Electron transfer activation parameters

Amjad and McAuley (1974) have investigated the oxidation of malic (2-hydroxy-butane-1,4-dicarboxylic) acid in perchloric add. The reaction occurs at stopped-flow lifetimes and is unaffected by either Ce(III) or nitrate. The Michaelis-Menten plot is linear with finite positive intercept indicating precursor complex formation. The resolved values for the formation constants are comparable to those reported for the hydroxy and keto monocarboxylates, suggesting that the second carboxylate group is not bound in the activated complex. The intramolecular electron transfer rate parameters are less than those reported by Hanna and Fenton for the cyclic a-hydroxycarboxylic acids. 

Attached leaves of pumpkin (Cucurbita pepo L.) were photoinhibited at 750, 1500 or 2500 umol PAR m 2s either at room temperature (RT) or at 1 0, in saturated humidity. After the treatment, different photosynthetic parameters were measured either from leaf discs (fluorescence induction at 77 K, apparent quantum yield of oxygen evolution), or from thylakoids isolated from treated leaves (electron transfer activities, fluorescence induction in the presence of DCMU or FeCN, atrazine binding). 

Modem electron transfer tlieory has its conceptual origins in activated complex tlieory, and in tlieories of nonradiative decay. The analysis by Marcus in tire 1950s provided quantitative connections between the solvent characteristics and tire key parameters controlling tire rate of ET. The Marcus tlieory predicts an adiabatic bimolecular ET rate as... 

The height of the potential barrier is lower than that for nonadiabatic reactions and depends on the interaction between the acceptor and the metal. However, at not too large values of the effective eiectrochemical Landau-Zener parameter the difference in the activation barriers is insignihcant. Taking into account the fact that the effective eiectron transmission coefficient is 1 here, one concludes that the rate of the adiabatic outer-sphere electron transfer reaction is practically independent of the electronic properties of the metal electrode. 

Instead of the quantity given by Eq. (15), the quantity given by Eq. (10) was treated as the activation energy of the process in the earlier papers on the quantum mechanical theory of electron transfer reactions. This difference between the results of the quantum mechanical theory of radiationless transitions and those obtained by the methods of nonequilibrium thermodynamics has also been noted in Ref. 9. The results of the quantum mechanical theory were obtained in the harmonic oscillator model, and Eqs. (9) and (10) are valid only if the vibrations of the oscillators are classical and their frequencies are unchanged in the course of the electron transition (i.e., (o k = w[). It might seem that, in this case, the energy of the transition and the free energy of the transition are equal to each other. However, we have to remember that for the solvent, the oscillators are the effective ones and the parameters of the system Hamiltonian related to the dielectric properties of the medium depend on the temperature. Therefore, the problem of the relationship between the results obtained by the two methods mentioned above deserves to be discussed. 

Each of these free energy relationships employs the intrinsic barrier AGo+ as the disposable parameter. [The intrinsic barrier represents the activation energy for electron transfer when the driving force is zero, i.e., AG = AGo at AG = 0 or the equili-... 

Electrochemistry, organic, structure and mechanism in, 12, 1 Electrode processes, physical parameters for the control of, 10, 155 Electron donor-acceptor complexes, electron transfer in the thermal and photochemical activation of, in organic and organometallic reactions. 29, 185 Electron spin resonance, identification of organic free radicals, 1, 284 Electron spin resonance, studies of short-lived organic radicals, 5, 23 Electron storage and transfer in organic redox systems with multiple electrophores, 28, 1 Electron transfer, 35, 117... 

B. Activation Parameters for Coupled Electron Transfer and Spin Change... 

A significant technical development is the pulsed-accelerated-flow (PAF) method, which is similar to the stopped-flow method but allows much more rapid reactions to be observed (1). Margerum s group has been the principal exponent of the method, and they have recently refined the technique to enable temperature-dependent studies. They have reported on the use of the method to obtain activation parameters for the outer-sphere electron transfer reaction between [Ti Clf ] and [W(CN)8]4. This reaction has a rate constant of 1x108M 1s 1 at 25°C, which is too fast for conventional stopped-flow methods. Since the reaction has a large driving force it is also unsuitable for observation by rapid relaxation methods. 

In a new twist on this subject, electrochemical activation parameters have been obtained for two series of redox couples that undergo coupled spin-state change and electron transfer (28). One series is [M(tacn)2]3+/2+ where M = Fe, Co, Ni, and Ru, and tacn = 1,4,7-triazacyclononane. The other is [Fe(pzb)2]+/0, where pzb-= hydrotris(pyrazol-l-yl)borate... 

The nature of the ligand donor atom and the stereochemistry at the metal ion can have a profound effect on the redox potential of redox-active metal ions. The standard redox potentials of Cu2+/Cu+, Fe3+/Fe2+, Mn3+/Mn2+, Co3+/Co2+, can be altered by more than 1.0 V by varying such parameters. A simple example of this effect is provided by the couple Cu2+/Cu+. These two forms of copper have quite different coordination geometries, and ligand environments, which are distorted towards the Cu(I) geometry, will raise the redox potential, as we will see later in the case of the electron transfer protein plastocyanin. 

Figure 9. Compensation plot of activation parameters for the electron transfer rate constant k2 taken from Table I...

Table I. Intramolecular Electron Transfer Rates and Activation Parameters ...

Diagnostic criteria to identify an irreversible dimerization reaction following a reversible electron transfer. In the presence of a chemical reaction following an electron transfer, the dependence of the cyclic voltammetric parameters from the concentration of the redox active species are sufficient by themselves to reveal preliminarily a second-order complication (a ten-fold change in concentration from = 2 10-4 mol dm-3 to 2 10-3 mol dm-3 represents a typical path). 

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