Reactions and Electron-Exchange Rates

Electron Transfer Accompanied by a Net Chemical Change 12.2.3.4.2. Cross Reactions and Electron-Exchange Rates... 

Table IV. Electron Transfer Cross-Reaction and Self-Exchange Rate Constants for Blue Copper Proteins (25°, /aO.IM, pH 7)a...

Fig. 2a-c. Kinetic zone diagram for the catalysis at redox modified electrodes a. The kinetic zones are characterized by capital letters R control by rate of mediation reaction, S control by rate of subtrate diffusion, E control by electron diffusion rate, combinations are mixed and borderline cases b. The kinetic parameters on the axes are given in the form of characteristic currents i, current due to exchange reaction, ig current due to electron diffusion, iji current due to substrate diffusion c. The signpost on the left indicates how a position in the diagram will move on changing experimental parameters c% bulk concentration of substrate c, Cq catalyst concentration in the film Dj, Dg diffusion coefficients of substrate and electrons k, rate constant of exchange reaction k distribution coefficient of substrate between film and solution d> film thickness (from ref. 

The electron exchange rate k (Eq. 10.5) is a function of the transmission coefficient k (approximately 1 for reactions with substantial electronic coupling (>4 kJ), i.e., for adiabatic reactions), the effective collision frequency in solution (Z 1011 M 1 s 1 Ar2) and the free energy term AG. ... 

Reactions of hydroquinone, catechol, and L-ascorbic acid with dicyanobis(l,10-phenan-thn>line)iion(III) were studied in dimethyl sulfoxide (DMSO). Application of the Marcus theory to the reactions of catechol and hydroquinone provided the electron exchange rate constant for the Fe(III/II) couple in DMSO. The self-exchange rate constant for the ascorbic acidAadical couple was estimated for the first time in DMSO. The one electron-oxidation process of L-ascorbic acid in an aprotic solvents such as DMSO may be completely different from that in aqueous solutions. 

The electron exchange rate constant of the iron(III) complex in DMSO was estimated from the cross reactions with hydroquinone and catechol, which was compared with the rate constant obtained electrochemically. The mechanism of the ascorbic acid oxidation reaction in DMSO is discussed based on the Marcus theory. 

The O2—H2O couple is the redox pair controlling reactions in aerated solutions, so that reaeration of anoxic soils drives reduced species (e.g., Fe " ) toward the oxidized state. The range of redox potentials over which Fe ", and NH4 have been found to oxidize and disappear on aeration of a reduced soil are denoted by the open boxes in Figure 7.5. Nitrate reappearance on aeration is also depicted by an open box. The measured redox potentials that follow re-aeration do not directly reflect the 02—H20 equilibrium state but rather the status of redox couples having faster electron exchange rates. Furthermore, while each redox couple would be expected (in theory) to undergo complete conversion to the reduced form (in flooded soils) or to the oxidized form (in re-aerated soils) before the adjacent redox couple on the Eh scale became active, actual behavior in soils is much less ideal. Several redox reactions are typically active simultaneously. This may reflect spatial variability in the aeration (and redox potential) of soil aggregates, caused by slow diffusion processes in micropores. 

Interfacial electron-transfer reactions between polymer-bonded metal complexes and the substrates in solution phase were studied to show colloid aspects of polymer catalysis. A polymer-bonded metal complex often shows a specifically catalytic behavior, because the electron-transfer reactivity is strongly affected by the pol)rmer matrix that surrounds the complex. The electron-transfer reaction of the amphiphilic block copol)rmer-bonded Cu(II) complex with Fe(II)(phenanthroline)3 proceeded due to a favorable entropic contribution, which indicated hydrophobic environmental effect of the copolymer. An electrochemical study of the electron-transfer reaction between a poly(xylylviologen) coated electrode and Fe(III) ion gave the diffusion constants of mass-transfer and electron-exchange and the rate constant of electron-transfer in the macromolecular domain. 

The electron exchange rate constants [Rh(dmpe)3]" / " couples have been determined to be 2 x 10 and 4 x 10 M" s", respectively, from the appliction of the Marcus cross-relationship to the reactions with several ruthenium(II) pentaammine complexes. The relative values are consistent with the differences in the M—P bond distance changes (Ado = 0.068 A for Tc and 0.054 A for Re) determined by EXAFS measurements. 

The kinetic data for a series of outer-sphere electron transfer reactions between the [Rh2(02CCH3)4(CH3CN)2] couple and nickel tetraaza macrocycles and iron and ruthenium tris(polypyridine) complexes in acetonitrile have been correlated in terms of the Marcus relationship, yielding a [Rh2] electron exchange rate constant of 3.0 1.7 x 10 M A somewhat smaller value of 5.3 1.3 x... 

The most important theoretical ideas concerning adiabatic outer sphere electron transfer reactions in solution are summarized. The kinetics of the reduction of a series of different tris-1,10,-phenanthroline complexes of Fe(III) by Fe(CN) were measured in order to test the influence of the redox-potential on these reactions. The lectron exchange rate of the complexes Fe(dipy), Ru(dipy) and 0 (dipy) was derived from the study of their reduction by Fe(CN). Using the edox reaction between the anionic complexes of Fe(CN) and the effect of added... 

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