Electron self-exchange reactions rate constants

Esr spectroscopy has also been used to study pure solvent dynamics in electron self-exchange reactions (Grampp et al., 1990a Grampp and Jaenicke, 1984a,b). When the systems are not linked by a spacer (i.e. TCNQ- /TCNQ (TCNQ = tetracyanoquinodimethane), the homogeneous bimolecular rate constants /chom are given by (10), with fcA the association constant and kET... 

The Marcus therory provides an appropriate formalism for calculating the rate constant of an outer-sphere redox reaction from a set of nonkinetic parameters1139"1425. The simplest possible process is a self-exchange reaction, where AG = 0. In an outer-sphere electron self-exchange reaction the electron is transferred within the precursor complex (Eq. 10.4). 

Electron self-exchange reactions in macrobicyclic cobalt complexes have intensively been investigated. The rate constant of such reactions obtained for a variety of complexes, listed in Table 52, differ by several orders of magnitude (from 0.011 and 0.02 for the [CoCdiMesAMHsar)] and [Co(diAMHsar)]° cations to 2.8x10 for the hexathioether macrobicyclic [Co(diMEsar-S6)] + cation). The available data allow one to determine certain rules for the variation in the rate of electron self-exchange in macrobicyclic cobalt complexes. 

It is called the cross-relation because it is algebraically derived from expressions for the two related electron self-exchange reactions shown inEquations 1.21 and 1.22.. Associated with these reactions are two self-exchange rate constants k and k22 and reorganization energies Xu and 22-... 

On the other hand, [Cu(I)L ]" (10,24), a stronger reducing agent than Cu" (aq), affects the rate constants of the reduction reactions in a more complex way. The rate constant of the outer-sphere reduction is nearly not affected, that is, the decrease in the rate constant of the electron self-exchange reaction compensates the increase in the redox potential. For the inner-sphere reactions, slows down the reaction with ClsCCOO" probably due to steric hindrance. On the other hand, accelerates considerably the reduction of NO2 probably by stabilizing the transient complex L Cu(I)N02 and has only a minor effect on the rate of reduction of [Co(III)(NH3)5Cl]. ... 

A mechanism involving the polarization of the ascorbate ligand by a Cu(II) central ion was proposed (138), though the involvement of Cu(I) cannot be ruled out (139). All these reactions proceed via the inner-sphere mechanism however, the copper-catalyzed reduction of superoxide boimd to a binuclear cobalt(III) complex by 2-aminoethanethiol proceeds via the outer-sphere mechanism (140). This is attributed to the effect of 2-aminoethanethiol as a hgand on the rate constant of the Cu(ll/1) electron self-exchange reaction which is suggested to proceed via the gated mechanism. 

Electron self-exchange reaction between O2 and 02 was then discussed, and developments before and after an experimentally determined rate constant for this reaction was published, were also summarized. Related to this, the problem of size differences between O2 or 02 and their typical metal-complex electron donors or acceptors was recently solved quantitatively by addition of a single experimentally accessible parameter, A, which corrected the outer-sphere reorganization energy used in the Marcus cross relation. When this was done, it was found that rate constants for one electron oxidations of the superoxide radical anion, 02 , by typical outer-sphere oxidants are successfiiUy described by the Marcus model for adiabatic outer-sphere electron transfer. 

Rate constants of electron self-exchange reactions in transition-metal complexes, measured in water at room temperature ... 

The electron self-exchange reaction for [Co(OH2)6] in water has an experimental rate constant for electron transfer = 2.4 M" sec". There is a low-lying electronic state that bypasses the spin-forbidden chaiacter of the reaction fiom the ground state, and the electron transfer can be considered adiabadc. The relevant structural data for the reaction is as follows ... 

Theoretically, however, non-spontaneous catalytic reactions with (E - E ) < 0 is anticipated if both rate constants of the electron self-exchange reactions of the catalyst and the substrate are sufficiently large . 

The rate constant and activation parameters for the outer-sphere electron self-exchange reaction of the [Ni(C5H5)2] couple have been measured in dichloromethane by means of H NMR line-shape analysis. The very rapid exchange process, = 2.2 x 10 s (a value of 1 x 10 Af s is... 

The electron self-exchange rate constants evaluated by the Marcus expressions (using cross-reaction data) and those determined experimentally differ in the following cases. Give possible reasons for these differences. 

The reversibility of the [Os(bpy)3]3+/2+ couple makes it useful for the determination of the electron self-exchange rates of other couples by application of the Marcus cross-reaction equation. Recently, this has been applied to the oxidation of S032- to S042- (622). The new rate constant for this reaction of 1.63 x 107 M-1 sec-1 is consistent with the... 

Since the metal ion is involved only for the acceptor part, the dependence of A-a on [M" ] should be responsible for the change in X.da with M"" concentration. The electron self-exchange between Fc NQ and the Fc—NQ /M" complex occurs via formation of the Fc NQ/M" complex as shown in Scheme 20. According to Scheme 20, the electron self- exchange rate constant (A ex) is given by Eq. 20, where Z is the frequency factor for the intermolecular ET reaction, (A,a )... 

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