Homogeneous ET Reactions

Homogeneous ET reactions take place in a homogeneous medium from a donor (D) to an acceptor (A). Electrons are exchanged between molecules or ions, for example, one electron may transfer from Fe + to Co +. As always, a metal complex (metal + ligands) is the smallest unity for our attention, not just the metal ions. Heterogeneous ET reactions take place between a solid material and ions in solution. One example is the well-known Daniell electrochemical cell, where Cu + ions in a solution receive electrons from a copper electrode and are deposited there as copper metal. 

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It was suggested that the absence of an inverted region for the ET reactions at spacer-covered metal electrodes is due to the availabihty of a continuum of electronic states in metal electrodes below the Fermi level. For the same reason, the inverted region is also not expected to be seen for the homogeneous intermolecular ET reactions because a continuum of electronic states are also available below and above the respective ground states of acceptor and donor ions in solutions involved in homogeneous ET reactions. 

The most elementary follow-up reaction is the homogeneous ET to another solution species, which may be identical with the donor molecule itself [52]. The equilibria of homogeneous ET reactions are governed by the standard potentials of the involved redox couples and are easily calculated with given data according to Eqs. (6-9) ... 

In the case of electrochemical ET, the relevant overlap was between the gaussian density-of-states function of the reacting species in solution and the Fermi-Dirac distribution function of the charge carriers in the electrode (Fig. 4.21). Figure 4B. 1 shows the analogous density-of-states functions for a homogeneous ET reaction. The rate... 

Fig. 22 Reaction-layer model for a homogeneous ET reaction between cytochrome c and the DMFc couple. Reprinted with permission from ref. 176. Copyright 2012 American Chemical Society.

Extending the formalism for ET in homogeneous phase, reactions at liquid-liquid interfaces can be described in terms of a series of elementary steps initiated by the approach of reactants to the interfacial region and the formation of the ET precursor complex [1,5,60],... 

Afonso, M. Dos Santos et al., Chem.Abs., 1987, 107, 175302 The thermal homogeneous chain reaction to give mainly octafluoropropane and hexafluoropropylene oxide becomes explosive above a minimum oxygen pressure of 26 mbar. 

Aromatic polycarbonates are currently manufactured either by the interfacial polycondensation of the sodium salt of diphenols such as bisphenol A with phosgene (Reaction 1, Scheme 22) or by transesterification of diphenyl carbonate (DPC) with diphenols in the presence of homogeneous catalysts (Reaction 2, Scheme 22). DPC is made by the oxidative carbonylation of dimethyl carbonate. If DPC can be made from cyclic carbonates by transesterification with solid catalysts, then an environmentally friendlier route to polycarbonates using C02 (instead of COCl2/CO) can be established. Transesterifications are catalyzed by a variety of materials K2C03, KOH, Mg-containing smectites, and oxides supported on silica (250). Recently, Ma et al. (251) reported the transesterification of dimethyl oxalate with phenol catalyzed by Sn-TS-1 samples calcined at various temperatures. The activity was related to the weak Lewis acidity of Sn-TS-1 (251). 

Elliott et a/.71,72,78,96 conducted extensive investigations on the homogeneous WGS reaction using various metal carbonates as well as other basic catalysts. Testing was carried out in a 1-liter autoclave at 500 psig CO pressure ( 35 atm) with 500 g of H20 and 0.32 M solutions of salts in the temperature range 200 100 °C. They... 

In theoretical kinetics today there are still no serious competitors to the transition state theory of Eyring and co-workers (Glasstone et al., 1941). In its most stringent sense it applies only to simple homogeneous gas reactions. The treatment of simple reactions in solution requires additional knowledge of the properties of liquids, and the theory becomes less rigorous and less fundamental. In the extension... 

Later, Newton made a quantum chemical computation of transmission coefficient and reported a value of 1.1 x 10 for an ET reaction between Fe(H20)f redox couple in a homogeneous solution using the... 

Quite often the resulting anionic species undergoes a fast follow-up reaction. Thus, homogeneous ET becomes a crucial step in many electrode reactions a well-known example is the cathodic reduction of organic halides. In mercury, heterogeneous ET to most halides is slow. This kinetic... 

The reduction of organic halides in the presence of aromatic hydrocarbons, the subject of detailed kinetic studies, provide rate constants for the homogeneous ET [147-150] and the follow-up reaction [151]. The theoretical basis for this kind of experiment ( homogeneous redox catalysis ) was laid by Saveant s group in a series of papers during the years 1978-80 [152-157]. Homogeneous ET also plays an important role in the protonation of anion radicals [158]. 

CathodicaUy reduced hydrocarbons not only undergo homogeneous ET and nucleophilic attack but also coupling reactions resulting in hydrodimerization and polymerization. 

The final step of the convolution analysis is the determination of the transfer coefficient a. This coefficient, sometimes called the symmetry factor, describes how variations in the reaction free energy affect the activation free energy (equation 26). The value of a does not depend on whether the reaction is a heterogeneous or a homogeneous ET (or even a different type of reaction such as a proton transfer, where a is better known as the Bronsted coefficient). Since the ET rate constant may be described by equation (4), the experimental determination of a is carried out by derivatization of the ln/Chet-AG° and thus of the experimental Inkhei- plots (AG° = F E — E°)) (equation 27). 

Fig. 9 Plots showing the variation in the logarithm of the homogeneous rate constant for ET, iog(A fion,), with the driving force, —AG", for the ET reactions of a number of aromatic radical anions with (a) ascaridole (O) and (b) (t-BuO)2 ( ) at...

A more recent investigation has been carried out on the homogeneous ET to an extended series of diaryl disulfides (X = NH2, OMe, H, F, C02Et, CN. NO2) in DMF. The redox catalysis approach was applied extensively. The mechanism of the homogeneous reaction between electrogenerated radical anion donors D and (ArS)2 takes place according to the sequence (equations 80-83) ... 

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