Cyclic chronocoulemetry: Disproportionation and dimerization reactions coupled to electron transfer

An extension of double-potential-step chronocoulemetry to multiple or cyclic techniques has been developed, and its potential applications in the study of coupled chemical reactions in electrochemistry are discussed. Disproportionation and dimerization mechnisms are considered. Wroking curves have been calculated with the use of the finite difference digital simulation method. It is shown that better resolution for disproportionation and dimerization reactions can be obtained with cyclic chronocoulometry than with double-potential-step chronocoulemetry. The method has been experimentally verified measuring the disproportionation reaction U(V) in 1 mol dm^?3 sodium hydrogen carbonate solutions. A rate coefficient of 15.6 dm³ mol^?1 was calculated for this reaction.     

Water-mediated electron transfer between protein redox centers

Recent experimental and theoretical investigations show that water molecules between or near redox partners can significantly affect their electron-transfer (ET) properties. Here we study the effects of intervening water molecules on the electron self-exchange reaction of azurin (Az), by performing a conformational sampling on the water medium and by using a newly developed ab initio method to calculate transfer integrals between molecular redox sites. We show that the insertion of water molecules at the interface between the copper active sites of Az dimers slightly increases the overall ET rate, while some favorable water conformations can considerably enhance the ET kinetics. These features are traced back to the interplay of two competing factors: the electrostatic interaction between the water and protein subsystems (mainly opposing the ET process for the water arrangements drawn from MD simulations) and the effectiveness of water in mediating ET coupling pathways. Such an interplay provides a physical basis for the found absence of correlation between the electronic couplings derived through ab initio electronic structure calculations and the related quantities obtained through the Empirical Pathways (EP) method. In fact, the latter does not account for electrostatic effects on the transfer integrals. Thus, we conclude that the water-mediated electron tunneling is not controlled by the geometry of a single physical pathway. We discuss the results in terms of the interplay between different ET pathways controlled by the conformational changes of one of the water molecules via its electrostatic influence. Finally, we examine the dynamical effects of the interfacial water and check the validity of the Condon approximation.     

Proton coupled electron transfer and redox active tyrosines in Photosystem II

In this article, progress in understanding proton coupled electron transfer (PCET) in Photosystem II is reviewed. Changes in acidity/basicity may accompany oxidation/reduction reactions in biological catalysis. Alterations in the proton transfer pathway can then be used to alter the rates of the electron transfer reactions. Studies of the bioenergetic complexes have played a central role in advancing our understanding of PCET. Because oxidation of the tyrosine results in deprotonation of the phenolic oxygen, redox active tyrosines are involved in PCET reactions in several enzymes. This review focuses on PCET involving the redox active tyrosines in Photosystem II. Photosystem II catalyzes the light-driven oxidation of water and reduction of plastoquinone. Photosystem II provides a paradigm for the study of redox active tyrosines, because this photosynthetic reaction center contains two tyrosines with different roles in catalysis. The tyrosines, YZ and YD, exhibit differences in kinetics and midpoint potentials, and these differences may be due to noncovalent interactions with the protein environment. Here, studies of YD and YZ and relevant model compounds are described.     

An electron density based analysis to establish the electronic adiabaticity of proton coupled electron transfer reactions

Electrons and protons are the main actors in play in proton coupled electron transfer (PCET) reactions, which are fundamental in many biological (i.e., photosynthesis and enzymatic reactions) and electrochemical processes. The mechanism, energetics and kinetics of PCET reactions are strongly controlled by the coupling between the transferred electrons and protons. Concerted PCET reactions are classified according to the electronical adiabaticity degree of the process. To discriminate among different mechanisms, we propose a new analysis based on the use of electron density based indexes. We choose, as test case, the 3-Methylphenoxyl/phenol system in two different conformations to show how the proposed analysis is a suitable tool to discriminate between the different degree of adiabaticity of PCET processes. The very low computational cost of this procedure is extremely promising to analyze and provide evidences of PCET mechanisms ruling the reactivity of many biological and catalytic systems.     

Competition between dissolution and redox reactions in the electrochemistry ofn-GaP

The lowering of the photocorrosion ofn-GaP in 1M KOH in the presence of several redox systems was investigated using neutron activation analysis. After thermal neutron irradiation of GaP and annihilation of irradiation induced defects by annealing processes the photodissolution was investigated by measuring the activity of the corresponding electrolyte solutions. The dependence of the stabilization of the photoelectrode on concentration and redox potential of the reduced form of the redox couples I^–, Fe (CN) ₆ ^4– and SO ₃ ^2– was measured. It was found that the stabilization is growing with growing concentration and lowering of the redox potential of the corresponding redox couple.     

Ion/electron coupled transport through polyaniline membrane: fast transmembrane redox reactions at neutral pH

Different oxidizing and reducing agents can be separated by a solid polymer membrane, but they still react. The possibility to conduct this type of transmembrane redox reactions coupled with electroneutral electron/chloride countertransport is demonstrated here for polyaniline (PANI) membrane doped with camphor sulfonic acid (CSA). For the 80 microm film, the transmembrane reaction rate can be as high as 5 x 10 (-8) mol/(cm (2) s) with FeCl 3 as the oxidizing and ascorbic acid as the reducing agents, which is approximately 25 times higher than described by us earlier with HCl-doped PANI membrane. Both solutions separated by the membrane with CSA can have pH near neutral, which is impossible with HCl-doping. Advanced kinetic mechanism of electron/ion coupled transport including chloride equilibrium at the interface is proposed, and major kinetic parameters are estimated. Interfacial redox rate constants (cm/s) are compared with the rate constants determined for polarized membranes by electrochemical methods and also with usual first and second order rate constants in the bulk volume. Possibility to conduct different redox reactions between substances which are not mixed and are separated by a membrane makes this new process very attractive for chemical and environmental engineering.     

Electroanalytical simulations. Orthogonal collocation simulation of fast second-order chemical reactions coupled to an electron transfer with a heterogeneous equivalent formulation

The combination of orthogonal collocation and the heterogeneous equivalent technique is extended to simulate cyclic voltammograms of fast second-order follow-up reactions coupled to an electron transfer at an electrode surface. The $\text{ED}$ (reversible electron transfer with irreversible follow-up dimerization) and $\text{EC}$₂ (reversible electron transfer with irreversible second-order follow-up reaction) models are considered. The non-linear boundary equations are solved numerically. No linear approximation of the concentration profiles is required. The use of non-linear space coordinate transformations is described. Peak potential and peak current function results are compared with literature values and agreement is found. The transition between the second-order $\text{EC}$₂ and the corresponding first-order mechanisms is discussed.     

Extension of the improved “heterogeneous equivalent” method to the digital simulation of the slow psuedo-first order homogeneous reactions coupled to the electrode reaction

The improved “Heterogeneous Equivalent” method for simulation of very fast first and pseudo-first order homogeneous reactions coupled with electrode reaction has been extended for the treatment of moderately fast and slow homogeneous reactions. New algorithms have been developed avoiding the simulation of homogeneous kinetics through finite difference approximation. Instead, the analytical expression for the term containing the kinetic parameter has been included in combination with other constant parameters (such as the diffusion coefficient).     

Metals in biology: Low-lying states of iron-oxygen and iron-sulfur complexes modeling electron transfer in redox reactions

Energy surfaces of low-lying states of planar complexes [Fe(C₂H₂X₂)₂]ⁿ with X=O and S and total charges n varying between + 3 and − 2 have been investigated by quantum chemical ab initio MO-SCF calculations of double zeta quality. Through studies of such properties as geometry, electron density distribution and molecular orbital energies it has been concluded that some of the states can be referred to as Fe (III) and others as Fe (II) states. The lowest states are found to be those with n = − 1 and 0 for X = O and n = − 1 and − 2 for X = S. For both the oxygen and the sulfur complexes Fe (III) and Fe (II) states are very close in energy. Supposing other electron acceptors and donors to be available, possible schemes for redox reactions between metal and ligand are suggested.     

Mechanistic implications of proton transfer coupled to electron transfer.

The kinetics of electron transfer for the reactions cis-[Ru(IV)(bpy)2(py)(O)]2+ + H+ + [Os(II)(bpy)3]2+ <==> cis-[Ru(III)(bpy)2(py)(OH)]2+ + [Os(III)(bpy)3]3+ and cis-[Ru(III)(bpy)2(py)(OH)]2+ + H+ + [Os(II)(bpy)3]2+ <==> cis-[Ru(II)(bpy)2(py)(H2O)]2+ + [Os(III)(bpy)3]3+ have been studied in both directions by varying the pH from 1 to 8. The kinetics are complex but can be fit to a double "square scheme" involving stepwise electron and proton transfer by including the disproportionation equilibrium, 2cis-[Ru(III)(bpy)2(py)(OH)]2+ <==> (3 x 10(3) M(-1) x s(-1) forward, 2.1 x 10(5) M(-1) x s(-1) reverse) cis-[Ru(IV)(bpy)2(py)(O)]2+ + cis-[Ru(II)(bpy)2(py)(H2O)]2+. Electron transfer is outer-sphere and uncoupled from proton transfer. The kinetic study has revealed (1) pH-dependent reactions where the pH dependence arises from the distribution between acid and base forms and not from variations in the driving force; (2) competing pathways involving initial electron transfer or initial proton transfer whose relative importance depends on pH; (3) a significant inhibition to outer-sphere electron transfer for the Ru(IV)=O2+/Ru(III)-OH2+ couple because of the large difference in pK(a) values between Ru(IV)=OH3+ (pK(a) < 0) and Ru(III)-OH2+ (pK(a) > 14); and (4) regions where proton loss from cis-[Ru(II)(bpy)2(py)(H2O)]2+ or cis-[Ru(III)(bpy)2(py)(OH)]2+ is rate limiting. The difference in pK(a) values favors more complex pathways such as proton-coupled electron transfer.     

Proton-coupled electron transfer in solution, proteins, and electrochemistry

Recent advances in the theoretical treatment of proton-coupled electron transfer (PCET) reactions are reviewed. These reactions play an important role in a wide range of biological processes, as well as in fuel cells, solar cells, chemical sensors, and electrochemical devices. A unified theoretical framework has been developed to describe both sequential and concerted PCET, as well as hydrogen atom transfer (HAT). A quantitative diagnostic has been proposed to differentiate between HAT and PCET in terms of the degree of electronic nonadiabaticity, where HAT corresponds to electronically adiabatic proton transfer and PCET corresponds to electronically nonadiabatic proton transfer. In both cases, the overall reaction is typically vibronically nonadiabatic. A series of rate constant expressions have been derived in various limits by describing the PCET reactions in terms of nonadiabatic transitions between electron-proton vibronic states. These expressions account for the solvent response to both electron and proton transfer and the effects of the proton donor-acceptor vibrational motion. The solvent and protein environment can be represented by a dielectric continuum or described with explicit molecular dynamics. These theoretical treatments have been applied to numerous PCET reactions in solution and proteins. Expressions for heterogeneous rate constants and current densities for electrochemical PCET have also been derived and applied to model systems.     

Theoretical and mechanistic aspects of proton-coupled electron transfer in electrochemistry

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Homogeneous redox catalysis of electrochemical reactions: Part III. Rate determining electron transfer. Kinetic characterization of follow-up chemical reactions

Electrochemical systems involving moderately fast charge transfers and very fast irreversible follow-up chemical reactions usually escape from kinetic and mechanistic characterization through the standard use of electrochemical techniques. It is shown that these difficulties can be overcome using an indirect approach which involves the homogeneous redox catalysis of the considered electrochemical reaction. A procedure for determining the rate constant of such fast follow-up reaction is presented. It is illustrated by the determination of the cleavage rate constant of the chlorobenzene anion radical in DMF which reaches a value on the order of 10⁷ s^?1.     

The energy of the homogeneous electron gas to second order

The energy of an infinite, homogeneous electron gas is examined by second order perturbation theory using a Hartee-Fock rather than a noninteracting particle unperturbed state. The second order energy still diverges for small promotions k , albert than as ln|ln k| rather than as In k.     

Nonperturbative quantum simulation of time-resolved nonlinear spectra: Methodology and application to electron transfer reactions in the condensed phase

A quantum dynamical method is presented to accurately simulate time-resolved nonlinear spectra for complex molecular systems. The method combines the nonpertubative approach to describe nonlinear optical signals with the multilayer multiconfiguration time-dependent Hartree theory to calculate the laser-induced polarization for the overall field–matter system. A specific nonlinear optical signal is obtained by Fourier decomposition of the overall polarization. The performance of the method is demonstrated by applications to photoinduced ultrafast electron transfer reactions in mixed-valence compounds and at dye–semiconductor interfaces.     

Application of second order BWEN perturbation theory to some simple reactions

The second order Brillouin-Wigner perturbation expansion with the Epstein-Nesbet partitioning is applied to some isomerization and insertion reactions, using the 6-31G^* basis set. BWEN2 is found to be comparable in accuracy with RSMP2 for predictions of energy barriers and isomerization energies.     

The effect of 19-electron formation constants on the electrochemistry and electron transfer induced substitution reactions of cyclopentadienylmetal halide complexes

The reduction of cyclopentadienylmetal halide complexes is generally considered to involve addition of an electron to an orbital that is antibonding with respect to the metal-halide bond. Subsequent metal-halide bond cleavage yields the halide and an organometallic radical. At inert electrodes, this radical is reduced further to an 18-electron anion. This series of reactions constitutes a prototypical ECE mechanism. Chemical reduction can be used to divert the radical into other pathways such as electron transfer chain catalyzed substitution. Attempts to initiate such reductively induced substitution reactions of CpFe(CO)₂I and Cp′Mo(CO)₃I give very different results, suggesting that these very similar complexes are reduced via substantially different mechanisms. Very likely, the molybdenum complex reacts via a DISP mechanism instead of ECE. The difference in electrochemical reduction mechanism as well as the different reactivity toward reductively induced substitution are explained in terms of a difference in the formation constants of 19-electron intermediates.