Single-electron-transfer processes description

However, there is evidence that reactions of aluminium hydride produced in situ involve single-electron-transfer (SET) processesThe reactions described by Trost and Ghadiri have most likely not been studied in sufficient detail to permit an adequate description of the reaction mechanism to be given at this stage. It is, however, quite likely that the Grignard reactions catalyzed by copper(II) and nickel(II) complexes , as developed by julia - and by Masaki , do involve SET processes, although, if this is so, the preservation of stereochemistry in some of the examples described by these workers is quite remarkable. (In this context, the reader s attention is drawn to Reference 196, end of this section.)... 

This conception of an 8, 2 reaction as an electron-shift process is obviously equivalent to its conception as an inner sphere electron transfer, i.e. a single electron transfer concerted with the breaking of the R—X bond and the formation of the R—Nu bond. Faced with an experimental system, however, the first question—ET or 8 2 —still remains, whatever intimate description of the 8, 2 reaction one may consider most appropriate. If this is thought of in terms of inner sphere electron transfer, the question thus raised is part of the more general problem of distinguishing outer sphere from inner sphere electron-transfer processes (Lexa et ai, 1981), an actively investigated question in other areas of chemistry, particularly that of coordination complex chemistry (Taube, 1970 Espenson, 1986). 

Taking a single parameter q to describe the electron transfer process in a solvent is certainly a crude simplification. Actually there are billions of variables in the game describing the degrees of freedom of the water molecules in the first and further hydration shells. One of the important steps toward successful description of the electron transfer reaction was the Marcus postulate. that... 

A semi-quantitative description of the core level spectrum and the charge-transfer process can be obtained from a simple two-level MOLCAO-model based on the sudden approximation 155 157,160). Here, we follow the formulation of Larsson157) and consider the influence of a core hole on a single electron in an MO formed by linear combination of AO s Ul and uM centred on the ligands (L) and the central metal ion (M). In the ground state, before ionization, the electron is in a bonding orbital... 

However it turned out that the structural, chemical and dynamical details are essential for complex descriptions of long-range proton transport. These parameters appear to be distinctly different for different families of compounds, preventing proton conduction processes from being described by a single model or concept as is the case for electron transfer reactions in solutions (described within Marcus theory [23]) or hydrogen diffusion in metals (incoherent phonon assisted tunneling [24]). 

An Sn2 reaction, depicted in equation 1, seems to come about by an electron pair on the nucleophile displacing a second electron pair—the R-X a bond. Four valence electrons appear to be involved. The problem with this representation is that the electronic rearrangement seems radically different from that in a SET process, such as the initiation step of the SRN1 process (7), equation 2. Clearly, just a single electron has been transferred from the nucleophile to RX. As a consequence of these quite different descriptions, the relationship between the two processes becomes obscure. What factors encourage one pathway over the other is not clear. 

Besides its practical importance, photodissociation — especially of small polyatomic molecules — provides an ideal opportunity for the study of molecular dynamics on a detailed state-to-state level. We associate with molecular dynamics processes such as energy transfer between the various molecular modes, the breaking of chemical bonds and the creation of new ones, transitions between different electronic states etc. One goal of modern physical chemistry is the microscopical understanding of molecular reactivity beyond purely kinetic descriptions (Levine and Bernstein 1987). Because the initial conditions can be well defined (absorption of a single monochromatic photon, preparation of the parent molecule in selected quantum states), photodissociation is ideally suited to address questions which are unprecedented in chemistry. The last decade has witnessed an explosion of new experimental techniques which nowadays makes it possible to tackle questions which before were beyond any practical realization (Ashfold and Baggott 1987). 

In the current understanding of PCET reactions, both electron and proton are treated quantum-mechanically, and therefore the tunnelling probability must be accounted for both particles. In fact, concerted processes can be described as double tunnelling (proton and electron), with a single transition state. " For a description of the reaction coordinate, four adiabatic states (reactants, products and intermediates) described by paraboloids, are usually considered. The expression for the semi-classical rate constant in this case incorporates elements derived from electron and proton transfer theories... 

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