Dynamics of adiabatic electron transfer reactions at metal electrodes

We have investigated the dynamics of adiabatic electron transfer reactions at metal electrodes using a Hamiltonian suggested by Schmickler (J. Electroanal. Chem., 204 (1986) 31). We show that in the adiabatic limit the problem reduces to that of dynamics of a single variable, the shift of the ionic orbital caused by its interaction with the solvent. This variable is identified as the reaction co-ordinate for the problem and we show that in certain limits, it obeys a non-linear Volterra type integral equation, with a stochastic inhomogeneous term. For an inhomogenous term with the autocorrelation function decaying exponentially, this may be converted into a differential equation for Brownian motion. This equation can be analysed to obtain the rate, through the associated Fokker-Planck equation. The rate so obtained, has a correction to the pre-exponential factor obtained by Schmickler. A possible extension to inner sphere reactions is also discussed.     

Proton transfer reactions on semiconductor surfaces

The concept of proton affinity on semiconductor surfaces has been explored through an investigation of the chemistry of amines on the Ge(100)-2 x 1, Si(100)-2 x 1, and C(100)-2 x 1 surfaces. Multiple internal reflection Fourier transform infrared (MIR-FTIR) spectroscopy, temperature programmed desorption (TPD), and density functional theory (DFT) calculations were used in the studies. We find that methylamine, dimethylamine, and trimethylamine undergo molecular chemisorption on the Ge(100)-2 x 1 surface through the formation of Ge-N dative bonds. In contrast, primary and secondary amines react on the Si(100)-2 x 1 surface via N-H dissociation. Since N-H dissociation of amines at semiconductor surfaces mimics a proton-transfer reaction, the difference in chemical reactivities of the Ge(100)-2 x 1 and Si(100)-2 x 1 surfaces toward N-H dissociation can be interpreted as a decrease of proton affinity down a group in the periodic table. The trend in proton affinities of the two surfaces is explained in terms of thermodynamics and kinetics. Solid-state effects on the C(100)-2 x 1 surface and the surface proton affinity concept are discussed based on our theoretical predictions.     

Dissociative electron transfer on ionic surfaces

A theory of electron transfer between adsorbed ions and electrically charged ionic surfaces is developed. Electron transfer from a charged AgBr surface to an Ag⁺ ion, as well as from Br^? ion to AgBr surface holes is studied. The Ag and Br atoms resulting from the process are shown to be dissociated from the surface.     

Dissociative electron transfer reactions

We consider recent data on dissociative electron transfer reactions in which the electron transfer causes practically concerted dissociation of the chemical bond in the reagent. We discuss considerable experimental data on reactions in the gas phase and in solutions, and also existing theoretical models for describing the kinetics of these complex processes. Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 34, No. 2, pp. 67–78, March–April, 1998.     

Light-induced electron transfer reactions

The reactions of the luminescent excited states of the polypyridine-ruthenium(II) complexes (^*RuL₃²⁺) with electron acceptors and donors are discussed. These electron transfer reactions convert the excited state into RuL₃³⁺ and RuL₃⁺, respectively. The former ruthenium complex is a more powerful oxidant and the latter is a more powerful reductant than the excited state itself. Some applications of these complexes in the conversion and storage of solar energy are presented. Theoretical models for electron transfer reactions are described and the implications of these models for the quenching and back electron transfer reactions are discussed. It is pointed out that the exploitation of the inverted region may provide a useful means of slowing down back electron transfer reactions.     

Kinetics of electron transfer reactions of metal complexes at impermeable redox active polymeric films on electrode surfaces and charge transport within the polymer film

At rotated Pt disk electrodes coated with thin films of the redox polymer poly-[Ru(vbpy)₃]²⁺, ruthenium and iron bipyridine complexes dissolved in acetonitrile can become oxidized by two pathways. The first is diffusion of the solute complex through the polymeric film to react at its normal potential E_sub⁰, at the Pt/polymer interface. The second is a mediated electron transfer cross-reaction between the solute complex and poly-[Ru(vbpy) ₃]²⁺ sites generated in the film at adequately positive potentials. The mediated reaction, as judged from the lack of variation of its rate k_crsΓ with the quantity of polymer mediator sites present in the multimolecular layer film, and from other evidence, is confined to the outer few (one?) monolayers of ruthenium polymer film sites. The mediated reaction becomes the dominant pathway for films with Γ_T ～2×10^?9 mol/cm² of ruthenium polymer sites, owing to the low permeability (measured independently) of the solutecomplexes into the film. The rate k_crsΓ could be measured when the solute complex oxidation potential is more positive than that of the redox film, and is too fast to measure when E_sub⁰, is more negative than the redox film E_cal?⁰, New theory is presented and evaluated to describe the rising portions of the voitammetric waves for the nine solute complexes studied. The rate of charge transport through the poly-[Ru(vbpy)₃]²⁺ film becomes controlling under certain conditions and can be thereby measured as well.     

Gold sputtered electrode surfaces enhance direct electron transfer reactions of human cytochrome P450s

We immobilized human cytochrome P450 (CYP), a membrane-bound enzyme, onto both smooth and nanostructured surfaces of gold electrodes via a naphthalene thiolate monolayer film. Rapid electron transfer of CYP with an electrode as a redox partner took place when the enzyme was immobilized onto an electrode surface with nanostructures. This structure was easily prepared by conventional sputtering techniques. A well-defined pair of peaks was observed at ? 0.175 V (vs. SHE) with the largest heterogeneous electron transfer rate constant of 340 s^? 1 for human CYP. The positive redox potential shift of 45 mV upon drug (testosterone) binding was clearly detected, which corresponded to a change in the spin states of heme iron in CYP. The present study showed that gold sputtered surfaces are very useful for direct electron transfer reactions of human CYP isoforms.     

Photochemical electron transfer reactions in alkylcobaloximes

It is shown that photolysis with visible light (λ > 420 nm) of any alkylcobaloxime procedes via a mechanism involving an initial electron transfer reaction from an equatorial ligand to the central metal to produce a cobalt(II) species which retains both original axial ligands. In a subsequent rearrangement of the equatorial ligand a hydrogen atom is ejected.     

Photochemical electron transfer reactions of tirapazamine

The absorption and fluorescence spectra of 3-aminobenzo-1,2,4-triazine di-N-oxide (tirapazamine) have been recorded and exhibit a dependence on solvent that correlates with the Dimroth ET30 parameter. Time-dependent density functional theory calculations reveal that the transition of tirapazamine in the visible region is pi-->pi* in nature. The fluorescence lifetime is 98+/-2 ps in water. The fluorescence quantum yield is approximately 0.002 in water. The fluorescence of tirapazamine is efficiently quenched by electron donors via an electron-transfer process. Linear Stern-Volmer fluorescence quenching plots are observed with sodium azide, potassium thiocyanate, guanosine monophosphate and tryptophan (Trp) methyl ester hydrochloride. Guanosine monophosphate, tyrosine (Tyr) methyl ester hydrochloride and Trp methyl ester hydrochloride appear to quench the fluorescence at a rate greater than diffusion control implying that these substrates complex with tirapazamine in its ground state. This complexation was detected by absorption spectroscopy.     

Barrierless electron transfer bond fragmentation reactions

The ultrafast N-O bond fragmentation in a series of N-methoxypyridyl radicals, formed by one-electron reduction of the corresponding N-methoxypyridiniums, has been investigated as potentially barrierless electron-transfer-initiated chemical reactions. A model for the reaction involving the electronic and geometric factors that control the shape of the potential energy surface for the reaction is described. On the basis of this model, molecular structural features appropriate for ultrafast reactivity are proposed. Femtosecond kinetic measurements on these reactions are consistent with a kinetic definition of an essentially barrierless reaction, i.e., that the lifetime of the radical is a few vibrational periods of the fragmenting bond, for the p-methoxy-N-methoxypyridyl radical.     

Activation entropy of electron transfer reactions

We report microscopic calculations of free energies and entropies for intramolecular electron transfer reactions. The calculation algorithm combines the atomistic geometry and charge distribution of a molecular solute obtained from quantum calculations with the microscopic polarization response of a polar solvent expressed in terms of its polarization structure factors. The procedure is tested on a donor–acceptor complex in which ruthenium donor and cobalt acceptor sites are linked by a four-proline polypeptide. The reorganization energies and reaction energy gaps are calculated as a function of temperature by using structure factors obtained from our analytical procedure and from computer simulations. Good agreement between two procedures and with direct computer simulations of the reorganization energy is achieved. The microscopic algorithm is compared to the dielectric continuum calculations. We found that the strong dependence of the reorganization energy on the solvent refractive index predicted by continuum models is not supported by the microscopic theory. Also, the reorganization and overall solvation entropies are substantially larger in the microscopic theory compared to continuum models.     

Re-defining the kinetics of redox reactions on passive metal surfaces

It is known that the kinetics of redox reactions occurring on the surfaces of passive metals depend upon the properties of the passive film, ostensibly due to quantum mechanical tunnelling (QMT) of electrons and holes between the metal and the redox couple at the barrier layer/solution (bl/s) interface. In this paper, the tunnelling probability is used to inter-convert the exchange current densities for the redox reactions occurring at the bl/s interface and on the hypothetical bare metal surface. We review our previous work on combining QMT theory with the point defect model (PDM), which provides an analytical expression for the bl thickness as a function of voltage. By combining QMT theory and the PDM, we derive a modified form of the generalized Butler-Volmer equation that requires as input only the kinetic parameters for the redox reaction on the hypothetical bare surface and parameters contained in the PDM. The application of the theory is illustrated with reference to the corrosion of carbon steel in concrete pore solution, to calculating the corrosion potential of, and crack growth rate in, sensitized type 304 SS in boiling water reactor (BWR) coolant circuits, and the use of hydrogen oxidation on platinum to determine the thickness of the bl as a function of voltage and temperature. This illustrates a new, powerful technique for probing the formation of passive films on metal surfaces.

     

Kinetic measurements of hydrocarbon conversion reactions on model metal surfaces

Examples from recent studies in our laboratory are presented to illustrate the main tools available to surface scientists for the determination of the kinetics of surface reactions. Emphasis is given here to hydrocarbon conversions and studies that rely on the use of model systems, typically single crystals and controlled (ultrahigh vacuum) environments. A detailed discussion is provided on the use of temperature-programmed desorption for the determination of activation energies as well as for product identification and yield estimations. Isothermal kinetic measurements are addressed next by focusing on studies under vacuum using molecular beams and surface-sensitive spectroscopies. That is followed by a review of the usefulness of high-pressure cells and other reactor designs for the emulation of realistic catalytic conditions. Finally, an analysis of the power of isotope labeling and chemical substitutions in mechanistic research on surface reactions is presented.     

Photocatalytic single electron transfer reactions on TiO2 semiconductor

The use of inorganic semiconductor particles such as titanium dioxide(TiO_2) has received relatively less attention in organic chemistry, although semiconductor particles have been widely used as a single electron transfer photocatalyst in waterpurification, air-cleaning, and self-cleaning. In recent years, the photocatalysis on semiconductor particles has become an active area of research even in organic chemistry, since the heterogeneous semiconductor photocatalysis leads to the unique redox organic reactions. In an early stage, the semiconductor photocatalysis was applied to the oxidation of organic molecules.Semiconductor particles have also the potential to induce the reductive chemical transformations in the absence of oxygen(O_2),by using the suitable sacrificial hole scavenger. In this review, we summarize the representative examples of the reductive and oxidative organic reactions using semiconductor particles and the recent applications to the stereoselective reactions.     

1,3-二碘六氟丙烷的电子转移反应

在四醋酸铅的催化下,二氟二碘甲烷(CF2I2,1)与四氟乙烯加成生成1,3-二碘六氟丙烷(ICF2CF2CF2I,3).3与烯烃、炔烃和丙二酸二乙酯阴离子发生电子转移反应。     

Photochemical initiation of electron transfer reactions.

It is quite apparent that the use of photoinitiated electron transfer has become a powerful, if not dominating, technique in the study of biological electron transfer. It provides a means to measure directly very fast processes and, through the choice of approach (flavin semiquinones or related, metal substitution in hemes or modification with ruthenium) and experimental conditions, provides the ability to probe different features of the electron transfer mechanism. Nevertheless, much remains to be done to fully understand biological electron transfer. The use of photoinitiated electron transfer has clearly established a role for a number of factors involved in controlling the kinetics of electron transfer, including driving force, distance, intervening media, dynamics (conformational gating) and orientation of redox centers. However, we have only scratched the surface in regard to understanding in molecular terms the details of electron transfer in physiologically relevant systems. Thus, even relatively simple and well characterized systems like cytochrome c-cytochrome c peroxidase remain obscure in terms of the through-protein electron paths (intervening media) and the role of protein dynamics in controlling electron transfer kinetics. Indeed, it is the through-protein paths and conformational gating that are unique to biological systems and provide nature with the capability of modulating electron transfer kinetics to optimize biological function. Of the techniques described here, the use of flavin semiquinones is clearly the least invasive in that there is no evidence that flavin semiquinones bind to or perturb physiologically relevant systems. However, this approach is constrained in that precise distances and orientations are not always known, and the range of driving forces available is limited. In contrast, metal substitution and ruthenation allow the positions of interacting redox centers to be reasonably well defined and can provide a very large range of driving force. This latter point is particularly important since it provides a means to discriminate between rate limiting electron transfer and conformational gating. Nevertheless, chemically modifying redox proteins runs the risk of structural and electrostatic alterations which can be difficult to detect but have profound effects on the redox kinetics. Moreover, the intrinsic protein dynamics can be affected, resulting, in the worst case, in changes in conformational gating which cannot be resolved from rate limiting electron transfer. Given the early stage of development of photo-initiated electron transfer, substantial progress can be expected in the next few years. No doubt new approaches will be developed and existing approaches further refined. Especially important, the theoretical basis for interpreting and understanding electron transfer will continue to evolve.(ABSTRACT TRUNCATED AT 400 WORDS)     

Salt effects in photoinduced electron transfer reactions

Metal salts and oxygen react synergistically to inhibit back-electron-transfer in photoinduced reactions.     

Semi-empirical potential energy surfaces for hydrogen-atom transfer reactions

A method for constructing potential energy surfaces previously proposed by the author has been extended to hydrogen transfer reactions between halide, oxygen, and carbon atoms. A qualitative relation was found between the repulsive energy and the number of anti-bonding electrons. In general, the calculated kinetic isotope effect is in satisfactory agreement with observed values and the contributions by H-atom tunneling to the rate of reaction is smaller than that obtained from other surfaces.