

Electron transfer (ET) reactions at soft interfaces, similar to assisted ion transfer reactions, can occur either heterogeneously or homogeneously in the vicinity of the interface following the transfer of one of the reactants (see Eigure 16.2). 

The Nernst equation for a monoelectronic heterogeneous ET reaction is given by 

In this way, it is only necessary to determine the standard redox potential of the fer-rocinium/ferrocene couple in the organic phase with respect to the aqueous SHE by means of a thermodynamic cycle. 

One very interesting class of ET reactions at soft interfaces are those that are photoini-tiated. Following the pioneering studies of the Russian school, including those of Volkov [6] and Kuzmin [7], it has been shown that photosensitizers soluble in one phase are often adsorbed at the interface and can be quenched by electron donor or acceptors. This class of reaction offers interesting perspectives to design biomimetic approaches to artificial photosynthesis. Photoelectrochemistry at the interfaces between two immiscible electrolyte solutions (ITIES) is rather analogous to photoelectrochemistry at a semi-conductor electrode, where the potential drop within the semi-conductor should be considered. 

Kinetic analysis of the substitution reactions indicate that they follow a dissociative mechanism. It has also been shown that two water molecules in [Cr(H20)5I]2+ undergo exchange with labeled water. It is interesting that one exchange is rapid and occurs before I- leaves. However, this is not true of the chloride compound. Therefore, it appears that the iodide ion labilizes the water trans to it, but the chloride does not. 

In addition to the indications of an octahedral trans effect presented, there exists structural information in the form of bond lengths and spectral data similar to that described earlier for square planar complexes. Although the trans effect in octahedral complexes is not the dominant influence that it is in square planar complexes, there is no doubt that there is such an effect. 

An aqueous solution containing complexes of two different metal ions may make it possible for a redox reaction to occur. In such cases, electrons are transferred from the metal ion being oxidized to the metal ion being reduced. For example, 

In this reaction, an electron is transferred from Cr2+ to Fe3+, and such reactions are usually called electron transfer or electron exchange reactions. Electron transfer reactions may also occur in cases where only one type of metal ion is involved. For example, the reaction 

Electron transfer between metal ions contained in complexes can occur in two different ways, depending on the nature of the metal complexes that are present. If the complexes are inert, electron transfer occurring faster than the substitution processes must occur without breaking the bond between the metal and ligand. Such electron transfers are said to take place by an outer sphere mechanism. Thus, each metal ion remains attached to its original ligands and the electron is transferred through the coordination spheres of the metal ions. 

You may not have noticed it, but we have considered two kinds of reactions in previous chapters. In some, such as (9.3), 

In a purely formal manner the description of an electron transfer event, such as the reduction in solution of Fe(III) ion, can be written in two ways, depending on whether the reduction is operated by a chemical agent or by an electrode  

In both cases, the adopted symbolism only gives a picture of the overall process. In fact, from a mechanistic viewpoint, the redox reactions (as with any other type of reaction) proceed by a series of intermediate steps involving phenomena such as  

Commonly, oxidation-reduction reactions in a homogenous phase are classified as  

In the inner-sphere reactions, the process involves a transition state in which a mutual strong penetration of the coordination spheres of the reagents occurs (and, therefore, strong interaction between reagents), whereas in the outer-sphere reactions there is no overlap of the coordination spheres of the reagents (and, therefore, there is weak interaction between reagents). 

The fact that from a chloro-cobalt complex a chloro-chromium complex is formed, suggests that the reaction must proceed through an intermediate state that enables the transfer of a chlorine atom from cobalt to chromium. The proposed mechanism for this reaction is  

Here the reaction has been shown to proceed via electron transfer to and from the metal surface such that the metal particle acts as a highly dispersed electrode. 

Organic free radical reactions are also catalyzed by metal colldds. For example, although 1-hydroxy-l-methyethyl, (CH3)2COH , radicals generated by the photolysis of 50-propanol/acetone mixtures are unreactive towards thallium(I) ions or methylene chloride in water, the presence of colloidal silver induces them to reduce T1 to colloidal thallium (a one electron process) and CH2O2 to CHQ3 and Q (a two electron process Eqn. 6.9). [273] 

The net reaction is the liberation of hydrogen from the organic radical, however, many radicals are oxidized before hydrogen liberation begins. By measuring the pH change of the reaction during irradiation, (Eqn. 6.10), it was shown that the silver partides absorb many electrons before water is reduced. For the 7 nm diver partides used in this study, 450 stored electrons per partide were measured at the stationary state. [279] The colloidal silver partides thus act as electron pools from which electrons can be released to appropriate acceptors, in this case water. 

Dioxygen in its ground (triplet) state reacts as a rather stable diradical. One of its most important radical reactions is [4] in which radicals R are converted into peroxyl radicals R-OO which generally have quite different reactivities from those of the parent R species. This reaction is, in general, reversible, and stable radicals such as nitrox-ides fail to undergo reaction [1.1] to any measurable extent. 

In contrast, lJ02 oxygen, often just referred to as singlet oxygen , reacts as an electrophile, via the empty n orbital. Thus its reactions 

3 Describe and explain oxidation and reduction in terms of electron transfer. 

4 Given an oxidation half-reaction equation and a reduction half-reaction equation, combine them to form a net ionic equation for an oxidation-reduction reaction. 

Where do the electrons entering the lightbulb come from, and where do they go when they leave the bulb Four measurable observations answer those questions. After the cell has operated for a period of time (1) the mass of the zinc electrode decreases, (2) the Zn concentration increases, (3) the mass of the copper electrode increases, and (4) the Cu concentration decreases. The first two observations indicate that neutral zinc atoms lose two electrons to become zinc ions. Stated another way, zinc atoms are being divided into zinc ions and two electrons  

The electrons flow through the wire and the lightbulb to the copper electrode, where they join a copper ion to become a copper atom  

The chemical change that occurs at the zinc electrode is oxidation oxidation is defined as the loss of electrons. The reaetion is described as a half-reaction because it eannot occur by itself There must be a second half-reaction. The electrons lost by the substance oxidized must have someplace to go. In this ease they go to the copper ion, which is reduced. Reduction is a gain of electrons. 

In this chapter, we will study the fundamentals of electron transfer (ET) reactions. In ET reactions, the only moving particle appears to be an electron. No bonds are broken or formed. Spectral changes are noticeable after transfer of an electron, and this gives a possibility to study the kinetics of very fast ET reactions by time-resolved spectroscopy. Still, small structural changes do occur at ET, and these changes are the main reason why ET reactions need activation energy. In this chapter and in Chapter 11, we primarily have thermal activation in mind. In Chapter 13, we will discuss photoinduced ET (PIET) reactions. 

More useful mechanistic information is obtained from intramolecular electron-transfer reactions if the kinetics for the electron-transfer step can be isolated from the effects of diffusion. The main stimulus for making such studies is the urge to design systems that mimic some of the essential features of the photosynthetic reaction centre complex and much attention has focussed on the study of porphyrin-based photoactive dyads. Thus, a series of N-alkylporphyrins linked to a quinolinium cation has been synthesized and found to display a rich variety of photoreactions. The singlet excited state of the quinolinium cation operates in both intramolecular energy- and electron-transfer reactions while the excited singlet state of the porphyrin transfers an electron to the appended quinolinium cation. Several new porphyrin-quinone dyads have been studied,including cyclophane-derived systems where the reactants are held in a face-to-face orienta- 

In this reaction, an electron is transfer from [ Fe(CN)6]4- to Fe3+ in [Fe(CN)6]3- (where Fe and Fe are different isotopes of iron). This is an electron transfer in which the product differs from the reactants only in that a different isotope of Fe is contained in the +2 and +3 oxidation states. 

For an outer sphere electron transfer, the coordination spheres of each complex ion remain intact. Thus, the transferred electron must pass through both coordination spheres. Reactions such as the following are of this type (where represents a different isotope)  

The simplest class of reaction of coordination compounds which has been studied is that of electron-transfer reactions. Suppose that a solution of potassium ferrocyanide (hexacyanoferrate(II)) is mixed with one of potassium ferricyanide (hexacyanoferrate(III)), then if an [Fe(CN)g] anion loses an electron and an [Fe(CN)6] anion gains one, a chemical reaction has occurred, although there is no change in the composition of the mixture. If one of the atoms in just one of the complex ions is labelled in some way—with for example—then the reaction may be studied by seeing 

It is not essential that two matched anions are in contact at the instant of electron transfer. The question which then arises is Just how far apart can two ions be and yet participate in an electron-transfer reaction The answer to this question is particularly important in bioinorganic chemistry (Chapter 16), for there two potential participants may be unable to approach each other very closely because of the constraints imposed by the large molecule(s) within which they are found. For most systems the answer to the above question seems to be less than 10 A but it is becoming clear that much 

At first sight, an attempt to calculate rate constants for electron transfers of the sort we have been discussing would seem an impossible task. Charged species, separated by varying amounts of solvent, a solvent which will react 

The equilibrium constant for this last reaction, obtained from emf measurements, is Ki2- For reactants and products of the same size and charge type the simplest form of the Marcus cross-relationship is 

The reaction is carried out in water and the final cobalt(II) product is actually [Co(H20)g], but this is immaterial to our discussion. It was found that if the solution contained labelled chloride ion ( CP) none of the activity appeared in the [Cr(H20)5Cl] product the reaction does not involve the free chloride ions present in the solution. This observation indicates that there must be an intimate contact between the reacting species, a Cl ion being transferred from cobalt(III) to chromium(II) and an electron migrating in the opposite direction. It therefore seems likely that a species something similar to that shown in Fig. 14.4 must be involved. Similar transfer of the ligand X from [Co(NH3)5X] to chromium(II) occurs for X = Cl , Br, N3, acetate, SOj and POj . That there is a transient intermediate something like that in Fig. 14.4 is supported by the observation that for X = NCS (the complex having a Co-N bond) the initial product is 

Valence d electrons of transition metals impart special properties (e.g., color and substitution reactivity) to coordination complexes. These valence electrons can also be removed completely from (oxidation) or added to (reduction) metal d orbitals with relative facility. Such oxidation-reduction (redox) reactions, like substitution reactions, are integral to metal complex reactivity. Consider the role of redox chemistry in the synthesis of [Co(NH3)5C1]+, equation (1.8). In general, the preparation of cobalt(III) complexes (Chapters 2 and 5) starts with substitutionally labile cobalt(II) salts that are combined with appropriate ligands with subsequent oxidation of the metal by H202 or 02 to the substitutionally inert (robust) +3 state. 

Equation (1.8) also shows how redox reactions are intricately linked to substitution. As Taube stated in his Nobel lecture While substitution reactions can be discussed without concern for oxidation reduction reactions, the reverse is not true. The substitution properties of both cobalt(III) and cobalt(II) metal ions provides the rationale for this synthetic methodology. 

The study of electron transfer reactions began in earnest when radioactive isotopes, produced for nuclear research and the atom bomb program during World War II, became accessible. Glen Seaborg, in a 1940 review of artificial radioactivity, noted the first attempt to measure the self-exchange reaction between aqueous iron(III) and iron(II), equation (1.9).1  

By 1950, such isotopic tracer methods began to revolutionize the study of redox reactions as color alone could not always be used to distinguish product formation see equation (1.9). The importance of H+ and other ions on electron transfer rates was soon discovered. A symposium on electron transfer took place in 1951 at the University of Notre Dame, during which a distinction between outer- and inner-sphere electron transfer was made. 

Equation (1.9) is an example of a self-exchange electron transfer reaction for which AG° = 0. One cannot tell the difference between reactant and product by color alone, hence the need for isotopic labeling indicated by the asterisk. 

Important progress was made when it was realized that there are, in principle, two types of mechanisms of electron-transfer reactions, outer-sphere and inner-sphere mechanisms.  

In outer-sphere redox processes, the interaction between an oxidizing and a reducing agent at the moment of electron transfer is very small. The coordination shell remains intact, though perturbed. 

Redox reactions are very exciting from the theoretical point of view some publications in this field have provided the basis for today s understanding of the electron-transfer mechanisms.  

The metal exchange reaction rates, which are much higher than those of the replacement reactions, often reveal the electron-transfer mechanism. E.g., this holds for the reaction  

This means that 10 molar complex solutions exchange iron more than 99% in 13 ms. o 

In the following sections the effect of pressure on different types of electron-transfer processes is discussed systematically. Some of our work in this area was reviewed as part of a special symposium devoted to the complementarity of various experimental techniques in the study of electron-transfer reactions (124). Swaddle and Tregloan recently reviewed electrode reactions of metal complexes in solution at high pressure (125). The main emphasis in this section is on some of the most recent work that we have been involved in, dealing with long-distance electron-transfer processes involving cytochrome c. However, by way of introduction, a short discussion on the effect of pressure on self-exchange (symmetrical) and nonsymmetrical electron-transfer reactions between transition metal complexes that have been reported in the literature, is presented. 

The above-quoted reactions all proceed via an outer-sphere electron-transfer mechanism. By way of comparison, the volume of activa- 

For the mechanistic interpretation of activation volume data for nonsymmetrical electron-transfer reactions, it is essential to have information on the overall volume change that can occur during such a process. This can be calculated from the partial molar volumes of reactant and product species, when these are available, or can be determined from density measurements. Efforts have in recent years focused on the electrochemical determination of reaction volume data from the pressure dependence of the redox potential. Tregloan and coworkers (139, 140) have demonstrated how such techniques can reveal information on the magnitude of intrinsic and solvational volume changes associated with electron-transfer reactions of transition 

It has in general been the objective of many mechanistic studies dealing with inorganic electron-transfer reactions to distinguish between outer- and inner-sphere mechanisms. Along these lines high-pressure kinetic methods and the construction of reaction volume profiles have also been employed to contribute toward a better understanding of the intimate mechanisms involved in such processes. The differentiation between outer- and inner-sphere mechanisms depends 

The coordination sphere of the reactants remains intact in the former case and is modified by ligand substitution in the latter, which will naturally affect the associated volume changes. 

In competition with this radical coupling reaction is electron transfer between the alkyl radical and thianthrene radical cation. If the radical R- is of sufficiently low oxidation potential (and the reorganization energy permits), e.g., terf-butyl radical, then electron transfer occurs to yield the corresponding cation R+. Evidence for electron-transfer in these reactions was also convincingly demonstrated by the use of unsymmetrical organometallic species. Kochi [29] has shown that the alkyl group that is preferentially cleaved 

This suggests that the mechanism for the formation of arylsulfonium salt 7, R=Ar involves the complexation mechanism [31] further discussed below, but illustrated for the first part of this reaction in Eqs. (4-6). 

Thianthrene radical cation is also an excellent one-electron oxidant of iron porphyrin complexes. Such oxidation of Fem(0Cl03)(TPP), where TPP is meso-tetraphenylporphyrin, provides the corresponding porphyrin 7r-cation radical analytically pure [32]. Similar oxidation of the AT-methyl porphyrin complex (N-MeTPP)FenCl, where AT-MeTPP is AT-methyl-meso-tetraphenylporphyrin, afforded [N-MeTPPFemCl]+ which was not further oxidized [33]. Thus thianthrene radical cation selectively oxidized the aromatic porphyrin ligand in one case and the metal center in the other. Ligand oxidation at a phenolic moiety has also been reported [34] on treatment of a 1,4,7-triazacyclononane appended with one or two phenol moieties ligated to Cu(II) complex with thianthrene radical cation. 

Formation of 7r-cation radicals has also been suggested [35] in the reaction of stable enols with thianthrene radical cation and other one-electron oxidants. Thus enol 10 on treatment with thianthrene radical cation affords benzofuran derivative 11 in 87 % yield. The initial step in this reaction is suggested to be one-electron transfer forming the cation radical of 10, which has been unequivocally identified in a related system [36]. 

This cation radical rapidly deprotonates, undergoes further oxidation (or disproportionates) to the corresponding a-carbonyl cation. Cyclization of this cation and rearrangement [37] yield ultimately 11. It should be noted that an alternative reaction path has been identified in the reaction apparently of enols with thianthrene radical cation. Ketones [38, 39] and aldehydes [40] on treatment with thianthrene radical cation form sulfonium salts 12 and thianthrene. 

In the respiratory chain, electrons from the powerful reducing agents NADH and FADH2 pass through four membrane-bound protein complexes and two mobile electron carriers before reducing O2 to H2O. We shall see that the electron transfer reactions drive the synthesis of ATP at three of the membrane protein complexes. 

The respiratory chain begins in complex I (NADH-Q oxidoreductase), where NADH is oxidized by coenzyme Q (Q, Atlas M5) in a two-electron reaction  

Reduced Q migrates to complex III (Q-cytochrome c oxidoreductase), which catalyzes the reduction of the protein cytochrome c (Cyt c). Cytochrome c contains the heme c group, the central iron ion of which can exist in oxidation states -1-3 and +2. The net reaction catalyzed by complex III is 

Reduced cytochrome c carries electrons from complex III to complex IV (cytochrome c oxidase), where O2 is reduced to H2O  

The reactions that occur in complexes I, II, III, and IV are exergonic and together could drive the synthesis of ATP  

Reduction of Fe(CN)g ions at n-type semiconductor ZnO electrodes involves the conduction band with reaction orders 1 and 0 with respect to fer-ricyanide and ferrocyanide ions, respectively [55]. The electrode reaction can be described by a simple model of direct electron transfer from the conduction band with no surface states being involved. The potential distri- 

On the basis of electrode kinetic data obtained in 1M NaOH for oxides in the range 0.1 x 0.5, van Buren et al. [77] concluded that the solid state electronic properties of these mixed oxies have no observable effect on the electron transfer kinetics and the oxides can be considered as pseudo-metallic from an electrochemical point of view. There are, however, several observations that make this conclusion questionable (a) Characterization data for the oxide electrode surfaces were not presented. In particular, the electrochemical real surface area (capacity, or BET) of the electrodes, and therefore comparison of apparent rate coefficients, are uncertain, (b) The 

Kinetic data for the hexacyanoferrate system at various oxide electrodes 

It has been shown [78] that, for fast electrode kinetics at a hydrodynamic electrode 

If the ring electrode of the rotating ring-disc system monitors the flux of the oxidized or reduced components of the redox couple, then deconvolution of both Faradaic and pseudo-capacitive currents may be done [85] 

A fundamental understanding of oxidation-reduction reactions is vital to the inorganic chemist in contexts ranging from energy transduction - chemical to electrical and the converse, in technical matters in corrosion processes and metallurgy, redox processes in environmental chemistry and metalloenzymes and metallo-proteins involved in electron transfer. Electron-transfer reactions of transition metal complexes are accompanied by a change in the oxidation state of the metal 

As an example, photochemical excitation of donor-acceptor complexes may be considered. Irradiaiion into the CT band of the anthracene-tetracyano-ethylene complex leads directly to the radical ion pair, the components of which are identifiable from their UV-visible spectra. The transient absorptions decay in 60 ps after excitation, as the radical ion pairs undergo rapid back electron transfer to afford the original donor-acceptor complex (Hilinski et al., 1984). With tetranitromethane as acceptor, however, an addition product is obtained in both high quantum and chemical yield. This is due to the fact that the tetranitromethane radical anion undergoes spontaneous fragmentation lo a NO, radical and a trinitromethyl anion, which is not able to reduce the anthracene radical cation (Masnovi et al., 1985)  

A detailed study showed that after dissociation of the radical anion a contact ion pair E) C(NO,)f ] in a solvent cage is initially formed. It transforms into 

The photoreduction of carbonyl compounds or aromatic hydrocarbons by amines was one of the early electron-transfer reactions to be studied. Observation of products from primary electron transfer depends on the facility of a deprotonation of the amine, which must be fast compared to back electron transfer. For amines without a hydrogens, quenching by back electron transfer is observed exclusively (Cohen et al., 1973). The solvent plays a quite important role since it determines the yield of radical ion pairs formed from the exciplex (Hirata and Mataga, 1984). 

With singlet excited trans-stilbene (151) and tertiary alkyl amines only products characteristic for radical coupling are observed (Lewis et al., 1982). 

With unsymmetrical trialkylamines selective formation of the least-substituted a-amino radical is observed. The stereoselectivity is thought to be stereoelectronic in origin, as can be most easily seen in highly substituted amines such as diisopropylmethylamine, where a deprotonation occurs 

Metal-Metal Bonding in M3E2 and M4E4 Clusters 

MjEj cluster Z I M-M bond orders M4E4 cluster z M-M bond orders 

M4E4 (38-41) compounds where the total number of valence electrons, Z, is directly related to the degree of metal-metal bonding in the M3 resp. M4 frameworks. Table I summarizes their results. 

Whereas in ligand bridged dinuclear complexes, removal or addition of two electrons makes or breaks one metal-metal bond (15) this does not seem to be the case for clusters, presumably because of their delocalized bonding. At least for one case, however, two-electron reduction can induce a significant change in cluster shape (18,42) the 84-electron cluster Os6(CO),g with framework 1 is easily reduced to the 86-electron anion Os6(CO) g with framework 2, in accordance with skeletal electron counting rules. 

In trying to establish fixed, or at least constrained, geometry for the donor-acceptor pair many researchers have turned their attention towards supramole-cular chemistry wherein the principles of molecular recognition are used to assemble suitable entities without covalent interactions. Thus, the photophysical properties of biacetyl imprisoned inside a hemicarcerand have been measured in the presence of about twenty different quenchers but the framework of the 

Related molecular dyads have been constructed in which a metal complex, often ruthenium(II) tris(2,2 -bipyridine) or similar, functions as chromophore and an appended organic moiety acts as redox partner. Other systems have been built from two separate metal complexes. Each of these systems shows selective intramolecular electron transfer under illumination. Rates of charge separation and recombination have been measured in each case and, on the basis of transient spectroscopic studies, the reaction mechanism has been elucidated. The results are of extreme importance for furthering our understanding of electron-transfer reactions and for developing effective molecular-scale electronic devices. The field is open and still highly active. 

The formation of excited NO2 is particularly interesting in that the geometry of the approach of the reactants can be correlated with the product symmetry, the excited state being linear and the ground state bent. The differing probability of formation of each of the states can be demonstrated very nicely by the correlation between the values of the pre-exponential (log A) factors and the continuity of the PE surfaces as determined by the state symmetry. 

Another example, discussed with the dioxetans later, which involves cage reaction of triplet radicals is that of the decomposition of hyponitrite esters [48]. Hydrogen abstraction from one radical R2CH-0 by another leads to triplet carbonyl products in high yield. 

All of the reactions discussed above produce excited carbonyl products which are expected [49] to have a partially tetrahedral configuration about the trigonal C atom rather than the planar geometry of the ground state carbonyl. Since these products are derived from tetrahedral sp precursors, geometrical influences may be at work. The assessment of the relative worth of each of these factors in determining whether or not excited states are formed is not possible at present. 

The strongly exothermic transfer of electrons between fluorescent organic molecules represents one of the most general mechanisms in chemiluminescence [50, 51]. It can be found in electroluminescence, radical ion annihilation and peroxide decomposition. The basic concept was introduced by Hercules [39] following the general theory of Marcus [52]. The reaction co-ordinate can be roughly indicated by the potential energy curves shown. 

Originally invoked for electroluminescent reactions, this idea has now been developed for the reaction of peroxides with fluorescent compounds of low ionisation potential [51]. Many of these reactions are discussed in Chap. XI, but Fig. 2 can be most succintly exemplified by the radical ion annihilation shown, where Ar is a fluorescent aromatic hydrocarbon such as diphenylanthracene, LUMO is the lowest, normally unoccupied molecular orbital and HOMO is the highest occupied molecular orbital. 

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Within this framework, by considering the physical situation of the electrode double layer, the free energy of activation of an electron transfer reaction can be identified with the reorganization energy of the solvation sheath around the ion. This idea will be carried through in detail for the simple case of the strongly solvated... 

In our simple model, the expression in A2.4.135 corresponds to the activation energy for a redox process in which only the interaction between the central ion and the ligands in the primary solvation shell is considered, and this only in the fonn of the totally synnnetrical vibration. In reality, the rate of the electron transfer reaction is also infiuenced by the motion of molecules in the outer solvation shell, as well as by other... 

Several processes are unique to ions. A common reaction type in which no chemical rearrangement occurs but rather an electron is transferred to a positive ion or from a negative ion is tenued charge transfer or electron transfer. Proton transfer is also conunon in both positive and negative ion reactions. Many proton- and electron-transfer reactions occur at or near the collision rate [72]. A reaction pertaining only to negative ions is associative detaclunent [73, 74],... 

The discussion thus far in this chapter has been centred on classical mechanics. However, in many systems, an explicit quantum treatment is required (not to mention the fact that it is the correct law of physics). This statement is particularly true for proton and electron transfer reactions in chemistry, as well as for reactions involving high-frequency vibrations. 

The radical cation of 1 (T ) is produced by a photo-induced electron transfer reaction with an excited electron acceptor, chloranil. The major product observed in the CIDNP spectrum is the regenerated electron donor, 1. The parameters for Kaptein s net effect rule in this case are that the RP is from a triplet precursor (p. is +), the recombination product is that which is under consideration (e is +) and Ag is negative. This leaves the sign of the hyperfine coupling constant as the only unknown in the expression for the polarization phase. Roth et aJ [10] used the phase and intensity of each signal to detemiine the relative signs and magnitudes of the... 

Sekiguchi S, Kobori Y, Akiyama K and Tero-Kubota S 1998 Marcus free energy dependence of the sign of exchange interactions in radical ion pairs generated by photoinduced electron transfer reactions J. Am. Chem. Soc. 120 1325-6... 

For a simple electron transfer reaction containing low concentrations of a redox couple in an excess of electrolyte, the potential established at an inert electrode under equilibrium conditions will be governed by the Nemst equation and the electrode will take up the equilibrium potential for the couple 0/R. In temis of... 

A number of different types of experiment can be designed, in which disc and ring can either be swept to investigate the potential region at which the electron transfer reactions occur, or held at constant potential (under mass-transport control), depending on the infomiation sought. 

Boxer S G, Goldstein R A, Lockhart D J, Middendorf T R and Takiff L 1989 Excited states, electron-transfer reactions, and intermediates in bacterial photosynthetic reaction centers J. Rhys. Chem. 93 8280-94... 

Sauer M, Drexhage K H, Lieberwirth U, Muller R, Nerd S and Zander C 1998 Dynamics of the electron transfer reaction between an oxazine dye and DNA oligonucleotides motored on the single-molecule level Chem. Phys. Lett. 284 153-63... 

Electron transfer reactions are conceptually simple. The coupled stmctural changes may be modest, as in tire case of outer-sphere electron transport processes. Otlier electron transfer processes result in bond fonnation or... 

Much of tills chapter concerns ET reactions in solution. However, gas phase ET processes are well known too. See figure C3.2.1. The Tiarjioon mechanism by which halogens oxidize alkali metals is fundamentally an electron transfer reaction [2]. One might guess, from tliis simple reaction, some of tlie stmctural parameters tliat control ET rates relative electron affinities of reactants, reactant separation distance, bond lengtli changes upon oxidation/reduction, vibrational frequencies, etc. 

Figure C3.2.1. A slice tlirough tlie intersecting potential energy curves associated witli tlie K-l-Br2 electron transfer reaction. At tlie crossing point between tlie curves (Afy, electron transfer occurs, tlius Tiarjiooning tlie species,...

A powerful application of outer-sphere electron transfer theory relates the ET rate between D and A to the rates of self exchange for the individual species. Self-exchange rates correspond to electron transfer in D/D (/cjj) and A/A (/c22)- These rates are related through the cross-relation to the D/A electron transfer reaction by the expression... 

Electron transfer reaction rates can depend strongly on tire polarity or dielectric properties of tire solvent. This is because (a) a polar solvent serves to stabilize botli tire initial and final states, tluis altering tire driving force of tire ET reaction, and (b) in a reaction coordinate system where the distance between reactants and products (DA and... 

Early studies showed tliat tire rates of ET are limited by solvation rates for certain barrierless electron transfer reactions. However, more recent studies showed tliat electron-transfer rates can far exceed tire rates of diffusional solvation, which indicate critical roles for intramolecular (high frequency) vibrational mode couplings and inertial solvation. The interiDlay between inter- and intramolecular degrees of freedom is particularly significant in tire Marcus inverted regime [45] (figure C3.2.12)). 

Calculations within tire framework of a reaction coordinate degrees of freedom coupled to a batli of oscillators (solvent) suggest tliat coherent oscillations in the electronic-state populations of an electron-transfer reaction in a polar solvent can be induced by subjecting tire system to a sequence of monocliromatic laser pulses on tire picosecond time scale. The ability to tailor electron transfer by such light fields is an ongoing area of interest [511 (figure C3.2.14). 

Figure C3.2.14. Electron population difference x(t) = - P (0 for tliree electron transfer reactions in the...

Daizadeh I, Guo J-X and Stuchebrukhov A 1999 Vortex structure of the tunneling flow in long-range electron transfer reactions J. Chem. Phys. 110 8865-8... 

The proper quantumdynamical treatment of fast electronic transfer reactions and reactions involving electronically excited states is very complex, not only because the Born-Oppenheimer approximation brakes down but... 

Electron-transfer reactions appear to be inherently capable of producing excited products when sufficient energy is released (154—157). This abiUty may be related to the speed of electron transfer, which is fast relative to atomic motion, so that vibrational excitation is inhibited (158). 

Electron-transfer reactions producing triplet excited states can be diagnosed by a substantial increase in luminescence intensity produced by a magnetic field (170). The intensity increases because the magnetic field reduces quenching of the triplet by radical ions (157). 

Eig. 2. Electron-transfer reaction rate, vs exoergicity of reaction the dashed line is according to simple Marcus theory the soUd line and data poiats are... 

H2 or O2 from water in the presence of a sacrificial reductant or oxidant employ a mthenium complex, typically [Ru(bipy)2], as the photon absorber (96,97). A series of mixed binuclear mthenium complexes having a variety of bridging ligands have been the subject of numerous studies into the nature of bimolecular electron-transfer reactions and have been extensively reviewed (99—102). The first example of this system, reported in 1969 (103), is the Creutz-Taube complex [35599-57-6] [Ru2(pyz)(NH3. 

Photopolymerization. In many cases polymerization is initiated by ittadiation of a sensitizer with ultraviolet or visible light. The excited state of the sensitizer may dissociate directiy to form active free radicals, or it may first undergo a bimoleculat electron-transfer reaction, the products of which initiate polymerization (14). TriphenylaLkylborate salts of polymethines such as (23) ate photoinitiators of free-radical polymerization. The sensitivity of these salts throughout the entire visible spectral region is the result of an intra-ion pair electron-transfer reaction (101). 

Metal oxide electrodes have been coated with a monolayer of this same diaminosilane (Table 3, No. 5) by contacting the electrodes with a benzene solution of the silane at room temperature (30). Electroactive moieties attached to such silane-treated electrodes undergo electron-transfer reactions with the underlying metal oxide (31). Dye molecules attached to sdylated electrodes absorb light coincident with the absorption spectmm of the dye, which is a first step toward simple production of photoelectrochemical devices (32) (see Photovoltaic cells). 

A review of the role of thiols as electron donors in photoinduced electron-transfer reactions has been compiled (49). 

Metal-Catalyzed Oxidation. Trace quantities of transition metal ions catalyze the decomposition of hydroperoxides to radical species and greatiy accelerate the rate of oxidation. Most effective are those metal ions that undergo one-electron transfer reactions, eg, copper, iron, cobalt, and manganese ions (9). The metal catalyst is an active hydroperoxide decomposer in both its higher and its lower oxidation states. In the overall reaction, two molecules of hydroperoxide decompose to peroxy and alkoxy radicals (eq. 5). 

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