Electron exchange factor

A recently proposed semiclassical model, in which an electronic transmission coefficient and a nuclear tunneling factor are introduced as corrections to the classical activated-complex expression, is described. The nuclear tunneling corrections are shown to be important only at low temperatures or when the electron transfer is very exothermic. By contrast, corrections for nonadiabaticity may be significant for most outer-sphere reactions of metal complexes. The rate constants for the Fe(H20)6 +-Fe(H20)6 +> Ru(NH3)62+-Ru(NH3)63+ and Ru(bpy)32+-Ru(bpy)33+ electron exchange reactions predicted by the semiclassical model are in very good agreement with the observed values. The implications of the model for optically-induced electron transfer in mixed-valence systems are noted. 

A quantitative determination of such matrix elements (to be elaborated below) is of crucial importance because it not only allows an absolute evaluation of the desired rate constants but also helps to reveal the qualitative aspects of the mechanism. In particular, questions regarding the magnitude of electronic transmission factors and the relative importance of ligands and metal ions in facilitating electron exchange between transition metal complexes can be assessed from a knowledge of... 

The coefficient of (1 / /2) is simply a normalization factor. This expression builds in a physical description of electron exchange implicitly it changes sign if two electrons are exchanged. This expression has other advantages. For example, it does not distinguish between electrons and it disappears if two electrons have the same coordinates or if two of the one-electron wave functions are the same. This means that the Slater determinant satisfies... 

Aryl and Alkyl Halides. Although the factors governing photodissociation of alkyl halides are not well known, there exists clear experimental evidence for electron exchange between the radical fragments formed by C-X homolysis. For example, the ultraviolet photolysis of 1-octyliodide shows photorearrangement to the 2-halogenated isomer, eq. 58 (181) ... 

The corresponding procedure is carried out in the case of hole transfer, now with use of the ionization energies of the symmetric and antisymmeric linear combination of HOMO S. From eq.(29) follows that the total energy difference at the avoided crossing, equal to the electronic factor, can be obtained as an orbital energy difference between two MO s which are sum and difference of the electron exchanging donor and acceptor MO s < >d and < >a, respectively. 

Since gas phase ionization potentials indicate that secondary alcohols should be more easily oxidized than primary alcohols, the relative reactivity cannot be governed by thermodynamic factors associated with primary electron exchange. A logical explanation for the ordering of the reactivity may relate to the preferential adsorption of the primary alcohol on the metal oxide surface. 

Harriman and Ziessel recently reported the acetylide bridged bimetallic complex [(mpt)Os(tCCb)Ru(bpy)2]4+ (tCCb shown in Scheme 4) [62]. Excitation of the Ru(II) center resulted in nearly 100% efficient energy transfer to the Os to mpt MLCT state with a rate constant of 7 2 x 10iO s-1. Analysis of the Forster overlap factor led to the prediction of a much lower rate constant for resonant energy transfer and the authors concluded that the process was dominated by an electron exchange transfer mechanism. 

It should be pointed out that in many cases it seems uncertain which substance it is that constitutes the veritable catalyst. Particularly for metallic and oxidic catalysts each separate case must be investigated for formation of a monomolecular layer of compounds or adsorbates, for instance, sulfides, carbides, hydrides, etc., which constitutes the real catalyst after an individual activation period of the metal or the oxide. In such cases the electron exchange between the film and the substrate will, of course, be the decisive factor 13). 

The rate constants (kex) of the electron exchange reactions between ZnTPP+ and ZnTPP [Eq. (1)] were determined using Eq. (2), where AHms( and AH°msi are the maximum slope linewidths of the ESR spectra in the presence and absence of ZnTPP+, respectively, and P, is a statistical factor [14]. From the linear plots of (AHmsi - Afi°msl) and [ZnTPP] at various temperatures are obtained the self-exchange electron-transfer rate constant (k ). The Arrhenius plots are shown in Fig. 13.3 together with the observed activation enthalpies (AHols ), where the effect of diffusion (kdiff) is taken into account. The AHol/ values are all positive and decrease in order toluene > MeCN > CH2C12 [16],... 

The formation of electronic surface states in a semiconductor band gap by metal nanoparticles is the major factor that determine the efficiency of electron exchange between metal particles and a semiconductor matrix. It also influences the efficiency of electro-... 

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