Rate Of Electron Transfer Reaction Research Articles

Density Functional Theory Based Study of the Electron Transfer Reaction at the Lithium Metal Anode in a Lithium–Air Battery with Ionic Liquid Electrolytes

Room temperature ionic liquids, which have unique properties such as a relatively wide electrochemical stability window and negligible vapor pressure, are promising candidates as electrolytes for developing lithium–air batteries with enhanced performance. The local current density, a crucial parameter in determining the performance of lithium–air batteries, is directly proportional to the rate constant of the electron transfer reaction at the surface of the anode that involves the oxidation of pure lithium metal into lithium ion (Li+). The electrochemical properties of ionic liquid based electrolytes, which can be molecularly tailored on the basis of the structure of their constituent cations and anions, play a crucial role in determining the reaction rate at the anode. In this paper, we present a novel approach, based on Marcus theory, to evaluate the effect of varying length of the alkyl side chain of model imidazolium based cations on the rates of electron transfer reaction at the anode. Density functi...

Photoinduced electron transfer between 2-methylanthraquinone and triethylamine in an ionic liquid: Time-resolved EPR and transient absorption spectroscopy study

Photoinduced electron transfer between 2-methylanthraquinone (MeAQ) and triethylamine (TEA) in a room-temperature ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), was investigated by comparing the time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy and the transient absorption spectroscopy. The results of TR-EPR spectroscopy, in which MeAQ was 8mmolL−1 and TEA was 150mmolL−1, indicated that the transient radical would exist longer time in [bmim][PF6] than in acetonitrile. At the delay time of 8μs after laser excitation, the TR-EPR signal transformed from an emissive peak into an absorptive peak when the experiment was performed in [bmim][PF6]. The results of the transient absorption spectroscopy, in which MeAQ was 0.1mmolL−1 and TEA was 2.2mmolL−1, showed that the efficiency and the rate of the photoinduced electron transfer reaction in [bmim][PF6] were obviously lower than that in acetonitrile. It was concluded that various factors, such as concentration, viscosity and local structural transformation of the solution, have an influence on the process of photoinduced electron transfer in [bmim][PF6].

Synthesis of Ag nanoparticles in Span/Span-Tween mixed surfactant system and its optical, kinetic and fluorimetric studies

Abstract Span and Span-Tween mixed surfactants have been used to stabilize the silver nanoparticles synthesized chemically and photochemically. This synthetic process of silver sol preparation has the advantage of producing homogeneous particles of very small size (∼1.65 nm) and finally aggregate brilliantly in a symmetric manner to form cubic clusters. Electromagnetic interaction in the close-packed assembly of silver aggregates generates its optical property that has been explained in the light of Maxwell-Garnett effective medium theory. A small sized silver nanoparticle has quenching properties on the fluorophore probes of methylene blue and cresol red. The particle size dependent fluorescence quenching property has been rationalized considering its surface area. Steady state circular dichroism and time resolved spectroscopic studies of Ag nanoparticles with bovine serum albumin (BSA) and human adult hemoglobin (Hb) proves the formation of a ground state BSA-Ag complex. The effect of different sizes of silver nanoparticle on the rate of electron transfer reaction between [Co III (NH 3 ) 5 Br](NO 3 ) 2 and Mohr's salt (Fe II ) has also been explained rationally.

Studies on electrochemical behaviour on NpO2 2+/NpO2 + redox couple at single walled carbon nanotube modified glassy carbon electrode (SWCNT-GC)

Abstract Electrochemistry of NpO2 2+/NpO2 + couple in 1 M H2SO4 was studied on bare and modified glassy carbon (GC) electrodes by cyclic voltammetry (CV), differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS). The modified electrode (SWCNTs-GC) was characterized by scanning electron microscopy (SEM). Electrocatalysis of NpO2 2+/NpO2 +redox reaction was observed on SWCNT-GC electrode. The lower charge transfer resistance at SWCNT-GC reflects faster rate of electron transfer reaction of Np(VI)/Np(V) couple. These results are interesting to develop electroanalytical methodologies for neptunium determination using SWCNT modified electrode. To the best of our knowledge, this is the first study on the electrocatalysis of neptunium on SWCNT modified electrode.

Mechanisms for control of biological electron transfer reactions

Electron transfer (ET) through and between proteins is a fundamental biological process. The rates and mechanisms of these ET reactions are controlled by the proteins in which the redox centers that donate and accept electrons reside. The protein influences the magnitudes of the ET parameters, the electronic coupling and reorganization energy that are associated with the ET reaction. The protein can regulate the rates of the ET reaction by requiring reaction steps to optimize the system for ET, leading to kinetic mechanisms of gated or coupled ET. Amino acid residues in the segment of the protein through which long range ET occurs can also modulate the ET rate by serving as staging points for hopping mechanisms of ET. Specific examples are presented to illustrate these mechanisms by which proteins control rates of ET reactions.

Key Interactions for Electron Transfer from Cytochrome C to Cytochrome c Oxidase in Respiratory Chain: Thermodynamic Characterization of Complex Formation for Electron Transfer

The electron transfer (ET) from cytochrome bc 1 to cytochrome c oxidase (CcO) is the final and crucial ET process in the respiratory chain, which is mediated by a small hemoprotein, cytochrome c (Cyt c) (Fig.1). Despite its biological significance, the lack of the structural information prevents us from understanding the molecular mechanism for the electron transfer. Recently, we utilized 1H-15N HSQC spectra of 15N-labeled Cyt c in the absence and presence of unlabeled CcO, and successfully identified the interaction site for CcO on Cyt c 1. As we previously reported1, the interaction site is constituted by the charged and hydrophobic residues in Cyt c, suggesting that both of electrostatic and hydrophobic interactions contribute to the formation of the Cyt c – CcO complex. Although the mutations of the charged residue resulted in the significant defects of the electron transfer activity in Cyt c, the substitution of the hydrophobic residues did not affect the dissociation constant for the complex formation and electron transfer rate from Cyt c to CcO. To examine the contribution of the hydrophobic interactions to the electron transfer reaction from Cyt c to CcO, we determined the thermodynamic parameters for the complex formation between Cyt c and CcO. The isothermal titration calorimetry (ITC) measurements have revealed that the complex formation of Cyt c with CcO induces the positive enthalpy (ΔH > 0) and positive entropy changes (ΔS > 0), showing that the complex formation is an endothermic and entropy-driven reaction. It is quite interesting that the protein-protein complex formation reaction is entropy-drivenand the analysis of the entropy change suggests that one of the factors to increase the entropy in the complex formation is dehydration of hydrophobic amino acid residues located in the interaction site. While the experimental confirmation of the dehydration of hydrophobic amino acid residues is rather difficult, we examined osmotic pressure effects on the dissociation constant for the complex formation between Cyt c and CcO. Based on thermodynamics, the dependence of the dissociation constant on the osmotic pressure is correlated with the volume change in the reaction of the complex formation. Such a volume change in the complex formation can be supposed to originate from the dissociation of hydrated water molecules from the protein-protein interaction site, associated with the complex formation between Cyt c and CcO. The dissociation constant was decreased with the increase of the osmotic pressure, corresponding to the negative volume change in the complex formation. Because the negative volume change is one of the characteristics of the dehydration of hydrophobic amino acid residues, the osmotic pressure dependence of the dissociation constant for the complex formation between Cyt c and CcO clearly demonstrated that water molecules are dehydrated from hydrophobic amino acid residues in the interaction site between Cyt c and CcO, confirming that the hydrophobic interactions are key interactions for the formation of the electron transfer complex. To determine the amino acid residues responsible for the dehydration and examine the contribution of the dehydration to the electron transfer reaction rate from Cyt c to CcO, some mutational studies on Cyt c are now in progress.

The influence of nanoparticles on enzymatic bioelectrocatalysis

Detailed experimental evidences that neither an overpotential of bioelectrocatalysis, nor direct electron transfer and bioelectrocatalytic reaction rates for an adsorbed enzyme depend on the size of gold nanoparticles, are presented.

Influence of the charge of amino group on rate and direction of intramolecular electron transfer reaction in shortlived oxidized peptides containing tryptophan and tyrosine

Influence of the charge of amino group on rate and direction of intramolecular electron transfer reaction in shortlived oxidized peptides containing tryptophan and tyrosine

Simultaneous detection of hydroxylamine and phenol using p-aminophenol-modified carbon nanotube paste electrode

A carbon paste electrode that was chemically modified with multiwall carbon nanotubes and p -aminophenol was used as a selective electrochemical sensor for the simultaneous detection of hydroxylamine (HX) and phenol. Cyclic voltammetry, double potential-step chronoamperometry, square wave voltammetry (SWV), and electrochemical impedance spectroscopy were used to investigate the use of p -aminophenol in the carbon nanotubes paste matrixes as a mediator for the electrocatalytic oxidation of HX and phenol in aqueous solution. The coefficient of electron transfer and catalytic reaction rate constant were determined using the electrochemical methods. Under optimized conditions, the electrocatalytic oxidation current peaks for HX and phenol increased linearly with concentration in the range of 0.5–180.0 and 10.0–650.0 μmol/L for HX and phenol, respectively. The detection limits for HX and phenol were 0.15 and 7.1 μmol/L, respectively. The anodic potential peaks of HX and phenol were separated by 0.65 V in SWV. Because of good selectivity and sensitivity, the present method provides a simple method for the selective detection of HX and phenol in practical samples such as water samples. A carbon paste electrode chemically modified with multiwall carbon nanotubes and p -aminophenol was prepared and used as a selective electrochemical sensor for the simultaneous determination of hydroxylamine and phenol in water samples.

Electrochemical studies of U(VI)/U(IV) redox reaction in 1M H2SO4at single-walled carbon nanotubes (SWCNTs) modified gold (Au) electrode

AbstractElectrochemistry of U(VI)/U(IV) couple in 1 M H2SO4was studied on bare and SWCNT modified gold (Au) electrodes by cyclic voltammetry (CV), differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS). The gold electrode modified with single-walled carbon nanotubes (SWCNTs-Au) was characterized by scanning electron microscopy (SEM). Electrocatalysis of U(VI)/U(IV) redox reaction was observed on SWCNT-Au. The lower charge transfer resistance at SWCNT-Au promoted the rate of electron transfer reaction of U(VI)/U(IV) couple. These results are interesting to develop electroanalytical methodologies for uranium determination using SWCNT modified electrodes. To the best of our knowledge, this is the first study on the electrocatalysis of uranium on SWCNT modified electrode.

Electron transfer/ion transfer mode of scanning electrochemical microscopy (SECM): a new tool for imaging and kinetic studies

A new mode of the scanning electrochemical microscope (SECM) operation was developed that combines reagent delivery from the nanopipette with electron transfer at the conductive substrate and ion transfer across the liquid/liquid interface supported at the nanopipette tip. This approach offers potential advantages for measurements of heterogeneous electron transfer kinetics and reaction rate imaging by enabling straightforward separation of the contributions of surface topography and reactivity features to the tip current. It addresses some other long-standing technical issues, including sensitive probing of low signal sources (e.g., immobilized enzymes or catalyst particles) without diffusional broadening and the elimination of the elevated background signal in generation/collection-type experiments. The high spatial resolution attainable in electron transfer/ion transfer mode experiments and the absence of redox mediator species in the bulk solution are advantageous for studies of biological cells.

Kinetics and Mechanism of Oxidation of L-Ascorbic Acid by Pt(IV)(aq) in Aqueous Hydrochloric Acid Medium

Reduction of [PtCl6]2− by L-ascorbic acid (H2ASc) in 0.1 M aqueous acid medium has been investigated spectrophotometrically under pseudo-first order condition at [PtCl6]2− = 0.005–0.007 mol dm−3, 0.05 ≤ [H2ASc]/mol dm−3 ≤ 0.3, 298 K ≤ T ≤ 308 K, [H+] = 0.14 mol dm−3, I=0.5 mol dm−3. The redox reaction follows the rate law: d[Pt(IV)]/dt = k[H2ASc][Pt(IV)], where k is the second-order rate constant and [H2ASc] is the total concentration of ascorbic acid. Electron transfer from [H2ASc] to Pt(IV) center leading to the release of two halide ions and formation of the reaction products, square planner Pt(II) halide complex, and dehydrated ascorbic acid is suggested. This redox reaction follows an outersphere mechanism as Pt(IV) complex is substituted inert. Activation parameters were calculated corresponding to rate of electron transfer reaction k. Activation parameters favor the electron transfer reaction.

Electron Transfer Reaction Dynamics of p-Nitroaniline in Water from Liquid to Supercritical Conditions

Photoexcitation dynamics of p-nitroaniline (pNA) have been investigated by femto-second transient absorption spectroscopy in water from liquid to supercritical conditions; along the isochoric line from the ambient condition to 664 K at 40.1 MPa and along the isothermal line from 40.1 to 36.1 MPa at 664 K. The rates of the back electron transfer reaction from the photoexcited charge transfer state to the electronic ground state was determined by the bleach recovery of the ground state absorption, and the successive vibrational relaxation in the electronic ground state was determined by the hot-band decay which was apparent at the red edge of the absorption. The variation of the back electron transfer rate was compared with the prediction based on the electron transfer theory including the Franck-Condon active vibrational modes. The results indicated that both the free energy change of the reaction and the change of the intramolecular vibrational reorganization energy cause the characteristic density (or temperature) dependence of the back electron transfer rate. The density dependence of the vibrational relaxation rate was compared with the collision frequency and the coordination number of the solvent molecule around the solute estimated by the molecular dynamics simulations. The density dependence of the coordination of a water oxygen atom to an amino hydrogen atom of pNA was found to be correlated with the density dependence of vibrational relaxation rate.

A mechanistic study on the electrocatalysis of the Pu(iv)/Pu(iii) redox reaction at a platinum electrode modified with single-walled carbon nanotubes (SWCNTs) and polyaniline (PANI)

The electrochemistry of the Pu(IV)/Pu(III) couple in 1 M sulphuric acid solution was studied on bare and modified platinum electrodes by cyclic voltammetry and electrochemical impedance spectroscopy. The platinum electrode was separately modified with single-walled carbon nanotubes (SWCNT-Pt) and polyaniline (PANI-Pt). The modified electrodes were characterized by scanning electron microscopy (SEM) and energy dispersive X-ray fluorescence (EDXRF). Electrocatalysis of the Pu(IV)/Pu(III) redox reaction was observed on both SWCNT-Pt and PANI-Pt. However, PANI-Pt showed better catalytic action for the electron transfer reaction of the Pu(IV)/Pu(III) couple. The Pu(IV)/Pu(III) couple showed quasi-reversible electron transfer behavior on a bare platinum electrode because of the PtO layer formation by the Pu(IV) solution at the electrode–electrolyte interface. In SWCNT-Pt, the direct interaction between Pu(IV) and platinum was blocked by SWCNTs and it diminished the oxide layer formation at the interface. The lower charge transfer resistance at SWCNT-Pt also promoted the rate of electron transfer reaction of the Pu(IV)/Pu(III) couple. Electrocatalysis of the Pu(IV)/Pu(III) couple on PANI-Pt was attributed to the cumulative effect of the Donnan interaction between the PANI and Pu(IV) anionic complex, specific adsorption of Pu(IV) on the reactive centres, low charge transfer resistance across the electrode–electrolyte interface and a catalytic chemical reaction coupled with the electron transfer reaction. To the best of our knowledge, this paper presents the first evidence of the electrocatalysis of actinides on SWCNT and PANI modified electrodes along with the detailed investigations of their electrocatalysis mechanisms.

Electronic coupling for charge transfer in donor–bridge–acceptor systems. Performance of the two-state FCD model

Electronic coupling is a key parameter that determines the rate of electron transfer reactions and electrical conductivity of molecular wires. To examine the performance of a two-state approach based on the orthogonal transformation of adiabatic states to diabatic states, we compare the effective donor-acceptor coupling V(DA) computed with three different approaches in model donor-bridge-acceptor (D-B-A) systems. It is found that V(DA) derived with the two-state method accounts properly for both the direct and superexchange interactions. The approach becomes, however, less accurate with the increasing energy difference of the donor and acceptor states. We suggest a simple diagnostic to identify the situation when the estimated coupling might be inaccurate and consider how to improve the performance of the two-state scheme in such a case.

Light harvesting zinc naphthalocyanine–perylenediimide supramolecular dyads: long-lived charge-separated states in nonpolar media

Photoinduced electron-transfer dynamics of self-assembled donor-acceptor dyads formed by axial coordination of zinc naphthalocyanine, ZnNc, and perylenediimide (PDI) bearing either pyridine (py) or imidazole (im) coordinating ligands were investigated. The PDIim unit was functionalized with tert-octylphenoxy groups at the bay positions, which avoid aggregation providing solubility, to examine the effect of the bulky substituents at the bay positions on the rates of electron-transfer reactions. The combination between zinc naphthalocyanine and perylenediimide entities absorbs light over a wide region of the visible and near infrared (NIR) spectrum. The binding constants of the self-assembled ZnNc:PDIpy (1) and ZnNc:PDIim (2) in toluene were found to be 2.40 × 10(4) and 1.10 × 10(5) M(-1), respectively, from the steady-state absorption and emission measurements, indicating formation of moderately stable complexes. The geometric and electronic calculations by using an ab initio B3LYP/6-311G method showed the majority of the highest occupied frontier molecular orbital (HOMO) on the zinc naphthalocyanine entity, while the lowest unoccupied molecular orbital (LUMO) was on the perylenediimide entities, suggesting that the charge-separated states of the supramolecular dyads are ZnNc˙(+):PDI˙(-). The electrochemical results suggest the exothermic charge-separation process via the singlet states of both ZnNc and PDI entities in nonpolar toluene. Upon coordination of perylenediimide to ZnNc, the main quenching pathway involved charge separation via the singlet-excited states of ZnNc and PDIs. Clear evidence of the intramolecular electron transfer from the singlet-excited state of ZnNc to PDI within the supramolecular dyads in toluene was monitored by the femtosecond laser photolysis by observing the characteristic absorption band of the PDI radical anion (PDI˙(-)) and the ZnNc radical cation (ZnNc˙(+)) in the visible and NIR regions. The rate constants of charge-separation (k(CS)) processes of the self-assembled dyads 1 and 2 were determined to be 4.05 × 10(10) and 1.20 × 10(9) s(-1), respectively. The rate constant of charge recombination (k(CR)) and the lifetime of charge-separated states (τ(CS)) of dyad 1 were determined to be 2.34 × 10(8) s(-1) and 4.30 ns, respectively. Interestingly, a slower charge recombination (2.20 × 10(7) s(-1)) and a longer lifetime of the charge separated state (45 ns) were observed in dyad 2 in nonpolar toluene by utilizing the nanosecond transient measurements. The absorption in a wide section of the solar spectrum and the high charge-separation/charge-recombination ratio suggest the usefulness of the self-assembled zinc naphthalocyanine-perylenediimide dyads as good photosynthetic models.

Temperature independence of the primary electron transfer reaction rate constants in photosynthetic bacterial reaction centers

Temperature independence of the primary electron transfer reaction rate constants in photosynthetic bacterial reaction centers

Direct simulation of electron transfer using ring polymer molecular dynamics: Comparison with semiclassical instanton theory and exact quantum methods

The use of ring polymer molecular dynamics (RPMD) for the direct simulation of electron transfer (ET) reaction dynamics is analyzed in the context of Marcus theory, semiclassical instanton theory, and exact quantum dynamics approaches. For both fully atomistic and system-bath representations of condensed-phase ET, we demonstrate that RPMD accurately predicts both ET reaction rates and mechanisms throughout the normal and activationless regimes of the thermodynamic driving force. Analysis of the ensemble of reactive RPMD trajectories reveals the solvent reorganization mechanism for ET that is anticipated in the Marcus rate theory, and the accuracy of the RPMD rate calculation is understood in terms of its exact description of statistical fluctuations and its formal connection to semiclassical instanton theory for deep-tunneling processes. In the inverted regime of the thermodynamic driving force, neither RPMD nor a related formulation of semiclassical instanton theory capture the characteristic turnover in the reaction rate; comparison with exact quantum dynamics simulations reveals that these methods provide inadequate quantization of the real-time electronic-state dynamics in the inverted regime.

Effect of Anion Ligation on Electron Transfer of Double-Linked Zinc Porphyrin−Fullerene Dyad

The interaction between metalloporphyrins and their axial ligands plays an important role in the electron transfer (ET) processes in which the excited porphyrin participates. An efficient photoinduced ET reaction of a double-linked zinc(II) porphyrin-fullerene dyad was demonstrated in ionic environment. The chloride ion of tetrabutylammonium chloride (TBACl) electrolyte solution ligates the zinc porphyrin moiety in the dyad which results in a red shift of the absorption bands and lowers the energy of the charge-separated state by about 0.26 eV as compared to the nonligated dyad. Excitation of the porphyrin chromophore results in ET from porphyrin to fullerene in a moderately polar solvent, anisole. In nonionic and nonligating ionic environments, the ET reaction occurs through an intermediate state, an intramolecular exciplex, which has emission in the near-infrared region of the spectrum. This emission is not observed directly for the dyad in TBACl/anisole solution, but evidence of the exciplex intermediate was seen in the time-resolved measurements. The lower energy of the charge-separated state in the ligated environment explains the different ET reaction rates determined in the spectroscopic studies: the charge recombination process of the ligated dyad is about 5 times faster than that of the nonligated one.

Effect of Ortho Substitution on the Charge Localization of Dinitrobenzene Radical Anions

Optical and electron paramagnetic resonance spectroscopies were used to study the radical anions of several m-dinitrobenzenes and p-dinitrobenzenes with substituents on ortho positions relative to the nitro groups. 1,4-Dinitrobenzene, 1,4-dimethyl-2,5-dinitrobenzene, and 2,5-dinitrobenzene-1,4-diamine radical anions are delocalized (class III) mixed valence species, but in the dinitrodurene radical anion the nitro groups are forced out of the ring plane due to the steric hindrance, which results in localization of the charge. The radical anions m-dinitrobenzene, 2,6-dinitrotoluene, and dinitromesitylene are all localized (class II) mixed valence species, as is common for m-dinitrobenzenes, and the rate of intramolecular electron transfer reaction strongly decreases with the number of methyl substituents. The same mechanism of rotation of the nitro groups out of the ring plane due to steric hindrance caused by neighboring methyl groups is also responsible for slowing the reaction. However, 2,6-dinitroaniline radical anion and 2,6-dinitrophenoxide radical dianion are charge-delocalized because the strong electron releasing amino and oxido groups increase the conjugation between the two charge-bearing units.