Historical development electron-transfer

Chromium(II) is a very effective and important reducing agent that has played a significant and historical role in the development of redox mechanisms (Chap. 5). It has a facile ability to take part in inner-sphere redox reactions (Prob. 9). The coordinated water of Cr(II) is easily replaced by the potential bridging group of the oxidant, and after intramolecular electron transfer, the Cr(III) carries the bridging group away with it and as it is an inert product, it can be easily identified. There have been many studies of the interaction of Cr(II) with Co(III) complexes (Tables 2.6 and 5.7) and with Cr(III) complexes (Table 5.8). Only a few reductions by Cr(II) are outer-sphere (Table 5.7). By contrast, Cr(edta) Ref. 69 and Cr(bpy)3 are very effective outer-sphere reductants (Table 5.7). 

In order to understand the principles involved in electron-transfer catalysis and also in order to appreciate the historical development of the subject, we must treat hole catalysis and electron transfer between metal atoms and ions and organic substrates before examining catalytic reactions in more detail. This review is intended to cover the basic principles involved in these three areas and to provide a conceptual framework for electron-transfer catalysis. 

Historically, the potential sweep technique and cyclic voltammetry were developed for analysis (as successors to polarography) and much of the theoretical development is concerned with the situation under conditions of diffusion control, for that is where the analytical applications are most readily made. In many of these approaches, the underlying assumption is that the electron transfer that must necessarily occur at the interface is a fast process and plays little part in determining the dependence of the observed current upon potential or upon the concentration of the reactant. However, these assumptions may not always apply. 

Historically, the development of ET theory has been based on inorganic systems, in which the (metal-ion) redox centers are surrounded by coordinated ligands [52]. In those cases in which no new metal-ligand bonds are formed or bond breakage is observed, the interaction between the redox centers is weak (usually Hda < 200 cm-1), and such reactions are conventionally designated as outer-sphere (OS) electron transfer [52, 53]. 

The importance of Marcus theoretical work on electron transfer reactions was recognized with a Nobel Prize in Chemistry in 1992, and its historical development is outlined in his Nobel Lecture.3 The aspects of his theoretical work most widely used by experimentalists concern outer-sphere electron transfer reactions. These are characterized by weak electronic interactions between electron donors and acceptors along the reaction coordinate and are distinct from inner-sphere electron transfer processes that proceed through the formation of chemical bonds between reacting species. Marcus theoretical work includes intermolecular (often bimolecular) reactions, intramolecular electron transfer, and heterogeneous (electrode) reactions. The background and models presented here are intended to serve as an introduction to bimolecular processes. 

This order is the reverse of that of the historical development, which began with the problem of electron transfer the theory was later adapted to proton transfer and subsequently to group transfer (see Sections 8.1 seq., 8.3). 

In this paper we shall begin by a short historical overview of the different theories of electron transfer. This overview will be of course limited, in order to emphasize the physical principles involved in electron transfer. For additional details, exhaustive reviews of the different theoretical treatments can be found in refs. Moreover, we shall restrict ourselves to theories of quantum mechanical nature. Thus, stochastic models (cf for instance the Kramers model) are not discussed here, but they are treated elsewhere in this book. We shall then focus on recent developments in intramolecular electron transfer and its solvent influence. 

Electron transfers are another prototype of chemical reactions as paradigmatic as H-atom or proton-transfer reactions. Furthermore, these are chemical processes ubiquitous in chemistry and biology. They are important from a historical point of view in the development of chemical kinetics, and are scientifically relevant in many physical, chanical, biological and technological processes. 

The theoretical description of photochemistry is historically based on the diabatic representation, where the diabatic models have been given the generic label desorption induced by electronic transitions (DIET) [91]. Such theories were originally developed by Menzel, Gomer and Redhead (MGR) [92,93] for repulsive excited states and later generalized to attractive excited states by Antoniewicz [94]. There are many mechanisms by which photons can induce photochemistry/desorption direct optical excitation of the adsorbate, direct optical excitation of the metal-adsorbate complex (i.e., via a charge-transfer band) or indirectly via substrate mediated excitation (e-h pairs). The differences in these mechanisms lie principally in how localized the relevant electron and hole created by the light are on the adsorbate. 

Historically, the field of electronic structure calculations has seen two largely independent lines of development on the one hand molecular quantum chemists have based most work on wavefunction techniques (the Hartree Fock [2, 3] and post-HF theories [4]) on the other, condensed-matter physicists had their reference method in Density Functional Theory (DFT) [5-7]. This formal division between molecular and solid-state communities has been due to the poor transferability of the standard computational methods between the two fields early DFT functionals underperform post-HF techniques in reproducing the known properties of small molecules... 

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