Rate constant electron-transfer reactions

When the above factors are put under control, the possibility of changing the ligand L in the pentacyano(L)ferrate complexes adds a further dimension for studying systematic reactivity changes, brought out by the controlled modification of the redox potentials of the Fe(II)-Fe(III) redox couples. In this way, the rates of electron transfer reactions between a series of [Fen(CN)5L]re complexes toward a common oxidant like [Coin(NH3)5(dmso)]3+ showed a variation in agreement with Marcus predictions for outer-sphere electron transfer processes, as demonstrated by linear plots of the rate constants versus the redox potentials (123). 

It was shown that the rate of electron-transfer reactions substantially changes via the introduction of an additional electrolyte into solution [363, 367]. The variation of reaction rate constants depends on the type of anion added. Thus, this effect is not caused by ionic strength variations and is related to the formation of the outer-sphere complexes. 

Because of the unique nature of electron-transfer reactions, these have been of great theoretical interest. More recently, research has centered on a microscopic picture of the electron-transfer reactions and predicting reaction rate constants (5,6). 

Among the dynamical properties the ones most frequently studied are the lateral diffusion coefficient for water motion parallel to the interface, re-orientational motion near the interface, and the residence time of water molecules near the interface. Occasionally the single particle dynamics is further analyzed on the basis of the spectral densities of motion. Benjamin studied the dynamics of ion transfer across liquid/liquid interfaces and calculated the parameters of a kinetic model for these processes [10]. Reaction rate constants for electron transfer reactions were also derived for electron transfer reactions [11-19]. More recently, systematic studies were performed concerning water and ion transport through cylindrical pores [20-24] and water mobility in disordered polymers [25,26]. 

Thiols react more rapidly with nucleophilic radicals than with electrophilic radicals. They have very large Ctr with S and VAc, but near ideal transfer constants (C - 1.0) with acrylic monomers (Table 6.2). Aromatic thiols have higher C,r than aliphatic thiols but also give more retardation. This is a consequence of the poor reinitiation efficiency shown by the phenylthiyl radical. The substitution pattern of the alkanethiol appears to have only a small (<2-fokl) effect on the transfer constant. Studies on the reactions of small alkyl radicals with thiols indicate that the rate of the transfer reaction is accelerated in polar solvents and, in particular, water.5 Similar trends arc observed for transfer to 1 in S polymerization with Clr = 1.4 in benzene 3.6 in CUT and 6.1 in 5% aqueous CifiCN.1 In copolymerizations, the thiyl radicals react preferentially with electron-rich monomers (Section 3.4.3.2). 

Use die activated complex theory for explaining clearly how the applied potential affects the rate constant of an electron-transfer reaction. Draw free energy curves and use proper equations for your explanation. 

Let us consider the electron transfer between two rigid metal ions located some distance x from each other in the bulk of the solution. It is assumed that the inner-sphere reorganization of the donor D and acceptor A does not take place. The experiments show that the rate constants of these reactions differ by many orders of magnitude and the processes have an activated character even for identical ions D and A. The questions to be answered are Why does the electron exchange between identical ions in the solution require activation What is the reaction coordinate ... 

The usual Tafel evaluation yielded a transfer coefficient a = 0.52 and a rate constant k of 4x 10 cm s at the standard potential of the MV /MV couple. This k value corresponds to a moderately fast electrochemical reaction. In this electrode-kinetic treatment the changes in the rate of electron transfer with pH were attributed only to the changes in the overpotential. A more exact treatment should also take into account the electrostatic effect on the rate of reaction which also changes with pH. 

Packer, J.E., Willson, R.L., Hahnemann, D. and Asmus, K.-D. (1980). Electron transfer reactions of halogenated aliphatic peroxyl radicals measurements of absolute rate constants by pulse radiolysis. J. Chem. Soc. Perkins Transact. II, 296-299. 

The rate constants for the reactions of the arylperoxyl radicals with carotenoids were determined from the first-order kinetics of the formation of the carotenoid radicals produced (using a range of carotenoid concentrations). The three arylperoxyl radicals were all observed to react with carotenoids to yield the carotenoid radical cations via electron transfer. 

From Table 14.6 it can be seen that, with the exception of astaxanthin (ASTA), the rate constants for the electron transfer reactions decrease for each carotenoid in the order 9-phenanthryl peroxyl > 1-naphthyl peroxyl > 2-naphthyl peroxyl. This order of reactivity should be related to the reduction potentials of the radicals, with 9-phenanthryl peroxyl having the highest reduction potential. The same order of reactivity for these three arylperoxyl radicals reacting with Trolox was shown by Neta and coworkers (Alfassi et al. 1995). The reactivities of all the carotenoids studied are similar... 

Thus, if the lifetime of a spin state is 81, the energy level is broadened by an amount H/81, with consequences for ESR line widths. Ward and Weissman1 added some unreduced naphthalene to a solution of the radical anion, and, from the observed broadening, computed 81, and from 8/ the rate constant for the electron transfer reaction ... 

Further experimental studies involved the determination of the rate constant of the reaction of several alkyl halides with a series of electrochemically generated anion radicals so as to construct activation driving force plots.39,40,179 Such plots were later used to test the theory of dissociative electron transfer (Section 2),22,49 assuming, in view of the stereochemical data,178 that the Sn2 pathway may be neglected before the ET pathway in their competition for controlling the kinetics of the reaction. 

Our problem now is to determine the functional form of this experimental free energy curve for the intrinsic rate constant ki for electron transfer. In addition to the Marcus eq 4, two other relationships are currently in use to relate the activation free energy to the free energy change in electron transfer reactions (15, JL6). 

Rate Constants and Reactivity. Electron-transfer reactions of plastocyanin (and other metalloproteins) are so efficient that only a narrow range of redox partners (having small driving force) can be employed. Rates are invariably in the stopped-flow range, Table I. Unless otherwise stated parsley plastocyanin... 

Cyclic chain termination with aromatic amines also occurs in the oxidation of tertiary aliphatic amines (see Table 16.1). To explain this fact, a mechanism of the conversion of the aminyl radical into AmH involving the (3-C—H bonds was suggested [30]. However, its realization is hampered because this reaction due to high triplet repulsion should have high activation energy and low rate constant. Since tertiary amines have low ionization potentials and readily participate in electron transfer reactions, the cyclic mechanism in systems of this type is realized apparently as a sequence of such reactions, similar to that occurring in the systems containing transition metal complexes (see below). 

Photosensitization of diaryliodonium salts by anthracene occurs by a photoredox reaction in which an electron is transferred from an excited singlet or triplet state of the anthracene to the diaryliodonium initiator.13"15,17 The lifetimes of the anthracene singlet and triplet states are on the order of nanoseconds and microseconds respectively, and the bimolecular electron transfer reactions between the anthracene and the initiator are limited by the rate of diffusion of reactants, which in turn depends upon the system viscosity. In this contribution, we have studied the effects of viscosity on the rate of the photosensitization reaction of diaryliodonium salts by anthracene. Using steady-state fluorescence spectroscopy, we have characterized the photosensitization rate in propanol/glycerol solutions of varying viscosities. The results were analyzed using numerical solutions of the photophysical kinetic equations in conjunction with the mathematical relationships provided by the Smoluchowski16 theory for the rate constants of the diffusion-controlled bimolecular reactions. 

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