Electron transfer kinetic scheme

The quantitative effects of steric encumbrance on the electron-transfer kinetics reinforce the notion that the inner-sphere character of the contact ion pair D+, A- is critical to the electron-transfer paradigm in Scheme 1. Charge-transfer bonding as established in the encounter complex (see above) is doubtless an important consideration in the quantitative treatment of the energetics. None the less, the successful application of the electron-transfer paradigm to the... 

As with the other reaction schemes involving the coupling of electron transfer with a follow-up homogeneous reaction, the kinetics of electron transfer may interfere in the rate control of the overall process, similar to what was described earlier for the EC mechanism. Under these conditions a convenient way of obtaining the rate constant for the follow-up reaction with no interference from the electron transfer kinetics is to use double potential chronoamperometry in place of cyclic voltammetry. The variations of normalized anodic-to-cathodic current ratio with the dimensionless rate parameter are summarized in Figure 2.15 for all four electrodimerization mechanisms. 

FIGURE 4.1 3. a RDEV response of a monolayer catalytic coating for the reaction scheme in Figure 4.10 with a slow P/Q electron transfer. Kinetic parameter [equation (4.5)] kr°8/DA = 5. The same electrode transfer MHL law as in Figure 1.18. Dotted line Nemstian limiting case. Solid lines from left to right, e (5r0DAC = 1, 0.1, 0.01. h Derivation of the catalytic rate constant, c Derivation of the kinetic law. 

Triad formation in Scheme 10 is a two-step process (88) involving the metastable A-nitropyridinyl radical, whereas the adiabatic electron transfer in Scheme 19 is likely to occur irreversibly with the simultaneous cleavage of the N-N02 bond, as in (89). As a result, the nascent pair (Py and N02) in (88) can suffer greater diffusive separation from ArH+- compared with that in (89). If so, the complexation of the aromatic cation radical by pyridine (90), as recently delineated by Reitstoen and Parker (1991) is (kinetically)... 

Fiomogeneous cross-reaction electron-transfer kinetic studies suggest that many other Cu(II/I) systems obey Scheme 1. However, few Cu(II/I) systems have been subjected to sufficiently low temperature or rapid-scan CV measurements to demonstrate the presence of rate-limiting conformational changes. 

The remarkable solvent isotope effect on the kinetics of oxidation of guanine by 2AP radicals has been detected in H2O and D2O solutions [14]. In H2O, the rate constants of G(-H) formation are larger than those in D2O by a factor of 1.5-2.0 (Table 1). This kinetic isotope effect indicates that the electron transfer reaction from guanine to 2AP radicals is coupled to deprotonation/ protonation reactions of the primary electron-transfer products (Scheme 1). 

Reaction of Cytochrome cimu with Tris(oxalato)cobalt(III) The cytochrome c protein was also used as reductant in a study of the redox reaction with tris (oxalato)cobalt(III).284 Selection of the anionic cobalt(III) species, [Conl(ox)3]3 was prompted, in part, because it was surmised that it would form a sufficiently stable precursor complex with the positively charged cyt c so that the equilibrium constant for precursor complex formation (K) would be of a magnitude that would permit it to be separated in the kinetic analysis of an intermolecular electron transfer process from the actual electron transfer kinetic step (kET).2S5 The reaction scheme for oxidation of cyt c11 may be outlined ... 

No activation (energy) barrier separates the donor and the acceptor from the ET products (and vice versa). The electron transfer in Scheme 18 is not a kinetic process, but is dependent on the thermodynamics, whereby electron redistribution is concurrent with complex formation. Accordingly, the rate-limiting activation barrier is simply given by the sum of the energy gain from complex formation and the driving force for electron transfer, i.e. ... 

The observed rate constant, obs, never equals the rate constant 2 for forward electron transfer in these simple models. The presence of multiple steps in the electron transfer mechanism [keeping in mind that Eq. (20) represents a minimal scheme for an electron transfer reaction] emphasizes the difficulties in extracting 2 values from measurements of obs under steady-state conditions. Rapid kinetic studies provide a more powerful approach for separating the actual kinetics of electron transfer from the association and dissociation steps, but the analysis may still be complex. Owing to difficulties associated with bimolecular kinetics, many recent studies of electron transfer have emphasized unimolecular processes. Physiologically, however, the bimolecular processes can be of considerable importance for the overall electron transfer kinetics. 

This method operating with the time window of about 100 to 3000 s is suitable to study reactions with rate constants, k, of the order of 10" to 10"" s" This voltammetric technique enables the determination of reaction products by the electroanalysis of the solution following the electrolysis and hence the interpretation of the overall reaction scheme. Moreover, the experiments are carried out at potentials of the limiting current this is why the electron transfer kinetics does not disturb the estimation of the results. The interpretation of the coulometric responses is usually much simpler than those of other voltammetric methods. The experimental parameters to be evaluated are as follows ... 

The mechanism proposed for the production of radicals from the N,N-dimethylaniline/BPO couple179,1 involves reaction of the aniline with BPO by a Sn-2 mechanism to produce an intermediate (44). This thermally decomposes to benzoyloxy radicals and an amine radical cation (46) both of which might, in principle, initiate polymerization (Scheme 3.29). Pryor and Hendrikson181 were able to distinguish this mechanism from a process involving single electron transfer through a study of the kinetic isotope effect. 

Heterogeneous electron reactions at liquid liquid interfaces occur in many chemical and biological systems. The interfaces between two immiscible solutions in water-nitrobenzene and water 1,2-dichloroethane are broadly used for modeling studies of kinetics of electron transfer between redox couples present in both media. The basic scheme of such a reaction is... 

Most radiation-chemical reactions are thermal in nature those considered in the diffusion-kinetic scheme are essentially thermal reactions (see Chapter 7). In polar media, electron thermalization is presumed to occur before solvation (Mozumder, 1988). However, ionization processes usually involve transfer of energy in excess of the ionization potential (see Chapter 4). Therefore, mechanisms of thermalization are important for radiation-chemical effects. 

The problem, from the point of view of the investigation of the mechanism, is that since the first electron transfer step is rate limiting, conventional kinetic techniques for studying the reaction are useless as a means of elucidating the overall mechanism and conventional spectroscopic techniques have also proved to be of little help. Gressin and colleagues (1979) were therefore driven to the careful study of the nature of the products as a function of reaction conditions. This work is seminal in the study of the direct reduction of C02 since the authors were able to produce a general scheme for the reaction of CO 2 that is now widely accepted. 

If the EDA and CT pre-equilibria are fast relative to such a (follow-up) process, the overall second-order rate constant is k2 = eda c e In this kinetic situation, the ion-radical pair might not be experimentally observed in a thermally activated adiabatic process. However, photochemical (laser) activation via the deliberate irradiation of the charge-transfer absorption (hvct) will lead to the spontaneous generation of the ion-radical pair (equations 4, 5) that is experimentally observable if the time-resolution of the laser pulse exceeds that of the follow-up processes (kf and /tBet)- Indeed, charge-transfer activation provides the basis for the experimental demonstration of the viability of the electron-transfer paradigm in Scheme l.21... 

We emphasize that the critical ion pair stilbene+, CA in the two photoactivation methodologies (i.e., charge-transfer activation as well as chloranil activation) is the same, and the different multiplicities of the ion pairs control only the timescale of reaction sequences.14 Moreover, based on the detailed kinetic analysis of the time-resolved absorption spectra and the effect of solvent polarity (and added salt) on photochemical efficiencies for the oxetane formation, it is readily concluded that the initially formed ion pair undergoes a slow coupling (kc - 108 s-1). Thus competition to form solvent-separated ion pairs as well as back electron transfer limits the quantum yields of oxetane production. Such ion-pair dynamics are readily modulated by choosing a solvent of low polarity for the efficient production of oxetane. Also note that a similar electron-transfer mechanism was demonstrated for the cycloaddition of a variety of diarylacetylenes with a quinone via the [D, A] complex56 (Scheme 12). 

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