Homogeneous Electron Exchange Research Articles

Ab Initio QM/MM Simulation of Ferrocene Homogeneous Electron-Transfer Reaction.

Here, we demonstrate the feasibility of hybrid computational methods to predict the homogeneous electron exchange between the ferrocene and its oxidized (ferrocenium) state. The free energy for ferrocene oxidation was determined from thermodynamic cycles and implicit solvation strategies within density functional theory (DFT) methods leading to no more than 15% of deviation (in the range of 0.1-0.2 eV) when compared to absolute redox free energies obtained experimentally. Reorganization energy, as defined according to the Marcus theory of electron-transfer rate, was obtained by sampling the vertical ionization/electron affinity energies using hybrid quantum/classical (QM/MM) Born-Oppenheimer molecular dynamics trajectories. Calculated reorganization energies show a subtle but noteworthy dependence with the nature and the localization of the compensating countercharge. We concluded that the adopted hybrid computational strategy, to simulate homogeneous redox reactions, was successfully demonstrated and it further permits applications in more complex systems (required in daily life applications), where the electron transfer occurs heterogeneously.

Using trimethylamine dehydrogenase in an enzyme linked amperometric electrode

A rational design for the site-specific immobilization of the protein trimethylamine dehydrogenase (TMADH) to facilitate charge transfer between enzyme and an electrode is described. Protein engineering and site-specific chemical modification have been used to extend the electron pathway from the protein surface to redox mediators. The kinetics of TMADH mutants (V344C and Y442C) modified with the iodoacetamide and 4-iodoacetamido 1-naphthole (IAN) showed that modification at position 344 has a more profound influence on intra- and inter-molecular electron transport, and the catalytic parameters kcat and KMapp became a function of chemical modification. Ferricenium ion was shown to act as an electron acceptor for both mutants, but as its site of interaction is the residue 344, it was rejected for wiring in favour of [Fe(5-NH2-phen)3]2+, the latter showing similar very fast homogeneous electron exchange kinetics, ideal for ‘wire’ construction. The Y442C mutant was successfully immobilised on to an electrode surface which had been chemically modified with the redox polymer, poly-[Fe(5-NH2-phen)3]2+. This design enabled direct electrical communication between the enzyme and electrode. Using a partly oxidized polymer to limit the supply of oxidised electron acceptor, gave evidence for transition from the fast “0/2- cycle” to the “1/3-cycle” for the TMADH.

A Mediated Thin-Layer Voltammetry Method for the Study of Redox Protein Electrochemistry

A novel mediated thin-layer voltammetry technique that allows the rapid determination of midpoint potentials and electron transfer rate constants for small quantities of redox active proteins is described. Thin-layer voltammograms simulated for an electrolyte containing a redox active protein and an electron transfer mediator show that the rapid homogeneous electron exchange reaction between the protein and the mediator serves to mediate the charge transfer of the protein at the electrode, which does not take place in the absence of the mediator, and results in the observation of an apparently reversible redox couple. Both theoretical and experimental data are presented which suggest that the thin-layer voltammetry method will be generally applicable for the determination of protein redox potentials with the proper selection of mediators. Rate constants for the electron transfer between metalloproteins and mediators can be evaluated by comparing experimental voltammograms with theoretical data from simulations. The technique is demonstrated for the metalloproteins cytochrome c, ferredoxin, and the iron protein of nitrogenase.

Estimation of the outer-sphere contribution to the activation volume for electron exchange reactions using the mean spherical approximation

The outer-sphere contribution to the volume of activation of homogeneous electron exchange reactions is estimated for selected solvents on the basis of the mean spherical approximation (MSA), and the calculated values are compared with those estimated by the Strank-Hush-Marcus (SHM) theory and with activation volumes obtained experimentally for the electron exchange reaction between tris(hexafluoroacetylacetonato)ruthenium(III) and -(II) in acetone, acetonitrile, methanol and chloroform. The MSA treatment, which recognizes the molecular nature of the solvent, does not improve significantly upon the continuous-dielectric SHM theory, which represents the experimental data adequately for the more polar solvents.

Electron exchange kinetics in a tetrahedrally coordinated copper(II)/(I) couple

The four-coordinate anion CuI(dpym)2− (Hdpym = 3,3′,5,5′-tetramethyl-4,4′-dicarboethoxydipyrromethene) can be prepared in solution in acetone either by electrochemical reduction of the known tetrahedral complex CuII(dpym)20 (E0 = −290 mV vs. SCE) or by the quantitative reaction of 2Hdpym with Cu(CH3CN)4+ in the absence of O2. The latter reaction does not go to completion in solvents that bind relatively strongly to CuI or that are poor proton acceptors. Ligand exchange between CuI(dpym)2− excess Hdpym in acetone is "fast" in the 1H NMR timeframe, with k1 = 1.4 × 107 L mol−1 s−1at 298 K (first order in each reactant), ΔH‡1 = 3.4 ± 0.6 kJ mol−1, and ΔS‡1 = −97 ± 3 J K−1 mol−1. In the absence of excess Hdpym, dissociation of CuI(dpym)2− in acetone remains negligible. Homogeneous electron exchange between CuI(dpym)2− and CuII(dpym)20 in acetone falls in the "slow" 1H NMR timeframe, with kex = 5.9 × 103 L mol−1 s−1, ΔH‡ex = 48.5 ± 3.0 kJ mol−1, and ΔS‡ex = −10 ± 10 J K−1, at ionic strength I ≈ 0.007 mol L−1 and 298 K, while for the same self-exchange on a Pt electrode the heterogeneous rate constant kel = 0.16±0.04 cm s−1 at I ≈ 0.1 mol−1L−1 and 298 K, according to AC voltammetry. These values of Kex and Kel are of the order expected for CuII/I couples in which no significant change in coordination number or geometry accompanies electron transfer. Keywords: Electron transfer, copper complexes, ligand substitution kinetics, dynamic NMR.

Solvent effect on the rate of homogeneous electron exchange of the Eu(III)-Eu(II) system in water-DMF mixtures

The kinetics of homogeneous electron exchange Eu3+/Eu2+ has been investigated in 1M HClO4, water+dimethylformamide (DMF) mixed solvent by using labeled europium. Non-monotonic variations of the electron exchange rate constants as a function of the solvent composition was observed.

Mediated enzyme/wall‐jet amperometric electrode in flow‐injection monitoring of glucose

AbstractTetrathiafulvalene serves as a useful mediator for glucose oxidase in its application in second‐generation enzyme electrodes for monitoring glucose. Among its useful characteristics are a low redox potential, fast homogeneous electron exchange kinetics with glucose oxidase, high stability, and low solubility in aqueous solutions. The fabrication and characterization of tetrathiafulvalene‐glucose oxidase enzyme electrodes in flow‐injected and stopped‐flow analysis is described. One of the conclusions is that the homogeneous rate constant for tetrathiafulvalene is higher than that of dimethyl ferrocene. Tetrathiafulvalene is, however, more susceptible to pH variations. The characteristics and advantages of the mediated enzyme/wall‐jet amperometric detection are described.

Electrochemical reactions with protonations at equilibrium: Part XIII. Experimental study of the homogeneous electron exchange in quinone/dihydroquinone systems

The rate of the electron (and proton) exchange reaction ▪, where BQ and BQH 2 are benzoquinones and dihydrobenzoquinone, and XQ and XQH 2 their corresponding monosubstituted forms (substituents CH 3, C 6H 5, or Cl) is studied as a function of pH. A W-shaped curve is obtained for log k f and log k b, whose segments have slopes of ±1 (or 2); the curves parallel that predicted for log k app ( k app being the apparent isotopic rate constant for the system BQ + BQH 2 ⇄ BQH 2 + BQ) on the basis of the theory of the homogeneous nine-member scheme with fast protonations (E. Laviron, J. Electroanal. Chem., 169 (1984) 29) and that of the homogeneous e −– e − exchange (E. Laviron, J. Electroanal. Chem., 148 (1983) 1). The sequence of proton and electron exchange is discussed in detail. It can be concluded in particular that when the pH increases (from H 0 = −2 to pH 7) the global rate for the BQ/BQH 2 system is controlled successively by the reproportionation reaction BQH + + BQH 2 ⇄ BQH + BQH + 2, by the isotopic reactions BQ + BQ − ⇄ BQ − + BQ and BQH 2 + BQH + 2 ⇄ BQH + 2 + BQH 2, and by the reproportionation reactions BQH − + BQ ⇄ BQH + BQ − and BQ 2− + BQ ⇆ 2 BQ − .

Role of solvent reorganization dynamics in electron-transfer processes. Theory-experiment comparisons for electrochemical and homogeneous electron exchange involving metallocene redox couples

Abstract : Electrochemical Rate Constants and Activation Parameters are reported for the electron exchange of five metallocene couples and dibenzenechromium (I)/ (O) in eight solvents at mercury electrodes. The solvents (acetonitrile, acetone, methylene chloride, formamide, N-methylformamide, N,N'- dimethylformamide, dimethylsulfoxide, and benzonitrile) were chosen so to provide substantial variations in their dynamical as well as dielectric properties. The metallocene couples are of the form M(Cp)2(+/o), where M = Fe, Co, or Mn, and Cp = cyclopentadiene or pentamethylcyclopentadiene. The inner- shell (i.e., bond distortional) barriers are calculated for the metallocene and arene couples from bond-distance and vibrational data to be small (< or approx, 0.25 kcal/mol) yet metal-dependent. Detailed comparisons of the observed solvent-dependent kinetics are made with the rate parameters calculated from contemporary theoretical treatments of outer-sphere electron transfer. Considerably better agreement between the experimental and theoretical kinetic parameters was obtained when the latter take into account the influence of solvent friction upon the barrier-crossing frequency. These results indicate that the conventional transition-state theory may not apply to electron-transfer reactions where the free-energy barrier is due chiefly to solvent reorganization, at least in 'high friction' media where concerted solvent relaxation is slow.

Kinetics of electron exchange in N,N,N′,N′-tetramethyl-p-phenylenediamine in aprotic solvents and the semiclassical theory of electron transfer

The homogeneous electron exchange of the N,N,N′N′-tetramethyl-p-phenylenediamine radical (S˙+) with the reduced (R) and the totally oxidized (T2+) forms has been measured in aprotic solvents by means of e.s.r. line-broadening. Solvent polarity and temperature were varied. For each temperature the logarithm of the rate constant k1(S˙+⇌ R) is a linear function of the solvent parameter γ= 1/n2–1/ε(where n is the refractive index and ε the dielectric constant) and at constant γ(0.27 ⩽γ⩽ 0.53) a linear function of 1/T. The outer reorganization energy can be calculated using an ellipsoidal model of the reactants and a reaction distance equal to the molecular distance in the radical dimer. The inner reorganization energy is calculated from i.r. and crystallographic data. Nuclear tunnelling is considered since the intramolecular frequencies are high. The experimental function ln k1(γ, T–1) cannot be reproduced by the theory unless additional interaction energies such as ion association S˙+⋯ClO–4 and especially ion-molecule interactions S˙+⋯R are allowed for. The preexponential factors of the theoretical expressions are considerably greater than the experimental values, especially if a precursor equilibrium is assumed. The rate constant k2(S˙+⇌ T2+) is small compared with k1. The difference is mainly caused by the Coulomb repulsion term.

Electrochemical reactions with protonations at equilibrium: Part VI. The homogeneous electron exchange reaction between a monoelectronic and A 1 e, 1 H + system

The rate of homogeneous electron exchange between a 1 e redox couple and a 1 e, 1 H + redox system is studied theoretically when the protonations are much faster than the electron exchange reactions, i.e. when they can be assumed to be at equilibrium. It is shown that the whole system is equivalent to a reaction between two simple 1 e couples, with apparent rate constants for the electron exchange. Variations of these constants are complex functions of the difference of the standard potentials of the monoelectronic system and those of the 1 e, 1 H + system. They also vary with pH in a less complicated way. The reaction path and the reaction sequence (order of addition or loss of electrons and protons) are studied. A potential-pH diagram, which allows the results to be visualized, is given. It is shown that it is not possible to accelerate at the same time the reduction and oxidation of the members of the square scheme by the same monoelectronic system. Applications to redox polymer electrodes are discussed.

Homogeneous Electron Exchange in Catalytic Polarographic Reduction.

Homogeneous Electron Exchange in Catalytic Polarographic Reduction.

Homogeneous and heterogeneous electron exchange in the reduction of aromatic hydrocarbons

Homogeneous electron exchange rates, determined by a modified e.s.r. method in both the slow and fast exchange limits, are compared with published heterogeneous rates at mercury for aromatic hydrocarbon reduction in dimethylformamide; the ratio is approximately constant. This constant can be derived from theories of Marcus and Hush on a model in which image forces are negligible in the electrode process and homogeneous exchange occurs only on contact. Potential-step methods are used to study the source of electrons transferred to radicals formed by protonation of the hydrocarbon radical anions (e.c.e. mechanism) : electron transfer occurs from other radical anions for anthracene and from the electrode for naphthalene. This result is interpreted in terms of relative rates of cross-exchange reactions as predicted by Marcus theory from the “isotopic” exchange rates and redox potentials of the systems. General considerations imply that cross-exchange reactions will predominate in many aromatic systems. Proton addition from phenol to the radical anions appears to involve direct contact of acid and base rather than the free (solvated) proton.

Stereochemical effects in the electrochemistry of some hindered stilbenes in dimethylformamide

A series of progressively hindered stilbenes shows a decreasing rate of electrode reaction for radical-ion formation in dimethylformamide, which is mainly a reflection of the Arrhenius prexponential term. The effect is discussed in terms of entropy effects and geometry changes. Increasing hindrance leads to shifts in reduction and oxidation potential, which are discussed in terms of the effects of non-planarity on the electronic energy levels. For the geometrical isomers cis and trans stilbene, electron transfer to the former is slower than to the latter and isomerization of the anion radical to the planar trans configuration is slow. Reaction of the anions in their isomeric forms with carbon dioxide is discussed in terms of non-bonded interactions and rotation about the central carbon-carbon bond in the reaction intermediates, whose lifetimes are less than 1 msec. Results for heterogeneous electron transfer are paralleled by homogeneous electron exchange rates.

Homogeneous Electron Exchange Reactions of Aromatic Molecules

RECENT reviews summarize the large body of information available on the rates and mechanisms of inorganic electron exchange processes1–3. In contrast, purely organic systems have received minor attention. The homogeneous electron exchange rates (kexc) for a few aromatic hydrocarbons and other individual systems have been measured4,5. We have been interested in the values of kexc for aromatic molecules containing polar functional groups and particularly the effect of solvents, substituents, and molecular geometry on these rates. We wish to report some preliminary results which show that we are able to measure kexc with a precision which is capable of detecting subtle differences due to molecular structure and environment.