Electron transfer, activation control diffusion limit

General base catalysis of the cyclization of ethyl 2-hydroxymethyl 4 nitrobenzoate [35], where the acyl group is further activated by electron withdrawal by the nitro-substituent, is characterized by a Bronsted coefficient of 0 97, i.e. unity within the limits of error, suggesting that proton transfer is a diffusion-controlled process (Fife... 

The proximity of the diffusion limit also inhibits a detailed discussion of the data in Table 7, but a significant difference to the substituent effects discussed in Section III.D.4 is obvious. Whereas the reactivities of terminal alkenes, dienes, and styrenes toward AnPhCH correlate with the stabilities of the new carbenium ions and not with the ionization potentials of the 7r-nucleophiles [69], the situation is different for the reactions of enol ethers with (p-ClC6H4)2CH+ [136]. In this reaction series, methyl groups at the position of electrophilic attack activate the enol ether double bonds more than methyl groups at the new carbocationic center, i.e., the relative activation free enthalpies are not controlled any longer by the stabilities of the intermediate carbocations but by the ionization potentials of the enol ethers (Fig. 20). An interpretation of the correlation in Fig. 20 has not yet been given, but one can alternatively discuss early transition states which are controlled by frontier orbital interactions or the involvement of outer sphere electron transfer processes [220]. 

Let (R)o and (P)o be the activities of R and P at Xq and (R)d> and (P)d, those at x = Xd> (the electron activity is not considered because it is implicitly included in the potentials). For all the electron transfer reactions investigated up to now, the rates of the diffusion-migration steps in Scheme 5 do not limit the overall process. Although this may be of importance for the prediction of the maximal rate of electron transfer at an electrode, we do not consider the possible kinetic limitation by either of these physical processes (compare the diffusion control of an homogeneous electron transfer discussed earlier). Thus, within this restriction, the existence of the diffusion-migration processes involves only thermodynamic contributions. Thus the overall rate constants in Scheme 5, defined in Eq. (98),... 

Quenching of the ( CT)[Ru(bipy)3] by [Cr(bipy)]3 has been studied. This is via electron transfer to the Cr complex and a rapid back reaction. The ruthenium complex will also quench the 727 nm emission of the metal-centred doublet excited state of the chromium species, by a similar mechanism. Evidently both ligand- and metal-centred excited states can be quenched by bimolecular redox processes. A number of Ru complexes, e.g. [Ru(bipy)3] and [Ru(phen)3] also have their luminescence quenched by electron transfer to Fe or paraquat. Both the initial quenching reactions and back reactions are close to the diffusion-controlled limit. These mechanisms involve initial oxidation of Ru to Ru [equation (1)]. However, the triplet excited state is more active than the ground state towards reductants as well as... 

The large positive AV values observed for the quenching by B and TMB are due to the diffusion limit that applies, such that the change in viscosity of the solvent with pressure leads to decreased kq. In the activation-controlled limit, two terms contribute to the observed value of AK, namely, the volume change for the association of the precursor and that associated with the electron-transfer process. The latter contributions can partially cancel each other and account for the rather small pressure effects sometimes observed under such conditions. A more detailed analysis revealed that changes in the dielectric constant of the medium can account for the observed effects in the case of activation-controlled electron transfer [62],... 

As a first step, it is important to define which reactions are susceptible to be catalyzed. In principle, the reaction rates of any process can be increased. In a heterogeneous process, such as electrode reactions, the diffusion of the active species to the electrode may be the rate-determining step of the whole process. In that case, any improvement in the rate of the electron-transfer step would not produce any change in the overall rate of the process, since the mass-transfer process is still the limiting step. The electrode reaction will behave then as a reversible diffusion-controlled reaction. Whenever the reaction in the actual experimental conditions is not diffusion controlled, it may be interesting to find a better electrocatalyst for it. The criteria for defining a reaction as diffusion controlled depends on the technique employed for the study. According to the technique, several... 

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