Transfer, electron

Electron-Transfer and Electrochemical Reactions Photochemical and Other Energized Reactions 

Contents v. 1-2. The formation of bonds to hydrogen— —v. 15. Electron-transfer and electrochemical reactions photochemical and other energized reactions. 

Collected works. 2. Chemistry, Inorganic—Synthesis— Collected works. I. Zuckerman, Jerry J. 

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Alkali metals can donate electrons to the double bonds yielding anion radicals and positively charged, alkali-metal counterions. This may result either from direct attack of the monomer on the alkali metal, or from attack on the metal through an intermediate compound such as naphthalene. Both result in bifunctional initiation, that is, formation of species with two carbanionic ends. 

Alkali Metals Initiation by direct attack on the alkali metal involves transfer of the loosely held 5 electron from a Group lA metal atom to the monomer. A radical ion (i.e., a species having both ionic and radical centers) is formed  

The initiation process thus results in a bifunctional dicarbanion species capable of propagating at both of its ends. 

Free alkah metals may be employed as solutions in certain ether solvents, in liquid anunonia, or as ne suspensions in inert solvents. The latter are prepared by heating the metal above its melting point in the solvent, stirring vigorously to form an emulsion, and then cooling to obtain a ne dispersion of the metal. 

Alkali Metal Complexes Polycyclic aromatic compoimds can react with alkali metals in ether solution to produce radical ions (Szwarc, 1968). The reaction involves the transfer of an electron 

Anodic and cathodic electron transfer is the elementary process underlying all electrolytic reactions, and is followed by chemical reactions of the intermediates formed in the majority of cases. In cases where the intermediate is stable under the prevailing reaction conditions, anodic or cathodic preparation of radical ions, radicals, or ions is possible. The electrolytic method is here often superior to chemical methods, since work-up is not complicated by the presence of any inorganic redox couple. 

Anodic and cathodic electron transfer is the elementary act in all electrochemical reactions of interest here (atom transfers are possible too, especially in electrocatalytic reactions Piersma and Gileadi, 1966) and results in the formation of radical ions from neutral molecules [(2) and (3)] and neutral radicals from charged species [(4) and (5)]. In the overwhelming majority of cases, radical ions 

Certain aromatic hydrocarbons, such as 9,10-diphenylanthracene, give relatively stable radicals and cation radicals upon electrochemical reduction and oxidation, respectively. If one arranges to have the radical ions from both processes mixed, either by normal DC electrolysis in a suitably designed cell or by using an alternating current for the electrolysis, the phenomenon of electrochemiluminescence appears (Hercules, 1971 McCapra, 1973). 

The physiological pathway of electron transfer in flavocytochrome is from bound lactate to FMN, then FMN to 52-heme, and finally 52-heme to cytochrome c (Fig. 9) (2,11, 80,102). The first step, oxidation of L-lactate to pyruvate with concomitant electron transfer to FMN, is the slowest step in the enzyme turnover (103). With the enzyme from S. cerevisiae, a steady-state kinetic isotope effect (with ferricyanide as electron acceptor) of around 5 was obtained for the oxidation of dl-lactate deuterated at the C position, consistent with the major ratedetermining step being cleavage of the C -H bond (103). Flavocytochrome 52 reduction by [2- H]lactate measured by stopped-flow spectrophotometry resulted in isotope effects of 8 and 6 for flavin and heme reduction, respectively, indicating that C -H bond cleavage is not totally rate limiting (104). 

the first acceptor in the electron-transfer pathway, has been clearly shown to be essential for lactate dehydrogenase activity (105). What is the role of the heme Forestier and Baudras found a linear relationship between heme content and enzyme activity extrapolation to zero heme content indicated zero cytochrome c reductase activity and a lowered ferricyanide reductase activity (106). Thus electron 

The physiological pathway of electron transfer in flavocytochrome 62. Fox, Oxidized FMN F, flavosemiquinone F,ed, reduced FMN H x, oxidized heme Hrej, reduced heme Cyt c, cytochrome c. 

The conversion of L-lactate to pyruvate is a two-electron redox process. One could consider this occurring as two one-electron steps (a radical mechanism) or as one two-electron step. There are two options for a single two-electron step, and these are hydride transfer (H ) or proton (H+) abstraction followed by a two-electron transfer from a carbanion intermediate. These two alternatives for lactate are shown formally in Eqs. (1) and (2) for hydride transfer and the carbanion mechanism, respectively. 

In the case of flavocytochrome 62 and related flavoenzymes, there is a sufficient body of evidence to indicate that the carbanion mechanism operates. The formation of a carbanion is not, of course, an oxidation and two electrons need to be transferred from the carbanion intermediate to the flavin cofactor. This could occur possibly via a covalent intermediate or by sequential one-electron transfers. These possibilities will be discussed in detail later in this section. 

Both intra and inter molecular electron transfer are fundamental processes in electron transport in polymers. This in turn determines the operation of organic optoelectronic devices. Electron transfer between molecular complexes is also an important process in many biological systems. In this section we give a brief review of the nonadiabatic electron transfer that is relevant to electron transport. Since this process is nonadiabatic, the configuration coordinates of the initiai and final electronic states are different. In practice, they are often taken to have the same vaiues namely the position at which the potential energy surfaces cross (called the reaction coordinates), as illustrated in Fig. 9.6. 

In this section we derive expressions for the rates of electron transfer within the Fermi Golden Rule approximation. As we described for exciton transport in Section 9.2.4, these rates can be used to model charge transport using the density matrix formalism. There is a wide and thorough discussion of this topic in May and Kiihn (2000). 

An electrochemical reaction with electron transfer between electrode and electrolyte is called a redox reaction. The simplest case is described by the equation 

This is a one-valent electrode reaction. The stoichiometric number of the electron is one, which is usually assumed for the elementary charge transfer step. Reduced and oxidized components have different solvation numbers. The general kinetic equations vsdll be developed for this reaction. 

In a more general description one can define an electrode reaction with n electrons, whereupon either the elementary electrode reaction occurs n times or there is a more complex situation of multi-step electrode reactions for which there is much treatment in the literatiue.  

In the usual manner of chemical kinetics the reaction rates of the forward (oxidation) and backward (reduction) reactions are defined as 

The reaction rates can be expressed as current densities, an anodic current density for oxidation 

Photoionization (pathway viii, Fig. 1) represents the complete removal of an electron from a molecule as a result of the absorption of light, and it is the analogue of photo dissociation processes, but where the electron and positive ion are the products. 

Intermolecular electron transfer seems normally to require virtually direct contact between the donor and acceptor molecules. According to a widely accepted theory, the rate constant for transfer decreases exponentially with the separation of the donor and acceptor species. In fluids, the internu-clear separation fluctuates with time, so that transfer is dominated by the short-distance events. Under such circumstances, the transfer process can be regarded as a normal bimolecular reaction, for which in a transition-state formulation the rate coefficient, kt, is characterized by a free energy of activation, A G, equated somewhat arbitrarily with the activation energy, a, of the conventional Arrhenius expression  

The efficiency of photochemical processes has been alluded to several times in the preceding sections. It is now appropriate to provide a way of expressing these efficiencies quantitatively. 

the thermodynamics and kinetics of electron-transfer reactions (redox potentials and electron-transfer rates) have steric contributions, and molecular mechanics calculations have been used to identify these. A large amount of data has been assembled on Co /Co couples, and the majority of the molecular mechanics calculations reported to date have dealt with hexaaminecobalt(III/II) complexes. 

The Gibbs free energy ofphotoinduced electron transfer, AetG°, in an excited encounter complex (D -A) can be estimated from Equation 5.1, where Zs°(D + /D) is the standard electrode potential of the donor radical cation, E° A/A ) that of the acceptor A and A/i0 o is the 0 0 excitation energy of the excited molecule (D or A ) that participates in the reaction. 

In the general case, the electrostatic work terms w that account for the Coulombic attraction of reactants (D, A) and products (D +, A ) are w D, A) = zDzA 2/(Ansosgi) and ir(D +, A ) zD + zA e2/(4neoegi), where zD and zA are the charge numbers of donor D and acceptor A prior to electron transfer and zD + and zA are those after electron transfer. For neutral species D and A, zD = zA = 0. Choosing convenient units and collecting the constants we obtain the scaled Equation 5.2. 

Equation 5.2 Scaled equation for the free energy of electron transfer 

Mixing electron donor and acceptor molecules in apolar solution may be accompanied by the formation of weakly associated charge-transfer or electron-donor acceptor(EDA) complexes in the ground state.342 In a simple quantum mechanical treatment, the ground-state wavefunction of EDA complexes may be described as a resonance hybrid of a no-bond ground state (A- D) and a charge-transfer state (A D + ) (Equation 5.3), where 

The charge-transfer excited state is then represented by Equation 5.4, with cl no bond 0 and C dative 1  

The curves in Fig. 11.1b demonstrate that the situation is more complex when more than one isomer or conformer is present in the system. With the three con-formers of [Co(tra s-diammac)]3+/2+ three distinct redox potentials are expected, 

The determination of the structure of the encounter complex (relative orientation of the two reactants) and the ensuing information on the stereoselectivity of the electron transfer is a further possible application of molecular mechanics in this field, but this has not yet been evaluated. 

The voltammogram for a simple oxidative electron transfer process (19)  

The relationship between all these parameters and the current is given by the Butler-Volmer equation (20), 

An alternative theory to the Butler-Volmer theory for electron transfer is provided by the Marcus-Hush theory (Marcus, 1968 Hush, 1968) which assumes a potential-dependent a. Since in most cases a is essentially independent of potential, use of the simpler Butler-Volmer equation is usually adequate. 

Many reactions catalyzed by metaUoenzymes involve electron transfer. On the simplest level, one can consider electron transfer reactions to be complemen- 

Noncomplementary reactions, as shown in equation 1.28, involve unequal numbers of oxidants and reductants because the number of electrons gained or lost by each metal differs. Noncomplementary reactions, especially for large biomolecules, must proceed through a number of bimolecular steps since the possibility of termolecular or higher-order collisions is very small. 

Two types of electron transfer mechanisms are defined for transition metal species. Outer-sphere electron transfer occurs when the outer, or solvent, coordination sphere of the metal centers is involved in transferring electrons. No reorganization of the inner coordination sphere of either reactant takes place during electron transfer. A reaction example is depicted in equation 1.29  

Harry B. Gray and Walther Ellis, writing in Chapter 6 of reference 15, describe three types of oxidation-reduction centers found in biological systems. The first of these, protein side chains, may undergo oxidation-reduction reactions such as the transformation of two cysteine residues to form the cystine dimer as shown in equation 1.30  

Other electron transferases include the rubredoxin and ferredoxin iron-sulfur proteins, so named because they contain iron-sulfur clusters of various sizes. Rubredoxins are found in anaerobic bacteria and contain iron ligated to four cysteine sulfurs. Ferredoxins are found in plant chloroplasts and mammalian tissue and contain spin-coupled [2Fe-2S] clusters. Further discussion of rubredoxin and ferredoxin proteins can be found in Chapters 6 and 7 of reference 15, and cytochromes will be extensively discussed in Chapter 7 of this text. 

This equation corresponds to the situation when the rate of the total electron transfer process is said to be at the diffusion limit, and so is as perfect as possible At another extreme where fc i 2 ET + i[A] Equation (8.113) reduces to 

Specific redox characteristics of a catalyst derived from CV scans are also used to confirm an enzyme s ability for bioelectrocatalysis by either direct electron transfer (DET) or mediated electron transfer (MET) to the electrode. DET and MET are two distinct mechanisms of bioelectrocatalysis. MET has the advantage of being compatible with almost all naturally occurring oxidoreductase enzymes and coenzymes, but it requires additional components (either smaU-molecule redox mediators or redox polymers) because the enzymes cannot efficiently transfer electrons to the electrode. These additional components make the system more complex and less stable [8]. The vast majority of oxidoreductase enzymes that require MET to an electrode are nicotinamide adenine dinucleotide (NAD ) dependent. Two of the most commonly encountered NAD -dependent enzymes in BFC anodes are glucose dehydrogenase (GDH) and alcohol dehydrogenase (ADH). These enzymes have been thoroughly characterized in respect to half-cell electrochemistry and have been demonstrated for operation in BFC. More information about MET can be found in Chapter 9. 

Researchers have also used voltammetry to evaluate enzyme inhibition mechanisms. One of the most commonly studied enzyme inhibition systems is hydrogenases (see Chapter 6), primarily because these enzymes are inhibited by a wide variety of species, and experimental difficulty comes from the inherent instability of the enzymes in low oxygen concentrations [9]. 

Although the redox potential of-OH is very high [E( OI I/OE I ) = +1.9 V (Klan-inget al. 1985) E(-OH, H+/H20) = 2.73 (Wardman 1989)], direct ET is rarely observed in OH-reactions, and where it occurs intermediate complexes are likely to be involved. For example, in its reaction with thiocyanate, where the final product is the three-electron bonded dirhodane radical anion [reaction (31) for other three-electron bonded systems, see Chaps. 5 and 7], a similar three-electron bonded intermediate might precede ET [reactions (29) and (30)]. 

The reaction of OH with thiolate ions, taken as an overall reaction, is an ET reaction [reaction (36)]. One must, however, again take into account that a three-electron bonded intermediate is formed in the first step (Chap. 7). In semi-de-protonated dithiothreitol reaction (32) dominates over the H-abstraction reaction (33) (Akhlaq and von Sonntag 1987), although the rate constant for the reaction of OH with a thiol and a thiolate ion are both diffusion controlled (k = 1.5 x 1010 dm3 mol-1 s 1). This is another example of the potentially high regio-selectivity of OH reactions. 

Although an ET from phenolates is highly exothermic (for reduction potentials Lind et al. 1990 Jonsson et al. 1993) and ET is thermodynamically favored over addition (Lundqvist and Eriksson 2000), the usually preferred mode of reaction is addition rather than ET. Yet, addition and ET are in competition (Tripathi 1998), and, when the ortho- and the para-positions which are the relevant positions of addition for the electrophilic OH are blocked by a bulky substituent [e.g., reaction (34)] ET may become dominant (Table 3.3). Thus, also for these reactions a short-lived tx-complex [cf. reaction (6)] may be postulated as common precursor wherefrom the competition between addition and ET occurs. 

in the oxidation of transition-metal ions, adducts have been established as intermediates [e.g., reaction (35) O Neill and Schulte-Frohlinde 1975 Asmus et al. 1978 for the equilibrium of Tl2+ and OH, see Schwarz and Dodson 1984], 

A similar situation holds for the reaction of OH with Cu2+. The reaction proceeds by the replacement of a water molecule of its solvation shell (Cohen et al. 1990) rather than by ET. In neutral solution, the intermediate formed carries zero charge [reaction (37) Barker and Fowles 1970 Asmus et al. 1978 Ulanski and von Sonntag 2000], and only in more acid solutions more positively charged species start to dominate (Fig. 3.1), but real aqua-Cu3+ may not to be formed to any major extent, because at below pH 3 the reaction becomes increasingly reversible [reaction (40) Meyerstein 1971 Ulanski and von Sonntag 2000]. 

According to the Franck-Condon principle, the photoexcitation triggers a vertical transition to the excited state, which is followed by a rapid nuclear equilibration. Without donor excitation, the electron transfer process would be highly endothermic. However, after exciting the donor, electron transfer occurs at the crossing of the equilibrated excited state surface and the product state. 

The change in Gibbs free energy associated with the electron transfer event is given by the following relation.19 

FIGURE 14. Photo-induced electron transfer process. 

FIGURE 15. Potential energy surfaces for the ground state DA), the excited state DA, reactant state), and the charge-separated state D+-A, product state), proposed by Marcus s theory. X, total reorganization energy TS, transition state. (Modified from Ref. 19.) 

The transmission factor is related to the transition probability (P0) at the intersection of two potential energy surfaces, as given by the Landau-Zener theory.24 

The simple collision theory for bimolecular gas phase reactions is usually introduced to students in the early stages of their courses in chemical kinetics. They learn that the discrepancy between the rate constants calculated by use of this model and the experimentally determined values may be interpreted in terms of a steric factor, which is defined to be the ratio of the experimental to the calculated rate constants Despite its inherent limitations, the collision theory introduces the idea that molecular orientation (molecular shape) may play a role in chemical reactivity. We now have experimental evidence that molecular orientation plays a crucial role in many collision processes ranging from photoionization to thermal energy chemical reactions. Usually, processes involve a statistical distribution of orientations, and information about orientation requirements must be inferred from indirect experiments. Over the last 25 years, two methods have been developed for orienting molecules prior to collision (1) orientation by state selection in inhomogeneous electric fields, which will be discussed in this chapter, and (2) bmte force orientation of polar molecules in extremely strong electric fields. Several chemical reactions have been studied with one of the reagents oriented prior to collision.  

An example of enhanced ion production. The chemical equilibrium exists in a solution of an amine (RNH2). With little or no acid present, the equilibrium lies well to the left, and there are few preformed protonated amine molecules (ions, RNH3+) the FAB mass spectrum (a) is typical. With more or stronger acid, the equilibrium shifts to the right, producing more protonated amine molecules. Thus, addition of acid to a solution of an amine subjected to FAB usually causes a large increase in the number of protonated amine species recorded (spectrum b). 

Szwarc and coworker have studied the interesting and useful polymerizations initiated by aromatic radical-anions such as sodium naphthalene [Szwarc, 1968, 1974, 1983]. Initiation proceeds by the prior formation of the active initiator, the naphthalene radical-anion (XVIII) 

The reaction involves the transfer of an electron from the alkali metal to naphthalene. The radical nature of the anion-radical has been established from electron spin resonance spectroscopy and the carbanion nature by their reaction with carbon dioxide to form the carboxylic acid derivative. The equilibrium in Eq. 5-65 depends on the electron affinity of the hydrocarbon and the donor properties of the solvent. Biphenyl is less useful than naphthalene since its equilibrium is far less toward the anion-radical than for naphthalene. Anthracene is also less useful even though it easily forms the anion-radical. The anthracene anion-radical is too stable to initiate polymerization. Polar solvents are needed to stabilize the anion-radical, primarily via solvation of the cation. Sodium naphthalene is formed quantitatively in tetrahy-drofuran (THF), but dilution with hydrocarbons results in precipitation of sodium and regeneration of naphthalene. For the less electropositive alkaline-earth metals, an even more polar solent than THF [e.g., hexamethylphosphoramide (HMPA)] is needed. 

The naphthalene anion-radical (which is colored greenish-blue) transfers an electron to a monomer such as styrene to form the styryl radical-anion (XIX) 

The styryl radical-anion is shown as a resonance hybrid of the forms wherein the anion and radical centers are alternately on the a- and ])-carbon atoms. The styryl radical-anion dimerizes to form the dicarbanion (XX) 

That this reaction occurs is shown by electron spin resonance measurements, which indicate the complete disappearance of radicals in the system immediately after the addition of monomer. The dimerization occurs to form the styryl dicarbanion instead of - CH2CH4 CH4 CH2 -, since the former is much more stable. The styryl dianions so-formed are colored red (the same as styryl monocarbanions formed via initiators such as n-butyl-lithium). Anionic propagation occurs at both carbanion ends of the styryl dianion 

1 External Factors which Affect Photoinduced Charge Injection 275 

Interfacial Supramolecular Assemblies comprise an electrochemically addressable solid surface functionalized with a film which incorporates molecular components that can be addressed electrochemically or photochemically. In these assemblies, specific bonding interactions exist between the surface and film and they are generally in contact with a solution. Typical of a supramolecular assembly, the individual building blocks retain much of their molecular character, but the overall assembly exhibits new properties, or is capable of performing a specific function beyond that possible when using the individual components. 

This has led to a well-developed biochemistry, and has resulted in a much improved understanding of the properties of enzymes, natural photosynthesis, respiration, etc. These studies revealed that the structure of natural systems is controlled by intermolecular forces and the importance of organization and self-assembly was soon recognized. 

One of the fascinations of scientists has long been the ability of nature to use supramolecular forces to create molecular assemblies for carrying out particular functions. As a result, one of the ultimate aims of supramolecular chemistry is to create molecular devices. 

1 A selection of suitable texts for further reading on this subject are presented at the end of this chapter. 

The electrode reaction may involve the formation of a new phase (e.g. the electrodeposition of metals in plating, refining and winning or bubble formation when the product is a gas) or the transformation of one solid phase to another (e.g. reaction (1.5)). The formation of a new phase is itself a multistep process requiring both nucleation and subsequent growth, and crystal growth may involve both surface diffusion and lattice growth. 

Since electrode reactions commonly involve the transfer of several electrons, the complications (a)—(c) can occur sandwiched between as well as preceding or following electron transfer. Moreover very complex situations do arise. Thus, for example, reaction (1.5) is likely to involve electron transfer, diffusion, chemical reactions (protonation and hydration equilibria as well as sulphation), phase transformation and adsorbed intermediates In this chapter, however, we shall take the approach of considering each fundamental type of process in turn. The equations that will arise must be regarded as idealistic and simplistic but will generally be sufficient for us to understand most cells in industrial practice provided we can recognize which of the fundamental steps in the overall electrode processes predominantly determine the cell characteristics. 

1 ELECTRON TRANSFER We shall discuss the electrode reaction 

As with any chemical process, it is necessary to consider both the thermodynamics and the kinetics of the electrode process. If we connect the two electrodes and monitor the cell potential while allowing no current to flow, the potential of the working electrode will eventually reach a steady-state value indicating that the cell 

While no current is flowing and there is no net chemical change in the cell, there must be a dynamic equilibrium at the surface of the WE, i.e. the reduction of 0 and oxidation of R are both occurring but the processes are of equal rate 

In this section, the thermodynaniics and kinetics of the electrode reaction  

As with any chemical process, it is logical first to consider the thermodyi mics. Suppose the potential of the working electrode vs. the reference electrode is monitored white no current is allowed to flow. Under these circumstances no chemical change can occur at the surface and the solution composition will remain unchanged and uniform. The working electrode will take up its equilibrium (or reversible) potential which may also be calculated from the Nemst equation  

Strictly this equation should be written in terms of the activities of O and R but for simplicity it will be assumed that the ratio of activity coefficient is L It should be recognized, however, that in many industrial cells with high concentrations of electroactive species and maybe no additional electrolyte, such an assumption would introduce an error and it would be better to include the appropriate activity coefficients or fugacities (section L8). Also in general, the Nernst equation should be written in terms of activities or coimntrations at the electrode surface (cq and but throughout this section it is also assumed that the currents are low enough for the surface and bulk concentrations to be essentially the same. Certainly, when no current flows, -no approximation is involved. 

At the equilibrium potential, although no net current is observed, there will be a dynamic equilibrium at the electrode surface. Both reduction of O and oxidation of R will be taking place, but these processes will have an equal rate so 

The equilibrium potential . and the exchange current density /q together totally characterize the equilibrium situation at an electrode. 

Now it is necessary to formulate the equations which describe the kinetics of an electron transfer reaction such as reaction (1.19). It is normal to assume that electron-transfer processes are first-order reactions and then the rate of reduction of O depends only on a rate constant and the concentration of O at the site of electron transfer (at the electrode surface). As noted above, only situations where the bulk and surface concentrations are similar will be considered for the present. This is equivalent to assuming that mass transport plays no role in determining the overall rate (see p. 30). Then we may write  

The second term of Eq. (38) represents the difference between the redox scales in the two solvents, a and /5. Indeed, we have 

One of the original aspects of electron transfer reactions at liquid-liquid interfaces is that, contrary to what happens in bulk solutions, a strong reducing agent in one phase can coexist in contact with an easily reducible species in the other phase, if the Galvani potential difference between the two phases is such that the Gibbs energy for equilibrium is positive. 

Very few experimental observations, or electrochemical studies, of electron transfer across a liquid-liquid interface have been reported. This paucity is due to the difficulties associated with the choice of systems for which the Gibbs energy of transfer of both the reactants and products are well known, so as to ensure that the measured currents are due to electron transfer and not ion transfer. Other difficulties, associated with the nature of the supporting electrolytes often used in the organic phase, also hindered early progress in this area (e.g., tetraphenylarsonium can oxidize ferrocene, or tetraphenylborate can be easily oxidized thereby reducing the size of the potential window). The pioneering work of Samec et. was based on the oxidation of ferrocene in the 

Geblewicz and Schiffrin have studied the system [Fe(CN)6] / in water-Lutetium biphthalocyanine in 1,2-dichloroethane, and very recently Cheng and Schiffrin investigated the systems [Fe(CN)6] / in water-bis(pyridine) mejo-tetraphenylporphyrinato iron(II) and ruthenium(III) in 1,2-dichloroethane. These systems have the advantage that none of the products of the reaction would cross the interface, thereby impeding the measurements. 

Kihara et investigated a series of redox couples and analyzed their data using the steady-state current-potential expression derived by Samec  

Mannich-type reactions, an organocatalyst such as pyrrolidine/trifluoroacetic acid (TFA) or L-proline was employed to assist in the additions of ketones. 

A = electron acceptor D = electron donor CB - conductance band VB = valence band 

Konig and co-workers also studied CdS-mediated coupling of benzylic alcohols, ethers, and amines. Using high-power blue LEDs, the authors were 

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SAMs are generating attention for numerous potential uses ranging from chromatography [SO] to substrates for liquid crystal alignment [SI]. Most attention has been focused on future application as nonlinear optical devices [49] however, their use to control electron transfer at electrochemical surfaces has already been realized [S2], In addition, they provide ideal model surfaces for studies of protein adsorption [S3]. 

Metal to ceramic (oxide) adhesion is very important to the microelectronics industry. An electron transfer model by Burlitch and co-workers [75] shows the importance of electron donating capability in enhancing adhesion. Their calculations are able to explain the enhancement in adhesion when a NiPt layer is added to a Pt-NiO interface. 

Much use has been made of micellar systems in the study of photophysical processes, such as in excited-state quenching by energy transfer or electron transfer (see Refs. 214-218 for examples). In the latter case, ions are involved, and their selective exclusion from the Stem and electrical double layer of charged micelles (see Ref. 219) can have dramatic effects, and ones of potential imfKntance in solar energy conversion systems. 

Electrochemistry is concerned with the study of the interface between an electronic and an ionic conductor and, traditionally, has concentrated on (i) the nature of the ionic conductor, which is usually an aqueous or (more rarely) a non-aqueous solution, polymer or superionic solid containing mobile ions (ii) the structure of the electrified interface that fonns on inunersion of an electronic conductor into an ionic conductor and (iii) the electron-transfer processes that can take place at this interface and the limitations on the rates of such processes. 

At low currents, the rate of change of die electrode potential with current is associated with the limiting rate of electron transfer across the phase boundary between the electronically conducting electrode and the ionically conducting solution, and is temied the electron transfer overpotential. The electron transfer rate at a given overpotential has been found to depend on the nature of the species participating in the reaction, and the properties of the electrolyte and the electrode itself (such as, for example, the chemical nature of the metal). 

At higher current densities, the primary electron transfer rate is usually no longer limiting instead, limitations arise tluough the slow transport of reactants from the solution to the electrode surface or, conversely, the slow transport of the product away from the electrode (diffusion overpotential) or tluough the inability of chemical reactions coupled to the electron transfer step to keep pace (reaction overpotential). 

Examples of the lader include the adsorption or desorption of species participating in the reaction or the participation of chemical reactions before or after the electron transfer step itself One such process occurs in the evolution of hydrogen from a solution of a weak acid, HA in this case, the electron transfer from the electrode to die proton in solution must be preceded by the acid dissociation reaction taking place in solution. 

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... 

Similarly, changes must take place in the outer solvation shell diirmg electron transfer, all of which implies that the solvation shells themselves inliibit electron transfer. This inliibition by the surrounding solvent molecules in the iimer and outer solvation shells can be characterized by an activation free energy AG. ... 

The point at which electron transfer takes place clearly corresponds to the condition equating equations (A2.4.132) for the states and/we find that... 

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. 

Many of the most interesting current developments in electronic spectroscopy are addressed in special chapters of their own in this encyclopedia. The reader is referred especially to sections B2.1 on ultrafast spectroscopy. Cl.5 on single molecule spectroscopy, C3.2 on electron transfer, and C3.3 on energy transfer. Additional topics on electronic spectroscopy will also be found in many other chapters. 

Zhong Y and McHale J L 1997 Resonance Raman study of solvent dynamics in electron transfer. II. Betaine-30 in... 

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... 

Utilizing FT-EPR teclmiques, van Willigen and co-workers have studied the photoinduced electron transfer from zinc tetrakis(4-sulfonatophenyl)porphyrin (ZnTPPS) to duroquinone (DQ) to fonn ZnTPPS and DQ in different micellar solutions [34, 63]. Spin-correlated radical pairs [ZnTPPS. . . DQ ] are fomied initially, and the SCRP lifetime depends upon the solution enviromnent. The ZnTPPS is not observed due to its short T2 relaxation time, but the spectra of DQ allow for the detemiination of the location and stability of reactant and product species in the various micellar solutions. While DQ is always located within the micelle, tire... 

Figrue BE 16.20 shows spectra of DQ m a solution of TXlOO, a neutral surfactant, as a function of delay time. The spectra are qualitatively similar to those obtained in ethanol solution. At early delay times, the polarization is largely TM while RPM increases at later delay times. The early TM indicates that the reaction involves ZnTPPS triplets while the A/E RPM at later delay times is produced by triplet excited-state electron transfer. Calculation of relaxation times from spectral data indicates that in this case the ZnTPPS porphyrin molecules are in the micelle, although some may also be in the hydrophobic mantle of the micelle. Furtlier,... 

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... 

Levanon H and Mobius K 1997 Advanced EPR spectroscopy on electron transfer processes in photosynthesis and biomimetic model systems Ann. Rev. Biophys. Biomol. Struct. 26 495-540... 

Levstein P R and van Willigen H 1991 Photoinduced electron transfer from porphyrins to quinones in micellar systems an FT-EPR study Chem. Phys. Lett. 187 415-22... 

Electrode processes are a class of heterogeneous chemical reaction that involves the transfer of charge across the interface between a solid and an adjacent solution phase, either in equilibrium or under partial or total kinetic control. A simple type of electrode reaction involves electron transfer between an inert metal electrode and an ion or molecule in solution. Oxidation of an electroactive species corresponds to the transfer of electrons from the solution phase to the electrode (anodic), whereas electron transfer in the opposite direction results in the reduction of the species (cathodic). Electron transfer is only possible when the electroactive material is within molecular distances of the electrode surface thus for a simple electrode reaction involving solution species of the fonn... 

The nature of electrode processes can, of course, be more complex and also involve phase fonnation, homogeneous chemical reactions, adsorption or multiple electron transfer [1, 2, 3 and 4],... 

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... 

Cyclic voltammetry provides a simple method for investigating the reversibility of an electrode reaction (table Bl.28.1). The reversibility of a reaction closely depends upon the rate of electron transfer being sufficiently high to maintain the surface concentrations close to those demanded by the electrode potential through the Nemst equation. Therefore, when the scan rate is increased, a reversible reaction may be transfomied to an irreversible one if the rate of electron transfer is slow. For a reversible reaction at a planar electrode, the peak current density, fp, is given by... 

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