The Mechanism of Electron Transfer

There are two ways of describing the transfer of an electron from its site in the solvent into the orbitals of a substrate. One is the classical mechanism, which involves an overlap of the orbitals of the substrate and those of e, and the formation of a transition state followed by a rapid electron transfer. The other is a non-classical electron tunnelling 

The tunnelling process evidently may have a transmission coefficient smaller than unity, especially when there is insufficient orbital overlap between e q and the acceptor orbital (geometrical hindrance or thickness of barrier). In such cases, the transmission coefficient depends strongly on the gain in free energy on electron transfer AO). When there is a 

The tunnelling mechanism has been strongly indicated when many diffusion-controlled reactions have been examined quantitatively (Anbar and Hart, 1968). Of a large number of diffusion-controlled reactions examined, over 80% exhibited a rate in excellent agreement with that predicted by the Smoluchovsky-Debye formula (Debye, 1942) 

The diffusion coefficient of the hydrated electron Z) w. = 4-7 cm2 sec-1 was determined from its ionic mobility (Schmidt and Buck, 1966). The radius of e-, rei r = 2-5 A was assumed on the basis of theoretical predictions. Dx and rx are the diffusion coefficient and the van der Waals radius of the substrate molecule. 

The dependence of the reactivity on AO°, which is expected from both mechanisms, may help to understand the good correlation with Hammett and Taft s a functions. These may be, therefore, regarded as a measure of the effect of different substituents on the overall electron affinities of organic molecules in aqueous solution. The latter conclusion, if accepted and verified, may be regarded as a major contribution of e q reactions to physical organic chemistry. 

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N2 recognized as a bridging ligand in ((NH3)5RuN2Ru(NH3)5] by D. F. Harrison, E. Weissterger, and H. Taute. (H. Taute, 1983 Nobel Prize for chemistry for his work on the mechanisms of electron transfer reactions especially in metal complexes ). 

The main goal of the present study was to discover new ecl-ac-tive complexes. But the first example may demonstrate that complexes known to show eel can serve to gain more insight into the mechanism of electron transfer processes. 

Nowadays, studies of direct electrochemistry of redox proteins at the electrodesolution interface have held more and more scientists interest. Those studies are a convenient and informative means for understanding the kinetics and thermodynamics of biological redox processes. And they may provide a model for the study of the mechanism of electron transfer between enzymes in biological systems, and establish a foundation for fabricating new kinds of biosensors or enzymatic bioreactors. 

Marcus theory is based on certain assumptions that will be discussed later. The main goal of computer simulations of electron transfer is to check some of these assumptions and to provide additional microscopic insight into the mechanism of electron transfer and the microscopic factors that influence the rate of transfer. We discuss these issues in the following section for the simple case of outer-sphere electron transfer reactions. 

At this point it is necessary to consider the mechanism of electron-transfer luminescence in solutions which cannot involve ion-radical annihilation because both cation and anion of the fluorescer are not formed. Such emission can be achieved by treating anion radicals with chemical oxidants or electrochemically under conditions where the corresponding cation cannot be produced, and it may also be achieved by electrochemical reduction of cations without producing the corresponding anion. In addition to triplets, three types of processes and pathways have been proposed to help explain why such emission occurs. These may be described as (7) impurities, (2) ion-radical aggregates, and (5) heterogeneous electron transfer. It is evident63 that impurities,... 

Harry Gray Two points in Prof. Taube s paper quoted as experiments in progress haven t been mentioned. Both are concerned with the mechanism of electron transfer, because the transmission in the ligand, wherever the attack is, is through the 7r-system, and in cobalt(III) in the detectable radical ion intermediate, because of the improbability of resonance transfer from tt to electron resonance experiment in which one tries the reduction by chromous and looks for the ESR signal of the radical ion. 

Shiga, K., Tollin, G. Studies on the mechanism of electron transfer in flavodoxins. In Flavins and flavoproteins (Singer, T. P. ed.) pp. 422-433. Amsterdam, Elsevier 1976... 

The mechanism of electron transfer over the long distances (of the order of 1000 pm or more) necessitated by the large size of redox enzymes is one that is not completely clear despite much current study. These transfers are critical whether one is considering the photosynthetic center (page 917) or electron carriers such as the... 

It might be assumed that there would be little to study in the mechanism of electron transfer—that the reducing agent and the oxidizing agent would simply bump into each other and electron transfer would take place. Reactions in solution are complicated, however, by the fact that the oxidized and reduced species are often metal ions surrounded by shields of ligands and solvating molecules. Electron transfer reactions... 

An argument in favour of the correctness of the values of R calculated with the help of eqn. (26) is a correlation detected in ref. 28 between the values of R found in liquid solutions and the values of Rtatt 10 1 s found in solid solutions for the same acceptors (Fig. 25). Such a correlation must exist if the mechanism of electron transfer in either case is tunneling. The lower values of R, as compared with those of Rt, for the reactions of eaq and e(j with the same acceptor are accounted for in a natural way in terms of the tunneling mechanism by the difference of the characteristic times during which there occurs a tunneling. In liquids the characteristic time is the time of a diffusion jump at T = 300 K, 10 10 s, in solids, it is the time between the end of irradiation and the measurement of the radiation yield of et r, t 103s. 

Electron transfer from the excited states of Fe(II) to the H30 f cation in aqueous solutions of H2S04 which results in the formation of Fe(III) and of H atoms has been studied by Korolev and Bazhin [36, 37]. The quantum yield of the formation of Fe(III) in 5.5 M H2S04 at 77 K has been found to be only two times smaller than at room temperature. Photo-oxidation of Fe(II) is also observed at 4.2 K. The actual very weak dependence of the efficiency of Fe(II) photo-oxidation on temperature points to the tunneling mechanism of this process [36, 37]. Bazhin and Korolev [38], have made a detailed theoretical analysis in terms of the theory of radiationless transitions of the mechanism of electron transfer from the excited ions Fe(II) to H30 1 in solutions. In this work a simple way is suggested for an a priori estimation of the maximum possible distance, RmSiX, of tunneling between a donor and an acceptor in solid matrices. This method is based on taking into account the dependence... 

The present chapter discusses briefly modern ideas on the mechanisms of electron transfer during photosynthesis and the experimental data pointing to the important part played by electron tunneling reactions in the operation of the reaction centres of photosynthesizing systems. 

EVOLUTION OF THE IDEAS ABOUT THE MECHANISMS OF ELECTRON TRANSFER IN BIOLOGICAL SYSTEMS... 

The high conformational mobility of porphyrin-quinone compounds with flexible bonding makes it difficult to elucidate in sufficient detail the mechanism of electron transfer between porphyrin and quinone fragments. Far greater possibilities for determining the role of mutual orientation of P and Q and the distance between them are offered by P-Q compounds in which the P and Q fragments are linked by several bridges. A P-Q compound of this... 

These quotes were chosen to introduce this chapter on chemically modified electrodes because they are from some of the earliest papers in the field and because they review the concepts and objectives of this research area. We learn that the field of chemically modified electrodes involves attaching specific molecules to the surfaces of conventional inert electrodes. We also discover the two major reasons for wanting to attach molecules to electrode surfaces. As explained by Lane and Hubbard, one objective is to obtain fundamental information about the mechanism of electron transfer at electrode surfaces. The second objective, as expressed by Watkins et al. and Elliott and Murray, is to impart to the electrode surface some chemical specificity not available at the unmodified electrode. For example, the modified electrode might catalyze a specific chemical reaction. Alternatively, the modified electrode might be able to recognize a specific molecule present in a contacting solution phase. 

A large number of investigations of the mechanism of electron transfer reactions of macromolecule-metal complexes in biological systems has been reported. These investigations were concerned with not only natural metalloenzymes such as cytochromes, ferredoxin, blue coppers, oxygenase, peroxidase, catalase, hemoglobin, and ruberodoxin, but also modified metalloenzymes 47). 

Studies of such systems provided a better understanding of the mechanism of electron transfer processes in general. This reaction type is also the basis of almost any type of natural or artificial photosynthesis. Hence it is not surprising that many investigations have been devoted to excited state electron transfer reactions. On the contrary, the reversal of excited state electron transfer has found much less attention although it is certainly not less interesting. 

The use of ac electrolysis in all its variations is certainly an interesting and valuable technique for study of the mechanism of electron transfer reactions. The generation of a short-lived redox pair as chemical intermediates is an important feature of the ac electrolysis. In the future it may even be developed to synthetic applications irrespective of the mechanistic details. In some cases it could be a convenient alternative to photochemical reactions. In other cases it represents a new reaction type which has no precedent. 

A basic understanding of the electronic structures of iron bearing clays and oxides is needed before one can understand the mechanisms of electron transfer and photochemical reactions associated with these minerals. This chapter will discuss the electronic structures of iron bearing clays and oxides (primarily from cluster molecular orbital calculations) and compare theoretical results with experiment. The discussion will be... 

Electrochemical studies on SAMs have proven invaluable in elucidating the impact of various molecular parameters such as bridge structure, molecular orientation or the distance between the electroactive species and electrode surface. As described above in Section 5.2.1, the kinetics of heterogeneous electron transfer have been studied as a function of bond length for many systems. Similarly, the impact of bridge structure and inter-site distances have been studied for various supramolecu-lar donor-acceptor systems undergoing photoinduced electron transfer in solution. In both types of study, electron transfer is observed to increase as the distance between the donor and acceptor decreases. As discussed earlier in Chapter 2, the functional relationship between the donor-acceptor distance and the electron transfer rate depends on the mechanism of electron transfer, which in turn depends on the electronic nature of the bridge. 

In recent years, there has been a great deal of interest in the mechanisms of electron transfer processes.52-60 It is now recognized that oxidation-reduction reactions involving metal ions and their complexes are mainly of two types inner-sphere (ligand transfer) and outer-sphere (electron transfer) reactions. Prototypes of these two processes are represented by the following reactions. 

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