Homogeneous Electron Transfer Reactions Research Articles

Estimation of the outer-sphere contribution to the activation parameters for homogeneous electron-transfer reactions using the mean spherical approximation

Estimation of the outer-sphere contribution to the Gibbs energy and enthalpy of activation for homogeneous electron transfer is described on the basis of the mean spherical approximation (MSA). It is shown that these quantities for spherical reactants are significantly smaller than those estimated by the Born continuum model. MSA estimates and experimental values of the activation enthalpy for the cobaltacene/cobaltacenium cation redox couple are found to agree with one another. The assumptions used in applying the MSA in the present paper and in earlier work are discussed.

Investigation of electron transfer processes in organic molecules and metal complexes by means of electrolysis-EPR

Kinetics of electrochemical reduction of trialkylphosphates, slow homogeneous electron transfer reaction with the participation of paramagnetic intermediates and multispin Co(II)-, Cu(II)-, Zn(II),-nitroxyde radical systems reduction processes are studied with the help of combining electrochemical method and EPR.

Solvent effects on simple electron transfer reactions: A comparison of results for homogeneous and heterogeneous systems

Solvent effects on the rate constants for both homogeneous and heterogeneous electron transfer reactions have been analyzed on the basis of current models which consider the role of dynamic relaxation processes in determining the magnitude of the pre-exponential factor. A statistical method for separating the effects of the solvent longitudinal relaxation time τ L from those of the solvent permittivity parameter γ is described and applied to 15 sets of experimental data for which results are available in at least four solvents. The degree to which the explained variation in the logarithm of the rate constant could be attributed to either of these effects varied all the way from 0 to 100% depending on the degree of reaction adiabaticity and the relative sizes of the inner and outer sphere components of the Gibbs energy of activation. Data for the limiting cases in which there is no τ l dependence in the pre-exponential factor or in which the pre-exponential factor is proportional to τ L −1 were analyzed further to obtain the size-distance parameter and the components of the pre-exponential factor relevant to the encounter pre-equilibrium model. These parameters have been discussed with respect to current developments in electron transfer theory. Problems in estimating the longitudinal relaxation time in the solvent, required for the analysis, are also considered.

Photochemical ion transfer across the interface between two immiscible electrolyte solutions

Irradiation of the interface between two immiscible electrolyte solutions (ITIES) by visible or UV light can give rise to a photocurrent [l-3] or photopotential [4]. This effect has been ascribed either to the photoexcited electron transfer between a sensitizer in one solvent phase and an electron acceptor in the other solvent phase [1,3], or to the transfer of an ionic product of the homogeneous photoassociated electron transfer reaction [2]. Eventually, the non-radiative dissipation of the absorbed light can result in a microscopic thermal gradient at the ITIES bringing about a change in the interfacial potential difference - a sort of ~e~~lect~c effect [2]. The purpose of this communication is to show that the base electrolyte ions themselves can yield a photocurrent which is closely related to their excited states. This applies in particular to aromatic ions like tetraarylborate anions or tetraarylarsonium cations. The interface between water and 1,2-~chloroeth~e (geometric area of 2.5 cm2) was formed in an all-glass four-electrode cell [5], with the organic solvent phase placed in the bottom part of the cell. The top of the cell was left open, so that the light from a 150 W xenon arc source (Applied Photophysics, Model 4060) reflected by a mirror at 45’ passed only through the layer of the non-absorbing aqueous phase prior to reaching the interface. Mon~hromatic light was obtained from a monochromator (Applied Photophysics) placed between the xenon lamp and the reflection mirror. The incident intensity of the monochromatic light was measured by means of a calibrated photodiode detector (Macam Photometrics, Model SD

Spatially resolved spectroelectrochemistry for examining an electrochemically initiated homogeneous electron transfer reaction

The title technique provides a means to determine time resolved concentration vs. distance profiles for electrogenerated absorbers as they diffuse away from the electrode. Such profiles are perturbed by any homogeneous chemical reactions of the absorber, as is well known from studies with amperometric and normal incidence spectroelectrochemical techniques. The present paper considers the utility of experimentally determined concentration profiles for deducing the stoichiometry, kinetics, and mechanisms of homogeneous electron transfer reactions between two redox systems. The one electron oxidation of methoxypromazine by electrogenerated chlorpromazine cation radical was examined as an example of a redox catalysis mechanism. Concentration profiles were sensitive to the stoichiometry, rate, and equilibrium constant for the reaction. In addition, the spatially resolved absorption probe could be used to monitor either methoxypromazine or chlorpromazine cation radical, or kinetic results could be determined from the sum of the absorbances of both radicals at a single wavelength. The technique amounts to a superset of previous methods, with either the chronoamperometric or chronoabsorptometric responses being mathematical derivatives of the spatially resolved absorption data set. The increased dimensionality of the new technique should prove useful for mechanism diagnosis.

Fourier Transform EPR Measurement of Homogeneous Electron Transfer Rates

AbstractFourier‐Transform Electron Paramagnetic Resonance has measured the rates of homogeneous electron transfer reactions involving duroquinone radical anions. The radicals are generated by laser photolysis of duroquinone in methanol solution containing 10% triethylamine. The duroquinone concentration was varied over a factor of 1000. The rate constant for electron transfer between the radical anion and the neutral duroquinone is 1.5×108M−1s−1 and the intrinsic spin relaxation rates, T−11 and T−12 are 0.32 MHz and 0.44 MHz, respectively. Two‐Dimensional magnetization transfer spectra show that the reaction is a homogeneous electron transfer reaction. The electron transfer rates are measured by two novel pulse sequences designed for more efficient data acquisition in samples with no unresolved inhomogeneous broadening. These two sequences result in a more rapid, accurate determination of the second‐order chemical rate constant since the second‐order rate constant is obtained directly and is not derived from a single measurement of a pseudo‐first‐order rate. The transient duroquinone radical anions studied here appear to have the same T1, T2 and electron transfer rates as stable duroquinone radical anions in alkaline solutions.

Use of macrocyclic polythiaether ligands in structure-reactivity studies of copper (II)/(I)

Abstract

Homogeneous and Electrochemical Electron-Transfer Reaction of Nitrobenzene Anion Radical Dissolved in Nitrobenzene

Abstract Nitrobenzene anion radical was stably prepared by the electrolytic reduction of nitrobenzene solution containing various kinds and amounts of tetraalkylammonium perchlorates. The rate constants of homogeneous electron-transfer reactions of these nitrobenzene anion radicals with nitrobenzene molecules as a solvent were determined by ESR method at various temperatures. These rate constants at 25 °C were about 107 dm3 mol−1 s−1 and comparable with the rate constant of electron-transfer reation between nitrobenzene and its anion radial in N,N-dimethylformamide. The quasi-first order rate constants evaluated from these rate constants were about 108 s−1 and were larger than the rate constants of the intramolecular electron-transfer reactions of the anion radical of bis(p-nitrophenyl) compounds except for bis(p-nitrophenyl)methane. The absorption spectrum of the solution of nitrobenzene anion radical in nitrobenzene containing 0.1 M tetrabutylammonium perchlorate or 0.1 M tetraethylammonium perchlorate showed an additional weak peak at about 800 or 900 nm as well as the ordinary peak. The light energies of these additional peaks were in good agreement with the energy values of the optical electron-transfer reactions evaluated according to the theory of Hush from the activation energies of corresponding thermal electron-transfer reactions. The rate constants of electrochemical electron-transfer reactions and the diffusion coefficients of nitrobenzene anion radical in nitrobenzene were also measured.

Simulated Data for Simple Electrochemical Determination of Rate Constants for Homogeneous Electron Transfer (SET) Reactions and Competition Parameter for Second-Order Follow-up Reactions. II. Coupling and Second SET Reaction between Mediator and Reduced Form of the Substrate.

Methode simple de determination de constante de vitesse dans le cas ou le transfert d'electrons homogene entre le mediateur et le substrat est suivi par deux processus du 2eme ordre en competition

Mechanistic studies on the electrosynthesis of sulfones

The mechanism of the electrosynthesis of sulfones based on the reduction of sulfur dioxide in aprotic solvents in the presence of organic halides has been studied in detail. The sequence of reactions postulated earlier (J. Appl. Electrochem., 3 (1973) 291) SO 2 + e − → SO ∓− 2 SO ∓− 2 + RX → R-SO ∓ 2 + X − R-SO ∓ 2 + SO − 2 → R-SO − 2 + SO 2 R-SO − 2 + RX → R-SO 2 - R + X − has been confirmed by evaluating kinetic and other data making use of electrochemical, spectrophotometric and ESR technigues. The second and fourth steps are S N2 substitution reactions, while the third step is a homogeneous electron transfer reaction.

Catalyse redox homogene de reduction des p-toluenesulfonamides. Determination des parametres cinetiques et thermodynamiques

The redox catalysis of the reduction of tertiary p-toluenesulfonamides and gem- N-di- p-toluenesulfonamides by electrogenerated organic anion radicals, on mercury cathode, was studied by cyclic voltammetry and controlled-potential electrolysis in aprotic DMF. Tertiary tosylamides, not directly reducible by electrochemical means in presence of LiClO 4 as a supporting electrolyte, were cleaved by electrogenerated pyren anion radical. The new theoretical treatments, recently developed by Savéant and coworkers and extended to a SET-type mechanism and cyclic voltammetry results, allowed us to determine the rate controlling step of the catalytic process and to calculate the standard rate constant of the homogeneous electron transfer reaction and the standard potential in respect to the tosylamide anion radical formation.

Simulated Data for Electrochemical Determination of Rate Constants for Homogeneous Electron Transfer Reactions with a Second Order Homogeneous Follow-up Reaction. I: Coupling between Mediator and Reduced Form of the Substrate.

Simulated Data for Electrochemical Determination of Rate Constants for Homogeneous Electron Transfer Reactions with a Second Order Homogeneous Follow-up Reaction. I: Coupling between Mediator and Reduced Form of the Substrate.

The solvent effect on the electro-oxidation of 1,4-phenylenediamine. The influence of the solvent reorientation dynamics on the one-electron transfer rate

The one-electron electro-oxidation of 1,4-phenylenediamine to the corresponding radical cation has been studied by cyclic voltammetry at a Pt electrode in perchlorate solutions in a wide range of aprotic and hydrogen-bonded solvents. A linear relationship between the redox potential of this system and the donor number of a given solvent has been found. It has been shown that the dynamics of solvent reorientation strongly affects the heterogeneous electron transfer rate in the studied case: an almost linear relationship between the standard rate constant and the reciprocal of the longitudinal dielectric relaxation time of a given solvent has been found. It has been suggested that the solvent dynamics affects the rate of the analogous homogeneous electron-transfer reaction.

Preceding chemical reaction mechanisms in homogeneous electron transfer reactions. Mediated electrochemical reduction of highly reactive benzylic halides

La reduction de chloro-9 [α-(fluorenylidene-9) benzyl]-9 fluorene et la reduction de chloro-9 mesytyl-9 fluorene dans l'acetonitrile peut etre catalysee par des ferrocenes et des couples organiques reversibles a des potentiels jusqu'a 2 V avec le processus electrochimique direct conduisant aux radicaux stables correspondants, tandis que les carbanions sont obtenus par reaction non catalysee

ESR‐Spectroscopic Investigation of the Homogeneous Electron Transfer Reactions between Substituted p‐Phenylenediamines and Quinonediimines, and the Validity of Marcus' Theory. I. Measurements at 293 K

AbstractThe rate constants k1, and k2 of the self exchange reactions between the half oxidized and the totally oxidized or the reduced forms of some p‐phenylenediamine derivatives are investigated at 293 K by ESR line‐broadening in various aprotic solvents (dielectric constants ϵ between 4.7 and 49). The dimerization constants of the radicals at 183 K and the enthalpies of dimerization are measured, too. The results are discussed in terms of Marcus' theory. In k1, is found to depend linearly on γ = 1/n2 ‐ 1/ϵ (n = refractive index), the covered range being 0.27 < γ < 0.53. The distance parameter g is obtained from the slope and compared with values derived from different molecular models. The results are represented satisfactorily by ellipsoidal molecules with half axes obtained from crystallographic data, the distance in the activated complex being approximately the same as in the radical dimers. The inner reorganization energy is calculated by Hückel methods and compared with experimental values obtained from the intercept of the straight line ln k1 (γ). The deviations are considerable. The calculated Marcus activation free enthalpies are smaller then the measured ones. — The slow rate constant k2 of the second step can be computed, if the Coulomb forces between the radical cation and the dication of the quinone diimine are considered. The inner reorganization energy of the transformation between aromatic and quinoidic structures is negligible. The rate constants of different cross reactions with p‐phenylene diamines are calculated with Marcus' formula which, however, has to be extended by work terms. The symproportionation rate is nearly diffusion controlled. With some other reactions the conformity of measured and calculated rate constants is excellent.

ESR‐Spectroscopic Investigation of the Homogeneous Electron Transfer Reactions between Substituted p‐Phenylenediamines and Quinonediimines, and the Validity of Marcus' Theory. II. Temperature Dependence and Activation Parameters

AbstractUsing ESR‐line broadening the rate constants of electron self exchange between five substituted p‐phenylenediamines, semiquinonediimines, and quinonediimines in six aprotic solvents are measured in the temperature range 230 K < T < 360 K. A linear dependence of ln k on the solvent parameter γ = 1/n2 ‐ 1/ϵ (n = refractive index, ϵ = permittivity) is observed for each temperature as expected from Marcus' theory, also a linear dependence of ln kT−1/2 on T−1 for each solvent. The experimental surface ln k(γ, T), however, is not in agreement with Marcus' theory, the resulting activation energies and ‐entropies being proportional to γ and increasing from negative values at low γ (extrapolated) to positive at high γ. The deviations are explained by a temperature‐dependent reaction distance and a preceding association between the radical cations and the counterions which increases strongly with decreasing polarity of the solvent.

Homogeneous redox catalysis of the electroreduction of benzyl chloride

The reactions between benzyl chloride and radical anions derived from anthracene and 9,10-diphenylanthracene were studied by cyclic voltammetry and controlled-potential electrolyses in N,N-dimethylformamide solutions, for several concentrations of the aromatic hydrocarbons. The controlled-potential electrolyses for direct and catalytic homogeneous reduction of benzyl chloride on mercury showed toluene as being the main product in both cases. These results were used in the formulation of the overall catalytic mechanisms. The application of a new theoretical treatment for the cyclic voltammetric results, developed recently by Savéant and coworkers, allowed the establishment of the rate determining step and the calculation of the rate constant for the homogeneous electron transfer reaction between the anthracene radical anion and benzyl chloride. This resulted in a value of 1.8 × 104 M−1 s−1. For 9,10-diphenylanthracene it was established the existence of a mixed kinetic control under normal experimental conditions. Using very low initial concentrations of the depolarizer it was possible to obtain for the rate constant the value of 5.0 × 103 M−1 s−1.

Theory of highly exothermic electron transfer reactions

The theory of highly exothermic homogeneous outer-sphere electron transfer reactions is discussed for transfers occurring over a range of distances. A finite rate of diffusion of reactants and their long-range force are treated by solving the diffusion equation numerically for the reactant pair distribution function. Steady-state solutions for the bimolecular rate constant are compared with experimental data as well as with our recent approximate analytic solution, which is found to agree in the present case. On the basis of short-time solutions, it is proposed that experiments which measure electron transfer rates at short times following the onset of reaction improve the possibility of observing the inverted effect in bimolecular systems. The chance of seeing it in linked systems (unimolecular reactions) is even greater. The relation between the prediction of an “inverted region” in the rate constant vs. ΔG° plot and the existence of a maximum in charge transfer spectral plots of intensity vs. absorption frequency is pointed out.

Electrode kinetics of Eu3+/Eu2+ reaction by cyclic voltammetry

The paper is a detailed study of the cyclic voltammetric behaviour of Eu3+ at HMDE in molar solutions of KCl, KBr, KI, KSCN and in 0.1M-EDTA solution with an indigenously built equipment. The computed values of the rate constants at various scan rates show good agreement with those reported by other electrochemical methods. In addition, the results indicate participation of a bridged activated complex in the electron-transfer step, the rate constants showing the trend SCN- > I- > Br- > Cl- usually observed for bridging order of these anions in homogeneous electron-transfer reactions. The results for Eu-EDTA system, however, indicate involvement of an outer sphere activated complex in the electrode reaction.

A study of rearrangement energies of redox species

A large body of rate constant data from homogeneous electron-transfer reaction kinetics studies was analyzed by use of the complete Marcus theory. The goal of the analysis was to obtain reorganization energies to be used in kinetic analyses of reaction rates at semiconductor electrodes. In general, quite consistent results were found further supporting the basic tenets of the Marcus theory.