Rate Of Electron Transfer Reaction Research Articles

Electrochemical Visualization of Defect-induced Density of States at Single Graphene Flake

The density of states (DOS) at the Fermi level plays crucial role in the property and application of graphene-based material. We introduce an electrochemical approach that allows straightforwardly probing the defect-induced DOS at single flake of chemically reduced graphene oxide (r-GO). The idea is based on the earlier belief that the non-adiabatic electron transfer (ET) reaction rate at electrode/electrolyte interface increases with the Fermi-level DOS of electrode. By investigating the voltammetric responses of defect-rich and defect-lean r-GO nanoflakes using a variety of redox probes, we observe strong correlation between the ET kinetics and the defect densities on r-GO flakes, which are tuned by varying the flake sizes and preparation procedures. Our results thus validate a few general beliefs, namely, the defects can boost the Fermi-level DOS in graphene, the ET reaction on graphene-based material is non-adiabatic, and the non-adiabatic electrochemical ET kinetics indeed increases with the DOS of electrode.

Comparative study of low-index {1 0 1}-TiO2, {0 0 1}-TiO2, {1 0 0}-TiO2 and high-index {2 0 1}-TiO2 on glyphosate adsorption and photo-degradation

TiO2 has attracted widespread concern about its photo-catalysis in regard to water contaminant removal. However, TiO2 with different exposed facets shows considerable variation in adsorption and photo-catalysis performance. This study aims at comparatively investigating the adsorption and photo-catalysis performance of low-index {1 0 1}-TiO2, {0 0 1}-TiO2, {1 0 0}-TiO2 and high-index {2 0 1}-TiO2. The results showed that, compared to low-index TiO2, {2 0 1}-TiO2 exhibited a higher effectiveness on glyphosate (PMG) removal. In detail, the highest Langmuir adsorption capacity was observed on {2 0 1}-TiO2 (0.88, molecules/nm2). Based on 2D-COS and ATR-FTIR analysis, it is proved that the transformation sequence of each PMG functional group adsorbed on TiO2 followed different order. In addition, PMG photo-degradation on {2 0 1}-TiO2 had a higher apparent pseudo-first-order reaction constant (kapp = 3.0h−1) than other three TiO2. This is mainly attributed to a higher generation of reactive oxygen species (ROS), a larger electron transfer and oxygen reduction reaction (ORR) rate of {2 0 1}-TiO2, compared to other three TiO2, as investigated by ESR, photo-current and linear sweep voltammetry measurement, respectively. Furthermore, PMG photo-degradation on the four TiO2 was primarily due to OH. Thus, it can be reasonably inferred that the comparative investigation of high-index and low-index TiO2 can provide an integrated insight into similar adsorption and photo-catalysis behavior in the environment.

Non enzymatic NADH-dependent reduction of Cis-[Co(en)2(H2O)2]3+ in aqueous medium

The kinetics of the electron transfer reaction of NADH with Cis-[Co(en)2(H2O)2]3+ has been studied over the range 1.0 ≤ 102 [NADH] ≤ 3.0 mol dm-3, 7.0 ≤ pH ≤ 8.0 and 200C ≤ t ≤ 350C in aqueous medium. The rate of electron transfer reaction was found to be first-order dependence each in [NADH]T and Cis-[Co(en)2(H2O)2]3+T. The products of the reaction were found to be NAD+ and Co(II). The corresponding activation parameters of the electron transfer reactions were found to be as ΔH#=27.55 kJ mol-1 and  ΔS#= -189.35 JK-1mol-1.Â

Flavanols react preferentially with quinones through an electron transfer reaction, stimulating rather than preventing wine browning

Wine oxidation changes the chemistry, sensory profile and color of wines. In wine oxidation, phenolics are oxidized to quinones and these reactive compounds can be quenched by sacrificial nucleophiles, such as the A-ring on flavanoids, preventing oxidative damage from the loss of desirable flavor molecules. The “catechol” B-ring on flavanoids, in contrast, can be oxidized by quinones through electron transfer reactions that lead to flavanoid quinones, precursors of browning products. Here we compared the rate of flavanoids reacting by either nucleophilic quinone quenching, or by electron transfer to generate flavanoid quinones. Our approach is based on mathematical modeling of reaction data to derive the rate constants of reactions of A-ring quenching vs B-ring electron transfer with caffeic acid quinone, by fitting the predicted loss of precursors and the appearance of products (or derivatives) with experimental data collected by LC/MS. The rate constant of the electron transfer reaction of caffeic acid quinone towards 4-methyl-catechol was fast (k4MC = 3.43E-2 mLmol−1sec−1) but nucleophilic reactions with afzelechin (kAfz = 2.53E-3 mLmol−1sec−1) or malvidin-3-glucoside (kMal = 5.34E-3 mLmol−1sec−1), were much slower. No reaction was detected between caffeic acid quinone and isorhamnetin. Additionally, the electron transfer reaction rate of catechin and caffeic acid quinone was much faster at pH 7 (1.22E-02 mLmol−1sec−1) vs pH 3.5 (1.79E-03 mLmol−1sec−1). These results help explain why the reaction of catechin and caffeic acid quinone favors the formation of browning products, and more so at higher pH values. Furthermore, bisulfite reacted with quinones faster than the electron transfer reactions, preventing the browning observed in the reaction of catechin with caffeic acid quinone in the absence of bisulfite.

Limitations in Estimation of E1/2 from Cyclic Voltammetric Data

It is common practice to estimate the standard (E0) or formal (E0’) potential from cyclic voltammetric data. A frequent estimate of E0 or E0’ is taken from either the peak potential (Ep), the half wave potential (E1/2), or the average of the cathodic and anodic peak potentials (Epc+Epa)/2. Often, this suffices to identify the standard or formal potential within 100 mV, but confidence in such estimates is not always justified. Mechanisms that include both electron transfer (E) and chemical (C) steps can impact the quality of estimated standard and formal potentials. In some systems, limitations to the estimated standard and formal potentials depend on the rates of heterogeneous electron transfer and homogenous reactions. Here, the quality of estimated standard and formal potentials based on peak potentials, half-wave potentials, and average potentials for the cathodic and anodic sweeps are described for various mechanisms. Cases where the standard and formal potentials are well estimated from measured cyclic voltammetric data are identified and the quality of the estimated standard and formal potentials are characterized. That is, specify whether the estimate is within 100 mV, 50 mV, or 10 mV for a given mechanism. Situations where the estimate fails are identified and alternative means to estimate the standard and formal potential are provided. Several reaction schemes that include EC, EE, CE, and the various ECE mechanisms are characterized.

Redox Equivalents and Mitochondrial Bioenergetics.

Mitochondrial energy metabolism depends upon high-flux and low-flux electron transfer pathways. The former provide the energy to support chemiosmotic coupling for oxidative phosphorylation. The latter provide mechanisms for signaling and control of mitochondrial functions. Few practical methods are available to measure rates of individual mitochondrial electron transfer reactions; however, a number of approaches are available to measure steady-state redox potentials (E h) of donor/acceptor couples, and these can be used to gain insight into rate controlling reactions as well as mitochondrial bioenergetics. Redox changes within the respiratory electron transfer pathway are quantified by optical spectroscopy and measurement of changes in autofluorescence. Low-flux pathways involving thiol/disulfide redox couples are measured by redox Western blot and mass spectrometry-based redox proteomics. Together, the approaches provide the opportunity to develop integrated systems biology descriptions of mitochondrial redox signaling and control mechanisms.

Rutile TiO2 nanorods/MWCNT composites for enhanced simultaneous photocatalytic oxidation of organic dyes and reduction of metal ions

This work aims to enhance the charge separation and the photocatalytic performance of TiO2 by combining two ways of modification synthesizing TiO2 Nanorods with modified morphology and anchoring on the surface of multi-walled carbon nanotubes (MWCNT). Simple hydrothermal method is performed to synthesize pure rutile TiO2 Nanorods/MWCNT composite. The photocatalytic activity of the nanocomposite is evaluated for the degradation of the organic dye Bromophenol blue (BPB). An enhancement in the photocatalytic activity is observed using TiO2 Nanorods/MWCNT composite as compared to pure TiO2 Nanorods. A synergism between the MWCNT and the TiO2 Nanorods is expected to suppress the recombination of photogenerated electron–hole pairs and hence the photocatalytic activity. Moreover, simultaneous degradation of BPB and reduction of Ag(I) is studied using TiO2 Nanorods/MWCNT nanocomposite. Enhancement in the photocatalytic degradation of BPB is observed in the presence of Ag(I) as compared to O2 as electron acceptor due to higher TiO2 electron transfer reaction rate to the Ag(I) ion as compared to its rate to the dissolved O2. The results provide an effective method for dual benefits for the wastewater purification from organic and inorganic pollutants.

Theoretical study on interaction of cytochrome f and plastocyanin complex by a simple coarse-grained model with molecular crowding effect

ABSTRACTWe present a simple coarse-grained model with the molecular crowding effect in solvent to investigate the structure and dynamics of protein complexes including association and/or dissociation processes and investigate some physical properties such as the structure and the reaction rate from the viewpoint of the hydrophobic intermolecular interactions of protein complex. In the present coarse-grained model, a function depending upon the density of hydrophobic amino acid residues in a binding area of the complex is introduced, and the function involves the molecular crowding effect for the intermolecular interactions of hydrophobic amino acid residues between proteins. We propose a hydrophobic intermolecular potential energy between proteins by using the density-dependent function. The present coarse-grained model is applied to the complex of cytochrome f and plastocyanin by using the Langevin dynamics simulation to investigate some physical properties such as the complex structure, the electron transfer reaction rate constant from plastocyanin to cytochrome f and so on. We find that for proceeding the electron transfer reaction, the distance between metals in their active sites is necessary within about 18 Å. We discuss some typical complex structures formed in the present simulation in relation to the molecular crowding effect on hydrophobic interactions.

Driving Force Dependence of Electron Transfer from Electronically Excited [Ir(COD)(μ-Me2pz)]2 to Photo-Acid Generators.

We report the rates of electron transfer (ET) reactions of electronically excited [Ir(COD)(μ-Me2pz)]2 with onium salt photoacid generators (PAGs). The reduction potentials of the PAGs span a large electrochemical window that allows determination of the driving force dependence of the ET reactions. Rate constants of ET from electronically excited [Ir(COD)(μ-Me2pz)]2 to onium PAGs are determined by the reaction driving force until the diffusion limit in acetonitrile is reached.

The Kinetics of Electron Transfer Reaction of Methylene Blue and Titanium Trichloride in different Solvents

The kinetics of the electron transfer reaction of methylene blue and titanium trichloride was studied in water and aqueousalcoholic solvents at various temperatures by spectrophotometry. The rate of reaction was observed by taking change inabsorbance as a function of time at λmax 660 nm. The reaction is pseudo-first order, dependent on concentration of titaniumtrichloride at fixed concentration of methylene blue.The effect of solvent was studied in the pH/Ho range from 4 to 7. It was observed that the rate of reaction increased withincrease in polarity of the reaction medium. The rate of reaction was high in acidic condition and decreased with furtherincrease in hydrogen ions activity. The increase in temperature increased the rate of electron transfer reaction of methyleneblue and titanium trichloride. Activation energy (Ea) was calculated by Arrhenius relation. The absence of any reactionintermediate was confirmed by spectroscopic and kinetic investigations. A plausible mechanism for the reaction in line withouter-sphere reaction pathway has been proposed. Thermodynamic parameters such as activation energy (Ea), enthalpychange (∆H), free energy change (∆G) and entropy change (∆S) were also evaluated.

(Invited) Evidence for Magnetic Effects on Electron Transfer

Electron transfer events are commonly considered in light of the transfer of charge. But, electrons also have properties of spin. Here, various systems are evaluated for impact of spin on rates of electron transfer. Literature and experimental data are considered. Electron spins interact with magnetic fields. In our experimental studies, magnetic microparticles are introduced to the electrode surface. From the literature, correlations between measured parameters and magnetic properties of electrodes and electroactive species are presented. This includes rate constants for heterogeneous and homogeneous electron transfer. From laboratory studies with micromagnets held to the electrode with a Nafion coating, impacts on homogeneous and heterogeneous electron transfer are found. For transition metal complexes such as M(bpy)₃z+ present at near contact in Nafion, electron hopping rates are measured as self exchange rates, k₁₁. When compared to simple Nafion films, rates of electron electron hopping are enhanced in the presence of magnets; effects scale with the number of spin (unpaired electrons) involved in the reaction. Temperature studies chart variation in the energy of activation (reorganization energy) and the pre-exponential factor. For adsorbate reactions, magnetic effects are substantial for reactions such as CO oxidation and hydride formation on palladium. Rates are enhanced > 100 fold. Hydrogen evolution reaction (HER) rates for poorly electrocatalytic electrodes are enhanced substantially on introduction of micromagnets. Because transfer of electrons requires transfer of both charge and spin, electron transfer rates can be slowed when rates of spin transfer are slow. Facilitating rates of spin transfer as by introduction of micromagnets increases rates of electron transfer reactions.

Electron transfer in pH-sensitive nitroxide radicals

Two pH-sensitive stable nitroxide radicals are characterized by EPR spectroscopy and cyclic voltammetry in aqueous solution at different pH values. The rates of the photochemically-induced electron transfer reactions from the protonated and deprotonated forms of the radicals to a ruthenium complex are determined.

Cavity-Controlled Chemistry in Molecular Ensembles.

The demonstration of strong and ultrastrong coupling regimes of cavity QED with polyatomic molecules has opened new routes to control chemical dynamics at the nanoscale. We show that strong resonant coupling of a cavity field with an electronic transition can effectively decouple collective electronic and nuclear degrees of freedom in a disordered molecular ensemble, even for molecules with high-frequency quantum vibrational modes having strong electron-vibration interactions. This type of polaron decoupling can be used to control chemical reactions. We show that the rate of electron transfer reactions in a cavity can be orders of magnitude larger than in free space for a wide class of organic molecular species.

Effects of Operating Temperature on the Electrical Performance of a Li-air Battery operated with Ionic Liquid Electrolyte

Li-air cell operated with ionic liquid electrolytes is a very promising energy storage technology for electric vehicle and plug-in hybrid electric vehicle due to several favorable characteristics of ionic liquids. However, Li-air cells that employ room temperature ionic liquid (RTIL) electrolytes exhibit poor performance due to limited oxygen solubility and low reactant species mobility. To circumvent these aforementioned drawbacks, we investigated the electrical performance of a Li-air cell with ionic liquid electrolytes operating at high temperature. A continuum based model is used to quantify the performance of the Li-air cell, with an ionic liquid (MPPY-TFSI) electrolyte, as a function of operating temperature. Simulations at the atomistic scale, such as molecular dynamics (MD) and density functional theory (DFT) calculations are used to obtain key properties for the continuum model. These properties include electron transfer reaction rate constant, species diffusivity and oxygen solubility. The MD simulations indicate that oxygen solubility in ionic liquid increases with temperature, which is very favorable for high temperature operation. The continuum based cell level simulation results show that the battery performance can be improved significantly by increasing operating temperature. For instance, specific capacity as high as 3000mAh/g can be achieved at 110°C operating temperature, which is almost 25 times higher than its counterpart at room temperature. Simulation results also reveal that by increasing the operating temperature, the specific capacity can be improved significantly for high load current density, which is one of the most critical drawbacks in RTIL based Li-air battery. We also studied the effect of cathode thickness on the performance of Li-air battery at different operating temperatures. The transport limitation of oxygen and lithium ions can be alleviated at higher operating temperatures, suggesting that even thicker cathode materials can be used to enhance the cell capacity at elevated temperature.

A dynamical approach to non-adiabatic electron transfers at the bio-inorganic interface.

A methodology is proposed to investigate electron transfer reactions between redox-active biomolecular systems (e.g. a protein) and inorganic surfaces. The whole system is modelled at the atomistic level using classical molecular dynamics - making an extensive sampling of the system's configurations possible - and the energies associated with the redox-active complex reduction are calculated using a hybrid quantum/classical approach along the molecular dynamics trajectory. The non-adiabaticity is introduced a posteriori using a Monte Carlo approach based on the Landau-Zener theory extended to treat a metal surface. This approach thus allows us to investigate the role of the energy fluctuations, determined by the dynamical evolution of the system, as well as the role of non-adiabaticity in affecting the kinetic rate of the electron transfer reaction. Most notably, it allows us to investigate the two contributions separately, hence achieving a detailed picture of the mechanisms that determine the rate. The analysis of the system configurations also allows us to relate the estimated electronic coupling to the structural and dynamic properties of the system. As a test case, the methodology is here applied to study the electron transfer reaction between cytochrome c and a gold surface. The results obtained explain the different electron transfer rates experimentally measured for two different concentrations of proteins on the electrode surface.

Deriving the exact nonadiabatic quantum propagator in the mapping variable representation.

We derive an exact quantum propagator for nonadiabatic dynamics in multi-state systems using the mapping variable representation, where classical-like Cartesian variables are used to represent both continuous nuclear degrees of freedom and discrete electronic states. The resulting Liouvillian is a Moyal series that, when suitably approximated, can allow for the use of classical dynamics to efficiently model large systems. We demonstrate that different truncations of the exact Liouvillian lead to existing approximate semiclassical and mixed quantum-classical methods and we derive an associated error term for each method. Furthermore, by combining the imaginary-time path-integral representation of the Boltzmann operator with the exact Liouvillian, we obtain an analytic expression for thermal quantum real-time correlation functions. These results provide a rigorous theoretical foundation for the development of accurate and efficient classical-like dynamics to compute observables such as electron transfer reaction rates in complex quantized systems.

Ring-polymer instanton theory of electron transfer in the nonadiabatic limit.

We take the golden-rule instanton method derived in the previous paper [J. O. Richardson, R. Bauer, and M. Thoss, J. Chem. Phys. 143, 134115 (2015)] and reformulate it using a ring-polymer instanton approach. This gives equations which can be used to compute the rates of electron-transfer reactions in the nonadiabatic (golden-rule) limit numerically within a semiclassical approximation. The multidimensional ring-polymer instanton trajectories are obtained efficiently by minimization of the action. In this form, comparison with Wolynes' quantum instanton method [P. G. Wolynes, J. Chem. Phys. 87, 6559 (1987)] is possible and we show that our semiclassical approach is the steepest-descent limit of this method. We discuss advantages and disadvantages of both methods and give examples of where the new approach is more accurate.

Temperature- and pressure-dependent kinetico-mechanistic studies on the formation of mixed-valence {(tetraamine)CoIIINCFeII(CN)5}− units

The reduction of CoIII in the tetraamine-encapsulating ligand complex [CoIII{(μ-ET)(Me2)cyclen}(H2O)2]3+ by [FeII(CN)6]4− has been studied kinetico-mechanistically at different pH, temperatures, and pressures. The process agrees with the expected outer-sphere redox mechanism, with the value of the encounter-complex equilibrium constant large enough to allow for kinetic determination of the first-order electron transfer reaction rate constant. The value of the encounter-complex equilibrium constant, Kpre-eq, is not only dependent on the charge of the redox partners, but also on the establishment of an important network of hydrogen bonds. These can also explain the differences obtained in the activation volumes determined for the diaqua and bis-hydroxo complexes. Neither the leaching of CoII nor the presence of [FeIII(CN)6]3− is observed in the final reaction medium, which indicates that a fast sequence involving substitution on the transient CoII complex followed by a fast inner-sphere electron transfer takes place. This sort of mechanism has already been established for encapsulating pentaamine ligand complexes, but this is the first example of such a sequential reaction occurring on a tetradentate ligand complex. Preliminary UV–Vis and electrochemical characterization experiments have been conducted on the final reaction mixtures, suggesting the formation of a stable cyanide-bridged CoIII/FeII mixed-valence complex of the same type reported in the literature for encapsulating {CoIII(N)5} skeletons.

Real Time Quantification of Ultrafast Photoinduced Bimolecular Electron Transfer Rate: Direct Probing of the Transient Intermediate.

Fluorescence quenching studies through steady-state and time-resolved measurements are inadequate to quantify the bimolecular electron transfer rate in bulk homogeneous solution due to constraints from diffusion. To nullify the effect of diffusion, direct evaluation of the rate of formation of a transient intermediate produced upon the electron transfer is essential. Methyl viologen, a well-known electron acceptor, produces a radical cation after accepting an electron, which has a characteristic strong and broad absorption band centered at 600 nm. Hence it is a good choice to evaluate the rate of photoinduced electron transfer reaction employing femtosecond broadband transient absorption spectroscopy. The time constant of the aforementioned process between pyrene and methyl viologen in methanol has been estimated to be 2.5 ± 0.4 ps using the same technique. The time constant for the backward reaction was found to be 14 ± 1 ps. These values did not change with variation of concentration of quencher, i.e., methyl viologen. Hence, we can infer that diffusion has no contribution in the estimation of rate constants. However, on changing the solvent from methanol to ethanol, the time constant of the electron transfer reaction has been found to increase and has accounted for the change in solvent reorganization energy.

Study of the ion-channel behavior on glassy carbon electrode supported bilayer lipid membranes stimulated by perchlorate anion.

In this paper, a kind of didodecyldimethylammonium bromide (DDAB) layer membranes was supported on a glassy carbon electrode (GCE). We studied the ion channel behavior of the supported bilayer lipid membrane by scanning electrochemical microscopy (SCEM) in tris(2,2'-bipyridine) ruthenium(II) solution. Perchlorate anion was used as a presence of stimulus and ruthenium(II) complex cations as the probing ions for the measurement of SECM, the lipid membrane channel was opened and exhibited the behavior of distinct SECM positive feedback curve. The channel was in a closed state in the absence of perchlorate anions while reflected the behavior of SECM negative feedback curve. The rates of electron transfer reaction in the lipid membranes surface were detected and it was dependant on the potential of SECM.