Electron transfer fundamental concepts

Kavarnos, G. J. Fundamental Concepts of Photoinduced Electron Transfer. 156, 21-58 (1990). 

CNT-based inorganic hybrid materials are part of carbon-based inorganic hybrid materials as anodic electrodes in LIBs. The concept has been proven to be successful at least at laboratory scale, and is promising as a potential alternative to replace graphite-based anodes. However, little is known about the interface structure between CNT and the supported active materials, and thus the electron transfer between the two components. More detailed fundamental research on the interface and interaction between CNTs and active materials at atomic level is needed for a better understanding of the abovementioned improvement. 

The electrical contact of redox proteins is one of the most fundamental concepts of bioelectronics. Redox proteins usually lack direct electrical communication with electrodes. This can be explained by the Marcus theory16 that formulates the electron transfer (ET) rate, ket, between a donor-acceptor pair (Eq. 12.1), where d0 and d are the van der Waals and actual distances separating the donor-acceptor pair, respectively, and AG° and X correspond to the free energy change and the reorganization enery accompanying the electron transfer process, respectively. 

In the last two decades, much has been learned about fundamental aspects of electron transfer in organic and inorganic systems in homogeneous solution. More recendy, the attention of many laboratories has been attracted by the extraordinary potential applications of these fundamental concepts for building real devices which operate on a molecular level. 

Kavamos GJ (1990) Fundamental Concepts of Photoinduced Electron Transfer. 156 21-58 Kelly JM, see Kirsch-De-Mesmaeker A (1996) 177 25-76 Kerr RG, see Baker BJ (1993) 167 1-32 KhairutdinovRF,see ZamaraevKI (1992) 163 1-94... 

The stoichiometric calculations of Chapters 12 and 13 are based on the mole as the fundamental chemical unit in reactions. An alternative method of calculation utilizes the equivalent as a fundamental chemical unit. There are two kinds of equivalents, the type depending on the reaction in question we shall refer to them as acid-base equivalents (or simply as equivalents) and electron-transfer equivalents (or E-T equivalents). The concept of an equivalent is particularly useful when dealing with complex or unknown mixtures, or when working out the structure and properties of unknown compounds. In addition, it emphasizes a basic characteristic of all chemical reactions that is directly applicable to all types of titration analyses. 

If nothing else has been accomplished, it should be very clear from Chapters 3 to 5 that there is an extraordinary number of finite current electroana-lytical techniques. There is no doubt that this can cause considerable confusion for novices. Fortunately, all of these methods are based on relatively few fundamental concepts. It must be understood that (1) electron transfer rates and equilibrium constants vary with potential, (2) mass transport to an electrode surface is precisely defined and reproducible, and (3) the charge required to establish an electrode potential can be temporally distinguished from that utilized by a redox couple. These concepts are addressed in Chapter 2. Now that we have covered the more important electrochemical techniques, it is strongly recommended that Chapter 2 be reviewed with these techniques in mind. 

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. 

Chemisorption [9] is an adsorptive interaction between a molecule and a surface in which electron density is shared by the adsorbed molecule and the surface. Electrochemical investigations of molecules that are chemisorbed to electrode surfaces have been conducted for at least three decades. Why is it, then, that the papers that are credited with starting the chemically modified electrode field (in 1973) describe chemisorption of olefinic substances on platinum electrodes [10,11] What is it about these papers that is different from the earlier work The answer to this question lies in the quote by Lane and Hubbard at the start of this chapter. Lane and Hubbard raised the possibility of using carefully designed adsorbate molecules to probe the fundamentals of electron-transfer reactions at electrode surfaces. It is this concept of specifically tailoring an electrode surface to achieve a particularly desired goal that distinguishes this work from the prior literature on chemisorption, and it is this concept that launched the chemically modified electrode field. 

II. Fundamental concepts of catalysis on photoinduced electron transfer 110... 

Despite these apparent difficulties, there are now a number of examples for photoinduced electron transfer reactions that are significantly catalyzed. It is the purpose of this chapter to present fundamental concepts and the application of catalysis of photoinduced electron transfer reactions. The photochemical redox reactions, which would otherwise be unlikely to occur, are made possible to proceed efficiently by the catalysis on the photoinduced electron transfer steps. First, the fundamental concepts of catalysis on photoinduced electron transfer are presented. Subsequently, the mechanistic viability is described by showing a number of examples of photochemical reactions that involve catalyzed electron transfer processes as the ratedetermining steps. 

II. FUNDAMENTAL CONCEPTS OF CATALYSIS ON PHOTOINDUCED ELECTRON TRANSFER... 

This brief review attempts to summarize the salient features of chemically modified electrodes, and, of necessity, does not address many of the theoretical and practical concepts in any real detail. It is clear, however, that this field will continue to grow rapidly in the future to provide electrodes for a variety of purposes including electrocatalysis, electrochromic displays, surface corrosion protection, electrosynthesis, photosensitization, and selective chemical concentration and analysis. But before many of these applications are realized, numerous unanswered questions concerning surface orientation, bonding, electron-transfer processes, mass-transport phenomena and non-ideal redox behavior must be addressed. This is a very challenging area of research, and the potential for important contributions, both fundamental and applied, is extremely high. 

Several of the key issues are reflected in the debate over the appropriate use of pe to describe redox conditions in natural waters (129-131). The parameter is defined in terms of the activity of solvated electrons in solution (i.e., pe = - log e ), but the species e aq does not exist under environmental conditions to any significant degree. The related concept of pe (132), referring to the activity of electrons in the electrode material, may have a more realistic physical basis with respect to electrode potentials, but it does not provide an improved basis for describing redox transformations in solution. The fundamental problem is that the mechanisms of oxidation and reduction under environmental conditions do not involve electron transfer from solution (or from electrode materials, except in a few remediation applications). Instead, these mechanisms involve reactions with specific oxidant or reductant molecules, and it is these species that define the half-reactions on which estimates of environmental redox reactions should be based. 

Electron transfer was interpreted in Ref. [54] in terms of the nonradiative decay process [20-24,44]. For an up-to-date review of theoretical works on electron transfer see the relevant chapter in this volume (R. A. Marcus — Recent developments in fundamental concepts of PET in biological systems). 

The molecular concept has become so central in chemistry that understanding of chemical events is commonly assumed to consist of relating experimental observations to micro events at the molecular level, which means changes in molecular structure. In this sense molecular structure is a fundamental theoretical concept in chemistry. As the micro changes are invariably triggered by electron transfer, the correct theory at the molecular level must be quantum mechanics. It is therefore surprising that a quantum theory of molecular structure has never developed. This failure stems from the fact that physics and chemistry operate at different levels and that grafting the models of physics onto chemistry produces an incomplete picture. 

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