Kinetics Of Electron Transfer Reactions Research Articles

Structural characterization of a new manganese(III)–salen complex [H 2salen= N, N′-bis(salicylidene)ethane-1,2-diamine] and study of its electron transfer kinetics with hydroquinone and catechol

The octahedral manganese(III) complex, [Mn(salen)Cl(H 2O)]·H 2O (H 2salen= N, N′-bis(salicylidene)ethane-1,2-diamine), has been synthesized and structurally characterized by single-crystal X-ray diffraction study. The X-ray structural analysis shows that the metal center is six-coordinate with distorted octahedral geometry. In this compound, the MnCl and MnOH 2 bond lengths are 2.621(6) and 2.333(2) Å, respectively, slightly longer than the usual values. The kinetics of electron transfer reaction of the complex with catechol and hydroquinone has been followed spectrophotometrically over the pH range 5.75–8.15 at 30 °C. The nature of the kinetic trace is indicative of a biphasic reaction with each reductant. Detailed analysis of kinetic data supports the formation of a relatively stable intermediate complex between manganese(III) and the dihydroxybenzenes (H 2A) at the initial stage. The decay of the intermediate is the bimolecular electron transfer process involving another molecule of reductants leading to the manganese(II) complex and benzoquinones. Mn III(salen)(H 2O)(OH) generated in situ from the aquation and proton equilibrium of the parent complex has been assigned to be the reactive species during the formation of the intermediate. The corresponding species for the redox step is Mn III(L)(H 2O)(HA), sufficient experimental evidence for which has been gathered from pH-rate profiles.

Electrochemical corrosion kinetics of tin and tin amalgams in NaCl aqueous solutions

Electrochemical techniques were used to determine the corrosion rate of pure tin metal as compared to 80 Sn/20 Hg tin amalgam. X-ray diagrams showed that this amalgam was a crystalline γ2 phase, whereas a 50 Sn/50 Hg amalgam contained liquid alloy embedded in the same γ2 phase. Open circuit potential measurements, combined with narrow range potential scanning voltammetry, lead to the conclusion that amalgamation resulted in enhancement of the corrosion current, mainly by increasing the cathodic electron transfer reaction kinetics both in deaerated and in oxygen-saturated NaCl solution. When maintained at zero current potential in a solution containing dissolved O2 gas, the samples were gradually covered with an insulating oxide layer which was identified by a series of electrochemical impedance diagrams recorded at different time intervals. The oxide layer was firmly adherent to the bulk tin metal but was poor at protecting the amalgam electrode. Finally, at potential values where the anodic current reached a few mA/cm2, the pure tin metal surface was suddenly deteriorated by the formation of extremely deep pinhole corrosion pits, while this effect was smoothed down by amalgamation.

Electrochemical study of the intermolecular electron transfer to Pseudomonas aeruginosa cytochrome cd 1 nitrite reductase

The kinetics of electron transfer reaction between cytochrome cd 1 nitrite reductase (NiR) from Pseudomonas aeruginosa and various physiological/non physiological redox partners was investigated using cyclic voltammetry at the pyrolytic graphite electrode. While NiR did not exchange electron with the electrode, cytochrome c 551 and azurin, both from Ps. aeruginosa, behaved as fast electrochemical systems. The intermolecular electron transfers between NiR and cytochrome c 551 or azurin as electron shuttles, in the presence of nitrite, were studied. Second order rate constants of 2×10 6 and 1.4×10 5 M −1 s −1 are calculated for cytochrome c 551 and azurin, respectively. The dependence of the second-order rate constant on ionic strength and pH is discussed. Finally, the effect of the global charge of the electron shuttles was explored using differently charged species (proteins or small ions). The experimental results suggest involvement of polar interactions as well as of hydrophobic contacts in the protein recognition prior to the intermolecular electron transfer. As the cross-reaction between Ps. nautica cytochrome c 552 and Ps. aeruginosa NiR was shown to be as efficient as the catalytic reaction involving the physiological partners, it is concluded to a ‘pseudo-specificity’ in the recognition between NiR and the electron donor.

Kinetics and mechanism of electron transfer reactions of iron(III) perchlorate with aldohexoses in presence of 2,2' -bipyridyl

The kinetics of electron transfer reactions of iron(III) perchlorate with D-glucose, D-galactose and D-mannoses in presence of complexing 2,2'-bipyridyl have been studied under the pseudo-first order conditions. The results suggest the formation of 1 : 2 complex between Fe I I I and 2,2'-bipyridyl. The complex reacts with aldohexose to form an intermediate complex which disproportionates in a slow step to form aldopentose by a free radical mechanism. A rate law has been derived.

The role of different reorganization energies within the Zusman theory of electron transfer

We consider the kinetics of electron transfer reactions in condensed media with different reorganization energies for the forward and backward processes. The starting point of our analysis is an extension of the well-known Zusman equations to the case of parabolic diabatic curves with different curvatures. A generalized master equation for the populations as well as formal expressions for their long-time limit is derived. We discuss the conditions under which the time evolution of the populations of reactants and products can be described at all times by a single exponential law. In the limit of very small tunnel splitting, a novel rate formula for the nonadiabatic transitions is obtained. It generalizes previous results derived within the contact approximation. For larger values of the tunnel splitting, we make use of the consecutive step approximation leading to a rate formula that bridges between the nonadiabatic and solvent-controlled adiabatic regimes. Finally, the analytical predictions for the long-time populations and for the rate constant are tested against precise numerical solutions of the starting set of partial differential equations.

Effects of vibrational energy relaxation and reverse reaction on electron transfer kinetics and fluorescence line shapes in solution

The existing theoretical formulations of electron transfer reactions (ETR) neglect the effects of vibrational energy relaxation (VER) and do not include higher vibrational states in both the reactant and the product surfaces. Both of these aspects can be important for photo-induced electron transfer reactions, particularly for those which are in the Marcus inverted regime. In this article, a theoretical formulation is presented which describes the two aspects. The formalism requires an extension of the hybrid model introduced earlier by Barbara et al. [Science 256, 975 (1992)]. We model a general electron transfer as a two-surface reaction where overlap between the vibrational levels of the two surfaces create multiple, broad reaction windows. The strength and the accessibility of each window is determined by many factors. We find that when VER and reverse transfer are present, the time dependence of the survival probability of the reactant differs significantly (from the case when they are assumed to be absent) for a large range of values of the solvent reorganization energy (λX), quantum mode reorganization energy (λq), electronic coupling constant (Vel) and vibrational energy relaxation rate (kVER). Several interesting results, such as a transient rise in the population of the zeroth vibrational level of the reactant surface, a Kramers (or Grote–Hynes) type recrossing due to back reaction and a pronounced role of the initial Gaussian component of the solvation time correlation function in the dynamics of electron transfer reaction, are observed. Significant dependence of the electron transfer rate on the ultrafast Gaussian component of solvation dynamics is predicted for a range of values of Vel, although dependence on average solvation time can be weak. Another result is that, although VER alters relaxation dynamics in both the product and the reactant surfaces noticeably, the average rate of electron transfer is found to be weakly dependent on kVER for a range of values of Vel; this independence breaks down only at very small values of Vel. In addition, the hybrid model is employed to study the time resolved fluorescence line shape for the electron transfer reactions. It is found that VER can have a significant influence on the fluorescence spectrum. The possibility of vibrational state resolved spectra is investigated.

Optical control of electrons during electron transfer.

The dynamics of electron transfer reactions in solution can be controlled with the use of a sequence of femtosecond laser pulses. In the charge transfer to solvent (CTTS) reaction of sodide (Na-) in tetrahydrofuran, an initial light pulse launched the CTTS reaction, ejecting an electron into either an immediate or a solvent-separated Na0:solvated electron contact pair. A second pulse was used to excite the electrons in the contact pairs, and a third pulse monitored the amount of Na- produced through the back electron transfer. Excitation of the electrons in immediate contact pairs shut off the back electron transfer, whereas excitation of the electrons in solvent-separated pairs both enhanced and hindered the back electron transfer.

Electron transfer reactions at gold nanoparticles.

It is demonstrated that scanning electrochemical microscopy can be used to investigate the kinetics of electron transfer reactions catalysed by metal nanoparticles supported on an insulating substrate.

Dynamics and Mechanisms of Ultrafast Fluorescence Quenching Reactions of Flavin Chromophores in Protein Nanospace

We have studied excited-state dynamics of “nonfluorescent” flavoproteins including riboflavin binding protein (RBP), d-amino acid oxidase benzoate complex (DAOB), and others by means of femtosecond fluorescence up-conversion method and have observed ultrafast fluorescence quenching dynamics for the first time. We have interpreted the fluorescence quenching mechanisms of these flavoproteins as due to the ultrafast electron transfer (ET) to flavin chromophore (F) in the excited electronic state from nearby tryptophan (Trp.NH) or tyrosine (Tyr.OH) residues placed in the protein nanospace (PNS), on the basis of their X-ray structures. Extremely fast fluorescence quenching in RBP (τf ∼ 90−100 fs) could be attributed to the compact stacked arrangement, Trp.NH.....F.....Tyr.OH, supremely favorable for the ultrafast ET reaction dynamics. Comparisons of fluorescence time profiles and spectral characteristics of F in solution with those in PNS have indicated the existence of extremely fast FC (Franck−Condon) → Fl (fluorescence) state conversion in PNS within the time resolution of the apparatus. The ultrafast FC → Fl conversion may be a coherent process coupled with intra-chromophore high-frequency modes leading to formation of vibrationally nonrelaxed or only partially relaxed Fl state, from which barrierless ET seems to occur. Fluorescence dynamics of DAOB have indicated faster initial decay in both blue and red sides of the spectrum contrary to other flavoproteins which showed practically wavelength-independent fluorescence dynamics. This result of DAOB is similar to those of photoactive yellow protein and visual rhodopsin although their reaction mechanism (twisting) is different from DAOB (ET). We have proposed a possible mechanism for this fluorescence dynamics of DAOB on the basis of an extremely compact stacked configuration of F...benzoate-...Tyr.OH which seems to undergo moderate frequency intermolecular vibration coupled with intra-chromophore high-frequency modes of F in the course of ET from Tyr.OH to excited F.

Influence of interionic separation in electron transfer reactions

A theoretical study is presented on the influence of the interionic separation between the reacting metal cations on the kinetics of electron transfer reactions. The results of some simulations are presented to characterize the dependence of the activation free energy of the electron transfer process relatively to the interionic separation as well as the potential of mean force between the two cations considered in this study, Cu + and Cu 2+, in aqueous medium. The use of truncation of coulombic interactions is compared with the use of Ewald sums in the treatment of long range interactions. The truncation of coulombic interactions in the simulations is observed to have a strong influence in the final results.

Electron transfer reaction dynamics in non-Debye solvents

The dynamics of electron transfer in a non-Debye solvent is described by multidimensional Markovian reaction-diffusion equation. To highlight differences with existing approaches in the simplest possible context, the irreversible outer-sphere reaction in a solvent with a biexponential energy-gap autocorrelation function, Δ(t), is studied in detail. In a Debye solvent, Δ(t)=exp(−t/τL) and the rate can be rigorously expressed as an explicit functional of exp(−t/τL). It has been suggested that the exact rate in a non-Debye solvent can be found by replacing exp(−t/τL) with the appropriate (nonexponential) Δ(t). For a “biexponential” solvent, our approach is based on an anisotropic diffusion equation for motion on a harmonic surface in the presence of a two-dimensional delta function sink. Three approximations, which reduce the solution of this equation to effective one-dimensional ones, are considered and compared with exact Brownian dynamics simulation results. The crudest approximation replaces the non-Debye solvent with an effective Debye one with τeff−1=(−dΔ/dt)t=0. The second is obtained by invoking the Wilemski–Fixman-type closure approximation for the equivalent two-dimensional integral equation. This approximation turns out to be identical to the above mentioned “substitution” procedure. When the relaxation times of the two exponentials are sufficiently different, it is shown how the two-dimensional problem can be reduced to a one-dimensional one with a nonlocal sink function. This anisotropic relaxation time approximation is in excellent agreement with simulations when the relaxation times differ by at least a factor of three and the activation energy is greater than kBT. Finally, it is indicated how the influence of intramolecular vibrational modes (i.e., nonlocal sink functions) can be treated within the framework of this formalism.

Stereoselectivity in reactions of metal complexes part XXI. Kinetics and mechanism of electron transfer between rac-[Co(L)H 2O] + (L = N,N′-[(pyridine-2,6-diyl)-bis(methylene)]bis[amino acid]) and optically active iron (II) complexes

The kinetics of electron-transfer reactions between chiral Co III and optically active Fe II complexes has been studied using circular dichroism (CD) spectroscopy. The ligands used all have the same basics structure of N,N′-[(pyridine-2,6-diyl)bis(methylene)]bis[amino acid]. According to the substitution pattern on this linear pentadentate framework, the complex formation with inert Co III and Fe II is considered to be stereospecific. The substituents were systematically varied in an effort to provide evidence for the intimate mechanism of the electron transfer. Observed kinetic stereoselectivities of the reduction of the aqua-Co III complexes varied from 1.7 to 2.4, and, with one exception, heterochiral couples of the complexes reacted faster. Considering experimental data and models, an inner-sphere mechanism is proposed, with a transition state where the carboxylate groups on the inert Co III complex act as the bridge. The question of bridging between the two metal centers by the carboxylate group is discussed.

Stopped-flow kinetic studies of low potential electron carriers of the photosynthetic bacterium, Rhodobacter capsulatus: ferredoxin I and NifF

The kinetics of electron-transfer reactions involving nif-specific proteins from Rhodobacter capsulatus; ferredoxin I, NifF, Fe-protein of nitrogenase and dithionite were studied using stopped-flow spectrophotometry. Kinetic evidence was obtained for the formation of a tight (0.44 μM) complex between NifF and Fe-protein. Under the same conditions, FdI interacted only weakly ( K d>325 μM) with Fe-protein. There was no evidence for complex formation between NifF and FdI since the reaction NifF SQ+FdI red had a bimolecular rate constant of 12.5±1.2×10 3 M −1 s −1. These results suggest that NifF, which is present in only small quantities in the cell, can make a significant contribution to the overall rate of nitrogen fixation due its high reactivity with Fe-protein. Moreover, the apparent lack of specific interaction between NifF and FdI suggest that they act in vivo in parallel to reduce Fe-protein and not in series.

Application of the transient grating technique to the study of the energetics and dynamics of electron transfer reactions: separation of an ion pair into free ions

The enthalpy of formation of the free ions generated by the electron transfer between benzophenone in the lower triplet state and 1,4-diazabicyclo[2,2,2]octane in acetonitrile has been measured using transient thermal phase grating. This enthalpy is smaller by more than 0.14±0.08 eV than the enthalpy of formation of the geminate ion pair obtained from a previous ps thermal phase grating investigation. Therefore, the dissociation of a geminate ion pair into free ions in endothermic. However, separation is exergonic if translational entropy in taken into account. A small and positive value of C, the correction term in the Rehm—Weller equation, is obtained if this term is considered as a free energy. The principles of the transient grating technique as a tool for investigating photoinduced processes in solution are briefly reviewed.

Kinetics and mechanism of the oxidation of alkyl and aryl methyl ketones by permanganate ion in aqueous ethanoic acid

The kinetics of electron-transfer reactions between permanganate ion and ethyl and aryl methyl ketones have been studied in aqueous MeCO2H acid medium in the presence of HClO4 at different temperatures. For ethyl methyl ketone and XC6H4COMe (X = p-Cl, p-Br or p-NO2) the reaction obeys the rate law −d[MnO4−]/dt = (kKe[H+][MnO4−][RCO Me])/(1 + Ke[H+][RCOMe]).␣But the oxidations of XC6H4COMe (X = p-Me and p-OMe)␣follow the rate equation −d[MnO4−]/dt = k3[H+][MnO4−][RCOMe]. The reaction involves a fast pre-equilibrium with intermediate formation of a permanganate ester before the two-electron transfer, rate-determining, step. A number of thermodynamic parameters have been evaluated.

Effects of viscosity and temperature on the kinetics of the electron-transfer reaction between the triplet state of zinc cytochrome c and cupriplastocyanin.

This is a study of the effects of viscosity (in the range of 0.8-790 cP), of temperature (in the range of 260.7-307.7 K), and of ionic strength (in the range of 2.5-20.0 mM) on the kinetics of photoinduced electron-transfer reaction 3Zncyt/pc(II) --> Zncyt+/pc(I) within the electrostatic complex of zinc cytochrome c and cupriplastocyanin at pH 7.0. The unimolecular rate constant is kF. The apparent activation parameters DeltaH*, DeltaS*, and DeltaG* for this reaction were obtained in experiments with aqueous glycerol solutions having a constant composition. The interpolation of kF values obtained at the constant composition into the dependence of kF on temperature at constant viscosity gave the proper activation parameters, which agree with those obtained in experiments with solutions having a constant viscosity. This agreement validates the latter method, which is more efficient than the former, for determining activation parameters of processes that are modulated by viscosity. The smooth change in kF is governed by the change in viscosity, not in other properties of the solvent, and it does not depend on the choice of the viscosigen. Donor/acceptor electronic coupling (HAB) and reorganizational energy (lambda), obtained by fitting of the temperature dependence of kF to the Marcus equation, are consistent with true electron transfer and with electron transfer that is coupled to, or gated by, a preceding structural rearrangement of the diprotein complex 3Zncyt/pc(II). The fact that at very high viscosity kF approaches zero shows that the reaction is probably gated throughout the investigated range of viscosity. Kinetic effects and noneffects of ionic strength, viscosity, and thermodynamic driving force indicate, but do not prove, that the reaction under consideration is gated. The kinetic effect of viscosity is analyzed in terms of two models. Because ln kF is a nonlinear function of ln eta, protein friction has to be considered in the analysis of viscosity effects on kinetics.

Correlation formula for the couplings at crossings between ionic and covalent molecular states

From an analysis of empirical and theoretical information on ionic and covalent interactions, a relationship is established between the coupling matrix elements (which measure the nonadia-batic splitting at avoided crossings and strongly influence transition probabilities) and basic quantities of separated partners (ionization potentials, electron affinities, polarizabilities). Although the systems considered here are limited to alkali atoms interacting with simple atoms and mole-cules, the proposed correlation rule paves the way for extensions to deal with electron transfer reaction dynamics for more general donor-acceptor couples.

Dynamics of hot-electron transfer in oriented methyl halides on GaAs(110)

Abstract The ordered molecular orientation of CH3X (X = Cl, Br and I) on GaAs(110) has been studied by angle-resolved time-of-flght (TOF) spectra of methyl fragments ejected by dissociation due to photoinduced electron transfer processes. A hot electron, produced in the substrate by UV irradiation, induces scission of CX bond in a manner similar to gas phase dissociation electron attachment (DEA). The methyl fragments ejected from a 1 ML layer have a unique angular distribution peaked about −45° ± 5° in the [ 0 1 ] direction defined by the GaAs(110) surface. The dynamics of electron transfer reactions in these systems, including the wavelength (193, 248 and 351 nm) dependence of the DEA reaction and the dependence of the dissociation cross section on the different methyl-halide molecules are discussed. The molecular orientation has been probed by near-edge X-ray adsorption fine structure (NEXAFS) experiments which indicate that the tilted orientation of absorbed molecules accounts for the angular distribution of methyl fragments ejected from hot-electron transfer reactions in these systems.

Excitation transfer, charge transfer and hydrogen abstraction reaction, kinetic studies of modification of DNA components

Laser induced acetone-sensitized excitation of DNA components offered, for the first time, T-T absorption spectra and direct measurement of triplet states kinetics of cytosine, cytidine, and dCMP, adenine, adenosine and dAMP and guanosine. Mechanisms and kinetics of electron transfer reactions between triplet acetone and purine bases and triplet thymine and electrophilic modifiers have been elucidated. Rapid electron transfer from hydroxycinnamic acid derivatives to oxidizing OH adducts of pyrimidines have been performed. Fast reduction and repair of oxidizing damage of pyrimidines have been achieved using pulse radiolysis techniques.

Reduction Potentials and Kinetics of Electron Transfer Reactions of Phenylthiyl Radicals: Comparisons with Phenoxyl Radicals

The reduction potentials relative to the standard hydrogen electrode (SHE) for a number of para-substituted phenylthiyl radicals (E°(p-XC6H4S•/p-XC6H4S-)) have been derived from pulse radiolytic studies of electron transfer equilibria which compare their values to those of radicals of known reduction potentials. A ladder combining the reduction potentials for both phenylthiyl and phenoxyl radicals has been established. These reduction potentials have been shown to be self-consistent and are intermediate between those of p-benzosemiquinone radical anion at 0.02 V and phenoxyl radical at 0.79 V. The reduction potential decreases as the electron donating power of the para substituent rises. The substituent effect is, however, much weaker for the phenylthiyl radicals than for their oxygen analogs. These observations demonstrate that the electronic interaction between the sulfur atoms and the aromatic ring system is much less than that which occurs with oxygen atoms. Examination of the rates of electron transf...