Science.gov

Sample records for electron transfer model

  1. Modeling electron transfer in photosystem I.

    PubMed

    Makita, Hiroki; Hastings, Gary

    2016-06-01

    Nanosecond to millisecond time-resolved absorption spectroscopy has been used to study electron transfer processes in photosystem I particles from Synechocystis sp. PCC 6803 with eight different quinones incorporated into the A1 binding site, at both 298 and 77K. A detailed kinetic model was constructed and solved within the context of Marcus electron transfer theory, and it was found that all of the data could be well described only if the in situ midpoint potentials of the quinones fell in a tightly defined range. For photosystem I with phylloquinone incorporated into the A1 binding site all of the time-resolved optical data is best modeled when the in situ midpoint potential of phylloquinone on the A/B branch is -635/-690 mV, respectively. With the midpoint potential of the F(X) iron sulfur cluster set at -680 mV, this indicates that forward electron transfer from A(1)(-) to F(X) is slightly endergonic/exergonic on the A/B branch, respectively. Additionally, for forward electron transfer from A(1)(-) to F(X), on both the A and B branches the reorganization energy is close to 0.7 eV. Reorganization energies of 0.4 or 1.0 eV are not possible. For the eight different quinones incorporated, the same kinetic model was used, allowing us to establish in situ redox potentials for all of the incorporated quinones on both branches. A linear correlation was found between the in situ and in vitro midpoint potentials of the quinones on both branches. PMID:26994812

  2. Analytical model for rates of electron attachment and intramolecular electron transfer in electron transfer dissociation mass spectrometry.

    PubMed

    Simons, Jack

    2010-05-26

    A new physical model is put forth to allow the prediction of electron transfer rates and distances for (i) intramolecular transfer from an n > or = 3 Rydberg orbital on a positive site to a disulfide or amide bond site and (ii) intermolecular transfer from an anion donor to an n > or = 3 Rydberg orbital of a positively charged polypeptide. Although ab initio methods have proven capable of handling such electron transfer events when the Rydberg orbital has principal quantum number n = 3, they have proven to be incapable of handling Rydberg states having quantum number n > 3, so having a new tool capable of handling n > 3 Rydberg states is important. The model (i) focuses on each Rydberg orbital's large peak of high amplitude, (ii) approximates the electron density within this peak as constant within a radial shell characterized by a radius and thickness T both of which depend on the quantum number n, and (iii) assumes that strong coupling (either with an orbital of an anion donor or to a disulfide sigma* or a backbone amide pi* orbital) occurs when the valence orbital penetrates fully within the radial shell of the Rydberg orbital. These assumptions permit a derivation of the ratios of rates of electron transfer for n > 3 to those for n = 3. Combining these ratios with ab initio rates for n = 3 allows one to make rate predictions for inter- and intramolecular electron transfer involving Rydberg orbitals appropriate to the electron transfer dissociation process. One important prediction of this model is that the combination of large-penetration and Landau-Zener surface-crossing conditions places very severe limitations on which Rydberg levels can initially be populated in electron transfer dissociation. Another prediction is that a Rydberg orbital of a given principal quantum number n has a limited range of distances over which it can transfer an electron; sigma* or pi* orbitals either too far from or too close to a given Rydberg orbital cannot accept an electron

  3. Modeling biofilms with dual extracellular electron transfer mechanisms

    PubMed Central

    Renslow, Ryan; Babauta, Jerome; Kuprat, Andrew; Schenk, Jim; Ivory, Cornelius; Fredrickson, Jim; Beyenal, Haluk

    2013-01-01

    Electrochemically active biofilms have a unique form of respiration in which they utilize solid external materials as terminal electron acceptors for their metabolism. Currently, two primary mechanisms have been identified for long-range extracellular electron transfer (EET): a diffusion- and a conduction-based mechanism. Evidence in the literature suggests that some biofilms, particularly Shewanella oneidensis, produce the requisite components for both mechanisms. In this study, a generic model is presented that incorporates the diffusion- and the conduction-based mechanisms and allows electrochemically active biofilms to utilize both simultaneously. The model was applied to S. oneidensis and Geobacter sulfurreducens biofilms using experimentally generated data found in the literature. Our simulation results show that 1) biofilms having both mechanisms available, especially if they can interact, may have a metabolic advantage over biofilms that can use only a single mechanism; 2) the thickness of G. sulfurreducens biofilms is likely not limited by conductivity; 3) accurate intrabiofilm diffusion coefficient values are critical for current generation predictions; and 4) the local biofilm potential and redox potential are two distinct parameters and cannot be assumed to have identical values. Finally, we determined that simulated cyclic and squarewave voltammetry based on our model are currently not capable of determining the specific percentages of extracellular electron transfer mechanisms in a biofilm. The developed model will be a critical tool for designing experiments to explain EET mechanisms. PMID:24113651

  4. Modeling biofilms with dual extracellular electron transfer mechanisms.

    PubMed

    Renslow, Ryan; Babauta, Jerome; Kuprat, Andrew; Schenk, Jim; Ivory, Cornelius; Fredrickson, Jim; Beyenal, Haluk

    2013-11-28

    Electrochemically active biofilms have a unique form of respiration in which they utilize solid external materials as terminal electron acceptors for their metabolism. Currently, two primary mechanisms have been identified for long-range extracellular electron transfer (EET): a diffusion- and a conduction-based mechanism. Evidence in the literature suggests that some biofilms, particularly Shewanella oneidensis, produce the requisite components for both mechanisms. In this study, a generic model is presented that incorporates the diffusion- and the conduction-based mechanisms and allows electrochemically active biofilms to utilize both simultaneously. The model was applied to S. oneidensis and Geobacter sulfurreducens biofilms using experimentally generated data found in the literature. Our simulation results show that (1) biofilms having both mechanisms available, especially if they can interact, may have a metabolic advantage over biofilms that can use only a single mechanism; (2) the thickness of G. sulfurreducens biofilms is likely not limited by conductivity; (3) accurate intrabiofilm diffusion coefficient values are critical for current generation predictions; and (4) the local biofilm potential and redox potential are two distinct parameters and cannot be assumed to have identical values. Finally, we determined that simulated cyclic and squarewave voltammetry based on our model are currently not capable of determining the specific percentages of extracellular electron transfer mechanisms in a biofilm. The developed model will be a critical tool for designing experiments to explain EET mechanisms. PMID:24113651

  5. Modeling biofilms with dual extracellular electron transfer mechanisms

    SciTech Connect

    Renslow, Ryan S.; Babauta, Jerome T.; Kuprat, Andrew P.; Schenk, Jim; Ivory, Cornelius; Fredrickson, Jim K.; Beyenal, Haluk

    2013-11-28

    Electrochemically active biofilms have a unique form of respiration in which they utilize solid external materials as their terminal electron acceptor for metabolism. Currently, two primary mechanisms have been identified for long-range extracellular electron transfer (EET): a diffusion- and a conduction-based mechanism. Evidence in the literature suggests that some biofilms, particularly Shewanella oneidensis, produce components requisite for both mechanisms. In this study, a generic model is presented that incorporates both diffusion- and conduction-based mechanisms and allows electrochemically active biofilms to utilize both simultaneously. The model was applied to Shewanella oneidensis and Geobacter sulfurreducens biofilms using experimentally generated data found the literature. Our simulation results showed that 1) biofilms having both mechanisms available, especially if they can interact, may have metabolic advantage over biofilms that can use only a single mechanism; 2) the thickness of Geobacter sulfurreducens biofilms is likely not limited by conductivity; 3) accurate intrabiofilm diffusion coefficient values are critical for current generation predictions; and 4) the local biofilm potential and redox potential are two distinct measurements and cannot be assumed to have identical values. Finally, we determined that cyclic and squarewave voltammetry are currently not good tools to determine the specific percentage of extracellular electron transfer mechanisms used by biofilms. The developed model will be a critical tool in designing experiments to explain EET mechanisms.

  6. Molecular Models for Conductance in Junctions and Electrochemical Electron Transfer

    NASA Astrophysics Data System (ADS)

    Mazinani, Shobeir Khezr Seddigh

    This thesis develops molecular models for electron transport in molecular junctions and intra-molecular electron transfer. The goal is to identify molecular descriptors that afford a substantial simplification of these electronic processes. First, the connection between static molecular polarizability and the molecular conductance is examined. A correlation emerges whereby the measured conductance of a tunneling junction decreases as a function of the calculated molecular polarizability for several systems, a result consistent with the idea of a molecule as a polarizable dielectric. A model based on a macroscopic extension of the Clausius-Mossotti equation to the molecular domain and Simmon's tunneling model is developed to explain this correlation. Despite the simplicity of the theory, it paves the way for further experimental, conceptual and theoretical developments in the use of molecular descriptors to describe both conductance and electron transfer. Second, the conductance of several biologically relevant, weakly bonded, hydrogen-bonded systems is systematically investigated. While there is no correlation between hydrogen bond strength and conductance, the results indicate a relation between the conductance and atomic polarizability of the hydrogen bond acceptor atom. The relevance of these results to electron transfer in biological systems is discussed. Hydrogen production and oxidation using catalysts inspired by hydrogenases provides a more sustainable alternative to the use of precious metals. To understand electrochemical and spectroscopic properties of a collection of Fe and Ni mimics of hydrogenases, high-level density functional theory calculations are described. The results, based on a detailed analysis of the energies, charges and molecular orbitals of these metal complexes, indicate the importance of geometric constraints imposed by the ligand on molecular properties such as acidity and electrocatalytic activity. Based on model calculations of

  7. Molecular structures of porphyrin-quinone models for electron transfer

    SciTech Connect

    Fajer, J.; Barkigia, K.M.; Melamed, D.; Sweet, R.M.; Kurreck, H.; Gersdorff, J. von; Plato, M.; Rohland, H.C.; Elger, G.; Moebius, K.

    1996-08-15

    Synthetic porphyrin-quinone complexes are commonly used to mimic electron transport in photosynthetic reaction centers and to probe the effects of energetics, distances, and relative orientations on rates of electron transfer between donor-acceptor couples. The structures of two such models have been determined by X-ray diffraction. The redox pairs consist of a zinc porphyrin covalently linked to benzoquinone in cis and trans configurations via a cyclohexanediyl bridge. The crystallographic studies were undertaken to provide a structural foundation for the extensive body of experimental and theoretical results that exists for these compounds in both the ground and photoinduced charge-separated states. The results validate conclusions reached from theoretical calculations, EPR and two-dimensional NMR results for these states. 15 refs., 6 figs., 2 tabs.

  8. A stochastic reorganizational bath model for electronic energy transfer

    SciTech Connect

    Fujita, Takatoshi E-mail: aspuru@chemistry.harvard.edu; Huh, Joonsuk; Aspuru-Guzik, Alán E-mail: aspuru@chemistry.harvard.edu

    2014-06-28

    Environmentally induced fluctuations of the optical gap play a crucial role in electronic energy transfer dynamics. One of the simplest approaches to incorporate such fluctuations in energy transfer dynamics is the well known Haken-Strobl-Reineker (HSR) model, in which the energy-gap fluctuation is approximated as white noise. Recently, several groups have employed molecular dynamics simulations and excited-state calculations in conjunction to account for excitation energies’ thermal fluctuations. On the other hand, since the original work of HSR, many groups have employed stochastic models to simulate the same transfer dynamics. Here, we discuss a rigorous connection between the stochastic and the atomistic bath models. If the phonon bath is treated classically, time evolution of the exciton-phonon system can be described by Ehrenfest dynamics. To establish the relationship between the stochastic and atomistic bath models, we employ a projection operator technique to derive the generalized Langevin equations for the energy-gap fluctuations. The stochastic bath model can be obtained as an approximation of the atomistic Ehrenfest equations via the generalized Langevin approach. Based on this connection, we propose a novel scheme to take account of reorganization effects within the framework of stochastic models. The proposed scheme provides a better description of the population dynamics especially in the regime of strong exciton-phonon coupling. Finally, we discuss the effect of the bath reorganization in the absorption and fluorescence spectra of ideal J-aggregates in terms of the Stokes shifts. We find a simple expression that relates the reorganization contribution to the Stokes shifts – the reorganization shift – to the ideal or non-ideal exciton delocalization in a J-aggregate. The reorganization shift can be described by three parameters: the monomer reorganization energy, the relaxation time of the optical gap, and the exciton delocalization length

  9. REFLECTIONS ON THE TWO-STATE ELECTRON TRANSFER MODEL.

    SciTech Connect

    Brunschwig, B.S.

    2000-01-12

    There is general agreement that the two most important factors determining electron transfer rates in solution are the degree of electronic interaction between the donor and acceptor sites, and the changes in the nuclear configurations of the donor, acceptor, and surrounding medium that occur upon the gain or loss of an electron Ll-51. The electronic interaction of the sites will be very weak, and the electron transfer slow, when the sites are far apart or their interaction is symmetry or spin forbidden. Since electron motion is much faster than nuclear motion, energy conservation requires that, prior to the actual electron transfer, the nuclear configurations of the reactants and the surrounding medium adjust from their equilibrium values to a configuration (generally) intermediate between that of the reactants and products. In the case of electron transfer between , two metal complexes in a polar solvent, the nuclear configuration changes involve adjustments in the metal-ligand and intraligand bond lengths and angles, and changes in the orientations of the surrounding solvent molecules. In common with ordinary chemical reactions, an electron transfer reaction can then be described in terms of the motion of the system on an energy surface from the reactant equilibrium configuration (initial state) to the product equilibrium configuration (final state) via the activated complex (transition state) configuration.

  10. Nonadiabatic anharmonic electron transfer.

    PubMed

    Schmidt, P P

    2013-03-28

    The effect of an inner sphere, local mode vibration on an electron transfer is modeled using the nonadiabatic transition probability (rate) expression together with both the anharmonic Morse and the harmonic oscillator potential. For an anharmonic inner sphere mode, a variational analysis uses harmonic oscillator basis functions to overcome the difficulties evaluating Morse-model Franck-Condon overlap factors. Individual matrix elements are computed with the use of new, fast, robust, and flexible recurrence relations. The analysis therefore readily addresses changes in frequency and/or displacement of oscillator minimums in the different electron transfer states. Direct summation of the individual Boltzmann weighted Franck-Condon contributions avoids the limitations inherent in the use of the familiar high-temperature, gaussian form of the rate constant. The effect of harmonic versus anharmonic inner sphere modes on the electron transfer is readily seen, especially in the exoergic, inverted region. The behavior of the transition probability can also be displayed as a surface for all temperatures and values of the driving force/exoergicity Δ = -ΔG. The temperature insensitivity of the transfer rate is clearly seen when the exoergicity equals the collective reorganization energy (Δ = Λ(s)) along a maximum ln (w) vs. Δ ridge of the surface. The surface also reveals additional regions for Δ where ln (w) appears to be insensitive to temperature, or effectively activationless, for some kinds of inner sphere contributions. PMID:23556710

  11. Nonadiabatic anharmonic electron transfer

    SciTech Connect

    Schmidt, P. P.

    2013-03-28

    The effect of an inner sphere, local mode vibration on an electron transfer is modeled using the nonadiabatic transition probability (rate) expression together with both the anharmonic Morse and the harmonic oscillator potential. For an anharmonic inner sphere mode, a variational analysis uses harmonic oscillator basis functions to overcome the difficulties evaluating Morse-model Franck-Condon overlap factors. Individual matrix elements are computed with the use of new, fast, robust, and flexible recurrence relations. The analysis therefore readily addresses changes in frequency and/or displacement of oscillator minimums in the different electron transfer states. Direct summation of the individual Boltzmann weighted Franck-Condon contributions avoids the limitations inherent in the use of the familiar high-temperature, Gaussian form of the rate constant. The effect of harmonic versus anharmonic inner sphere modes on the electron transfer is readily seen, especially in the exoergic, inverted region. The behavior of the transition probability can also be displayed as a surface for all temperatures and values of the driving force/exoergicity {Delta}=-{Delta}G. The temperature insensitivity of the transfer rate is clearly seen when the exoergicity equals the collective reorganization energy ({Delta}={Lambda}{sub s}) along a maximum ln (w) vs. {Delta} ridge of the surface. The surface also reveals additional regions for {Delta} where ln (w) appears to be insensitive to temperature, or effectively activationless, for some kinds of inner sphere contributions.

  12. CLASSICAL MODEL FOR ELECTRONICALLY NON-ADIABATIC COLLISION PROCESSES: RESONANCE EFFECTS IN ELECTRONIC-VIBRATIONAL ENERGY TRANSFER

    SciTech Connect

    Orel, Ann E.; Miller, William H.

    1980-11-01

    A recently developed classical model for electronically nonadiabatic collision processes is applied to electronic-vibrational energy transfer in a collinear atom~diatom system, A + BC(v=1) + A*+ BC(v=0), which closely resembles Br-H{sub 2}. This classical model, which treats electronic as well as heavy particle (i.e., translation, rotation, and vibration) degrees of freedom by classical mechanics, is found to describe the resonance features in this process reasonably well. The usefulness of the approach is that it allows one to extend standard Monte Carlo classical trajectory methodology to include electronically non-adiabatic processes in a dynamically consistent way,

  13. A "parallel plate" electrostatic model for bimolecular rate constants applied to electron transfer proteins.

    PubMed Central

    Watkins, J. A.; Cusanovich, M. A.; Meyer, T. E.; Tollin, G.

    1994-01-01

    A "parallel plate" model describing the electrostatic potential energy of protein-protein interactions is presented that provides an analytical representation of the effect of ionic strength on a biomolecular rate constant. The model takes into account the asymmetric distribution of charge on the surface of the protein and localized charges at the site of electron transfer that are modeled as elements of a parallel plate condenser. Both monopolar and dipolar interactions are included. Examples of simple (monophasic) and complex (biphasic) ionic strength dependencies obtained from experiments with several electron transfer protein systems are presented, all of which can be accommodated by the model. The simple cases do not require the use of both monopolar and dipolar terms (i.e., they can be fit well by either alone). The biphasic dependencies can be fit only by using dipolar and monopolar terms of opposite sign, which is physically unreasonable for the molecules considered. Alternatively, the high ionic strength portion of the complex dependencies can be fit using either the monopolar term alone or the complete equation; this assumes a model in which such behavior is a consequence of electron transfer mechanisms involving changes in orientation or site of reaction as the ionic strength is varied. Based on these analyses, we conclude that the principal applications of the model presented here are to provide information about the structural properties of intermediate electron transfer complexes and to quantify comparisons between related proteins or site-specific mutants. We also conclude that the relative contributions of monopolar and dipolar effects to protein electron transfer kinetics cannot be evaluated from experimental data by present approximations. PMID:7703857

  14. Constraint-Based Modeling of Carbon Fixation and the Energetics of Electron Transfer in Geobacter metallireducens

    SciTech Connect

    Feist, AM; Nagarajan, H; Rotaru, AE; Tremblay, PL; Zhang, T; Nevin, KP; Lovley, DR; Zengler, K

    2014-04-24

    Geobacter species are of great interest for environmental and biotechnology applications as they can carry out direct electron transfer to insoluble metals or other microorganisms and have the ability to assimilate inorganic carbon. Here, we report on the capability and key enabling metabolic machinery of Geobacter metallireducens GS-15 to carry out CO2 fixation and direct electron transfer to iron. An updated metabolic reconstruction was generated, growth screens on targeted conditions of interest were performed, and constraint-based analysis was utilized to characterize and evaluate critical pathways and reactions in G. metallireducens. The novel capability of G. metallireducens to grow autotrophically with formate and Fe(III) was predicted and subsequently validated in vivo. Additionally, the energetic cost of transferring electrons to an external electron acceptor was determined through analysis of growth experiments carried out using three different electron acceptors (Fe(III), nitrate, and fumarate) by systematically isolating and examining different parts of the electron transport chain. The updated reconstruction will serve as a knowledgebase for understanding and engineering Geobacter and similar species. Author Summary The ability of microorganisms to exchange electrons directly with their environment has large implications for our knowledge of industrial and environmental processes. For decades, it has been known that microbes can use electrodes as electron acceptors in microbial fuel cell settings. Geobacter metallireducens has been one of the model organisms for characterizing microbe-electrode interactions as well as environmental processes such as bioremediation. Here, we significantly expand the knowledge of metabolism and energetics of this model organism by employing constraint-based metabolic modeling. Through this analysis, we build the metabolic pathways necessary for carbon fixation, a desirable property for industrial chemical production. We

  15. Calculation of electron transfer reorganization energies using the finite difference Poisson-Boltzmann model.

    PubMed Central

    Sharp, K A

    1998-01-01

    A description is given of a method to calculate the electron transfer reorganization energy (lambda) in proteins using the linear or nonlinear Poisson-Boltzmann (PB) equation. Finite difference solutions to the linear PB equation are then used to calculate lambda for intramolecular electron transfer reactions in the photosynthetic reaction center from Rhodopseudomonas viridis and the ruthenated heme proteins cytochrome c, myoglobin, and cytochrome b and for intermolecular electron transfer between two cytochrome c molecules. The overall agreement with experiment is good considering both the experimental and computational difficulties in estimating lambda. The calculations show that acceptor/donor separation and position of the cofactors with respect to the protein/solvent boundary are equally important and, along with the overall polarizability of the protein, are the major determinants of lambda. In agreement with previous studies, the calculations show that the protein provides a low reorganization environment for electron transfer. Agreement with experiment is best if the protein polarizability is modeled with a low (<8) average effective dielectric constant. The effect of buried waters on the reorganization energy of the photosynthetic reaction center was examined and found to make a contribution ranging from 0.05 eV to 0.27 eV, depending on the donor/acceptor pair. PMID:9512022

  16. Picosecond electron transfer in diporphyrin models of Photosystem II of green plants

    SciTech Connect

    Netzel, T L; Fujita, I; Wang, C B; Fajer, J

    1980-01-01

    Green plants and photosynthetic bacteria efficiently transform the energy of an absorbed photon into redox products. Current in vivo and in vitro studies on Photosystem II (PS II) suggest the electron donor is a chlorophyll monomer, ligated to produce the high oxidation potential of P680, and the electron acceptor is pheophytin, a metal-free chlorophyll. This study probes the behavior of this PS II model in solvents of high dielectric constant and tests the sensitivity of its charge transfer reactions to increases in linking chain length as well as to changes in the relative orientation of the porphyrin subunits. (ACR)

  17. A general theoretical model for electron transfer reactions in complex systems.

    PubMed

    Amadei, Andrea; Daidone, Isabella; Aschi, Massimiliano

    2012-01-28

    In this paper we present a general theoretical-computational model for treating electron transfer reactions in complex atomic-molecular systems. The underlying idea of the approach, based on unbiased first-principles calculations at the atomistic level, utilizes the definition and the construction of the Diabatic Perturbed states of the involved reactive partners (i.e. the quantum centres in our perturbation approach) as provided by the interaction with their environment, including their mutual interaction. In this way we reconstruct the true Adiabatic states of the reactive partners characterizing the electron transfer process as the fluctuation of the electronic density due to the fluctuating perturbation. Results obtained by using a combination of Molecular Dynamics simulation and the Perturbed Matrix Method on a prototypical intramolecular electron transfer (from 2-(9,9'-dimethyl)fluorene to the 2-naphthalene group separated by a steroidal 5-α-androstane skeleton) well illustrate the accuracy of the method in reproducing both the thermodynamics and the kinetics of the process. PMID:22158942

  18. Model for the charge-transfer probability in helium nanodroplets following electron-impact ionization

    SciTech Connect

    Ellis, Andrew M.; Yang Shengfu

    2007-09-15

    A theoretical model has been developed to describe the probability of charge transfer from helium cations to dopant molecules inside helium nanodroplets following electron-impact ionization. The location of the initial charge site inside helium nanodroplets subject to electron impact has been investigated and is found to play an important role in understanding the ionization of dopants inside helium droplets. The model is consistent with a charge migration process in small helium droplets that is strongly directed by intermolecular forces originating from the dopant, whereas for large droplets (tens of thousands of helium atoms and larger) the charge migration increasingly takes on the character of a random walk. This suggests a clear droplet size limit for the use of electron-impact mass spectrometry for detecting molecules in helium droplets.

  19. Modeling Electronic-Nuclear Interactions for Excitation Energy Transfer Processes in Light-Harvesting Complexes.

    PubMed

    Lee, Mi Kyung; Coker, David F

    2016-08-18

    An accurate approach for computing intermolecular and intrachromophore contributions to spectral densities to describe the electronic-nuclear interactions relevant for modeling excitation energy transfer processes in light harvesting systems is presented. The approach is based on molecular dynamics (MD) calculations of classical correlation functions of long-range contributions to excitation energy fluctuations and a separate harmonic analysis and single-point gradient quantum calculations for electron-intrachromophore vibrational couplings. A simple model is also presented that enables detailed analysis of the shortcomings of standard MD-based excitation energy fluctuation correlation function approaches. The method introduced here avoids these problems, and its reliability is demonstrated in accurate predictions for bacteriochlorophyll molecules in the Fenna-Matthews-Olson pigment-protein complex, where excellent agreement with experimental spectral densities is found. This efficient approach can provide instantaneous spectral densities for treating the influence of fluctuations in environmental dissipation on fast electronic relaxation. PMID:27472379

  20. Charge-transfer model for the electronic structure of layered ruthenates

    NASA Astrophysics Data System (ADS)

    Rościszewski, Krzysztof; Oleś, Andrzej M.

    2015-04-01

    Motivated by the earlier experimental results and ab initio studies on the electronic structure of layered ruthenates (Sr2RuO4 and Ca2RuO4 ) we introduce and investigate the multiband d -p charge transfer model describing a single RuO4 layer, similar to the charge transfer model for a single CuO2 plane including apical oxygen orbitals in high Tc cuprates. The present model takes into account nearest-neighbor anisotropic ruthenium-oxygen d -p and oxygen-oxygen p -p hopping elements, crystal-field splittings, and spin-orbit coupling. The intraorbital Coulomb repulsion and Hund's exchange are defined not only at ruthenium but also at oxygen ions. Our results demonstrate that the RuO4 layer cannot be regarded to be a pure ruthenium t2 g system. We examine a different scenario in which ruthenium eg orbitals are partly occupied and highlight the significance of oxygen orbitals. We point out that the predictions of an idealized model based on ionic configuration (with n0=4 +4 ×6 =28 electrons per RuO4 unit) do not agree with the experimental facts for Sr2RuO4 which support our finding that the electron number in the d -p states is significantly smaller. In fact, we find the electron occupation of d and p orbitals for a single RuO4 unit n =28 -x , being smaller by at least 1-1.5 electrons from that in the ionic model and corresponding to self-doping with x ≃1.5 .

  1. Doping Dependent Charge Transfer Gap and Realistic Electronic Model of n-type Cuprate Superconductors

    SciTech Connect

    Xiang, T.

    2010-05-03

    Based on the analysis of the measurement data of angle-resolved photoemission spectroscopy (ARPES) and optics, we show that the charge transfer gap is significantly smaller than the optical one and is reduced by doping in electron doped cuprate superconductors. This leads to a strong charge fluctuation between the Zhang-Rice singlet and the upper Hubbard bands. The basic model for describing this system is a hybridized two-band t-J model. In the symmetric limit where the corresponding intra- and inter-band hopping integrals are equal to each other, this two-band model is equivalent to the Hubbard model with an antiferromagnetic exchange interaction (i.e. the t-U-J model). The mean-field result of the t-U-J model gives a good account for the doping evolution of the Fermi surface and the staggered magnetization.

  2. A wave packet model for electron transfer and its implications for the semiconductor-liquid interface

    SciTech Connect

    Smith, B.B.; Nozik, A.J.

    1999-11-11

    This paper establishes the computational feasibility and examines the implications of a particular technique for simulations of time dependent electron transfer (ET) at semiconductor-liquid interfaces (SLIs). The methodology uses a one electron formalism employing wave packets, pseudopotentials, and molecular dynamics, which the authors dub WPMD. They describe a detailed mechanism for SLI ET by using the methodology. The model is versatile enough to address conventional SLI ET, surface state and adsorption mediated ET, photoexcited ET, and ET between quantum dots and other microstructures. They contrast the perspectives of their WPMD model of SLI ET with those in traditional literature and find substantial differences. The use of standard Landau-Zener theory for SLI ET is found particularly problematic.

  3. Two-Electron Transfer Pathways.

    PubMed

    Lin, Jiaxing; Balamurugan, D; Zhang, Peng; Skourtis, Spiros S; Beratan, David N

    2015-06-18

    The frontiers of electron-transfer chemistry demand that we develop theoretical frameworks to describe the delivery of multiple electrons, atoms, and ions in molecular systems. When electrons move over long distances through high barriers, where the probability for thermal population of oxidized or reduced bridge-localized states is very small, the electrons will tunnel from the donor (D) to acceptor (A), facilitated by bridge-mediated superexchange interactions. If the stable donor and acceptor redox states on D and A differ by two electrons, it is possible that the electrons will propagate coherently from D to A. While structure-function relations for single-electron superexchange in molecules are well established, strategies to manipulate the coherent flow of multiple electrons are largely unknown. In contrast to one-electron superexchange, two-electron superexchange involves both one- and two-electron virtual intermediate states, the number of virtual intermediates increases very rapidly with system size, and multiple classes of pathways interfere with one another. In the study described here, we developed simple superexchange models for two-electron transfer. We explored how the bridge structure and energetics influence multielectron superexchange, and we compared two-electron superexchange interactions to single-electron superexchange. Multielectron superexchange introduces interference between singly and doubly oxidized (or reduced) bridge virtual states, so that even simple linear donor-bridge-acceptor systems have pathway topologies that resemble those seen for one-electron superexchange through bridges with multiple parallel pathways. The simple model systems studied here exhibit a richness that is amenable to experimental exploration by manipulating the multiple pathways, pathway crosstalk, and changes in the number of donor and acceptor species. The features that emerge from these studies may assist in developing new strategies to deliver multiple

  4. Polar solvation and electron transfer

    SciTech Connect

    Not Available

    1993-04-13

    The report is divided into the following sections: completion of previous studies on solvation dynamics, dipole lattice studies, inertial components of solvation response, simple models of solvation dynamics, rotational dynamics and dielectric friction, intramolecular electron transfer reactions, and intermolecular donor-acceptor complexes.

  5. Golden rule kinetics of transfer reactions in condensed phase: The microscopic model of electron transfer reactions in disordered solid matrices

    SciTech Connect

    Basilevsky, M. V.; Mitina, E. A.; Odinokov, A. V.; National Research Nuclear University “MEPhI,” 31, Kashirskoye shosse, Moscow ; Titov, S. V.

    2013-12-21

    The algorithm for a theoretical calculation of transfer reaction rates for light quantum particles (i.e., the electron and H-atom transfers) in non-polar solid matrices is formulated and justified. The mechanism postulated involves a local mode (an either intra- or inter-molecular one) serving as a mediator which accomplishes the energy exchange between the reacting high-frequency quantum mode and the phonon modes belonging to the environment. This approach uses as a background the Fermi golden rule beyond the usually applied spin-boson approximation. The dynamical treatment rests on the one-dimensional version of the standard quantum relaxation equation for the reduced density matrix, which describes the frequency fluctuation spectrum for the local mode under consideration. The temperature dependence of a reaction rate is controlled by the dimensionless parameter ξ{sub 0}=ℏω{sub 0}/k{sub B}T where ω{sub 0} is the frequency of the local mode and T is the temperature. The realization of the computational scheme is different for the high/intermediate (ξ{sub 0} < 1 − 3) and for low (ξ{sub 0}≫ 1) temperature ranges. For the first (quasi-classical) kinetic regime, the Redfield approximation to the solution of the relaxation equation proved to be sufficient and efficient in practical applications. The study of the essentially quantum-mechanical low-temperature kinetic regime in its asymptotic limit requires the implementation of the exact relaxation equation. The coherent mechanism providing a non-vanishing reaction rate has been revealed when T→ 0. An accurate computational methodology for the cross-over kinetic regime needs a further elaboration. The original model of the hopping mechanism for electronic conduction in photosensitive organic materials is considered, based on the above techniques. The electron transfer (ET) in active centers of such systems proceeds via local intra- and intermolecular modes. The active modes, as a rule, operate beyond the

  6. Electron shuttling in electron transfer dissociation

    NASA Astrophysics Data System (ADS)

    Neff, Diane; Smuczynska, Sylwia; Simons, Jack

    2009-06-01

    Ab initio electronic structure calculations have been performed on two model systems containing a disulfide linkage and one or two positively charged sites, aimed at gaining further insight into how and where electrons attach to positively charged peptides under electron capture (ECD) and electron transfer dissociation (ETD) mass spectroscopy conditions. Couplings among electronic states involving (i) an entrance-channel with the excess electron residing on a donor anion interacting with the positively charged peptide, (ii) a state in which the electron has been transferred to the SS [sigma]* orbital to cause bond cleavage, and (iii) a manifold of states in which the electron has been transferred to a ground- or excited-Rydberg orbital on a positive site. The results of this study suggest that specific excited Rydberg states play a key role in effecting electron shuttling to the SS [sigma]* orbital. The excited-Rydberg orbitals close in energy to the SS [sigma]* orbital and with sufficient radial extent to span the distance between the positive site and the SS [sigma]* orbital play the key role. Then, when the anion donor, excited-Rydberg, and SS [sigma]* orbitals achieve spatial proximity and similarity in energies, one can have what is termed here a shuttle of an electron from the donor to the SS [sigma]* orbital, which results in SS bond cleavage. For the singly and doubly charged systems studied here, it was the 3p and 3d Rydberg orbitals, respectively, that met these criteria of spatial and energetic proximity. For other peptides having different charge states, it will be other Rydberg orbitals that meet these criteria because the relative energies of the SS [sigma]* and Rydberg orbitals are governed by the (different) Coulomb stabilizations these orbitals experience. However, the evidence suggests that it is not very high-energy Rydberg states but states with 3 < n < 10 that are involved in the rate limiting steps in ECD, ETD, and ECID experiments.

  7. Electron transfer in proteins.

    PubMed

    Gray, H B; Winkler, J R

    1996-01-01

    Electron-transfer (ET) reactions are key steps in a diverse array of biological transformations ranging from photosynthesis to aerobic respiration. A powerful theoretical formalism has been developed that describes ET rates in terms of two parameters: the nuclear reorganization energy (lambda) and the electronic-coupling strength (HAB). Studies of ET reactions in ruthenium-modified proteins have probed lambda and HAB in several metalloproteins (cytochrome c, myoglobin, azurin). This work has shown that protein reorganization energies are sensitive to the medium surrounding the redox sites and that an aqueous environment, in particular, leads to large reorganization energies. Analyses of electronic-coupling strengths suggest that the efficiency of long-range ET depends on the protein secondary structure: beta sheets appear to mediate coupling more efficiently than alpha-helical structures, and hydrogen bonds play a critical role in both. PMID:8811189

  8. Modeling Charge Transfer in Fullerene Collisions via Real-Time Electron Dynamics.

    PubMed

    Jakowski, Jacek; Irle, Stephan; Sumpter, Bobby G; Morokuma, Keiji

    2012-06-01

    An approach for performing real-time dynamics of electron transfer in a prototype redox reaction that occurs in reactive collisions between neutral and ionic fullerenes is discussed. The quantum dynamical simulations show that the electron transfer occurs within 60 fs directly preceding the collision of the fullerenes, followed by structural changes and relaxation of electron charge. The consequences of real-time electron dynamics are fully elucidated for the far from equilibrium processes of collisions between neutral and multiply charged fullerenes. PMID:26285634

  9. Theory and computational modeling: Medium reorganization and donor/acceptor coupling in electron transfer processes

    SciTech Connect

    Newton, M.D.; Feldberg, S.W.; Smalley, J.F.

    1998-03-01

    The continuing goal is to convert the rapidly accumulating mechanistic information about electron transfer (et) kinetics (often representable in terms of simple rate constants) into precise tools for fine-tuned control of the kinetics and for design of molecular-based systems which meet specified et characteristics. The present treatment will be limited to the kinetic framework defined by the assumption of transition state theory (TST). The primary objective of this paper is to report recent advances in the theoretical formulation, calculation, and analysis of energetics and electronic coupling pertinent to et in complex molecular aggregates. The control of et kinetics (i.e., enhancing desired processes, while inhibiting others) involves, of course, both system energetics (especially reorganization energies (E{sub r}) and free energy changes ({Delta}G{sup 0})) and electronic coupling of local D and A sites, which for thermal processes is most directly relevant only after the system has reached the appropriate point (or region) along the reaction coordinate (i.e., the transition state). The authors first discuss TST rate constant models, emphasizing genetic features, but also noting some special features arising when metal electrodes are involved. They then turn to a consideration of detailed aspects of medium reorganization and donor/acceptor coupling. With these theoretical tools in hand, they examine the results of recent applications to complex molecular systems using the techniques of computational quantum chemistry and electrostatics, together with detailed analysis of the numerical results and comparison with recent electrochemical kinetic data.

  10. Electronic energy transfer in model photosynthetic systems: Markovian vs. non-Markovian dynamics.

    PubMed

    Singh, Navinder; Brumer, Paul

    2011-01-01

    A simple numerical algorithm for solving the non-Markovian master equation in the second Born approximation is developed and used to propagate the traditional dimer system that models electronic energy transfer in photosynthetic systems. Specifically, the coupled integro-differential equations for the reduced density matrix are solved by an efficient auxiliary function method in both the energy and site representations. In addition to giving exact results to this order, the approach allows us to access the range of the reorganization energy and decay rates of the phonon auto-correlation function for which the Markovian Redfield theory and the second-order approximation is useful. For example, the use of Redfield theory for lambda > 10 cm(-1) in Fenna-Mathews-Olson (FMO) type systems is shown to be fundamentally inaccurate. PMID:22452072

  11. Simulations of fluorescence quenching using theoretical models of energy and electron transfer in random arrays

    NASA Astrophysics Data System (ADS)

    Boulu, Laurent G.; Kozak, John J.

    A master equation is solved numerically for investigating energy transfer and trapping in two-dimensional disordered systems of chlorophylls and quinones. Quenching of the excitation occurs both by electron transfer from a chlorophyll to a neighbouring quinone and by energy transfer to self-quenching traps consisting of statistical pairs of chlorophyll molecules closer than a critical distance. The quinone concentration dependence of the average lifetime of the calculated fluorescence decay is determined for different values of the Förster transfer radius 0 and A, the microscopic electron transfer rate at 'zero distance'. Quasi-Stern-Volmer behaviour is obtained. The half-quenching concentration and the quenching rate kQ depend strongly on A; they increase little with faster energy transfer because of competing self-quenching and slow electron transfer. Our results are compared to recent fluorescence quenching data that Chauvet and Patterson obtained from real-time measurements in monolayers of chlorophyll a and vitamin K1 diluted in dioleylphosphatidylcholine (DOL). Our convoluted decays fit the experimental data if A = 50-100 ns-1 and tMPH0193_images = 60-70 Å. Accordingly, kQ = 3-5 × 10-5 cm2/molecules·s. These values are in close agreement with those reported in the literature.

  12. The electronic couplings in electron transfer and excitation energy transfer.

    PubMed

    Hsu, Chao-Ping

    2009-04-21

    The transport of charge via electrons and the transport of excitation energy via excitons are two processes of fundamental importance in diverse areas of research. Characterization of electron transfer (ET) and excitation energy transfer (EET) rates are essential for a full understanding of, for instance, biological systems (such as respiration and photosynthesis) and opto-electronic devices (which interconvert electric and light energy). In this Account, we examine one of the parameters, the electronic coupling factor, for which reliable values are critical in determining transfer rates. Although ET and EET are different processes, many strategies for calculating the couplings share common themes. We emphasize the similarities in basic assumptions between the computational methods for the ET and EET couplings, examine the differences, and summarize the properties, advantages, and limits of the different computational methods. The electronic coupling factor is an off-diagonal Hamiltonian matrix element between the initial and final diabatic states in the transport processes. ET coupling is essentially the interaction of the two molecular orbitals (MOs) where the electron occupancy is changed. Singlet excitation energy transfer (SEET), however, contains a Frster dipole-dipole coupling as its most important constituent. Triplet excitation energy transfer (TEET) involves an exchange of two electrons of different spin and energy; thus, it is like an overlap interaction of two pairs of MOs. Strategies for calculating ET and EET couplings can be classified as (1) energy-gap-based approaches, (2) direct calculation of the off-diagonal matrix elements, or (3) use of an additional operator to describe the extent of charge or excitation localization and to calculate the coupling value. Some of the difficulties in calculating the couplings were recently resolved. Methods were developed to remove the nondynamical correlation problem from the highly precise coupled cluster

  13. The XAS model of dissolved Cu(II) and its significance to biological electron transfer

    NASA Astrophysics Data System (ADS)

    Frank, Patrick; Benfatto, Maurizio; Hedman, Britt; Hodgson, Keith O.

    2009-11-01

    The standard model for dissolved Cu(II) portrays the complex ion as an axially elongated, equatorially planar octahedron. Using EXAFS and MXAN analyses of copper K-edge XAS spectra, new structural models for dissolved [Cu(aq)]2+ and [Cu(amm)]2+ have been determined. These structures uniformly depart from the octahedral model in favour of an axially elongated square pyramidal core. MXAN results also indicate that the equatorial ligands need not be coplanar with copper. Further structural elements include a -z axially localized scatterer at ~3 Å. Even more distant scatterers imply second shell solvent organization, which can vary with the medium. Preliminary results from new extended, k = 18 Å-1, higher resolution copper K-edge XAS data sets are reported. The low symmetry of dissolved Cu(II) ion contradicts the central thesis of the rack-induced bonding hypothesis of copper electron transfer proteins. The asymmetry of biological copper is not a frozen vibronic excited state enforced by a rigid protein scaffold, but is entirely in harmony with the structural ground state of the dissolved aqueous Cu(II) complex ion.

  14. Constraint-Based Modeling of Carbon Fixation and the Energetics of Electron Transfer in Geobacter metallireducens

    PubMed Central

    Feist, Adam M.; Nagarajan, Harish; Rotaru, Amelia-Elena; Tremblay, Pier-Luc; Zhang, Tian; Nevin, Kelly P.; Lovley, Derek R.; Zengler, Karsten

    2014-01-01

    Geobacter species are of great interest for environmental and biotechnology applications as they can carry out direct electron transfer to insoluble metals or other microorganisms and have the ability to assimilate inorganic carbon. Here, we report on the capability and key enabling metabolic machinery of Geobacter metallireducens GS-15 to carry out CO2 fixation and direct electron transfer to iron. An updated metabolic reconstruction was generated, growth screens on targeted conditions of interest were performed, and constraint-based analysis was utilized to characterize and evaluate critical pathways and reactions in G. metallireducens. The novel capability of G. metallireducens to grow autotrophically with formate and Fe(III) was predicted and subsequently validated in vivo. Additionally, the energetic cost of transferring electrons to an external electron acceptor was determined through analysis of growth experiments carried out using three different electron acceptors (Fe(III), nitrate, and fumarate) by systematically isolating and examining different parts of the electron transport chain. The updated reconstruction will serve as a knowledgebase for understanding and engineering Geobacter and similar species. PMID:24762737

  15. The ‘porin–cytochrome’ model for microbe-to-mineral electron transfer

    SciTech Connect

    Richardson, David J.; Butt, Julea N.; Fredrickson, Jim K.; Zachara, John M.; Shi, Liang; Edwards, Marcus J.; White, Gaye F.; Baiden, Nanakow; Gates, Andrew J.; Marritt, Sophie; Clarke, Thomas A.

    2012-05-30

    Many species of bacteria can couple anaerobic growth to the respiratory reduction of insoluble minerals containing Fe(III) or Mn(III/IV). It has been suggested that in Shewanella species electrons cross the outer membrane to extracellular substrates via 'porin-cytochrome' electron transport modules. The molecular structure of an outer-membrane extracellular-facing deca-haem terminus for such a module has recently been resolved. It is debated how, once outside the cells, electrons are transferred from outer-membrane cytochromes to insoluble electron sinks. This may occur directly or by assemblies of cytochromes, perhaps functioning as 'nanowires', or via electron shuttles. Here we review recent work in this field and explore whether it allows for unification of the electron transport mechanisms supporting extracellular mineral respiration in Shewanella that may extend into other genera of Gram-negative bacteria.

  16. Hierarchical coarse-graining model for photosystem II including electron and excitation-energy transfer processes.

    PubMed

    Matsuoka, Takeshi; Tanaka, Shigenori; Ebina, Kuniyoshi

    2014-03-01

    We propose a hierarchical reduction scheme to cope with coupled rate equations that describe the dynamics of multi-time-scale photosynthetic reactions. To numerically solve nonlinear dynamical equations containing a wide temporal range of rate constants, we first study a prototypical three-variable model. Using a separation of the time scale of rate constants combined with identified slow variables as (quasi-)conserved quantities in the fast process, we achieve a coarse-graining of the dynamical equations reduced to those at a slower time scale. By iteratively employing this reduction method, the coarse-graining of broadly multi-scale dynamical equations can be performed in a hierarchical manner. We then apply this scheme to the reaction dynamics analysis of a simplified model for an illuminated photosystem II, which involves many processes of electron and excitation-energy transfers with a wide range of rate constants. We thus confirm a good agreement between the coarse-grained and fully (finely) integrated results for the population dynamics. PMID:24418347

  17. Bridge mediated ultrafast heterogeneous electron transfer

    NASA Astrophysics Data System (ADS)

    Ramakrishna, S.; Willig, F.; May, V.

    2002-01-01

    Bridge mediated photoinduced ultrafast heterogeneous electron transfer (ET) from a molecularly anchored chromophore to a semiconductor surface is modelled theoretically. The continuum levels of the semiconductor substrate are taken into account in the numerical calculations via a polynomial expansion. Electron transfer for the direct injection case in the strong coupling limit is studied and compared with cases where intermediate bridging states are successively introduced to weaken the effective electronic coupling. The role of vibronic coherences in the strong electronic coupling limit as well as in off-resonant bridge mediated electron transfer is also discussed.

  18. Solvent free energy curves for electron transfer reactions: A nonlinear solvent response model

    NASA Astrophysics Data System (ADS)

    Ichiye, Toshiko

    1996-05-01

    Marcus theory for electron transfer assumes a linear response of the solvent so that both the reactant and product free energy curves are parabolic functions of the solvent polarization, each with the same solvent force constant k characterizing the curvature. Simulation data by other workers indicate that the assumption of parabolic free energy curves is good for the Fe2+-Fe3+ self-exchange reaction but that the k of the reactant and product free energy curves are different for the reaction D0+A0→D1-+A1+. However, the fluctuations sampled in these simulations were not large enough to reach the activation barrier region, which was thus treated either by umbrella sampling or by parabolic extrapolation. Here, we present free energy curves calculated from a simple model of ionic solvation developed in an earlier paper by Hyun, Babu, and Ichiye, which we refer to here as the HBI model. The HBI model describes the nonlinearity of the solvent response due to the orientation of polar solvent molecules. Since it is a continuum model, it may be considered the first-order nonlinear correction to the linear response Born model. Moreover, in the limit of zero charge or infinite radius, the Born model and the Marcus relations are recovered. Here, the full free energy curves are calculated using analytic expressions from the HBI model. The HBI reactant and product curves have different k for D0+A0→D1-+A1+ as in the simulations, but examining the full curves shows they are nonparabolic due to the nonlinear response of the solvent. On the other hand, the HBI curves are close to parabolic for the Fe2+-Fe3+ reaction, also in agreement with simulations, while those for another self-exchange reaction D0-A1+ show greater deviations from parabolic behavior than the Fe2+-Fe3+ reaction. This indicates that transitions from neutral to charged species will have the largest deviations. Thus, the second moment of the polarization is shown to be a measure of the deviation from Marcus

  19. Electron transfer within a reaction path model calibrated by constrained DFT calculations: application to mixed-valence organic compounds.

    PubMed

    Mangaud, E; de la Lande, A; Meier, C; Desouter-Lecomte, M

    2015-12-14

    The quantum dynamics of electron transfer in mixed-valence organic compounds is investigated using a reaction path model calibrated by constrained density functional theory (cDFT). Constrained DFT is used to define diabatic states relevant for describing the electron transfer, to obtain equilibrium structures for each of these states and to estimate the electronic coupling between them. The harmonic analysis at the diabatic minima yields normal modes forming the dissipative bath coupled to the electronic states. In order to decrease the system-bath coupling, an effective one dimensional vibronic Hamiltonian is constructed by partitioning the modes into a linear reaction path which connects both equilibrium positions and a set of secondary vibrational modes, coupled to this reaction coordinate. Using this vibronic model Hamiltonian, dissipative quantum dynamics is carried out using Redfield theory, based on a spectral density which is determined from the cDFT results. In a first benchmark case, the model is applied to a series of mixed-valence organic compounds formed by two 1,4-dimethoxy-3-methylphenylene fragments linked by an increasing number of phenylene bridges. This allows us to examine the coherent electron transfer in extreme situations leading to a ground adiabatic state with or without a barrier and therefore to the trapping of the charge or to an easy delocalization. PMID:26041466

  20. Proton-Coupled Electron Transfer

    SciTech Connect

    Weinberg, Dave; Gagliardi, Christopher J.; Hull, Jonathan F; Murphy, Christine Fecenko; Kent, Caleb A.; Westlake, Brittany C.; Paul, Amit; Ess, Daniel H; McCafferty, Dewey Granville; Meyer, Thomas J

    2012-07-11

    Proton-Coupled Electron Transfer (PCET) describes reactions in which there is a change in both electron and proton content between reactants and products. It originates from the influence of changes in electron content on acid-base properties and provides a molecular-level basis for energy transduction between proton transfer and electron transfer. Coupled electron-proton transfer or EPT is defined as an elementary step in which electrons and protons transfer from different orbitals on the donor to different orbitals on the acceptor. There is (usually) a clear distinction between EPT and H-atom transfer (HAT) or hydride transfer, in which the transferring electrons and proton come from the same bond. Hybrid mechanisms exist in which the elementary steps are different for the reaction partners. EPT pathways such as PhO•/PhOH exchange have much in common with HAT pathways in that electronic coupling is significant, comparable to the reorganization energy with H{sub DA} ~ λ. Multiple-Site Electron-Proton Transfer (MS-EPT) is an elementary step in which an electron-proton donor transfers electrons and protons to different acceptors, or an electron-proton acceptor accepts electrons and protons from different donors. It exploits the long-range nature of electron transfer while providing for the short-range nature of proton transfer. A variety of EPT pathways exist, creating a taxonomy based on what is transferred, e.g., 1e-/2H+ MS-EPT. PCET achieves “redox potential leveling” between sequential couples and the buildup of multiple redox equivalents, which is of importance in multielectron catalysis. There are many examples of PCET and pH-dependent redox behavior in metal complexes, in organic and biological molecules, in excited states, and on surfaces. Changes in pH can be used to induce electron transfer through films and over long distances in molecules. Changes in pH, induced by local electron transfer, create pH gradients and a driving

  1. Proton-Coupled Electron Transfer Reactions at a Heme-Propionate in an Iron-Protoporphyrin-IX Model Compound

    PubMed Central

    2011-01-01

    A heme model system has been developed in which the heme-propionate is the only proton donating/accepting site, using protoporphyrin IX-monomethyl esters (PPIXMME) and N-methylimidazole (MeIm). Proton-coupled electron transfer (PCET) reactions of these model compounds have been examined in acetonitrile solvent. (PPIXMME)FeIII(MeIm)2-propionate (FeIII~CO2) is readily reduced by the ascorbate derivative 5,6-isopropylidine ascorbate to give (PPIXMME)FeII(MeIm)2-propionic acid (FeII~CO2H). Excess of the hydroxylamine TEMPOH or of hydroquinone similarly reduce FeIII~CO2, and TEMPO and benzoquinone oxidize FeII~CO2H to return to FeIII~CO2. The measured equilibrium constants, and the determined pKa and E1/2 values, indicate that FeII~CO2H has an effective bond dissociation free energy (BDFE) of 67.8 ± 0.6 kcal mol–1. In these PPIX models, electron transfer occurs at the iron center and proton transfer occurs at the remote heme propionate. According to thermochemical and other arguments, the TEMPOH reaction occurs by concerted proton-electron transfer (CPET), and a similar pathway is indicated for the ascorbate derivative. Based on these results, heme propionates should be considered as potential key components of PCET/CPET active sites in heme proteins. PMID:21524059

  2. Influence of environment induced correlated fluctuations in electronic coupling on coherent excitation energy transfer dynamics in model photosynthetic systems

    NASA Astrophysics Data System (ADS)

    Huo, Pengfei; Coker, David F.

    2012-03-01

    Two-dimensional photon-echo experiments indicate that excitation energy transfer between chromophores near the reaction center of the photosynthetic purple bacterium Rhodobacter sphaeroides occurs coherently with decoherence times of hundreds of femtoseconds, comparable to the energy transfer time scale in these systems. The original explanation of this observation suggested that correlated fluctuations in chromophore excitation energies, driven by large scale protein motions could result in long lived coherent energy transfer dynamics. However, no significant site energy correlation has been found in recent molecular dynamics simulations of several model light harvesting systems. Instead, there is evidence of correlated fluctuations in site energy-electronic coupling and electronic coupling-electronic coupling. The roles of these different types of correlations in excitation energy transfer dynamics are not yet thoroughly understood, though the effects of site energy correlations have been well studied. In this paper, we introduce several general models that can realistically describe the effects of various types of correlated fluctuations in chromophore properties and systematically study the behavior of these models using general methods for treating dissipative quantum dynamics in complex multi-chromophore systems. The effects of correlation between site energy and inter-site electronic couplings are explored in a two state model of excitation energy transfer between the accessory bacteriochlorophyll and bacteriopheophytin in a reaction center system and we find that these types of correlated fluctuations can enhance or suppress coherence and transfer rate simultaneously. In contrast, models for correlated fluctuations in chromophore excitation energies show enhanced coherent dynamics but necessarily show decrease in excitation energy transfer rate accompanying such coherence enhancement. Finally, for a three state model of the Fenna-Matthews-Olsen light

  3. Influence of environment induced correlated fluctuations in electronic coupling on coherent excitation energy transfer dynamics in model photosynthetic systems.

    PubMed

    Huo, Pengfei; Coker, David F

    2012-03-21

    Two-dimensional photon-echo experiments indicate that excitation energy transfer between chromophores near the reaction center of the photosynthetic purple bacterium Rhodobacter sphaeroides occurs coherently with decoherence times of hundreds of femtoseconds, comparable to the energy transfer time scale in these systems. The original explanation of this observation suggested that correlated fluctuations in chromophore excitation energies, driven by large scale protein motions could result in long lived coherent energy transfer dynamics. However, no significant site energy correlation has been found in recent molecular dynamics simulations of several model light harvesting systems. Instead, there is evidence of correlated fluctuations in site energy-electronic coupling and electronic coupling-electronic coupling. The roles of these different types of correlations in excitation energy transfer dynamics are not yet thoroughly understood, though the effects of site energy correlations have been well studied. In this paper, we introduce several general models that can realistically describe the effects of various types of correlated fluctuations in chromophore properties and systematically study the behavior of these models using general methods for treating dissipative quantum dynamics in complex multi-chromophore systems. The effects of correlation between site energy and inter-site electronic couplings are explored in a two state model of excitation energy transfer between the accessory bacteriochlorophyll and bacteriopheophytin in a reaction center system and we find that these types of correlated fluctuations can enhance or suppress coherence and transfer rate simultaneously. In contrast, models for correlated fluctuations in chromophore excitation energies show enhanced coherent dynamics but necessarily show decrease in excitation energy transfer rate accompanying such coherence enhancement. Finally, for a three state model of the Fenna-Matthews-Olsen light

  4. Diving into the redox properties of Geobacter sulfurreducens cytochromes: a model for extracellular electron transfer.

    PubMed

    Santos, Telma C; Silva, Marta A; Morgado, Leonor; Dantas, Joana M; Salgueiro, Carlos A

    2015-05-28

    Geobacter bacteria have a remarkable respiratory versatility that includes the dissimilatory reduction of insoluble metal oxides in natural habitats and electron transfer to electrode surfaces from which electricity can be harvested. In both cases, electrons need to be exported from the cell interior to the exterior via a mechanism designated as extracellular electron transfer (EET). Several c-type cytochromes from G. sulfurreducens (Gs) were identified as key players in this process. Biochemical and biophysical data have been obtained for ten Gs cytochromes, including inner-membrane associated (MacA), periplasmic (PpcA, PpcB, PpcC, PpcD, PpcE and GSU1996) and outer membrane-associated (OmcF, OmcS and OmcZ). The redox properties of these cytochromes have been determined, except for PpcC and GSU1996. In this perspective, the reduction potentials of these two cytochromes were determined by potentiometric redox titrations followed by visible spectroscopy. The data obtained are taken together with those available for other key cytochromes to present a thorough overview of the current knowledge of Gs EET mechanisms and provide a possible rationalization for the existence of several multiheme cytochromes involved in the same respiratory pathways. PMID:25906375

  5. Electron transfer in biological molecules

    SciTech Connect

    Gray, H.B.

    1995-12-01

    Electron-transfer reactions are key stemps in photosynthesis, respiration, drug metabolism, and many other biochemical processes. These reactions commonly occur between protein-bound prosthetic groups that are separated by large molecular distances (often greater than 10 {Angstrom}). Although the electron donors and acceptors are expected to be weakly coupled, the reactions are remarkably fast and proceed with high specificity. Recent work on structurally engineered iron and cooper proteins has shown that the chemical bonds in the intervening medium potentially can control the rates of these electron-transfer reactions.

  6. Rate-promoting vibrations and coupled hydrogen-electron transfer reactions in the condensed phase: A model for enzymatic catalysis

    NASA Astrophysics Data System (ADS)

    Mincer, Joshua S.; Schwartz, Steven D.

    2004-04-01

    A model is presented for coupled hydrogen-electron transfer reactions in condensed phase in the presence of a rate promoting vibration. Large kinetic isotope effects (KIEs) are found when the hydrogen is substituted with deuterium. While these KIEs are essentially temperature independent, reaction rates do exhibit temperature dependence. These findings agree with recent experimental data for various enzyme-catalyzed reactions, such as the amine dehydrogenases and soybean lipoxygenase. Consistent with earlier results, turning off the promoting vibration results in an increased KIE. Increasing the barrier height increases the KIE, while increasing the rate of electron transfer decreases it. These results are discussed in light of other views of vibrationally enhanced tunneling in enzymes.

  7. Coherence in electron transfer pathways.

    PubMed

    Skourtis, Spiros S; Beratan, David N; Waldeck, David H

    2011-01-01

    Central to the view of electron-transfer reactions is the idea that nuclear motion generates a transition state geometry at which the electron/hole amplitude propagates coherently from the electron donor to the electron acceptor. In the weakly coupled or nonadiabatic regime, the electron amplitude tunnels through an electronic barrier between the donor and acceptor. The structure of the barrier is determined by the covalent and noncovalent interactions of the bridge. Because the tunneling barrier depends on the nuclear coordinates of the reactants (and on the surrounding medium), the tunneling barrier is highly anisotropic, and it is useful to identify particular routes, or pathways, along which the transmission amplitude propagates. Moreover, when more than one such pathway exists, and the paths give rise to comparable transmission amplitude magnitudes, one may expect to observe quantum interferences among pathways if the propagation remains coherent. Given that the effective tunneling barrier height and width are affected by the nuclear positions, the modulation of the nuclear coordinates will lead to a modulation of the tunneling barrier and hence of the electron flow. For long distance electron transfer in biological and biomimetic systems, nuclear fluctuations, arising from flexible protein moieties and mobile water bridges, can become quite significant. We discuss experimental and theoretical results that explore the quantum interferences among coupling pathways in electron-transfer kinetics; we emphasize recent data and theories associated with the signatures of chirality and inelastic processes, which are manifested in the tunneling pathway coherence (or absence of coherence). PMID:23833692

  8. Bound Flavin-Cytochrome Model of Extracellular Electron Transfer in Shewanella oneidensis: Analysis by Free Energy Molecular Dynamics Simulations.

    PubMed

    Hong, Gongyi; Pachter, Ruth

    2016-06-30

    Flavins are known to enhance extracellular electron transfer (EET) in Shewanella oneidensis MR-1 bacteria, which reduce electron acceptors through outer-membrane (OM) cytochromes c. Free-shuttle and bound-redox cofactor mechanisms were proposed to explain this enhancement, but recent electrochemical reports favor a flavin-bound model, proposing two one-electron reductions of flavin, namely, oxidized (Ox) to semiquinone (Sq) and semiquinone to hydroquinone (Hq), at anodic and cathodic conditions, respectively. In this work, to provide a mechanistic understanding of riboflavin (RF) binding at the multiheme OM cytochrome OmcA, we explored binding configurations at hemes 2, 5, 7, and 10. Subsequently, on the basis of molecular dynamics (MD) simulations, binding free energies and redox potential shifts upon RF binding for the Ox/Sq and Sq/Hq reductions were analyzed. Our results demonstrated an upshift in the Ox/Sq and a downshift in the Sq/Hq redox potentials, consistent with a bound RF-OmcA model. Furthermore, binding free energy MD simulations indicated an RF binding preference at heme 7. MD simulations of the OmcA-MtrC complex interfacing at hemes 5 revealed a small interprotein redox potential difference with an electron transfer rate of 10(7)-10(8)/s. PMID:27266856

  9. Extension of Hopfield's Electron Transfer Model To Accommodate Site-Site Correlation.

    PubMed

    Newton, Marshall D

    2015-11-19

    Extension of the Förster analogue for the ET rate constant (based on virtual intermediate electron detachment or attachment states) with inclusion of site-site correlation due to coulomb terms associated with solvent reorganization energy and the driving force, has been developed and illustrated for a simple three-state, two-mode model. The model is applicable to charge separation (CS), recombination (CR), and shift (CSh) ET processes, with or without an intervening bridge. The model provides a unified perspective on the role of virtual intermediate states in accounting for the thermal Franck-Condon weighted density of states (FCWD), the gaps controlling superexchange coupling, and mean absolute redox potentials, with full accommodation of site-site coulomb interactions. Two types of correlation have been analyzed: aside from the site-site correlation due to coulomb interactions, we have emphasized the intrinsic "nonorthogonality" which generally pertains to reaction coordinates (RCs) for different ET processes involving multiple electronic states, as may be expressed by suitably defined direction cosines (cos(θ)). A pair of RCs may be nonorthogonal even when the site-site coulomb correlations are absent. While different RCs are linearly independent in the mathematical sense for all θ ≠ 0°, they are independent in the sense of being "uncorrelated" only in the limit of orthogonality (θ = 90°). Application to more than two coordinates is straightforward and may include both discrete and continuum contributions. PMID:26501566

  10. A phenomenological model of dynamical arrest of electron transfer in solvents in the glass-transition region

    SciTech Connect

    Matyushov, Dmitry V.

    2005-02-22

    A phenomenological model of electron transfer reactions in solvents undergoing glass transition is discussed. The reaction constant cuts off slow polarization modes from the spectrum of nuclear thermal motions active on the observation time scale. The arrest of nuclear solvation in turn affects the reaction activation barrier making it dependent on the rate. The resultant rate constant is sought from a self-consistent equation. The model describes well the sharp change in the solvent Stokes shift of optical lines in the glass-transition region. It is also applied to describe the temperature dependence of primary charge separation and reduction of primary pair in photosynthetic reaction centers. The model shows that a weak dependence of the primary charge separation rate on temperature can be explained by dynamical arrest of nuclear solvation on the picosecond time scale of electron transfer. For reduction of primary pair by cytochrome, the model yields a sharp turnover of the reaction kinetics at the transition temperature when nuclear solvation freezes in.

  11. The generalized spin-boson model for electron-transfer reactions involving two harmonic potentials with a different force constant

    SciTech Connect

    Tang, J.

    1994-01-01

    The generalized spin-model is employed to analyze the electron-transfer reactions involving two harmonic potentials with a different force constant. An analytical expression for the nonadiabatic rate constant is derived with fill consideration of the effects of quantum modes. For a single dominant solvent mode at low frequency, the result of the high temperature regime is reduced to the formula derived earlier based on the stochastic Liouville theory. For multiple soft solvent modes, the rate constant is a convoluted integral of a rate function for each individual single mode.

  12. Photoinduced electron transfer in ordered polymers

    SciTech Connect

    Jones, G. II.

    1991-12-01

    Long range photoinduced electron transfer between electron donor and acceptor groups is of considerable current interest in terms of strategies for artificial photosynthesis and studies regarding the redox properties of proteins. As part of an extensive study of long range electron transfer involving biopolymers, we have carried out this year investigations of the assembly of electron transfer agents in a system of model short peptides. Also studied is a polyelectrolyte that can adopt a helical conformation when electrostatically complexed with organic dye counter-ions. The principal interest in these systems has to do with the well ordered secondary structures adopted by peptide polymers, and the capabilities for synthetic modification of peptide side chains and end groups with chromophores or electroactive substituents. The present report gives a brief account of the following elements of work related to photochemical electron transfer themes: (1) the synthesis and photochemical characterization of chromophore-bound peptides and amino acid model compounds based on the amino acids, tryptophan and the spacer residue, alanine (Ala); (2) the study of binding of the cationic organic dye to a peptide electrolyte, for which cooperative dye loading and helix formation is important; and (3) completion of the synthesis of a new series of acridinium chromophores that have rod-like'' arrangements of inked aryl rings for assembly of electron donor-acceptor systems that will exhibit especially long lived charge separation.

  13. Protein electron transfer: Dynamics and statistics.

    PubMed

    Matyushov, Dmitry V

    2013-07-14

    Electron transfer between redox proteins participating in energy chains of biology is required to proceed with high energetic efficiency, minimizing losses of redox energy to heat. Within the standard models of electron transfer, this requirement, combined with the need for unidirectional (preferably activationless) transitions, is translated into the need to minimize the reorganization energy of electron transfer. This design program is, however, unrealistic for proteins whose active sites are typically positioned close to the polar and flexible protein-water interface to allow inter-protein electron tunneling. The high flexibility of the interfacial region makes both the hydration water and the surface protein layer act as highly polar solvents. The reorganization energy, as measured by fluctuations, is not minimized, but rather maximized in this region. Natural systems in fact utilize the broad breadth of interfacial electrostatic fluctuations, but in the ways not anticipated by the standard models based on equilibrium thermodynamics. The combination of the broad spectrum of static fluctuations with their dispersive dynamics offers the mechanism of dynamical freezing (ergodicity breaking) of subsets of nuclear modes on the time of reaction/residence of the electron at a redox cofactor. The separation of time-scales of nuclear modes coupled to electron transfer allows dynamical freezing. In particular, the separation between the relaxation time of electro-elastic fluctuations of the interface and the time of conformational transitions of the protein caused by changing redox state results in dynamical freezing of the latter for sufficiently fast electron transfer. The observable consequence of this dynamical freezing is significantly different reorganization energies describing the curvature at the bottom of electron-transfer free energy surfaces (large) and the distance between their minima (Stokes shift, small). The ratio of the two reorganization energies

  14. Electron-transfer reactions in polymer matrices

    NASA Astrophysics Data System (ADS)

    Vannikov, Anatolii V.; Grishina, Antonina D.

    1989-12-01

    This paper discusses the dark reactions and photoreactions that occur with transfer of an electron from a donor to an acceptor in polymer matrices under electron tunnelling conditions and when forming change-transfer complexes. The main emphasis is on an analysis of the factors that determine the rate of electron transfer, which, in accordance with the advanced theory of electron transfer, are the magnitude of the exchange interaction, the free energy of the process, and the reorganisation energies of the medium and the reacting donor and acceptor molecules. The existing models for the movement of charge carriers between single-type transport sites are discussed. The limits of applicability of the different models have been determined. The reorganisation energy of a polymer matrix is shown to have a considerable effect on the rate of movement of charge carriers on introduced transport molecules. The effect of the dielectric properties and free volume of polymer matrices on the characteristics of electron phototransfer in donor-acceptor complexes is discussed. The bibliography includes 126 references.

  15. Experimental Approaches to Studying Biological Electron Transfer.

    ERIC Educational Resources Information Center

    Scott, Robert A.; And Others

    1985-01-01

    Provides an overview on biological electron-transfer reactions, summarizing what is known about how distance, spatial organization, medium, and other factors affect electron transfer. Experimental approaches, including studies of bimolecular electron transfer reactions (electrostatic effects and precursor complexes), are considered. (JN)

  16. Electron Transfer versus Proton Transfer in Gas-Phase Ion/Ion Reactions of Polyprotonated Peptides

    PubMed Central

    Gunawardena, Harsha P.; He, Min; Chrisman, Paul A.; Pitteri, Sharon J.; Hogan, Jason M.; Hodges, Brittany D. M.; McLuckey, Scott A.

    2005-01-01

    The ion/ion reactions of several dozen reagent anions with triply protonated cations of the model peptide KGAILKGAILR have been examined to evaluate predictions of a Landau–Zener-based model for the likelihood for electron transfer. Evidence for electron transfer was provided by the appearance of fragment ions unique to electron transfer or electron capture dissociation. Proton transfer and electron transfer are competitive processes for any combination of anionic and cationic reactants. For reagent anions in reactions with protonated peptides, proton transfer is usually significantly more exothermic than electron transfer. If charge transfer occurs at relatively long distances, electron transfer should, therefore, be favored on kinetic grounds because the reactant and product channels cross at greater distances, provided conditions are favorable for electron transfer at the crossing point. The results are consistent with a model based on Landau–Zener theory that indicates both thermodynamic and geometric criteria apply for electron transfer involving polyatomic anions. Both the model and the data suggest that electron affinities associated with the anionic reagents greater than about 60–70 kcal/mol minimize the likelihood that electron transfer will be observed. Provided the electron affinity is not too high, the Franck–Condon factors associated with the anion and its corresponding neutral must not be too low. When one or the other of these criteria is not met, proton transfer tends to occur essentially exclusively. Experiments involving ion/ion attachment products also suggest that a significant barrier exists to the isomerization between chemical complexes that, if formed, lead to either proton transfer or electron transfer. PMID:16144411

  17. Electronic and Nuclear Factors in Charge and Excitation Transfer

    SciTech Connect

    Piotr Piotrowiak

    2004-09-28

    We report the and/or state of several subprojects of our DOE sponsored research on Electronic and Nuclear Factors in Electron and Excitation Transfer: (1) Construction of an ultrafast Ti:sapphire amplifier. (2) Mediation of electronic interactions in host-guest molecules. (3) Theoretical models of electrolytes in weakly polar media. (4) Symmetry effects in intramolecular excitation transfer.

  18. Solvent gating of intramolecular electron transfer

    SciTech Connect

    Miller, R.M. ); Spears, K.G.; Gong, J.H.; Wach, M. )

    1994-02-03

    The rates for ionic photodissociation of malachite green leucocyanide to form cyanide ion and a malachite green carbonium ion were measured as a function of solvent and temperature. The observed rates in mixtures of polar and nonpolar solvents all had an activation energy of about 1 kcal/mol for a wide range of dielectric constants. This dissociative intramolecular electron transfer (DIET) is unusual because it is the first example where solvent configurational entropy changes are required to enable a large amplitude molecular distortion leading to a nonadiabatic electron transfer and ionic dissociation. This solvent gated intramolecular electron-transfer mechanism is supported by analysis of the preexponential and activation energy trends in dipolar aprotic solven mixtures and alcohol solvents. The large amplitude motion is not separately measurable due to the slow gating rates, but viscosity effects on both the preexponential and the activation energy are analyzed to demonstrate consistency with a barrierless diffusion model having a structural dependence on electron-transfer rate. The rate has an inverse dependence on viscosity raised to the 0.53 power. 36 refs., 6 figs., 4 tabs.

  19. Electron transfer kinetics in molecular photovoltaic devices

    NASA Astrophysics Data System (ADS)

    Heimer, Todd Anthony

    1997-11-01

    Solar energy devices based on the sensitization of nanostructured titanium dioxide semiconductors have proven to be efficient converters of photons into electricity. However, many questions remain concerning the factors which govern conversion efficiency, stability, and photovoltage in these molecular level devices. The energetics and kinetics of electron transfer at the semiconductor/sensitizer interface play a critical role in solar cell photoelectrochemical properties. Through electrochemical and spectroscopic techniques the kinetics of the interfacial electron transfer processes have been measured. The kinetics are in general complex, and various models based on distributions of first order reaction rates or higher order reactions have been used to describe the experimental observations. The unique molecular nature of these devices allows the energy levels involved in electron transfer to be tuned through synthetic modification of the sensitizer. Systematic studies of chemically related sensitizers allow conclusions to be drawn about the factors which govern interfacial electron transfer processes and therefore determine the photoelectrochemical properties of the device. Hopefully, this thesis presents a foundation of knowledge which can successfully direct future development of sensitizers and semiconductors for efficient conversion of light to electricity.

  20. Reaction coordinates for electron transfer reactions

    SciTech Connect

    Rasaiah, Jayendran C.; Zhu Jianjun

    2008-12-07

    The polarization fluctuation and energy gap formulations of the reaction coordinate for outer sphere electron transfer are linearly related to the constant energy constraint Lagrangian multiplier m in Marcus' theory of electron transfer. The quadratic dependence of the free energies of the reactant and product intermediates on m and m+1, respectively, leads to similar dependence of the free energies on the reaction coordinates and to the same dependence of the activation energy on the reorganization energy and the standard reaction free energy. Within the approximations of a continuum model of the solvent and linear response of the longitudinal polarization to the electric field in Marcus' theory, both formulations of the reaction coordinate are expected to lead to the same results.

  1. Modular electron transfer circuits for synthetic biology

    PubMed Central

    Agapakis, Christina M

    2010-01-01

    Electron transfer is central to a wide range of essential metabolic pathways, from photosynthesis to fermentation. The evolutionary diversity and conservation of proteins that transfer electrons makes these pathways a valuable platform for engineered metabolic circuits in synthetic biology. Rational engineering of electron transfer pathways containing hydrogenases has the potential to lead to industrial scale production of hydrogen as an alternative source of clean fuel and experimental assays for understanding the complex interactions of multiple electron transfer proteins in vivo. We designed and implemented a synthetic hydrogen metabolism circuit in Escherichia coli that creates an electron transfer pathway both orthogonal to and integrated within existing metabolism. The design of such modular electron transfer circuits allows for facile characterization of in vivo system parameters with applications toward further engineering for alternative energy production. PMID:21468209

  2. Kinetic ion thermometers for electron transfer dissociation.

    PubMed

    Pepin, Robert; Tureček, František

    2015-02-19

    Peptide fragment ions of the z-type were used as kinetic ion thermometers to gauge the internal energy of peptide cation-radicals produced by electron transfer in the gas-phase. Electron transfer dissociation (ETD)-produced z2 ions containing the leucine residue, z2(Leu-Lys) and z2(Leu-Arg), were found to undergo spontaneous dissociation by loss of C3H7 that was monitored by time-resolved kinetic measurements on the time scale of the linear ion trap mass spectrometer. Kinetic modeling of the dissociations, including collisional cooling and product loss by neutralization, provided unimolecular rate constants for dissociation that were converted to the z ion internal energies using RRKM theory. The internal energy of z2(Leu-Lys) and z2(Leu-Arg) fragment ions was found to decrease with the increasing size of the precursor peptide ion, indicating vibrational energy partitioning between the ion and neutral fragments and ergodic behavior. The experimentally determined excitation in the peptide cation-radicals upon electron transfer (285-327 kJ mol(-1)) was found to be lower than that theoretically calculated from the reaction exothermicity. The reasons for this missing energy are discussed. PMID:25594857

  3. Photoinitiated electron transfer in multichromophoric species: Synthetic tetrads and pentads

    SciTech Connect

    1993-03-01

    This project involves the design, synthesis and study of molecules which mimic some of the important aspects of photosynthetic electron and energy transfer. This research project is leading to a better understanding of the energy conserving steps of photosynthesis via the study of synthetic model systems which abstract features of the natural photosynthetic apparatus. The knowledge gained from these studies will aid in the design of artificial photosynthetic reaction centers which employ the basic chemistry and physics of photosynthesis to help meet mankind`s energy needs. The approach to artificial photosynthesis employed in this project is to use synthetic pigments, electron donors, and electron acceptors similar to those found in biological reaction centers, but to replace the protein component with covalent bonds. These chemical linkages determine the electronic coupling between the various moieties by controlling separation, relative orientation, and overlap of electronic orbitals. The model systems are designed to mimic the following aspects of natural photosynthetic electron transfer: electron donation from a tetrapyrrole excited single state, electron transfer between tetrapyrroles, electron transfer from tetrapyrroles to quinones, and electron transfer between quinones with different redox properties. In addition, they mimic carotenoid antenna function in photosynthesis (singlet-singlet energy transfer from carotenoid polyenes to chlorophyll) and carotenoid photoprotection from singlet oxygen damage (triplet-triplet energy transfer from chlorophyll to carotenoids).

  4. Electronic aspects of the hydride transfer mechanism. Ab initio analytical gradient studies of the cyclopropenyl-cation/lithium hydride model reactant system

    NASA Astrophysics Data System (ADS)

    Tapia, O.; Andres, J.; Aullo, J. M.; Bränden, C.-I.

    1985-11-01

    The electronic mechanisms of a model hydride transfer reaction are theoretically studied with ab inito RHF and UHF SCF MO procedures at the 4-31G basis set level and analytical gradient methods. The model system describes the reduction of cyclopropenyl cation to cyclopropene by the oxidation of lithium hydride to lithium cation. The molecular fragments corresponding to the asymptotic reactive channels characterizing the stepwise mechanisms currently discussed in the literature have been characterized. The binding energy between the fragments is estimated within a simple electrostatic approximate scheme. The results show that a hydride-ion mechanism is a likely pathway for this particular system. The system is thereafter thoroughly studied from the supermolecule approach. Reaction paths for the ground and first triplet electronic states have been calculated. The hypersurface is explored from a geometrical disposition of the reactants that mimics the one found in several dehydrogenases (perpendicular configuration). A hydride ion is found to be the particle transferred on the unconstrained as well as the constrained reaction pathways in the ground electronic state. In the triplet state (perpendicular configuration) the mechanism is stepwise: electron transfer followed by a hydrogen atom transfer. It has been noticed that the perpendicular geometrical disposition of the reactants plays an important role by polarizing the susceptible cyclopropene C-H bond in the sense of increasing the electronic density at the hydrogen nucleus. This provides a clue to rationalize several dehydrogenase's active site structure and mechanism. The reactant molecular complex found in the inverted potential energy curves, namely the LiH---Cp+ association has an electronic distribution which can be described as a hydride ion cementing two electron deficient centers corresponding to the cyclopropenyl and the lithium cations. Direct CI calculations confirm the overall picture obtained above.

  5. Quantum Monte Carlo Treatment of the Charge Transfer and Diradical Electronic Character in a Retinal Chromophore Minimal Model

    PubMed Central

    2015-01-01

    The penta-2,4-dieniminium cation (PSB3) displays similar ground state and first excited state potential energy features as those of the retinal protonated Schiff base (RPSB) chromophore in rhodopsin. Recently, PSB3 has been used to benchmark several electronic structure methods, including highly correlated multireference wave function approaches, highlighting the necessity to accurately describe the electronic correlation in order to obtain reliable properties even along the ground state (thermal) isomerization paths. In this work, we apply two quantum Monte Carlo approaches, the variational Monte Carlo and the lattice regularized diffusion Monte Carlo, to study the energetics and electronic properties of PSB3 along representative minimum energy paths and scans related to its thermal cis–trans isomerization. Quantum Monte Carlo is used in combination with the Jastrow antisymmetrized geminal power ansatz, which guarantees an accurate and balanced description of the static electronic correlation thanks to the multiconfigurational nature of the antisymmetrized geminal power term, and of the dynamical correlation, due to the presence of the Jastrow factor explicitly depending on electron–electron distances. Along the two ground state isomerization minimum energy paths of PSB3, CASSCF calculations yield wave functions having either charge transfer or diradical character in proximity of the two transition state configurations. Here, we observe that at the quantum Monte Carlo level of theory, only the transition state with charge transfer character can be located. The conical intersection, which becomes highly sloped, is observed only if the path connecting the two original CASSCF transition states is extended beyond the diradical one, namely by increasing the bond-length-alternation (BLA). These findings are in good agreement with the results obtained by MRCISD+Q calculations, and they demonstrate the importance of having an accurate description of the static and

  6. Quantum Monte Carlo Treatment of the Charge Transfer and Diradical Electronic Character in a Retinal Chromophore Minimal Model.

    PubMed

    Zen, Andrea; Coccia, Emanuele; Gozem, Samer; Olivucci, Massimo; Guidoni, Leonardo

    2015-03-10

    The penta-2,4-dieniminium cation (PSB3) displays similar ground state and first excited state potential energy features as those of the retinal protonated Schiff base (RPSB) chromophore in rhodopsin. Recently, PSB3 has been used to benchmark several electronic structure methods, including highly correlated multireference wave function approaches, highlighting the necessity to accurately describe the electronic correlation in order to obtain reliable properties even along the ground state (thermal) isomerization paths. In this work, we apply two quantum Monte Carlo approaches, the variational Monte Carlo and the lattice regularized diffusion Monte Carlo, to study the energetics and electronic properties of PSB3 along representative minimum energy paths and scans related to its thermal cis–trans isomerization. Quantum Monte Carlo is used in combination with the Jastrow antisymmetrized geminal power ansatz, which guarantees an accurate and balanced description of the static electronic correlation thanks to the multiconfigurational nature of the antisymmetrized geminal power term, and of the dynamical correlation, due to the presence of the Jastrow factor explicitly depending on electron–electron distances. Along the two ground state isomerization minimum energy paths of PSB3, CASSCF calculations yield wave functions having either charge transfer or diradical character in proximity of the two transition state configurations. Here, we observe that at the quantum Monte Carlo level of theory, only the transition state with charge transfer character can be located. The conical intersection, which becomes highly sloped, is observed only if the path connecting the two original CASSCF transition states is extended beyond the diradical one, namely by increasing the bond-length-alternation (BLA). These findings are in good agreement with the results obtained by MRCISD+Q calculations, and they demonstrate the importance of having an accurate description of the static and

  7. Ab Initio Modeling of Fe(II) Adsorption and Interfacial Electron Transfer at Goethite (α-FeOOH) Surfaces

    SciTech Connect

    Alexandrov, Vitali Y.; Rosso, Kevin M.

    2015-01-01

    Goethite (α-FeOOH) surfaces represent one of the most ubiquitous redox-active interfaces in the environment, playing an important role in biogeochemical metal cycling and contaminant residence in the subsurface. Fe(II)-catalyzed recrystallization of goethite is a fundamental process in this context, but the proposed Fe(II)aq-Fe(III)goethite electron and iron atom exchange mechanism of recrystallization remains poorly understood at the atomic level. We examine the adsorption of aqueous Fe(II) and subsequent interfacial electron transfer (ET) between adsorbed Fe(II) and structural Fe(III) at the (110) and (021) goethite surfaces using density functional theory calculations including Hubbard U corrections (DFT+U) aided by ab initio molecular dynamics simulations. We investigate various surface sites for the adsorption of Fe2+(H2O)6 in different coordination environments. Calculated energies for adsorbed complexes at both surfaces favor monodentate complexes with reduced 4- and 5-fold coordination over higher-dentate structures and 6- fold coordination. The hydrolysis of H2O ligands is observed for some pre-ET adsorbed Fe(II) configurations. ET from the adsorbed Fe(II) into the goethite lattice is calculated to be energetically uphill always, but simultaneous proton transfer from H2O ligands of the adsorbed complexes to the surface oxygen species stabilizes post-ET states. We find that surface defects such as oxygen vacancies near the adsorption site also can stabilize post-ET states, enabling the Fe(II)aq-Fe(III)goethite interfacial electron transfer reaction implied from experiments to proceed.

  8. Photo-induced electron transfer method

    DOEpatents

    Wohlgemuth, Roland; Calvin, Melvin

    1984-01-01

    The efficiency of photo-induced electron transfer reactions is increased and the back transfer of electrons in such reactions is greatly reduced when a photo-sensitizer zinc porphyrin-surfactant and an electron donor manganese porphyrin-surfactant are admixed into phospho-lipid membranes. The phospholipids comprising said membranes are selected from phospholipids whose head portions are negatively charged. Said membranes are contacted with an aqueous medium in which an essentially neutral viologen electron acceptor is admixed. Catalysts capable of transfering electrons from reduced viologen electron acceptor to hydrogen to produce elemental hydrogen are also included in the aqueous medium. An oxidizable olefin is also admixed in the phospholipid for the purpose of combining with oxygen that coordinates with oxidized electron donor manganese porphyrin-surfactant.

  9. Photo-induced electron transfer method

    DOEpatents

    Wohlgemuth, R.; Calvin, M.

    1984-01-24

    The efficiency of photo-induced electron transfer reactions is increased and the back transfer of electrons in such reactions is greatly reduced when a photo-sensitizer zinc porphyrin-surfactant and an electron donor manganese porphyrin-surfactant are admixed into phospholipid membranes. The phospholipids comprising said membranes are selected from phospholipids whose head portions are negatively charged. Said membranes are contacted with an aqueous medium in which an essentially neutral viologen electron acceptor is admixed. Catalysts capable of transferring electrons from reduced viologen electron acceptor to hydrogen to produce elemental hydrogen are also included in the aqueous medium. An oxidizable olefin is also admixed in the phospholipid for the purpose of combining with oxygen that coordinates with oxidized electron donor manganese porphyrin-surfactant.

  10. Photoinduced electron transfer in ordered polymers

    SciTech Connect

    Jones, G. II.

    1990-10-20

    Photochemical studies on organic polymers or biopolymers (particularly synthetic peptides) that have been modified by covalent attachment (or other means of binding) of organic chromophores and electron transfer agents are described. Specific projects involve are: peptide conjugates bearing electroactive residues such as tryptophan and specifically labeled at the N- or C-terminus of peptide chains; the electrostatic binding of organic dyes to poly-electrolytes (polyacrylates) for which the formation of dimeric aggregates of bound dye that display unusual photophysical and electron transfer properties is important; a study of the binding of dyes and electron transfer agents to the protein mimic,'' polyvinyl-2-pyrrolidinone (PVP), in hydrophobic domains that depend on specific H-bond interaction; and completion of an earlier study having to do with the triplet state properties of charge-transfer (CT) complexes of a high potential quinone and various electron donors (investigation of the properties of triplet (contact) radical-ion pairs). 13 refs., 5 figs., 2 tabs.

  11. Dynamics of electron transfer in amine photooxidation

    SciTech Connect

    Peters, K.S.; Freilich, S.C.; Schaeffer, C.G.

    1980-08-13

    Studies were initiated utilizing picosecond (ps) absorption spectroscopy, to directly monitor the dynamics of electron transfer from 1,4-diazabicyclo(2.2.2)octane (Dabco) to the excited states of benzophenone and fluorenone. These two systems were chosen because of their contrasting photochemistry. The quantum yield for photoreduction of benzophenone in polar solvents is generally greater than 0.1, while that of fluorenone is zero. In polar solvents, the proposed mechanism dictates that an electron is transferred to the excited singlet state fluorenone, which then back-transfers the electron, regenerating ground-state fluorenone and amine. Photolysis of benzophenone in the presence of an amine transfers an electron to an excited triplet state, forming an ion pair that is stable relative to diffusional separation. The results of this study verify this proposal.

  12. Vectorial electron transfer in spatially ordered arrays

    SciTech Connect

    Fox, M.A.

    1993-02-01

    Progress was made on synthesis of new materials for directional electron transfer (block copolymers and helical oligopeptides), preparation and characterization of anisotropic composites bearing organics and inorganics, electrocatalysis (redox-activated catalysts), and surface modifications of metals and semiconductors.

  13. New insights into the nonadiabatic state population dynamics of model proton-coupled electron transfer reactions from the mixed quantum-classical Liouville approach

    NASA Astrophysics Data System (ADS)

    Shakib, Farnaz A.; Hanna, Gabriel

    2016-01-01

    In a previous study [F. A. Shakib and G. Hanna, J. Chem. Phys. 141, 044122 (2014)], we investigated a model proton-coupled electron transfer (PCET) reaction via the mixed quantum-classical Liouville (MQCL) approach and found that the trajectories spend the majority of their time on the mean of two coherently coupled adiabatic potential energy surfaces. This suggested a need for mean surface evolution to accurately simulate observables related to ultrafast PCET processes. In this study, we simulate the time-dependent populations of the three lowest adiabatic states in the ET-PT (i.e., electron transfer preceding proton transfer) version of the same PCET model via the MQCL approach and compare them to the exact quantum results and those obtained via the fewest switches surface hopping (FSSH) approach. We find that the MQCL population profiles are in good agreement with the exact quantum results and show a significant improvement over the FSSH results. All of the mean surfaces are shown to play a direct role in the dynamics of the state populations. Interestingly, our results indicate that the population transfer to the second-excited state can be mediated by dynamics on the mean of the ground and second-excited state surfaces, as part of a sequence of nonadiabatic transitions that bypasses the first-excited state surface altogether. This is made possible through nonadiabatic transitions between different mean surfaces, which is the manifestation of coherence transfer in MQCL dynamics. We also investigate the effect of the strength of the coupling between the proton/electron and the solvent coordinate on the state population dynamics. Drastic changes in the population dynamics are observed, which can be understood in terms of the changes in the potential energy surfaces and the nonadiabatic couplings. Finally, we investigate the state population dynamics in the PT-ET (i.e., proton transfer preceding electron transfer) and concerted versions of the model. The PT

  14. Photoinitiated electron transfer in multi-chromophoric species: Synthetic tetrads and pentads

    SciTech Connect

    Not Available

    1990-02-14

    This research project involves the design, synthesis and study of the molecules which mimic many of the important aspects of photosynthetic electron and energy transfer. Specifically, the molecules are designed to mimic the following aspects of natural photosynthetic multistep electron transfer: electron donation from a tetrapyrrole excited singlet state, electron transfer between tetrapyrroles, electron transfer from tetrapyrroles to quinones, and electron transfer between quinones with different redox properties. In addition, they model carotenoid antenna function in photosynthesis (singlet-singlet energy transfer from carotenoid polyenes to chlorophyll) and carotenoid photoprotection from singlet oxygen damage (triplet-triplet energy transfer from chlorophyll to carotenoids).

  15. Long-range electron transfer

    PubMed Central

    Gray, Harry B.; Winkler, Jay R.

    2005-01-01

    Recent investigations have shed much light on the nuclear and electronic factors that control the rates of long-range electron tunneling through molecules in aqueous and organic glasses as well as through bonds in donor–bridge–acceptor complexes. Couplings through covalent and hydrogen bonds are much stronger than those across van der Waals gaps, and these differences in coupling between bonded and nonbonded atoms account for the dependence of tunneling rates on the structure of the media between redox sites in Ru-modified proteins and protein–protein complexes. PMID:15738403

  16. The Symmetrical Quasi-Classical Model for Electronically Non-Adiabatic Processes Applied to Energy Transfer Dynamics in Site-Exciton Models of Light-Harvesting Complexes.

    PubMed

    Cotton, Stephen J; Miller, William H

    2016-03-01

    In a recent series of papers, it has been illustrated that a symmetrical quasi-classical (SQC) windowing model applied to the Meyer-Miller (MM) classical vibronic Hamiltonian provides an excellent description of a variety of electronically non-adiabatic benchmark model systems for which exact quantum results are available for comparison. In this paper, the SQC/MM approach is used to treat energy transfer dynamics in site-exciton models of light-harvesting complexes, and in particular, the well-known 7-state Fenna-Mathews-Olson (FMO) complex. Again, numerically "exact" results are available for comparison, here via the hierarchical equation of motion (HEOM) approach of Ishizaki and Fleming, and it is seen that the simple SQC/MM approach provides very reasonable agreement with the previous HEOM results. It is noted, however, that unlike most (if not all) simple approaches for treating these systems, because the SQC/MM approach presents a fully atomistic simulation based on classical trajectory simulation, it places no restrictions on the characteristics of the thermal baths coupled to each two-level site, e.g., bath spectral densities (SD) of any analytic functional form may be employed as well as discrete SD determined experimentally or from MD simulation (nor is there any restriction that the baths be harmonic), opening up the possibility of simulating more realistic variations on the basic site-exciton framework for describing the non-adiabatic dynamics of photosynthetic pigment complexes. PMID:26761191

  17. Electron transfer across a thermal gradient.

    PubMed

    Craven, Galen T; Nitzan, Abraham

    2016-08-23

    Charge transfer is a fundamental process that underlies a multitude of phenomena in chemistry and biology. Recent advances in observing and manipulating charge and heat transport at the nanoscale, and recently developed techniques for monitoring temperature at high temporal and spatial resolution, imply the need for considering electron transfer across thermal gradients. Here, a theory is developed for the rate of electron transfer and the associated heat transport between donor-acceptor pairs located at sites of different temperatures. To this end, through application of a generalized multidimensional transition state theory, the traditional Arrhenius picture of activation energy as a single point on a free energy surface is replaced with a bithermal property that is derived from statistical weighting over all configurations where the reactant and product states are equienergetic. The flow of energy associated with the electron transfer process is also examined, leading to relations between the rate of heat exchange among the donor and acceptor sites as functions of the temperature difference and the electronic driving bias. In particular, we find that an open electron transfer channel contributes to enhanced heat transport between sites even when they are in electronic equilibrium. The presented results provide a unified theory for charge transport and the associated heat conduction between sites at different temperatures. PMID:27450086

  18. Respiratory electron transfer pathways in plant mitochondria

    PubMed Central

    Schertl, Peter; Braun, Hans-Peter

    2014-01-01

    The respiratory electron transport chain (ETC) couples electron transfer from organic substrates onto molecular oxygen with proton translocation across the inner mitochondrial membrane. The resulting proton gradient is used by the ATP synthase complex for ATP formation. In plants, the ETC is especially intricate. Besides the “classical” oxidoreductase complexes (complex I–IV) and the mobile electron transporters cytochrome c and ubiquinone, it comprises numerous “alternative oxidoreductases.” Furthermore, several dehydrogenases localized in the mitochondrial matrix and the mitochondrial intermembrane space directly or indirectly provide electrons for the ETC. Entry of electrons into the system occurs via numerous pathways which are dynamically regulated in response to the metabolic state of a plant cell as well as environmental factors. This mini review aims to summarize recent findings on respiratory electron transfer pathways in plants and on the involved components and supramolecular assemblies. PMID:24808901

  19. Photoinduced Electron Transfer in Organic Solar Cells.

    PubMed

    Song, Peng; Li, Yuanzuo; Ma, Fengcai; Pullerits, Tõnu; Sun, Mengtao

    2016-04-01

    Electron transfer (ET) is the key process in light-driven charge separation reactions in organic solar cells. The current review summarizes the progress in theoretical modelling of ET in these materials. First we give an account of ET, with a description originating from Marcus theory. We systematically go through all the relevant parameters and show how they depend on different material properties, and discuss the consequences such dependencies have for the performance of the devices. Finally, we present a set of visualization methods which have proven to be very useful in analyzing the elementary processes in absorption and charge separation events. Such visualization tools help us to understand the properties of the photochemical and photobiological systems in solar cells. PMID:26853631

  20. Single Molecule Spectroscopy of Electron Transfer

    SciTech Connect

    Michael Holman; Ling Zang; Ruchuan Liu; David M. Adams

    2009-10-20

    The objectives of this research are threefold: (1) to develop methods for the study electron transfer processes at the single molecule level, (2) to develop a series of modifiable and structurally well defined molecular and nanoparticle systems suitable for detailed single molecule/particle and bulk spectroscopic investigation, (3) to relate experiment to theory in order to elucidate the dependence of electron transfer processes on molecular and electronic structure, coupling and reorganization energies. We have begun the systematic development of single molecule spectroscopy (SMS) of electron transfer and summaries of recent studies are shown. There is a tremendous need for experiments designed to probe the discrete electronic and molecular dynamic fluctuations of single molecules near electrodes and at nanoparticle surfaces. Single molecule spectroscopy (SMS) has emerged as a powerful method to measure properties of individual molecules which would normally be obscured in ensemble-averaged measurement. Fluctuations in the fluorescence time trajectories contain detailed molecular level statistical and dynamical information of the system. The full distribution of a molecular property is revealed in the stochastic fluctuations, giving information about the range of possible behaviors that lead to the ensemble average. In the case of electron transfer, this level of understanding is particularly important to the field of molecular and nanoscale electronics: from a device-design standpoint, understanding and controlling this picture of the overall range of possible behaviors will likely prove to be as important as designing ia the ideal behavior of any given molecule.

  1. Final Report: Vibrational Dynamics in Photoinduced Electron Transfer

    SciTech Connect

    Kenneth G. Spears

    2006-04-19

    The objective of this grant was to understand how molecular vibrational states (geometry distortions) are involved in photoinduced electron transfer rates of molecules. This subject is an important component of understanding how molecular absorbers of light convert that energy into charge separation. This is important because the absorption usually excites molecular vibrations in a new electronic state prior to electron transfer to other molecules or semiconductor nanoparticles, as in some types of solar cells. The speeds of charge separation and charge recombination are key parameters that require experiments such as those in this work to test the rules governing electron transfer rates. Major progress was made on this goal. Some of the molecular structures selected for developing experimental data were bimolecular charge transfer complexes that contained metals of cobalt or vanadium. The experiments used the absorption of an ultrafast pulse of light to directly separate charges onto the two different molecular parts of the complex. The charge recombination then proceeds naturally, and one goal was to measure the speed of this recombination for different types of molecular vibrations. We used picosecond and femtosecond duration pulses with tunable colors at infrared wavelengths to directly observe vibrational states and their different rates of charge recombination (also called electron transfer). We discovered that different contact geometries in the complexes had very different electron transfer rates, and that one geometry had a significant dependence on the amount of vibration in the complex. This is the first and only measurement of such rates, and it allowed us to confirm our interpretation with a number of molecular models and test the sensitivity of electron transfer to vibrational states. This led us to develop a general theory, where we point out how molecular distortions can change the electron transfer rates to be much faster than prior theories

  2. Photoinduced electron transfer in a porphyrin dyad

    SciTech Connect

    Gust, D.; Moore, T.A.; Moore, A.L.; Leggett, L.; Lin, S.; DeGraziano, J.M.; Hermant, R.M.; Nicodem, D.; Craig, P.; Seely, G.R.; Nieman, R.A. )

    1993-07-29

    A prophyrin dyad designed to facilitate vectorial interporphyrin electron transfer has been synthesized and studied using steady-state and time-resolved absorption and emission spectroscopies. The dyad features a zinc tetraaylporphyrin bearing electron-donating substituents linked by an amide bond to a free base porphyrin carrying electron-withdrawing groups. Excitation of the zinc porphyrin moiety in dichloromethane solution is followed by singlet energy transfer to the free base and concurrent electron transfer to the same moiety to yield a charge-separated state. The free base first excited singlet state decays by accepting an electron from the zinc porphyrin to form the same charge-separated state. Similar results are observed in butyronitrile. Transient absorption studies in butyronitrile verify the formation of a short-lived (8 ps) charge-separated state from the porphyrin first excited singlet states. The results support the suggestion that fluorescence quenching in related porphyrin dyads and carotenoid--diporphyrin triads is due to photoinduced electron transfer, rather than some other decay process. 17 refs., 8 refs.

  3. Theory of plasmon enhanced interfacial electron transfer

    NASA Astrophysics Data System (ADS)

    Wang, Luxia; May, Volkhard

    2015-04-01

    A particular attempt to improve the efficiency of a dye sensitized solar cell is it's decoration with metal nano-particles (MNP). The MNP-plasmon induced enhancement of the local field enlarges the photoexcitation of the dyes and a subsequent improvement of the charge separation efficiency may result. In a recent work (2014 J. Phys. Chem. C 118 2812) we presented a theory of plasmon enhanced interfacial electron transfer for perylene attached to a TiO2 surface and placed in the proximity of a spherical MNP. These earlier studies are generalized here to the coupling of to up to four MNPs and to the use of somewhat altered molecular parameters. If the MNPs are placed close to each other strong hybridization of plasmon excitations appears and a broad resonance to which molecular excitations are coupled is formed. To investigate this situation the whole charge injection dynamics is described in the framework of the density matrix theory. The approach accounts for optical excitation of the dye coupled to the MNPs and considers subsequent electron injection into the rutile TiO2-cluster. Using a tight-binding model for the TiO2-system with about 105 atoms the electron motion in the cluster is described. We again consider short optical excitation which causes an intermediate steady state with a time-independent overall probability to have the electron injected into the cluster. This probability is used to introduce an enhancement factor which rates the influence of the MNP. Values larger than 500 are obtained.

  4. Theory of plasmon enhanced interfacial electron transfer.

    PubMed

    Wang, Luxia; May, Volkhard

    2015-04-10

    A particular attempt to improve the efficiency of a dye sensitized solar cell is it's decoration with metal nano-particles (MNP). The MNP-plasmon induced enhancement of the local field enlarges the photoexcitation of the dyes and a subsequent improvement of the charge separation efficiency may result. In a recent work (2014 J. Phys. Chem. C 118 2812) we presented a theory of plasmon enhanced interfacial electron transfer for perylene attached to a TiO2 surface and placed in the proximity of a spherical MNP. These earlier studies are generalized here to the coupling of to up to four MNPs and to the use of somewhat altered molecular parameters. If the MNPs are placed close to each other strong hybridization of plasmon excitations appears and a broad resonance to which molecular excitations are coupled is formed. To investigate this situation the whole charge injection dynamics is described in the framework of the density matrix theory. The approach accounts for optical excitation of the dye coupled to the MNPs and considers subsequent electron injection into the rutile TiO2-cluster. Using a tight-binding model for the TiO2-system with about 10(5) atoms the electron motion in the cluster is described. We again consider short optical excitation which causes an intermediate steady state with a time-independent overall probability to have the electron injected into the cluster. This probability is used to introduce an enhancement factor which rates the influence of the MNP. Values larger than 500 are obtained. PMID:25764984

  5. Electron Transfer Dissociation of Oligonucleotide Cations.

    PubMed

    Smith, Suncerae I; Brodbelt, Jennifer S

    2009-06-01

    Electron transfer dissociation (ETD) of multi-protonated 6 - 20-mer oligonucleotides and 12- and 14-mer duplexes is compared to collision activated dissociation (CAD). ETD causes efficient charge reduction of the multi-protonated oligonucleotides in addition to limited backbone cleavages to yield sequence ions of low abundance. Subsequent CAD of the charge-reduced oligonucleotides formed upon electron transfer, in a net process termed electron transfer collision activated dissociation (ETcaD), results in rich fragmentation in terms of w, a, z, and d products, with a marked decrease in the abundance of base loss ions and internal fragments. Complete sequencing was possible for nearly all oligonucleotides studied. ETcaD of an oligonucleotide duplex resulted in specific backbone cleavages, with conservation of weaker non-covalent bonds. PMID:20161288

  6. Unusual distance dependences of electron transfer rates.

    PubMed

    Kuss-Petermann, Martin; Wenger, Oliver S

    2016-07-28

    Usually the rates for electron transfer (kET) decrease with increasing donor-acceptor distance, but Marcus theory predicts a regime in which kET is expected to increase when the transfer distance gets longer. Until recently, experimental evidence for such counter-intuitive behavior had been very limited, and consequently this effect is much less well-known than the Gaussian free energy dependence of electron transfer rates leading to the so-called inverted driving-force effect. This article presents the theoretical concepts that lead to the prediction of electron transfer rate maxima at large donor-acceptor distances, and it discusses conditions that are expected to favor experimental observations of such behavior. It continues with a consideration of specific recent examples in which electron transfer rates were observed to increase with increasing donor-acceptor distance, and it closes with a discussion of the importance of this effect in the context of light-to-chemical energy conversion. PMID:27353891

  7. A benchmark test suite for proton transfer energies and its use to test electronic structure model chemistries

    NASA Astrophysics Data System (ADS)

    Nachimuthu, Santhanamoorthi; Gao, Jiali; Truhlar, Donald G.

    2012-05-01

    We present benchmark calculations of nine selected points on potential energy surfaces describing proton transfer processes in three model systems, H5O2+, CH3OH…H+…OH2, and CH3COOH…OH2. The calculated relative energies of these geometries are compared to those calculated by various wave function and density functional methods, including the polarized molecular orbital (PMO) model recently developed in our research group and other semiempirical molecular orbital methods. We found that the SCC-DFTB and PMO methods (the latter available so far only for molecules consisting of only O and H and therefore only for the first of the three model systems) give results that are, on average, within 2 kcal/mol of the benchmark results. Other semiempirical molecular orbital methods have mean unsigned errors (MUEs) of 3-8 kcal/mol, local density functionals have MUEs in the range 0.7-3.7 kcal/mol, and hybrid density functionals have MUEs of only 0.3-1.0 kcal/mol, with the best density functional performance obtained by hybrid meta-GGAs, especially M06 and PW6B95.

  8. Proton-Coupled Electron Transfer Reactions with Photometric Bases Reveal Free Energy Relationships for Proton Transfer.

    PubMed

    Eisenhart, Thomas T; Howland, William C; Dempsey, Jillian L

    2016-08-18

    The proton-coupled electron transfer (PCET) oxidation of p-aminophenol in acetonitrile was initiated via stopped-flow rapid-mixing and spectroscopically monitored. For oxidation by ferrocenium in the presence of 7-(dimethylamino)quinoline proton acceptors, both the electron transfer and proton transfer components could be optically monitored in the visible region; the decay of the ferrocenium absorbance is readily monitored (λmax = 620 nm), and the absorbance of the 2,4-substituted 7-(dimethylamino)quinoline derivatives (λmax = 370-392 nm) red-shifts substantially (ca. 70 nm) upon protonation. Spectral analysis revealed the reaction proceeds via a stepwise electron transfer-proton transfer process, and modeling of the kinetics traces monitoring the ferrocenium and quinolinium signals provided rate constants for elementary proton and electron transfer steps. As the pKa values of the conjugate acids of the 2,4-R-7-(dimethylamino)quinoline derivatives employed were readily tuned by varying the substituents at the 2- and 4-positions of the quinoline backbone, the driving force for proton transfer was systematically varied. Proton transfer rate constants (kPT,2 = (1.5-7.5) × 10(8) M(-1) s(-1), kPT,4 = (0.55-3.0) × 10(7) M(-1) s(-1)) were found to correlate with the pKa of the conjugate acid of the proton acceptor, in agreement with anticipated free energy relationships for proton transfer processes in PCET reactions. PMID:27500804

  9. Excited-state intramolecular hydrogen transfer (ESIHT) of 1,8-dihydroxy-9,10-anthraquinone (DHAQ) characterized by ultrafast electronic and vibrational spectroscopy and computational modeling.

    PubMed

    Mohammed, Omar F; Xiao, Dequan; Batista, Victor S; Nibbering, Erik T J

    2014-05-01

    We combine ultrafast electronic and vibrational spectroscopy and computational modeling to investigate the photoinduced excited-state intramolecular hydrogen-transfer dynamics in 1,8-dihydroxy-9,10-anthraquinone (DHAQ) in tetrachloroethene, acetonitrile, dimethyl sulfoxide, and methanol. We analyze the electronic excited states of DHAQ with various possible hydrogen-bonding schemes and provide a general description of the electronic excited-state dynamics based on a systematic analysis of femtosecond UV/vis and UV/IR pump-probe spectroscopic data. Upon photoabsorption at 400 nm, the S2 electronic excited state is initially populated, followed by a rapid equilibration within 150 fs through population transfer to the S1 state where DHAQ exhibits ESIHT dynamics. In this equilibration process, the excited-state population is distributed between the 9,10-quinone (S2) and 1,10-quinone (S1) states while undergoing vibrational energy redistribution, vibrational cooling, and solvation dynamics on the 0.1-50 ps time scale. Transient UV/vis pump-probe data in methanol also suggest additional relaxation dynamics on the subnanosecond time scale, which we tentatively ascribe to hydrogen bond dynamics of DHAQ with the protic solvent, affecting the equilibrium population dynamics within the S2 and S1 electronic excited states. Ultimately, the two excited singlet states decay with a solvent-dependent time constant ranging from 139 to 210 ps. The concomitant electronic ground-state recovery is, however, only partial because a large fraction of the population relaxes to the first triplet state. From the similarity of the time scales involved, we conjecture that the solvent plays a crucial role in breaking the intramolecular hydrogen bond of DHAQ during the S2/S1 relaxation to either the ground or triplet state. PMID:24684387

  10. Systems approach to excitation-energy and electron transfer reaction networks in photosystem II complex: model studies for chlorophyll a fluorescence induction kinetics.

    PubMed

    Matsuoka, Takeshi; Tanaka, Shigenori; Ebina, Kuniyoshi

    2015-09-01

    Photosystem II (PS II) is a protein complex which evolves oxygen and drives charge separation for photosynthesis employing electron and excitation-energy transfer processes over a wide timescale range from picoseconds to milliseconds. While the fluorescence emitted by the antenna pigments of this complex is known as an important indicator of the activity of photosynthesis, its interpretation was difficult because of the complexity of PS II. In this study, an extensive kinetic model which describes the complex and multi-timescale characteristics of PS II is analyzed through the use of the hierarchical coarse-graining method proposed in the authors׳ earlier work. In this coarse-grained analysis, the reaction center (RC) is described by two states, open and closed RCs, both of which consist of oxidized and neutral special pairs being in quasi-equilibrium states. Besides, the PS II model at millisecond scale with three-state RC, which was studied previously, could be derived by suitably adjusting the kinetic parameters of electron transfer between tyrosine and RC. Our novel coarse-grained model of PS II can appropriately explain the light-intensity dependent change of the characteristic patterns of fluorescence induction kinetics from O-J-I-P, which shows two inflection points, J and I, between initial point O and peak point P, to O-J-D-I-P, which shows a dip D between J and I inflection points. PMID:26025316

  11. 75 FR 33681 - Electronic Fund Transfers

    Federal Register 2010, 2011, 2012, 2013, 2014

    2010-06-15

    ... document in the Federal Register of June 4, 2010 (75 FR 31665). The document (FR Doc. 2010-13280) amended... number 2. In the final rule, FR Doc. 2010-13280, published on June 4, 2010 (75 FR 31665) make the... CFR Part 205 Electronic Fund Transfers June 4, 2010. AGENCY: Board of Governors of the Federal...

  12. 75 FR 66644 - Electronic Fund Transfers

    Federal Register 2010, 2011, 2012, 2013, 2014

    2010-10-29

    ...The Board is amending Regulation E, which implements the Electronic Fund Transfer Act, and the official staff commentary to the regulation, in order to implement legislation that modifies the effective date of certain disclosure requirements in the gift card provisions of the Credit Card Accountability Responsibility and Disclosure Act of...

  13. Activation entropy of electron transfer reactions

    NASA Astrophysics Data System (ADS)

    Milischuk, Anatoli A.; Matyushov, Dmitry V.; Newton, Marshall D.

    2006-05-01

    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.

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

    PubMed

    Hammes-Schiffer, Sharon; Soudackov, Alexander V

    2008-11-13

    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. PMID:18842015

  15. Electronic coherence in electronic energy transfer despite fast dephasing

    NASA Astrophysics Data System (ADS)

    Scholes, Gregory

    2009-03-01

    F"orster resonance energy transfer (FRET) is a common and fundamental photophysical process in life and materials sciences. FRET is an interchromophore relaxation process that transmits the electronic excitation from an initially excited donor to a ground state acceptor chromophore (light-absorbing moleule). FRET is used, for example, to harvest light in photosynthesis, measure distances in proteins, and it accelerates the photodegradation of polymers. In recent years attention has turned to the study of FRET in complex assemblies of molecules. While F"orster theory has enabled the efficiency of FRET to be predicted and analyzed in numerous and diverse areas of study, recent work has aimed to discover ways beyond the F"oster mechanism by which electronic energy can be transferred. The talk will compare and contrast theoretical and experimental studies of excitation relaxation in photosynthetic antenna systems with the conjugated polymer poly[2-methoxy,5-(2'-ethyl-hexoxy)-1,4-phenylenevinylene] (MEH-PPV). I will report new work where we have used a new anisotropy experiment to examine coherent energy transfer and a complementary technique using two-dimensional electronic spectroscopy expose the role of coherence transfer in the fastest time dynamics. We find that coherent energy transfer occurs for many tens of femtoseconds, even at room temperature. That leads us to examine the nature and implications of the so-called intermediate coupling regime for EET.

  16. Mechanism of Intermolecular Electron Transfer in Bionanostructures

    NASA Astrophysics Data System (ADS)

    Gruodis, A.; Galikova, N.; Šarka, K.; Saulė, R.; Batiuškaitė, D.; Saulis, G.

    Hepatocellular carcinoma (HCC) is one of the most common malignant tumors worldwide. Most patients are inoperable and hepatoma cells are resistant to conventional chemotherapies. Thus, the development of novel therapies for HCC treatment is of paramount importance. Amongst different alimentary factors, vitamin C and vitamin K3 In the present work, it has been shown that the treatment of mouse hepatoma MH-22A cells by vitamin C and vitamin K3 at the ratio of 100:1 greatly enhanced their cytotoxicity. When cells were subjected to vitamin C at 200 μM or to vitamin K3 at 2 μM separately, their viability reduced by only about 10%. However, when vitamins C and K3 were combined at the same concentrations, they killed more than 90% of cells. To elucidate the mechanism of the synergistic cytotoxicity of the C&K3 mixture, theoretical quantum-chemical analysis of the dynamics of intermolecular electron transfer (IET) processes within the complexes containing C (five forms) and K3 (one form) has been carried out. Optimization of the ground state complex geometry has been provided by means of GAUSSIAN03 package. Simulation of the IET has been carried out using NUVOLA package, in the framework of molecular orbitals (MO). The rate of IET has been calculated using Fermi Golden rule. The results of simulations allow us to create the preliminary model of the reaction pathway.

  17. Electronic decoherence for electron transfer in blue copper proteins

    NASA Astrophysics Data System (ADS)

    Lockwood, Daren M.; Cheng, Yuen-Kit; Rossky, Peter J.

    2001-09-01

    We present a molecular dynamics investigation of the electronic decoherence rate for electron transfer (ET) in a solvated protein molecule. We find that decoherence occurs on an ultrafast time scale of 2.4 fs, considerably faster than fluctuations in the electronic coupling. Both protein and solvent dynamics play important roles. Solvent alone would give rise to a decoherence time of 3.0 fs, as compared to 4.1 fs from the protein matrix alone. This implies that both solvation and protein dynamics can strongly affect both the rate and mechanism of ET.

  18. Effect of protein relaxation on electron transfer from the cytochrome subunit to the bacteriochlorophyll dimer in Rps. sulfoviridis reaction centers within mixed adiabatic/nonadiabatic model.

    PubMed

    Kotelnikov, A I; Ortega, J M; Medvedev, E S; Psikha, B L; Garcia, D; Mathis, P

    2002-05-15

    The broad set of nonexponential electron transfer (ET) kinetics in reaction centers (RC) from Rhodopseudomonas sulfoviridis in temperature range 297-40 K are described within a mixed adiabatic/nonadiabatic model. The key point of the model is the combination of Sumi-Marcus and Rips-Jortner approaches which can be represented by the separate contributions of temperature-independent vibrational (v) and temperature-dependent diffusive (d) coordinates to the preexponential factor, to the free energy of reaction DeltaG=DeltaG(v)+DeltaG(d)(T) and to the reorganization energy lambda=lambda(v)+lambda(d)(T). The broad distribution of protein dielectric relaxation times along the diffusive coordinate is considered within the Davidson-Cole formalism. PMID:12009432

  19. Theory of ultrafast heterogeneous electron transfer: Contributions of direct charge transfer excitations to the absorbance

    NASA Astrophysics Data System (ADS)

    Wang, Luxia; Willig, Frank; May, Volkhard

    2007-04-01

    Absorption spectra related to heterogeneous electron transfer are analyzed with the focus on direct charge transfer transition from the surface attached molecule into the semiconductor band states. The computations are based on a model of reduced dimensionality with a single intramolecular vibrational coordinate but a complete account for the continuum of conduction band states. The applicability of this model to perylene on TiO2 has been demonstrated in a series of earlier papers. Here, based on a time-dependent formulation, the absorbance is calculated with the inclusion of charge transfer excitations. A broad parameter set inspired by the perylene TiO2 systems is considered. In particular, the description generalizes the Fano effect to heterogeneous electron transfer reactions. Preliminary simulations of measured spectra are presented for perylene-catechol attached to TiO2.

  20. Electron transfer in silver telluride melt

    SciTech Connect

    Glazov, V.M.; Burkhanov, A.S.

    1987-06-01

    Electron transfer in silver telluride melt was studied experimentally at different temperatures. The method used to study electron transfer and thermodiffusion is based on Onsager's theory and consists of measuring the electrodiffusion potential which varies as a function of time in the system formed by the liquid semiconductor and the neutral metallic electrodes. The effective charges and the average coefficients of diffusion of silver ions were calculated and the ionic component of the total electrical conductivity of Ag/sub 2/Te melt was evaluated. It was observed that the indicated characteristics vary systematically in the series of silver chalcogenides with anionic substitution. The negative temperature coefficient of electrical conductivity in silver sulfide and silver selenide melts was explained.

  1. Biotechnological Aspects of Microbial Extracellular Electron Transfer

    PubMed Central

    Kato, Souichiro

    2015-01-01

    Extracellular electron transfer (EET) is a type of microbial respiration that enables electron transfer between microbial cells and extracellular solid materials, including naturally-occurring metal compounds and artificial electrodes. Microorganisms harboring EET abilities have received considerable attention for their various biotechnological applications, in addition to their contribution to global energy and material cycles. In this review, current knowledge on microbial EET and its application to diverse biotechnologies, including the bioremediation of toxic metals, recovery of useful metals, biocorrosion, and microbial electrochemical systems (microbial fuel cells and microbial electrosynthesis), were introduced. Two potential biotechnologies based on microbial EET, namely the electrochemical control of microbial metabolism and electrochemical stimulation of microbial symbiotic reactions (electric syntrophy), were also discussed. PMID:26004795

  2. Electron transfer control in soluble methane monooxygenase.

    PubMed

    Wang, Weixue; Iacob, Roxana E; Luoh, Rebecca P; Engen, John R; Lippard, Stephen J

    2014-07-01

    The hydroxylation or epoxidation of hydrocarbons by bacterial multicomponent monooxygenases (BMMs) requires the interplay of three or four protein components. How component protein interactions control catalysis, however, is not well understood. In particular, the binding sites of the reductase components on the surface of their cognate hydroxylases and the role(s) that the regulatory proteins play during intermolecular electron transfer leading to the hydroxylase reduction have been enigmatic. Here we determine the reductase binding site on the hydroxylase of a BMM enzyme, soluble methane monooxygenase (sMMO) from Methylococcus capsulatus (Bath). We present evidence that the ferredoxin domain of the reductase binds to the canyon region of the hydroxylase, previously determined to be the regulatory protein binding site as well. The latter thus inhibits reductase binding to the hydroxylase and, consequently, intermolecular electron transfer from the reductase to the hydroxylase diiron active site. The binding competition between the regulatory protein and the reductase may serve as a control mechanism for regulating electron transfer, and other BMM enzymes are likely to adopt the same mechanism. PMID:24937475

  3. Technology Transfer Issues and a New Technology Transfer Model

    ERIC Educational Resources Information Center

    Choi, Hee Jun

    2009-01-01

    The following are major issues that should be considered for efficient and effective technology transfer: conceptions of technology, technological activity and transfer, communication channels, factors affecting transfer, and models of transfer. In particular, a well-developed model of technology transfer could be used as a framework for…

  4. Electron Transfer and Reaction Mechanism of Laccases

    PubMed Central

    Jones, Stephen M.; Solomon, Edward I.

    2015-01-01

    Laccases are part of the family of multicopper oxidases (MCOs), which couple the oxidation of substrates to the four electron reduction of O2 to H2O. MCOs contain a minimum of four Cu's divided into Type 1 (T1), Type 2 (T2), and binuclear Type 3 (T3) Cu sites that are distinguished based on unique spectroscopic features. Substrate oxidation occurs near the T1, and electrons are transferred approximately 13 Å through the protein via the Cys-His pathway to the T2/T3 trinuclear copper cluster (TNC) where dioxygen reduction occurs. This review outlines the electron transfer (ET) process in laccases, and the mechanism of O2 reduction as elucidated through spectroscopic, kinetic, and computational data. Marcus theory is used to describe the relevant factors which impact ET rates including the driving force (ΔG°), reorganization energy (λ), and electronic coupling matrix element (HDA). Then the mechanism of O2 reaction is detailed with particular focus on the intermediates formed during the two 2e− reduction steps. The first 2e− step forms the peroxide intermediate (PI), followed by the second 2e− step to form the native intermediate (NI), which has been shown to be the catalytically relevant fully oxidized form of the enzyme. PMID:25572295

  5. Numerical modeling of heat-transfer and the influence of process parameters on tailoring the grain morphology of IN718 in electron beam additive manufacturing

    DOE PAGESBeta

    Raghavan, Narendran; Dehoff, Ryan; Pannala, Sreekanth; Simunovic, Srdjan; Kirka, Michael; Turner, John; Carlson, Neil; Babu, Sudarsanam S.

    2016-04-26

    The fabrication of 3-D parts from CAD models by additive manufacturing (AM) is a disruptive technology that is transforming the metal manufacturing industry. The correlation between solidification microstructure and mechanical properties has been well understood in the casting and welding processes over the years. This paper focuses on extending these principles to additive manufacturing to understand the transient phenomena of repeated melting and solidification during electron beam powder melting process to achieve site-specific microstructure control within a fabricated component. In this paper, we have developed a novel melt scan strategy for electron beam melting of nickel-base superalloy (Inconel 718) andmore » also analyzed 3-D heat transfer conditions using a parallel numerical solidification code (Truchas) developed at Los Alamos National Laboratory. The spatial and temporal variations of temperature gradient (G) and growth velocity (R) at the liquid-solid interface of the melt pool were calculated as a function of electron beam parameters. By manipulating the relative number of voxels that lie in the columnar or equiaxed region, the crystallographic texture of the components can be controlled to an extent. The analysis of the parameters provided optimum processing conditions that will result in columnar to equiaxed transition (CET) during the solidification. Furthermore, the results from the numerical simulations were validated by experimental processing and characterization thereby proving the potential of additive manufacturing process to achieve site-specific crystallographic texture control within a fabricated component.« less

  6. Promoting Interspecies Electron Transfer with Biochar

    PubMed Central

    Chen, Shanshan; Rotaru, Amelia-Elena; Shrestha, Pravin Malla; Malvankar, Nikhil S.; Liu, Fanghua; Fan, Wei; Nevin, Kelly P.; Lovley, Derek R.

    2014-01-01

    Biochar, a charcoal-like product of the incomplete combustion of organic materials, is an increasingly popular soil amendment designed to improve soil fertility. We investigated the possibility that biochar could promote direct interspecies electron transfer (DIET) in a manner similar to that previously reported for granular activated carbon (GAC). Although the biochars investigated were 1000 times less conductive than GAC, they stimulated DIET in co-cultures of Geobacter metallireducens with Geobacter sulfurreducens or Methanosarcina barkeri in which ethanol was the electron donor. Cells were attached to the biochar, yet not in close contact, suggesting that electrons were likely conducted through the biochar, rather than biological electrical connections. The finding that biochar can stimulate DIET may be an important consideration when amending soils with biochar and can help explain why biochar may enhance methane production from organic wastes under anaerobic conditions. PMID:24846283

  7. Photon-to-electron quantum information transfer

    NASA Astrophysics Data System (ADS)

    Kosaka, Hideo

    2011-05-01

    Spin is a fundamental property of electrons and plays an important role in information storage. For spin-based quantum information technology, preparation and read-out of the electron spin state must be spin coherent, but both the traditional preparation and read-out of the spin state are projective to up/down spin states, which do not have spin coherence. We have recently demonstrated that the polarization coherence of light can be coherently transferred to the spin coherence of electrons in a semiconductor. We have also developed a new scheme named tomographic Kerr rotation (TKR) by generalizing the traditional KR to directly readout the spin coherence of optically prepared electrons without the need for the spin dynamics, which allows the spin projection measurement in an arbitrary set of basis states. These demonstrations were performed using g-factor-controlled semiconductor quantum wells with precessing and nonprecessing electrons. The developed scheme offers a tool for performing basis-independent preparation and read-out of a spin quantum state in a solid. These results encourage us to make a quantum media converter between flying photon qubits and stationary electron spin qubits in semiconductors.

  8. Electron transfer statistics and thermal fluctuations in molecular junctions

    NASA Astrophysics Data System (ADS)

    Goswami, Himangshu Prabal; Harbola, Upendra

    2015-02-01

    We derive analytical expressions for probability distribution function (PDF) for electron transport in a simple model of quantum junction in presence of thermal fluctuations. Our approach is based on the large deviation theory combined with the generating function method. For large number of electrons transferred, the PDF is found to decay exponentially in the tails with different rates due to applied bias. This asymmetry in the PDF is related to the fluctuation theorem. Statistics of fluctuations are analyzed in terms of the Fano factor. Thermal fluctuations play a quantitative role in determining the statistics of electron transfer; they tend to suppress the average current while enhancing the fluctuations in particle transfer. This gives rise to both bunching and antibunching phenomena as determined by the Fano factor. The thermal fluctuations and shot noise compete with each other and determine the net (effective) statistics of particle transfer. Exact analytical expression is obtained for delay time distribution. The optimal values of the delay time between successive electron transfers can be lowered below the corresponding shot noise values by tuning the thermal effects.

  9. Electron transfer statistics and thermal fluctuations in molecular junctions.

    PubMed

    Goswami, Himangshu Prabal; Harbola, Upendra

    2015-02-28

    We derive analytical expressions for probability distribution function (PDF) for electron transport in a simple model of quantum junction in presence of thermal fluctuations. Our approach is based on the large deviation theory combined with the generating function method. For large number of electrons transferred, the PDF is found to decay exponentially in the tails with different rates due to applied bias. This asymmetry in the PDF is related to the fluctuation theorem. Statistics of fluctuations are analyzed in terms of the Fano factor. Thermal fluctuations play a quantitative role in determining the statistics of electron transfer; they tend to suppress the average current while enhancing the fluctuations in particle transfer. This gives rise to both bunching and antibunching phenomena as determined by the Fano factor. The thermal fluctuations and shot noise compete with each other and determine the net (effective) statistics of particle transfer. Exact analytical expression is obtained for delay time distribution. The optimal values of the delay time between successive electron transfers can be lowered below the corresponding shot noise values by tuning the thermal effects. PMID:25725711

  10. Electron transfer statistics and thermal fluctuations in molecular junctions

    SciTech Connect

    Goswami, Himangshu Prabal; Harbola, Upendra

    2015-02-28

    We derive analytical expressions for probability distribution function (PDF) for electron transport in a simple model of quantum junction in presence of thermal fluctuations. Our approach is based on the large deviation theory combined with the generating function method. For large number of electrons transferred, the PDF is found to decay exponentially in the tails with different rates due to applied bias. This asymmetry in the PDF is related to the fluctuation theorem. Statistics of fluctuations are analyzed in terms of the Fano factor. Thermal fluctuations play a quantitative role in determining the statistics of electron transfer; they tend to suppress the average current while enhancing the fluctuations in particle transfer. This gives rise to both bunching and antibunching phenomena as determined by the Fano factor. The thermal fluctuations and shot noise compete with each other and determine the net (effective) statistics of particle transfer. Exact analytical expression is obtained for delay time distribution. The optimal values of the delay time between successive electron transfers can be lowered below the corresponding shot noise values by tuning the thermal effects.

  11. Four-body model for transfer excitation

    SciTech Connect

    Harris, A. L.; Peacher, J. L.; Madison, D. H.; Colgan, J.

    2009-12-15

    We present here a four-body model for transfer-excitation collisions, which we call the four-body transfer-excitation (4BTE) model. Each two-body interaction is explicitly included in the 4BTE model, allowing us to study the effects of individual two-body interactions. We apply our model to fully differential cross sections for proton+helium collisions, and study the effect of the incident projectile-atom interaction, the scattered projectile-ion interaction, the projectile-nuclear interaction, and electron correlation within the target atom.

  12. Numerical simulation of transient moisture transfer into an electronic enclosure

    NASA Astrophysics Data System (ADS)

    Nasirabadi, P. Shojaee; Jabbari, M.; Hattel, J. H.

    2016-06-01

    Electronic systems are sometimes exposed to harsh environmental conditions of temperature and humidity. Moisture transfer into electronic enclosures and condensation can cause several problems such as corrosion and alteration in thermal stresses. It is therefore essential to study the local climate inside the enclosures to be able to protect the electronic systems. In this work, moisture transfer into a typical electronic enclosure is numerically studied using CFD. In order to reduce the CPU-time and make a way for subsequent factorial design analysis, a simplifying modification is applied in which the real 3D geometry is approximated by a 2D axial symmetry one. The results for 2D and 3D models were compared in order to calibrate the 2D representation. Furthermore, simulation results were compared with experimental data and good agreement was found.

  13. Frontier orbital symmetry control of intermolecular electron transfer

    SciTech Connect

    Stevens, B.

    1990-11-01

    Research continued on the study of intermolecular electron transfer. This report discusses the following topics: fluorescence quenching by electron transfer and the modification of quenching dynamics by solvent properties and net free energy change; transient absorption measurements following selective excitation of 1:1 EDA complex isomers; selective quenching of dual fluorescence from linked EDA systems; electron-transfer sensitized cycloreversion of rubrene endoperoxide; and vibronic modification of adiabatic requirements for intermolecular electron transfer. (CBS)

  14. A unified diabatic description for electron transfer reactions, isomerization reactions, proton transfer reactions, and aromaticity.

    PubMed

    Reimers, Jeffrey R; McKemmish, Laura K; McKenzie, Ross H; Hush, Noel S

    2015-10-14

    While diabatic approaches are ubiquitous for the understanding of electron-transfer reactions and have been mooted as being of general relevance, alternate applications have not been able to unify the same wide range of observed spectroscopic and kinetic properties. The cause of this is identified as the fundamentally different orbital configurations involved: charge-transfer phenomena involve typically either 1 or 3 electrons in two orbitals whereas most reactions are typically closed shell. As a result, two vibrationally coupled electronic states depict charge-transfer scenarios whereas three coupled states arise for closed-shell reactions of non-degenerate molecules and seven states for the reactions implicated in the aromaticity of benzene. Previous diabatic treatments of closed-shell processes have considered only two arbitrarily chosen states as being critical, mapping these states to those for electron transfer. We show that such effective two-state diabatic models are feasible but involve renormalized electronic coupling and vibrational coupling parameters, with this renormalization being property dependent. With this caveat, diabatic models are shown to provide excellent descriptions of the spectroscopy and kinetics of the ammonia inversion reaction, proton transfer in N2H7(+), and aromaticity in benzene. This allows for the development of a single simple theory that can semi-quantitatively describe all of these chemical phenomena, as well as of course electron-transfer reactions. It forms a basis for understanding many technologically relevant aspects of chemical reactions, condensed-matter physics, chemical quantum entanglement, nanotechnology, and natural or artificial solar energy capture and conversion. PMID:26193994

  15. 48 CFR 18.124 - Electronic funds transfer.

    Code of Federal Regulations, 2013 CFR

    2013-10-01

    ... 48 Federal Acquisition Regulations System 1 2013-10-01 2013-10-01 false Electronic funds transfer. 18.124 Section 18.124 Federal Acquisition Regulations System FEDERAL ACQUISITION REGULATION... Electronic funds transfer. Electronic funds transfer payments may be waived for acquisitions to...

  16. 48 CFR 18.124 - Electronic funds transfer.

    Code of Federal Regulations, 2012 CFR

    2012-10-01

    ... 48 Federal Acquisition Regulations System 1 2012-10-01 2012-10-01 false Electronic funds transfer. 18.124 Section 18.124 Federal Acquisition Regulations System FEDERAL ACQUISITION REGULATION... Electronic funds transfer. Electronic funds transfer payments may be waived for acquisitions to...

  17. 31 CFR 208.3 - Payment by electronic funds transfer.

    Code of Federal Regulations, 2014 CFR

    2014-07-01

    ... 31 Money and Finance: Treasury 2 2014-07-01 2014-07-01 false Payment by electronic funds transfer... DISBURSEMENTS § 208.3 Payment by electronic funds transfer. Subject to § 208.4, and notwithstanding any other... electronic funds transfer....

  18. 31 CFR 208.3 - Payment by electronic funds transfer.

    Code of Federal Regulations, 2012 CFR

    2012-07-01

    ... 31 Money and Finance:Treasury 2 2012-07-01 2012-07-01 false Payment by electronic funds transfer... DISBURSEMENTS § 208.3 Payment by electronic funds transfer. Subject to § 208.4, and notwithstanding any other... electronic funds transfer....

  19. 48 CFR 18.124 - Electronic funds transfer.

    Code of Federal Regulations, 2014 CFR

    2014-10-01

    ... 48 Federal Acquisition Regulations System 1 2014-10-01 2014-10-01 false Electronic funds transfer. 18.124 Section 18.124 Federal Acquisition Regulations System FEDERAL ACQUISITION REGULATION... Electronic funds transfer. Electronic funds transfer payments may be waived for acquisitions to...

  20. 14 CFR 1260.69 - Electronic funds transfer payment methods.

    Code of Federal Regulations, 2011 CFR

    2011-01-01

    ... 14 Aeronautics and Space 5 2011-01-01 2010-01-01 true Electronic funds transfer payment methods... COOPERATIVE AGREEMENTS General Special Conditions § 1260.69 Electronic funds transfer payment methods. Electronic Funds Transfer Payment Methods October 2000 (a) Payments under this grant will be made by...

  1. 48 CFR 18.124 - Electronic funds transfer.

    Code of Federal Regulations, 2011 CFR

    2011-10-01

    ... 48 Federal Acquisition Regulations System 1 2011-10-01 2011-10-01 false Electronic funds transfer. 18.124 Section 18.124 Federal Acquisition Regulations System FEDERAL ACQUISITION REGULATION... Electronic funds transfer. Electronic funds transfer payments may be waived for acquisitions to...

  2. 31 CFR 208.3 - Payment by electronic funds transfer.

    Code of Federal Regulations, 2011 CFR

    2011-07-01

    ... 31 Money and Finance:Treasury 2 2011-07-01 2011-07-01 false Payment by electronic funds transfer... DISBURSEMENTS § 208.3 Payment by electronic funds transfer. Subject to § 208.4, and notwithstanding any other... electronic funds transfer....

  3. 14 CFR 1274.931 - Electronic funds transfer payment methods.

    Code of Federal Regulations, 2011 CFR

    2011-01-01

    ... 14 Aeronautics and Space 5 2011-01-01 2010-01-01 true Electronic funds transfer payment methods... COOPERATIVE AGREEMENTS WITH COMMERCIAL FIRMS Other Provisions and Special Conditions § 1274.931 Electronic funds transfer payment methods. Electronic Funds Transfer Payment Methods July 2002 Payments under...

  4. 14 CFR 1260.69 - Electronic funds transfer payment methods.

    Code of Federal Regulations, 2010 CFR

    2010-01-01

    ... 14 Aeronautics and Space 5 2010-01-01 2010-01-01 false Electronic funds transfer payment methods... COOPERATIVE AGREEMENTS General Special Conditions § 1260.69 Electronic funds transfer payment methods. Electronic Funds Transfer Payment Methods October 2000 (a) Payments under this grant will be made by...

  5. 14 CFR 1274.931 - Electronic funds transfer payment methods.

    Code of Federal Regulations, 2010 CFR

    2010-01-01

    ... 14 Aeronautics and Space 5 2010-01-01 2010-01-01 false Electronic funds transfer payment methods... COOPERATIVE AGREEMENTS WITH COMMERCIAL FIRMS Other Provisions and Special Conditions § 1274.931 Electronic funds transfer payment methods. Electronic Funds Transfer Payment Methods July 2002 Payments under...

  6. Theoretical studies of electron transfer and optical spectroscopy

    NASA Astrophysics Data System (ADS)

    Friesner, R. A.

    Progress was made in three areas. First, we have made major strides in developing our PSGVB electronic structure code including high level electron correlation methods needed to treat complex structures like semiconductor clusters. Secondly, we have developed a one-electron pseudopotential model for semiconductor clusters which reproduces the experimental bandgap as a function of cluster size without adjustable parameters. We are currently using the model to calculate absorption spectra as opposed to just the bandgap via a dispersed polaron formalism. Finally, we have developed two novel quantum dynamical methods, one involving surface hopping and the second utilizing Redfield theory. These methods will be applied to calculation of the relaxation of an excited electron in a semiconductor cluster and also to electron transfer through DNA.

  7. Protein dynamics modulated electron transfer kinetics in early stage photosynthesis

    NASA Astrophysics Data System (ADS)

    Kundu, Prasanta; Dua, Arti

    2013-01-01

    A recent experiment has probed the electron transfer kinetics in the early stage of photosynthesis in Rhodobacter sphaeroides for the reaction center of wild type and different mutants [Science 316, 747 (2007)]. By monitoring the changes in the transient absorption of the donor-acceptor pair at 280 and 930 nm, both of which show non-exponential temporal decay, the experiment has provided a strong evidence that the initial electron transfer kinetics is modulated by the dynamics of protein backbone. In this work, we present a model where the electron transfer kinetics of the donor-acceptor pair is described along the reaction coordinate associated with the distance fluctuations in a protein backbone. The stochastic evolution of the reaction coordinate is described in terms of a non-Markovian generalized Langevin equation with a memory kernel and Gaussian colored noise, both of which are completely described in terms of the microscopics of the protein normal modes. This model provides excellent fits to the transient absorption signals at 280 and 930 nm associated with protein distance fluctuations and protein dynamics modulated electron transfer reaction, respectively. In contrast to previous models, the present work explains the microscopic origins of the non-exponential decay of the transient absorption curve at 280 nm in terms of multiple time scales of relaxation of the protein normal modes. Dynamic disorder in the reaction pathway due to protein conformational fluctuations which occur on time scales slower than or comparable to the electron transfer kinetics explains the microscopic origin of the non-exponential nature of the transient absorption decay at 930 nm. The theoretical estimates for the relative driving force for five different mutants are in close agreement with the experimental estimates obtained using electrochemical measurements.

  8. Protein dynamics modulated electron transfer kinetics in early stage photosynthesis.

    PubMed

    Kundu, Prasanta; Dua, Arti

    2013-01-28

    A recent experiment has probed the electron transfer kinetics in the early stage of photosynthesis in Rhodobacter sphaeroides for the reaction center of wild type and different mutants [Science 316, 747 (2007)]. By monitoring the changes in the transient absorption of the donor-acceptor pair at 280 and 930 nm, both of which show non-exponential temporal decay, the experiment has provided a strong evidence that the initial electron transfer kinetics is modulated by the dynamics of protein backbone. In this work, we present a model where the electron transfer kinetics of the donor-acceptor pair is described along the reaction coordinate associated with the distance fluctuations in a protein backbone. The stochastic evolution of the reaction coordinate is described in terms of a non-Markovian generalized Langevin equation with a memory kernel and Gaussian colored noise, both of which are completely described in terms of the microscopics of the protein normal modes. This model provides excellent fits to the transient absorption signals at 280 and 930 nm associated with protein distance fluctuations and protein dynamics modulated electron transfer reaction, respectively. In contrast to previous models, the present work explains the microscopic origins of the non-exponential decay of the transient absorption curve at 280 nm in terms of multiple time scales of relaxation of the protein normal modes. Dynamic disorder in the reaction pathway due to protein conformational fluctuations which occur on time scales slower than or comparable to the electron transfer kinetics explains the microscopic origin of the non-exponential nature of the transient absorption decay at 930 nm. The theoretical estimates for the relative driving force for five different mutants are in close agreement with the experimental estimates obtained using electrochemical measurements. PMID:23387626

  9. Proton-Coupled Electron Transfer in a Series of Ruthenium-Linked Tyrosines with Internal Bases: Evaluation of a Tunneling Model for Experimental Temperature-Dependent Kinetics.

    PubMed

    Markle, Todd F; Zhang, Ming-Tian; Santoni, Marie-Pierre; Johannissen, Linus O; Hammarström, Leif

    2016-09-01

    Photoinitiated proton-coupled electron transfer (PCET) kinetics has been investigated in a series of four modified tyrosines linked to a ruthenium photosensitizer in acetonitrile, with each tyrosine bearing an internal hydrogen bond to a covalently linked pyridine or benzimidazole base. After correcting for differences in driving force, it is found that the intrinsic PCET rate constant still varies by 2 orders of magnitude. The differences in rates, as well as the magnitude of the kinetic isotope effect (KIE = kH/kD), both generally correlate with DFT calculated proton donor-acceptor distances. An Arrhenius analysis of temperature dependent data shows that the difference in reactivity arises primarily from differences in activation energies. We use this kinetic data to evaluate a commonly employed theoretical model for proton tunneling which includes a harmonic distribution of proton donor-acceptor distances due to vibrational motions of the molecule. Applying this model to the experimental data yields the conclusion that donor-acceptor compression is more facile in the compounds with shorter PT distance; however, this is contrary to independent calculations for the same compounds. This discrepancy is likely because the assumption in the model of Morse-shaped proton potential energy surfaces is inappropriate for (strongly) hydrogen-bonded systems. These results question the general applicability of this model. The results also suggest that a correlation of rate vs proton tunneling distance for the series of compounds is complicated by a concomitant variation of other relevant parameters. PMID:27490689

  10. Transmembrane Photoredox in Model Protocellular Systems. Polycyclic Aromatic Hydrocarbons as Plausible Light-harvesting/Electron Transfer Catalysts

    NASA Astrophysics Data System (ADS)

    Cape, J. L.; Monnard, P.-A.; Ziock, H.-J.; Boncella, J. M.

    2010-04-01

    Mechanistic studies of transmembrane photoredox in a model protocell system indicate a plausible role for membrane solublized PAH species as primitive ‘photosynthetic' energy transduction catalysts.

  11. Evidence for the purely electronic character of primary electron transfer in purple bacteria Rh. Sphaeroides

    NASA Astrophysics Data System (ADS)

    Glebov, I. O.; Poddubnyy, V. V.; Eremin, V. V.

    2015-11-01

    A quantum-chemical calculation of the excited electronic states of a Rh. Sphaeroides reaction centre was performed. We discovered a new excited electronic state which can participate in electron transfer (ET). The energy gradient calculations showed that photoexcitation activates only high-frequency vibrational modes. This contradicts the widely accepted picture of ET resulting from vibrational wave packet motion. An alternative model is suggested where ET has a purely dissipative character and occurs only due to pigment--protein interaction. With this model, we demonstrate that oscillations in the femtosecond spectra can be caused by the new electronic state and non-Markovian character of dissipative dynamics.

  12. Electronic energy transfer and electron transfer in flexible bichromophoric molecules studied in a supersonic jet

    NASA Astrophysics Data System (ADS)

    Wang, Xin

    1999-08-01

    The objective of this dissertation is to use laser spectroscopy in supersonic jets to investigate the dynamics and mechanism of Electronic Energy Transfer (EET) and Photon-induced Electron Transfer (PET) in bichromophoric molecules with flexible bridges. Molecular geometry computations using molecular mechanism methods have been applied to the interpretations of experimental results. In the investigations of a series of molecules: (1- naphthalene) -(CH 2)n- (9-anthracene) (n = 1, 3 and 6), EET rate from the naphthalene moiety to the anthracene moiety has been found to be much faster than the naphthalene fluorescence lifetime. The importance of exchange mechanism has been demonstrated in the n = 3 and 6 molecules, which is explained by the formation of face-to-face inter- chromophore configurations. In the investigation of a methyldisilane-linked bichromophoric molecule containing anthracene and dimethyl-aniline, it has been found that this molecule can be directly excited to the Charge Transfer state without being excited to the Locally Excited state first. The unusually low-lying Charge Transfer state indicates a significant contribution to the electron transfer mechanism from the through Si-Si bond coupling. The silicon molecular wire is therefore shown to be a better electron conductor than the carbon molecular wire.

  13. Photoinitiated electron transfer in multi-chromophoric species: Synthetic tetrads and pentads. Technical progress report, 1987--1990

    SciTech Connect

    Not Available

    1990-02-14

    This research project involves the design, synthesis and study of the molecules which mimic many of the important aspects of photosynthetic electron and energy transfer. Specifically, the molecules are designed to mimic the following aspects of natural photosynthetic multistep electron transfer: electron donation from a tetrapyrrole excited singlet state, electron transfer between tetrapyrroles, electron transfer from tetrapyrroles to quinones, and electron transfer between quinones with different redox properties. In addition, they model carotenoid antenna function in photosynthesis (singlet-singlet energy transfer from carotenoid polyenes to chlorophyll) and carotenoid photoprotection from singlet oxygen damage (triplet-triplet energy transfer from chlorophyll to carotenoids).

  14. Electron Transfer Interactome of Cytochrome c

    PubMed Central

    Volkov, Alexander N.; van Nuland, Nico A. J.

    2012-01-01

    Lying at the heart of many vital cellular processes such as photosynthesis and respiration, biological electron transfer (ET) is mediated by transient interactions among proteins that recognize multiple binding partners. Accurate description of the ET complexes – necessary for a comprehensive understanding of the cellular signaling and metabolism – is compounded by their short lifetimes and pronounced binding promiscuity. Here, we used a computational approach relying solely on the steric properties of the individual proteins to predict the ET properties of protein complexes constituting the functional interactome of the eukaryotic cytochrome c (Cc). Cc is a small, soluble, highly-conserved electron carrier protein that coordinates the electron flow among different redox partners. In eukaryotes, Cc is a key component of the mitochondrial respiratory chain, where it shuttles electrons between its reductase and oxidase, and an essential electron donor or acceptor in a number of other redox systems. Starting from the structures of individual proteins, we performed extensive conformational sampling of the ET-competent binding geometries, which allowed mapping out functional epitopes in the Cc complexes, estimating the upper limit of the ET rate in a given system, assessing ET properties of different binding stoichiometries, and gauging the effect of domain mobility on the intermolecular ET. The resulting picture of the Cc interactome 1) reveals that most ET-competent binding geometries are located in electrostatically favorable regions, 2) indicates that the ET can take place from more than one protein-protein orientation, and 3) suggests that protein dynamics within redox complexes, and not the electron tunneling event itself, is the rate-limiting step in the intermolecular ET. Further, we show that the functional epitope size correlates with the extent of dynamics in the Cc complexes and thus can be used as a diagnostic tool for protein mobility. PMID:23236271

  15. Reorganization energy of electron transfer processes in ionic fluids: A molecular Debye-Hueckel approach

    SciTech Connect

    Xiao Tiejun; Song Xueyu

    2013-03-21

    The reorganization energy of electron transfer processes in ionic fluids is studied under the linear response approximation using a molecule Debye-Hueckel theory. Reorganization energies of some model reactants of electron transfer reactions in molten salts are obtained from molecular simulations and a molecule Debye-Hueckel approach. Good agreements between simulation results and the results from our theoretical calculations using the same model Hamiltonian are found. Applications of our theory to electron transfer reactions in room temperature ionic liquids further demonstrate that our theoretical approach presents a reliable and accurate methodology for the estimation of reorganization energies of electron transfer reactions in ionic fluids.

  16. Rotational And Rovibrational Energy Transfer In Electron Collisions With Molecules

    NASA Technical Reports Server (NTRS)

    Thuemmel, Helmar T.; Langhoff, Stephen R. (Technical Monitor)

    1995-01-01

    Air flows around a hypervelocity reentry vehicle undergo dissociation, rovibrational excitation and ionization. More specifically the air, initially 80% N2 and 20% O2, in the shock layer consists of species such as N, O, N2, O2, NO, N+, O+, N+, O+, NO+ and 2 free electrons. It was pointed out in multi temperature models'' that the temperature of the rotational energy modes and the gas-kinetic translational temperature are quickly equilibrated by a few collisions and rise rapidly to high temperatures as 50000K before falling off to equilibrium value of 10000K. Contrary, the electronic and vibrational temperatures state energy distributions remain low (less than 15000K) because of the slow equilibration. Electron vibrational energy transfer is thought to play a crucial role in such a ionizing flow regime since chemical reaction rates and dissociation depend strongly on the vibrational temperatures. Modeling of these flowfields in principle require the rovibrational excitation and de-excitation cross section data for average electron energies from threshold up to several eV (leV=11605.4 K). In this lecture we focus on theoretical description of rotational effects i.e. energy transfer of electrons to molecules such that the molecular rotational (vojo goes to voj) or vibrational and rotational (v(sub 0)j(sub 0) goes to vj) states are changed. Excitation and de-excitation of electronic states was discussed in a previous talk at this conference.

  17. Transfer Printed Crystalline Nanomembrane for Versatile Electronic Applications

    NASA Astrophysics Data System (ADS)

    Seo, Jung-Hun

    Flexible electronics have traditionally been addressed low-frequency applications, since the materials for the traditional flexible electronics, such as polymer and non-crystalline inorganic semiconductors, have poor electronic properties. Fast flexible electronics that operate at radio frequencies (RF), particularly at microwave frequencies, could lead to a number of novel RF applications where rigid chip-based solid-state electronics cannot easily fulfill. Single-crystal semiconductor nanomembranes (NM) that can be released from a number of wafer sources are mechanically very flexible yet exhibit outstanding electronic properties that are equivalent to their bulky counterparts. These thin flexible single-crystal materials can furthermore be placed, via transfer printing techniques, to nearly any substrate, including flexible polymers, thus creating the opportunity to realize RF flexible electronics. In this thesis, various RF transistors made of semiconductor NMs on plastic substrates will be discussed. In addition, as a photonic application, the demonstration of large-area Si NM surface normal ultra-compact photonic crystal reflectors fabricated using the laser interference lithography technique (LIL) will be discussed. Particularly, the mechanism of LIL and NM transfer without using an adhesive layer will be introduced and their optical performance will be addressed. Lastly, the realization of selective substitutional boron doping, using heavily doped Si NM as a doping source, will be discussed. A detailed mechanism using computational modeling and experimental analyses will be provided. The fabrication of high voltage diamond p-i diodes and their performance will be discussed.

  18. Insights into Proton-Coupled Electron Transfer from Computation

    NASA Astrophysics Data System (ADS)

    Provorse, Makenzie R.

    Proton-coupled electron transfer (PCET) is utilized throughout Nature to facilitate essential biological processes, such as photosynthesis, cellular respiration, and DNA replication and repair. The general approach to studying PCET processes is based on a two-dimensional More O'Ferrall-Jencks diagram in which electron transfer (ET) and proton transfer (PT) occur in a sequential or concerted fashion. Experimentally, it is difficult to discern the contributing factors of concerted PCET mechanisms. Several theoretical approaches have arisen to qualitatively and quantitatively investigate these reactions. Here, we present a multistate density functional theory (MSDFT) method to efficiently and accurately model PCET mechanisms. The MSDFT method is validated against experimental and computational data previously reported on an isoelectronic series of small molecule self-exchange hydrogen atom transfer reactions and a model complex specifically designed to study long-range ET through a hydrogen-bonded salt-bridge interface. Further application of this method to the hydrogen atom abstraction of ascorbate by a nitroxyl radical demonstrates the sensitivity of the thermodynamic and kinetic properties to solvent effects. In particular, the origin of the unusual kinetic isotope effect is investigated. Lastly, the MSDFT is employed in a combined quantum mechanical/molecular mechanical (QM/MM) approach to explicitly model PCET in condensed phases.

  19. DNA Damage Induced by Low-Energy Electrons: Electron Transfer and Diffraction

    NASA Astrophysics Data System (ADS)

    Zheng, Yi; Wagner, J. Richard; Sanche, Léon

    2006-05-01

    Thin films of the short single strand of DNA, GCAT, in which guanine (G) or adenine (A) have been removed, were bombarded under vacuum by 4 to 15 eV electrons. The fragments corresponding to base release and strand breaks (SB) were analyzed by high performance liquid chromatography and their yields compared with those obtained from unmodified GCAT. From such a comparison, it is shown that, using GCAT as a model system, (1) most SB result from electron capture by DNA bases followed by electron transfer to the phosphate group and (2) the initial capture probability depends on the coherence of the electron wave within the tetramer.

  20. DNA Damage Induced by Low-Energy Electrons: Electron Transfer and Diffraction

    SciTech Connect

    Zheng Yi; Wagner, J. Richard; Sanche, Leon

    2006-05-26

    Thin films of the short single strand of DNA, GCAT, in which guanine (G) or adenine (A) have been removed, were bombarded under vacuum by 4 to 15 eV electrons. The fragments corresponding to base release and strand breaks (SB) were analyzed by high performance liquid chromatography and their yields compared with those obtained from unmodified GCAT. From such a comparison, it is shown that, using GCAT as a model system (1) most SB result from electron capture by DNA bases followed by electron transfer to the phosphate group and (2) the initial capture probability depends on the coherence of the electron wave within the tetramer.

  1. Interchain electron-electron scattering in a one-dimensional charge-transfer conductor

    SciTech Connect

    Lyo, S.K.

    1982-02-15

    The contribution of donor-acceptor interchain electron-electron scattering to the dc resistivity is calculated for one-dimensional charge-transfer metallic conductors. The interchain electron-electron interaction is due to Coulombic or phonon-exchange interactions. The resistivity arises from U processes when the relative signs of the slopes of the donor and acceptor bands are the same and from N processes when they are opposite. The results predict an upper bound for the strength of interchain and possibly on-site Coulomb interactions. Application of the model to TTF-TCNQ (tetrathiafulvalenium tetracyanoquinodimethanide) is discussed.

  2. Quantum tunneling resonant electron transfer process in Lorentzian plasmas

    SciTech Connect

    Hong, Woo-Pyo; Jung, Young-Dae

    2014-08-15

    The quantum tunneling resonant electron transfer process between a positive ion and a neutral atom collision is investigated in nonthermal generalized Lorentzian plasmas. The result shows that the nonthermal effect enhances the resonant electron transfer cross section in Lorentzian plasmas. It is found that the nonthermal effect on the classical resonant electron transfer cross section is more significant than that on the quantum tunneling resonant charge transfer cross section. It is shown that the nonthermal effect on the resonant electron transfer cross section decreases with an increase of the Debye length. In addition, the nonthermal effect on the quantum tunneling resonant electron transfer cross section decreases with increasing collision energy. The variation of nonthermal and plasma shielding effects on the quantum tunneling resonant electron transfer process is also discussed.

  3. GPU-accelerated computation of electron transfer.

    PubMed

    Höfinger, Siegfried; Acocella, Angela; Pop, Sergiu C; Narumi, Tetsu; Yasuoka, Kenji; Beu, Titus; Zerbetto, Francesco

    2012-11-01

    Electron transfer is a fundamental process that can be studied with the help of computer simulation. The underlying quantum mechanical description renders the problem a computationally intensive application. In this study, we probe the graphics processing unit (GPU) for suitability to this type of problem. Time-critical components are identified via profiling of an existing implementation and several different variants are tested involving the GPU at increasing levels of abstraction. A publicly available library supporting basic linear algebra operations on the GPU turns out to accelerate the computation approximately 50-fold with minor dependence on actual problem size. The performance gain does not compromise numerical accuracy and is of significant value for practical purposes. PMID:22847673

  4. Electron transfer reactions in microporous solids

    SciTech Connect

    Mallouk, T.E.

    1992-05-01

    We have studied electron transfer quenching of the excited state of Ru(bpy){sub 3}{sup 2+} in aqueous suspensions of zeolites Y, L, and mordenite. The internal pore network of the zeolite is ion-exchanged with methylviologen cations, which quench the excited state of the surface-bound sensitizer. A detailed study of the quenching and charge recombination kinetics, using time-resolved luminescence quenching and transient diffuse reflectance spectroscopies, shows to remarkable effects: first, the excited state quenching is entirely dynamic is large-pore zeolites (L and Y), even when they are prepared as apparently dry'' powders (which still contain significant amounts of internally sited water). Second, a lower limit for the diffusion coefficient of the MV{sup 2+} ion in these zeolites, determined by this technique, is 10{sup {minus}7} cm{sup 2}sec, i.e., only about one order of magnitude slower than a typical ion in liquid water, and 2--3 orders of magnitude faster than charge transfer diffusion of cations in polyelectrolyte films or membranes such as Nafion. Surface sensitization of internally platinized layered oxide semiconductors such as K{sub 4-x}H{sub x}Nb{sub 6}O{sub 17}{center dot}nH{sub 2}O (x {approx} 2.5) yields photocatalysts for the production of H{sub 2} and I{sub 3{minus}} in aqueous iodide solutions. Layered alkali niobates and titanates form a class of zeolitic wide-bandap semiconductors, and are the first examples of photocatalysts that evolve hydrogen from an electrochemically reversible (i.e., non-sacrificial) electron donor with visible light excitation.

  5. Electron transfer reactions in microporous solids

    NASA Astrophysics Data System (ADS)

    Mallouk, T. E.

    1992-05-01

    We have studied electron transfer quenching of the excited state of Ru3(2+) in aqueous suspensions of zeolites Y, L, and mordenite. The internal pore network of the zeolite is ion-exchanged with methylviologen cations, which quench the excited state of the surface-bound sensitizer. A detailed study of the quenching and charge recombination kinetics, using time-resolved luminescence quenching and transient diffuse reflectance spectroscopies, shows two remarkable effects: first, the excited state quenching is entirely dynamic is large-pore zeolites (L and Y), even when they are prepared as apparently 'dry' powders (which still contain significant amounts of internally sited water). Second, a lower limit for the diffusion coefficient of the MV(2+) ion in these zeolites, determined by this technique, is 10(exp -7) sq cm sec, i.e., only about one order of magnitude slower than a typical ion in liquid water, and 2 to 3 orders of magnitude faster than charge transfer diffusion of cations in polyelectrolyte films or membranes such as Nafion. Surface sensitization of internally platinized layered oxide semiconductors such as K(4-x)H(x)Nb6O17 - nH2O(x approx. = 2.5) yields photocatalysts for the production of H2 and I3(-) in aqueous iodide solutions. Layered alkali niobates and titanates form a class of zeolitic wide-bandap semiconductors, and are the first examples of photocatalysts that evolve hydrogen from an electrochemically reversible (i.e., non-sacrificial) electron donor with visible light excitation.

  6. Radiative transfer models

    NASA Technical Reports Server (NTRS)

    Horwitz, James L.

    1992-01-01

    The purpose of this work was to assist with the development of analytical techniques for the interpretation of infrared observations. We have done the following: (1) helped to develop models for continuum absorption calculations for water vapor in the far infrared spectral region; (2) worked on models for pressure-induced absorption for O2 and N2 and their comparison with available observations; and (3) developed preliminary studies of non-local thermal equilibrium effects in the upper stratosphere and mesosphere for infrared gases. These new techniques were employed for analysis of balloon-borne far infrared data by a group at the Harvard-Smithsonian Center for Astrophysics. The empirical continuum absorption model for water vapor in the far infrared spectral region and the pressure-induced N2 absorption model were found to give satisfactory results in the retrieval of the mixing ratios of a number of stratospheric trace constituents from balloon-borne far infrared observations.

  7. Visualization of electron transfer interactions of membrane proteins

    NASA Astrophysics Data System (ADS)

    Kawato, Suguru

    1991-08-01

    To visualize electron transfer interactions of proteins in the cellular nieinbrane, we have developed a polarized laser flash-induced anisotropy decay imaging. The time-resolved anisotropy is particularly sensitive to protein-protein interactions. This technique has been successfully applied to examine formation and dissociation of electron transfer complex in adrenal cortex and liver. Electron transfer plays a significant role for steroid hormone synthesis from cholesterol in adrenalcortex and for drug metabolism in liver such as detoxification of chemical compounds. Several redox partners perticipate in dynamic electron transfer interactions. The terminal enzyme cytochrome P-450 receives electrons to activate molecular oxygen, resulting in hydroxylation of various substrates.

  8. Geometric Electron Models.

    ERIC Educational Resources Information Center

    Nika, G. Gerald; Parameswaran, R.

    1997-01-01

    Describes a visual approach for explaining the filling of electrons in the shells, subshells, and orbitals of the chemical elements. Enables students to apply the principles of atomic electron configuration while using manipulatives to model the building up of electron configurations as the atomic numbers of elements increase on the periodic…

  9. Electronic Education System Model.

    ERIC Educational Resources Information Center

    Cloete, Elsabe

    2001-01-01

    Discusses electronic learning efforts and problems in implementing computers in schools. Defines and describes an electronic educational system model that was developed to assist the designers of different electronic learning settings to plan and successfully implement a specific learning situation, with the focus on the individual requirements of…

  10. Hardwiring microbes via direct interspecies electron transfer: mechanisms and applications.

    PubMed

    Cheng, Qiwen; Call, Douglas F

    2016-08-10

    Multicellular microbial communities are important catalysts in engineered systems designed to treat wastewater, remediate contaminated sediments, and produce energy from biomass. Understanding the interspecies interactions within them is therefore essential to design effective processes. The flow of electrons within these communities is especially important in the determination of reaction possibilities (thermodynamics) and rates (kinetics). Conventional models of electron transfer incorporate the diffusion of metabolites generated by one organism and consumed by a second, frequently referred to as mediated interspecies electron transfer (MIET). Evidence has emerged in the last decade that another method, called direct interspecies electron transfer (DIET), may occur between organisms or in conjunction with electrically conductive materials. Recent research has suggested that DIET can be stimulated in engineered systems to improve desired treatment goals and energy recovery in systems such as anaerobic digesters and microbial electrochemical technologies. In this review, we summarize the latest understanding of DIET mechanisms, the associated microorganisms, and the underlying thermodynamics. We also critically examine approaches to stimulate DIET in engineered systems and assess their effectiveness. We find that in most cases attempts to promote DIET in mixed culture systems do not yield the improvements expected based on defined culture studies. Uncertainties of other processes that may be co-occurring in real systems, such as contaminant sorption and biofilm promotion, need to be further investigated. We conclude by identifying areas of future research related to DIET and its application in biological treatment processes. PMID:27349520

  11. Vibrationally Assisted Electron Transfer Mechanism of Olfaction: Myth or Reality?

    PubMed Central

    Solov’yov, Ilia A.; Chang, Po-Yao; Schulten, Klaus

    2012-01-01

    Smell is a vital sense for animals. The mainstream explanation of smell is based on recognition of the odorant molecules through characteristics of their surface, e.g., shape, but certain experiments suggest that such recognition is complemented by recognition of vibrational modes. According to this suggestion an olfactory receptor is activated by electron transfer assisted through odorant vibrational excitation. The hundreds to thousands of different olfactory receptors in an animal recognize odorants over a discriminant landscape with surface properties and vibrational frequencies as the two major dimensions. In the present paper we introduce the vibrationally assisted mechanism of olfaction and demonstrate for several odorants that, indeed, a strong enhancement of an electron tunneling rate due to odorant vibrations can arise. We discuss in this regard the influence of odorant deuteration and explain, thereby, recent experiments performed on Drosophila melanogaster. Our demonstration is based on known physical properties of biological electron transfer and on ab initio calculations on odorants carried out for the purpose of the present study. We identify a range of physical characteristics which olfactory receptors and odorants must obey for the vibrationally assisted electron transfer mechanism to function. We argue that the stated characteristics are feasible for realistic olfactory receptors, noting, though, that the receptor structure presently is still unknown, but can be studied through homology modeling. PMID:22899100

  12. 48 CFR 18.123 - Electronic funds transfer.

    Code of Federal Regulations, 2010 CFR

    2010-10-01

    ... 48 Federal Acquisition Regulations System 1 2010-10-01 2010-10-01 false Electronic funds transfer. 18.123 Section 18.123 Federal Acquisition Regulations System FEDERAL ACQUISITION REGULATION CONTRACTING METHODS AND CONTRACT TYPES EMERGENCY ACQUISITIONS Available Acquisition Flexibilities 18.123 Electronic funds transfer. Electronic funds...

  13. Sodium heat transfer system modeling

    NASA Astrophysics Data System (ADS)

    Baker, A. F.; Fewell, M. E.

    1983-11-01

    The sodium heat transfer system of the international energy agency (IEA) small solar power systems (SSPS) central receiver system (CRS), which includes the heliostat field, receiver, hot and cold storage vessels, and sodium/water steam generator was modeled. The computer code SOLTES (simulator of large thermal energy systems), was used to model this system. The results from SOLTES are compared to measured data.

  14. Neutral histidine and photoinduced electron transfer in DNA photolyases.

    PubMed

    Domratcheva, Tatiana

    2011-11-16

    The two major UV-induced DNA lesions, the cyclobutane pyrimidine dimers (CPD) and (6-4) pyrimidine-pyrimidone photoproducts, can be repaired by the light-activated enzymes CPD and (6-4) photolyases, respectively. It is a long-standing question how the two classes of photolyases with alike molecular structure are capable of reversing the two chemically different DNA photoproducts. In both photolyases the repair reaction is initiated by photoinduced electron transfer from the hydroquinone-anion part of the flavin adenine dinucleotide (FADH(-)) cofactor to the photoproduct. Here, the state-of-the-art XMCQDPT2-CASSCF approach was employed to compute the excitation spectra of the respective active site models. It is found that protonation of His365 in the presence of the hydroquinone-anion electron donor causes spontaneous, as opposed to photoinduced, coupled proton and electron transfer to the (6-4) photoproduct. The resulting neutralized biradical, containing the neutral semiquinone and the N3'-protonated (6-4) photoproduct neutral radical, corresponds to the lowest energy electronic ground-state minimum. The high electron affinity of the N3'-protonated (6-4) photoproduct underlines this finding. Thus, it is anticipated that the (6-4) photoproduct repair is assisted by His365 in its neutral form, which is in contrast to the repair mechanisms proposed in the literature. The repair via hydroxyl group transfer assisted by neutral His365 is considered. The repair involves the 5'base radical anion of the (6-4) photoproduct which in terms of electronic structure is similar to the CPD radical anion. A unified model of the CPD and (6-4) photoproduct repair is proposed. PMID:21970417

  15. Solvent reorganizational red-edge effect in intramolecular electron transfer.

    PubMed Central

    Demchenko, A P; Sytnik, A I

    1991-01-01

    Polar solvents are characterized by statistical distributions of solute-solvent interaction energies that result in inhomogeneous broadening of the solute electronic spectra. This allows photoselection of the high interaction energy part of the distribution by excitation at the red (long-wavelength) edge of the absorption bands. We observe that intramolecular electron transfer in the bianthryl molecule from the locally excited (LE) to the charge-transfer (CT) state, which requires solvent relaxation and does not occur in vitrified polar solutions, is dramatically facilitated in low-temperature propylene glycol glass by the red-edge excitation. This allows one to obtain spectroscopically the pure CT form and observe its dependence upon the relaxational properties of the solvent. A qualitative potential model of this effect is presented. PMID:11607224

  16. Ultrafast Spectroscopic Signatures of Coherent Electron-Transfer Mechanisms in a Transition Metal Complex.

    PubMed

    Guo, Zhenkun; Giokas, Paul G; Cheshire, Thomas P; Williams, Olivia F; Dirkes, David J; You, Wei; Moran, Andrew M

    2016-07-28

    The prevalence of ultrafast electron-transfer processes in light-harvesting materials has motivated a deeper understanding of coherent reaction mechanisms. Kinetic models based on the traditional (equilibrium) form of Fermi's Golden Rule are commonly employed to understand photoinduced electron-transfer dynamics. These models fail in two ways when the electron-transfer process is fast compared to solvation dynamics and vibrational dephasing. First, electron-transfer dynamics may be accelerated if the photoexcited wavepacket traverses the point of degeneracy between donor and acceptor states in the solvent coordinate. Second, traditional kinetic models fail to describe electron-transfer transitions that yield products which undergo coherent nuclear motions. We address the second point in this work. Transient absorption spectroscopy and a numerical model are used to investigate coherent back-electron-transfer mechanisms in a transition metal complex composed of titanium and catechol, [Ti(cat)3](2-). The transient absorption experiments reveal coherent wavepacket motions initiated by the back-electron-transfer process. Model calculations suggest that the vibrationally coherent product states may originate in either vibrational populations or coherences of the reactant. That is, vibrational coherence may be produced even if the reactant does not undergo coherent nuclear motions. The analysis raises a question of broader significance: can a vibrational population-to-coherence transition (i.e., a nonsecular transition) accelerate electron-transfer reactions even when the rate is slower than vibrational dephasing? PMID:27362388

  17. A mechano-chemiosmotic model for the coupling of electron and proton transfer to ATP synthesis in energy-transforming membranes: a personal perspective.

    PubMed

    Kasumov, Eldar A; Kasumov, Ruslan E; Kasumova, Irina V

    2015-01-01

    ATP is synthesized using ATP synthase by utilizing energy either from the oxidation of organic compounds, or from light, via redox reactions (oxidative- or photo phosphorylation), in energy-transforming membranes of mitochondria, chloroplasts, and bacteria. ATP synthase undergoes several changes during its functioning. The generally accepted model for ATP synthesis is the well-known rotatory model (see e.g., Junge et al., Nature 459:364-370, 2009; Junge and Müller, Science 333:704-705, 2011). Here, we present an alternative modified model for the coupling of electron and proton transfer to ATP synthesis, which was initially developed by Albert Lester Lehninger (1917-1986). Details of the molecular mechanism of ATP synthesis are described here that involves cyclic low-amplitude shrinkage and swelling of mitochondria. A comparison of the well-known current model and the mechano-chemiosmotic model is also presented. Based on structural, and other data, we suggest that ATP synthase is a Ca(2+)/H(+)-K(+) Cl(-)-pump-pore-enzyme complex, in which γ-subunit rotates 360° in steps of 30°, and 90° due to the binding of phosphate ions to positively charged amino acid residues in the N-terminal γ-subunit, while in the electric field. The coiled coil b 2-subunits are suggested to act as ropes that are shortened by binding of phosphate ions to positively charged lysines or arginines; this process is suggested to pull the α 3 β 3-hexamer to the membrane during the energization process. ATP is then synthesized during the reverse rotation of the γ-subunit by destabilizing the phosphated N-terminal γ-subunit and b 2-subunits under the influence of Ca(2+) ions, which are pumped over from storage-intermembrane space into the matrix, during swelling of intermembrane space. In the process of ATP synthesis, energy is first, predominantly, used in the delivery of phosphate ions and protons to the α 3 β 3-hexamer against the energy barrier with the help of C-terminal alpha

  18. Four-electron model for singlet and triplet excitation energy transfers with inclusion of coherence memory, inelastic tunneling and nuclear quantum effects

    NASA Astrophysics Data System (ADS)

    Suzuki, Yosuke; Ebina, Kuniyoshi; Tanaka, Shigenori

    2016-08-01

    A computational scheme to describe the coherent dynamics of excitation energy transfer (EET) in molecular systems is proposed on the basis of generalized master equations with memory kernels. This formalism takes into account those physical effects in electron-bath coupling system such as the spin symmetry of excitons, the inelastic electron tunneling and the quantum features of nuclear motions, thus providing a theoretical framework to perform an ab initio description of EET through molecular simulations for evaluating the spectral density and the temporal correlation function of electronic coupling. Some test calculations have then been carried out to investigate the dependence of exciton population dynamics on coherence memory, inelastic tunneling correlation time, magnitude of electronic coupling, quantum correction to temporal correlation function, reorganization energy and energy gap.

  19. Participation of Electron Transfer Process in Rate-Limiting Step of Aromatic Hydroxylation Reactions by Compound I Models of Heme Enzymes.

    PubMed

    Asaka, Maaya; Fujii, Hiroshi

    2016-07-01

    Hydroxylation reactions of aromatic rings are key reactions in various biological and chemical processes. In spite of their significance, no consensus mechanism has been established. Here we performed Marcus plot analysis for aromatic hydroxylation reactions with oxoiron(IV) porphyrin π-cation radical complexes (compound I). Although many recent studies support the mechanism involving direct electrophilic attack of compound I, the slopes of the Marcus plots indicate a significant contribution of an electron transfer process in the rate-limiting step, leading us to propose a new reaction mechanism in which the electron transfer process between an aromatic compound and compound I is in equilibrium in a solvent cage and coupled with the subsequent bond formation process. PMID:27327623

  20. (Mechanistic examination of organometallic electron transfer reactions: Annual report, 1989)

    SciTech Connect

    Not Available

    1989-01-01

    Our mechanistic examination of electron transfer reactions between organometallic complexes has required data from our stopped-flow infrared spectrophotometer that was constructed in the first year. Our research on organometallic electron transfer reaction mechanisms was recognized by an invitation to the Symposium on Organometallic Reaction Mechanisms at the National ACS meeting in Miami. We have obtained a reasonable understanding of the electron transfer reactions between metal cations and anions and between metal carbonyl anions and metal carbonyl dimers. In addition we have begun to obtain data on the outer sphere electron transfer between metal carbonyl anions and coordination complexes and on reactions involving cluster anions.

  1. [Mechanistic examination of organometallic electron transfer reactions: Annual report, 1989

    SciTech Connect

    Not Available

    1989-12-31

    Our mechanistic examination of electron transfer reactions between organometallic complexes has required data from our stopped-flow infrared spectrophotometer that was constructed in the first year. Our research on organometallic electron transfer reaction mechanisms was recognized by an invitation to the Symposium on Organometallic Reaction Mechanisms at the National ACS meeting in Miami. We have obtained a reasonable understanding of the electron transfer reactions between metal cations and anions and between metal carbonyl anions and metal carbonyl dimers. In addition we have begun to obtain data on the outer sphere electron transfer between metal carbonyl anions and coordination complexes and on reactions involving cluster anions.

  2. Two-state model based on electron-transfer reactivity changes to quantify the noncovalent interaction between Co(NH3)5Cl2+ and 18-crown-6 ether: the effect of second-sphere coordination on electron-transfer processes.

    PubMed

    Borreguero, M; Prado-Gotor, R

    2008-04-01

    The electron-transfer reaction between [Fe(CN)6]4- and [CoCl(NH3)5]2+ was studied in the presence of 18-crown-6 ether (18C6) in different reaction media constituted by water and acetonitrile as organic cosolvent at 298.2 K. The results corresponding to this reaction show a clear influence of 18C6 on the kinetics: a positive catalytic effect. Trends in the observed reactivity are explained by a change in the degree of association of one of the reactants (the cobalt complex) with the 18C6. This association is governed by an equilibrium constant that depends on the dielectric constant of the medium. The results show an increase of the rate constants for the electron-transfer process as the 18-crown-ether concentration increases and an increase of the binding free energy of the cobalt complex to the 18C6 when the electrostatic field of the medium becomes weaker. An analysis of the experimental data allows not only the reactivity changes associated with adducts formation processes for an electron-transfer reaction but also information on the binding free energy of the cobalt complex to the 18C6 to be obtained, which can be quantified by using a two-state model. We have found a good correlation between the energy of binding and the Kosower's Z-value. The influence of the 18C6 in the intramolecular electron transfer in the binuclear complex [Fe(CN)5pzCo(NH3)5] has been also investigated. PMID:18311956

  3. The electron transfer system of syntrophically grown Desulfovibrio vulgaris

    SciTech Connect

    Walker, C.B.; He, Z.; Yang, Z.K.; Ringbauer, Jr., J.A.; He, Q.; Zhou, J.; Voordouw, G.; Wall, J.D.; Arkin, A.P.; Hazen, T.C.; Stolyar, S.; Stahl, D.A.

    2009-05-01

    Interspecies hydrogen transfer between organisms producing and consuming hydrogen promotes the decomposition of organic matter in most anoxic environments. Although syntrophic couplings between hydrogen producers and consumers are a major feature of the carbon cycle, mechanisms for energy recovery at the extremely low free energies of reactions typical of these anaerobic communities have not been established. In this study, comparative transcriptional analysis of a model sulfate-reducing microbe, Desulfovibrio vulgaris Hildenborough, suggested the use of alternative electron transfer systems dependent upon growth modality. During syntrophic growth on lactate with a hydrogenotrophic methanogen, D. vulgaris up-regulated numerous genes involved in electron transfer and energy generation when compared with sulfate-limited monocultures. In particular, genes coding for the putative membrane-bound Coo hydrogenase, two periplasmic hydrogenases (Hyd and Hyn) and the well-characterized high-molecular weight cytochrome (Hmc) were among the most highly expressed and up-regulated. Additionally, a predicted operon coding for genes involved in lactate transport and oxidation exhibited up-regulation, further suggesting an alternative pathway for electrons derived from lactate oxidation during syntrophic growth. Mutations in a subset of genes coding for Coo, Hmc, Hyd and Hyn impaired or severely limited syntrophic growth but had little affect on growth via sulfate-respiration. These results demonstrate that syntrophic growth and sulfate-respiration use largely independent energy generation pathways and imply that understanding of microbial processes sustaining nutrient cycling must consider lifestyles not captured in pure culture.

  4. The electron transfer system of synthrophically grown desulfovibrio vulgaris

    SciTech Connect

    Walker, Christopher; He, Zhili; Yang, Zamin Koo; Ringbauer, Joseph; HE, Qiang; Zhou, Jizhong; Voordouw, Gerrit; Wall, Judy; Arkin, Adam; Hazen, Terry; Stolyar, Sergey; Stahl, David

    2009-01-01

    Interspecies hydrogen transfer between organisms producing and consuming hydrogen promotes the decomposition of organic matter in most anoxic environments. Although syntrophic coupling between hydrogen producers and consumers is a major feature of the carbon cycle, mechanisms for energy recovery at the extremely low free energies of reactions typical of these anaerobic communities have not been established. In this study, comparative transcriptional analysis of a model sulfate-reducing microbe, Desulfovibrio vulgaris Hildenborough, suggested the use of alternative electron transfer systems dependent on growth modality. During syntrophic growth on lactate with a hydrogenotrophic methanogen, numerous genes involved in electron transfer and energy generation were upregulated in D. vulgaris compared with their expression in sulfate-limited monocultures. In particular, genes coding for the putative membrane-bound Coo hydrogenase, two periplasmic hydrogenases (Hyd and Hyn), and the well-characterized high-molecular-weight cytochrome (Hmc) were among the most highly expressed and upregulated genes. Additionally, a predicted operon containing genes involved in lactate transport and oxidation exhibited upregulation, further suggesting an alternative pathway for electrons derived from lactate oxidation during syntrophic growth. Mutations in a subset of genes coding for Coo, Hmc, Hyd, and Hyn impaired or severely limited syntrophic growth but had little effect on growth via sulfate respiration. These results demonstrate that syntrophic growth and sulfate respiration use largely independent energy generation pathways and imply that to understand microbial processes that sustain nutrient cycling, lifestyles not captured in pure culture must be considered.

  5. The Electron Transfer System of Syntrophically Grown Desulfovibrio vulgaris

    SciTech Connect

    PBD; ENIGMA; GTL; VIMSS; Walker, Christopher B.; He, Zhili; Yang, Zamin K.; Ringbauer Jr., Joseph A.; He, Qiang; Zhou, Jizhong; Voordouw, Gerrit; Wall, Judy D.; Arkin, Adam P.; Hazen, Terry C.; Stolyar, Sergey; Stahl, David A.

    2009-06-22

    Interspecies hydrogen transfer between organisms producing and consuming hydrogen promotes the decomposition of organic matter in most anoxic environments. Although syntrophic couplings between hydrogen producers and consumers are a major feature of the carbon cycle, mechanisms for energy recovery at the extremely low free energies of reactions typical of these anaerobic communities have not been established. In this study, comparative transcriptional analysis of a model sulfate-reducing microbe, Desulfovibrio vulgaris Hildenborough, suggested the use of alternative electron transfer systems dependent upon growth modality. During syntrophic growth on lactate with a hydrogenotrophic methanogen, D. vulgaris up-regulated numerous genes involved in electron transfer and energy generation when compared with sulfate-limited monocultures. In particular, genes coding for the putative membrane-bound Coo hydrogenase, two periplasmic hydrogenases (Hyd and Hyn) and the well-characterized high-molecular weight cytochrome (Hmc) were among the most highly expressed and up-regulated. Additionally, a predicted operon coding for genes involved in lactate transport and oxidation exhibited up-regulation, further suggesting an alternative pathway for electrons derived from lactate oxidation during syntrophic growth. Mutations in a subset of genes coding for Coo, Hmc, Hyd and Hyn impaired or severely limited syntrophic growth but had little affect on growth via sulfate-respiration. These results demonstrate that syntrophic growth and sulfate-respiration use largely independent energy generation pathways and imply that understanding of microbial processes sustaining nutrient cycling must consider lifestyles not captured in pure culture.

  6. Electronic transfer of sensitive patient data.

    PubMed

    Detterbeck, A M W; Kaiser, J; Hirschfelder, U

    2015-01-01

    The purpose of this study was to develop decision-making aids and recommendations for dental practitioners regarding the utilization and sharing of sensitive digital patient data. In the current environment of growing digitization, healthcare professionals need detailed knowledge of secure data management to maximize confidentiality and minimize the risks involved in both archiving patient data and sharing it through electronic channels. Despite well-defined legal requirements, an all-inclusive technological solution does not currently exist. The need for a preliminary review and critical appraisal of common practices of data transfer prompted a search of the literature and the Web to identify viable methods of secure data exchange and to develop a flowchart. A strong focus was placed on the transmission of datasets both smaller than and larger than 10 MB, and on secure communication by smartphone. Although encryption of patient-related data should be routine, it is often difficult to implement. Pretty Good Privacy (PGP) and Secure/Multipurpose Internet Mail Extensions (S/MIME) are viable standards for secure e-mail encryption. Sharing of high-volume data should be accomplished with the help of file encryption. Careful handling of sensitive patient data is mandatory, and it is the end-user's responsibility to meet any requirements for encryption, preferably by using free, open-source (and hence transparent) software. PMID:25911828

  7. Electronic overfill protection for crude oil transfer

    SciTech Connect

    Kilgore, D.R.; Miles, D.C.

    1995-12-31

    There are many considerations involved in the transfer of crude oil, but the most catastrophic consequences may come as the result of a spill during loading or unloading. The safety and well-being of personnel in the vicinity is of the utmost concern. Should one be fortunate enough that an explosion or fire is not the results of a spill, the one must contend with the dilemma of containment. Preserving environmental integrity is a subject that is high on everyone`s list. The phrase ``reportable spill`` can send chills up and down anyone`s back. The repercussions continue: Ground water contamination; Soil remediation; Regulatory fines and penalties; Litigation. And this is all topped off by the ``black eye`` that the company receives with the perception of the public. For these reasons, and more, the carriers of crude oil are choosing self imposed compliances to reduce the frequency of spills. Electronic Overfill Protection has been modified to meet the specific needs and requirements of the crude oil industry. Here, the authors will examine how this type of system evolved, how it functions, and where it may lead.

  8. Gaussian fluctuations and linear response in an electron transfer protein

    PubMed Central

    Simonson, Thomas

    2002-01-01

    In response to charge separation or transfer, polar liquids respond in a simple linear fashion. A similar linear response for proteins might be expected from the central limit theorem and is postulated in widely used theories of protein electrostatics, including the Marcus electron transfer theory and dielectric continuum theories. Although these theories are supported by a variety of experimental data, the exact validity of a linear protein dielectric response has been difficult to determine. Molecular dynamics simulations are presented that establish a linear dielectric response of both protein and surrounding solvent over the course of a biologically relevant electron transfer reaction: oxido-reduction of yeast cytochrome c in solution. Using an umbrella-sampling free energy approach with long simulations, an accurate treatment of long-range electrostatics and both classical and quantum models of the heme, good agreement is obtained with experiment for the redox potential relative to a heme–octapeptide complex. We obtain a reorganization free energy that is only half that for heme–octapeptide and is reproduced with a dielectric continuum model where the heme vicinity has a dielectric constant of only 1.1. This value implies that the contribution of protein reorganization to the electron transfer free energy barrier is reduced almost to the theoretical limit (a dielectric of one), and that the fluctuations of the electrostatic potential on the heme have a simple harmonic form, in accord with Marcus theory, even though the fluctuations of many individual protein groups (especially at the protein surface) are anharmonic. PMID:12011418

  9. Integrated modeling, data transfers, and physical models

    NASA Astrophysics Data System (ADS)

    Brookshire, D. S.; Chermak, J. M.

    2003-04-01

    Difficulties in developing precise economic policy models for water reallocation and re-regulation in various regional and transboundary settings has been exacerbated not only by climate issues but also by institutional changes reflected in the promulgation of environmental laws, changing regional populations, and an increased focus on water quality standards. As complexity of the water issues have increased, model development at a micro-policy level is necessary to capture difficult institutional nuances and represent the differing national, regional and stakeholders' viewpoints. More often than not, adequate "local" or specific micro-data are not available in all settings for modeling and policy decisions. Economic policy analysis increasingly deals with this problem through data transfers (transferring results from one study area to another) and significant progress has been made in understanding the issue of the dimensionality of data transfers. This paper explores the conceptual and empirical dimensions of data transfers in the context of integrated modeling when the transfers are not only from the behavioral, but also from the hard sciences. We begin by exploring the domain of transfer issues associated with policy analyses that directly consider uncertainty in both the behavioral and physical science settings. We then, through a stylized, hybrid, economic-engineering model of water supply and demand in the Middle Rio Grand Valley of New Mexico (USA) analyze the impacts of; (1) the relative uncertainty of data transfers methods, (2) the uncertainty of climate data and, (3) the uncertainly of population growth. These efforts are motivated by the need to address the relative importance of more accurate data both from the physical sciences as well as from demography and economics for policy analyses. We evaluate the impacts by empirically addressing (within the Middle Rio Grand model): (1) How much does the surrounding uncertainty of the benefit transfer

  10. 75 FR 75897 - Electronic Funds Transfer of Depository Taxes

    Federal Register 2010, 2011, 2012, 2013, 2014

    2010-12-07

    ... published in the Federal Register (75 FR 51707) proposed amendments to the regulations (REG-153340-09) to... Internal Revenue Service 26 CFR Parts 1, 31, 40, and 301 RIN 1545-BJ13 Electronic Funds Transfer of... Electronic Funds Transfer (EFT). In response to the decision of the Financial Management Service...

  11. 76 FR 709 - Electronic Funds Transfer of Depository Taxes; Correction

    Federal Register 2010, 2011, 2012, 2013, 2014

    2011-01-06

    ... Federal Register on Tuesday, December 7, 2010 (75 FR 75897) providing guidance relating to Federal tax deposits (FTDs) by Electronic Funds Transfer (EFT). The temporary and final regulations provide rules under... Internal Revenue Service 26 CFR Parts 40 and 301 RIN 1545-BJ13 Electronic Funds Transfer of...

  12. 78 FR 66251 - Electronic Fund Transfers(Regulation E)

    Federal Register 2010, 2011, 2012, 2013, 2014

    2013-11-05

    ... PROTECTION 12 CFR Part 1005 RIN 3170-AA33 Electronic Fund Transfers (Regulation E) AGENCY: Bureau of Consumer... countries that qualify for an exception in subpart B of Regulation E, which implements the Electronic Fund....consumerfinance.gov/remittances-transfer-rule-amendment-to-regulation-e/ . SUPPLEMENTARY INFORMATION: The...

  13. Concerted proton-electron transfers: electrochemical and related approaches.

    PubMed

    Costentin, Cyrille; Robert, Marc; Savéant, Jean-Michel

    2010-07-20

    Proton-coupled electron transfers (PCETs) are omnipresent in natural and artificial chemical processes. Given the contemporary challenges associated with energy conversion, pollution abatement, and the development of high-performance sensors, a greater understanding of the mechanisms that underlie the practical efficiency of PCETs is a timely research topic. In contrast to hydrogen-atom transfers, proton and electron transfers involve different centers in PCET reactions. The reaction may go through an electron- or proton-transfer intermediate, giving rise to the electron-proton transfer (EPT) and the proton-electron transfer (PET) pathways. When the proton and electron transfers are concerted (the CPET pathway), the high-energy intermediates of the stepwise pathways are bypassed, although this thermodynamic benefit may have a kinetic cost. The primary task of kinetics-based mechanism analysis is therefore to distinguish the three pathways, quantifying the factors that govern the competition between them, which requires modeling of CPET reactivity. CPET models of varying sophistication have appeared, but the large number of parameters involved and the uncertainty of the quantum chemical calculations they may have to resort to make experimental confrontation and inspiration a necessary component of model testing and refinement. Electrochemical PCETs are worthy of particular attention, if only because most applications in which PCET mechanisms are operative involve collection or injection of electricity through electrodes. More fundamentally, changing the electrode potential is an easy and continuous means of varying the driving force of the reaction, whereas the current flowing through the electrode is a straightforward measure of its rate. Consequently, the current-potential response in nondestructive techniques (such as cyclic voltammetry) can be read as an activation-driving force relationship, provided the contribution of diffusion has been taken into account

  14. Single Molecule Electron Transfer Process of Ruthenium Complexes.

    SciTech Connect

    Hu, Dehong; Lu, H PETER.

    2006-03-01

    Transition metal complexes such as ruthenium complexes, having metal-to-ligand charge transfer states, are extensively used in solar energy conversion and electron transfer in biological systems and at interfaces. The dynamics of metal-to-ligand charge transfer and subsequent intermolecular, intramolecular, and interfacial electron transfer processes can be highly complex and inhomogeneous, especially when molecules are involved in interactions and perturbations from heterogeneous local environments and gated by conformation fluctuations. We have employed the single-molecule spectroscopy, a powerful approach for inhomogeneous systems to study the electron transfer dynamics of ruthenium complexes. We have applied a range of statistical analysis methods to reveal nonclassical photon emission behavior of the single ruthenium complex, i.e., photon antibunching, and photophysical ground-state recovering dynamics on a microsecond time scale. The use of photon antibunching to measure phosphorescence lifetimes and single-molecule electron transfer dynamics at room temperature is demonstrated.

  15. Electronic energy transfer: Localized operator partitioning of electronic energy in composite quantum systems

    NASA Astrophysics Data System (ADS)

    Khan, Yaser; Brumer, Paul

    2012-11-01

    A Hamiltonian based approach using spatially localized projection operators is introduced to give precise meaning to the chemically intuitive idea of the electronic energy on a quantum subsystem. This definition facilitates the study of electronic energy transfer in arbitrarily coupled quantum systems. In particular, the decomposition scheme can be applied to molecular components that are strongly interacting (with significant orbital overlap) as well as to isolated fragments. The result defines a consistent electronic energy at all internuclear distances, including the case of separated fragments, and reduces to the well-known Förster and Dexter results in their respective limits. Numerical calculations of coherent energy and charge transfer dynamics in simple model systems are presented and the effect of collisionally induced decoherence is examined.

  16. Theory of ultrafast photoinduced heterogeneous electron transfer: Decay of vibrational coherence into a finite electronic-vibrational quasicontinuum

    NASA Astrophysics Data System (ADS)

    Ramakrishna, S.; Willig, F.; May, V.

    2001-08-01

    Photo-induced electron transfer from a surface attached dye molecule to the band levels of a semiconductor is modeled via an electronic-vibronic quasicontinuum. The description enables one to obtain a fairly accurate expression for the decay of the excited molecular state, including initial vibronic coherences. The model accounts for (a) the effect of a finite band width, (b) variations in reorganization energy and electronic coupling, (c) various energetic positions for the injecting level, (d) different initial vibrational wave packets in the excited state, and (e) two vibrational modes participating in the electron transfer process. Most cases are studied numerically and can be reasonably well understood from the obtained decay expression.

  17. Probing active electron transfer branch in photosystem I reaction center.

    NASA Astrophysics Data System (ADS)

    Savikhin, Sergei; Dashdorj, Naranbaatar; Xu, Wu; Martinsson, Peter; Chitnis, Parag

    2003-03-01

    Complimentary point mutations were introduced at the primary electron acceptor sites in A and B branches of the photosystem I (PS I) reaction center (RC) from Synechocystis sp. PCC 6803 and their effect on the kinetics of the electron transfer process was studied by means of ultrafast pump-probe spectroscopy. The results indicate that in these species the electron transfer occurs primarily along the A-branch. Previous optical experiments on PS I complexes from Chlorella sorokiniana demonstrated that both branches of RC are equally active. That suggests that the directionality of electron transfer in PS I is species dependent.

  18. Photoinduced Electron Transfer Based Ion Sensing within an Optical Fiber

    PubMed Central

    Englich, Florian V.; Foo, Tze Cheung; Richardson, Andrew C.; Ebendorff-Heidepriem, Heike; Sumby, Christopher J.; Monro, Tanya M.

    2011-01-01

    We combine suspended-core microstructured optical fibers with the photoinduced electron transfer (PET) effect to demonstrate a new type of fluorescent optical fiber-dip sensing platform for small volume ion detection. A sensor design based on a simple model PET-fluoroionophore system and small core microstructured optical fiber capable of detecting sodium ions is demonstrated. The performance of the dip sensor operating in a high sodium concentration regime (925 ppm Na+) and for lower sodium concentration environments (18.4 ppm Na+) is explored and future approaches to improving the sensor’s signal stability, sensitivity and selectivity are discussed. PMID:22163712

  19. Composition, size distribution, optical properties, and radiative effects of laboratory-resuspended PM10 from geological dust of the Rome area, by electron microscopy and radiative transfer modelling

    NASA Astrophysics Data System (ADS)

    Pietrodangelo, A.; Salzano, R.; Bassani, C.; Pareti, S.; Perrino, C.

    2015-11-01

    In this work, new information has been gained on the laboratory-resuspended PM10 fraction from geological topsoil and outcropped rocks representative of the Rome area (Latium). Mineralogical composition, size distribution, optical properties and the surface radiative forcing efficiency (RFE) of dust types representing the compositional end members of this geological area have been addressed. A multi-disciplinary approach was used, based on chamber resuspension of raw materials and sampling of the PM10 fraction, to simulate field sampling at dust source, scanning electron microscopy/X-ray energy-dispersive microanalysis (SEM XEDS) of individual mineral particles, X-ray diffraction (XRD) analysis of bulk dust samples, building of number and volume size distribution (SD) from microanalysis data of mineral particles and fitting to a log-normal curve, and radiative transfer modelling (RTM) to retrieve optical properties and radiative effects of the compositional end-member dust samples. The mineralogical composition of Rome lithogenic PM10 varies between an end-member dominated by silicate minerals (from volcanics lithotypes), and one mostly composed of calcite (from travertine or limestones). Lithogenic PM10 with intermediate composition derives mainly from siliciclastic rocks or marlstones. Size and mineral species of PM10 particles of silicate-dominated dust types are tuned mainly by rock weathering and, to lesser extent, by debris formation or crystallization; chemical precipitation of CaCO3 plays a major role in calcite-dominated types. These differences are reflected in the diversity of volume distributions, either within dust types or mineral species. Differences are also observed between volume distributions of calcite from travertine (natural source; SD unimodal at 5 μm a.d.) and from road dust (anthropic source; SD bimodal at 3.8 and 1.8 μm a.d.). The volcanics and travertine dusts differently affect the single scattering albedo (SSA) and the asymmetry

  20. Harvesting singlet fission for solar energy conversion: one versus two-electron transfer electron transfer from the quantum superposition state

    NASA Astrophysics Data System (ADS)

    Chan, Wai-Lun; Tritsch, John; Zhu, Xiaoyang

    2013-03-01

    Singlet fission (SF) is being explored to increase the efficiency of organic photovoltaics. A key question is how to effectively extract multiple electron-hole pairs from multiple excitons with the presence of other competing channels such as electron transfer from the singlet state. Recent experiments on the pentacene and tetracene show that a quantum superposition of the singlet (S1) and multiexciton (ME) state is formed during SF. However, little is known about the kinetics of electron transfer from this quantum superposition. Here, we apply time-resolved photoemission spectroscopy to the tetracene/C60 interface to probe one and two electron transfer from S1 and ME states, respectively. Because of the relatively slow (7 ps) SF in tetracene, both one- and two-electron transfer are allowed. We show evidence for the formation of two distinct charge transfer states due to electron transfer from photo-excited tetracene to the lowest unoccupied molecular orbital (LUMO) and the LUMO+1 levels in C60. Kinetic analysis shows that 60% of the quantum superposition transfers one electron through the S1 state to C60 while 40% undergoes two-electron transfer through the ME state.

  1. Integrating proton coupled electron transfer (PCET) and excited states

    SciTech Connect

    Gagliardi, Christopher J.; Westlake, Brittany C.; Kent, Caleb A.; Paul, Jared J.; Papanikolas, John M.; Meyer, Thomas J.

    2010-11-01

    In many of the chemical steps in photosynthesis and artificial photosynthesis, proton coupled electron transfer (PCET) plays an essential role. An important issue is how excited state reactivity can be integrated with PCET to carry out solar fuel reactions such as water splitting into hydrogen and oxygen or water reduction of CO2 to methanol or hydrocarbons. The principles behind PCET and concerted electron–proton transfer (EPT) pathways are reasonably well understood. In Photosystem II antenna light absorption is followed by sensitization of chlorophyll P680 and electron transfer quenching to give P680+. The oxidized chlorophyll activates the oxygen evolving complex (OEC), a CaMn4 cluster, through an intervening tyrosine–histidine pair, YZ. EPT plays a major role in a series of four activation steps that ultimately result in loss of 4e-/4H+ from the OEC with oxygen evolution. The key elements in photosynthesis and artificial photosynthesis – light absorption, excited state energy and electron transfer, electron transfer activation of multiple-electron, multiple-proton catalysis – can also be assembled in dye sensitized photoelectrochemical synthesis cells (DS-PEC). In this approach, molecular or nanoscale assemblies are incorporated at separate electrodes for coupled, light driven oxidation and reduction. Separate excited state electron transfer followed by proton transfer can be combined in single semi-concerted steps (photo-EPT) by photolysis of organic charge transfer excited states with H-bonded bases or in metal-to-ligand charge transfer (MLCT) excited states in pre-associated assemblies with H-bonded electron transfer donors or acceptors. In these assemblies, photochemically induced electron and proton transfer occur in a single, semi-concerted event to give high-energy, redox active intermediates.

  2. Electron Transfer Reactions in Colloidal Quantum Dot-Ligand Complexes

    NASA Astrophysics Data System (ADS)

    Morris-Cohen, Adam Joshua

    This thesis describes a quantitative analysis of the chemical composition of colloidal II-VI quantum dot (QD)-ligand complexes and transient absorption experiments analyzing the rates of electron transfer reactions in these complexes functionalized with redox active ligands. Chemical analysis reveals that phosphonate impurities in the surfactants used to synthesize CdSe QDs are the dominant ligands on the surface of the QDs, and these phosphonate impurities cause size-dependent Cd-enrichment of the QD surface. A study of the adsorption equilibrium of solution-phase CdS quantum dots and acid-derivatized viologen ligands (V2+) reveals that the structure of the surfaces of the QDs depends on the concentration of the QDs. A new model based on the Langmuir isotherm that treats both the number of adsorbed ligands per QD and the number of available binding sites per QD as binomially-distributed quantities is described. Transient absorption spectroscopy of solution-phase mixtures of colloidal CdS QDs and V2+ indicates electron transfer occurs from the conduction band of the QD to the LUMO of V2+. The rate constant for photoinduced electron transfer (PET) is independent of the number of methylene groups in the alkyl chain on the acid-derivatized viologen. The insensitivity of the electron transfer rate constant to the length of the functional groups on the viologen suggests a van der Waals (vdW) pathway for PET, where the electron bypasses the alkylcarboxylate and tunnels through the orbitals of the QD and of the bipyridinium core. The rate of PET from colloidal CdSe quantum dots (QDs) to oxo-centered triruthenium clusters (Ru 3O) depends on the structure of the chemical headgroup by which the Ru3O clusters adsorb to the QDs. Complexes comprising QDs and Ru 3O clusters adsorbed through a pyridine-4-carboxylic acid ligand have a PET rate constant of (4.9 ± 0.9)×109 s -1 whereas complexes comprising QDs and Ru3O clusters adsorbed through a 4-mercaptopyridine ligand have an

  3. Electron Transfer Pathways in Cholesterol Synthesis.

    PubMed

    Porter, Todd D

    2015-10-01

    Cholesterol synthesis in the endoplasmic reticulum requires electron input at multiple steps and utilizes both NADH and NADPH as the electron source. Four enzymes catalyzing five steps in the pathway require electron input: squalene monooxygenase, lanosterol demethylase, sterol 4α-methyl oxidase, and sterol C5-desaturase. The electron-donor proteins for these enzymes include cytochrome P450 reductase and the cytochrome b5 pathway. Here I review the evidence for electron donor protein requirements with these enzymes, the evidence for additional electron donor pathways, and the effect of deletion of these redox enzymes on cholesterol and lipid metabolism. PMID:26344922

  4. Bridge-mediated hopping or superexchange electron-transfer processes in bis(triarylamine) systems

    NASA Astrophysics Data System (ADS)

    Lambert, Christoph; Nöll, Gilbert; Schelter, Jürgen

    2002-09-01

    Hopping and superexchange are generally considered to be alternative electron-transfer mechanisms in molecular systems. In this work we used mixed-valence radical cations as model systems for the investigation of electron-transfer pathways. We show that substituents attached to a conjugated bridge connecting two triarylamine redox centres have a marked influence on the near-infrared absorption spectra of the corresponding cations. Spectral analysis, followed by evaluation of the electron-transfer parameters using the Generalized Mulliken-Hush theory and simulation of the potential energy surfaces, indicate that hopping and superexchange are not alternatives, but are both present in the radical cation with a dimethoxybenzene bridge. We found that the type of electron-transfer mechanism depends on the bridge-reorganization energy as well as on the bridge-state energy. Because superexchange and hopping follow different distance laws, our findings have implications for the design of new molecular and polymeric electron-transfer materials.

  5. Near-infrared-induced electron transfer of an uranyl macrocyclic complex without energy transfer to dioxygen.

    PubMed

    Davis, Christina M; Ohkubo, Kei; Ho, I-Ting; Zhang, Zhan; Ishida, Masatoshi; Fang, Yuanyuan; Lynch, Vincent M; Kadish, Karl M; Sessler, Jonathan L; Fukuzumi, Shunichi

    2015-04-21

    Photoexcitation of dichloromethane solutions of an uranyl macrocyclic complex with cyclo[1]furan[1]pyridine[4]-pyrrole () at the near-infrared (NIR) band (1177 nm) in the presence of electron donors and acceptors resulted in NIR-induced electron transfer without producing singlet oxygen via energy transfer. PMID:25791126

  6. K-shell Analysis Reveals Distinct Functional Parts in an Electron Transfer Network and Its Implications for Extracellular Electron Transfer.

    PubMed

    Ding, Dewu; Li, Ling; Shu, Chuanjun; Sun, Xiao

    2016-01-01

    Shewanella oneidensis MR-1 is capable of extracellular electron transfer (EET) and hence has attracted considerable attention. The EET pathways mainly consist of c-type cytochromes, along with some other proteins involved in electron transfer processes. By whole genome study and protein interactions inquisition, we constructed a large-scale electron transfer network containing 2276 interactions among 454 electron transfer related proteins in S. oneidensis MR-1. Using the k-shell decomposition method, we identified and analyzed distinct parts of the electron transfer network. We found that there was a negative correlation between the k s (k-shell values) and the average DR_100 (disordered regions per 100 amino acids) in every shell, which suggested that disordered regions of proteins played an important role during the formation and extension of the electron transfer network. Furthermore, proteins in the top three shells of the network are mainly located in the cytoplasm and inner membrane; these proteins can be responsible for transfer of electrons into the quinone pool in a wide variety of environmental conditions. In most of the other shells, proteins are broadly located throughout the five cellular compartments (cytoplasm, inner membrane, periplasm, outer membrane, and extracellular), which ensures the important EET ability of S. oneidensis MR-1. Specifically, the fourth shell was responsible for EET and the c-type cytochromes in the remaining shells of the electron transfer network were involved in aiding EET. Taken together, these results show that there are distinct functional parts in the electron transfer network of S. oneidensis MR-1, and the EET processes could achieve high efficiency through cooperation through such an electron transfer network. PMID:27148219

  7. K-shell Analysis Reveals Distinct Functional Parts in an Electron Transfer Network and Its Implications for Extracellular Electron Transfer

    PubMed Central

    Ding, Dewu; Li, Ling; Shu, Chuanjun; Sun, Xiao

    2016-01-01

    Shewanella oneidensis MR-1 is capable of extracellular electron transfer (EET) and hence has attracted considerable attention. The EET pathways mainly consist of c-type cytochromes, along with some other proteins involved in electron transfer processes. By whole genome study and protein interactions inquisition, we constructed a large-scale electron transfer network containing 2276 interactions among 454 electron transfer related proteins in S. oneidensis MR-1. Using the k-shell decomposition method, we identified and analyzed distinct parts of the electron transfer network. We found that there was a negative correlation between the ks (k-shell values) and the average DR_100 (disordered regions per 100 amino acids) in every shell, which suggested that disordered regions of proteins played an important role during the formation and extension of the electron transfer network. Furthermore, proteins in the top three shells of the network are mainly located in the cytoplasm and inner membrane; these proteins can be responsible for transfer of electrons into the quinone pool in a wide variety of environmental conditions. In most of the other shells, proteins are broadly located throughout the five cellular compartments (cytoplasm, inner membrane, periplasm, outer membrane, and extracellular), which ensures the important EET ability of S. oneidensis MR-1. Specifically, the fourth shell was responsible for EET and the c-type cytochromes in the remaining shells of the electron transfer network were involved in aiding EET. Taken together, these results show that there are distinct functional parts in the electron transfer network of S. oneidensis MR-1, and the EET processes could achieve high efficiency through cooperation through such an electron transfer network. PMID:27148219

  8. A molecular shift register based on electron transfer

    NASA Technical Reports Server (NTRS)

    Hopfield, J. J.; Onuchic, Josenelson; Beratan, David N.

    1988-01-01

    An electronic shift-register memory at the molecular level is described. The memory elements are based on a chain of electron-transfer molecules and the information is shifted by photoinduced electron-transfer reactions. This device integrates designed electronic molecules onto a very large scale integrated (silicon microelectronic) substrate, providing an example of a 'molecular electronic device' that could actually be made. The design requirements for such a device and possible synthetic strategies are discussed. Devices along these lines should have lower energy usage and enhanced storage density.

  9. A Comparison of Electron-Transfer Dynamics inIonic Liquids and Neutral Solvents

    SciTech Connect

    Wishart J. F.; Lee, H.Y.; Issa, J.B.; Isied, S.S.; Castner, Jr., E.W.; Pan, Y.; Hussey, C.L.; Lee, K.S.

    2012-03-01

    The effect of ionic liquids on photoinduced electron-transfer reactions in a donor-bridge-acceptor system is examined for two ionic liquid solvents, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide and tributylmethylammonium bis(trifluoromethylsulfonyl)amide. The results are compared with those for the same system in methanol and acetonitrile solution. Electron-transfer rates were measured using time-resolved fluorescence quenching for the donor-bridge-acceptor system comprising a 1-N,1-N-dimethylbenzene-1,4-diamine donor, a proline bridge, and a coumarin 343 acceptor. The photoinduced electron-transfer processes are in the inverted regime (-{Delta}G > {lambda}) in all four solvents, with driving forces of -1.6 to -1.9 eV and estimated reorganization energies of about 1.0 eV. The observed electron-transfer kinetics have broadly distributed rates that are generally slower in the ionic liquids compared to the neutral solvents, which also have narrower rate distributions. To describe the broad distributions of electron-transfer kinetics, we use two different models: a distribution of exponential lifetimes and a discrete sum of exponential lifetimes. Analysis of the donor-acceptor electronic coupling shows that for ionic liquids this intramolecular electron-transfer reaction should be treated using a solvent-controlled electron-transfer model.

  10. Electron acceptor dependence of electron shuttle secretion and extracellular electron transfer by Shewanella oneidensis MR-1.

    PubMed

    Wu, Chao; Cheng, Yuan-Yuan; Li, Bing-Bing; Li, Wen-Wei; Li, Dao-Bo; Yu, Han-Qing

    2013-05-01

    Shewanella oneidensis MR-1 is an extensively studied dissimilatory metal-reducing bacterium with a great potential for bioremediation and electricity generation. It secretes flavins as electron shuttles which play an important role in extracellular electron transfer. However, the influence of various environmental factors on the secretion of flavins is largely unknown. Here, the effects of electron acceptors, including fumarate, ferrihydrite, Fe(III)-nitrilotriacetic acid (NTA), nitrate and trimethylamine oxide (TMAO), on the secretion of flavins were investigated. The level of riboflavin and riboflavin-5'-phosphate (FMN) secreted by S. oneidensis MR-1 varied considerably with different electron acceptors. While nitrate and ferrihydrite suppressed the secretion of flavins in relative to fumarate, Fe(III)-NTA and TMAO promoted such a secretion and greatly enhanced ferrihydrite reduction and electricity generation. This work clearly demonstrates that electron acceptors could considerably affect the secretion of flavins and consequent microbial EET. Such impacts of electron acceptors in the environment deserve more attention. PMID:23558182

  11. KOtBu: A Privileged Reagent for Electron Transfer Reactions?

    PubMed

    Barham, Joshua P; Coulthard, Graeme; Emery, Katie J; Doni, Eswararao; Cumine, Florimond; Nocera, Giuseppe; John, Matthew P; Berlouis, Leonard E A; McGuire, Thomas; Tuttle, Tell; Murphy, John A

    2016-06-15

    Many recent studies have used KOtBu in organic reactions that involve single electron transfer; in the literature, the electron transfer is proposed to occur either directly from the metal alkoxide or indirectly, following reaction of the alkoxide with a solvent or additive. These reaction classes include coupling reactions of halobenzenes and arenes, reductive cleavages of dithianes, and SRN1 reactions. Direct electron transfer would imply that alkali metal alkoxides are willing partners in these electron transfer reactions, but the literature reports provide little or no experimental evidence for this. This paper examines each of these classes of reaction in turn, and contests the roles proposed for KOtBu; instead, it provides new mechanistic information that in each case supports the in situ formation of organic electron donors. We go on to show that direct electron transfer from KOtBu can however occur in appropriate cases, where the electron acceptor has a reduction potential near the oxidation potential of KOtBu, and the example that we use is CBr4. In this case, computational results support electrochemical data in backing a direct electron transfer reaction. PMID:27183183

  12. Intramolecular Long-Distance Electron Transfer in Organic Molecules

    NASA Astrophysics Data System (ADS)

    Closs, Gerhard L.; Miller, John R.

    1988-04-01

    Intramolecular long-distance electron transfer (ET) has been actively studied in recent years in order to test existing theories in a quantitative way and to provide the necessary constants for predicting ET rates from simple structural parameters. Theoretical predictions of an ``inverted region,'' where increasing the driving force of the reaction will decrease its rate, have begun to be experimentally confirmed. A predicted nonlinear dependence of ET rates on the polarity of the solvent has also been confirmed. This work has implications for the design of efficient photochemical charge-separation devices. Other studies have been directed toward determining the distance dependence of ET reactions. Model studies on different series of compounds give similar distance dependences. When different stereochemical structures are compared, it becomes apparent that geometrical factors must be taken into account. Finally, the mechanism of coupling between donor and acceptor in weakly interacting systems has become of major importance. The theoretical and experimental evidence favors a model in which coupling is provided by the interaction with the orbitals of the intervening molecular fragments, although more experimental evidence is needed. Studies on intramolecular ET in organic model compounds have established that current theories give an adequate description of the process. The separation of electronic from nuclear coordinates is only a convenient approximation applied to many models, but in long-distance ET it works remarkably well. It is particularly gratifying to see Marcus' ideas finally confirmed after three decades of skepticism. By obtaining the numbers for quantitative correlations between rates and distances, these experiments have shown that saturated hydrocarbon fragments can ``conduct'' electrons over tens of angstroms. A dramatic demonstration of this fact has recently been obtained by tunneling electron microscopy on Langmuir-Blodgett films, showing in a

  13. Electrochemical Electron Transfer and Proton-Coupled Electron Transfer: Effects of Double Layer and Ionic Environment on Solvent Reorganization Energies.

    PubMed

    Ghosh, Soumya; Soudackov, Alexander V; Hammes-Schiffer, Sharon

    2016-06-14

    Electron transfer and proton coupled electron transfer (PCET) reactions at electrochemical interfaces play an essential role in a broad range of energy conversion processes. The reorganization energy, which is a measure of the free-energy change associated with solute and solvent rearrangements, is a key quantity for calculating rate constants for these reactions. We present a computational method for including the effects of the double layer and ionic environment of the diffuse layer in calculations of electrochemical solvent reorganization energies. This approach incorporates an accurate electronic charge distribution of the solute within a molecular-shaped cavity in conjunction with a dielectric continuum treatment of the solvent, ions, and electrode using the integral equations formalism polarizable continuum model. The molecule-solvent boundary is treated explicitly, but the effects of the electrode-double layer and double layer-diffuse layer boundaries, as well as the effects of the ionic strength of the solvent, are included through an external Green's function. The calculated total reorganization energies agree well with experimentally measured values for a series of electrochemical systems, and the effects of including both the double layer and ionic environment are found to be very small. This general approach was also extended to electrochemical PCET and produced total reorganization energies in close agreement with experimental values for two experimentally studied PCET systems. PMID:27111050

  14. 76 FR 29901 - Electronic Fund Transfers

    Federal Register 2010, 2011, 2012, 2013, 2014

    2011-05-23

    ... Board anticipates that final rules on remittance transfers will be issued by the Bureau. \\26\\ 75 FR... be edited to remove any identifying or contact information. Public comments may also be viewed... a.m. and 5 p.m. on weekdays. FOR FURTHER INFORMATION CONTACT: Dana Miller, Mandie Aubrey or...

  15. Promoting Knowledge Transfer with Electronic Note Taking

    ERIC Educational Resources Information Center

    Katayama, Andrew D.; Shambaugh, R. Neal; Doctor, Tasneem

    2005-01-01

    We investigated the differences between (a) copying and pasting text versus typed note-taking methods of constructing study notes simultaneously with (b) vertically scaffolded versus horizontally scaffold notes on knowledge transfer. Forty-seven undergraduate educational psychology students participated. Materials included 2 electronic…

  16. Extracellular electron transfer mechanisms between microorganisms and minerals.

    PubMed

    Shi, Liang; Dong, Hailiang; Reguera, Gemma; Beyenal, Haluk; Lu, Anhuai; Liu, Juan; Yu, Han-Qing; Fredrickson, James K

    2016-10-01

    Electrons can be transferred from microorganisms to multivalent metal ions that are associated with minerals and vice versa. As the microbial cell envelope is neither physically permeable to minerals nor electrically conductive, microorganisms have evolved strategies to exchange electrons with extracellular minerals. In this Review, we discuss the molecular mechanisms that underlie the ability of microorganisms to exchange electrons, such as c-type cytochromes and microbial nanowires, with extracellular minerals and with microorganisms of the same or different species. Microorganisms that have extracellular electron transfer capability can be used for biotechnological applications, including bioremediation, biomining and the production of biofuels and nanomaterials. PMID:27573579

  17. Electronic excitation energy transfer between nucleobases of natural DNA.

    PubMed

    Vayá, Ignacio; Gustavsson, Thomas; Douki, Thierry; Berlin, Yuri; Markovitsi, Dimitra

    2012-07-18

    Transfer of the electronic excitation energy in calf thymus DNA is studied by time-resolved fluorescence spectroscopy. The fluorescence anisotropy, after an initial decay starting on the femtosecond time scale, dwindles down to ca. 0.1. The in-plane depolarized fluorescence decays are described by a stretched exponential law. Our observations are consistent with one-dimensional transfer mediated by charge-transfer excited states. PMID:22765050

  18. The dynamical correlation in spacer-mediated electron transfer couplings

    SciTech Connect

    Yang, C.-H.; Hsu, C.-P.

    2006-06-28

    The dynamical correlation effect in electron transfer (ET) coupling was studied in this work, for cases where electrons tunnel through a many-electron environment. The ET couplings for three different bridge-mediated model systems were calculated: (I) trans-alkyl chains [H{sub 2}C-(CH{sub 2}){sub n}-CH{sub 2}, n=2-10], (II) two isomers of trans-1,4-dimethylenecyclohexane, and (III) two ethylenes spaced by a saturated ethane molecule. The couplings were calculated as half energy gaps of the two lowest adiabatic states. The dynamical correlation was included with spin-flip (SF) and ionization potential or electron affinity coupled-cluster singles and doubles (SF-CCSD and IP/EA-CCSD) and a {delta}CCSD scheme. The direct coupling (DC) scheme is also used as a way to obtain a solution with nondynamical correlation, since DC uses approximated eigenstates that are symmetry-restoring linear combinations of two symmetry-broken unrestricted Hartree-Fock configurations. For all cases tested except for one, results from the DC scheme closely follow the CCSD data, indicating that the dual-configuration solutions can be a good approximation of wave functions with nondynamical correlation included, but there exist exceptions. Comparing the DC results with SF-CCSD and IP or EA-CCSD data, we concluded that the dynamical correlation effect is small for most of the cases we tested.

  19. A facile measurement of heterogeneous electron transfer kinetics.

    PubMed

    Bueno, Paulo R; Benites, Tiago Azevedo; Góes, Márcio Sousa; Davis, Jason J

    2013-11-19

    This work introduces a simple, single-step, impedance-derived capacitance spectroscopic approach as a convenient and direct way of reporting the heterogeneous rate of electron-transfer between an electrode and solution-phase redox species. The proposed methodology requires no equivalent circuit analysis or data fitting and is equally applicable to the strong coupling (diffusion-mediated) or weak coupling electron-transfer regimes. PMID:24187916

  20. Photoinduced electron transfer from DABCO to trans-nitrostilbenes

    NASA Astrophysics Data System (ADS)

    Görner, Helmut; Schulte-Frohlinde, Dietrich

    The anion radical of the trans isomers of 4-nitro-, 4,4'-dinitro-, and 4-nitro-4'-methoxystilbene was generated by triplet quenching with 1,4-diazabicyclo[2.2.2]octane (DABCO) in polar solvents at room temperature using laser flash photolysis. Electron transfer and trans → cis photoisomerization are competing processes. The radical ions decay by electron back-transfer yielding the initial ground states.

  1. MANAGING ELECTRONIC DATA TRANSFER IN ENVIRONMENTAL CLEANUPS

    EPA Science Inventory

    The use of computers and electronic information poses a complex problem for potential litigation in space law. The problem currently manifests itself in at least two ways. First, the Environmental Protection Agency (EPA) enforcement of Comprehensive Environmental Response, Compen...

  2. Improved heterogeneous electron transfer kinetics of fluorinated graphene derivatives

    NASA Astrophysics Data System (ADS)

    Boopathi, Sidhureddy; Narayanan, Tharangattu N.; Senthil Kumar, Shanmugam

    2014-08-01

    Though graphitic carbons are commercially available for various electrochemical processes, their performance is limited in terms of various electrochemical activities. Recent experiments on layered carbon materials, such as graphene, demonstrated an augmented performance of these systems in all electrochemical activities due to their unique electronic properties, enhanced surface area, structure and chemical stabilities. Moreover, flexibility in controlling electronic, as well as electrochemical activities by heteroatom doping brings further leverage in their practical use. Here, we study the electron transfer kinetics of fluorinated graphene derivatives, known as fluorinated graphene oxide (FGO) and its reduced form, RFGO. Enhanced electron transfer kinetics (heterogeneous electron transfer (HET)) is observed from these fluorinated systems in comparison to their undoped systems such as graphene oxide (GO) and reduced GO. A detailed study has been conducted using standard redox probes and biomolecules revealing the enhanced electro-catalytic activities of FGO and RFGO, and electron transfer rates are simulated theoretically. This study reveals that fluorine not only induces defects in graphitic lattice leading to an enhanced HET process but also can modify the electronic structure of graphene surface.Though graphitic carbons are commercially available for various electrochemical processes, their performance is limited in terms of various electrochemical activities. Recent experiments on layered carbon materials, such as graphene, demonstrated an augmented performance of these systems in all electrochemical activities due to their unique electronic properties, enhanced surface area, structure and chemical stabilities. Moreover, flexibility in controlling electronic, as well as electrochemical activities by heteroatom doping brings further leverage in their practical use. Here, we study the electron transfer kinetics of fluorinated graphene derivatives, known as

  3. Modeling of Radiative Transfer in Protostellar Disks

    NASA Technical Reports Server (NTRS)

    VonAllmen, Paul; Turner, Neal

    2007-01-01

    This program implements a spectral line, radiative transfer tool for interpreting Spitzer Space Telescope observations by matching them with models of protostellar disks for improved understanding of planet and star formation. The Spitzer Space Telescope detects gas phase molecules in the infrared spectra of protostellar disks, with spectral lines carrying information on the chemical composition of the material from which planets form. Input to the software includes chemical models developed at JPL. The products are synthetic images and spectra for comparison with Spitzer measurements. Radiative transfer in a protostellar disk is primarily affected by absorption and emission processes in the dust and in molecular gases such as H2, CO, and HCO. The magnitude of the optical absorption and emission is determined by the population of the electronic, vibrational, and rotational energy levels. The population of the molecular level is in turn determined by the intensity of the radiation field. Therefore, the intensity of the radiation field and the population of the molecular levels are inter-dependent quantities. To meet the computational challenges of solving for the coupled radiation field and electronic level populations in disks having wide ranges of optical depths and spatial scales, the tool runs in parallel on the JPL Dell Cluster supercomputer with C++ and Fortran compiler with a Message Passing Interface. Because this software has been developed on a distributed computing platform, the modeling of systems previously beyond the reach of available computational resources is possible.

  4. Quantum effects in ultrafast electron transfers within cryptochromes.

    PubMed

    Firmino, Thiago; Mangaud, Etienne; Cailliez, Fabien; Devolder, Adrien; Mendive-Tapia, David; Gatti, Fabien; Meier, Christoph; Desouter-Lecomte, Michèle; de la Lande, Aurélien

    2016-08-21

    Cryptochromes and photolyases are flavoproteins that may undergo ultrafast charge separation upon electronic excitation of their flavin cofactors. Charge separation involves chains of three or four tryptophan residues depending on the protein of interest. The molecular mechanisms of these processes are not completely clear. In the present work we investigate the relevance of quantum effects like the occurrence of nuclear tunneling and of coherences upon charge transfer in Arabidopsis thaliana cryptochromes. The possible breakdown of the Condon approximation is also investigated. We have devised a simulation protocol based on the realization of molecular dynamics simulations on diabatic potential energy surfaces defined at the hybrid constrained density functional theory/molecular mechanics level. The outcomes of the simulations are analyzed through various dedicated kinetics schemes related to the Marcus theory that account for the aforementioned quantum effects. MD simulations also provide a basic material to define realistic model Hamiltonians for subsequent quantum dissipative dynamics. To carry out quantum simulations, we have implemented an algorithm based on the Hierarchical Equations of Motion. With this new tool in hand we have been able to model the electron transfer chain considering either two- or three-state models. Kinetic models and quantum simulations converge to the conclusion that quantum effects have a significant impact on the rate of charge separation. Nuclear tunneling involving atoms of the tryptophan redox cofactors as well as of the environment (protein atoms and water molecules) is significant. On the other hand non-Condon effects are negligible in most simulations. Taken together, the results of the present work provide new insights into the molecular mechanisms controlling charge separation in this family of flavoproteins. PMID:27427185

  5. Nuclear interlevel transfer driven by electronic transitions

    SciTech Connect

    Solem, J.C.; Rinker, G.

    1985-01-01

    We show how a gamma-ray laser might be made by optically exciting a transfer of population from a long-lived isomer to an energetically adjacent short-lived state of the same nucleus. We compare the advantages of using transitions of high multipolarity versus transitions of low multi-polarity. Preliminary numerical investigations of the mechanism show it to be somewhat favorable. 35 refs., 4 figs.

  6. Electron transfer reactions in microporous solids

    SciTech Connect

    Mallouk, T.E.

    1993-01-01

    Basic thrust the research program involves use of microporous solids (zeolites, clays, layered and tunnel structure oxide semiconductors) as organizing media for artificial photosynthetic systems. Purpose of the microporous solid is twofold. First, it induces spatial organization of photoactive and electroactive components (sensitizers, semiconductor particles, electron relays, and catalysts) at the solid-solution interface, enhancing the quantum efficiency of charge separation and separating physically the ultimate electron donor and acceptor in the electron transport chain. Second, since the microcrystalline solid admits only molecules of a certain charge and size, it is possible to achieve permanent charge separation by sieving chemical photoproducts (e.g., H[sub 2] and I[sub 3][sup [minus

  7. (Validity of environmental transfer models)

    SciTech Connect

    Blaylock, B.G.; Hoffman, F.O.; Gardner, R.H.

    1990-11-07

    BIOMOVS (BIOspheric MOdel Validation Study) is an international cooperative study initiated in 1985 by the Swedish National Institute of Radiation Protection to test models designed to calculate the environmental transfer and bioaccumulation of radionuclides and other trace substances. The objective of the symposium and workshop was to synthesize results obtained during Phase 1 of BIOMOVS (the first five years of the study) and to suggest new directions that might be pursued during Phase 2 of BIOMOVS. The travelers were an instrumental part of the development of BIOMOVS. This symposium allowed the travelers to present a review of past efforts at model validation and a synthesis of current activities and to refine ideas concerning future development of models and data for assessing the fate, effect, and human risks of environmental contaminants. R. H. Gardner also visited the Free University, Amsterdam, and the National Institute of Public Health and Environmental Protection (RIVM) in Bilthoven to confer with scientists about current research in theoretical ecology and the use of models for estimating the transport and effect of environmental contaminants and to learn about the European efforts to map critical loads of acid deposition.

  8. Does electron-transfer theory explain large rate differences in singlet and triplet excited state electron-transfer reactions?

    SciTech Connect

    Zusman, L.D.; Kurnikov, I.V.; Beratan, D.N.

    1995-12-31

    Gray and coworkers have shown that intramolecular electron-transfer rates from singlet and triplet excited states in iridium(spacer)pyridinium complexes can be vastly different (>5 orders of magnitude). We have analyzed the possible sources of these differences, including effects that may arise from reorganization energies, free energies, and tunneling matrix elements. When distance dependent reorganization energies and energy dependent tunneling matrix elements are included, a systematic framework emerges to describe these electron-transfer reactions.

  9. Vectorial electron transfer in spatially ordered arrays

    SciTech Connect

    Fox, M.A.

    1992-01-01

    Progress has been made in four areas: the synthesis of new materials for directional electron; the preparation and characterization of anisotropic composites bearing organic and inorganic components; the elaboration of mechanisms of electrocatalysis; and the development of new methods for surface modification of metals and semiconductors.

  10. Frontier orbital symmetry control of intermolecular electron transfer. Final report, September 15, 1988--December 31, 1994

    SciTech Connect

    Stevens, B.

    1997-07-01

    This report discusses the following topics: the recovery of intermolecular transfer parameters from fluorescence quenching in liquids; photoinduced intramolecular electron transfer in flexible donor/space/acceptor systems containing an extended unsaturated spacer; electron transfer sensitized reaction; the recovery of solute and fractal dimensions from electron transfer quenching data; and frontier orbital symmetry control of back electron transfer.

  11. Mass transfer of electron acceptor aross the capillary fringe

    NASA Astrophysics Data System (ADS)

    Liu, S.; Piepenbrink, M.; Grathwohl, P.

    2005-12-01

    Transverse dispersion has been identified as a potentially limiting parameter controlling the mixing of electron donors and electron acceptors for natural attenuation of plumes originating from continuously emitting sources, however determining reactive transverse dispersion coefficients is not a simple task. The objective of this work is to elaborate the mass transfer of electron acceptor across the capillary fringe. A two-dimensional numerical reactive transport model and a fully controlled tank experiment are set up to investigate the mass transfer across the capillary and reactive fringe, where the oxygen supply is the limiting factor. The tank (77.9 times 14 times 0.8 cm) is made from acrylic-glass and filled with glass beads (0.5-0.75mm). Sodium dithionite, an easily oxidizable compound, is used as a surrogate for contaminants and is continuously injected from the inlets of the tank and reaches a steady state flow. Air circulates on the top of the glass beads. The oxygen concentrations as well as the reactive products (sulfate) are measured at the outlets of the tank with an oxygen sensor and via IC. In addition to that, resazurine, a redox indicator, is added to visualize the redox zones. These two-dimensional experimental results show quantitatively and qualitatively how the oxygen concentrations decrease at the plume fringe. Two dimensional numerical simulations with Min3P predicted oxygen distributions are compared with the experimental results. Acknowledgements: This work was funded by Helmholtz Association and Helmholtz Research Center UFZ; Project: `Virtual Institute for isotope biogeochemistry-biologically mediated processes at geochemical gradients and interfaces in soil - aquifer systems', Contract VH-VI-155.

  12. Electron transfer pathways in photosystem I reaction centers

    NASA Astrophysics Data System (ADS)

    Ivashin, Nikolaj; Larsson, Sven

    2003-07-01

    Electron transfer following charge separation in the photosystem I (PSI) reaction center of Synechococcus elongatus is studied using theoretical methods. The difference in rate between two almost symmetrical A- and B-branches is caused by a difference in a single residue (Trp B673 versus Gly A693), close to the F X iron-sulfur cluster. Partly due to its polar environment, Trp B673 acts as an electron acceptor in its π-system. The rate increases on the B-side due to shortened distances for electron transfer.

  13. Calculation of electron transfer in ruthenium-modified derivatives of cytochrome b562

    NASA Astrophysics Data System (ADS)

    Glukhova, O. E.; Prytkova, T. R.; Shunaev, V. V.

    2016-03-01

    Quantitative theoretical studies of long-range electron transfer are still quite rare and require further development of computational methods for the analysis of such reactions. We considered the electron transfer reaction in rutenium-modified derivatives of cytochrome b562 with advanced modeling techniques. We conducted a series of ab initio calculations of the donor/acceptor interaction in protein fragments and compared the calculated electron velocity with available experimental data. Our approach takes into account the co-factor of the electronic structure and the impact of the solution on a donor-acceptor interaction. This allows us to predict the absolute values of the electron transfer rate unlike other computational methods which provide only qualitative results. Our estimates with good accuracy repeat the experimental values of electron transfer rate. It was found that the electron transfer in certain derivatives of cytochrome b562 is mainly caused by "shortcut" conformations in which the donor/acceptor interactions are mediated by the interaction of Ru-unbound ligands with groups of the protein surface. We argue that a quantitative theoretical analysis is essential for detailed understanding of electron transfer in proteins and mechanisms of biological redox reactions.

  14. Structural effects on photoinduced electron transfer in carotenoid-porphyrin-quinone triads

    SciTech Connect

    Kuciauskas, D.; Liddell, P.A.; Hung, S.C.; Lin, S.; Stone, S.; Seely, G.R.; Moore, A.L.; Moore, T.A.; Gust, D.

    1997-01-16

    meso-Polyarylporphyrins are often used as components of molecules that mimic photosynthetic reaction centers by carrying out photoinduced electron-transfer reactions. Studies of these systems have raised questions concerning the role of alkyl substituents at the `{beta}-pyrrolic` positions on the porphyrin periphery in limiting {pi}-{pi} overlap between the macrocycle and the aryl rings. The degree of overlap affects electronic coupling and, therefore, the rates of electron-transfer reactions. There is also evidence that when the linkages joining porphyrins to electron-acceptor or -donor moieties contain amide bonds, the sense of the amide linkage may strongly affect electron-transfer rate constants. In this study, three carotenoid-porphyrin-quinone molecular triads and various model compounds have been prepared, and electron-transfer has been studied using time-resolved emission and absorption techniques. The results show that steric hindrance due to methyl groups at the {beta}-pyrrolic positions reduces electron-transfer rate constants by a factor of approximately 1/5. In addition, amide-containing donor-acceptor linkages having the nitrogen atom attached to the porphyrin meso-aryl ring demonstrate electron-transfer rate constants approximately 30 times larger than those for similar linkages with the amide reversed, after correction for thermodynamic effects. 52 refs., 7 figs., 2 tabs.

  15. A tetrastable naphthalenediimide: anion induced charge transfer, single and double electron transfer for combinational logic gates.

    PubMed

    Ajayakumar, M R; Hundal, Geeta; Mukhopadhyay, Pritam

    2013-09-11

    Herein we demonstrate the formation of the first tetrastable naphthalenediimide (NDI, 1a) molecule having multiple distinctly readable outputs. Differential response of 1a to fluoride anions induces intramolecular charge transfer (ICT), single/double electron transfer (SET/DET) leading to a set of combinational logic gates for the first time with a NDI moiety. PMID:23752683

  16. Dynamics of ultrafast photoinduced heterogeneous electron transfer, implications for recent solar energy conversion scenarios

    NASA Astrophysics Data System (ADS)

    Gundlach, Lars; Burfeindt, Bernd; Mahrt, Jürgen; Willig, Frank

    2012-08-01

    The general case of a heterogeneous electron transfer reaction is realized by ultrafast electron transfer from a photo-excited molecule to a wide continuum of electronic acceptor states. Two different theoretical model calculations addressing the injection dynamics have recently been presented. The first scenario predicts a wide energy distribution for the injected electron via excitations of high-energy vibrational modes in the ionized molecule, whereas the second scenario ascribes the width to thermal fluctuations. We present experimental data at different temperatures and identify the valid injection scenario for perylene/TiO2 systems. The results are discussed in view of recent solar energy conversion scenarios.

  17. Lewis Acid Coupled Electron Transfer of Metal-Oxygen Intermediates.

    PubMed

    Fukuzumi, Shunichi; Ohkubo, Kei; Lee, Yong-Min; Nam, Wonwoo

    2015-12-01

    Redox-inactive metal ions and Brønsted acids that function as Lewis acids play pivotal roles in modulating the redox reactivity of metal-oxygen intermediates, such as metal-oxo and metal-peroxo complexes. The mechanisms of the oxidative CH bond cleavage of toluene derivatives, sulfoxidation of thioanisole derivatives, and epoxidation of styrene derivatives by mononuclear nonheme iron(IV)-oxo complexes in the presence of triflic acid (HOTf) and Sc(OTf)3 have been unified as rate-determining electron transfer coupled with binding of Lewis acids (HOTf and Sc(OTf)3 ) by iron(III)-oxo complexes. All logarithms of the observed second-order rate constants of Lewis acid-promoted oxidative CH bond cleavage, sulfoxidation, and epoxidation reactions of iron(IV)-oxo complexes exhibit remarkably unified correlations with the driving forces of proton-coupled electron transfer (PCET) and metal ion-coupled electron transfer (MCET) in light of the Marcus theory of electron transfer when the differences in the formation constants of precursor complexes were taken into account. The binding of HOTf and Sc(OTf)3 to the metal-oxo moiety has been confirmed for Mn(IV) -oxo complexes. The enhancement of the electron-transfer reactivity of metal-oxo complexes by binding of Lewis acids increases with increasing the Lewis acidity of redox-inactive metal ions. Metal ions can also bind to mononuclear nonheme iron(III)-peroxo complexes, resulting in acceleration of the electron-transfer reduction but deceleration of the electron-transfer oxidation. Such a control on the reactivity of metal-oxygen intermediates by binding of Lewis acids provides valuable insight into the role of Ca(2+) in the oxidation of water to dioxygen by the oxygen-evolving complex in photosystem II. PMID:26404482

  18. Role of coherence and delocalization in photo-induced electron transfer at organic interfaces.

    PubMed

    Abramavicius, V; Pranculis, V; Melianas, A; Inganäs, O; Gulbinas, V; Abramavicius, D

    2016-01-01

    Photo-induced charge transfer at molecular heterojunctions has gained particular interest due to the development of organic solar cells (OSC) based on blends of electron donating and accepting materials. While charge transfer between donor and acceptor molecules can be described by Marcus theory, additional carrier delocalization and coherent propagation might play the dominant role. Here, we describe ultrafast charge separation at the interface of a conjugated polymer and an aggregate of the fullerene derivative PCBM using the stochastic Schrödinger equation (SSE) and reveal the complex time evolution of electron transfer, mediated by electronic coherence and delocalization. By fitting the model to ultrafast charge separation experiments, we estimate the extent of electron delocalization and establish the transition from coherent electron propagation to incoherent hopping. Our results indicate that even a relatively weak coupling between PCBM molecules is sufficient to facilitate electron delocalization and efficient charge separation at organic interfaces. PMID:27605035

  19. Role of coherence and delocalization in photo-induced electron transfer at organic interfaces

    PubMed Central

    Abramavicius, V.; Pranculis, V.; Melianas, A.; Inganäs, O.; Gulbinas, V.; Abramavicius, D.

    2016-01-01

    Photo-induced charge transfer at molecular heterojunctions has gained particular interest due to the development of organic solar cells (OSC) based on blends of electron donating and accepting materials. While charge transfer between donor and acceptor molecules can be described by Marcus theory, additional carrier delocalization and coherent propagation might play the dominant role. Here, we describe ultrafast charge separation at the interface of a conjugated polymer and an aggregate of the fullerene derivative PCBM using the stochastic Schrödinger equation (SSE) and reveal the complex time evolution of electron transfer, mediated by electronic coherence and delocalization. By fitting the model to ultrafast charge separation experiments, we estimate the extent of electron delocalization and establish the transition from coherent electron propagation to incoherent hopping. Our results indicate that even a relatively weak coupling between PCBM molecules is sufficient to facilitate electron delocalization and efficient charge separation at organic interfaces. PMID:27605035

  20. Ultrafast spectroscopy of electron transfer dynamics in liquids; excitation transfer studies of phase transitions

    NASA Astrophysics Data System (ADS)

    Goun, Alexei A.

    The transfer of an electron from a donor to an acceptor is the fundamental step in a wide range of chemical and biological processes. As a result, electron-transfer reactions have been the focus of numerous theoretical and experimental efforts aimed at understanding the kinetics and mechanism of the transfer event. Liquid solvents are an important medium for electron-transfer processes. The influences of the distance dependence, diffusion, the radial distribution function, and the hydrodynamic effect have been incorporated into the theory of electron transfer in solution, as well as into the theory of electron transfer between donors and acceptors in the head group regions of micelles. The development of new laser system with a pulse duration of tens of femtoseconds, with tunable wavelength allowed us to study these processes on a considerably shorter time scale than previous studies. This allowed us to observe not only the diffusion controlled but also the kinetics of electron transfer for donor/acceptor pairs that are in close proximity. In one set of experiments we have studied the kinetics of electron transfer in electron accepting molecule (rhodamine 3B) dissolved in electron donating solvent (N,N-dimethylaniline). The data for the forward electron transfer and geminate recombination are approximated by the statistical theory of the electron transfer. Optical anisotropy observed in the experiment demonstrates the orientation dependence of the electron transfer rate. In further experiments we investigated the electron transfer in non-hydrogen bonding liquids of increasing viscosity. The effective value of the donor/acceptor electronic coupling was found to decrease with viscosity. Electron transfer experiments were also carried out on the surface of micelles. The systems studied are the hole donor octadecyl-rhodamine B (ODRB) and the hole acceptor N,N-dimethyl-aniline (DMA) in micelles made of dodecyltrimethylammonium bromide (DTAB) and

  1. Electron transfer through rigid organic molecular wires enhanced by electronic and electron-vibration coupling

    NASA Astrophysics Data System (ADS)

    Sukegawa, Junpei; Schubert, Christina; Zhu, Xiaozhang; Tsuji, Hayato; Guldi, Dirk M.; Nakamura, Eiichi

    2014-10-01

    Electron transfer (ET) is a fundamental process in a wide range of biological systems, photovoltaics and molecular electronics. Therefore to understand the relationship between molecular structure and ET properties is of prime importance. For this purpose, photoinduced ET has been studied extensively using donor-bridge-acceptor molecules, in which π-conjugated molecular wires are employed as bridges. Here, we demonstrate that carbon-bridged oligo-p-phenylenevinylene (COPV), which is both rigid and flat, shows an 840-fold increase in the ET rate compared with the equivalent flexible molecular bridges. A 120-fold rate enhancement is explained in terms of enhanced electronic coupling between the electron donor and the electron acceptor because of effective conjugation through the COPVs. The remainder of the rate enhancement is explained by inelastic electron tunnelling through COPV caused by electron-vibration coupling, unprecedented for organic molecular wires in solution at room temperature. This type of nonlinear effect demonstrates the versatility and potential practical utility of COPVs in molecular device applications.

  2. Quantifying electron transfer reactions in biological systems: what interactions play the major role?

    PubMed Central

    Sjulstok, Emil; Olsen, Jógvan Magnus Haugaard; Solov’yov, Ilia A.

    2015-01-01

    Various biological processes involve the conversion of energy into forms that are usable for chemical transformations and are quantum mechanical in nature. Such processes involve light absorption, excited electronic states formation, excitation energy transfer, electrons and protons tunnelling which for example occur in photosynthesis, cellular respiration, DNA repair, and possibly magnetic field sensing. Quantum biology uses computation to model biological interactions in light of quantum mechanical effects and has primarily developed over the past decade as a result of convergence between quantum physics and biology. In this paper we consider electron transfer in biological processes, from a theoretical view-point; namely in terms of quantum mechanical and semi-classical models. We systematically characterize the interactions between the moving electron and its biological environment to deduce the driving force for the electron transfer reaction and to establish those interactions that play the major role in propelling the electron. The suggested approach is seen as a general recipe to treat electron transfer events in biological systems computationally, and we utilize it to describe specifically the electron transfer reactions in Arabidopsis thaliana cryptochrome–a signaling photoreceptor protein that became attractive recently due to its possible function as a biological magnetoreceptor. PMID:26689792

  3. Quantifying electron transfer reactions in biological systems: what interactions play the major role?

    NASA Astrophysics Data System (ADS)

    Sjulstok, Emil; Olsen, Jógvan Magnus Haugaard; Solov'Yov, Ilia A.

    2015-12-01

    Various biological processes involve the conversion of energy into forms that are usable for chemical transformations and are quantum mechanical in nature. Such processes involve light absorption, excited electronic states formation, excitation energy transfer, electrons and protons tunnelling which for example occur in photosynthesis, cellular respiration, DNA repair, and possibly magnetic field sensing. Quantum biology uses computation to model biological interactions in light of quantum mechanical effects and has primarily developed over the past decade as a result of convergence between quantum physics and biology. In this paper we consider electron transfer in biological processes, from a theoretical view-point; namely in terms of quantum mechanical and semi-classical models. We systematically characterize the interactions between the moving electron and its biological environment to deduce the driving force for the electron transfer reaction and to establish those interactions that play the major role in propelling the electron. The suggested approach is seen as a general recipe to treat electron transfer events in biological systems computationally, and we utilize it to describe specifically the electron transfer reactions in Arabidopsis thaliana cryptochrome-a signaling photoreceptor protein that became attractive recently due to its possible function as a biological magnetoreceptor.

  4. Alternative ground states enable pathway switching in biological electron transfer

    PubMed Central

    Abriata, Luciano A.; Álvarez-Paggi, Damián; Ledesma, Gabriela N.; Blackburn, Ninian J.; Vila, Alejandro J.; Murgida, Daniel H.

    2012-01-01

    Electron transfer is the simplest chemical reaction and constitutes the basis of a large variety of biological processes, such as photosynthesis and cellular respiration. Nature has evolved specific proteins and cofactors for these functions. The mechanisms optimizing biological electron transfer have been matter of intense debate, such as the role of the protein milieu between donor and acceptor sites. Here we propose a mechanism regulating long-range electron transfer in proteins. Specifically, we report a spectroscopic, electrochemical, and theoretical study on WT and single-mutant CuA redox centers from Thermus thermophilus, which shows that thermal fluctuations may populate two alternative ground-state electronic wave functions optimized for electron entry and exit, respectively, through two different and nearly perpendicular pathways. These findings suggest a unique role for alternative or “invisible” electronic ground states in directional electron transfer. Moreover, it is shown that this energy gap and, therefore, the equilibrium between ground states can be fine-tuned by minor perturbations, suggesting alternative ways through which protein–protein interactions and membrane potential may optimize and regulate electron–proton energy transduction. PMID:23054836

  5. Imaging population transfer in atoms with ultrafast electron pulses

    NASA Astrophysics Data System (ADS)

    Shao, Hua-Chieh; Starace, Anthony F.

    2016-05-01

    Ultrafast electron diffraction and microscopy have made significant progress recently in investigating atomic-scale structural dynamics in gas-phase and condensed materials. With these advances, direct imaging of electronic motions in atoms and molecules by ultrafast electron diffraction is anticipated. We propose imaging a laser-driven coherent population transfer in lithium atoms by femtosecond ultrafast electron pulses. Valuable information and insight can be obtained from studying such a system in order to refine ultrafast electron techniques and to interpret experimental results. Adiabatic passage by level crossing is used to transfer the electron population from the 2 s to the 2 p state. Our simulations demonstrate the ability of ultrafast electron diffraction to image this population transfer, as the time-dependent diffraction images reflect the electronic motion in the scattering intensity and angular distribution. Furthermore, asymmetric diffraction patterns indicate that even the relative phases of the electronic wave function can be resolved, provided there is sufficient temporal resolution. This work has been supported in part by DOE Award No. DE-SC0012193 [H.-C.S.] and by NSF Grant No. PHYS-1505492 [A.F.S.].

  6. Rates and Routes of Electron Transfer of [NiFe]-Hydrogenase in an Enzymatic Fuel Cell.

    PubMed

    Petrenko, Alexander; Stein, Matthias

    2015-10-29

    Hydrogenase enzymes are being used in enzymatic fuel cells immobilized on a graphite or carbon electrode surface, for example. The enzyme is used for the anodic oxidation of molecular hydrogen (H2) to produce protons and electrons. The association and orientation of the enzyme at the anode electrode for a direct electron transfer is not completely resolved. The distal FeS-cluster in [NiFe]-hydrogenases contains a histidine residue which is known to play a critical role in the intermolecular electron transfer between the enzyme and the electrode surface. The [NiFe]-hydrogenase graphite electrode association was investigated using Brownian Dynamics simulations. Residues that were shown to be in proximity to the electrode surface were identified (His184, Ser196, Glu461, Glu464), and electron transfer routes connecting the distal FeS-cluster with the surface residues were investigated. Several possible pathways for electron transfer between the distal FeS-cluster and the terminal amino acid residues were probed in terms of their rates of electron transfer using DFT methods. The reorganization energies λ of the distal iron-sulfur cluster and coronene as a molecular model for graphite were calculated. The reorganization energy of the distal (His)(Cys)3 cluster was found to be not very different from that of a standard cubane clusters with a (Cys)4 coordination. Electronic coupling matrix elements and rates of electron transfer for the different pathways were calculated according to the Marcus equation. The rates for glutamate-mediated electrode binding were found to be incompatible with experimental data. A direct electron transfer from the histidine ligand of the distal FeS-cluster to the electrode yielded rates of electron transfer in excellent agreement with experiment. A second pathway, however, from the distal FeS-cluster to the Ser196 residue was found to be equally efficient and feasible. PMID:26218232

  7. Collisional energy transfer and quenching of electronic excitation

    PubMed Central

    Lin, S. H.; Eyring, H.

    1975-01-01

    The purpose of this paper has been to explore in a preliminary way the nature and mechanism of collisional energy transfer and quenching of electronic excitation. For this purpose, the Born approximation has been used, and the triplet-triplet and singlet-singlet transfer, and the triplet-triplet and singlet-singlet quenching have been studied. It has been shown theoretically that (i) the singlet-singlet transfer constants (or cross sections) are always larger than the triplet-triplet transfer constants (or cross sections) for the same system of donor and acceptor; (ii) for the singlet-singlet transfer, the observed cross section varies linearly with respect to the spectral overlap between the donor emission and the acceptor absorption; (iii) the reason that the quenching constants (or cross sections) are always smaller than the energy transfer constants (or cross sections) is due to the fact that for the quenching the vibration of the acceptor hardly participates in accepting the electronic excitation and for the energy transfer only part of the excited electron energy of the donor is converted into the energy of nuclear motion; and (iv) the polar acceptor molecules are better quenchers than nonpolar acceptor molecules. PMID:16592281

  8. Non-Markovian electron transfer reactions with frequency-dependent friction

    SciTech Connect

    Tang, J.

    1993-12-31

    A modified non-Markovian Zusman equation for electron transfer reactions with frequency-dependent friction is presented. The derivation is based on the spin-boson model with a two-level system coupled to a non-Debye polar solvent bath with frequency-dependent friction. The diffusion constant in the Smoluchowski diffusion operator of the ordinary Zusman equation should be replaced by a convolution of a retarded time-dependent diffusion constant. An analytical expression for the electron transfer rate constant was derived using the Green`s function method. In the adiabatic regime, electron transfer process is generally nonexponential. Because of the time-retardation, initial electron transfer reaction is influenced more by the higher frequency components in the solvent relaxation.

  9. Plugging in or going wireless: strategies for interspecies electron transfer

    PubMed Central

    Shrestha, Pravin Malla; Rotaru, Amelia-Elena

    2014-01-01

    Interspecies exchange of electrons enables a diversity of microbial communities to gain energy from reactions that no one microbe can catalyze. The first recognized strategies for interspecies electron transfer were those that relied on chemical intermediates that are recycled through oxidized and reduced forms. Well-studied examples are interspecies H2 transfer and the cycling of sulfur intermediates in anaerobic photosynthetic communities. Direct interspecies electron transfer (DIET) in which two species establish electrical contact is an alternative. Electrical contacts documented to date include electrically conductive pili, as well as conductive iron minerals and conductive carbon moieties such as activated carbon and biochar. Interspecies electron transfer is central to the functioning of methane-producing microbial communities. The importance of interspecies H2 transfer in many methanogenic communities is clear, but under some circumstances DIET predominates. It is expected that further mechanistic studies and broadening investigations to a wider range of environments will help elucidate the factors that favor specific forms of interspecies electron exchange under different environmental conditions. PMID:24904551

  10. The electron transfer complex between nitrous oxide reductase and its electron donors.

    PubMed

    Dell'acqua, Simone; Moura, Isabel; Moura, José J G; Pauleta, Sofia R

    2011-12-01

    Identifying redox partners and the interaction surfaces is crucial for fully understanding electron flow in a respiratory chain. In this study, we focused on the interaction of nitrous oxide reductase (N(2)OR), which catalyzes the final step in bacterial denitrification, with its physiological electron donor, either a c-type cytochrome or a type 1 copper protein. The comparison between the interaction of N(2)OR from three different microorganisms, Pseudomonas nautica, Paracoccus denitrificans, and Achromobacter cycloclastes, with their physiological electron donors was performed through the analysis of the primary sequence alignment, electrostatic surface, and molecular docking simulations, using the bimolecular complex generation with global evaluation and ranking algorithm. The docking results were analyzed taking into account the experimental data, since the interaction is suggested to have either a hydrophobic nature, in the case of P. nautica N(2)OR, or an electrostatic nature, in the case of P. denitrificans N(2)OR and A. cycloclastes N(2)OR. A set of well-conserved residues on the N(2)OR surface were identified as being part of the electron transfer pathway from the redox partner to N(2)OR (Ala495, Asp519, Val524, His566 and Leu568 numbered according to the P. nautica N(2)OR sequence). Moreover, we built a model for Wolinella succinogenes N(2)OR, an enzyme that has an additional c-type-heme-containing domain. The structures of the N(2)OR domain and the c-type-heme-containing domain were modeled and the full-length structure was obtained by molecular docking simulation of these two domains. The orientation of the c-type-heme-containing domain relative to the N(2)OR domain is similar to that found in the other electron transfer complexes. PMID:21739254

  11. Optimization of Plasmon Decay Through Scattering and Hot Electron Transfer

    NASA Astrophysics Data System (ADS)

    DeJarnette, Drew

    Light incident on metal nanoparticles induce localized surface oscillations of conductive electrons, called plasmons, which is a means to control and manipulate light. Excited plasmons decay as either thermal energy as absorbed phonons or electromagnetic energy as scattered photons. An additional decay pathway for plasmons can exist for gold nanoparticles situated on graphene. Excited plasmons can decay directly to the graphene as through hot electron transfer. This dissertation begins by computational analysis of plasmon resonance energy and bandwidth as a function of particle size, shape, and dielectric environment in addition to diffractive coupled in lattices creating a Fano resonance. With this knowledge, plasmon resonance was probed with incident electrons using electron energy loss spectroscopy in a transmission electron microscope. Nanoparticles were fabricated using electron beam lithography on 50 nanometer thick silicon nitride with some particles fabricated with a graphene layer between the silicon nitride and metal structure. Plasmon resonance was compared between ellipses on and off graphene to characterize hot electron transfer as a means of plasmon decay. It was observed that the presence of graphene caused plasmon energy to decrease by as much as 9.8% and bandwidth to increase by 25%. Assuming the increased bandwidth was solely from electron transfer as an additional plasmon decay route, a 20% efficiency of plasmon decay to graphene was calculated for the particular ellipses analyzed.

  12. Photoinduced electron transfer processes in homogeneous and microheterogeneous solutions

    SciTech Connect

    Whitten, D.G.

    1991-10-01

    The focus of the work described in this report is on single electron transfer reactions of excited states which culminate in the formation of stable or metastable even electron species. For the most part the studies have involved even electron organic substrates which are thus converted photochemically to odd electron species and then at some stage reconvert to even electron products. These reactions generally fall into two rather different categories. In one set of studies we have examined reactions in which the metastable reagents generated by single electron transfer quenching of an excited state undergo novel fragmentation reactions, chiefly involving C-C bond cleavage. These reactions often culminate in novel and potentially useful chemical reactions and frequently have the potential for leading to new chemical products otherwise unaffordable by conventional reaction paths. In a rather different investigation we have also studied reactions in which single electron transfer quenching of an excited state is followed by subsequent reactions which lead reversibly to metastable two electron products which, often stable in themselves, can nonetheless be reacted with each other or with other reagents to regenerate the starting materials with release of energy. 66 refs., 9 figs., 1 tab.

  13. Complexes with Tunable Intramolecular Ferrocene to Ti(IV) Electronic Transitions: Models for Solid State Fe(II) to Ti(IV) Charge Transfer.

    PubMed

    Turlington, Michael D; Pienkos, Jared A; Carlton, Elizabeth S; Wroblewski, Karlee N; Myers, Alexis R; Trindle, Carl O; Altun, Zikri; Rack, Jeffrey J; Wagenknecht, Paul S

    2016-03-01

    Iron(II)-to-titanium(IV) metal-to-metal-charge transfer (MMCT) is important in the photosensitization of TiO2 by ferrocyanide, charge transfer in solid-state metal-oxide photocatalysts, and has been invoked to explain the blue color of sapphire, blue kyanite, and some lunar material. Herein, a series of complexes with alkynyl linkages between ferrocene (Fc) and Ti(IV) has been prepared and characterized by UV-vis spectroscopy and electrochemistry. Complexes with two ferrocene substituents include Cp2Ti(C2Fc)2, Cp*2Ti(C2Fc)2, and Cp2Ti(C4Fc)2. Complexes with a single ferrocene utilize a titanocene with a trimethylsilyl derivatized Cp ring, (TMS)Cp, and comprise the complexes (TMS)Cp2Ti(C2Fc)(C2R), where R = C6H5, p-C6H4CF3, and CF3. The complexes are compared to Cp2Ti(C2Ph)2, which lacks the second metal. Cyclic voltammetry for all complexes reveals a reversible Ti(IV/III) reduction wave and an Fe(II/III) oxidation that is irreversible for all complexes except (TMS)Cp2Ti(C2Fc)(C2CF3). All of the complexes with both Fc and Ti show an intense absorption (4000 M(-1)cm(-1) < ε < 8000 M(-1)cm(-1)) between 540 and 630 nm that is absent in complexes lacking a ferrocene donor. The energy of the absorption tracks with the difference between the Ti(IV/III) and Fe(III/II) reduction potentials, shifting to lower energy as the difference in potentials decreases. Reorganization energies, λ, have been determined using band shape analysis (2600 cm(-1) < λ < 5300 cm(-1)) and are in the range observed for other donor-acceptor complexes that have a ferrocene donor. Marcus-Hush-type analysis of the electrochemical and spectroscopic data are consistent with the assignment of the low-energy absorption as a MMCT band. TD-DFT analysis also supports this assignment. Solvatochromism is apparent for the MMCT band of all complexes, there being a bathochromic shift upon increasing polarizability of the solvent. The magnitude of the shift is dependent on both the electron density at Ti

  14. Dynamics of photoinduced electron transfer from adsorbed molecules into solids

    NASA Astrophysics Data System (ADS)

    Gundlach, L.; Ernstorfer, R.; Willig, F.

    2007-08-01

    Ultrafast interfacial electron transfer from the donor orbital of organic chromophores into empty electronic acceptor states of a semiconductor and of a metal was investigated by two-photon photoemission spectroscopy (2PPE). Experimental tools and procedures have been developed for carrying out wet-chemistry preparation of the molecule/solid interface. The organic chromophore perylene was investigated with several different bridge/anchor groups on TiO2(110). One perylene compound was investigated for comparison on Ag(110). Angle and polarization dependent 2PPE measurements revealed the orientation of the perylene chromophore on the surface as controlled by the adsorption geometry of the respective anchor group on TiO2. UPS measurements gave the position of the HOMO level of the chromophore with respect to the Fermi level of the solid. The donor level of each molecule was found high enough to fulfill the “wide band limit” of heterogeneous electron transfer dynamics. Time constants for heterogeneous electron transfer were extracted from 2PPE transients. A difference by a factor of four was found, 13 fs against 47 fs, when a conjugated bond was exchanged for a saturated bond in the otherwise identical bridge group. The two different contributions to the 2PPE transients arising firstly from the excited state of the chromophore and secondly from the injected electrons were separated by measuring the latter contribution separately in the case of instantaneous interfacial electron transfer realized with catechol as adsorbate. The time scales measured for the electron transfer step and for the subsequent electron escape process from the surface into the bulk of TiO2 showed both good agreement with recent theoretical predictions of other groups for these systems.

  15. Effects of quantum coherence in metalloprotein electron transfer

    NASA Astrophysics Data System (ADS)

    Dorner, Ross; Goold, John; Heaney, Libby; Farrow, Tristan; Vedral, Vlatko

    2012-09-01

    Many intramolecular electron transfer (ET) reactions in biology are mediated by metal centers in proteins. This process is commonly described by a model of diffusive hopping according to the semiclassical theories of Marcus and Hopfield. However, recent studies have raised the possibility that nontrivial quantum mechanical effects play a functioning role in certain biomolecular processes. Here, we investigate the potential effects of quantum coherence in biological ET by extending the semiclassical model to allow for the possibility of quantum coherent phenomena using a quantum master equation based on the Holstein Hamiltonian. We test the model on the structurally defined chain of seven iron-sulfur clusters in nicotinamide adenine dinucleotide plus hydrogen:ubiquinone oxidoreductase (complex I), a crucial respiratory enzyme and one of the longest chains of metal centers in biology. Using experimental parameters where possible, we find that, in limited circumstances, a small quantum mechanical contribution can provide a marked increase in the ET rate above the semiclassical diffusive-hopping rate. Under typical biological conditions, our model reduces to well-known diffusive behavior.

  16. Nanoantioxidant-driven plasmon enhanced proton-coupled electron transfer

    NASA Astrophysics Data System (ADS)

    Sotiriou, Georgios A.; Blattmann, Christoph O.; Deligiannakis, Yiannis

    2015-12-01

    Proton-coupled electron transfer (PCET) reactions involve the transfer of a proton and an electron and play an important role in a number of chemical and biological processes. Here, we describe a novel phenomenon, plasmon-enhanced PCET, which is manifested using SiO2-coated Ag nanoparticles functionalized with gallic acid (GA), a natural antioxidant molecule that can perform PCET. These GA-functionalized nanoparticles show enhanced plasmonic response at near-IR wavelengths, due to particle agglomeration caused by the GA molecules. Near-IR laser irradiation induces strong local hot-spots on the SiO2-coated Ag nanoparticles, as evidenced by surface enhanced Raman scattering (SERS). This leads to plasmon energy transfer to the grafted GA molecules that lowers the GA-OH bond dissociation enthalpy by at least 2 kcal mol-1 and therefore facilitates PCET. The nanoparticle-driven plasmon-enhancement of PCET brings together the so far unrelated research domains of nanoplasmonics and electron/proton translocation with significant impact on applications based on interfacial electron/proton transfer.Proton-coupled electron transfer (PCET) reactions involve the transfer of a proton and an electron and play an important role in a number of chemical and biological processes. Here, we describe a novel phenomenon, plasmon-enhanced PCET, which is manifested using SiO2-coated Ag nanoparticles functionalized with gallic acid (GA), a natural antioxidant molecule that can perform PCET. These GA-functionalized nanoparticles show enhanced plasmonic response at near-IR wavelengths, due to particle agglomeration caused by the GA molecules. Near-IR laser irradiation induces strong local hot-spots on the SiO2-coated Ag nanoparticles, as evidenced by surface enhanced Raman scattering (SERS). This leads to plasmon energy transfer to the grafted GA molecules that lowers the GA-OH bond dissociation enthalpy by at least 2 kcal mol-1 and therefore facilitates PCET. The nanoparticle-driven plasmon

  17. Electron transfer dynamics: Zusman equation versus exact theory.

    PubMed

    Shi, Qiang; Chen, Liping; Nan, Guangjun; Xu, Ruixue; Yan, YiJing

    2009-04-28

    The Zusman equation has been widely used to study the effect of solvent dynamics on electron transfer reactions. However, application of this equation is limited by the classical treatment of the nuclear degrees of freedom. In this paper, we revisit the Zusman equation in the framework of the exact hierarchical equations of motion formalism, and show that a high temperature approximation of the hierarchical theory is equivalent to the Zusman equation in describing electron transfer dynamics. Thus the exact hierarchical formalism naturally extends the Zusman equation to include quantum nuclear dynamics at low temperatures. This new finding has also inspired us to rescale the original hierarchical equations and incorporate a filtering algorithm to efficiently propagate the hierarchical equations. Numerical exact results are also presented for the electron transfer reaction dynamics and rate constant calculations. PMID:19405605

  18. Transferring the Learning of Teaching Models.

    ERIC Educational Resources Information Center

    Martin, William C.

    1981-01-01

    Ascertains the amount of transfer which takes place between on-campus training in teaching models and field application. Three models of teaching were selected--inquiry; role-playing; and synectics. The results illustrated that a preservice social studies program of model-based instruction fosters a decidedly high potential for transferring the…

  19. Photo-initiated ion formation from octaethyl-porphyrin and its zinc chelate as a model for electron transfer in reaction centers.

    PubMed Central

    Ballard, S G; Mauzerall, D

    1978-01-01

    Ion formation from the reaction of triplet (T) and ground state (P) octaethyl-porphyrin (OEP) and zinc octaethyl porphyrin (ZnOEP) and the corresponding cross-reactions have been measured in dry acetonitrile. A uniquely sensitive and fast conductance apparatus and a pulsed dye laser allowed the measurements to be made at the necessarily very low concentrations of T. The hemogeneous reaction of T (ZnOEP) and P (ZnOEP) occurs with rat constant k(1) = 2.0 x 10(8) M(-1)s(-1) and an ion yield of 67%. The similar homogeneous reaction of OEP has k(2) = 1.3 x 10(8)M(-1)s(-1) but an ion yield of only 3%. The cross-reaction of T (OEP) with P (ZnOEP) has k(3) = 1.5 x 10(8) M(-1)s(-1) and an ion yield of 27%, while the inverse cross-reaction of T (ZnOEP) with P (OEP) has k(4) = 3 x 10(8) M(-1)s(-1) and an ion yield of 20%. Thus, the rate constants are only slightly affected but the yields are sensitive to the porphyrin. The possible formation of the heterogeneous ions ZnOEP+ + OEP-, thermodynamically favored by 0.3 V over the homogeneous ions, has little influence on the observed yields. The data are explained by electron transfer and Coulomb field-electon spin-controlled escape of the initial ion-pair. PMID:708837

  20. Proton-coupled electron transfer with photoexcited metal complexes.

    PubMed

    Wenger, Oliver S

    2013-07-16

    Proton-coupled electron transfer (PCET) plays a crucial role in many enzymatic reactions and is relevant for a variety of processes including water oxidation, nitrogen fixation, and carbon dioxide reduction. Much of the research on PCET has focused on transfers between molecules in their electronic ground states, but increasingly researchers are investigating PCET between photoexcited reactants. This Account describes recent studies of excited-state PCET with d(6) metal complexes emphasizing work performed in my laboratory. Upon photoexcitation, some complexes release an electron and a proton to benzoquinone reaction partners. Others act as combined electron-proton acceptors in the presence of phenols. As a result, we can investigate photoinduced PCET involving electron and proton transfer in a given direction, a process that resembles hydrogen-atom transfer (HAT). In other studies, the photoexcited metal complexes merely serve as electron donors or electron acceptors because the proton donating and accepting sites are located on other parts of the molecular PCET ensemble. We and others have used this multisite design to explore so-called bidirectional PCET which occurs in many enzymes. A central question in all of these studies is whether concerted proton-electron transfer (CPET) can compete kinetically with sequential electron and proton transfer steps. Short laser pulses can trigger excited-state PCET, making it possible to investigate rapid reactions. Luminescence spectroscopy is a convenient tool for monitoring PCET, but unambiguous identification of reaction products can require a combination of luminescence spectroscopy and transient absorption spectroscopy. Nevertheless, in some cases, distinguishing between PCET photoproducts and reaction products formed by simple photoinduced electron transfer (ET) (reactions that don't include proton transfer) is tricky. Some of the studies presented here deal directly with this important problem. In one case study we

  1. Investigation of transferred-electron oscillations in diamond

    NASA Astrophysics Data System (ADS)

    Suntornwipat, N.; Majdi, S.; Gabrysch, M.; Isberg, J.

    2016-05-01

    The recent discovery of Negative Differential Mobility (NDM) in intrinsic single-crystalline diamond enables the development of devices for high frequency applications. The Transferred-Electron Oscillator (TEO) is one example of such devices that uses the benefit of NDM to generate continuous oscillations. This paper presents theoretical investigations of a diamond TEO in the temperature range of 110 to 140 K where NDM has been observed. Our simulations map out the parameter space in which transferred-electron oscillations are expected to occur for a specific device geometry. The results are promising and indicate that it is possible to fabricate diamond based TEO devices.

  2. Electron Transfer Dissociation Mass Spectrometry of Hemoglobin on Clinical Samples

    NASA Astrophysics Data System (ADS)

    Coelho Graça, Didia; Lescuyer, Pierre; Clerici, Lorella; Tsybin, Yury O.; Hartmer, Ralf; Meyer, Markus; Samii, Kaveh; Hochstrasser, Denis F.; Scherl, Alexander

    2012-10-01

    A mass spectrometry-based assay combining the specificity of selected reaction monitoring and the protein ion activation capabilities of electron transfer dissociation was developed and employed for the rapid identification of hemoglobin variants from whole blood without previous proteolytic cleavage. The analysis was performed in a robust ion trap mass spectrometer operating at nominal mass accuracy and resolution. Subtle differences in globin sequences, resulting with mass shifts of about one Da, can be unambiguously identified. These results suggest that mass spectrometry analysis of entire proteins using electron transfer dissociation can be employed on clinical samples in a workflow compatible with diagnostic applications.

  3. Modulating the electronic structure of chromophores by chemical substituents for efficient energy transfer: application to fluorone.

    PubMed

    Sand, Andrew M; Liu, Claire; Valentine, Andrew J S; Mazziotti, David A

    2014-08-01

    Strong electron correlation within a quasi-spin model of chromophores was recently shown to enhance exciton energy transfer significantly. Here we investigate how the modulation of the electronic structure of the chromophores by chemical substitution can enhance energy-transfer efficiency. Unlike previous work that does not consider the direct effect of the electronic structure on exciton dynamics, we add chemical substituents to the fluorone dimer to study the effect of electron-donating and electron-withdrawing substituents on exciton energy transfer. The exciton dynamics are studied from the solution of a quantum Liouville equation for an open system whose model Hamiltonian is derived from excited-state electronic structure calculations. Both van der Waals energies and coupling energies, arising from the Hellmann-Feynman force generated upon transferring the dimers from infinity to a finite separation, are built into the model Hamiltonian. Though these two effects are implicitly treated in dipole-based models, their explicit and separate treatment as discussed here is critical to forging the correct connection with the electronic structure calculations. We find that the addition of electron-donating substituents to the fluorone system results in an increase in exciton-transfer rates by factors ranging from 1.3-1.9. The computed oscillator strength is consistent with the recent experimental results on a larger heterodimer system containing fluorone. The oscillator strength increases with the addition of electron-donating substituents. Our results indicate that the study of chromophore networks via electronic structure will help in the future design of efficient synthetic light-harvesting systems. PMID:25062094

  4. Electron transfer in peptides: on the formation of silver nanoparticles.

    PubMed

    Kracht, Sonja; Messerer, Matthias; Lang, Matthieu; Eckhardt, Sonja; Lauz, Miriam; Grobéty, Bernard; Fromm, Katharina M; Giese, Bernd

    2015-03-01

    Some microorganisms perform anaerobic mineral respiration by reducing metal ions to metal nanoparticles, using peptide aggregates as medium for electron transfer (ET). Such a reaction type is investigated here with model peptides and silver as the metal. Surprisingly, Ag(+) ions bound by peptides with histidine as the Ag(+)-binding amino acid and tyrosine as photoinducible electron donor cannot be reduced to Ag nanoparticles (AgNPs) under ET conditions because the peptide prevents the aggregation of Ag atoms to form AgNPs. Only in the presence of chloride ions, which generate AgCl microcrystals in the peptide matrix, does the synthesis of AgNPs occur. The reaction starts with the formation of 100 nm Ag@AgCl/peptide nanocomposites which are cleaved into 15 nm AgNPs. This defined transformation from large nanoparticles into small ones is in contrast to the usually observed Ostwald ripening processes and can be followed in detail by studying time-resolved UV/Vis spectra which exhibit an isosbestic point. PMID:25663127

  5. Electron-Wavepacket Reaction Dynamics in Proton Transfer of Formamide

    NASA Astrophysics Data System (ADS)

    Nagashima, Kengo; Takatsuka, Kazuo

    2009-10-01

    We apply the semiclassical Ehrenfest theory, which provides electron wavepacket dynamics coupled to nuclear motion, to a study of water-assisted proton relay in formamide compared with a forced proton transfer in gas phase, both of which are associated with the tautomerization. We start with the enol (imidic acid) form HO-CH═NH and track its proton transfer process to the keto (amide) form O═CH-NH2. Identifying the fact that this is indeed a "proton transfer process" rather than hydrogen-atom migration in terms of radical character on the proton, we show a collective quantum flux of electrons, which flows backward against the proton motion. This backward flux compensates the electrons tightly covering the proton, as represented in the Mulliken charge. The enol form formamide is one of the simplest species in the group O═CR1-NHR2, which is a unit of polypeptide. In the gas phase, the nitrogen atom may have a pyramidal structure as in the ammonium molecule; therefore, the C-N bond may allow low barrier rotation along it. This rotation is strongly prohibited by the formation of the double bond C═N induced by the proton transfer. Not only the dynamical process of proton transfer itself but also the electronic structures left behind are greatly affected by the presence of water molecule(s) and polar solvents. In discussing the relative stability of the formamide after the proton transfer, the following resonance structures are frequently mentioned, O--CH═N+H2 ↔ O═CH-NH2. Here we address the dynamical manifestation of the resonance structures in terms of our dynamical electron theory.

  6. Electron transfer at semiconducting metal dichalcogenide/liquid electrolyte interfaces

    SciTech Connect

    Howard, J.N.

    1992-01-01

    Charge transfer at semiconductor/electrolyte interfaces is the critical process in photoelectrochemical systems. Many aspects of the theory for these interfaces have yet to be experimentally verified. There are few reliable measurements of the fundamental electron transfer rate at nonilluminated semiconductors. This situation stems from experimental limitations imposed by most semiconductor electrode surfaces. Layered metal dichalcogenide semiconductors have excellent properties as semiconductor electrodes, but edge sites and crystal defects must be masked so only the defect-free basal plane of the two-dimensional material is exposed to solution. Conventional epoxy encapsulation of the crystal epoxy can introduce deleterious effects. A minielectrochemical cell was developed to perform experiments in a single drop of electrolyte held against the working electrode. The electrochemical behavior and operational considerations of the cell for aqueous and nonaqueous systems were investigated. Spatially-resolved electrochemistry was demonstrated for n-WSe[sub 2] and highly ordered pyrolytic graphite. The minicell was used to investigate electron transfer at nonilluminated n-WSe[sub 2]/dimethylferrocene[sup +/0] interfaces. This semiconductor is resistant to corrosion and has stable interfacial energetics. Interfaces with excellent diode behavior could be obtained by probing different regions of the surface. Electron transfer at these high quality surfaces was studied over an extensive solution concentration range. The rate of electron transfer was independent of solution acceptor concentration from 5 [mu]M to 0.25 M. The electron transfer data can be explained by assuming a surface-state mediate mechanism. A second metal dichalcogenide, n-SnS[sub 2], was investigated to compare the behavior of this wide band gap material to the narrow band gap n-WSe[sub 2]. The n-SnS[sub 2] electrodes displayed undesirable electrochemical effects in several solvent systems.

  7. Excitation of the ligand-to-metal charge transfer band induces electron tunnelling in azurin

    SciTech Connect

    Baldacchini, Chiara; Bizzarri, Anna Rita; Cannistraro, Salvatore

    2014-03-03

    Optical excitation of azurin blue copper protein immobilized on indium-tin oxide, in resonance with its ligand-to-metal charge transfer absorption band, resulted in a light-induced current tunnelling within the protein milieu. The related electron transport rate is estimated to be about 10{sup 5} s{sup −1}. A model based on resonant tunnelling through an azurin excited molecular state is proposed. The capability of controlling electron transfer processes through light pulses opens interesting perspectives for implementation of azurin in bio-nano-opto-electronic devices.

  8. Cycloreversion of β-lactams via photoinduced electron transfer.

    PubMed

    Pérez-Ruiz, Raúl; Sáez, Jose A; Jiménez, M Consuelo; Miranda, Miguel A

    2014-11-14

    The radical anions of β-lactams, photogenerated in the presence of DABCO as an electron donor, undergo cycloreversion via N-C4 bond cleavage, back electron transfer and final C2-C3 bond cleavage, leading to olefins. The involved intermediates are 1,4-radical anions and 1,4-biradicals. The experimental observations are consistent with the results of DFT calculations. PMID:25223340

  9. Vibrational and Electronic Energy Transfer and Dissociation of Diatomic Molecules by Electron Collisions

    NASA Technical Reports Server (NTRS)

    Huo, Winifred M.; Langhoff, Stephen R. (Technical Monitor)

    1995-01-01

    At high altitudes and velocities equal to or greater than the geosynchronous return velocity (10 kilometers per second), the shock layer of a hypersonic flight will be in thermochemical nonequilibrium and partially ionized. The amount of ionization is determined by the velocity. For a trans atmospheric flight of 10 kilometers per second and at an altitude of 80 kilometers, a maximum of 1% ionization is expected. At a velocity of 12 - 17 kilometer per second, such as a Mars return mission, up to 30% of the atoms and molecules in the flow field will be ionized. Under those circumstances, electrons play an important role in determining the internal states of atoms and molecules in the flow field and hence the amount of radiative heat load and the distance it takes for the flow field to re-establish equilibrium. Electron collisions provide an effective means of transferring energy even when the electron number density is as low as 1%. Because the mass of an electron is 12,760 times smaller than the reduced mass of N2, its average speed, and hence its average collision frequency, is more than 100 times larger. Even in the slightly ionized regime with only 1% electrons, the frequency of electron-molecule collisions is equal to or larger than that of molecule-molecule collisions, an important consideration in the low density part of the atmosphere. Three electron-molecule collision processes relevant to hypersonic flows will be considered: (1) vibrational excitation/de-excitation of a diatomic molecule by electron impact, (2) electronic excitation/de-excitation, and (3) dissociative recombination in electron-diatomic ion collisions. A review of available data, both theory and experiment, will be given. Particular attention will be paid to tailoring the molecular physics to the condition of hypersonic flows. For example, the high rotational temperatures in a hypersonic flow field means that most experimental data carried out under room temperatures are not applicable. Also

  10. A new semiclassical decoupling scheme for electronic transitions in molecular collisions - Application to vibrational-to-electronic energy transfer

    NASA Technical Reports Server (NTRS)

    Lee, H.-W.; Lam, K. S.; Devries, P. L.; George, T. F.

    1980-01-01

    A new semiclassical decoupling scheme (the trajectory-based decoupling scheme) is introduced in a computational study of vibrational-to-electronic energy transfer for a simple model system that simulates collinear atom-diatom collisions. The probability of energy transfer (P) is calculated quasiclassically using the new scheme as well as quantum mechanically as a function of the atomic electronic-energy separation (lambda), with overall good agreement between the two sets of results. Classical mechanics with the new decoupling scheme is found to be capable of predicting resonance behavior whereas an earlier decoupling scheme (the coordinate-based decoupling scheme) failed. Interference effects are not exhibited in P vs lambda results.

  11. Fluctuations in Biological and Bioinspired Electron-Transfer Reactions

    PubMed Central

    Skourtis, Spiros S.; Waldeck, David H.; Beratan, David N.

    2010-01-01

    Central to theories of electron transfer (ET) is the idea that nuclear motion generates a transition state that enables electron flow to proceed, but nuclear motion also induces fluctuations in the donor-acceptor (DA) electronic coupling that is the rate-limiting parameter for nonadiabatic ET. The interplay between the DA energy gap and DA coupling fluctuations is particularly noteworthy in biological ET, where flexible protein and mobile water bridges take center stage. Here, we discuss the critical timescales at play for ET reactions in fluctuating media, highlighting issues of the Condon approximation, average medium versus fluctuation-controlled electron tunneling, gated and solvent relaxation controlled electron transfer, and the influence of inelastic tunneling on electronic coupling pathway interferences. Taken together, one may use this framework to establish principles to describe how macromolecular structure and structural fluctuations influence ET reactions. This framework deepens our understanding of ET chemistry in fluctuating media. Moreover, it provides a unifying perspective for biophysical charge-transfer processes and helps to frame new questions associated with energy harvesting and transduction in fluctuating media. PMID:20192814

  12. ELUCIDATING THE ROLE OF ELECTRON TRANSFER MEDIATORS IN REDUCTIVE TRANSFORMATIONS IN NATURAL SEDIMENTS

    EPA Science Inventory

    To study the identity and reactivity of electron transfer mediators (ETMs) in natural sediments, the reduction kinetics of a glass bead-azo dye complex were measured in abiotic and biotic model systems, as well as in natural sediments. In abiotic model systems, the bead-dye comp...

  13. Monitoring the Coherent Vibrational Control of Electronic Excitation Transfer Using Ultrafast Pump-Probe Polarization Spectroscopy

    NASA Astrophysics Data System (ADS)

    Biggs, Jason; Cina, Jeffrey

    2010-03-01

    The interplay between nuclear and electronic degrees of freedom in molecular energy-transfer complexes is a subject of current interest. We have proposed a method to use coherent nuclear motion to control the transfer of electronic excitation energy between donor and acceptor moieties in electronically coupled dimers. The underlying electronic and nuclear motion at the level of quantum mechanical amplitudes can be observed using nonlinear wave-packet interferometry(nl-WPI), a form of fluorescence-detected multidimensional electronic spectroscopy. In our control scheme, coherent nuclear motion is induced in the acceptor chromophore prior to direct electronic excitation of the donor. This nuclear motion affects the instantaneous resonance conditions between donor and acceptor moieties and thus affects subsequent energy transfer dynamics. We have developed the framework to simulate four-pulse nl-WPI experiments, and the pump-probe limit thereof, on energy-transfer systems after interaction with a control pulse that induces nuclear motion. We present simulations in the pump-probe limit from model energy-transfer systems subjected to prior impulsive vibrational excitation, and show how pulse polarization can be used to infer electronic dynamics from isotropically oriented dimers.

  14. Tryptophan-to-heme electron transfer in ferrous myoglobins

    PubMed Central

    Monni, Roberto; Al Haddad, André; van Mourik, Frank; Auböck, Gerald; Chergui, Majed

    2015-01-01

    It was recently demonstrated that in ferric myoglobins (Mb) the fluorescence quenching of the photoexcited tryptophan 14 (*Trp14) residue is in part due to an electron transfer to the heme porphyrin (porph), turning it to the ferrous state. However, the invariance of *Trp decay times in ferric and ferrous Mbs raises the question as to whether electron transfer may also be operative in the latter. Using UV pump/visible probe transient absorption, we show that this is indeed the case for deoxy-Mb. We observe that the reduction generates (with a yield of about 30%) a low-valence Fe–porphyrin π [FeII(porph●−)] -anion radical, which we observe for the first time to our knowledge under physiological conditions. We suggest that the pathway for the electron transfer proceeds via the leucine 69 (Leu69) and valine 68 (Val68) residues. The results on ferric Mbs and the present ones highlight the generality of Trp–porphyrin electron transfer in heme proteins. PMID:25902517

  15. 77 FR 40459 - Electronic Fund Transfers (Regulation E); Correction

    Federal Register 2010, 2011, 2012, 2013, 2014

    2012-07-10

    ..., the Bureau published the Final Rule (77 FR 6194), which implements the Electronic Fund Transfer Act... changes made to Sec. 1005.3(a) in the interim final rule published on December 27, 2011 (76 FR 81020). The... on December 27, 2011 (76 FR 81020) for which the Bureau found good cause to conclude that...

  16. 75 FR 52485 - Electronic Funds Transfer of Depository Taxes; Correction

    Federal Register 2010, 2011, 2012, 2013, 2014

    2010-08-26

    ... 23, 2010 (75 FR 51707), contain errors that may prove to be misleading and are in need of... notice of public hearing (REG-153340-09), which was the subject of FR Doc. 2010-20737, is corrected as... Internal Revenue Service 26 CFR Parts 1, 31, 40, and 301 RIN 1545-BJ13 Electronic Funds Transfer...

  17. 76 FR 708 - Electronic Funds Transfer of Depository Taxes; Correction

    Federal Register 2010, 2011, 2012, 2013, 2014

    2011-01-06

    ... regulations (TD 9507) that were published in the Federal Register on Tuesday, December 7, 2010 (75 FR 75897... Accordingly, the final and temporary regulations (TD 9507), that are the subject of FR Doc. 2010-30526, are... Internal Revenue Service 26 CFR Parts 1, 31, 40, and 301 RIN 1545-BJ13 Electronic Funds Transfer...

  18. Ultrafast electron transfer via a bridge-extended donor orbital

    NASA Astrophysics Data System (ADS)

    Ernstorfer, R.; Gundlach, L.; Felber, S.; Storck, W.; Eichberger, R.; Zimmermann, C.; Willig, F.

    Electron transfer from the excited aromatic donor perylene to TiO2 occurred with 10 fs time constant via the conjugated -CH=CH- bridge unit compared to 57 fs in the presence of the saturated -CH2-CH2- bridge unit.

  19. HCFA proposal takes zip out of EFTs (electronic fund transfers).

    PubMed

    Gardner, E

    1991-09-01

    HCFA recently proposed reimbursing providers through electronic fund transfer, a low-cost, fast, efficient means of deposit and payment now commonplace throughout the country. EFTs promise hospitals benefits such as clerical savings through computerization, but regulations proposed by HCFA would inhibit the increased efficiency and cash flow such transactions also can offer. PMID:10112504

  20. Photoinduced electron transfer in binary blends of conjugated polymers

    SciTech Connect

    Jenekhe, A.A.; Paor, L.R. de; Chen, X.L.; Tarkka, R.M.

    1996-10-01

    The authors report observations concerning the intermolecular photoinduced electron transfer through blends of n-type/p-type {pi}-conjugated organic polymers. The results of transient absorption spectroscopy, fluorescence quenching analysis, and delocalized radical ion pair generation studies imply that these materials are supramolecular materials.

  1. 31 CFR 208.3 - Payment by electronic funds transfer.

    Code of Federal Regulations, 2010 CFR

    2010-07-01

    ... 31 Money and Finance: Treasury 2 2010-07-01 2010-07-01 false Payment by electronic funds transfer. 208.3 Section 208.3 Money and Finance: Treasury Regulations Relating to Money and Finance (Continued) FISCAL SERVICE, DEPARTMENT OF THE TREASURY FINANCIAL MANAGEMENT SERVICE MANAGEMENT OF FEDERAL...

  2. 75 FR 51707 - Electronic Funds Transfer of Depository Taxes

    Federal Register 2010, 2011, 2012, 2013, 2014

    2010-08-23

    ...This document contains proposed regulations relating to Federal tax deposits (FTDs) by Electronic Funds Transfer (EFT). The proposed regulations affect all taxpayers that currently use FTD coupons. In response to the decision of the Financial Management Service to discontinue the system that processes FTD coupons, the proposed regulations provide rules under which depositors must use EFT for......

  3. A molecularly based theory for electron transfer reorganization energy

    NASA Astrophysics Data System (ADS)

    Zhuang, Bilin; Wang, Zhen-Gang

    2015-12-01

    Using field-theoretic techniques, we develop a molecularly based dipolar self-consistent-field theory (DSCFT) for charge solvation in pure solvents under equilibrium and nonequilibrium conditions and apply it to the reorganization energy of electron transfer reactions. The DSCFT uses a set of molecular parameters, such as the solvent molecule's permanent dipole moment and polarizability, thus avoiding approximations that are inherent in treating the solvent as a linear dielectric medium. A simple, analytical expression for the free energy is obtained in terms of the equilibrium and nonequilibrium electrostatic potential profiles and electric susceptibilities, which are obtained by solving a set of self-consistent equations. With no adjustable parameters, the DSCFT predicts activation energies and reorganization energies in good agreement with previous experiments and calculations for the electron transfer between metallic ions. Because the DSCFT is able to describe the properties of the solvent in the immediate vicinity of the charges, it is unnecessary to distinguish between the inner-sphere and outer-sphere solvent molecules in the calculation of the reorganization energy as in previous work. Furthermore, examining the nonequilibrium free energy surfaces of electron transfer, we find that the nonequilibrium free energy is well approximated by a double parabola for self-exchange reactions, but the curvature of the nonequilibrium free energy surface depends on the charges of the electron-transferring species, contrary to the prediction by the linear dielectric theory.

  4. CORRELATING ELECTRONIC AND VIBRATIONAL MOTIONS IN CHARGE TRANSFER SYSTEMS

    SciTech Connect

    Khalil, Munira

    2014-06-27

    The goal of this research program was to measure coupled electronic and nuclear motions during photoinduced charge transfer processes in transition metal complexes by developing and using novel femtosecond spectroscopies. The scientific highlights and the resulting scientific publications from the DOE supported work are outlined in the technical report.

  5. 76 FR 81019 - Electronic Fund Transfers (Regulation E)

    Federal Register 2010, 2011, 2012, 2013, 2014

    2011-12-27

    ... this rule. See 76 FR 43569 (July 21, 2011). \\4\\ Public Law 111-203, section 1002(14) (defining... response to a notice published at 76 FR 75825 (Dec. 5, 2011) concerning its efforts to identify priorities... that provide electronic fund transfer services jointly may contract among themselves to comply with...

  6. Nanoantioxidant-driven plasmon enhanced proton-coupled electron transfer.

    PubMed

    Sotiriou, Georgios A; Blattmann, Christoph O; Deligiannakis, Yiannis

    2016-01-14

    Proton-coupled electron transfer (PCET) reactions involve the transfer of a proton and an electron and play an important role in a number of chemical and biological processes. Here, we describe a novel phenomenon, plasmon-enhanced PCET, which is manifested using SiO2-coated Ag nanoparticles functionalized with gallic acid (GA), a natural antioxidant molecule that can perform PCET. These GA-functionalized nanoparticles show enhanced plasmonic response at near-IR wavelengths, due to particle agglomeration caused by the GA molecules. Near-IR laser irradiation induces strong local hot-spots on the SiO2-coated Ag nanoparticles, as evidenced by surface enhanced Raman scattering (SERS). This leads to plasmon energy transfer to the grafted GA molecules that lowers the GA-OH bond dissociation enthalpy by at least 2 kcal mol(-1) and therefore facilitates PCET. The nanoparticle-driven plasmon-enhancement of PCET brings together the so far unrelated research domains of nanoplasmonics and electron/proton translocation with significant impact on applications based on interfacial electron/proton transfer. PMID:26505730

  7. A molecularly based theory for electron transfer reorganization energy

    SciTech Connect

    Zhuang, Bilin; Wang, Zhen-Gang

    2015-12-14

    Using field-theoretic techniques, we develop a molecularly based dipolar self-consistent-field theory (DSCFT) for charge solvation in pure solvents under equilibrium and nonequilibrium conditions and apply it to the reorganization energy of electron transfer reactions. The DSCFT uses a set of molecular parameters, such as the solvent molecule’s permanent dipole moment and polarizability, thus avoiding approximations that are inherent in treating the solvent as a linear dielectric medium. A simple, analytical expression for the free energy is obtained in terms of the equilibrium and nonequilibrium electrostatic potential profiles and electric susceptibilities, which are obtained by solving a set of self-consistent equations. With no adjustable parameters, the DSCFT predicts activation energies and reorganization energies in good agreement with previous experiments and calculations for the electron transfer between metallic ions. Because the DSCFT is able to describe the properties of the solvent in the immediate vicinity of the charges, it is unnecessary to distinguish between the inner-sphere and outer-sphere solvent molecules in the calculation of the reorganization energy as in previous work. Furthermore, examining the nonequilibrium free energy surfaces of electron transfer, we find that the nonequilibrium free energy is well approximated by a double parabola for self-exchange reactions, but the curvature of the nonequilibrium free energy surface depends on the charges of the electron-transferring species, contrary to the prediction by the linear dielectric theory.

  8. Electronic Coupling Dependence of Ultrafast Interfacial Electron Transfer on Nanocrystalline Thin Films and Single Crystal

    SciTech Connect

    Lian, Tianquan

    2014-04-22

    The long-term goal of the proposed research is to understand electron transfer dynamics in nanoparticle/liquid interface. This knowledge is essential to many semiconductor nanoparticle based devices, including photocatalytic waste degradation and dye sensitized solar cells.

  9. Electron Transfer Dissociation: Effects of Cation Charge State on Product Partitioning in Ion/Ion Electron Transfer to Multiply Protonated Polypeptides

    PubMed Central

    Liu, Jian; McLuckey, Scott A.

    2012-01-01

    The effect of cation charge state on product partitioning in the gas-phase ion/ion electron transfer reactions of multiply protonated tryptic peptides, model peptides, and relatively large peptides with singly charged radical anions has been examined. In particular, partitioning into various competing channels, such as proton transfer (PT) versus electron transfer (ET), electron transfer with subsequent dissociation (ETD) versus electron transfer with no dissociation (ET,noD), and fragmentation of backbone bonds versus fragmentation of side chains, was measured quantitatively as a function of peptide charge state to allow insights to be drawn about the fundamental aspects of ion/ion reactions that lead to ETD. The ET channel increases relative to the PT channel, ETD increases relative to ET,noD, and fragmentation at backbone bonds increases relative to side-chain cleavages as cation charge state increases. The increase in ET versus PT with charge state is consistent with a Landau-Zener based curve-crossing model. An optimum charge state for ET is predicted by the model for the ground state-to-ground state reaction. However, when the population of excited product ion states is considered, it is possible that a decrease in ET efficiency as charge state increases will not be observed due to the possibility of the population of excited electronic states of the products. Several factors can contribute to the increase in ETD versus ET,noD and backbone cleavage versus side-chain losses. These factors include an increase in reaction exothermicity and charge state dependent differences in precursor and product ion structures, stabilities, and sites of protonation. PMID:23264749

  10. Electron Transfer Dissociation: Effects of Cation Charge State on Product Partitioning in Ion/Ion Electron Transfer to Multiply Protonated Polypeptides.

    PubMed

    Liu, Jian; McLuckey, Scott A

    2012-12-15

    The effect of cation charge state on product partitioning in the gas-phase ion/ion electron transfer reactions of multiply protonated tryptic peptides, model peptides, and relatively large peptides with singly charged radical anions has been examined. In particular, partitioning into various competing channels, such as proton transfer (PT) versus electron transfer (ET), electron transfer with subsequent dissociation (ETD) versus electron transfer with no dissociation (ET,noD), and fragmentation of backbone bonds versus fragmentation of side chains, was measured quantitatively as a function of peptide charge state to allow insights to be drawn about the fundamental aspects of ion/ion reactions that lead to ETD. The ET channel increases relative to the PT channel, ETD increases relative to ET,noD, and fragmentation at backbone bonds increases relative to side-chain cleavages as cation charge state increases. The increase in ET versus PT with charge state is consistent with a Landau-Zener based curve-crossing model. An optimum charge state for ET is predicted by the model for the ground state-to-ground state reaction. However, when the population of excited product ion states is considered, it is possible that a decrease in ET efficiency as charge state increases will not be observed due to the possibility of the population of excited electronic states of the products. Several factors can contribute to the increase in ETD versus ET,noD and backbone cleavage versus side-chain losses. These factors include an increase in reaction exothermicity and charge state dependent differences in precursor and product ion structures, stabilities, and sites of protonation. PMID:23264749

  11. Single cell activity reveals direct electron transfer in methanotrophic consortia.

    PubMed

    McGlynn, Shawn E; Chadwick, Grayson L; Kempes, Christopher P; Orphan, Victoria J

    2015-10-22

    Multicellular assemblages of microorganisms are ubiquitous in nature, and the proximity afforded by aggregation is thought to permit intercellular metabolic coupling that can accommodate otherwise unfavourable reactions. Consortia of methane-oxidizing archaea and sulphate-reducing bacteria are a well-known environmental example of microbial co-aggregation; however, the coupling mechanisms between these paired organisms is not well understood, despite the attention given them because of the global significance of anaerobic methane oxidation. Here we examined the influence of interspecies spatial positioning as it relates to biosynthetic activity within structurally diverse uncultured methane-oxidizing consortia by measuring stable isotope incorporation for individual archaeal and bacterial cells to constrain their potential metabolic interactions. In contrast to conventional models of syntrophy based on the passage of molecular intermediates, cellular activities were found to be independent of both species intermixing and distance between syntrophic partners within consortia. A generalized model of electric conductivity between co-associated archaea and bacteria best fit the empirical data. Combined with the detection of large multi-haem cytochromes in the genomes of methanotrophic archaea and the demonstration of redox-dependent staining of the matrix between cells in consortia, these results provide evidence for syntrophic coupling through direct electron transfer. PMID:26375009

  12. Single cell activity reveals direct electron transfer in methanotrophic consortia

    NASA Astrophysics Data System (ADS)

    McGlynn, Shawn E.; Chadwick, Grayson L.; Kempes, Christopher P.; Orphan, Victoria J.

    2015-10-01

    Multicellular assemblages of microorganisms are ubiquitous in nature, and the proximity afforded by aggregation is thought to permit intercellular metabolic coupling that can accommodate otherwise unfavourable reactions. Consortia of methane-oxidizing archaea and sulphate-reducing bacteria are a well-known environmental example of microbial co-aggregation; however, the coupling mechanisms between these paired organisms is not well understood, despite the attention given them because of the global significance of anaerobic methane oxidation. Here we examined the influence of interspecies spatial positioning as it relates to biosynthetic activity within structurally diverse uncultured methane-oxidizing consortia by measuring stable isotope incorporation for individual archaeal and bacterial cells to constrain their potential metabolic interactions. In contrast to conventional models of syntrophy based on the passage of molecular intermediates, cellular activities were found to be independent of both species intermixing and distance between syntrophic partners within consortia. A generalized model of electric conductivity between co-associated archaea and bacteria best fit the empirical data. Combined with the detection of large multi-haem cytochromes in the genomes of methanotrophic archaea and the demonstration of redox-dependent staining of the matrix between cells in consortia, these results provide evidence for syntrophic coupling through direct electron transfer.

  13. Polarization transfer of bremsstrahlung arising from spin-polarized electrons.

    PubMed

    Märtin, R; Weber, G; Barday, R; Fritzsche, Y; Spillmann, U; Chen, W; DuBois, R D; Enders, J; Hegewald, M; Hess, S; Surzhykov, A; Thorn, D B; Trotsenko, S; Wagner, M; Winters, D F A; Yerokhin, V A; Stöhlker, Th

    2012-06-29

    We report on a study of the polarization transfer between transversely polarized incident electrons and the emitted x rays for electron-atom bremsstrahlung. By means of Compton polarimetry we performed for the first time an energy-differential measurement of the complete properties of bremsstrahlung emission related to linear polarization, i.e., the degree of linear polarization as well as the orientation of the polarization axis. For the high-energy end of the bremsstrahlung continuum the experimental results for both observables show a high sensitivity on the initial electron spin polarization and prove that the polarization orientation is virtually independent of the photon energy. PMID:23004992

  14. Photoinduced Electron Transfer to Engineered Surface Traps in CdSe Nanocrystals

    NASA Astrophysics Data System (ADS)

    Califano, Marco; Zhu, Haiming; Yang, Ye; Hyeon-Deuk, Kim; Song, Nianhui; Wang, Youwei; Zhang, Wenqing; Prezhdo, Oleg; Lian, Tianquan

    2014-03-01

    Quantum confined nanomaterials, such as semiconductor nanocrystals (NCs), have emerged in the past decade as a new class of materials for solar energy conversion. An appropriate model for describing photoinduced charge transfer in these systems is, however, still lacking. Recently we observed that the rate of photoinduced electron transfer from CdSe NCs to molecular acceptors increased with decreasing NC size (and increasing driving force) exhibiting a lack of Marcus inverted regime behaviour over an apparent driving force range of 0-1.3 V. Our atomistic semiempirical pseudopotential calculations show that an Auger assisted ET mechanism, in which the transfer of the electron is coupled to the excitation of the hole, can circumvent the unfavourable Frank-Condon overlap (that is a signature of inter- or intra- molecular electron transfer) in the Marcus inverted regime, reproducing our observed ET rates with remarkable accuracy. We conclude that electron transfer from quantum dots differs from electron transfer originating from both molecules and bulk semiconductors. It proceeds via a novel Auger-assisted pathway which we believe is available to most excitonic nanomaterials. This new finding will have a major impact on the design of next generation solar energy harvesting devices.

  15. 76 FR 35219 - Federal Acquisition Regulation; Information Collection; Payment by Electronic Fund Transfer

    Federal Register 2010, 2011, 2012, 2013, 2014

    2011-06-16

    ... contract by electronic fund transfer (EFT). The information necessary to make the EFT transaction is... Regulation; Information Collection; Payment by Electronic Fund Transfer AGENCY: Department of Defense (DOD... extension of a previously approved information collection requirement concerning payment by electronic...

  16. Photocurrents from photosystem II in a metal oxide hybrid system: Electron transfer pathways.

    PubMed

    Brinkert, Katharina; Le Formal, Florian; Li, Xiaoe; Durrant, James; Rutherford, A William; Fantuzzi, Andrea

    2016-09-01

    We have investigated the nature of the photocurrent generated by Photosystem II (PSII), the water oxidizing enzyme, isolated from Thermosynechococcus elongatus, when immobilized on nanostructured titanium dioxide on an indium tin oxide electrode (TiO2/ITO). We investigated the properties of the photocurrent from PSII when immobilized as a monolayer versus multilayers, in the presence and absence of an inhibitor that binds to the site of the exchangeable quinone (QB) and in the presence and absence of exogenous mobile electron carriers (mediators). The findings indicate that electron transfer occurs from the first quinone (QA) directly to the electrode surface but that the electron transfer through the nanostructured metal oxide is the rate-limiting step. Redox mediators enhance the photocurrent by taking electrons from the nanostructured semiconductor surface to the ITO electrode surface not from PSII. This is demonstrated by photocurrent enhancement using a mediator incapable of accepting electrons from PSII. This model for electron transfer also explains anomalies reported in the literature using similar and related systems. The slow rate of the electron transfer step in the TiO2 is due to the energy level of electron injection into the semiconducting material being below the conduction band. This limits the usefulness of the present hybrid electrode. Strategies to overcome this kinetic limitation are discussed. PMID:26946088

  17. Syntrophic growth via quinone-mediated interspecies electron transfer

    PubMed Central

    Smith, Jessica A.; Nevin, Kelly P.; Lovley, Derek R.

    2015-01-01

    The mechanisms by which microbial species exchange electrons are of interest because interspecies electron transfer can expand the metabolic capabilities of microbial communities. Previous studies with the humic substance analog anthraquinone-2,6-disulfonate (AQDS) suggested that quinone-mediated interspecies electron transfer (QUIET) is feasible, but it was not determined if sufficient energy is available from QUIET to support the growth of both species. Furthermore, there have been no previous studies on the mechanisms for the oxidation of anthrahydroquinone-2,6-disulfonate (AHQDS). A co-culture of Geobacter metallireducens and G. sulfurreducens metabolized ethanol with the reduction of fumarate much faster in the presence of AQDS, and there was an increase in cell protein. G. sulfurreducens was more abundant, consistent with G. sulfurreducens obtaining electrons from acetate that G. metallireducens produced from ethanol, as well as from AHQDS. Co-cultures initiated with a citrate synthase-deficient strain of G. sulfurreducens that was unable to use acetate as an electron donor also metabolized ethanol with the reduction of fumarate and cell growth, but acetate accumulated over time. G. sulfurreducens and G. metallireducens were equally abundant in these co-cultures reflecting the inability of the citrate synthase-deficient strain of G. sulfurreducens to metabolize acetate. Evaluation of the mechanisms by which G. sulfurreducens accepts electrons from AHQDS demonstrated that a strain deficient in outer-surface c-type cytochromes that are required for AQDS reduction was as effective at QUIET as the wild-type strain. Deletion of additional genes previously implicated in extracellular electron transfer also had no impact on QUIET. These results demonstrate that QUIET can yield sufficient energy to support the growth of both syntrophic partners, but that the mechanisms by which electrons are derived from extracellular hydroquinones require further investigation. PMID

  18. Time prediction model of subway transfer.

    PubMed

    Zhou, Yuyang; Yao, Lin; Gong, Yi; Chen, Yanyan

    2016-01-01

    Walking time prediction aims to deduce waiting time and travel time for passengers and provide a quantitative basis for the subway schedule management. This model is founded based on transfer passenger flow and type of pedestrian facilities. Chaoyangmen station in Beijing was taken as the learning set to obtain the relationship between transfer walking speed and passenger volume. The sectional passenger volume of different facilities was calculated related to the transfer passage classification. Model parameters were computed by curve fitting with respect to various pedestrian facilities. The testing set contained four transfer stations with large passenger volume. It is validated that the established model is effective and practical. The proposed model offers a real-time prediction method with good applicability. It can provide transfer scheme reference for passengers, meanwhile, improve the scheduling and management of the subway operation. PMID:26835224

  19. Domain motions and electron transfer dynamics in 2Fe-superoxide reductase.

    PubMed

    Horch, Marius; Utesch, Tillmann; Hildebrandt, Peter; Mroginski, Maria Andrea; Zebger, Ingo

    2016-08-17

    Superoxide reductases are non-heme iron enzymes that represent valuable model systems for the reductive detoxification of reactive oxygen species. In the present study, we applied different theoretical methods to study the structural dynamics of a prototypical 2Fe-superoxide reductase and its influence on electron transfer towards the active site. Using normal mode and essential dynamics analyses, we could show that enzymes of this type are capable of well-defined, electrostatically triggered domain movements, which may allow conformational proofreading for cellular redox partners involved in intermolecular electron transfer. Moreover, these global modes of motion were found to enable access to molecular configurations with decreased tunnelling distances between the active site and the enzyme's second iron centre. Using all-atom classical molecular dynamics simulations and the tunnelling pathway model, however, we found that electron transfer between the two metal sites is not accelerated under these conditions. This unexpected finding suggests that the unperturbed enzymatic structure is optimized for intramolecular electron transfer, which provides an indirect indication of the biological relevance of such a mechanism. Consistently, efficient electron transfer was found to depend on a distinct route, which is accessible via the equilibrium geometry and characterized by a quasi conserved tyrosine that could enable multistep-tunnelling (hopping). Besides these explicit findings, the present study demonstrates the importance of considering both global and local protein dynamics, and a generalized approach for the functional analysis of these aspects is provided. PMID:27491757

  20. Asymmetric electron transfer in cyanobacterial Photosystem I: charge separation and secondary electron transfer dynamics of mutations near the primary electron acceptor A0.

    PubMed

    Dashdorj, Naranbaatar; Xu, Wu; Cohen, Rachel O; Golbeck, John H; Savikhin, Sergei

    2005-02-01

    Point mutations were introduced near the primary electron acceptor sites assigned to A0 in both the PsaA and PsaB branches of Photosystem I in the cyanobacterium Synechocystis sp. PCC 6803. The residues Met688PsaA and Met668PsaB, which provide the axial ligands to the Mg2+ of the eC-A3 and eC-B3 chlorophylls, were changed to leucine and asparagine (chlorophyll notation follows Jordan et al., 2001). The removal of the ligand is expected to alter the midpoint potential of the A0/A0- redox pair and result in a change in the intrinsic charge separation rate and secondary electron transfer kinetics from A0- to A1. The dynamics of primary charge separation and secondary electron transfer were studied at 690 nm and 390 nm in these mutants by ultrafast optical pump-probe spectroscopy. The data reveal that mutations in the PsaB branch do not alter electron transfer dynamics, whereas mutations in the PsaA branch have a distinct effect on electron transfer, slowing down both the primary charge separation and the secondary electron transfer step (the latter by a factor of 3-10). These results suggest that electron transfer in cyanobacterial Photosystem I is asymmetric and occurs primarily along the PsaA branch of cofactors. PMID:15542554

  1. Improving data transfer for model coupling

    NASA Astrophysics Data System (ADS)

    Zhang, C.; Liu, L.; Yang, G.; Li, R.; Wang, B.

    2015-10-01

    Data transfer, which means transferring data fields between two component models or rearranging data fields among processes of the same component model, is a fundamental operation of a coupler. Most of state-of-the-art coupler versions currently use an implementation based on the point-to-point (P2P) communication of the Message Passing Interface (MPI) (call such an implementation "P2P implementation" for short). In this paper, we reveal the drawbacks of the P2P implementation, including low communication bandwidth due to small message size, variable and big number of MPI messages, and jams during communication. To overcome these drawbacks, we propose a butterfly implementation for data transfer. Although the butterfly implementation can outperform the P2P implementation in many cases, it degrades the performance in some cases because the total message size transferred by the butterfly implementation is larger than that by the P2P implementation. To make the data transfer completely improved, we design and implement an adaptive data transfer library that combines the advantages of both butterfly implementation and P2P implementation. Performance evaluation shows that the adaptive data transfer library significantly improves the performance of data transfer in most cases and does not decrease the performance in any cases. Now the adaptive data transfer library is open to the public and has been imported into a coupler version C-Coupler1 for performance improvement of data transfer. We believe that it can also improve other coupler versions.

  2. Electron Transfer Mechanisms of DNA Repair by Photolyase

    NASA Astrophysics Data System (ADS)

    Zhong, Dongping

    2015-04-01

    Photolyase is a flavin photoenzyme that repairs two DNA base damage products induced by ultraviolet (UV) light: cyclobutane pyrimidine dimers and 6-4 photoproducts. With femtosecond spectroscopy and site-directed mutagenesis, investigators have recently made significant advances in our understanding of UV-damaged DNA repair, and the entire enzymatic dynamics can now be mapped out in real time. For dimer repair, six elementary steps have been characterized, including three electron transfer reactions and two bond-breaking processes, and their reaction times have been determined. A unique electron-tunneling pathway was identified, and the critical residues in modulating the repair function at the active site were determined. The dynamic synergy between the elementary reactions for maintaining high repair efficiency was elucidated, and the biological nature of the flavin active state was uncovered. For 6-4 photoproduct repair, a proton-coupled electron transfer repair mechanism has been revealed. The elucidation of electron transfer mechanisms and two repair photocycles is significant and provides a molecular basis for future practical applications, such as in rational drug design for curing skin cancer.

  3. A model for laboratory tech transfer investment

    SciTech Connect

    Otey, G.R.; Carson, C.C.; Bomber, T.M.; Rogers, J.D.

    1994-06-01

    A simple model has been developed to address a pragmatic question: What fraction of its research and development budget should a national laboratory devote to enhancing technology in the private sector? In dealing with lab-wide budgets in an aggregate sense, the model uses three parameters - fraction of lab R&D transferable to industry, transfer efficiency and payback to laboratory missions - to partition fixed R&D resources between technology transfer and core missions. It is a steady-state model in that the transfer process is assumed to work in equilibrium with technology generation. The results presented should be of use to those engaged in managing and overseeing federal laboratory technology transfer activities.

  4. Electron transfer, decoherence, and protein dynamics: insights from atomistic simulations.

    PubMed

    Narth, Christophe; Gillet, Natacha; Cailliez, Fabien; Lévy, Bernard; de la Lande, Aurélien

    2015-04-21

    Electron transfer in biological systems drives the processes of life. From cellular respiration to photosynthesis and enzymatic catalysis, electron transfers (ET) are chemical processes on which essential biological functions rely. Over the last 40 years, scientists have sought understanding of how these essential processes function in biology. One important breakthrough was the discovery that Marcus theory (MT) of electron transfer is applicable to biological systems. Chemists have experimentally collected both the reorganization energies (λ) and the driving forces (ΔG°), two parameters of Marcus theory, for a large variety of ET processes in proteins. At the same time, theoretical chemists have developed computational approaches that rely on molecular dynamics and quantum chemistry calculations to access numerical estimates of λ and ΔG°. Yet another crucial piece in determining the rate of an electron transfer is the electronic coupling between the initial and final electronic wave functions. This is an important prefactor in the nonadiabatic rate expression, since it reflects the probability that an electron tunnels from the electron donor to the acceptor through the intervening medium. The fact that a protein matrix supports electron tunneling much more efficiently than vacuum is now well documented, both experimentally and theoretically. Meanwhile, many chemists have provided examples of the rich physical chemistry that can be induced by protein dynamics. This Account describes our studies of the dynamical effects on electron tunneling. We present our analysis of two examples of natural biological systems through MD simulations and tunneling pathway analyses. Through these examples, we show that protein dynamics sustain efficient tunneling. Second, we introduce two time scales: τcoh and τFC. The former characterizes how fast the electronic coupling varies with nuclear vibrations (which cause dephasing). The latter reflects the time taken by the system

  5. Mixed-valence trinuclear manganese clusters: Influence of the electronic transfer on the magnetic properties

    NASA Astrophysics Data System (ADS)

    Gomez-Garcia, C. J.; Coronado, E.; Pourroy, G.

    1990-05-01

    The magnetic behavior of mixed-valence trinuclear clusters d4-d4-d5 is discussed on the basis of a model which takes into account valence delocalization and Heisenberg exchange. This model considers the competing effect between the electronic transfer and the localization of the extra electron due to an asymmetry of the triangular entity. The magnetic properties of the mixed-valence oxo-centered Mn(II)-Mn(III)-Mn(III) complexes formulated as Mn3O(O2CR)6L3 (R=Me, L=pyridine) are discussed on the basis of the developed model, supporting a significant intramolecular electron transfer. A comparison of these results to those previously reported by assuming a valence-trapped model is given.

  6. A predictive theoretical model for electron tunneling pathways in proteins

    NASA Technical Reports Server (NTRS)

    Onuchic, Jose Nelson; Beratan, David N.

    1990-01-01

    A practical method is presented for calculating the dependence of electron transfer rates on details of the protein medium intervening between donor and acceptor. The method takes proper account of the relative energetics and mutual interactions of the donor, acceptor, and peptide groups. It also provides a quantitative search scheme for determining the important tunneling pathways (specific sequences of localized bonding and antibonding orbitals of the protein which dominate the donor-acceptor electronic coupling) in native and tailored proteins, a tool for designing new proteins with prescribed electron transfer rates, and a consistent description of observed electron transfer rates in existing redox labeled metalloproteins and small molecule model compounds.

  7. A redox beginning: Which came first phosphoryl, acyl, or electron transfer ?. [Abstract only

    NASA Technical Reports Server (NTRS)

    Weber, Arthur L.

    1994-01-01

    Thermodynamic and kinetic information available on the synthesis of prebiotic monomers and polymers will be examined in order to illuminate the prebiotic plausibility of polymer syntheses based on (a) phosphoryl transfer that yields phosphodiester polymers, (b) acyl transfer that gives polyamides, and (c) electron transfer that produces polydisulfide or poly(thio)ester polymers. New experimental results on the oxidative polymerization of 2,3-dimercaptopropanol by ferric ions on the surface of ferric hydroxide oxide will be discussed as a chemical model of polymerization by electron transfer. This redox polymerization that yields polymers with a polydisulfide backbone was found to give oligomers up to the 15-mer from 1 mM of 2,3-dimercaptopropanol after one day at 25 C. High pressure liquid chromatography (HPLC) analysis of the oligomers was carried out on an Alltech OH-100 column eluted with acetonitrile-water.

  8. Electron transfer beyond the static picture: A TDDFT/TD-ZINDO study of a pentacene dimer

    SciTech Connect

    Reslan, Randa; Lopata, Kenneth A.; Arntsen, Christopher D.; Govind, Niranjan; Neuhauser, Daniel

    2012-12-14

    We use time-dependent density functional theory and time-dependent ZINDO (a semi-empirical method) to study transfer of an extra electron between a pair of pentacene dimers. A measure of the electronic transfer integral is computed in a dynamic picture via the vertical excitation energy from a delocalized anionic ground state. With increasing dimer separation, this dynamical measurement of charge transfer is shown to be significantly larger than the commonly used static approximation (i.e., LUMO+1 - LUMO of the neutral dimer, or HOMO - LUMO of the charged dimer), up to an order of magnitude higher at 6 Å. These results offer a word of caution for calculations involving large separations, as in organic photovoltaics, where care must be taken when using a static picture to model charge transfer.

  9. DNA Intercalated Psoralen Undergoes Efficient Photoinduced Electron Transfer.

    PubMed

    Fröbel, Sascha; Reiffers, Anna; Torres Ziegenbein, Christian; Gilch, Peter

    2015-04-01

    The interaction of psoralens with DNA has been used for therapeutic and research purposes for decades. Still the photoinduced behavior of psoralens in DNA has never been observed directly. Femtosecond transient absorption spectroscopy is used here to gain direct insight into the photophysics of a DNA-intercalated psoralen (4'-aminomethyl-4,5',8-trimethyl-psoralen (AMT)). Intercalation reduces the excited singlet lifetime of AMT to 4 ps compared with 1400 ps for AMT in water. This singlet quenching prohibits the population of the triplet state that is accessed in free AMT. Instead, a DNA to AMT electron transfer takes place. The resulting radical pair decays primarily via charge recombination with a time constant of 30 ps. The efficient electron transfer observed here reveals a completely new aspect of the psoralen-DNA interaction. PMID:26262984

  10. Catalytic Olefin Hydroamidation Enabled by Proton-Coupled Electron Transfer

    PubMed Central

    2015-01-01

    Here we report a ternary catalyst system for the intramolecular hydroamidation of unactivated olefins using simple N-aryl amide derivatives. Amide activation in these reactions occurs via concerted proton-coupled electron transfer (PCET) mediated by an excited state iridium complex and weak phosphate base to furnish a reactive amidyl radical that readily adds to pendant alkenes. A series of H-atom, electron, and proton transfer events with a thiophenol cocatalyst furnish the product and regenerate the active forms of the photocatalyst and base. Mechanistic studies indicate that the amide substrate can be selectively homolyzed via PCET in the presence of the thiophenol, despite a large difference in bond dissociation free energies between these functional groups. PMID:26439818

  11. Large scale oil lease automation and electronic custody transfer

    SciTech Connect

    Price, C.R.; Elmer, D.C.

    1995-12-31

    Typically, oil field production operations have only been automated at fields with long term production profiles and enhanced recovery. The automation generally consists of monitoring and control at the wellhead and centralized facilities. However, Union Pacific Resources Co. (UPRC) has successfully implemented a large scale automation program for rapid-decline primary recovery Austin Chalk wells where purchasers buy and transport oil from each individual wellsite. This project has resulted in two significant benefits. First, operators are using the system to re-engineer their work processes. Second, an inter-company team created a new electronic custody transfer method. This paper will describe: the progression of the company`s automation objectives in the area; the field operator`s interaction with the system, and the related benefits; the research and development of the new electronic custody transfer method.

  12. Intramolecular electronic energy transfer in bichromophoric molecular macrocyclic systems

    NASA Astrophysics Data System (ADS)

    Speiser, Shammai

    1993-01-01

    The structures and spectral properties of several bichromophoric molecules, suitable for optical data processing, are presented. The bichromophoric molecules are composed of an aromatic ring connected by two methylene chains to an (alpha) -diketone moiety. Both the absorption and emission spectra of these compounds can be attributed to a superposition of the individual spectra of the separate chromophores. The critical transfer radia for electronic energy transfer from the aromatic (donor) chromophore to the (alpha) -diketone (acceptor) chromophore was calculated from the spectral overlap between the fluorescence spectrum of the aromatic ring with the absorption spectrum of the (alpha) -diketone chromophore. The results show that this series of molecules is well suited for a mechanistic study of short-range intramolecular electronic energy transfer (intra-EET). The temperature and the molecular structure dependence of the intra-EET efficiency in this series was measured and analyzed for both singlet-singlet and triplet-triplet routes. The results show that the transfer efficiency is strongly temperature and structure dependent, indicating that exchange interaction is responsible for intra-EET between close chromophores in a bichromophoric molecule. The relative contributions of interchromophoric distance and that of the relative orientation of the two chromophores to exchange interaction are discussed.

  13. Marcus wins nobel prize in chemistry for electron transfer theory

    SciTech Connect

    Levi, B.G.

    1993-01-01

    This article describes the work of Rudolf Marcus of Caltech leading to his receipt of the 1992 Nobel Prize in Chemistry [open quotes]for his contributions to the theory of electron transfer reactions in chemical systems.[close quotes] Applications of Marcus' theory include such diverse phenomena as photosynthesis, electrically conducting polymers, chemiluminescence, and corrosion. Historical aspects of his career are given. 10 refs., 1 fig.

  14. Polar solvation and electron transfer. Annual progress report, July 1, 1992--June 30, 1993

    SciTech Connect

    Not Available

    1993-04-13

    The report is divided into the following sections: completion of previous studies on solvation dynamics, dipole lattice studies, inertial components of solvation response, simple models of solvation dynamics, rotational dynamics and dielectric friction, intramolecular electron transfer reactions, and intermolecular donor-acceptor complexes.

  15. Study of intermediates from transition metal excited-state electron- transfer reactions

    SciTech Connect

    Hoffman, M.Z.

    1993-03-31

    Progress on 6 projects is reported: excited state absorption spectrum of Ru(bpy)[sub 3][sup 2+], solvent cage model for electron transfer quenching, reductive quenching of [sup *]Cr(III) complexes, solution medium effects in oxidative quenching of [sup *]Ru(II) complexes, photosensitized oxidation of phenol in aqueous solution, and quenching of Ru(II) complexes by oxygen.

  16. Studies of Photosynthetic Energy and Charge Transfer by Two-dimensional Fourier transform electronic spectroscopy

    NASA Astrophysics Data System (ADS)

    Ogilvie, Jennifer

    2010-03-01

    Two-dimensional (2D) Fourier transform electronic spectroscopy has recently emerged as a powerful tool for the study of energy transfer in complex condensed-phase systems. Its experimental implementation is challenging but can be greatly simplified by implementing a pump-probe geometry, where the two phase-stable collinear pump pulses are created with an acousto-optic pulse-shaper. This approach also allows the use of a continuum probe pulse, expanding the available frequency range of the detection axis and allowing studies of energy transfer and electronic coupling over a broad range of frequencies. We discuss several benefits of 2D electronic spectroscopy and present 2D data on the D1-D2 reaction center complex of Photosystem II from spinach. We discuss the ability of 2D spectroscopy to distinguish between current models of energy and charge transfer in this system.

  17. Electron transfer reactions within zeolites: Radical cation from benzonorbornadiene

    SciTech Connect

    Pitchumani, K.; Ramamurthy, V.; Corbin, D.R.

    1996-08-28

    Zeolites are being used as solid acid catalysts in a number of commercial processes. Occasionally zeolites are also reported to perform as electron transfer agents. Recently, we observed that radical cations of certain olefins and thiophene oligomers can be generated spontaneously within ZSM-5 zeolites. We noticed that these radical cations generated from diphenyl polyenes and thiophene oligomers were remarkably stable (at room temperature) within ZSM-5 and can be characterized spectroscopically at leisure. We have initiated a program on electron transfer processes within large pore zeolites. The basis of this approach is that once a cation radical is generated within a large pore zeolite, it will have sufficient room to undergo a molecular transformation. Our aim is to identify a condition under which electron transfer can be routinely and reliably carried out within large pore zeolites such as faujasites. To our great surprise, when benzonorbornadiene A and a number of olefins were included in divalent cation exchanged faujasites. they were transformed into products very quickly (<15 min). This observation allowed us to explore the use of zeolites as oxidants. Results of our studies on benzonorbornadiene are presented in this communication. 16 refs., 1 fig.

  18. Intercellular wiring enables electron transfer between methanotrophic archaea and bacteria.

    PubMed

    Wegener, Gunter; Krukenberg, Viola; Riedel, Dietmar; Tegetmeyer, Halina E; Boetius, Antje

    2015-10-22

    The anaerobic oxidation of methane (AOM) with sulfate controls the emission of the greenhouse gas methane from the ocean floor. In marine sediments, AOM is performed by dual-species consortia of anaerobic methanotrophic archaea (ANME) and sulfate-reducing bacteria (SRB) inhabiting the methane-sulfate transition zone. The biochemical pathways and biological adaptations enabling this globally relevant process are not fully understood. Here we study the syntrophic interaction in thermophilic AOM (TAOM) between ANME-1 archaea and their consortium partner SRB HotSeep-1 (ref. 6) at 60 °C to test the hypothesis of a direct interspecies exchange of electrons. The activity of TAOM consortia was compared to the first ANME-free culture of an AOM partner bacterium that grows using hydrogen as the sole electron donor. The thermophilic ANME-1 do not produce sufficient hydrogen to sustain the observed growth of the HotSeep-1 partner. Enhancing the growth of the HotSeep-1 partner by hydrogen addition represses methane oxidation and the metabolic activity of ANME-1. Further supporting the hypothesis of direct electron transfer between the partners, we observe that under TAOM conditions, both ANME and the HotSeep-1 bacteria overexpress genes for extracellular cytochrome production and form cell-to-cell connections that resemble the nanowire structures responsible for interspecies electron transfer between syntrophic consortia of Geobacter. HotSeep-1 highly expresses genes for pili production only during consortial growth using methane, and the nanowire-like structures are absent in HotSeep-1 cells isolated with hydrogen. These observations suggest that direct electron transfer is a principal mechanism in TAOM, which may also explain the enigmatic functioning and specificity of other methanotrophic ANME-SRB consortia. PMID:26490622

  19. Photoinduced electron transfer in ordered polymers: Progress report, May 1, 1988--December 31, 1988

    SciTech Connect

    Jones, G. II

    1988-12-31

    Long-range photoinduced electron transfer between potential electron donors and acceptors is of considerable current interest in terms of strategies for artificial photosynthesis and studies regarding the redox properties of proteins. Electron transfer over organized arrays of significant dimension (e.g., 10 nm) may also be important for molecule-based bistable switches or rectifiers. As part of a program of study of long range electron transfer involving biopolymers, we report early investigations of the interaction of the xanthene dye, eosin (EY), with synthetic peptides constructed of the amino acid residues, lysine (Lys), tryptophan (Trp), and tyrosine (Tyr). The principal interest in these systems has to do with the well ordered secondary structures (e.g., ..cap alpha..-helices) adopted by peptide polymers and the capabilities for synthetic modification of peptide side chains and end groups with chromophores or electroactive substituents. Work which is completed or well underway includes synthesis of Trp homopolymer (20-25 Trp amino acid residues) with EY attached specifically at the N-terminus. Model compounds in which the xanthene dye is attached to one or two Trp residues have been prepared along with copolymers of Trp or Tyr and Lys with randomally labeled EY at Lys side chains. Photoinduced electron transfer between variously attached eosin pendants and co-bound Trp units has been studied using fluorescence quenching and laser flash photolysis methods. Also, as a model for events occurring for the synthetic peptides, electron transfer quenching of eosin singlet and triplet excited states by substituted phenols, representative of the pendant groups of Tyr residues has been investigated. The observation of ultra-long-lived contact radical ion pairs which result from the quenching an excited quinone molecule and the photolysis of quinone charge-transfer complexes constitute another theme of continuing research.

  20. Supporting Students' Knowledge Transfer in Modeling Activities

    ERIC Educational Resources Information Center

    Piksööt, Jaanika; Sarapuu, Tago

    2014-01-01

    This study investigates ways to enhance secondary school students' knowledge transfer in complex science domains by implementing question prompts. Two samples of students applied two web-based models to study molecular genetics--the model of genetic code (n = 258) and translation (n = 245). For each model, the samples were randomly divided into…

  1. Mechanisms for control of biological electron transfer reactions

    PubMed Central

    Williamson, Heather R.; Dow, Brian A.; Davidson, Victor L.

    2014-01-01

    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. PMID:25085775

  2. Electrochemical proton-coupled electron transfer of an osmium aquo complex: theoretical analysis of asymmetric tafel plots and transfer coefficients.

    PubMed

    Ludlow, Michelle K; Soudackov, Alexander V; Hammes-Schiffer, Sharon

    2010-02-01

    Electrochemical proton-coupled electron transfer of an osmium aquo complex attached to a self-assembled monolayer on a gold electrode is studied with a recently developed theoretical formulation. The calculated hydrogen/deuterium kinetic isotope effect for the standard rate constant, the cathodic transfer coefficient at zero overpotential, and the Tafel plot are in excellent agreement with experimental data. The input quantities to the heterogeneous rate constant expressions were calculated with density functional theory in conjunction with dielectric continuum models, and no parameters were fit to experimental data. The theoretical calculations indicate that the asymmetry of the Tafel plot and the deviation of the transfer coefficient at zero overpotential from the standard value of one-half arise from the change in the equilibrium proton donor-acceptor distance upon electron transfer. The direction of the asymmetry and deviation from one-half is determined by the sign of this distance change, and the magnitude of these effects is determined by the magnitude of this distance change, as well as the reorganization energy and the distance dependence of the overlap between the initial and final proton vibrational wave functions. This theory provides experimentally testable predictions for the impact of specific system properties on the qualitative behavior of the Tafel plots. PMID:20067257

  3. Photoinduced Electron Transfer between Anionic Corrole and DNA.

    PubMed

    Wang, Li-Li; Zhang, Lei; Wang, Hui; Zhang, Yang; Huang, Jun-Teng; Zhu, He; Ying, Xiao; Ji, Liang-Nian; Liu, Hai-Yang

    2016-02-01

    The interaction between a water-soluble anionic Ga(III) corrole [Ga(tpfc)(SO3Na)2] and calf thymus DNA (ct-DNA) has been investigated by using femtosecond transient absorption spectroscopy. A significant broadening from 570 to 585 nm of positive absorption band of the blend of Ga(tpfc)(SO3Na)2 and ct-DNA (Ga(tpfc)(SO3Na)2-ctDNA) has been observed from 0.15 to 0.50 ps after photoexcitation of Ga(tpfc)(SO3Na)2 into the Soret band. The control experiment has been performed on the model DNA ([poly(dG-dC)]2) rich in guanine bases, which exhibits a similar spectral broadening, whereas it is absent for [poly(dA-dT)]2 without guanine bases. The molecular orbital calculation shows that HOMO of Ga(tpfc)(SO3Na)2 is lower than that of guanine bases. The results of the electrochemical experiment show the reversible electron transfer (ET) between Ga(tpfc)(SO3Na)2 and guanine bases of ct-DNA is thermodynamically favorable. The dynamical analysis of the transient absorption spectra reveals that an ultrafast forward ET from the guanine bases to Ga(tpfc)(SO3Na)2 occurs within the pulse duration (156 fs), leading to the formation of an intermediate state. The following back ET to the ground state of Ga(tpfc)(SO3Na)2 may be accomplished in 520 fs. PMID:26752116

  4. Application of Electron-Transfer Theory to Several Systems of Biological Interest

    DOE R&D Accomplishments Database

    Marcus, R. A.; Sutin, N.

    1985-03-23

    Electron-transfer reaction rates are compared with theoretically calculated values for several reactions in the bacterial photosynthetic reaction center. A second aspect of the theory, the cross-relation, is illustrated using protein-protein electron transfers.

  5. ELECTRON TRANSFER MECHANISM AT THE SOLID-LIQUID INTERFACE OF PHYLLOSILICATES

    EPA Science Inventory

    Interfacial electron transfer processes on clay minerals have significant impact in natural environments and geochemical systems. Nitrobenzene was used as molecular probes to study the electron transfer mechanism at the solid-water interfaces of Fe-containing phyllosicates. For...

  6. Simplified models for heat transfer in rooms

    NASA Astrophysics Data System (ADS)

    Graca, Guilherme C. C. Carrilho Da

    Buildings protect their occupants from the outside environment. As a semi-enclosed environment, buildings tend to contain the internally generated heat and air pollutants, as well as the solar and conductive heat gains that can occur in the facade. In the warmer months of the year this generally leads to overheating, creating a need for a cooling system. Ventilation air replaces contaminated air in the building and is often used as the dominant medium for heat transfer between indoor and outdoor environments. The goal of the research presented in this thesis is to develop a better understanding of the important parameters in the performance of ventilation systems and to develop simplified convective heat transfer models. The general approach used in this study seeks to capture the dominant physical processes for these problems with first order accuracy, and develop simple models that show the correct system behavior trends. Dimensional analysis, in conjunction with simple momentum and energy conservation, scaled model experiments and numerical simulations, is used to improve airflow and heat transfer rate predictions in both single and multi room ventilation systems. This study includes the three commonly used room ventilation modes: mixing, displacement and cross-ventilation. A new modeling approach to convective heat transfer between the building and the outside is presented: the concept of equivalent room heat transfer coefficient. The new model quantifies the reduction in heat transfer between ventilation air and internal room surfaces caused by limited thermal capacity and temperature variation of the air for the three modes studied. Particular emphasis is placed on cross-ventilation, and on the development of a simple model to characterize the airflow patterns that occur in this case. The implementation of the models in a building thermal simulation software tool is presented as well as comparisons between model predictions, experimental results and complex

  7. Itinerant electron model and conductance of DNA

    NASA Astrophysics Data System (ADS)

    Qu, Zhen; Kang, Da-Wei; Gao, Xu-Tuan; Xie, Shi-Jie

    2008-09-01

    DNA (Deoxyribonucleic acid) has recently caught the attention of chemists and physicists. A major reason for this interest is DNA’s potential use in nanoelectronic devices, both as a template for assembling nanocircuits and as an element of such circuits. However, the electronic properties of the DNA molecule remain very controversial. Charge-transfer reactions and conductivity measurements show a large variety of possible electronic behavior, ranging from Anderson and band-gap insulators to effective molecular wires and induced superconductors. In this review article, we summarize the wide-ranging experimental and theoretical results of charge transport in DNA. An itinerant electron model is suggested and the effect of the density of itinerant electrons on the conductivity of DNA is studied. Calculations show that a DNA molecule may show conductivity from insulating to metallic, which explains the controversial and profuse electric characteristics of DNA to some extent.

  8. Imaging charge and energy transfer in molecules using free-electron lasers

    NASA Astrophysics Data System (ADS)

    Rudenko, Artem

    2014-05-01

    Charge and energy transfer reactions drive numerous important processes in physics, chemistry and biology, with applications ranging from X-ray astrophysics to artificial photosynthesis and molecular electronics. Experimentally, the central goal in studies of transfer phenomena is to trace the spatial localization of charge at a given time. Because of their element and site sensitivity, ultrafast X-rays provide a promising tool to address this goal. In this talk I will discuss several experiments where free-electron lasers were employed to study charge and energy transfer dynamics in fragmenting molecules. In a first example, we used intense, 70 femtosecond 1.5 keV pulses from the Linac Coherent Light Source (LCLS) to study distance dependence of electron transfer in laser-dissociated methyl iodide molecules. Inducing well-localized positive charge on the heavy iodine atom, we observe signature of electron transition from the separated methyl group up to the distances of 35 atomic units. In a complementary experiment, we studied charge exchange between two partners in a dissociating molecular iodine employing a pump-probe arrangement with two identical 90 eV pulses from the Free-Electron LASer in Hamburg (FLASH). In both cases, the effective spatial range of the electron transfer can be reasonably described by a classical over-the-barrier model developed for ion-atom collisions. Finally, I will discuss a time-resolved measurement on non-local relaxation mechanism based on a long-range energy transfer, the so-called interatomic Coulombic decay. This work was supported by Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, US Department of Energy and by the Kansas NSF ``First Award'' program.

  9. Electronic Energy transfer in light-harvesting antenna complexes

    NASA Astrophysics Data System (ADS)

    Hossein-Nejad, Hoda

    The studies presented in this thesis explore electronic energy transfer (EET) in light-harvesting antenna complexes and investigate the role of quantum coherence in EET. The dynamics of energy transfer are investigated in three distinct length scales and a different formulation of the exciton transport problem is applied at each scale. These scales include: the scale of a molecular dimer, the scale of a single protein and the scale of a molecular aggregate. The antenna protein phycoerythrin 545 (PE545) isolated from the photosynthetic cryptophyte algae Rhodomonas CS4 is specifically studied in two chapters of this thesis. It is found that formation of small aggregates delocalizes the excitation across chromophores of adjacent proteins, and that this delocalization has a dramatic effect in enhancing the rate of energy transfer between pigments. Furthermore, we investigate EET from a donor to an acceptor via an intermediate site and observe that interference of coherent pathways gives a finite correction to the transfer rate that is sensitively dependent on the nature of the vibrational interactions in the system. The statistical fluctuations of a system exhibiting EET are investigated in the final chapter. The techniques of non-equilibrium statistical mechanics are applied to investigate the steady-state of a typical system exhibiting EET that is perturbed out of equilibrium due to its interaction with a fluctuating bath.

  10. Angular momentum transfer and polarization degree of ions with one-valence electron by electron impact

    NASA Astrophysics Data System (ADS)

    Akita, Kenichi; Kai, Takeshi; Nakazaki, Shinobu; Igarashi, Akinori

    2009-04-01

    We carry out the R-matrix calculations for electron-impact excitations of ions with one valence electron. The integral cross sections and polarization degree are obtained for the excitation process from the ground state to the first 2P° state of Li2+, B2+ and Al2+ as functions of electron incident energy. The differential cross sections and angular momentum transfer are also shown at non-resonant low-energy points. As for the angular momentum transfer (L⊥) at small scattering angles, they are negative for B2+ and Al2+, while it is positive for Li2+. Thus L⊥ of doubly charged ions with one-valence electron is not simple.

  11. Molecular control of electron and hole transfer processes: Theory and applications

    SciTech Connect

    Newton, M.D.; Cave, R.J.

    1996-02-01

    Recent decades have seen remarkable advances in microscopic understanding of electron transfer (ET) processes in widely ranging contexts, including solid-state, liquid solution, and complex biological assemblies. The primary goal of this chapter is to report recent advances in the modeling, calculation, and analysis of electronic coupling in complex molecular aggregates, thereby allowing an assessment of current progress toward the goal of molecular-level control and design. The control of electron transfer kinetics (i.e., enhancing desired processes, while inhibiting others) involves, of course, system energetics (especially activation and reorganization energies) as well as electronic coupling, which is most directly relevant only after the system has reached the appropriate point (or region) along the reaction coordinate. Nevertheless, to focus the discussion in this chapter, the authors will consider such energetics, and the associated molecular and solvent coordinates which control then, only to the extent that they bear on the analysis of the electronic coupling. In the following sections they first discuss the formulation of basic ET models, including the definition of initial and final states, the role of orbitals and 1-particle models in a many-electron context, the utility of various effective Hamiltonians, and the role of vibronic as well as purely electronic effects. With these theoretical tools in hand, they then examine very recent applications to complex molecular systems using the techniques of computational quantum chemistry, followed by detailed analysis of the numerical results. They then conclude with some comments regarding the current ``state of the art`` and remaining challenges.

  12. Inner reorganization limiting electron transfer controlled hydrogen bonding: intra- vs. intermolecular effects.

    PubMed

    Martínez-González, Eduardo; Frontana, Carlos

    2014-05-01

    In this work, experimental evidence of the influence of the electron transfer kinetics during electron transfer controlled hydrogen bonding between anion radicals of metronidazole and ornidazole, derivatives of 5-nitro-imidazole, and 1,3-diethylurea as the hydrogen bond donor, is presented. Analysis of the variations of voltammetric EpIcvs. log KB[DH], where KB is the binding constant, allowed us to determine the values of the binding constant and also the electron transfer rate k, confirmed by experiments obtained at different scan rates. Electronic structure calculations at the BHandHLYP/6-311++G(2d,2p) level for metronidazole, including the solvent effect by the Cramer/Truhlar model, suggested that the minimum energy conformer is stabilized by intramolecular hydrogen bonding. In this structure, the inner reorganization energy, λi,j, contributes significantly (0.5 eV) to the total reorganization energy of electron transfer, thus leading to a diminishment of the experimental k. PMID:24653999

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

    PubMed

    Zanetti-Polzi, Laura; Corni, Stefano

    2016-04-21

    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. PMID:27031179

  14. Microbial extracellular electron transfer and its relevance to iron corrosion.

    PubMed

    Kato, Souichiro

    2016-03-01

    Extracellular electron transfer (EET) is a microbial metabolism that enables efficient electron transfer between microbial cells and extracellular solid materials. Microorganisms harbouring EET abilities have received considerable attention for their various biotechnological applications, including bioleaching and bioelectrochemical systems. On the other hand, recent research revealed that microbial EET potentially induces corrosion of iron structures. It has been well known that corrosion of iron occurring under anoxic conditions is mostly caused by microbial activities, which is termed as microbiologically influenced corrosion (MIC). Among diverse MIC mechanisms, microbial EET activity that enhances corrosion via direct uptake of electrons from metallic iron, specifically termed as electrical MIC (EMIC), has been regarded as one of the major causative factors. The EMIC-inducing microorganisms initially identified were certain sulfate-reducing bacteria and methanogenic archaea isolated from marine environments. Subsequently, abilities to induce EMIC were also demonstrated in diverse anaerobic microorganisms in freshwater environments and oil fields, including acetogenic bacteria and nitrate-reducing bacteria. Abilities of EET and EMIC are now regarded as microbial traits more widespread among diverse microbial clades than was thought previously. In this review, basic understandings of microbial EET and recent progresses in the EMIC research are introduced. PMID:26863985

  15. Photoinduced electron transfer from dialkyl nitroxides to halogenated solvents

    SciTech Connect

    Chateauneuf, J. ); Lusztyk, J.; Ingold, K.U. )

    1990-02-02

    Laser flash photolysis (LFP) at wavelengths within the charge-transfer absorption present in CCl{sub 4} solutions of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) yields the oxoammonium chloride of TEMPO, 1 ({lambda}{sub max} = 460 nm), and the trichloromethyl radical in an essentially instantaneous ({le}18 ps) process. The primary photochemical event is an electron transfer from TEMPO to CCl{sub 4}, and this is followed by immediate decomposition of the CCl{sub 4}{sup {sm bullet}{minus}} radical anion to Cl{sup {minus}} and Cl{sub 3}C{sup {sm bullet}}. An independent synthesis of 1 confirmed that the absorption attributed to this species has been correctly assigned. The formation of Cl{sub 3}C{sup {sm bullet}} was inferred by its trapping by molecular oxygen. LFP of TEMPO in other halogenated solvents and of other nitroxides in halogenated solvents has confirmed the generality of these photoreactions.

  16. Single Electron Transfer Living Radical Polymerization via a New Initiator

    NASA Astrophysics Data System (ADS)

    Bai, Xiongxiong; Hu, Ying; Zhang, Xu; Ai, Lingling; Cheng, Chuanjie

    2014-08-01

    Research and development of novel initiating system such as single electron transfer living radical polymerization (SET-LRP) is of high importance in polymer chemistry. A new SET-LRP initiator was synthesized and applied to prepare end-functionalized poly(methyl methacrylate) (PMMA) in this study. α-Trichloromethyl benzyl alcohol was firstly synthesized, followed by preparation of PMMA under SET-LRP conditions. Conversion of MMA was 81.9%, and the molecular weight of PMMA was about 2.5 kDa at 60 °C for 1 h. Consistency of the number-average molecular weight of PMMA from NMR, GPC and theoretical calculation indicated that the polymerization featured controllable property. Broad molecular weight distribution (MWD) may be ascribed to branched polymers formed by initiation and chain transfer.

  17. The economic mobility in money transfer models

    NASA Astrophysics Data System (ADS)

    Ding, Ning; Xi, Ning; Wang, Yougui

    2006-07-01

    In this paper, we investigate the economic mobility in four money transfer models which have been applied into the research on wealth distribution. We demonstrate the mobility by recording the time series of agents’ ranks and observing their volatility. We also compare the mobility quantitatively by employing an index, “the per capita aggregate change in log-income”, proposed by economists. Like the shape of distribution, the character of mobility is also decided by the trading rule in these transfer models. It is worth noting that even though two models have the same type of distribution, their mobility characters may be quite different.

  18. Quantitative Probes of Electron-Phonon Coupling in an Organic Charge-Transfer Material

    NASA Astrophysics Data System (ADS)

    Rury, Aaron; Sorenson, Shayne; Driscoll, Eric; Dawlaty, Jahan

    While organic charge transfer (CT) materials may provide alternatives to inorganic materials in electronics and photonics applications, properties central to applications remain understudied in these organic materials. Specifically, electron-phonon coupling plays a pivotal role in electronic applications yet this coupling in CT materials remains difficult to directly characterize. To better understand the suitability of organic CT materials for electronic applications, we have devised an experimental technique that can directly assess electron-phonon coupling in a model organic CT material. Upon non-resonant interaction with an ultrafast laser pulse, we show that coherent excitation of Raman-active lattice vibrations of quinhydrone, a 1:1 co-crystal of the hydroquinone and p-benzoquinone, modulates the energies of electronic transitions probed by a white light pulse. Using a well-established theoretical framework of vibrational quantum beat spectra across the probe bandwidth, we quantitatively extract the parameters describing these electronic transitions to characterize electron-phonon coupling in this material. In conjunction with temperature-dependent resonance Raman measurements, we assess the hypothesis that several sharp transitions in the near-IR correspond to previously unknown excitonic states of this material. These results and their interpretation set the foundation for further elucidation of the one of the most important parameters in the application of organic charge-transfer materials to electronics and photonics.

  19. 12 CFR 205.15 - Electronic fund transfer of government benefits.

    Code of Federal Regulations, 2011 CFR

    2011-01-01

    ... 12 Banks and Banking 2 2011-01-01 2011-01-01 false Electronic fund transfer of government benefits. 205.15 Section 205.15 Banks and Banking FEDERAL RESERVE SYSTEM BOARD OF GOVERNORS OF THE FEDERAL RESERVE SYSTEM ELECTRONIC FUND TRANSFERS (REGULATION E) § 205.15 Electronic fund transfer of government benefits. (a) Government agency subject...

  20. 12 CFR 205.14 - Electronic fund transfer service provider not holding consumer's account.

    Code of Federal Regulations, 2011 CFR

    2011-01-01

    ... 12 Banks and Banking 2 2011-01-01 2011-01-01 false Electronic fund transfer service provider not holding consumer's account. 205.14 Section 205.14 Banks and Banking FEDERAL RESERVE SYSTEM BOARD OF GOVERNORS OF THE FEDERAL RESERVE SYSTEM ELECTRONIC FUND TRANSFERS (REGULATION E) § 205.14 Electronic fund transfer service provider not...

  1. Relationship between conformational dynamics and electron transfer in a desolvated peptide. Part II. Temperature dependence.

    PubMed

    Parks, Joel H; Semrouni, David; Clavaguéra, Carine; Ohanessian, Gilles

    2013-02-14

    Recent time-resolved lifetime measurements studied the quenching of the fluorescence emitted by a dye covalently bound to the desolvated peptide Dye-Pro(4)-Arg(+)-Trp. This peptide sequence was chosen for study since intramolecular interactions constrain all large-scale fluctuations except for those of the interacting dye and Trp side chain. It was shown that quenching occurred as a result of interaction between the excited dye and tryptophan side chain. These measurements exhibited a temperature dependence that suggested the quenching mechanism was related to electron transfer. This paper presents a comparison of the experimental quenching rate with the Marcus electron transfer model performed with molecular dynamics (MD) calculations. Taking advantage of the AMOEBA force field that explicitly includes polarizability ensures that the intramolecular electrostatic and polarization interactions in this desolvated peptide ion are treated realistically. MD calculations identify both large-scale fluctuations between conformations as well as small-scale fluctuations within a conformation that are shown to be correlated with torsional dynamics of the Trp side chain. Trajectories of the Dye-Trp distance identify the occurrence of close separations required for efficient electron transfer. The temperature dependence of the quenching rate closely follows the rate predicted by the Marcus electron transfer model within uncertainties resulting from statistical averages. Estimates of the energy parameters characterizing the Marcus model indicate the electronic coupling matrix element and the reaction free energy derived from the fits are consistent with published values for transfer in polyproline bridged peptides. These calculations help to provide a molecular basis for investigating conformational changes in desolvated biomolecular ions by fluorescence quenching measurements. PMID:23297809

  2. Suppression of electron transfer to dioxygen by charge transfer and electron transfer complexes in the FAD-dependent reductase component of toluene dioxygenase.

    PubMed

    Lin, Tzong-Yuan; Werther, Tobias; Jeoung, Jae-Hun; Dobbek, Holger

    2012-11-01

    The three-component toluene dioxygenase system consists of an FAD-containing reductase, a Rieske-type [2Fe-2S] ferredoxin, and a Rieske-type dioxygenase. The task of the FAD-containing reductase is to shuttle electrons from NADH to the ferredoxin, a reaction the enzyme has to catalyze in the presence of dioxygen. We investigated the kinetics of the reductase in the reductive and oxidative half-reaction and detected a stable charge transfer complex between the reduced reductase and NAD(+) at the end of the reductive half-reaction, which is substantially less reactive toward dioxygen than the reduced reductase in the absence of NAD(+). A plausible reason for the low reactivity toward dioxygen is revealed by the crystal structure of the complex between NAD(+) and reduced reductase, which shows that the nicotinamide ring and the protein matrix shield the reactive C4a position of the isoalloxazine ring and force the tricycle into an atypical planar conformation, both factors disfavoring the reaction of the reduced flavin with dioxygen. A rapid electron transfer from the charge transfer complex to electron acceptors further reduces the risk of unwanted side reactions, and the crystal structure of a complex between the reductase and its cognate ferredoxin shows a short distance between the electron-donating and -accepting cofactors. Attraction between the two proteins is likely mediated by opposite charges at one large patch of the complex interface. The stability, specificity, and reactivity of the observed charge transfer and electron transfer complexes are thought to prevent the reaction of reductase(TOL) with dioxygen and thus present a solution toward conflicting requirements. PMID:22992736

  3. A molecular Debye-Hückel approach to the reorganization energy of electron transfer reactions in an electric cell.

    PubMed

    Xiao, Tiejun; Song, Xueyu

    2014-10-01

    Electron transfer near an electrode immersed in ionic fluids is studied using the linear response approximation, namely, mean value of the vertical energy gap can be used to evaluate the reorganization energy, and hence any linear response model that can treat Coulomb interactions successfully can be used for the reorganization energy calculation. Specifically, a molecular Debye-Hückel theory is used to calculate the reorganization energy of electron transfer reactions in an electric cell. Applications to electron transfer near an electrode in molten salts show that the reorganization energies from our molecular Debye-Hückel theory agree well with the results from MD simulations. PMID:25296781

  4. A molecular Debye-Hückel approach to the reorganization energy of electron transfer reactions in an electric cell

    SciTech Connect

    Xiao, Tiejun; Song, Xueyu

    2014-10-07

    Electron transfer near an electrode immersed in ionic fluids is studied using the linear response approximation, namely, mean value of the vertical energy gap can be used to evaluate the reorganization energy, and hence any linear response model that can treat Coulomb interactions successfully can be used for the reorganization energy calculation. Specifically, a molecular Debye-Hückel theory is used to calculate the reorganization energy of electron transfer reactions in an electric cell. Applications to electron transfer near an electrode in molten salts show that the reorganization energies from our molecular Debye-Hückel theory agree well with the results from MD simulations.

  5. Modelling charge transfer reactions with the frozen density embedding formalism

    SciTech Connect

    Pavanello, Michele; Neugebauer, Johannes

    2011-12-21

    The frozen density embedding (FDE) subsystem formulation of density-functional theory is a useful tool for studying charge transfer reactions. In this work charge-localized, diabatic states are generated directly with FDE and used to calculate electronic couplings of hole transfer reactions in two {pi}-stacked nucleobase dimers of B-DNA: 5{sup '}-GG-3{sup '} and 5{sup '}-GT-3{sup '}. The calculations rely on two assumptions: the two-state model, and a small differential overlap between donor and acceptor subsystem densities. The resulting electronic couplings agree well with benchmark values for those exchange-correlation functionals that contain a high percentage of exact exchange. Instead, when semilocal GGA functionals are used the electronic couplings are grossly overestimated.

  6. Photoinduced electron transfer from phycoerythrin to colloidal metal semiconductor nanoparticles

    NASA Astrophysics Data System (ADS)

    Kathiravan, A.; Chandramohan, M.; Renganathan, R.; Sekar, S.

    2009-04-01

    Phycoerythrin is a water soluble pigment which absorbs in the visible region at 563 nm. The interaction of phycoerythrin with colloidal metal semiconductors was studied by absorption, FT-IR and fluorescence spectroscopy. Phycoerythrin adsorbed strongly on the surface of TiO 2 nanoparticles, the apparent association constant for the association between colloidal metal-TiO 2 nanoparticles and phycoerythrin was determined from fluorescence quenching data. The free energy change (Δ Get) for electron transfer process has been calculated by applying Rehm-Weller equation.

  7. Light induced electron transfer reactions of metal complexes

    SciTech Connect

    Sutin, N.; Creutz, C.

    1980-01-01

    Properties of the excited states of tris(2,2'-bipyridine) and tris(1,10-phenanthroline) complexes of chromium(III), iron(II), ruthenium(II), osmium(II), rhodium(III), and iridium(III) are described. The electron transfer reactions of the ground and excited states are discussed and interpreted in terms of the driving force for the reaction and the distortions of the excited states relative to the corresponding ground states. General considerations relevant to the conversion of light into chemical energy are presented and progress in the use of polypyridine complexes to effect the light decomposition of water into hydrogen and oxygen is reviewed.

  8. Modeling microscale heat transfer using Calore.

    SciTech Connect

    Gallis, Michail A.; Rader, Daniel John; Wong, Chung-Nin Channy; Bainbridge, Bruce L.; Torczynski, John Robert; Piekos, Edward Stanley

    2005-09-01

    Modeling microscale heat transfer with the computational-heat-transfer code Calore is discussed. Microscale heat transfer problems differ from their macroscopic counterparts in that conductive heat transfer in both solid and gaseous materials may have important noncontinuum effects. In a solid material, three noncontinuum effects are considered: ballistic transport of phonons across a thin film, scattering of phonons from surface roughness at a gas-solid interface, and scattering of phonons from grain boundaries within the solid material. These processes are modeled for polycrystalline silicon, and the thermal-conductivity values predicted by these models are compared to experimental data. In a gaseous material, two noncontinuum effects are considered: ballistic transport of gas molecules across a thin gap and accommodation of gas molecules to solid conditions when reflecting from a solid surface. These processes are modeled for arbitrary gases by allowing the gas and solid temperatures across a gas-solid interface to differ: a finite heat transfer coefficient (contact conductance) is imposed at the gas-solid interface so that the temperature difference is proportional to the normal heat flux. In this approach, the behavior of gas in the bulk is not changed from behavior observed under macroscopic conditions. These models are implemented in Calore as user subroutines. The user subroutines reside within Sandia's Source Forge server, where they undergo version control and regression testing and are available to analysts needing these capabilities. A Calore simulation is presented that exercises these models for a heated microbeam separated from an ambient-temperature substrate by a thin gas-filled gap. Failure to use the noncontinuum heat transfer models for the solid and the gas causes the maximum temperature of the microbeam to be significantly underpredicted.

  9. Proton-Coupled Electron Transfer in the Reduction of Carbonyls by Samarium Diiodide-Water Complexes.

    PubMed

    Chciuk, Tesia V; Anderson, William R; Flowers, Robert A

    2016-07-20

    Reduction of carbonyls by SmI2 is significantly impacted by the presence of water, but the fundamental step(s) of initial transfer of a formal hydrogen atom from the SmI2-water reagent system to produce an intermediate radical is not fully understood. In this work, we provide evidence consistent with the reduction of carbonyls by SmI2-water proceeding through proton-coupled electron transfer (PCET). Combined rate and computational studies show that a model aldehyde and ketone are likely reduced through an asynchronous PCET, whereas reduction of a representative lactone occurs through a concerted PCET. In the latter case, concerted PCET is likely a consequence of significantly endergonic initial electron transfer. PMID:27367158

  10. The role of electronic structure calculation in mechanistic analysis of electron transfer reactions in the liquid phase

    NASA Astrophysics Data System (ADS)

    Newton, M. D.

    1987-08-01

    Model calculations have been employed to elucidate the mechanism of electron transfer reactions in aqueous solution. The contribution of inner shell OH bonds to activation barriers has been estimated from calculation for metal ion hydrates. Calculated electron transfer matrix elements H sub if for redox processes of the type, ML sub 6 sup 2(+)+ ML sub 6 sup 3(+) in equilibrium ML sub 6 sup 3(+) + ML sub 6 sup 2(+), M = Fe, Co, or Ru, L = H2O or NH3, have been analyzed in terms of various orbital concepts. The matrix elements are based on ab initio wavefunctions for model supermolecule clusters of the type, (ML sub n ...L sub n M) sup 5(+), with n = 1 or 3. The analysis shows that the many-electron H sub if quantities can in fact be expressed to good approximation as effective 1-electron expressions of the type, H sub if proportional to lambda' sup 2 N sub c h sub L1Lr, where lambda' is the metal-ligand covalency parameter, h sub L1Lr is a local 1-electron matrix element for ligand orbitals in contact in the transition state, and N sub c is the number of such contacts. A least-squares fit of the data implies a value of approx. 5000/cm for h sub L1Lr, showing that significant coupling can occur in the absence of formal bonding between reactants.

  11. The electronic transfer of information and aerospace knowledge diffusion

    NASA Technical Reports Server (NTRS)

    Pinelli, Thomas E.; Bishop, Ann P.; Barclay, Rebecca O.; Kennedy, John M.

    1992-01-01

    Increasing reliance on and investment in information technology and electronic networking systems presupposes that computing and information technology will play a motor role in the diffusion of aerospace knowledge. Little is known, however, about actual information technology needs, uses, and problems within the aerospace knowledge diffusion process. The authors state that the potential contributions of information technology to increased productivity and competitiveness will be diminished unless empirically derived knowledge regarding the information-seeking behavior of the members of the social system - those who are producing, transferring, and using scientific and technical information - is incorporated into a new technology policy framework. Research into the use of information technology and electronic networks by U.S. aerospace engineers and scientists, collected as part of a research project designed to study aerospace knowledge diffusion, is presented in support of this assertion.

  12. Biochemical Mechanisms Controlling Terminal Electron Transfer in Geobacter sulfurreducens

    NASA Astrophysics Data System (ADS)

    Helmus, R.; Liermann, L. J.; Brantley, S. L.; Tien, M.

    2009-04-01

    The ability of Geobacter sulfurreducens to use a variety of metals as terminal electron acceptors (TEAs) for cellular respiration makes it attractive for use in bioremediation and implies its importance to mineral cycling in the environment. This study is aimed at understanding the biochemical mechanisms that allow Geobacter sulfurreducens to use soluble and insoluble iron and manganese forms as TEAs for cellular respiration and is the first of its kind to address the kinetics of manganese use as a TEA by G. sulfurreducens. First, G. sulfurreducens was conditioned to grow on various soluble and insoluble iron and manganese forms. G. sulfurreducens demonstrated enhanced growth rates when cultured using soluble TEAs compared with insoluble TEAs. However, the lower growth rate on insoluble iron compared with soluble iron was observed concomitantly with a 1-2 log lower cell density in stationary phase in insoluble iron cultures and a lower growth yield per electron donor used in log growth phase. Furthermore, the growth yield per electron was similar with both soluble and insoluble iron. These results suggest that the net amount of energy available for biomass production achieved from reducing insoluble iron is lower than with soluble iron, which may be due to a different biochemical mechanism catalyzing the electron transfer to TEA dependent upon the solubility of the TEA. One scenario consistent with this notion is that protein(s) in the outer membrane of G. sulfurreducens that transfers electrons to insoluble TEAs does so in a manner that uncouples electron flow from the proton pump in the cellular membrane, similar to what we have observed with Shewanella oneidensis MR-1. Both the growth rate and growth yield of G. sulfurreducens on insoluble manganese were higher than on insoluble iron, indicating that there is a difference in the flow of electrons to the TEA in these two situations. While the different redox potentials of these elements may affect these values

  13. Coupling capillary zone electrophoresis with electron transfer dissociation and activated ion electron transfer dissociation for top-down proteomics.

    PubMed

    Zhao, Yimeng; Riley, Nicholas M; Sun, Liangliang; Hebert, Alexander S; Yan, Xiaojing; Westphall, Michael S; Rush, Matthew J P; Zhu, Guijie; Champion, Matthew M; Mba Medie, Felix; Champion, Patricia A DiGiuseppe; Coon, Joshua J; Dovichi, Norman J

    2015-01-01

    Top-down proteomics offers the potential for full protein characterization, but many challenges remain for this approach, including efficient protein separations and effective fragmentation of intact proteins. Capillary zone electrophoresis (CZE) has shown great potential for separation of intact proteins, especially for differentially modified proteoforms of the same gene product. To date, however, CZE has been used only with collision-based fragmentation methods. Here we report the first implementation of electron transfer dissociation (ETD) with online CZE separations for top-down proteomics, analyzing a mixture of four standard proteins and a complex protein mixture from the Mycobacterium marinum bacterial secretome. Using a multipurpose dissociation cell on an Orbitrap Elite system, we demonstrate that CZE is fully compatible with ETD as well as higher energy collisional dissociation (HCD), and that the two complementary fragmentation methods can be used in tandem on the electrophoretic time scale for improved protein characterization. Furthermore, we show that activated ion electron transfer dissociation (AI-ETD), a recently introduced method for enhanced ETD fragmentation, provides useful performance with CZE separations to greatly increase protein characterization. When combined with HCD, AI-ETD improved the protein sequence coverage by more than 200% for proteins from both standard and complex mixtures, highlighting the benefits electron-driven dissociation methods can add to CZE separations. PMID:25893372

  14. Electron transfer kinetics in water-splitting dye-sensitized photoelectrochemical cells

    NASA Astrophysics Data System (ADS)

    Swierk, John R.

    steps of injection, recombination, and hole transfer among others. Finally, in Chapter 6 a model system that describes electron transfer between an oxidized sensitizer and water oxidation catalyst is demonstrated and provides insight into sensitizer regeneration in WS-DSPECs. Together the results in these chapters present a detailed picture of how electron scavenging, recombination, and transport combine to generate photocurrent in a fully characterized WS-DSPEC and serve as starting point for the further development of WS-DSPECs.

  15. MD studies of electron transfer at ambient and elevated pressures

    NASA Astrophysics Data System (ADS)

    Giles, Alex; Spooner, Jacob; Weinberg, Noham

    2013-06-01

    The effect of pressure on the rate constants of outer-sphere electron transfer reactions has often been described using the Marcus-Hush theory. This theory agrees well with experiment when internal reorganization of the ionic system is negligible, however it does not offer a recipe for calculation of the effects that result from significant solute restructuring. We have recently developed a molecular dynamics technique that accurately describes structural dependence of molecular volumes in non-polar and weakly polar systems. We are now extending this approach to the case of highly polar ionic systems where both solvent and solute restructuring components are important. For this purpose we construct pressure-dependent two-dimensional surfaces for electron transfer reactions in coordinate system composed of interionic distance and Marcus-type solvent polarization coordinate, and use these surfaces to describe pressure effects on reaction kinetics. R.A. Marcus. J. Chem. Phys. 24, 966 (1956); 24, 979 (1956); 26, 867 (1957). Discuss. Faraday Soc. 29, 21 (1960). Faraday Discuss. Chem. Soc. 74, 7 (1982); N.S. Hush. Trans. Faraday Soc. 57, 557 (1961).

  16. An ab initio study of specific solvent effects on the electronic coupling element in electron transfer reactions

    NASA Astrophysics Data System (ADS)

    Henderson, Thomas M.; Cave, Robert J.

    1998-11-01

    Specific solvent effects on the electronic coupling element for electron transfer are examined using two model donor-acceptor systems (Zn2+ and Li2+) and several model "solvent" species (He, Ne, H2O, and NH3). The effects are evaluated relative to the given donor-acceptor pair without solvent present. The electronic coupling element (Hab) is found to depend strongly on the identity of the intervening solvent, with He atoms decreasing Hab, whereas H2O and NH3 significantly increase Hab. The distance dependence (essentially exponential decay) is weakly affected by a single intervening solvent atom-molecule. However, when the donor-acceptor distance increases in concert with addition of successively greater numbers of solvent species, the decay with distance of Hab is altered appreciably. Effects due to varying the orientation of molecular solvent are found, somewhat surprisingly, to be quite modest.

  17. Electrochemical analysis of proton and electron transfer equilibria of the reducible moieties in humic acids.

    PubMed

    Aeschbacher, Michael; Vergari, Daniele; Schwarzenbach, René P; Sander, Michael

    2011-10-01

    Humic substances play a key role in biogeochemical and pollutant redox reactions. The objective of this work was to characterize the proton and electron transfer equilibria of the reducible moieties in different humic acids (HA). Cyclic voltammetry experiments demonstrated that diquat and ethylviologen mediated electron transfer between carbon working electrodes and HA. These compounds were used also to facilitate attainment of redox equilibria between redox electrodes and HA in potentiometric E(h) measurements. Bulk electrolysis of HA combined with pH-stat acid titration demonstrated that electron transfer to the reducible moieties in HA also resulted in proton uptake, suggesting decreasing reduction potentials E(h) of HA with increasing pH. This was confirmed by potentiometric E(h)-pH titrations of HA at different redox states. E(h) measurements of HA samples prereduced to different redox states by bulk electrolysis revealed reducible moieties in HA that cover a wide range of apparent standard reduction potentials at pH 7 from E(h)(0)* = +0.15 to -0.3 V. Modeling revealed an overall increase in the relative abundance of reducible moieties with decreasing E(h). The wide range of HA is consistent with its involvement in numerous environmental electron transfer reactions under various redox conditions. PMID:21823669

  18. Electron transfer of Pseudomonas aeruginosa CP1 in electrochemical reduction of nitric oxide.

    PubMed

    Zhou, Shaofeng; Huang, Shaobin; He, Jiaxin; Li, Han; Zhang, Yongqing

    2016-10-01

    This study reports catalytic electro-chemical reduction of nitric oxide (NO) enhanced by Pseudomonas aeruginosa strain CP1. The current generated in the presence of bacteria was 4.36times that in the absence of the bacteria. The strain was able to catalyze electro-chemical reduction of NO via indirect electron transfer with an electrode, revealed by a series of cyclic voltammetry experiments. Soluble electron shuttles secreted into solution by live bacteria were responsible for the catalytic effects. The enhancement of NO reduction was also confirmed by detection of nitrous oxide; the level of this intermediate was 46.4% higher in the presence of bacteria than in controls, illustrated that the electron transfer pathway did not directly reduce nitric oxide to N2. The findings of this study may offer a new model for bioelectrochemical research in the field of NO removal by biocatalysts. PMID:27426634

  19. Vibrational dynamics in photoinduced electron transfer. Progress report, December 1, 1992--November 30, 1993

    SciTech Connect

    Spears, K.G.

    1993-09-08

    Objective is to perform a new type of measurement for optically excited electron transfer processes that can provide unique experimental insight into the molecular mechanism of electron transfer. Measurements of optically excited electron transfer are done with picosecond infrared (IR) absorption spectroscopy to monitor the vibrational motions of the molecules immediately after electron transfer. Theory and experiment suggest that molecular vibrations and distortions are important controlling elements for electron transfer, and direct information has yet to be obtained on these elements of electron transfer mechanisms. The second period of funding has been dedicated to finishing technique development and performing studies of electron transfer in ion pair systems to identify if vibrational dependent electron transfer rates are present in this system. We have succeeded in measuring, for the first time, electron transfer rates as a function of vibrational state in an ion pair complex in solution. In a different area of electron transfer research we have proposed a new mechanism of solvent gated electron transfer.

  20. Interfacial Electron Transfer and Transient Photoconductivity Studied with Terahertz Spectroscopy

    NASA Astrophysics Data System (ADS)

    Milot, Rebecca Lee

    Terahertz spectroscopy is distinguished from other far infrared and millimeter wave spectroscopies by its inherent phase sensitivity and sub-picosecond time resolution making it a versatile technique to study a wide range of physical phenomena. As THz spectroscopy is still a relatively new field, many aspects of THz generation mechanisms have not been fully examined. Using terahertz emission spectroscopy (TES), THz emission from ZnTe(110) was analyzed and found to be limited by two-photon absorption and free-carrier generation at high excitation fluences. Due to concerns about the continued use of fossil fuels, solar energy has been widely investigated as a promising source of renewable energy. Dye-sensitized solar cells (DSSCs) have been developed as a low-cost alternative to conventional photovoltaic solar cells. To solve the issues of the intermittency and inefficient transport associated with solar energy, researchers are attempting to adapt DSSCs for water oxidation and chemical fuel production. Both device designs incorporate sensitizer molecules covalently bound to metal oxide nanoparticles. The sensitizer, which is comprised of a chromophore and anchoring group, absorbs light and transfers an electron from its excited state to the conduction band of the metal oxide, producing an electric current. Using time-resolved THz spectroscopy (TRTS), an optical pump/THz probe technique, the efficiency and dynamics of electron injection from sensitizers to metal oxides was evaluated as a function of the chromophore, its anchoring group, and the metal oxide identity. Experiments for studying fully functioning DSSCs and water oxidation devices are also described. Bio-inspired pentafluorophenyl porphyrin chromophores have been designed and synthesized for use in photoelectrochemical water oxidation cells. Influences on the efficiency and dynamics of electron injection from the chromophores into TiO2 and SnO2 nanoparticles due to changes in both the central substituent to

  1. Topological analyses of time-dependent electronic structures: application to electron-transfers in methionine enkephalin.

    PubMed

    Pilmé, Julien; Luppi, Eleonora; Bergès, Jacqueline; Houée-Lévin, Chantal; de la Lande, Aurélien

    2014-08-01

    We have studied electron transfers (ET) between electron donors and acceptors, taking as illustrative example the case of ET in methionine enkephalin. Recent pulse and gamma radiolysis experiments suggested that an ultrafast ET takes place from the C-terminal tyrosine residue to the N-terminal, oxidized, methionine residue. According to standard theoretical frameworks like the Marcus theory, ET can be decomposed into two successive steps: i) the achievement through thermal fluctuations, of a set of nuclear coordinates associated with degeneracy of the two electronic states, ii) the electron tunneling from the donor molecular orbital to the acceptor molecular orbital. Here, we focus on the analysis of the time-dependent electronic dynamics during the tunneling event. This is done by extending the approaches based on the topological analyses of stationary electronic density and of the electron localization function (ELF) to the time-dependent domain. Furthermore, we analyzed isosurfaces of the divergence of the current density, showing the paths that are followed by the tunneling electron from the donor to the acceptor. We show how these functions can be calculated with constrained density functional theory. Beyond this work, the topological tools used here can open up new opportunities for the electronic description in the time-dependent domain. PMID:25060148

  2. Experimental insights on the electron transfer and energy transfer processes between Ce{sup 3+}-Yb{sup 3+} and Ce{sup 3+}-Tb{sup 3+} in borate glass

    SciTech Connect

    Sontakke, Atul D. Katayama, Yumiko; Tanabe, Setsuhisa; Ueda, Jumpei; Dorenbos, Pieter

    2015-03-30

    A facile method to describe the electron transfer and energy transfer processes among lanthanide ions is presented based on the temperature dependent donor luminescence decay kinetics. The electron transfer process in Ce{sup 3+}-Yb{sup 3+} exhibits a steady rise with temperature, whereas the Ce{sup 3+}-Tb{sup 3+} energy transfer remains nearly unaffected. This feature has been investigated using the rate equation modeling and a methodology for the quantitative estimation of interaction parameters is presented. Moreover, the overall consequences of electron transfer and energy transfer process on donor-acceptor luminescence behavior, quantum efficiency, and donor luminescence decay kinetics are discussed in borate glass host. The results in this study propose a straight forward approach to distinguish the electron transfer and energy transfer processes between lanthanide ions in dielectric hosts, which is highly advantageous in view of the recent developments on lanthanide doped materials for spectral conversion, persistent luminescence, and related applications.

  3. Computational methods for intramolecular electron transfer in a ferrous-ferric iron complex.

    PubMed

    Zarzycki, Piotr; Kerisit, Sebastien; Rosso, Kevin

    2011-09-01

    The limitations of common theoretical and molecular computational approaches for predicting electron transfer quantities were assessed, using an archetypal bridged ferrous-ferric electron transfer system in aqueous solution. The basis set effect on the magnitude of the electronic coupling matrix element computed using the quasi-diabatic method was carefully examined, and it was found that the error related to a poor basis set could exceed the thermal energy at room temperature. A range of approaches to determining the external (solvent) reorganization energy were also investigated. Significant improvements from the Marcus continuum model can be obtained by including dipolar Born-Kirkwood-Onsager correction. In this regard, we also found that Klamt's Conductor-Like Screening Model (COSMO) yields estimations of the external reorganization energy similar to those obtained with explicit solvent molecular dynamics simulations if the fast-frequency modes are neglected, which makes it an attractive alternative to laborious umbrella sampling simulations. By using the COSMO model, we also confirm that a decrease in curvature of the potential energy surface is a manifestation of the dielectric saturation observed in the first solvation layer. The linearity of solvent response to the charge redistribution was assessed by analyzing the energy gap autocorrelation function as well as the solvent density and dipole moment fluctuations. Molecular dynamics was also used to evaluate the sign and magnitude of the solvent reorganization entropy and to determine its effect on the predicted electron transfer rate. Finally, we present a simple way of estimating the vibration frequency along the reaction coordinate, which also enables prediction of the mass-dependent isotopic signature of electron transfer reactions. PMID:21696749

  4. Electron transfer in a two-level system within a Cole-Davidson vitreous bath

    NASA Astrophysics Data System (ADS)

    Zarea, Mehdi; Ratner, Mark A.; Wasielewski, Michael R.

    2014-01-01

    We study electron transfer (ET) in a two level quantum system coupled to a glassy viscous bath. The bath is modeled by the Cole-Davidson (CD) spectral density. The ET in this model is compared to the ET in a normal Drude-Debye (DD) model. It is shown that at low temperatures and when the coupling to the bath is weak, the viscous bath preserves the quantum coherence for a longer time. However in the strong coupling regime, the tunneling rate is higher in the CD. In the classical high temperature limit the difference between the CD and DD models is negligible.

  5. Electron transfer in a two-level system within a Cole-Davidson vitreous bath

    SciTech Connect

    Zarea, Mehdi Ratner, Mark A.; Wasielewski, Michael R.

    2014-01-14

    We study electron transfer (ET) in a two level quantum system coupled to a glassy viscous bath. The bath is modeled by the Cole-Davidson (CD) spectral density. The ET in this model is compared to the ET in a normal Drude-Debye (DD) model. It is shown that at low temperatures and when the coupling to the bath is weak, the viscous bath preserves the quantum coherence for a longer time. However in the strong coupling regime, the tunneling rate is higher in the CD. In the classical high temperature limit the difference between the CD and DD models is negligible.

  6. ATP-induced electron transfer by redox-selective partner recognition

    NASA Astrophysics Data System (ADS)

    Hennig, Sandra E.; Goetzl, Sebastian; Jeoung, Jae-Hun; Bommer, Martin; Lendzian, Friedhelm; Hildebrandt, Peter; Dobbek, Holger

    2014-08-01

    Thermodynamically unfavourable electron transfers are enabled by coupling to an energy-supplying reaction. How the energy is transduced from the exergonic to the endergonic process is largely unknown. Here we provide the structural basis for an energy transduction process in the reductive activation of B12-dependent methyltransferases. The transfer of one electron from an activating enzyme to the cobalamin cofactor is energetically uphill and relies on coupling to an ATPase reaction. Our results demonstrate that the key to coupling is, besides the oxidation state-dependent complex formation, the conformational gating of the electron transfer. Complex formation induces a substitution of the ligand at the electron-accepting Co ion. Addition of ATP initiates electron transfer by provoking conformational changes that destabilize the complex. We show how remodelling of the electron-accepting Co2+ promotes ATP-dependent electron transfer; an efficient strategy not seen in other electron-transferring ATPases.

  7. Variation of geometries and electron properties along proton transfer in strong hydrogen-bond complexes

    NASA Astrophysics Data System (ADS)

    Pacios, L. F.; Gálvez, O.; Gómez, P. C.

    2005-06-01

    Proton transfer in hydrogen-bond systems formed by 4-methylimidazole in both neutral and protonated cationic forms and by acetate anion are studied by means of MP2/6-311++G(d,p) ab initio calculations. These two complexes model the histidine (neutral and protonated)-aspartate diad present in the active sites of enzymes the catalytic mechanism of which involves the formation of strong hydrogen bonds. We investigate the evolution of geometries, natural bond orbital populations of bonds and electron lone pairs, topological descriptors of the electron density, and spatial distributions of the electron localization function along the process N-H ⋯O→N⋯H⋯O→N⋯H-O, which represents the stages of the H-transfer. Except for a sudden change in the population of electron lone pairs in N and O at the middle N...H...O stage, all the properties analyzed show a smooth continuous behavior along the covalent → hydrogen bond transit inherent to the transfer, without any discontinuity that could identify a formation or breaking of the hydrogen bond. This way, the distinction between covalent or hydrogen-bonding features is associated to subtle electron rearrangement at the intermolecular space.

  8. Theory of ultrafast photoinduced electron transfer from a bulk semiconductor to a quantum dot

    SciTech Connect

    Rasmussen, Andrew M. Ramakrishna, S.; Weiss, Emily A.; Seideman, Tamar

    2014-04-14

    This paper describes analytical and numerical results from a model Hamiltonian method applied to electron transfer (ET) from a quasicontinuum (QC) of states to a set of discrete states, with and without a mediating bridge. Analysis of the factors that determine ET dynamics yields guidelines for achieving high-yield electron transfer in these systems, desired for instance for applications in heterogeneous catalysis. These include the choice of parameters of the laser pulse that excites the initial state into a continuum electronic wavepacket and the design of the coupling between the bridge molecule and the donor and acceptor. The vibrational mode on a bridging molecule between donor and acceptor has an influence on the yield of electron transfer via Franck-Condon factors, even in cases where excited vibrational states are only transiently populated. Laser-induced coherence of the initial state as well as energetic overlap is crucial in determining the ET yield from a QC to a discrete state, whereas the ET time is influenced by competing factors from the coupling strength and the coherence properties of the electronic wavepacket.

  9. Numerical Modeling of Ablation Heat Transfer

    NASA Technical Reports Server (NTRS)

    Ewing, Mark E.; Laker, Travis S.; Walker, David T.

    2013-01-01

    A unique numerical method has been developed for solving one-dimensional ablation heat transfer problems. This paper provides a comprehensive description of the method, along with detailed derivations of the governing equations. This methodology supports solutions for traditional ablation modeling including such effects as heat transfer, material decomposition, pyrolysis gas permeation and heat exchange, and thermochemical surface erosion. The numerical scheme utilizes a control-volume approach with a variable grid to account for surface movement. This method directly supports implementation of nontraditional models such as material swelling and mechanical erosion, extending capabilities for modeling complex ablation phenomena. Verifications of the numerical implementation are provided using analytical solutions, code comparisons, and the method of manufactured solutions. These verifications are used to demonstrate solution accuracy and proper error convergence rates. A simple demonstration of a mechanical erosion (spallation) model is also provided to illustrate the unique capabilities of the method.

  10. Evidence of short-range electron transfer of a redox enzyme on graphene oxide electrodes.

    PubMed

    Martins, Marccus V A; Pereira, Andressa R; Luz, Roberto A S; Iost, Rodrigo M; Crespilho, Frank N

    2014-09-01

    Direct electron transfer (DET) between redox enzymes and electrode surfaces is of growing interest and an important strategy in the development of biofuel cells and biosensors. Among the nanomaterials utilized at electrode/enzyme interfaces to enhance the electronic communication, graphene oxide (GO) has been identified as a highly promising candidate. It is postulated that GO layers decrease the distance between the flavin cofactor (FAD/FADH2) of the glucose oxidase enzyme (GOx) and the electrode surface, though experimental evidence concerning the distance dependence of the rate constant for heterogeneous electron-transfer (k(het)) has not yet been observed. In this work, we report the experimentally observed DET of the GOx enzyme adsorbed on flexible carbon fiber (FCF) electrodes modified with GO (FCF-GO), where the k(het) between GO and electroactive GOx has been measured at a structurally well-defined interface. The curves obtained from the Marcus theory were used to obtain k(het), by using the model proposed by Chidsey. In agreement with experimental data, this model proved to be useful to systematically probe the dependence of electron transfer rates on distance, in order to provide an empirical basis to understand the origin of interfacial DET between GO and GOx. We also demonstrate that the presence of GO at the enzyme/electrode interface diminishes the activation energy by decreasing the distance between the electrode surface and FAD/FADH2. PMID:24676540

  11. Characterizing the Locality of Diabatic States for Electronic Excitation Transfer by Decomposing the Diabatic Coupling

    SciTech Connect

    Vura-Weis, Josh; Newton, M. D.; Wasielewski, Michael R; Subotnik, J.E.

    2010-12-09

    A common strategy to calculate electronic coupling matrix elements for charge or energy transfer is to take the adiabatic states generated by electronic structure computations and rotate them to form localized diabatic states. In this paper, we show that, for intermolecular transfer of singlet electronic excitation, usually we cannot fully localize the electronic excitations in this way. Instead, we calculate putative initial and final states with small excitation tails caused by weak interactions with high energy excited states in the electronic manifold. These tails do not lead to substantial changes in the total diabatic coupling between states, but they do lead to a different partitioning of the total coupling between Coulomb (Förster), exchange (Dexter), and one-electron components. The tails may be reduced by using a multistate diabatic model or eliminated entirely by truncation (denoted as “chopping”). Without more information, we are unable to conclude with certainty whether the observed diabatic tails are a physical reality or a computational artifact. This research suggests that decomposition of the diabatic coupling between chromophores into Coulomb, exchange, and one-electron components may depend strongly on the number of states considered, and such results should be treated with caution.

  12. Structural basis of interprotein electron transfer in bacterial sulfite oxidation

    PubMed Central

    McGrath, Aaron P; Laming, Elise L; Casas Garcia, G Patricia; Kvansakul, Marc; Guss, J Mitchell; Trewhella, Jill; Calmes, Benoit; Bernhardt, Paul V; Kappler, Ulrike; Maher, Megan J

    2015-01-01

    Interprotein electron transfer underpins the essential processes of life and relies on the formation of specific, yet transient protein-protein interactions. In biological systems, the detoxification of sulfite is catalyzed by the sulfite-oxidizing enzymes (SOEs), which interact with an electron acceptor for catalytic turnover. Here, we report the structural and functional analyses of the SOE SorT from Sinorhizobium meliloti and its cognate electron acceptor SorU. Kinetic and thermodynamic analyses of the SorT/SorU interaction show the complex is dynamic in solution, and that the proteins interact with Kd = 13.5 ± 0.8 μM. The crystal structures of the oxidized SorT and SorU, both in isolation and in complex, reveal the interface to be remarkably electrostatic, with an unusually large number of direct hydrogen bonding interactions. The assembly of the complex is accompanied by an adjustment in the structure of SorU, and conformational sampling provides a mechanism for dissociation of the SorT/SorU assembly. DOI: http://dx.doi.org/10.7554/eLife.09066.001 PMID:26687009

  13. Structural basis of interprotein electron transfer in bacterial sulfite oxidation.

    PubMed

    McGrath, Aaron P; Laming, Elise L; Casas Garcia, G Patricia; Kvansakul, Marc; Guss, J Mitchell; Trewhella, Jill; Calmes, Benoit; Bernhardt, Paul V; Hanson, Graeme R; Kappler, Ulrike; Maher, Megan J

    2015-01-01

    Interprotein electron transfer underpins the essential processes of life and relies on the formation of specific, yet transient protein-protein interactions. In biological systems, the detoxification of sulfite is catalyzed by the sulfite-oxidizing enzymes (SOEs), which interact with an electron acceptor for catalytic turnover. Here, we report the structural and functional analyses of the SOE SorT from Sinorhizobium meliloti and its cognate electron acceptor SorU. Kinetic and thermodynamic analyses of the SorT/SorU interaction show the complex is dynamic in solution, and that the proteins interact with Kd = 13.5 ± 0.8 μM. The crystal structures of the oxidized SorT and SorU, both in isolation and in complex, reveal the interface to be remarkably electrostatic, with an unusually large number of direct hydrogen bonding interactions. The assembly of the complex is accompanied by an adjustment in the structure of SorU, and conformational sampling provides a mechanism for dissociation of the SorT/SorU assembly. PMID:26687009

  14. Coherent Transfer of Electronic Wavepacket Motion Between Atoms

    NASA Astrophysics Data System (ADS)

    Zhou, Tao; Richards, B. G.; Jones, R. R.

    2016-05-01

    We have shown that electron correlations, induced by controlled dipole-dipole (DD) interactions, can enable the coherent transfer of electronic wavepacket motion from atoms to their neighbors. In the experiment, a 5 ns tunable dye laser excites Rb atoms in a MOT to the 25s state in a weak static electric field for which the tunable 25s 33 s <--> 24p34p DD interaction is resonant. A picosecond THz pulse then further excites each Rydberg atom into a coherent superposition, of 25s and 24p states. The evolution of this mixed-parity wavepacket is characterized by time-dependent oscillations in the electric dipole moment, with a period of 2.9 ps. Approximately 5 ns after the wavepacket creation, a second 5 ns dye-laser promotes a second set of atoms from the 5p level into the 33s state. Because of the DD interaction, the second dye laser actually creates atom pairs whose electronic states are correlated via the resonant DD coupling. A 33 s + 34p wavepacket, oscillating with the same 2.9 ps period as the 25 s + 24p wavepacket, develops on the second set of atoms as a result of the correlation. A second, time-delayed ps THz pulse enables the detection of the coherent wavepacket motion on the two sets of atoms. This research has been supported by the NSF.

  15. Transfer equations for modeling interrill erosion

    NASA Astrophysics Data System (ADS)

    Bako Amina, Nouhou; Frédéric, Darboux; François, James; Carine, Lucas

    2016-04-01

    Numerous models are available for matter transfer along an hillslope. They are usually process-specific, requiring to use several models to simulate transfers along an hillslope. To overcome this issue, we develop a new model valid for chemical (nutrients, pollutants, dissolved carbon) and particle transfers by water. It is able to simulate both interrill and rill erosion. This new equation encompasses the previous models of Gao et al. (2004), Hairsine and Rose (1992, 1991) and Lajeunesse et al. (2013) in a single and unified form. We show that it can account for multi-class particle transport able to simulate both linear and non-linear behaviors. Surface conditions (crusts) is accounted for, making possible for space and time changes of soil properties. For the calibration of the model, specific laboratory experiments have been carried out to validate the effect of rainfall on travel distance of particles. These experiments allow to separate detachment by raindrops from the agitation of the flow by the drops. Different particle sizes and rainfall kinetic energies are investigated. The results assess the exact role of rainfall on sediment transport. Our new model is able to represent adequately these experimental results.

  16. Electron transfer in mixed-valence tetranuclear iron clusters. Orbital effects and magnetic properties

    NASA Astrophysics Data System (ADS)

    Borrás-Almenar, J. J.; Coronado, E.; Georges, R.; Gomez-García, C. J.

    1993-11-01

    The energy levels and magnetic properties of mixed-valence iron clusters with cubane structure are studied from a model that takes into account the different electron transfer paths between the e g-type metal orbitals via the p orbitals of the bridging atoms. The interplay between double exchange and superexchange on the magnetic properties are discussed, including distortion effects of the cubane structure from T d to D 2d symmetries.

  17. Magnetic properties of mixed-valence tetranuclear iron clusters: electron transfer versus exchange interactions

    NASA Astrophysics Data System (ADS)

    Borras-Almenar, J. J.; Coronado, E.; Georges, R.; Gómez-García, C. J.

    1992-02-01

    In order to obtain the energy levels of a mixed-valence [Fe 4S 4] 3+ cubane-type cluster, we develop a model that takes into account the electron delocalization within the cluster as well as the superexchange interactions. Trends of level pattern are discussed as a function of the sign and magnitude of transfer intergrals, and compared with those obtained on mixed-valence dimers and trimers.

  18. Fabrication and single-electron-transfer operation of a triple-dot single-electron transistor

    SciTech Connect

    Jo, Mingyu Uchida, Takafumi; Tsurumaki-Fukuchi, Atsushi; Arita, Masashi; Takahashi, Yasuo; Fujiwara, Akira; Nishiguchi, Katsuhiko; Ono, Yukinori; Inokawa, Hiroshi

    2015-12-07

    A triple-dot single-electron transistor was fabricated on silicon-on-insulator wafer using pattern-dependent oxidation. A specially designed one-dimensional silicon wire having small constrictions at both ends was converted to a triple-dot single-electron transistor by means of pattern-dependent oxidation. The fabrication of the center dot involved quantum size effects and stress-induced band gap reduction, whereas that of the two side dots involved thickness modulation because of the complex edge structure of two-dimensional silicon. Single-electron turnstile operation was confirmed at 8 K when a 100-mV, 1-MHz square wave was applied. Monte Carlo simulations indicated that such a device with inhomogeneous tunnel and gate capacitances can exhibit single-electron transfer.

  19. Unraveling the Interplay of Backbone Rigidity and Electron Rich Side-Chains on Electron Transfer in Peptides: The Realization of Tunable Molecular Wires

    PubMed Central

    2015-01-01

    Electrochemical studies are reported on a series of peptides constrained into either a 310-helix (1–6) or β-strand (7–9) conformation, with variable numbers of electron rich alkene containing side chains. Peptides (1 and 2) and (7 and 8) are further constrained into these geometries with a suitable side chain tether introduced by ring closing metathesis (RCM). Peptides 1, 4 and 5, each containing a single alkene side chain reveal a direct link between backbone rigidity and electron transfer, in isolation from any effects due to the electronic properties of the electron rich side-chains. Further studies on the linear peptides 3–6 confirm the ability of the alkene to facilitate electron transfer through the peptide. A comparison of the electrochemical data for the unsaturated tethered peptides (1 and 7) and saturated tethered peptides (2 and 8) reveals an interplay between backbone rigidity and effects arising from the electron rich alkene side-chains on electron transfer. Theoretical calculations on β-strand models analogous to 7, 8 and 9 provide further insights into the relative roles of backbone rigidity and electron rich side-chains on intramolecular electron transfer. Furthermore, electron population analysis confirms the role of the alkene as a “stepping stone” for electron transfer. These findings provide a new approach for fine-tuning the electronic properties of peptides by controlling backbone rigidity, and through the inclusion of electron rich side-chains. This allows for manipulation of energy barriers and hence conductance in peptides, a crucial step in the design and fabrication of molecular-based electronic devices. PMID:25122122

  20. Radiative transfer model: matrix operator method.

    PubMed

    Liu, Q; Ruprecht, E

    1996-07-20

    A radiative transfer model, the matrix operator method, is discussed here. The matrix operator method is applied to a plane-parallel atmosphere within three spectral ranges: the visible, the infrared, and the microwave. For a homogeneous layer with spherical scattering, the radiative transfer equation can be solved analytically. The vertically inhomogeneous atmosphere can be subdivided into a set of homogeneous layers. The solution of the radiative transfer equation for the vertically inhomogeneous atmosphere is obtained recurrently from the analytical solutions for the subdivided layers. As an example for the application of the matrix operator method, the effects of the cirrus and the stratocumulus clouds on the net radiation at the surface and at the top of the atmosphere are investigated. The relationship between the polarization in the microwave range and the rain rates is also studied. Copies of the FORTRAN program and the documentation of the FORTRAN program on a diskette are available. PMID:21102832

  1. Electronic shift register memory based on molecular electron-transfer reactions

    NASA Technical Reports Server (NTRS)

    Hopfield, J. J.; Onuchic, Jose Nelson; Beratan, David N.

    1989-01-01

    The design of a shift register memory at the molecular level is described in detail. The memory elements are based on a chain of electron-transfer molecules incorporated on a very large scale integrated (VLSI) substrate, and the information is shifted by photoinduced electron-transfer reactions. The design requirements for such a system are discussed, and several realistic strategies for synthesizing these systems are presented. The immediate advantage of such a hybrid molecular/VLSI device would arise from the possible information storage density. The prospect of considerable savings of energy per bit processed also exists. This molecular shift register memory element design solves the conceptual problems associated with integrating molecular size components with larger (micron) size features on a chip.

  2. Layered Black Phosphorus: Strongly Anisotropic Magnetic, Electronic, and Electron-Transfer Properties.

    PubMed

    Sofer, Zdeněk; Sedmidubský, David; Huber, Štěpán; Luxa, Jan; Bouša, Daniel; Boothroyd, Chris; Pumera, Martin

    2016-03-01

    Layered elemental materials, such as black phosphorus, exhibit unique properties originating from their highly anisotropic layered structure. The results presented herein demonstrate an anomalous anisotropy for the electrical, magnetic, and electrochemical properties of black phosphorus. It is shown that heterogeneous electron transfer from black phosphorus to outer- and inner-sphere molecular probes is highly anisotropic. The electron-transfer rates differ at the basal and edge planes. These unusual properties were interpreted by means of calculations, manifesting the metallic character of the edge planes as compared to the semiconducting properties of the basal plane. This indicates that black phosphorus belongs to a group of materials known as topological insulators. Consequently, these effects render the magnetic properties highly anisotropic, as both diamagnetic and paramagnetic behavior can be observed depending on the orientation in the magnetic field. PMID:26822395

  3. 27 CFR 27.48a - Payment of tax by electronic fund transfer.

    Code of Federal Regulations, 2010 CFR

    2010-04-01

    ... corporations, as defined in 26 U.S.C. 1563, and implementing regulations in 26 CFR 1.1563-1 through 1.1563-4.... (c) Electronic fund transfer or EFT means any transfer of funds, other than a transaction originated... electronic fund transfer. 27.48a Section 27.48a Alcohol, Tobacco Products and Firearms ALCOHOL AND...

  4. Proton-Coupled Electron Transfer: Moving Together and Charging Forward

    PubMed Central

    2016-01-01

    Proton-coupled electron transfer (PCET) is ubiquitous throughout chemistry and biology. This Perspective discusses recent advances and current challenges in the field of PCET, with an emphasis on the role of theory and computation. The fundamental theoretical concepts are summarized, and expressions for rate constants and kinetic isotope effects are provided. Computational methods for calculating reduction potentials and pKa’s for molecular electrocatalysts, as well as insights into linear correlations and non-innocent ligands, are also described. In addition, computational methods for simulating the nonadiabatic dynamics of photoexcited PCET are discussed. Representative applications to PCET in solution, proteins, electrochemistry, and photoinduced processes are presented, highlighting the interplay between theoretical and experimental studies. The current challenges and suggested future directions are outlined for each type of application, concluding with an overall view to the future. PMID:26110700

  5. Electron transfer kinetics at polarized nanoscopic liquid/liquid interfaces.

    PubMed

    Cai, Chenxin; Mirkin, Michael V

    2006-01-11

    Rapid kinetics of electron transfer (ET) reactions across the interface between water and 1,2-dichloroethane were measured by steady-state voltammetry at nanopipet electrodes (50- to 400-nm orifice radius). The origins of previously reported imperfect voltammetric responses of ET reactions at micropipets were investigated. Several new experimental systems were explored, and two of them yielded high-quality voltammograms suitable for kinetic experiments. The determined standard rate constants were compared to those measured previously at polarized and nonpolarized liquid/liquid interfaces. The effect of the interfacial dimensions on the magnitude of the apparent ET rate constant is discussed. A new approach to ET kinetic measurements based on the use of the scanning electrochemical microscope with a nanopipet tip and a metallic substrate has been developed and employed to check the validity of determined kinetic parameters. PMID:16390144

  6. Proton-Coupled Electron Transfer: Moving Together and Charging Forward.

    PubMed

    Hammes-Schiffer, Sharon

    2015-07-22

    Proton-coupled electron transfer (PCET) is ubiquitous throughout chemistry and biology. This Perspective discusses recent advances and current challenges in the field of PCET, with an emphasis on the role of theory and computation. The fundamental theoretical concepts are summarized, and expressions for rate constants and kinetic isotope effects are provided. Computational methods for calculating reduction potentials and pKa's for molecular electrocatalysts, as well as insights into linear correlations and non-innocent ligands, are also described. In addition, computational methods for simulating the nonadiabatic dynamics of photoexcited PCET are discussed. Representative applications to PCET in solution, proteins, electrochemistry, and photoinduced processes are presented, highlighting the interplay between theoretical and experimental studies. The current challenges and suggested future directions are outlined for each type of application, concluding with an overall view to the future. PMID:26110700

  7. Variation among Desulfovibrio species in electron transfer systems used for syntrophic growth.

    PubMed

    Meyer, Birte; Kuehl, Jennifer; Deutschbauer, Adam M; Price, Morgan N; Arkin, Adam P; Stahl, David A

    2013-03-01

    Mineralization of organic matter in anoxic environments relies on the cooperative activities of hydrogen producers and consumers linked by interspecies electron transfer in syntrophic consortia that may include sulfate-reducing species (e.g., Desulfovibrio). Physiological differences and various gene repertoires implicated in syntrophic metabolism among Desulfovibrio species suggest considerable variation in the biochemical basis of syntrophy. In this study, comparative transcriptional and mutant analyses of Desulfovibrio alaskensis strain G20 and Desulfovibrio vulgaris strain Hildenborough growing syntrophically with Methanococcus maripaludis on lactate were used to develop new and revised models for their alternative electron transfer and energy conservation systems. Lactate oxidation by strain G20 generates a reduced thiol-disulfide redox pair(s) and ferredoxin that are energetically coupled to H(+)/CO(2) reduction by periplasmic formate dehydrogenase and hydrogenase via a flavin-based reverse electron bifurcation process (electron confurcation) and a menaquinone (MQ) redox loop-mediated reverse electron flow involving the membrane-bound Qmo and Qrc complexes. In contrast, strain Hildenborough uses a larger number of cytoplasmic and periplasmic proteins linked in three intertwining pathways to couple H(+) reduction to lactate oxidation. The faster growth of strain G20 in coculture is associated with a kinetic advantage conferred by the Qmo-MQ-Qrc loop as an electron transfer system that permits higher lactate oxidation rates under elevated hydrogen levels (thereby enhancing methanogenic growth) and use of formate as the main electron-exchange mediator (>70% electron flux), as opposed to the primarily hydrogen-based exchange by strain Hildenborough. This study further demonstrates the absence of a conserved gene core in Desulfovibrio that would determine the ability for a syntrophic lifestyle. PMID:23264581

  8. Variation among Desulfovibrio Species in Electron Transfer Systems Used for Syntrophic Growth

    PubMed Central

    Meyer, Birte; Kuehl, Jennifer; Deutschbauer, Adam M.; Price, Morgan N.; Arkin, Adam P.

    2013-01-01

    Mineralization of organic matter in anoxic environments relies on the cooperative activities of hydrogen producers and consumers linked by interspecies electron transfer in syntrophic consortia that may include sulfate-reducing species (e.g., Desulfovibrio). Physiological differences and various gene repertoires implicated in syntrophic metabolism among Desulfovibrio species suggest considerable variation in the biochemical basis of syntrophy. In this study, comparative transcriptional and mutant analyses of Desulfovibrio alaskensis strain G20 and Desulfovibrio vulgaris strain Hildenborough growing syntrophically with Methanococcus maripaludis on lactate were used to develop new and revised models for their alternative electron transfer and energy conservation systems. Lactate oxidation by strain G20 generates a reduced thiol-disulfide redox pair(s) and ferredoxin that are energetically coupled to H+/CO2 reduction by periplasmic formate dehydrogenase and hydrogenase via a flavin-based reverse electron bifurcation process (electron confurcation) and a menaquinone (MQ) redox loop-mediated reverse electron flow involving the membrane-bound Qmo and Qrc complexes. In contrast, strain Hildenborough uses a larger number of cytoplasmic and periplasmic proteins linked in three intertwining pathways to couple H+ reduction to lactate oxidation. The faster growth of strain G20 in coculture is associated with a kinetic advantage conferred by the Qmo-MQ-Qrc loop as an electron transfer system that permits higher lactate oxidation rates under elevated hydrogen levels (thereby enhancing methanogenic growth) and use of formate as the main electron-exchange mediator (>70% electron flux), as opposed to the primarily hydrogen-based exchange by strain Hildenborough. This study further demonstrates the absence of a conserved gene core in Desulfovibrio that would determine the ability for a syntrophic lifestyle. PMID:23264581

  9. Computational methods for intramolecular electron transfer in a ferrous-ferric iron complex

    SciTech Connect

    Zarzycki, Piotr P.; Kerisit, Sebastien N.; Rosso, Kevin M.

    2011-07-29

    The limitations of common theoretical and molecular computational approaches for predicting electron transfer quantities were assessed, using an archetypal bridged ferrous-ferric electron transfer system in aqueous solution. The basis set effect on the magnitude of the electronic coupling matrix element computed using the quasi-diabatic method was carefully examined, and it was found that the error related to a poor basis set could exceed the thermal energy at room temperature. A range of approaches to determining the external (solvent) reorganization energy were also investigated. Significant improvements from the Marcus continuum model can be obtained by including dipolar Born-Kirkwood-Onsager correction. In this regard we also found that Klamt’s Conductor-Like Screening Model (COSMO) yields estimations of the external reorganization energy similar to those obtained with explicit solvent molecular dynamics simulations, if the fast-frequency modes are neglected, which makes it an attractive alternative to laborious umbrella sampling simulations. As expected, dielectric saturation observed in the first solvation shell decreases the curvature of the potential energy surface, but it nonetheless remains a quadratic function of the reaction coordinate. The linearity of solvent response to the charge redistribution was assessed by analyzing the energy gap autocorrelation function as well as the solvent density and dipole moment fluctuations. Molecular dynamics was also used to evaluate the sign and magnitude of the solvent reorganization entropy, to determine its effect on the predicted electron transfer rate. Finally, we present a simple way of estimating the vibration frequency along the reaction coordinate, which also enables prediction of the mass dependent isotopic signature of electron-transfer reactions.

  10. Electron transfer between [4Fe-4S] clusters

    NASA Astrophysics Data System (ADS)

    Voityuk, Alexander A.

    2010-07-01

    Iron-sulfur clusters [4Fe-4S] are major components in biological electron transport (ET). Using the DFT/B3LYP method, we calculate electronic coupling for low- and high-potential ET between [4Fe-4S] clusters and explore its sensitivity to structural parameters of the system and external electric field. As an example, we consider a model of the bacterial respiratory complex I and estimate the role of neighboring amino acids in facilitating the ET process between the clusters. Our results suggest that the superexchange mechanism rather than hole hopping should be operative in ET between [4Fe-4S] redox centers.

  11. Structural studies of photoinduced intramolecular electron transfer in cyclopentadienylnickelnitrosyl

    SciTech Connect

    Chen, L.X.; Bowman, M.K.; Wang, Zhiyu; Norris, J.R. |; Montano, P.A. |

    1994-03-01

    A structural study based on EXAFS, FTIR, and optical absorption spectroscopies has been conducted on a photogenerated, metastable state of cyclopentadienylnickelnitrosyl (CpNiNO) produced by a reversible photochemical reaction. The photogenerated, metastable state with distinctively different EXAFS, IR, and optical absorption spectra from those of the ground state molecules was created by irradiating the sample with the 365 nm line of a mercury lamp at 20K . At the same temperature, the reverse reaction was induced by irradiation with the 313 nm line from the mercury lamp. Based on the analysis of the EXAFS data, the photogenerated, metastable state of CpNiNO has undergone considerable nuclear rearrangements compared to its ground state. The nuclear movement is characterized by a 0.12{angstrom} elongation of Ni-N bond and by a bending of Ni-N-O. A shift of the N-O stretching frequency from 1824 to 1387 cm{sup {minus}1} was observed in the photoinduced reaction with 365 nm light, implying that a NO{sup {minus}} like species results from intramolecular electron transfer from Ni to NO. The changes in the absorption spectra for the same reaction showed reduced absorption of the 385 nm band and a newly generated broad band near IR region. Temperature dependence of the Debye-Waller factor of CpNiNO was in good agreement with the diatomic harmonic oscillator for the Ni-N bond, but deviated for the Ni-O and the Ni-C bonds. Based on the structures obtained from EXAFS, ZINDO calculations for both the ground state and the photogenerated, metastable state of CpNiNO reproduced the general features of the observed absorption spectra and qualitatively explained the wavelength dependence of the reaction. The calculated partial charges on each atom in the ground state and the photogenerated, metastable state of CpNiNO are consistent with intramolecular electron transfer upon photoexcitation by 365 nm light.

  12. Electron and energy transfer as probes of interparticle ion-exchange reactions in zeolite Y

    SciTech Connect

    Brigham, E.S.; Snowden, P.T.; Kim, Y.I.; Mallouk, T.E. )

    1993-08-19

    Photoinduced electron transfer and energy transfer reactions of tris(2,2[prime]-bipyridyl)ruthenium(II) (Ru(bpy)[sub 3][sup 2+]) with methylviologen (MV[sup 2+]) and tris(2,2[prime]-bipyridyl)osmium(II) (Os(bpy)[sub 3][sup 2+]) ion-exchanged onto/into separate zeolite Y particles were studied by emission spectroscopy. The kinetics of interparticle exchange were probed by observing the quenching of the MLCT excited state of-Ru(bpy)[sub 3][sup 2+] by mobile MV[sup 2+] or OS(bpy)[sub 3][sup 2+] ions. The exchange reactions occur on time scales of seconds to hours, depending on the ionic strength of the surrounding medium. The time-dependent luminescence data were fitted to a dispersed kinetics model, from which average rate constants for the exchange reactions could be extracted. Time constants for interparticle exchange of MV[sup 2+] and Os(bpy)[sub 3][sup 2+] ions, in the range 10[sup 3]-10[sup 5] s at electrolyte concentrations of 0.1-3 mM, are significantly longer than the time scales (10[sup [minus]7]-10[sup 1] s) of most electrochemical and photochemical intrazeolitic reactions involving these and similar electroactive ions. These results argue for reaction mechanisms that invoke intrazeolite electron transfer, rather than exchange of electroactive ions followed by solution-phase electron transfer, in these systems. 25 refs., 6 figs., 1 tab.

  13. Electron transfer from flavin to iron in the Pseudomonas oleovorans rubredoxin reductase-rubredoxin electron transfer complex.

    PubMed

    Lee, H J; Basran, J; Scrutton, N S

    1998-11-01

    Rubredoxin reductase (RR) and rubredoxin form a soluble and physiological eT complex. The complex provides reducing equivalents for a membrane-bound omega-hydroxylase, required for the hydroxylation of alkanes and related compounds. The gene (alkT) encoding RR has been overexpressed and the enzyme purified in amounts suitable for studies of eT by stopped-flow spectroscopy. The eT reactions from NADH to the flavin of RR and from reduced RR to the 1Fe and 2Fe forms of rubredoxin have been characterized by transient kinetic and thermodynamic analysis. The reductive half-reaction proceeds in a one-step reaction involving oxidized enzyme and a two-electron-reduced enzyme-NAD+ charge-transfer complex. Flavin reduction is observed at 450 nm and charge-transfer formation at 750 nm; both steps are hyperbolically dependent on NADH concentration. The limiting flavin reduction rate (180 +/- 4 s-1) is comparable to the limiting rate for charge-transfer formation (189 +/- 7 s-1) and analysis at 450 and 750 nm yielded enzyme-NADH dissociation constants of 36 +/- 2 and 43 +/- 5 microM, respectively. Thermodynamic analysis of the reductive half-reaction yielded values for changes in entropy (DeltaS = -65.8 +/- 2.2 J mol-1 K-1), enthalpy (DeltaH = 37.8 +/- 0.6 kJ mol-1) and Gibbs free energy (DeltaG = 57.5 +/- 0.7 kJ mol-1 at 298 K) during hydride ion transfer to the flavin N5 atom. Spectral analysis of mixtures of 1Fe or 2Fe rubredoxin and RR suggest that conformational changes accompany eT complex assembly. Both the 1Fe (nonphysiological) and 2Fe (physiological) forms of rubredoxin were found to oxidize two electron-reduced rubredoxin reductase with approximately equal facility. Rates for the reduction of rubredoxin are hyperbolically dependent on rubredoxin concentration and the limiting rates are 72. 7 +/- 0.6 and 55.2 +/- 0.3 s-1 for the 1Fe and 2Fe forms, respectively. Analysis of the temperature dependence of eT to rubredoxin using eT theory revealed that the reaction is not

  14. 12 CFR 205.14 - Electronic fund transfer service provider not holding consumer's account.

    Code of Federal Regulations, 2010 CFR

    2010-01-01

    ... 12 Banks and Banking 2 2010-01-01 2010-01-01 false Electronic fund transfer service provider not holding consumer's account. 205.14 Section 205.14 Banks and Banking FEDERAL RESERVE SYSTEM BOARD OF GOVERNORS OF THE FEDERAL RESERVE SYSTEM ELECTRONIC FUND TRANSFERS (REGULATION E) § 205.14 Electronic...

  15. Modeling Incoherent Electron Cloud Effects

    SciTech Connect

    Fischer, W.; Benedetto, E.; Rumolo, G.; Schulte, D.; Tomas, R.; Zimmermann, Frank; Franchetti, G.; Ohmi, Kazuhito; Sonnad, K.G.; Vay, Jean-Luc; Pivi, M.T.F.; Raubenheimer, Tor O.; /SLAC

    2008-01-24

    Incoherent electron effects could seriously limit the beam lifetime in proton or ion storage rings, such as LHC, SPS, or RHIC, or blow up the vertical emittance of positron beams, e.g., at the B factories or in linear-collider damping rings. Different approaches to modeling these effects each have their own merits and drawbacks. We describe several simulation codes which simplify the descriptions of the beam-electron interaction and of the accelerator structure in various different ways, and present results for a toy model of the SPS. In addition, we present evidence that for positron beams the interplay of incoherent electron-cloud effects and synchrotron radiation can lead to a significant increase in vertical equilibrium emittance. The magnitude of a few incoherent e{sup +}e{sup -} scattering processes is also estimated. Options for future code development are reviewed.

  16. Modeling Incoherent Electron Cloud Effects

    SciTech Connect

    Vay, Jean-Luc; Benedetto, E.; Fischer, W.; Franchetti, G.; Ohmi, K.; Schulte, D.; Sonnad, K.; Tomas, R.; Vay, J.-L.; Zimmermann, F.; Rumolo, G.; Pivi, M.; Raubenheimer, T.

    2007-06-18

    Incoherent electron effects could seriously limit the beam lifetime in proton or ion storage rings, such as LHC, SPS, or RHIC, or blow up the vertical emittance of positron beams, e.g., at the B factories or in linear-collider damping rings. Different approaches to modeling these effects each have their own merits and drawbacks. We describe several simulation codes which simplify the descriptions of the beam-electron interaction and of the accelerator structure in various different ways, and present results for a toy model of the SPS. In addition, we present evidence that for positron beams the interplay of incoherent electron-cloud effects and synchrotron radiation can lead to a significant increase in vertical equilibrium emittance. The magnitude of a few incoherent e+e- scattering processes is also estimated. Options for future code development are reviewed.

  17. 77 FR 71035 - Financial Management Service; Proposed Collection of Information: Electronic Funds Transfer (EFT...

    Federal Register 2010, 2011, 2012, 2013, 2014

    2012-11-28

    ... Transfer (EFT) Market Research Study AGENCY: Financial Management Service, Fiscal Service, Treasury. ACTION... Financial Management Service solicits comments concerning the ``Electronic Funds Transfer (EFT) Market Research Study.'' DATES: Written comments should be received on or before January 28, 2013....

  18. Role of Tensorial Electronic Friction in Energy Transfer at Metal Surfaces.

    PubMed

    Askerka, Mikhail; Maurer, Reinhard J; Batista, Victor S; Tully, John C

    2016-05-27

    An accurate description of nonadiabatic energy relaxation is crucial for modeling atomistic dynamics at metal surfaces. Interfacial energy transfer due to electron-hole pair excitations coupled to motion of molecular adsorbates is often simulated by Langevin molecular dynamics with electronic friction. Here, we present calculations of the full electronic friction tensor by using first order time-dependent perturbation theory at the density functional theory level. We show that the friction tensor is generally anisotropic and nondiagonal, as found for hydrogen atom on Pd(100) and CO on Cu(100) surfaces. This implies that electron-hole pair induced nonadiabatic coupling at metal surfaces leads to friction-induced mode coupling, therefore, opening an additional channel for energy redistribution. We demonstrate the robustness and accuracy of our results by direct comparison to established methods and experimental data. PMID:27284673

  19. Role of Tensorial Electronic Friction in Energy Transfer at Metal Surfaces

    NASA Astrophysics Data System (ADS)

    Askerka, Mikhail; Maurer, Reinhard J.; Batista, Victor S.; Tully, John C.

    2016-05-01

    An accurate description of nonadiabatic energy relaxation is crucial for modeling atomistic dynamics at metal surfaces. Interfacial energy transfer due to electron-hole pair excitations coupled to motion of molecular adsorbates is often simulated by Langevin molecular dynamics with electronic friction. Here, we present calculations of the full electronic friction tensor by using first order time-dependent perturbation theory at the density functional theory level. We show that the friction tensor is generally anisotropic and nondiagonal, as found for hydrogen atom on Pd(100) and CO on Cu(100) surfaces. This implies that electron-hole pair induced nonadiabatic coupling at metal surfaces leads to friction-induced mode coupling, therefore, opening an additional channel for energy redistribution. We demonstrate the robustness and accuracy of our results by direct comparison to established methods and experimental data.

  20. Communication: Predictive partial linearized path integral simulation of condensed phase electron transfer dynamics

    SciTech Connect

    Huo, Pengfei; Miller, Thomas F. III; Coker, David F.

    2013-10-21

    A partial linearized path integral approach is used to calculate the condensed phase electron transfer (ET) rate by directly evaluating the flux-flux/flux-side quantum time correlation functions. We demonstrate for a simple ET model that this approach can reliably capture the transition between non-adiabatic and adiabatic regimes as the electronic coupling is varied, while other commonly used semi-classical methods are less accurate over the broad range of electronic couplings considered. Further, we show that the approach reliably recovers the Marcus turnover as a function of thermodynamic driving force, giving highly accurate rates over four orders of magnitude from the normal to the inverted regimes. We also demonstrate that the approach yields accurate rate estimates over five orders of magnitude of inverse temperature. Finally, the approach outlined here accurately captures the electronic coherence in the flux-flux correlation function that is responsible for the decreased rate in the inverted regime.

  1. Study of Electron Transfer Processes between Simple Plasma Ions and Electron Attaching Gases

    NASA Astrophysics Data System (ADS)

    Williams, Ted; Adams, Nigel; Babcock, Lucia

    1998-11-01

    CCl4 and SF6 are gases that rapidly attach electrons. They are used in etchant plasmas and in high power switches to prevent breakdown. This attachment results in a simple negative ion chemistry that can be well characterized. A concurrent series of reactions involving positive ions also occurs, with ionization eventually being removed by ion-ion mutual neutralization. However, unlike the negative ion chemistry, the positive ion chemistry is more complex and has not been well characterized. Common plasma ions are those of the rare and diatomic gases, along with impurity ions such as H_2O^+ and H_3O^+. Reactions of these ions with CCl4 and SF6 generally occur rapidly by dissociative electron transfer. Some exceptions have been observed when the reactant ion contains an H-atom(s), such as the production of HCl when H_3^+ reacts with CCl_4. Since these reactions involve a fixed amount of energy, they bear similarity to photoelectron and photoelectron-photoion coincidence studies of these electron attaching gases in which only Franck-Condon transitions can occur. Comparision of product ions observed and rate coefficients gives better insights into the mechanism of the electron transfer process. Support by NSF AST-9415485 is gratefully acknowledged.

  2. Electron beam modeling on LTX

    NASA Astrophysics Data System (ADS)

    Szalkowski, Gregory; Majeski, Richard; Schmitt, John

    2014-10-01

    The lithium tokamak experiment (LTX) is a low aspect ratio tokamak with a steel clad copper shell that can be heated to 300-400 °C and coated with lithium. The lithium coating has been shown to decrease impurities in the plasma and decrease the recycling coefficient, improving plasma performance. The coating is applied to the walls by heating the shells, then using an electron beam to evaporate a pool of lithium located at the bottom of the shell. The beam is steered using the magnetic field generated by the field coils. This method allows for rapid evaporation of the lithium, producing a 50-100 nm coating in approximately 5 minutes. The current electron beam system can only coat half of the shell surface. A new electron beam system has been installed on LTX to coat the remaining shell surface. A model of this electron gun has been created using the AMaze program series (Field Precision LCC). The model will be used to find the magnetic fields needed to steer the electron beam produced by the gun to the lithium pool. The model will also show the electropotential produced both at the electron gun head and in the vessel. The model may also be used to find the dispersion of the beam and therefore the effective power density of the beam as it impacts the lithium pool. Supported by US DOE Contracts DE-AC02-09CH11466 and DE-AC52-07NA27344 and in part by the U.S. Department of Energy, Office of Science, Office of Workforce Development for Teachers and Scientists (WDTS) under the Science Undergraduate Laboratory Internship.

  3. Effect of friction on electron transfer: The two reaction coordinate case

    NASA Astrophysics Data System (ADS)

    Onuchic, José Nelson

    1987-04-01

    Electron transfer is a very important reaction in many biological processes such as photosynthesis and oxidative phosphorylation. In many of these reactions, most of the interesting dynamics can be included by using two reaction coordinates: one fast (local high frequency vibration modes) and one slow (outersphere modes such as solvent polarization). We report a model to describe this problem, which uses path integral techniques to calculate electron transfer rates, and also to obtain the Fokker-Planck equations associated with this model. Different limiting cases lead to qualitatively different results such as exponential or nonexponential time decay for the donor survival probability. Conditions for the validity of the adiabatic or the nonadiabatic limits will be discussed. Application of this model to real systems is proposed, in particular for a porphyrin rigidly linked to a quinone, which is a very interesting model compound for primary events of photosynthesis. This model can also be used for other multicoordinate biological reactions such as ligand binding to heme proteins. Also, in the concluding part of Sec. III, we discuss the important limit where the fast vibronic mode is much faster than all the other nuclear modes coupled to the problem. In this limit the fast mode ``renormalizes'' the electronic matrix element, and this considerably simplifies the treatment of the problem, reducing it to coupling only to the slow modes.

  4. Photoinduced electron transfer across fixed distances in chlorophyll donor-acceptor molecules

    SciTech Connect

    Wasielewski, M.R.; Johnson, D.G.; Svec, W.A.

    1987-06-01

    The primary events of photosynthesis are a series of rapid, unidirectional electron transfer events between donors and acceptors that are positioned in the reaction center protein at precise spatial orientations and distances relative to one another. Recent work suggests that electron transfer rates depend on distance and free energy of reaction in porphyrin-quinone models in which the distance and orientation of the donor relative to the acceptor is highly restricted. Spacer molecules were developed which were used to link chlorophyll donors with either chlorophyll or quinone acceptors to produce models in which the donor-acceptor distance is well-defined. Recent theoretical studies and photochemical hole-burning experiments have suggested that the actual primary event of photosynthesis is the production of an intramolecular charge transfer state involving the two bacteriochlorophyll molecules of the special pair dimer. This possibility was explored with symmetric, fixed distance chlorophyll dimer. The chlorophyll macrocycles share a common vinyl group at the 2-position. This linkage serves to increase the degree of electronic coupling between the macrocycles. This dimer exhibits a remarkable decrease in fluorescence quantum yield as the dielectric constant of the medium in which it is dissolved increases. This decrease is accompanied by a proportional decrease in the lowest excited singlet state lifetime as measured by picosecond fluorescence and absorption. 11 refs., 2 figs.

  5. Modeling heat transfer within porous multiconstituent materials

    NASA Astrophysics Data System (ADS)

    Niezgoda, Mathieu; Rochais, Denis; Enguehard, Franck; Rousseau, Benoit; Echegut, Patrick

    2012-06-01

    The purpose of our work has been to determine the effective thermal properties of materials considered heterogeneous at the microscale but which are regarded as homogenous in the macroscale environment in which they are used. We have developed a calculation code that renders it possible to simulate thermal experiments over complex multiconstituent materials from their numerical microstructural morphology obtained by volume segmentation through tomography. This modeling relies on the transient solving of the coupled conductive and radiative heat transfer in these voxelized structures.

  6. Ultrafast photoinduced electron transfer reactions in supramolecular arrays: Studies of electronic coupling and solvation

    SciTech Connect

    Wasielewski, M.R.; Wiederrecht, G.P.; Svec, W.A.

    1993-05-01

    Research in our laboratory focuses on developing supramolecular arrays that produce long-lived charge separation by limiting the electronic coupling between the separated charges, and on the role of solvation in determining the rates and energetics of photoinitiated electron transfer reactions. Arrays have been developed that closely mimic the electronic coupling that was observed only for long-lived radical pairs produced in photosynthetic glassy solids. A series of 36 fixed-distance donor-acceptor molecules using porphyrin donors, triptycene spacers, and 9 different acceptors has been prepared; these are used to probe the dependence of photoinduced charge separation rates on free energy of reaction as a function of solvent both in liquid and solid solution. Data were obtained on rates of charge separation in dioxane, MTHF, butyronitrile, toluene, chlorobenzene, and benzonitrile.

  7. Ultrafast photoinduced electron transfer reactions in supramolecular arrays: Studies of electronic coupling and solvation

    SciTech Connect

    Wasielewski, M.R.; Wiederrecht, G.P.; Svec, W.A.

    1993-01-01

    Research in our laboratory focuses on developing supramolecular arrays that produce long-lived charge separation by limiting the electronic coupling between the separated charges, and on the role of solvation in determining the rates and energetics of photoinitiated electron transfer reactions. Arrays have been developed that closely mimic the electronic coupling that was observed only for long-lived radical pairs produced in photosynthetic glassy solids. A series of 36 fixed-distance donor-acceptor molecules using porphyrin donors, triptycene spacers, and 9 different acceptors has been prepared; these are used to probe the dependence of photoinduced charge separation rates on free energy of reaction as a function of solvent both in liquid and solid solution. Data were obtained on rates of charge separation in dioxane, MTHF, butyronitrile, toluene, chlorobenzene, and benzonitrile.

  8. Photoinduced electron transfer reaction in polymer-surfactant aggregates: Photoinduced electron transfer between N,N-dimethylaniline and 7-amino coumarin dyes

    SciTech Connect

    Chakraborty, Anjan; Seth, Debabrata; Setua, Palash; Sarkar, Nilmoni

    2008-05-28

    Photoinduced electron transfer between coumarin dyes and N,N-dimethylaniline has been investigated by using steady state and picosecond time resolved fluorescence spectroscopy in sodium dodecyl sulphate (SDS) micelles and PVP-polyvinyl pyrrolidone (SDS) polymer-surfactant aggregates. A slower rate of electron transfer is observed in PVP-SDS aggregates than in polymer-free SDS micelles. A Marcus type inversion is observed in the correlation of free energy change in comparison with the electron transfer rate. The careful investigation reveals that C-151 deviates from the normal Marcus inverted region compared to its analogs C-152 and C-481 due to slower rotational relaxation and smaller translational diffusion coefficient.

  9. Tetramethoxybenzene is a Good Building Block for Molecular Wires: Insights from Photoinduced Electron Transfer.

    PubMed

    Heinz, Luisa G; Yushchenko, Oleksandr; Neuburger, Markus; Vauthey, Eric; Wenger, Oliver S

    2015-06-01

    Two donor bridge-acceptor molecules with terminal triarylamine and Ru(bpy)3(2+) (bpy = 2,2'-bipyridine) redox partners were synthesized and investigated by cyclic voltammetry, optical absorption, luminescence, and transient absorption spectroscopy. The two dyads differ only by the central bridging unit, which was tetramethoxybenzene (tmb) in one case and unsubstituted phenylene (ph) in the other case. Photoirradiation of the Ru(bpy)3(2+) complex of the two dyads triggers intramolecular electron transfer from the triarylamine to the (3)MLCT-excited metal complex, and this process occurs with time constants of 1.5 and 6.8 ns for the tmb- and ph-bridged dyads, respectively. Thermal electron transfer in the reverse direction then leads to disappearance of the photoproduct with a time constant of 10 ns in both dyads. The faster rate of photoinduced charge transfer in the tmb-bridged dyad can be understood in the framework of a hole-tunneling model in which the electron-rich tmb bridge imposes a more shallow barrier than the less electron-rich ph spacer. Until now tmb-based molecular wires have received very little attention, and alkoxy substituents have been mostly used for improving the solubility of oligo-p-phenylene vinylene (OPV) and oligo-p-phenylene ethynylene (OPE) wires. Our study illustrates how four alkoxy-substituents on a phenylene backbone can have a significant influence on the charge-transfer properties of a molecular wire, and this is relevant in the greater context of a future molecular electronics technology. PMID:25974891

  10. N-Cα Bond Cleavage of Zinc-Polyhistidine Complexes in Electron Transfer Dissociation Mediated by Zwitterion Formation: Experimental Evidence and Theoretical Analysis of the Utah-Washington Model.

    PubMed

    Asakawa, Daiki; Yamashita, Asuka; Kawai, Shikiho; Takeuchi, Takae; Wada, Yoshinao

    2016-02-11

    Electron capture dissociation (ECD) and electron transfer dissociation (ETD) of gas-phase ions are widely used for peptide/protein sequencing by mass spectrometry. To understand the general mechanism of ECD/ETD of peptides, we focused on the ETD fragmentation of metal-peptide complexes in the absence of remote protons. Since Zn(2+) strongly binds to neutral histidine residues in peptides, Zn(2+)-polyhistidine complexation does not generate any remote protons. However, in the absence of remote protons, electron transfer to the Zn(2+)-polyhistidine complex induced the N-Cα bond cleavage. The formation pathway for the ETD products was investigated by density functional theory calculations. The calculations showed that the charge-reduced zinc-peptide radical, [M + Zn](•+), can exist in the low-energy zwitterionic amide π* states, which underwent homolytic N-Cα bond dissociation. The homolytic cleavage resulted in the donation of an electron from the N-Cα bond to the nitrogen atom, producing an iminoenol c' anion. The counterpart z(•) radical contained a radical site on the α-carbon atom. The iminoenol c' anion then abstracted a proton to presumably form the more stable amide c' fragment. The current experimental and computational joint study strongly suggested that the N-Cα bond cleavage occurred through the aminoketyl radical-anion formation for Zn(2+)-polyhistidine complexes in ETD. PMID:26673038

  11. Heat transfer model for quenching by submerging

    NASA Astrophysics Data System (ADS)

    Passarella, D. N.; Varas, F.; Martín, E. B.

    2011-05-01

    In quenching by submerging the workpiece is cooled due to vaporization, convective flow and interaction of both mechanisms. The dynamics of these phenomena is very complex and the corresponding heat fluxes are strongly dependent on local flow variables such as velocity of fluid and vapor fraction. This local dependence may produce very different cooling rates along the piece, responsible for inappropriate metallurgical transformations, variability of material properties and residual stresses. In order to obtain an accurate description of cooling during quenching, a mathematical model of heat transfer is presented here. The model is based on the drift-flux mixture-model for multiphase flows, including an equation of conservation of energy for the liquid phase and specific boundary conditions that account for evaporation and presence of vapor phase on the surface of the piece. The model was implemented on Comsol Multiphysics software. Generation of appropriate initial and boundary conditions, as well as numerical resolution details, is briefly discussed. To test the model, a simple flow condition was analyzed. The effect of vapor fraction on heat transfer is assessed. The presence of the typical vapor blanket and its collapse can be recovered by the model, and its effect on the cooling rates on different parts of the piece is analyzed. Comparisons between numerical results and data from literature are made.

  12. Surface Activation of Electrocatalysis at Oxide Electrodes. Concerted Electron-Proton Transfer

    SciTech Connect

    Gagliardi, Christopher J.; Jurss, Jonah W.; Thorp, H. Holden; Meyer, Thomas J.

    2011-03-21

    Dramatic rate enhancements are observed for the oxidation of phenols, including tyrosine, at indium-tin oxide electrodes modified by the addition of the electron-transfer relays [MII(bpy)2(4,4'-(HO)2P(O)CH2)2bpy)]2+ (M = Ru, Os) with clear evidence for the importance of proton-coupled electron transfer and concerted electron-proton transfer.

  13. Introductory Tools for Radiative Transfer Models

    NASA Astrophysics Data System (ADS)

    Feldman, D.; Kuai, L.; Natraj, V.; Yung, Y.

    2006-12-01

    Satellite data are currently so voluminous that, despite their unprecedented quality and potential for scientific application, only a small fraction is analyzed due to two factors: researchers' computational constraints and a relatively small number of researchers actively utilizing the data. Ultimately it is hoped that the terabytes of unanalyzed data being archived can receive scientific scrutiny but this will require a popularization of the methods associated with the analysis. Since a large portion of complexity is associated with the proper implementation of the radiative transfer model, it is reasonable and appropriate to make the model as accessible as possible to general audiences. Unfortunately, the algorithmic and conceptual details that are necessary for state-of-the-art analysis also tend to frustrate the accessibility for those new to remote sensing. Several efforts have been made to have web- based radiative transfer calculations, and these are useful for limited calculations, but analysis of more than a few spectra requires the utilization of home- or server-based computing resources. We present a system that is designed to allow for easier access to radiative transfer models with implementation on a home computing platform in the hopes that this system can be utilized in and expanded upon in advanced high school and introductory college settings. This learning-by-doing process is aided through the use of several powerful tools. The first is a wikipedia-style introduction to the salient features of radiative transfer that references the seminal works in the field and refers to more complicated calculations and algorithms sparingly5. The second feature is a technical forum, commonly referred to as a tiki-wiki, that addresses technical and conceptual questions through public postings, private messages, and a ranked searching routine. Together, these tools may be able to facilitate greater interest in the field of remote sensing.

  14. Kinetics of electron transfer through the respiratory chain.

    PubMed Central

    Jin, Qusheng; Bethke, Craig M

    2002-01-01

    We show that the rate at which electrons pass through the respiratory chain in mitochondria and respiring prokaryotic cells is described by the product of three terms, one describing electron donation, one acceptance, and a third, the thermodynamic drive. We apply the theory of nonequilibrium thermodynamics in the context of the chemiosmotic model of proton translocation and energy conservation. This approach leads to a closed-form expression that predicts steady-state electron flux as a function of chemical conditions and the proton motive force across the mitochondrial inner membrane or prokaryotic cytoplasmic membrane. The rate expression, derived considering reverse and forward electron flow, is the first to account for both thermodynamic and kinetic controls on the respiration rate. The expression can be simplified under specific conditions to give rate laws of various forms familiar in cellular physiology and microbial ecology. The expression explains the nonlinear dependence of flux on electrical potential gradient, its hyperbolic dependence on substrate concentration, and the inhibiting effects of reaction products. It provides a theoretical basis for investigating life under unusual conditions, such as microbial respiration in alkaline waters. PMID:12324402

  15. Kinetics of electron transfer through the respiratory chain.

    PubMed

    Jin, Qusheng; Bethke, Craig M

    2002-10-01

    We show that the rate at which electrons pass through the respiratory chain in mitochondria and respiring prokaryotic cells is described by the product of three terms, one describing electron donation, one acceptance, and a third, the thermodynamic drive. We apply the theory of nonequilibrium thermodynamics in the context of the chemiosmotic model of proton translocation and energy conservation. This approach leads to a closed-form expression that predicts steady-state electron flux as a function of chemical conditions and the proton motive force across the mitochondrial inner membrane or prokaryotic cytoplasmic membrane. The rate expression, derived considering reverse and forward electron flow, is the first to account for both thermodynamic and kinetic controls on the respiration rate. The expression can be simplified under specific conditions to give rate laws of various forms familiar in cellular physiology and microbial ecology. The expression explains the nonlinear dependence of flux on electrical potential gradient, its hyperbolic dependence on substrate concentration, and the inhibiting effects of reaction products. It provides a theoretical basis for investigating life under unusual conditions, such as microbial respiration in alkaline waters. PMID:12324402

  16. Electron transfer, ionization, and excitation in atomic collisions. Progress report, June 15, 1992--June 14, 1995

    SciTech Connect

    Winter, T.G.; Alston, S.G.

    1995-08-01

    The research program of Winter and Alston addresses the fundamental processes of electron transfer, ionization, and excitation in ion-atom, ion-ion, and ion-molecule collisions. Attention is focussed on one- and two-electron systems and, more recently, quasi-one-electron systems whose electron-target-core interaction can be accurately modeled by one-electron potentials. The basic computational approaches can then be taken with few, if any, approximations, and the underlying collisional mechanisms can be more clearly revealed. Winter has focussed on intermediate collision energies (e.g., proton energies for p-He{sup +} collisions on the order of 100 kilo-electron volts), in which many electron states are strongly coupled during the collision and a coupled-state approach, such as a coupled-Sturmian-pseudostate approach, is appropriate. Alston has concentrated on higher collision energies (million electron-volt energies), or asymmetric collision systems, for which the coupling of the projectile is weaker with, however, many more target states being coupled together so that high-order perturbation theory is essential. Several calculations by Winter and Alston are described, as set forth in the original proposal.

  17. Electron Transfer Reactivity of the Aqueous Iron(IV)-Oxo Complex. Outer-Sphere vs Proton-Coupled Electron Transfer.

    PubMed

    Bataineh, Hajem; Pestovsky, Oleg; Bakac, Andreja

    2016-07-01

    The kinetics of oxidation of organic and inorganic reductants by aqueous iron(IV) ions, Fe(IV)(H2O)5O(2+) (hereafter Fe(IV)aqO(2+)), are reported. The substrates examined include several water-soluble ferrocenes, hexachloroiridate(III), polypyridyl complexes M(NN)3(2+) (M = Os, Fe and Ru; NN = phenanthroline, bipyridine and derivatives), HABTS(-)/ABTS(2-), phenothiazines, Co(II)(dmgBF2)2, macrocyclic nickel(II) complexes, and aqueous cerium(III). Most of the reductants were oxidized cleanly to the corresponding one-electron oxidation products, with the exception of phenothiazines which produced the corresponding oxides in a single-step reaction, and polypyridyl complexes of Fe(II) and Ru(II) that generated ligand-modified products. Fe(IV)aqO(2+) oxidizes even Ce(III) (E(0) in 1 M HClO4 = 1.7 V) with a rate constant greater than 10(4) M(-1) s(-1). In 0.10 M aqueous HClO4 at 25 °C, the reactions of Os(phen)3(2+) (k = 2.5 × 10(5) M(-1) s(-1)), IrCl6(3-) (1.6 × 10(6)), ABTS(2-) (4.7 × 10(7)), and Fe(cp)(C5H4CH2OH) (6.4 × 10(7)) appear to take place by outer sphere electron transfer (OSET). The rate constants for the oxidation of Os(phen)3(2+) and of ferrocenes remained unchanged in the acidity range 0.05 < [H(+)] < 0.10 M, ruling out prior protonation of Fe(IV)aqO(2+) and further supporting the OSET assignment. A fit to Marcus cross-relation yielded a composite parameter (log k22 + E(0)Fe/0.059) = 17.2 ± 0.8, where k22 and E(0)Fe are the self-exchange rate constant and reduction potential, respectively, for the Fe(IV)aqO(2+)/Fe(III)aqO(+) couple. Comparison with literature work suggests k22 < 10(-5) M(-1) s(-1) and thus E(0)(Fe(IV)aqO(2+)/Fe(III)aqO(+)) > 1.3 V. For proton-coupled electron transfer, the reduction potential is estimated at E(0) (Fe(IV)aqO(2+), H(+)/Fe(III)aqOH(2+)) ≥ 1.95 V. PMID:27320290

  18. RRTM: A rapid radiative transfer model

    SciTech Connect

    Mlawer, E.J.; Taubman, S.J.; Clough, S.A.

    1996-04-01

    A rapid radiative transfer model (RRTM) for the calculation of longwave clear-sky fluxes and cooling rates has been developed. The model, which uses the correlated-k method, is both accurate and computationally fast. The foundation for RRTM is the line-by-line radiative transfer model (LBLRTM) from which the relevant k-distributions are obtained. LBLRTM, which has been extensively validated against spectral observations e.g., the high-resolution sounder and the Atmospheric Emitted Radiance Interferometer, is used to validate the flux and cooling rate results from RRTM. Validations of RRTM`s results have been performed for the tropical, midlatitude summer, and midlatitude winter atmospheres, as well as for the four Intercomparison of Radiation Codes in Climate Models (ICRCCM) cases from the Spectral Radiance Experiment (SPECTRE). Details of some of these validations are presented below. RRTM has the identical atmospheric input module as LBLRTM, facilitating intercomparisons with LBLRTM and application of the model at the Atmospheric Radiation Measurement Cloud and Radiation Testbed sites.

  19. Nonadiabatic dynamics of photoinduced proton-coupled electron transfer: comparison of explicit and implicit solvent simulations.

    PubMed

    Auer, Benjamin; Soudackov, Alexander V; Hammes-Schiffer, Sharon

    2012-07-01

    Theoretical approaches for simulating the ultrafast dynamics of photoinduced proton-coupled electron transfer (PCET) reactions in solution are developed and applied to a series of model systems. These processes are simulated by propagating nonadiabatic surface hopping trajectories on electron-proton vibronic surfaces that depend on the solute and solvent nuclear coordinates. The PCET system is represented by a four-state empirical valence bond model, and the solvent is treated either as explicit solvent molecules or as a dielectric continuum, in which case the solvent dynamics is described in terms of two collective solvent coordinates corresponding to the energy gaps associated with electron and proton transfer. The explicit solvent simulations reveal two distinct solvent relaxation time scales, where the faster time scale relaxation corresponds to librational motions of solvent molecules in the first solvation shell, and the slower time scale relaxation corresponds to the bulk solvent dielectric response. The charge transfer dynamics is strongly coupled to both the fast and slow time scale solvent dynamics. The dynamical multistate continuum theory is extended to include the effects of two solvent relaxation time scales, and the resulting coupled generalized Langevin equations depend on parameters that can be extracted from equilibrium molecular dynamics simulations. The implicit and explicit solvent approaches lead to qualitatively similar charge transfer and solvent dynamics for model PCET systems, suggesting that the implicit solvent treatment captures the essential elements of the nonequilibrium solvent dynamics for many systems. A combination of implicit and explicit solvent approaches will enable the investigation of photoinduced PCET processes in a variety of condensed phase systems. PMID:22651684

  20. A hybrid approach to simulation of electron transfer in complex molecular systems.

    PubMed

    Kubař, Tomáš; Elstner, Marcus

    2013-10-01

    Electron transfer (ET) reactions in biomolecular systems represent an important class of processes at the interface of physics, chemistry and biology. The theoretical description of these reactions constitutes a huge challenge because extensive systems require a quantum-mechanical treatment and a broad range of time scales are involved. Thus, only small model systems may be investigated with the modern density functional theory techniques combined with non-adiabatic dynamics algorithms. On the other hand, model calculations based on Marcus's seminal theory describe the ET involving several assumptions that may not always be met. We review a multi-scale method that combines a non-adiabatic propagation scheme and a linear scaling quantum-chemical method with a molecular mechanics force field in such a way that an unbiased description of the dynamics of excess electron is achieved and the number of degrees of freedom is reduced effectively at the same time. ET reactions taking nanoseconds in systems with hundreds of quantum atoms can be simulated, bridging the gap between non-adiabatic ab initio simulations and model approaches such as the Marcus theory. A major recent application is hole transfer in DNA, which represents an archetypal ET reaction in a polarizable medium. Ongoing work focuses on hole transfer in proteins, peptides and organic semi-conductors. PMID:23883952

  1. A hybrid approach to simulation of electron transfer in complex molecular systems

    PubMed Central

    Kubař, Tomáš; Elstner, Marcus

    2013-01-01

    Electron transfer (ET) reactions in biomolecular systems represent an important class of processes at the interface of physics, chemistry and biology. The theoretical description of these reactions constitutes a huge challenge because extensive systems require a quantum-mechanical treatment and a broad range of time scales are involved. Thus, only small model systems may be investigated with the modern density functional theory techniques combined with non-adiabatic dynamics algorithms. On the other hand, model calculations based on Marcus's seminal theory describe the ET involving several assumptions that may not always be met. We review a multi-scale method that combines a non-adiabatic propagation scheme and a linear scaling quantum-chemical method with a molecular mechanics force field in such a way that an unbiased description of the dynamics of excess electron is achieved and the number of degrees of freedom is reduced effectively at the same time. ET reactions taking nanoseconds in systems with hundreds of quantum atoms can be simulated, bridging the gap between non-adiabatic ab initio simulations and model approaches such as the Marcus theory. A major recent application is hole transfer in DNA, which represents an archetypal ET reaction in a polarizable medium. Ongoing work focuses on hole transfer in proteins, peptides and organic semi-conductors. PMID:23883952

  2. When electron transfer meets electron transport in redox-active molecular nanojunctions.

    PubMed

    Janin, Marion; Ghilane, Jalal; Lacroix, Jean-Christophe

    2013-02-13

    A scanning electrochemical microscope (SECM) was used to arrange two microelectrodes face-to-face separated by a micrometric gap. Polyaniline (PANI) was deposited electrochemically from the SECM tip side until it bridged the two electrodes. The junctions obtained were characterized by following the current through the PANI as a function of its electrochemical potential measured versus a reference electrode acting as a gate electrode in a solid-state transistor. PANI nanojunctions showed conductances below 100 nS in the oxidized state, indicating control of the charge transport within the whole micrometric gap by a limited number of PANI wires. The SECM configuration makes it possible to observe in the same experiment and in the same current range the electron-transfer and electron-transport processes. These two phenomena are distinguished here and characterized by following the variation of the current with the bias voltage and the scan rate. The electron-transfer current changes with the scan rate, while the charge-transport current varies with the bias voltage. Finally, despite the initially micrometric gap, a junction where the conductance is controlled by a single oligoaniline strand is achieved. PMID:23331168

  3. Electron transfer mediation by aqueous C60 aggregates in H2O2/UV advanced oxidation of indigo carmine

    NASA Astrophysics Data System (ADS)

    Ge, Ling; Moor, Kyle; Zhang, Bo; He, Yiliang; Kim, Jae-Hong

    2014-10-01

    C60 fullerene has long been known to exhibit favorable electron accepting and shuttling properties, but little is known about the possibility of electron transfer mediation by fullerene aggregates (nC60) in water. In this study, we investigated the electron shuttling capabilities of nC60 using UV/H2O2 as a model oxidation process in the presence of an electron donor, indigo carmine (IC). nC60 addition to the IC/H2O2 system was found to drastically increase IC degradation and shift the reactive oxygen species (ROS) balance, favoring the formation of superoxide and perhydroxyl radical species compared to hydroxyl radicals. Results indicate that nC60 can act as an electron mediator, where the adsorbed IC donates an electron to nC60, which is subsequently transferred to H2O2 or perhydroxyl radical.C60 fullerene has long been known to exhibit favorable electron accepting and shuttling properties, but little is known about the possibility of electron transfer mediation by fullerene aggregates (nC60) in water. In this study, we investigated the electron shuttling capabilities of nC60 using UV/H2O2 as a model oxidation process in the presence of an electron donor, indigo carmine (IC). nC60 addition to the IC/H2O2 system was found to drastically increase IC degradation and shift the reactive oxygen species (ROS) balance, favoring the formation of superoxide and perhydroxyl radical species compared to hydroxyl radicals. Results indicate that nC60 can act as an electron mediator, where the adsorbed IC donates an electron to nC60, which is subsequently transferred to H2O2 or perhydroxyl radical. Electronic supplementary information (ESI) available. See DOI: 10.1039/c4nr03647f

  4. Challenges in reduction of dinitrogen by proton and electron transfer.

    PubMed

    van der Ham, Cornelis J M; Koper, Marc T M; Hetterscheid, Dennis G H

    2014-08-01

    Ammonia is an important nutrient for the growth of plants. In industry, ammonia is produced by the energy expensive Haber-Bosch process where dihydrogen and dinitrogen form ammonia at a very high pressure and temperature. In principle one could also reduce dinitrogen upon addition of protons and electrons similar to the mechanism of ammonia production by nitrogenases. Recently, major breakthroughs have taken place in our understanding of biological fixation of dinitrogen, of molecular model systems that can reduce dinitrogen, and in the electrochemical reduction of dinitrogen at heterogeneous surfaces. Yet for efficient reduction of dinitrogen with protons and electrons major hurdles still have to be overcome. In this tutorial review we give an overview of the different catalytic systems, highlight the recent breakthroughs, pinpoint common grounds and discuss the bottlenecks and challenges in catalytic reduction of dinitrogen. PMID:24802308

  5. Coherent electronic energy transfer and nonlinear polariton effects in anthracene-doped naphthalene crystals

    SciTech Connect

    Connolly, M.A.

    1987-01-01

    The nature of electronic energy transfer in strain-free mounted anthracene-doped naphthalene single crystals at frequencies near that of the (0.0) a-exciton in naphthalene was probed using sensitized resonant two-photon fluorescence excitation (TPE) and second harmonic generation (SHG) spectroscopies. The naphthalene-to-anthracene TPE intensity ratios were found to be 1.2 and 1.6, for excitation of the zero-phonon transition at 1.6 K and 20 K, respectively. This result is consistent with coherent exciton-polariton electronic energy transfer at liquid helium temperatures, and incoherent transfer at high temperatures. Energy transfer for the pure and the anthracene-doped naphthalene systems under investigation was determined to be in the capture limited regime. The polariton trapping event is discussed in terms of scattering off a trap with subsequent trapping (i.e., capture) and scattering off a trap without subsequent trapping (i.e., trap scattering). Thermal broadening and temperature dependences of the TPE and SHG signal intensities for both naphthalene and anthracene are presented and are discussed in terms of the polariton fusion model. The experimental data do not agree with the theoretical predictions; this discrepancy is discussed in terms of the anomalous crystal habit (diamond plates) observed in the present investigation. The first report of a longitudinal exciton in a doped organic molecular system also is reported.

  6. Probing concerted proton–electron transfer in phenol–imidazoles

    PubMed Central

    Markle, Todd F.; Rhile, Ian J.; DiPasquale, Antonio G.; Mayer, James M.

    2008-01-01

    A series of seven substituted 4,6-di-tert-butyl-2-(4,5-diarylimidazolyl)-phenols have been prepared and characterized, along with two related benzimidazole compounds. X-ray crystal structures of all of the compounds show that the phenol and imidazole rings are close to coplanar and are connected by an intramolecular ArOH⋯N hydrogen bond. One-electron oxidation of these compounds occurs with movement of the phenolic proton to the imidazole base by concerted proton–electron transfer (CPET) to yield fairly stable distonic radical cations. These phenol–base compounds are a valuable system in which to examine the key features of CPET. Kinetic measurements of bimolecular CPET oxidations, with Erxn between +0.04 and −0.33 V, give rate constants from (6.3 ± 0.6) × 102 to (3.0 ± 0.6) × 106 M−1 s−1. There is a good correlation of log(k) with ΔG°, with only one of the 15 rate constants falling more than a factor of 5.2 from the correlation line. Substituents on the imidazole affect the (O–H⋯N) hydrogen bond, as marked by variations in the 1H NMR and calculated vibrational spectra and geometries. Crystallographic dO⋯N values appear to be more strongly affected by crystal packing forces. However, there is almost no correlation of rate constants with any of these measured or computed parameters. Over this range of compounds from the same structural family, the dominant contributor to the differences in rate constant is the driving force ΔG°. PMID:18212121

  7. DETERMINATION OF HETEROGENEOUS ELECTRON TRANSFER RATE CONSTANTS AT MICROFABRICATED IRIDIUM ELECTRODES. (R825511C022)

    EPA Science Inventory

    There has been an increasing use of both solid metal and microfabricated iridium electrodes as substrates for various types of electroanalysis. However, investigations to determine heterogeneous electron transfer rate constants on iridium, especially at an electron beam evapor...

  8. Electron transfer-initiated Diels-Alder cycloadditions of 2'-hydroxychalcones.

    PubMed

    Cong, Huan; Ledbetter, Dustin; Rowe, Gerard T; Caradonna, John P; Porco, John A

    2008-07-23

    An efficient approach to cyclohexenyl chalcones employing highly electron rich 2'-hydroxychalcone dienophiles via electron transfer-initiated Diels-Alder cycloaddition is described. Using the methodology, the total synthesis of nicolaiodesin C has been accomplished. PMID:18576647

  9. Thermal transfer structures coupling electronics card(s) to coolant-cooled structure(s)

    DOEpatents

    David, Milnes P; Graybill, David P; Iyengar, Madhusudan K; Kamath, Vinod; Kochuparambil, Bejoy J; Parida, Pritish R; Schmidt, Roger R

    2014-12-16

    Cooling apparatuses and coolant-cooled electronic systems are provided which include thermal transfer structures configured to engage with a spring force one or more electronics cards with docking of the electronics card(s) within a respective socket(s) of the electronic system. A thermal transfer structure of the cooling apparatus includes a thermal spreader having a first thermal conduction surface, and a thermally conductive spring assembly coupled to the conduction surface of the thermal spreader and positioned and configured to reside between and physically couple a first surface of an electronics card to the first surface of the thermal spreader with docking of the electronics card within a socket of the electronic system. The thermal transfer structure is, in one embodiment, metallurgically bonded to a coolant-cooled structure and facilitates transfer of heat from the electronics card to coolant flowing through the coolant-cooled structure.

  10. Kinetic coupling between electron and proton transfer in cytochrome c oxidase: simultaneous measurements of conductance and absorbance changes.

    PubMed Central

    Adelroth, P; Sigurdson, H; Hallén, S; Brzezinski, P

    1996-01-01

    Bovine heart cytochrome c oxidase is an electron-current driven proton pump. To investigate the mechanism by which this pump operates it is important to study individual electron- and proton-transfer reactions in the enzyme, and key reactions in which they are kinetically and thermodynamically coupled. In this work, we have simultaneously measured absorbance changes associated with electron-transfer reactions and conductance changes associated with protonation reactions following pulsed illumination of the photolabile complex of partly reduced bovine cytochrome c oxidase and carbon monoxide. Following CO dissociation, several kinetic phases in the absorbance changes were observed with time constants ranging from approximately 3 microseconds to several milliseconds, reflecting internal electron-transfer reactions within the enzyme. The data show that the rate of one of these electron-transfer reactions, from cytochrome a3 to a on a millisecond time scale, is controlled by a proton-transfer reaction. These results are discussed in terms of a model in which cytochrome a3 interacts electrostatically with a protonatable group, L, in the vicinity of the binuclear center, in equilibrium with the bulk through a proton-conducting pathway, which determines the rate of proton transfer (and indirectly also of electron transfer). The interaction energy of cytochrome a3 with L was determined independently from the pH dependence of the extent of the millisecond-electron transfer and the number of protons released, as determined from the conductance measurements. The magnitude of the interaction energy, 70 meV (1 eV = 1.602 x 10(-19) J), is consistent with a distance of 5-10 A between cytochrome a3 and L. Based on the recently determined high-resolution x-ray structures of bovine and a bacterial cytochrome c oxidase, possible candidates for L and a physiological role for L are discussed. PMID:8901574

  11. Proton-coupled electron transfers: pH-dependent driving forces? Fundamentals and artifacts.

    PubMed

    Bonin, Julien; Costentin, Cyrille; Robert, Marc; Routier, Mathilde; Savéant, Jean-Michel

    2013-09-25

    Besides its own interest, tryptophan oxidation by photogenerated Ru complexes is one of the several examples where concerted proton-electron transfer (CPET) to water as proton acceptor endowed with a pH-dependent driving force has been invoked to explain the data. Since this notion is contrary to the very basic principles of chemical physics, it was interesting to attempt uncovering the source of this contradiction with an easily accessible substrate. Careful examination of the oxidation of the tryptophan (ethyl ester derivative) bearing a NH3(+)/NH2 group showed that there is no trace of such an unconventional H2O-CPET with a pH-dependent driving force. The reaction mechanism simply consists, with both the NH3(+) acid and NH2 basic forms of the tryptophan derivative, in a rate-determining electron-transfer step followed by deprotonation steps. The same is true with the ethyl ester-methyl amide derivative of tryptophan, whose behavior is even simpler since the molecule does not bear an acid-base group. No such unconventional H2O-CPET was found with phenol, another easily accessible substrate. It may thus be inferred that the same applies to less easily available systems in which electron transfer occurs intramolecularly. These observations help to rid the road of such artificial obstacles and improve present models of H2O-CPET reactions, a landmark towards the understanding of the role of water chains in natural systems. PMID:23972082

  12. Classical molecular dynamics simulation of the photoinduced electron transfer dynamics of plastocyanin.

    PubMed Central

    Ungar, L W; Scherer, N F; Voth, G A

    1997-01-01

    Classical molecular dynamics simulations are used to investigate the nuclear motions associated with photoinduced electron transfer in plastocyanin. The blue copper protein is modeled using a molecular mechanics potential; potential parameters for the copper-protein interactions are determined using an x-ray crystallographic structure and absorption and resonance Raman spectra. Molecular dynamics simulations yield a variety of information about the ground (oxidized) and optically excited (charge-transfer) states: 1) The probability distribution of the potential difference between the states, which is used to determine the coordinate and energy displacements, places the states well within the Marcus inverted region. 2) The two-time autocorrelation function of the difference potential in the ground state and the average of the difference potential after instantaneous excitation to the excited state are very similar (confirming linear response in this system); their decay indicates that vibrational relaxation occurs in about 1 ps in both states. 3) The spectral densities of various internal coordinates begin to identify the vibrations that affect the optical transition; the spectral density of the difference potential correlation function should also prove useful in quantum simulations of the back electron transfer. 4) Correlation functions of the protein atomic motions with the difference potential show that the nuclear motions are correlated over a distance of more than 20 A, especially along proposed electron transport paths. Images FIGURE 1 FIGURE 7 PMID:8994588

  13. The coupling of electron transfer and proton translocation: electrostatic calculations on Paracoccus denitrificans cytochrome c oxidase.

    PubMed Central

    Kannt, A; Lancaster, C R; Michel, H

    1998-01-01

    We have calculated the electrostatic potential and interaction energies of ionizable groups and analyzed the response of the protein environment to redox changes in Paracoccus denitrificans cytochrome c oxidase by using a continuum dielectric model and finite difference technique. Subsequent Monte Carlo sampling of protonation states enabled us to calculate the titration curves of all protonatable groups in the enzyme complex. Inclusion of a model membrane allowed us to restrict the calculations to the functionally essential subunits I and II. Some residues were calculated to have complex titration curves, as a result of strong electrostatic coupling, desolvation, and dipolar interactions. Around the heme a3-CuB binuclear center, we have identified a cluster of 18 strongly interacting residues that account for most of the proton uptake linked to electron transfer. This was calculated to be between 0.7 and 1.1 H+ per electron, depending on the redox transition considered. A hydroxide ion bound to CuB was determined to become protonated to form water upon transfer of the first electron to the binuclear site. The bulk of the protonation changes linked to further reduction of the heme a3-CuB center was calculated to be due to proton uptake by the interacting cluster and Glu(II-78). Upon formation of the three-electron reduced state (P1), His325, modeled in an alternative orientation away from CuB, was determined to become protonated. The agreement of these results with experiment and their relevance in the light of possible mechanisms of redox-coupled proton transfer are discussed. PMID:9533684

  14. The coupling of electron transfer and proton translocation: electrostatic calculations on Paracoccus denitrificans cytochrome c oxidase.

    PubMed

    Kannt, A; Lancaster, C R; Michel, H

    1998-02-01

    We have calculated the electrostatic potential and interaction energies of ionizable groups and analyzed the response of the protein environment to redox changes in Paracoccus denitrificans cytochrome c oxidase by using a continuum dielectric model and finite difference technique. Subsequent Monte Carlo sampling of protonation states enabled us to calculate the titration curves of all protonatable groups in the enzyme complex. Inclusion of a model membrane allowed us to restrict the calculations to the functionally essential subunits I and II. Some residues were calculated to have complex titration curves, as a result of strong electrostatic coupling, desolvation, and dipolar interactions. Around the heme a3-CuB binuclear center, we have identified a cluster of 18 strongly interacting residues that account for most of the proton uptake linked to electron transfer. This was calculated to be between 0.7 and 1.1 H+ per electron, depending on the redox transition considered. A hydroxide ion bound to CuB was determined to become protonated to form water upon transfer of the first electron to the binuclear site. The bulk of the protonation changes linked to further reduction of the heme a3-CuB center was calculated to be due to proton uptake by the interacting cluster and Glu(II-78). Upon formation of the three-electron reduced state (P1), His325, modeled in an alternative orientation away from CuB, was determined to become protonated. The agreement of these results with experiment and their relevance in the light of possible mechanisms of redox-coupled proton transfer are discussed. PMID:9533684

  15. Proton-Coupled Electron Transfer: Moving Together and Charging Forward

    SciTech Connect

    Hammes-Schiffer, Sharon

    2015-06-25

    Proton-coupled electron transfer (PCET) is ubiquitous throughout chemistry and biology. This Perspective discusses recent advances and current challenges in the field of PCET, with an emphasis on the role of theory and computation. The fundamental theoretical concepts are summarized, and expressions for rate constants and kinetic isotope effects are provided. Computational methods for calculating reduction potentials and pKa’s for molecular electrocatalysts, as well as methods for simulating the nonadiabatic dynamics of photoinduced processes, are also described. Representative applications to PCET in solution, proteins, electrochemistry, and photoinduced processes are presented, highlighting the interplay between theoretical and experimental studies. The current challenges and suggested future directions are outlined for each type of application, concluding with an overall view to the future. The work described herein was supported by National Science Foundation Grant CHE-13-61293 (theory development), National Institutes of Health Grant GM056207 (soybean lipoxygenase), Center for Chemical Innovation of the National Science Foundation Solar Fuels Grant CHE-1305124 (cobalt catalysts), Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (nickel catalysts), and Air Force Office of Scientific Research Award No. FA9550-14-1-0295 (photoinduced PCET).

  16. Revisiting direct electron transfer in nanostructured carbon laccase oxygen cathodes.

    PubMed

    Adam, Catherine; Scodeller, Pablo; Grattieri, Matteo; Villalba, Matías; Calvo, Ernesto J

    2016-06-01

    The biocatalytic electroreduction of oxygen has been studied on large surface area graphite and Vulcan® carbon electrodes with adsorbed Trametes trogii laccase. The electrokinetics of the O2 reduction reaction (ORR) was studied at different electrode potentials, O2 partial pressures and concentrations of hydrogen peroxide. Even though the overpotential at 0.25 mA·cm(-2) for the ORR at T1Cu of the adsorbed laccase on carbon is 0.8 V lower than for Pt of similar geometric area, the rate of the reaction and thus the operative current density is limited by the enzyme reaction rate at the T2/T3 cluster site for the adsorbed enzyme. The transition potential for the rate determining step from the direct electron transfer (DET) to the enzyme reaction shifts to higher potentials at higher oxygen partial pressure. Hydrogen peroxide produced by the ORR on bare carbon support participates in an inhibition mechanism, with uncompetitive predominance at high H2O2 concentration, non-competitive contribution can be detected at low inhibitor concentration. PMID:26883057

  17. Synthesis, Characterization, Photophysics and Photochemistry of Pyrylogen Electron Transfer Sensitizers

    SciTech Connect

    Clennan, Edward L.; Liao, Chen

    2014-01-01

    A series of new dicationic sensitizers that are hybrids of pyrylium salts and viologens has been synthesized. The electrochemical and photophysical properties of these "pyrylogen" sensitizers are reported in sufficient detail to allow rationale design of new photoinduced electron transfer reactions. The range of their reduction potentials (+0.37-+0.05V vs SCE) coupled with their range of singlet (48-63 kcal mol(-1)) and triplet (48-57kcalmol(-1)) energies demonstrate that they are potent oxidizing agents in both their singlet and triplet excited states, thermodynamically capable of oxidizing substrates with oxidation potentials as high as 3.1eV. The pyrylogens are synthesized in three steps from readily available starting materials in modest overall 11.4-22.3% yields. These sensitizers have the added advantages that: (1) their radical cations do not react on the CV timescale with oxygen bypassing the need to run reactions under nitrogen or argon and (2) have long wavelength absorptions between 413 and 523nm well out of the range where competitive absorbance by most substrates would cause a problem. These new sensitizers do react with water requiring special precautions to operate in a dry reaction environment.

  18. Electron transfer dissociation of dipositive uranyl and plutonyl coordination complexes.

    PubMed

    Rios, Daniel; Rutkowski, Philip X; Shuh, David K; Bray, Travis H; Gibson, John K; Van Stipdonk, Michael J

    2011-12-01

    Reported here is a comparison of electron transfer dissociation (ETD) and collision-induced dissociation (CID) of solvent-coordinated dipositive uranyl and plutonyl ions generated by electrospray ionization. Fundamental differences between the ETD and CID processes are apparent, as are differences between the intrinsic chemistries of uranyl and plutonyl. Reduction of both charge and oxidation state, which is inherent in ETD activation of [An(VI) O(2) (CH(3) COCH(3) )(4) ](2+) , [An(VI) O(2) (CH(3) CN)(4) ](2) , [U(VI) O(2) (CH(3) COCH(3) )(5) ](2+) and [U(VI) O(2) (CH(3) CN)(5) ](2+) (An = U or Pu), is accompanied by ligand loss. Resulting low-coordinate uranyl(V) complexes add O(2) , whereas plutonyl(V) complexes do not. In contrast, CID of the same complexes generates predominantly doubly-charged products through loss of coordinating ligands. Singly-charged CID products of [U(VI) O(2) (CH(3) COCH(3) )(4,5) ](2+) , [U(VI) O(2) (CH(3) CN)(4,5) ](2+) and [Pu(VI) O(2) (CH(3) CN)(4) ](2+) retain the hexavalent metal oxidation state with the addition of hydroxide or acetone enolate anion ligands. However, CID of [Pu(VI) O(2) (CH(3) COCH(3) )(4) ](2+) generates monopositive plutonyl(V) complexes, reflecting relatively more facile reduction of Pu(VI) to Pu(V). PMID:22223415

  19. Electron transfer precedes ATP hydrolysis during nitrogenase catalysis.

    PubMed

    Duval, Simon; Danyal, Karamatullah; Shaw, Sudipta; Lytle, Anna K; Dean, Dennis R; Hoffman, Brian M; Antony, Edwin; Seefeldt, Lance C

    2013-10-01

    The biological reduction of N2 to NH3 catalyzed by Mo-dependent nitrogenase requires at least eight rounds of a complex cycle of events associated with ATP-driven electron transfer (ET) from the Fe protein to the catalytic MoFe protein, with each ET coupled to the hydrolysis of two ATP molecules. Although steps within this cycle have been studied for decades, the nature of the coupling between ATP hydrolysis and ET, in particular the order of ET and ATP hydrolysis, has been elusive. Here, we have measured first-order rate constants for each key step in the reaction sequence, including direct measurement of the ATP hydrolysis rate constant: kATP = 70 s(-1), 25 °C. Comparison of the rate constants establishes that the reaction sequence involves four sequential steps: (i) conformationally gated ET (kET = 140 s(-1), 25 °C), (ii) ATP hydrolysis (kATP = 70 s(-1), 25 °C), (iii) Phosphate release (kPi = 16 s(-1), 25 °C), and (iv) Fe protein dissociation from the MoFe protein (kdiss = 6 s(-1), 25 °C). These findings allow completion of the thermodynamic cycle undergone by the Fe protein, showing that the energy of ATP binding and protein-protein association drive ET, with subsequent ATP hydrolysis and Pi release causing dissociation of the complex between the Fe(ox)(ADP)2 protein and the reduced MoFe protein. PMID:24062462

  20. Electron transfer mediation by aqueous C₆₀ aggregates in H₂O₂/UV advanced oxidation of indigo carmine.

    PubMed

    Ge, Ling; Moor, Kyle; Zhang, Bo; He, Yiliang; Kim, Jae-Hong

    2014-11-21

    C60 fullerene has long been known to exhibit favorable electron accepting and shuttling properties, but little is known about the possibility of electron transfer mediation by fullerene aggregates (nC60) in water. In this study, we investigated the electron shuttling capabilities of nC60 using UV/H2O2 as a model oxidation process in the presence of an electron donor, indigo carmine (IC). nC60 addition to the IC/H2O2 system was found to drastically increase IC degradation and shift the reactive oxygen species (ROS) balance, favoring the formation of superoxide and perhydroxyl radical species compared to hydroxyl radicals. Results indicate that nC60 can act as an electron mediator, where the adsorbed IC donates an electron to nC60, which is subsequently transferred to H2O2 or perhydroxyl radical. PMID:25268845

  1. Synthetic Applications of Proton-Coupled Electron Transfer.

    PubMed

    Gentry, Emily C; Knowles, Robert R

    2016-08-16

    Redox events in which an electron and proton are exchanged in a concerted elementary step are commonly referred to as proton-coupled electron transfers (PCETs). PCETs are known to operate in numerous important biological redox processes, as well as recent inorganic technologies for small molecule activation. These studies suggest that PCET catalysis might also function as a general mode of substrate activation in organic synthesis. Over the past three years, our group has worked to advance this hypothesis and to demonstrate the synthetic utility of PCET through the development of novel catalytic radical chemistries. The central aim of these efforts has been to demonstrate the ability of PCET to homolytically activate a wide variety of common organic functional groups that are energetically inaccessible using known molecular H atom transfer catalysts. To do so, we made use of a simple formalism first introduced by Mayer and co-workers that allowed us to predict the thermodynamic capacity of any oxidant/base or reductant/acid pair to formally add or remove H· from a given substrate. With this insight, we were able to rationally select catalyst combinations thermodynamically competent to homolyze the extraordinarily strong E-H σ-bonds found in many common protic functional groups (BDFEs > 100 kcal/mol) or to form unusually weak bonds to hydrogen via the reductive action of common organic π-systems (BDFEs < 35 kcal/mol). These ideas were reduced to practice through the development of new catalyst systems for reductive PCET activations of ketones and oxidative PCET activation of amide N-H bonds to directly furnish reactive ketyl and amidyl radicals, respectively. In both systems, the reaction outcomes were found to be successfully predicted using the effective bond strength formalism, suggesting that these simple thermochemical considerations can provide useful and actionable insights into PCET reaction design. The ability of PCET catalysis to control

  2. Generation of Phosphorescent Triplet States via Photoinduced Electron Transfer: Energy and Electron Transfer Dynamics in Pt Porphyrin-Rhodamine B Dyads

    PubMed Central

    Mani, Tomoyasu; Niedzwiedzki, Dariusz M.; Vinogradov, Sergei A.

    2012-01-01

    Control over generation and dynamics of excited electronic states is fundamental to their utilization in all areas of technology. We present the first example of multichromophoric systems in which emissive triplet states are generated via a pathway involving photoinduced electron transfer (ET), as opposed to local intrachromophoric processes. In model dyads, PtP-Phn-pRhB+ (1-3, n=1-3), comprising platinum(II) meso-tetraarylporphyrin (PtP) and rhodamine B piperazine derivative (pRhB+), linked by oligo-p-phenylene bridges (Phn), upon selective excitation of pRhB+ at a frequency below that of the lowest allowed transition of PtP, room-temperature T1→S0 phosphorescence of PtP was observed. The pathway leading to the emissive PtP triplet state includes excitation of pRhB+, ET with formation of the singlet radical pair, intersystem crossing within that pair and subsequent radical recombination. Due to the close proximity of the triplet energy levels of PtP and pRhB+, reversible triplet-triplet (TT) energy transfer between these states was observed in dyads 1 and 2. As a result, the phosphorescence of PtP was extended in time by the long decay of the pRhB+ triplet. Observation of ET and TT in the same series of molecules enabled direct comparison of the distance attenuation factors β between these two closely related processes. PMID:22400988

  3. Electronic resonance with anticorrelated pigment vibrations drives photosynthetic energy transfer outside the adiabatic framework

    PubMed Central

    Tiwari, Vivek; Peters, William K.; Jonas, David M.

    2013-01-01

    The delocalized, anticorrelated component of pigment vibrations can drive nonadiabatic electronic energy transfer in photosynthetic light-harvesting antennas. In femtosecond experiments, this energy transfer mechanism leads to excitation of delocalized, anticorrelated vibrational wavepackets on the ground electronic state that exhibit not only 2D spectroscopic signatures attributed to electronic coherence and oscillatory quantum energy transport but also a cross-peak asymmetry not previously explained by theory. A number of antennas have electronic energy gaps matching a pigment vibrational frequency with a small vibrational coordinate change on electronic excitation. Such photosynthetic energy transfer steps resemble molecular internal conversion through a nested intermolecular funnel. PMID:23267114

  4. Cooperative electrocatalytic alcohol oxidation with electron-proton-transfer mediators

    NASA Astrophysics Data System (ADS)

    Badalyan, Artavazd; Stahl, Shannon S.

    2016-07-01

    electron-proton-transfer mediators, such as TEMPO, may be used in combination with first-row transition metals, such as copper, to achieve efficient two-electron electrochemical processes, thereby introducing a new concept for the development of non-precious-metal electrocatalysts.

  5. Cooperative electrocatalytic alcohol oxidation with electron-proton-transfer mediators.

    PubMed

    Badalyan, Artavazd; Stahl, Shannon S

    2016-07-21

    electron-proton-transfer mediators, such as TEMPO, may be used in combination with first-row transition metals, such as copper, to achieve efficient two-electron electrochemical processes, thereby introducing a new concept for the development of non-precious-metal electrocatalysts. PMID:27350245

  6. Electric-field-driven electron-transfer in mixed-valence molecules.

    PubMed

    Blair, Enrique P; Corcelli, Steven A; Lent, Craig S

    2016-07-01

    Molecular quantum-dot cellular automata is a computing paradigm in which digital information is encoded by the charge configuration of a mixed-valence molecule. General-purpose computing can be achieved by arranging these compounds on a substrate and exploiting intermolecular Coulombic coupling. The operation of such a device relies on nonequilibrium electron transfer (ET), whereby the time-varying electric field of one molecule induces an ET event in a neighboring molecule. The magnitude of the electric fields can be quite large because of close spatial proximity, and the induced ET rate is a measure of the nonequilibrium response of the molecule. We calculate the electric-field-driven ET rate for a model mixed-valence compound. The mixed-valence molecule is regarded as a two-state electronic system coupled to a molecular vibrational mode, which is, in turn, coupled to a thermal environment. Both the electronic and vibrational degrees-of-freedom are treated quantum mechanically, and the dissipative vibrational-bath interaction is modeled with the Lindblad equation. This approach captures both tunneling and nonadiabatic dynamics. Relationships between microscopic molecular properties and the driven ET rate are explored for two time-dependent applied fields: an abruptly switched field and a linearly ramped field. In both cases, the driven ET rate is only weakly temperature dependent. When the model is applied using parameters appropriate to a specific mixed-valence molecule, diferrocenylacetylene, terahertz-range ET transfer rates are predicted. PMID:27394108

  7. Electric-field-driven electron-transfer in mixed-valence molecules

    NASA Astrophysics Data System (ADS)

    Blair, Enrique P.; Corcelli, Steven A.; Lent, Craig S.

    2016-07-01

    Molecular quantum-dot cellular automata is a computing paradigm in which digital information is encoded by the charge configuration of a mixed-valence molecule. General-purpose computing can be achieved by arranging these compounds on a substrate and exploiting intermolecular Coulombic coupling. The operation of such a device relies on nonequilibrium electron transfer (ET), whereby the time-varying electric field of one molecule induces an ET event in a neighboring molecule. The magnitude of the electric fields can be quite large because of close spatial proximity, and the induced ET rate is a measure of the nonequilibrium response of the molecule. We calculate the electric-field-driven ET rate for a model mixed-valence compound. The mixed-valence molecule is regarded as a two-state electronic system coupled to a molecular vibrational mode, which is, in turn, coupled to a thermal environment. Both the electronic and vibrational degrees-of-freedom are treated quantum mechanically, and the dissipative vibrational-bath interaction is modeled with the Lindblad equation. This approach captures both tunneling and nonadiabatic dynamics. Relationships between microscopic molecular properties and the driven ET rate are explored for two time-dependent applied fields: an abruptly switched field and a linearly ramped field. In both cases, the driven ET rate is only weakly temperature dependent. When the model is applied using parameters appropriate to a specific mixed-valence molecule, diferrocenylacetylene, terahertz-range ET transfer rates are predicted.

  8. Microscale gradients and their role in electron-transfer mechanisms in biofilms

    PubMed Central

    Beyenal, Haluk; Babauta, Jerome T.

    2014-01-01

    The chemical and electrochemical gradients in biofilms play a critical role in electron-transfer processes between cells and a solid electron acceptor. Most of the time, electron-transfer processes have been investigated in the bulk phase, for a biofilm electrode or for an isolated component of a biofilm. Currently, the knowledge of chemical and electrochemical gradients in living biofilms respiring on a solid surface is limited. We believe the chemical and electrochemical gradients are critical for explaining electron-transfer mechanisms. The bulk conditions, an isolated part of a biofilm or a single cell cannot be used to explain electron-transfer mechanisms in biofilm systems. In addition, microscale gradients explain how the reactor configuration plays a critical role in electron-transfer processes. PMID:23176474

  9. Microscale gradients and their role in electron-transfer mechanisms in biofilms.

    PubMed

    Beyenal, Haluk; Babauta, Jerome T

    2012-12-01

    The chemical and electrochemical gradients in biofilms play a critical role in electron-transfer processes between cells and a solid electron acceptor. Most of the time, electron-transfer processes have been investigated in the bulk phase, for a biofilm electrode or for an isolated component of a biofilm. Currently, the knowledge of chemical and electrochemical gradients in living biofilms respiring on a solid surface is limited. We believe the chemical and electrochemical gradients are critical for explaining electron-transfer mechanisms. The bulk conditions, an isolated part of a biofilm or a single cell cannot be used to explain electron-transfer mechanisms in biofilm systems. In addition, microscale gradients explain how the reactor configuration plays a critical role in electron-transfer processes. PMID:23176474

  10. Rf transfer in the Coupled-Cavity Free-Electron Laser Two-Beam Accelerator

    SciTech Connect

    Makowski, M.A.

    1991-01-01

    A significant technical problem associated with the Coupled-Cavity Free-Electron Laser Two-Beam Accelerator is the transfer of RF energy from the drive accelerator to the high-gradient accelerator. Several concepts have been advanced to solve this problem. This paper examines one possible solution in which the drive and high-gradient cavities are directly coupled to one another by means of holes in the cavity walls or coupled indirectly through a third intermediate transfer cavity. Energy cascades through the cavities on a beat frequency time scale which must be made small compared to the cavity skin time but large compared to the FEL pulse length. The transfer is complicated by the fact that each of the cavities in the system can support many resonant modes near the chosen frequency of operation. A generalized set of coupled-cavity equations has been developed to model the energy transfer between the various modes in each of the cavities. For a two cavity case transfer efficiencies in excess of 95% can be achieved. 3 refs., 2 figs.

  11. Long-range electron transfer across Peptide bridges: the transition from electron superexchange to hopping.

    PubMed

    Malak, Rouba Abdel; Gao, Zhinong; Wishart, James F; Isied, Stephan S

    2004-11-01

    Long-range electron transfer rate constants for complexes of the type [(bpy)2RuIIL-Pron-apyRuIII)(NH3)5]5++ proline residues (n) varying from 0 to 9 were determined by complementary electron pulse radiolysis and flash photolysis techniques from the picosecond to the millisecond time scales. The activationless kmax values from both techniques coalesce into one data set. The distance dependence of the reactions is consistent with a smooth transition from a superexchange mechanism with attenuation constant beta = 1.4 A-1 to a hopping mechanism with attenuation constant beta = 0.17 A-1. The transition occurs between n = 3 and 4 prolines, and the virtual hopping rate constant at the shortest distance is about 1 x 106 times slower than that observed for the superexchange value. PMID:15506726

  12. Electron-transfer acceleration investigated by time resolved infrared spectroscopy.

    PubMed

    Vlček, Antonín; Kvapilová, Hana; Towrie, Michael; Záliš, Stanislav

    2015-03-17

    Ultrafast electron transfer (ET) processes are important primary steps in natural and artificial photosynthesis, as well as in molecular electronic/photonic devices. In biological systems, ET often occurs surprisingly fast over long distances of several tens of angströms. Laser-pulse irradiation is conveniently used to generate strongly oxidizing (or reducing) excited states whose reactions are then studied by time-resolved spectroscopic techniques. While photoluminescence decay and UV-vis absorption supply precise kinetics data, time-resolved infrared absorption (TRIR) and Raman-based spectroscopies have the advantage of providing additional structural information and monitoring vibrational energy flows and dissipation, as well as medium relaxation, that accompany ultrafast ET. We will discuss three cases of photoinduced ET involving the Re(I)(CO)3(N,N) moiety (N,N = polypyridine) that occur much faster than would be expected from ET theories. [Re(4-N-methylpyridinium-pyridine)(CO)3(N,N)](2+) represents a case of excited-state picosecond ET between two different ligands that remains ultrafast even in slow-relaxing solvents, beating the adiabatic limit. This is caused by vibrational/solvational excitation of the precursor state and participation of high-frequency quantum modes in barrier crossing. The case of Re-tryptophan assemblies demonstrates that excited-state Trp → *Re(II) ET is accelerated from nanoseconds to picoseconds when the Re(I)(CO)3(N,N) chromophore is appended to a protein, close to a tryptophan residue. TRIR in combination with DFT calculations and structural studies reveals an interaction between the N,N ligand and the tryptophan indole. It results in partial electronic delocalization in the precursor excited state and likely contributes to the ultrafast ET rate. Long-lived vibrational/solvational excitation of the protein Re(I)(CO)3(N,N)···Trp moiety, documented by dynamic IR band shifts, could be another accelerating factor. The last

  13. Novel zinc phthalocyanine-benzoquinone rigid dyad and its photoinduced electron transfer properties.

    SciTech Connect

    Lee, C.-H.; Guo, J.; Chen, L. X.; Mandal, B. K.; Chemical Sciences and Engineering Division; Illinois Inst. of Tech.; Northwestern Univ.

    2008-10-10

    While preparing the first structurally rigid zinc phthalocyanine-benzoquinone (ZnPc-BQ) dyad as a model for photoinduced charge separation mimicking natural photosynthesis, a convenient method is developed for in situ generation of a benzoquinone chromophore in the dyad using an iso-butyryl mask. The dyad has no rotamers and possesses a fixed distance between ZnPc and BQ moieties (center-to-center and edge-to-edge distances are 9.40 and 2.14 {angstrom}, respectively). The dyad displays unusual electronic perturbation in the ground state, resulting from the interactions between Pc and BQ, and exhibits photoinduced electron transfer with a lifetime of 40 ps of the charged separated states. The steady-state fluorescence and electrochemical behavior of the dyad are evaluated. This study opens a route to subsequent dyads, triads, and complex architectures of electron donor-acceptor arrays with rigid structures and long charge separation states.

  14. Session on modeling of radiative transfer processes

    NASA Technical Reports Server (NTRS)

    Flatau, Piotr

    1993-01-01

    The session on modeling of radiative transfer processes is reviewed. Six critical issues surfaced in the discussion concerning scale-interactive radiative processes relevent to the mesoscale convective systems (MCS's). These issues are the need to expand basic knowledge of how MCS's influence climate through extensive cloud shields and increased humidity in the upper troposphere; to improve radiation parameterizations used in mesoscale and General Circulation Model (GCM) models; to improve our basic understanding of the influence of radiation on MCS dynamics due to diabatic heating, production of condensate, and vertical and horizontal heat fluxes; to quantify our understanding of radiative impacts of MCS's on the surface and free atmosphere energy budgets; to quantify and identify radiative and microphysical processes important in the evolution of MCS's; and to improve the capability to remotely sense MCS radiative properties from space and ground-based systems.

  15. Electron, proton and hydrogen-atom transfers in photosynthetic water oxidation.

    PubMed Central

    Tommos, Cecilia

    2002-01-01

    When photosynthetic organisms developed so that they could use water as an electron source to reduce carbon dioxide, the stage was set for efficient proliferation. Algae and plants spread globally and provided the foundation for our atmosphere and for O(2)-based chemistry in biological systems. Light-driven water oxidation is catalysed by photosystem II, the active site of which contains a redox-active tyrosine denoted Y(Z), a tetramanganese cluster, calcium and chloride. In 1995, Gerald Babcock and co-workers presented the hypothesis that photosynthetic water oxidation occurs as a metallo-radical catalysed process. In this model, the oxidized tyrosine radical is generated by coupled proton/electron transfer and re-reduced by abstracting hydrogen atoms from substrate water or hydroxide-ligated to the manganese cluster. The proposed function of Y(Z) requires proton transfer from the tyrosine site upon oxidation. The oxidation mechanism of Y(Z) in an inhibited and O(2)-evolving photosystem II is discussed. Domino-deprotonation from Y(Z) to the bulk solution is shown to be consistent with a variety of data obtained on metal-depleted samples. Experimental data that suggest that the oxidation of Y(Z) in O(2)-evolving samples is coupled to proton transfer in a hydrogen-bonding network are described. Finally, a dielectric-dependent model for the proton release that is associated with the catalytic cycle of photosystem II is discussed. PMID:12437877

  16. Heterogeneous electron transfer at nanoscopic electrodes: importance of electronic structures and electric double layers.

    PubMed

    Chen, Shengli; Liu, Yuwen; Chen, Junxiang

    2014-08-01

    Heterogeneous electron-transfer (ET) processes at solid electrodes play key roles in molecular electronics and electrochemical energy conversion and sensing. Electrode nanosization and/or nanostructurization are among the major current strategies for performance promotion in these fields. Besides, nano-sized/structured electrodes offer great opportunities to characterize electrochemical structures and processes with high spatial and temporal resolution. This review presents recent insights into the nanoscopic size and structure effects of electrodes and electrode materials on heterogeneous ET kinetics, by emphasizing the importance of the electric double-layer (EDL) at the electrode/electrolyte interface and the electronic structure of electrode materials. It is shown, by general conceptual analysis and recent example demonstrations of representative electrode systems including electrodes of nanometer sizes and gaps and of nanomaterials such as sp(2) hybridized nanocarbons and semiconductor quantum dots, how the heterogeneous ET kinetics, the electronic structures of electrodes, the EDL structures at the electrode/electrolyte interface and the nanoscopic electrode sizes and structures may be related. PMID:24871071

  17. Ab initio quantum mechanical/molecular mechanical simulation of electron transfer process: Fractional electron approach

    NASA Astrophysics Data System (ADS)

    Zeng, Xiancheng; Hu, Hao; Hu, Xiangqian; Cohen, Aron J.; Yang, Weitao

    2008-03-01

    Electron transfer (ET) reactions are one of the most important processes in chemistry and biology. Because of the quantum nature of the processes and the complicated roles of the solvent, theoretical study of ET processes is challenging. To simulate ET processes at the electronic level, we have developed an efficient density functional theory (DFT) quantum mechanical (QM)/molecular mechanical (MM) approach that uses the fractional number of electrons as the order parameter to calculate the redox free energy of ET reactions in solution. We applied this method to study the ET reactions of the aqueous metal complexes Fe(H2O)62+/3+ and Ru(H2O)62+/3+. The calculated oxidation potentials, 5.82 eV for Fe(II/III) and 5.14 eV for Ru(II/III), agree well with the experimental data, 5.50 and 4.96 eV, for iron and ruthenium, respectively. Furthermore, we have constructed the diabatic free energy surfaces from histogram analysis based on the molecular dynamics trajectories. The resulting reorganization energy and the diabatic activation energy also show good agreement with experimental data. Our calculations show that using the fractional number of electrons (FNE) as the order parameter in the thermodynamic integration process leads to efficient sampling and validate the ab initio QM/MM approach in the calculation of redox free energies.

  18. Modeling crossed-beam energy transfer for inertial confinement fusion

    NASA Astrophysics Data System (ADS)

    Marion, D. J. Y.; Debayle, A.; Masson-Laborde, P.-E.; Loiseau, P.; Casanova, M.

    2016-05-01

    We developed a numerical code that describes both the energy transfer occurring when two or more laser beams overlap in a weakly non-homogeneous plasma, and the beam energy losses associated with the electron-ion collisions. The numerical solutions are validated with both the exact analytical solutions in homogeneous plasmas, and with new approximate analytical solutions in non-homogeneous plasmas that include the aforementioned inverse bremsstrahlung effect. Comparisons with kinetic particle-in-cell simulations are satisfactory, provided the acoustic wave-breaking limit and the self-focusing regime are not reached. An application of the Cross-Beam Energy Transfer model is shown for a typical case of indirect-drive implosion in a gold hohlraum.

  19. Photoinduced electron transfer from semiconductor quantum dots to metal oxide nanoparticles

    PubMed Central

    Tvrdy, Kevin; Frantsuzov, Pavel A.; Kamat, Prashant V.

    2011-01-01

    Quantum dot-metal oxide junctions are an integral part of next-generation solar cells, light emitting diodes, and nanostructured electronic arrays. Here we present a comprehensive examination of electron transfer at these junctions, using a series of CdSe quantum dot donors (sizes 2.8, 3.3, 4.0, and 4.2 nm in diameter) and metal oxide nanoparticle acceptors (SnO2, TiO2, and ZnO). Apparent electron transfer rate constants showed strong dependence on change in system free energy, exhibiting a sharp rise at small driving forces followed by a modest rise further away from the characteristic reorganization energy. The observed trend mimics the predicted behavior of electron transfer from a single quantum state to a continuum of electron accepting states, such as those present in the conduction band of a metal oxide nanoparticle. In contrast with dye-sensitized metal oxide electron transfer studies, our systems did not exhibit unthermalized hot-electron injection due to relatively large ratios of electron cooling rate to electron transfer rate. To investigate the implications of these findings in photovoltaic cells, quantum dot-metal oxide working electrodes were constructed in an identical fashion to the films used for the electron transfer portion of the study. Interestingly, the films which exhibited the fastest electron transfer rates (SnO2) were not the same as those which showed the highest photocurrent (TiO2). These findings suggest that, in addition to electron transfer at the quantum dot-metal oxide interface, other electron transfer reactions play key roles in the determination of overall device efficiency. PMID:21149685

  20. Photochemical reactions of electron-deficient olefins with N,N,N‧,N‧-tetramethylbenzidine via photoinduced electron-transfer

    NASA Astrophysics Data System (ADS)

    Pan, Yang; Zhao, Junshu; Ji, Yuanyuan; Yan, Lei; Yu, Shuqin

    2006-01-01

    Photoinduced electron transfer reactions of several electron-deficient olefins with N, N, N', N'-tetramethylbenzidine (TMB) in acetonitrile solution have been studied by using laser flash photolysis technique and steady-state fluorescence quenching method. Laser pulse excitation of TMB yields 3TMB* after rapid intersystem crossing from 1TMB*. The triplet which located at 480 nm is found to undergo fast quenching with the electron acceptors fumaronitrile (FN), dimethyl fumarate (DMF), diethyl fumarate (DEF), cinnamonitrile (CN), α-acetoxyacrylonitrile (AAN), crotononitrile (CrN) and 3-methoxyacrylonitrile (MAN). Substituents binding to olefin molecule own different electron-donating/withdrawing powers, which determine the electron-deficient property (π-cloud density) of olefin molecule as well as control the electron transfer rate constant directly. The detection of ion radical intermediates in the photolysis reactions confirms the proposed electron transfer mechanism, as expected from thermodynamics. The quenching rate constants of triplet TMB by these olefins have been determined at 510 nm to avoid the disturbance of formed TMB cation radical around 475 nm. All the kqT values approach or reach to the diffusion-controlled limit. In addition, fluorescence quenching rate constants kqS have been also obtained by calculating with Stern-Volmer equation. A correlation between experimental electron transfer rate constants and free energy changes has been explained by Marcus theory of adiabatic outer-sphere electron transfer. Disharmonic kq values for CN and CrN in endergonic region may be the disturbance of exciplexs formation.

  1. 77 FR 10373 - Greenhouse Gas Reporting Program: Electronics Manufacturing: Revisions to Heat Transfer Fluid...

    Federal Register 2010, 2011, 2012, 2013, 2014

    2012-02-22

    ...The EPA is finalizing technical revisions to the electronics manufacturing source category of the Greenhouse Gas Reporting Rule related to fluorinated heat transfer fluids. More specifically, EPA is finalizing amendments to the definition of fluorinated heat transfer fluids and to the provisions to estimate and report emissions from fluorinated heat transfer fluids. This final rule is narrow......

  2. Electron Transfer at the Microbe-Mineral Interface: A Grand Challenge in Biogeochemistry

    SciTech Connect

    Fredrickson, Jim K.; Zachara, John M.

    2008-06-01

    The interplay between microorganisms and minerals is a complex and dynamic process that has sculpted the geosphere for nearly the entire history of the Earth. The work of Dr. Terry Beveridge and colleagues provided some of the first insights into metal-microbe and mineral-microbe interactions and established a foundation for subsequent detailed investigations of interactions between microorganisms and minerals. Beveridge also envisioned that interdisciplinary approaches and teams would be required to explain how individual microbial cells interact with their immediate environment at nano- or sub-molecular scales and that through such approaches and using emerging technologies that the details of such interactions would be revealed at the molecular level. With this vision as incentive and inspiration, a multidisciplinary, collaborative team-based investigation was initiated to probe the process of electron transfer at the microbe-mineral interface. This grand challenge to this team was to address the hypothesis that multi-heme c-type cytochromes of dissimilatory metal reducing bacteria localized to the cell exterior function as the terminal reductases in electron transfer to Fe(III) and Mn(IV) oxides. This question has been the subject of extensive investigation for years yet the answer has remained elusive. The team involves an integrated group of experimental and computational capabilities at DOE’s Environmental Molecular Sciences Laboratory, a national scientific user facility, as the collaborative focal point. The approach involves a combination of in vitro and in vivo biologic and biogeochemical experiments and computational analyses that, when integrated, provide a conceptual model of the electron transfer process. The resulting conceptual model will be evaluated by integrating and comparing various experimental, i.e., in vitro and in vivo ET kinetics, and theoretical results. Collectively the grand challenge will provide a detailed view of how organisms

  3. Enzymatic cellulose oxidation is linked to lignin by long-range electron transfer

    PubMed Central

    Westereng, Bjørge; Cannella, David; Wittrup Agger, Jane; Jørgensen, Henning; Larsen Andersen, Mogens; Eijsink, Vincent G.H.; Felby, Claus

    2015-01-01

    Enzymatic oxidation of cell wall polysaccharides by lytic polysaccharide monooxygenases (LPMOs) plays a pivotal role in the degradation of plant biomass. While experiments have shown that LPMOs are copper dependent enzymes requiring an electron donor, the mechanism and origin of the electron supply in biological systems are only partly understood. We show here that insoluble high molecular weight lignin functions as a reservoir of electrons facilitating LPMO activity. The electrons are donated to the enzyme by long-range electron transfer involving soluble low molecular weight lignins present in plant cell walls. Electron transfer was confirmed by electron paramagnetic resonance spectroscopy showing that LPMO activity on cellulose changes the level of unpaired electrons in the lignin. The discovery of a long-range electron transfer mechanism links the biodegradation of cellulose and lignin and sheds new light on how oxidative enzymes present in plant degraders may act in concert. PMID:26686263

  4. Activated-ion electron transfer dissociation improves the ability of electron transfer dissociation to identify peptides in a complex mixture.

    PubMed

    Ledvina, Aaron R; Beauchene, Nicole A; McAlister, Graeme C; Syka, John E P; Schwartz, Jae C; Griep-Raming, Jens; Westphall, Michael S; Coon, Joshua J

    2010-12-15

    Using a modified electron transfer dissociation (ETD)-enabled quadrupole linear ion trap (QLT) mass spectrometer, we demonstrate the utility of IR activation concomitant with ETD ion-ion reactions (activated-ion ETD, AI-ETD). Analyzing 12 strong cation exchanged (SCX) fractions of a LysC digest of human cell protein extract using ETD, collision-activated dissociation (CAD), and AI-ETD, we find that AI-ETD generates 13 405 peptide spectral matches (PSMs) at a 1% false-discovery rate (1% FDR), surpassing both ETD (7 968) and CAD (10 904). We also analyze 12 SCX fractions of a tryptic digest of human cell protein extract and find that ETD produces 6 234 PSMs, AI-ETD 9 130 PSMs, and CAD 15 209 PSMs. Compared to ETD with supplemental collisional activation (ETcaD), AI-ETD generates ∼80% more PSMs for the whole cell lysate digested with trypsin and ∼50% more PSMs for the whole cell lysate digested with LysC. PMID:21062032

  5. Structural and orientation effects on electronic energy transfer between silicon quantum dots with dopants and with silver adsorbates

    SciTech Connect

    Vinson, N.; Freitag, H.; Micha, D. A.

    2014-06-28

    Starting from the atomic structure of silicon quantum dots (QDs), and utilizing ab initio electronic structure calculations within the Förster resonance energy transfer (FRET) treatment, a model has been developed to characterize electronic excitation energy transfer between QDs. Electronic energy transfer rates, K{sub EET}, between selected identical pairs of crystalline silicon quantum dots systems, either bare, doped with Al or P, or adsorbed with Ag and Ag{sub 3}, have been calculated and analyzed to extend previous work on light absorption by QDs. The effects of their size and relative orientation on energy transfer rates for each system have also been considered. Using time-dependent density functional theory and the hybrid functional HSE06, the FRET treatment was employed to model electronic energy transfer rates within the dipole-dipole interaction approximation. Calculations with adsorbed Ag show that: (a) addition of Ag increases rates up to 100 times, (b) addition of Ag{sub 3} increases rates up to 1000 times, (c) collinear alignment of permanent dipoles increases transfer rates by an order of magnitude compared to parallel orientation, and (d) smaller QD-size increases transfer due to greater electronic orbitals overlap. Calculations with dopants show that: (a) p-type and n-type dopants enhance energy transfer up to two orders of magnitude, (b) surface-doping with P and center-doping with Al show the greatest rates, and (c) K{sub EET} is largest for collinear permanent dipoles when the dopant is on the outer surface and for parallel permanent dipoles when the dopant is inside the QD.

  6. 75 FR 59172 - Electronic Funds Transfer of Depository Taxes; Hearing Cancellation

    Federal Register 2010, 2011, 2012, 2013, 2014

    2010-09-27

    ... Register on Thursday, August ] 26, 2010, (75 FR 52485) announced that a public hearing was scheduled for... Internal Revenue Service 26 CFR Parts 1, 31, 40, and 301 RIN 1545-BJ13 Electronic Funds Transfer of... on proposed regulation relating to Federal tax deposits (FTDs) by Electronic Funds Transfer...

  7. 49 CFR 225.37 - Magnetic media transfer and electronic submission.

    Code of Federal Regulations, 2010 CFR

    2010-10-01

    ... 49 Transportation 4 2010-10-01 2010-10-01 false Magnetic media transfer and electronic submission..., AND INVESTIGATIONS § 225.37 Magnetic media transfer and electronic submission. (a) A railroad has the option of submitting the following reports, updates, and amendments by way of magnetic media...

  8. 27 CFR 53.158 - Payment of tax by electronic fund transfer.

    Code of Federal Regulations, 2010 CFR

    2010-04-01

    ... electronic fund transfer. 53.158 Section 53.158 Alcohol, Tobacco Products and Firearms ALCOHOL AND TOBACCO TAX AND TRADE BUREAU, DEPARTMENT OF THE TREASURY (CONTINUED) FIREARMS MANUFACTURERS EXCISE TAXES... Taxes § 53.158 Payment of tax by electronic fund transfer. (a) In general. For return periods...

  9. 27 CFR 24.272 - Payment of tax by electronic fund transfer.

    Code of Federal Regulations, 2010 CFR

    2010-04-01

    ... electronic fund transfer. 24.272 Section 24.272 Alcohol, Tobacco Products and Firearms ALCOHOL AND TOBACCO TAX AND TRADE BUREAU, DEPARTMENT OF THE TREASURY LIQUORS WINE Removal, Return and Receipt of Wine Taxpaid Removals § 24.272 Payment of tax by electronic fund transfer. (a) General. (1) During a...

  10. Distance dependence of electron transfer from liposome-embedded (alkanephosphocholine-porphinato) zinc

    SciTech Connect

    Tsuchida, E.; Kaneko, M.; Nishide, H.; Hoshino, M.

    1986-05-22

    (Alkanephosphocholine-porphinato)zinc forms a geometrically well-defined bilayer liposome with phospholipid. Electron transfer from the liposome-embedded (porphinato)zincs with different alkyl chain lengths to methylviologen present in the outer bulk solution is measured by laser flash photolysis: the intermolecular electron transfer was observed only when the porphyrin plane is located within 12 A from the surface.

  11. Theory for electron-transfer reactions involving two Marcus surfaces with a different force constant

    SciTech Connect

    Tang, Jau

    1994-02-01

    Theory for electron-transfer reactions at high temperature involving two Marcus parabolic surfaces with a different force constant is presented. The dynamic solvent effects are also considered using the stochastic Liouville equation, assuming an overdamped Debye solvent. An analytical expression for the adiabatic/nonadiabatic electron-transfer rate constant is derived.

  12. Aminotroponiminates as tunable, redox-active ligands: reversible single electron transfer and reductive dimerisation.

    PubMed

    Lichtenberg, C; Krummenacher, I

    2016-08-21

    Aminotroponiminates (atis) are shown to be redox-active ligands. Under strongly reducing conditions, the result of electron transfer can be controlled by the choice of the metal bound to the ati ligand. Either reversible electron transfer or a reductively induced dimerisation is observed. The latter reaction is (regio- and diastereo-) selective and chemically reversible. PMID:27452905

  13. 49 CFR 225.37 - Optical media transfer and electronic submission.

    Code of Federal Regulations, 2011 CFR

    2011-10-01

    ... 49 Transportation 4 2011-10-01 2011-10-01 false Optical media transfer and electronic submission..., AND INVESTIGATIONS § 225.37 Optical media transfer and electronic submission. (a) A railroad has the option of submitting the following reports, updates, and amendments by way of optical media (CD-ROM),...

  14. 27 CFR 41.63 - Payment of tax by electronic fund transfer.

    Code of Federal Regulations, 2010 CFR

    2010-04-01

    ... controlled group of corporations, as defined in 26 U.S.C. 1563, and implementing regulations in 26 CFR 1.1563... electronic fund transfer. 41.63 Section 41.63 Alcohol, Tobacco Products and Firearms ALCOHOL AND TOBACCO TAX... by electronic fund transfer. (a) Each importer who was liable, during a calendar year, for a...

  15. 41 CFR 102-118.70 - Must my agency make all payments via electronic funds transfer?

    Code of Federal Regulations, 2010 CFR

    2010-07-01

    ... 41 Public Contracts and Property Management 3 2010-07-01 2010-07-01 false Must my agency make all payments via electronic funds transfer? 102-118.70 Section 102-118.70 Public Contracts and Property... Services § 102-118.70 Must my agency make all payments via electronic funds transfer? Yes, under 31...

  16. 48 CFR 52.232-33 - Payment by Electronic Funds Transfer-Central Contractor Registration.

    Code of Federal Regulations, 2010 CFR

    2010-10-01

    ... CONTRACT CLAUSES Text of Provisions and Clauses 52.232-33 Payment by Electronic Funds Transfer—Central... Government under this contract shall be made by electronic funds transfer (EFT), except as provided in... Fedwire Transfer System. The rules governing Federal payments through the ACH are contained in 31 CFR...

  17. 77 FR 22066 - Proposed Collection of Information: “Notice of Reclamation Electronic Funds Transfer, Federal...

    Federal Register 2010, 2011, 2012, 2013, 2014

    2012-04-12

    ... comments concerning forms FMS-I33, ``Notice of Reclamation. Electronic Funds Transfer, Federal Recurring Payment'' and FMS-135, ``Request for Debit. Electronic Funds Transfer, Federal Recurring Payments.'' DATES... Number: FMS 133, FMS 135. ] Abstract: Program agencies authorize Treasury to recover payments that...

  18. Electron-transfer photochromism to switch bulk second-order nonlinear optical properties with high contrast.

    PubMed

    Li, Pei-Xin; Wang, Ming-Sheng; Zhang, Ming-Jian; Lin, Chen-Sheng; Cai, Li-Zhen; Guo, Sheng-Ping; Guo, Guo-Cong

    2014-10-20

    The first bulk electron-transfer photochromic compound with intrinsic second-order nonlinear optical (NLO) photoswitching properties has been synthesized. This system employs an electron-transfer photoactive asymmetric viologen ligand coordinated to a zinc(II) center. PMID:25195919

  19. Protein electron transfer: is biology (thermo)dynamic?

    PubMed

    Matyushov, Dmitry V

    2015-12-01

    Simple physical mechanisms are behind the flow of energy in all forms of life. Energy comes to living systems through electrons occupying high-energy states, either from food (respiratory chains) or from light (photosynthesis). This energy is transformed into the cross-membrane proton-motive force that eventually drives all biochemistry of the cell. Life's ability to transfer electrons over large distances with nearly zero loss of free energy is puzzling and has not been accomplished in synthetic systems. The focus of this review is on how this energetic efficiency is realized. General physical mechanisms and interactions that allow proteins to fold into compact water-soluble structures are also responsible for a rugged landscape of energy states and a broad distribution of relaxation times. Specific to a protein as a fluctuating thermal bath is the protein-water interface, which is heterogeneous both dynamically and structurally. The spectrum of interfacial fluctuations is a consequence of protein's elastic flexibility combined with a high density of surface charges polarizing water dipoles into surface nanodomains. Electrostatics is critical to the protein function and the relevant questions are: (i) What is the spectrum of interfacial electrostatic fluctuations? (ii) Does the interfacial biological water produce electrostatic signatures specific to proteins? (iii) How is protein-mediated chemistry affected by electrostatics? These questions connect the fluctuation spectrum to the dynamical control of chemical reactivity, i.e. the dependence of the activation free energy of the reaction on the dynamics of the bath. Ergodicity is often broken in protein-driven reactions and thermodynamic free energies become irrelevant. Continuous ergodicity breaking in a dense spectrum of relaxation times requires using dynamically restricted ensembles to calculate statistical averages. When applied to the calculation of the rates, this formalism leads to the nonergodic activated

  20. Protein electron transfer: is biology (thermo)dynamic?

    NASA Astrophysics Data System (ADS)

    Matyushov, Dmitry V.

    2015-12-01

    Simple physical mechanisms are behind the flow of energy in all forms of life. Energy comes to living systems through electrons occupying high-energy states, either from food (respiratory chains) or from light (photosynthesis). This energy is transformed into the cross-membrane proton-motive force that eventually drives all biochemistry of the cell. Life’s ability to transfer electrons over large distances with nearly zero loss of free energy is puzzling and has not been accomplished in synthetic systems. The focus of this review is on how this energetic efficiency is realized. General physical mechanisms and interactions that allow proteins to fold into compact water-soluble structures are also responsible for a rugged landscape of energy states and a broad distribution of relaxation times. Specific to a protein as a fluctuating thermal bath is the protein-water interface, which is heterogeneous both dynamically and structurally. The spectrum of interfacial fluctuations is a consequence of protein’s elastic flexibility combined with a high density of surface charges polarizing water dipoles into surface nanodomains. Electrostatics is critical to the protein function and the relevant questions are: (i) What is the spectrum of interfacial electrostatic fluctuations? (ii) Does the interfacial biological water produce electrostatic signatures specific to proteins? (iii) How is protein-mediated chemistry affected by electrostatics? These questions connect the fluctuation spectrum to the dynamical control of chemical reactivity, i.e. the dependence of the activation free energy of the reaction on the dynamics of the bath. Ergodicity is often broken in protein-driven reactions and thermodynamic free energies become irrelevant. Continuous ergodicity breaking in a dense spectrum of relaxation times requires using dynamically restricted ensembles to calculate statistical averages. When applied to the calculation of the rates, this formalism leads to the nonergodic

  1. Demonstration of Lignin-to-Peroxidase Direct Electron Transfer

    PubMed Central

    Sáez-Jiménez, Verónica; Baratto, Maria Camilla; Pogni, Rebecca; Rencoret, Jorge; Gutiérrez, Ana; Santos, José Ignacio; Martínez, Angel T.; Ruiz-Dueñas, Francisco Javier

    2015-01-01

    Versatile peroxidase (VP) is a high redox-potential peroxidase of biotechnological interest that is able to oxidize phenolic and non-phenolic aromatics, Mn2+, and different dyes. The ability of VP from Pleurotus eryngii to oxidize water-soluble lignins (softwood and hardwood lignosulfonates) is demonstrated here by a combination of directed mutagenesis and spectroscopic techniques, among others. In addition, direct electron transfer between the peroxidase and the lignin macromolecule was kinetically characterized using stopped-flow spectrophotometry. VP variants were used to show that this reaction strongly depends on the presence of a solvent-exposed tryptophan residue (Trp-164). Moreover, the tryptophanyl radical detected by EPR spectroscopy of H2O2-activated VP (being absent from the W164S variant) was identified as catalytically active because it was reduced during lignosulfonate oxidation, resulting in the appearance of a lignin radical. The decrease of lignin fluorescence (excitation at 355 nm/emission at 400 nm) during VP treatment under steady-state conditions was accompanied by a decrease of the lignin (aromatic nuclei and side chains) signals in one-dimensional and two-dimensional NMR spectra, confirming the ligninolytic capabilities of the enzyme. Simultaneously, size-exclusion chromatography showed an increase of the molecular mass of the modified residual lignin, especially for the (low molecular mass) hardwood lignosulfonate, revealing that the oxidation products tend to recondense during the VP treatment. Finally, mutagenesis of selected residues neighboring Trp-164 resulted in improved apparent second-order rate constants for lignosulfonate reactions, revealing that changes in its protein environment (modifying the net negative charge and/or substrate accessibility/binding) can modulate the reactivity of the catalytic tryptophan. PMID:26240145

  2. Photoinduced electron transfer in perylene-TiO2 nanoassemblies.

    PubMed

    Llansola-Portoles, Manuel J; Bergkamp, Jesse J; Tomlin, John; Moore, Thomas A; Kodis, Gerdenis; Moore, Ana L; Cosa, Gonzalo; Palacios, Rodrigo E

    2013-01-01

    The photosensitization effect of three perylene dye derivatives on titanium dioxide nanoparticles (TiO2 NPs) has been investigated. The dyes used, 1,7-dibromoperylene-3,4,9,10-tetracarboxy dianhydride (1), 1,7-dipyrrolidinylperylene-3,4,9,10-tetracarboxy dianhydride (2) and 1,7-bis(4-tert-butylphenyloxy)perylene-3,4,9,10-tetracarboxy dianhydride (3) have in common bisanhydride groups that convert into TiO2 binding groups upon hydrolysis. The different substituents on the bay position of the dyes enable tuning of their redox properties to yield significantly different driving forces for photoinduced electron transfer (PeT). Recently developed TiO2 NPs having a small average size and a narrow distribution (4 ± 1 nm) are used in this work to prepare the dye-TiO2 systems under study. Whereas successful sensitization was obtained with 1 and 2 as evidenced by steady-state spectral shifts and transient absorption results, no evidence for the attachment of 3 to TiO2 was observed. The comparison of the rates of PeT (kPeT ) for 1- and 2-TiO2 systems studied in this work with those obtained for previously reported analogous systems, having TiO2 NPs covered by a surfactant layer (Hernandez et al. [2012] J. Phys. Chem. B., 117, 4568-4581), indicates that kPeT for the former systems is slower than that for the later. These results are interpreted in terms of the different energy values of the conduction band edge in each system. PMID:23742178

  3. Non-Markov dissipative dynamics of electron transfer in a photosynthetic reaction center

    NASA Astrophysics Data System (ADS)

    Poddubnyy, V. V.; Glebov, I. O.; Eremin, V. V.

    2014-02-01

    We consider the dissipative dynamics of electron transfer in the photosynthetic reaction center of purple bacteria and propose a model where the transition between electron states arises only due to the interaction between a chromophore system and the protein environment and is not accompanied by the motion of nuclei of the reaction subsystem. We establish applicability conditions for the Markov approximation in the framework of this model and show that these conditions are not necessarily satisfied in the protein medium. We represent the spectral function of the "system+heat bath" interaction in the form of one or several Gaussian functions to study specific characteristics of non-Markov dynamics of the final state population, the presence of an induction period and vibrations. The consistency of the computational results obtained for non-Markov dynamics with experimental data confirms the correctness of the proposed approach.

  4. The effect of temperature and transmembrane potentials on the rates of electron transfer between membrane-bound biological redox components.

    PubMed

    Kuznetsov, A M; Ulstrup, J

    1981-06-12

    We have investigated rate data for the temperature and free energy dependence of the primary electron-transfer processes in bacterial photosynthesis. Rather than representing the whole electronic-nuclear coupling by a frequently applied discrete single-mode model, we have incorporated a continuum of modes characterized by a certain distribution function. In this way, we can illuminate the role of both a broad distribution of low-frequency modes representing the medium and a narrow distribution representing local nuclear modes. Furthermore, it emerges from the calculations that both sets are important in the overall scheme of primary photosynthetic electron-transfer processes. By means of this model and quantum-mechanical rate theory, we can reproduce a number of important features of the primary photosynthetic processes concerning in particular the temperature (tunnelling or thermally activated nuclear motion) and free energy dependence ('normal', 'activation-less', or 'inverted' regions) of the rate constants and estimate such parameters as nuclear-reorganization energy electron-exchange integrals and electron-transfer distances. We have finally considered some of the important factors which determine the potential drop across the membrane and estimated the extent to which variations in the potential drop affect the rate constants of the electron-transfer processes. PMID:7284345

  5. Efficiencies of photoinduced electron-transfer reactions: Role of the Marcus inverted region in return electron transfer within geminate radical-ion pairs

    SciTech Connect

    Gould, I.R.; Ege, D.; Moser, J.E.; Farid, S. )

    1990-05-23

    In photoinduced electron-transfer processes the primary step is conversion of the electronic energy of an excited state into chemical energy retained in the form of a redox (geminate radical-ion) pair (A + D {sup hv}{yields} A{sup {sm bullet}{minus}}/D{sup {sm bullet}+}). In polar solvents, separation of the geminate pair occurs with formation of free radical ions in solution. The quantum yields of product formation, from reactions of either the free ions, or of the geminate pair, are often low, however, due to the return electron transfer reaction (A{sup {sm bullet}{minus}}/D{sup {sm bullet}+} {yields} A + D), an energy-wasting step that competes with the useful reactions of the ion pair. The present study was undertaken to investigate the parameters controlling the rates of these return electron transfer reactions. Quantum yields of free radical ion formation were measured for ion pairs formed upon electron-transfer quenching of the first excited singlet states of cyanoanthracenes by simple aromatic hydrocarbon donors in aceonitrile at room temperature. The free-ion yields are determined by the competition between the rates of separation and return electron transfer.

  6. Quenching of triplet states of aromatic ketones by sulfur-containing amino acids in solution. Evidence for electron transfer

    SciTech Connect

    Marciniak, B.; Bobrowski, K.; Hug, G.L. )

    1993-11-18

    The mechanism for quenching triplet states of benzophenones by sulfur-containing amino acids in water/acetonitrile solution was investigated by laser flash photolysis. The amino acids in the study were methionine, S-methylcysteine, and S-carboxymethylcysteine, and the eight aromatic triplets were those of benzophenone and its derivatives possessing electron-withdrawing or electron-donating groups. The presence of radical ions in the transient spectra and correlations of the quenching rate constants with the free energy change for electron transfer are strong indications that the process involves an electron transfer. These correlations were displayed as Rehm-Weller plots (logarithm of quenching rate vs free energy). Classical theoretical formulations of the Rehm-Weller correlations were used to estimate the intrinsic barriers and the transmission coefficients for the electron-transfer processes. Applying both [open quotes]quadratic[close quotes] Marcus and [open quotes]asymptotic[close quotes] Agmon-Levine free energy relationships led to the values of intrinsic barriers lower than the solvent reorganization energy calculated within the framework of the dielectric continuum model. These relationships also led to low electronic transmission coefficients. The low values of the intrinsic barriers for electron transfer were also obtained using the recently developed Tachiya approach. 58 refs., 5 figs., 2 tabs.

  7. Molecular dynamics simulations give insight into the conformational change, complex formation, and electron transfer pathway for cytochrome P450 reductase

    PubMed Central

    Sündermann, Axel; Oostenbrink, Chris

    2013-01-01

    Cytochrome P450 reductase (CYPOR) undergoes a large conformational change to allow for an electron transfer to a redox partner to take place. After an internal electron transfer over its cofactors, it opens up to facilitate the interaction and electron transfer with a cytochrome P450. The open conformation appears difficult to crystallize. Therefore, a model of a human CYPOR in the open conformation was constructed to be able to investigate the stability and conformational change of this protein by means of molecular dynamics simulations. Since the role of the protein is to provide electrons to a redox partner, the interactions with cytochrome P450 2D6 (2D6) were investigated and a possible complex structure is suggested. Additionally, electron pathway calculations with a newly written program were performed to investigate which amino acids relay the electrons from the FMN cofactor of CYPOR to the HEME of 2D6. Several possible interacting amino acids in the complex, as well as a possible electron transfer pathway were identified and open the way for further investigation by site directed mutagenesis studies. PMID:23832577

  8. An electron energy-loss study of picene and chrysene based charge transfer salts

    SciTech Connect

    Müller, Eric; Mahns, Benjamin; Büchner, Bernd; Knupfer, Martin

    2015-05-14

    The electronic excitation spectra of charge transfer compounds built from the hydrocarbons picene and chrysene, and the strong electron acceptors F{sub 4}TCNQ (2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) and TCNQ (7,7,8,8-tetracyanoquinodimethan) have been investigated using electron energy-loss spectroscopy. The corresponding charge transfer compounds have been prepared by co-evaporation of the pristine constituents. We demonstrate that all investigated combinations support charge transfer, which results in new electronic excitation features at low energy. This might represent a way to synthesize low band gap organic semiconductors.

  9. Probing the ultrafast electron transfer at the CuPc/Au(111) interface

    SciTech Connect

    Chen Wei; Wang Li; Qi Dongchen; Chen Shi; Gao Xingyu; Wee, Andrew Thye Shen

    2006-05-01

    Core-hole clock spectroscopy and near-edge x-ray-absorption fine structure measurements have been used to investigate the ultrafast electron transfer dynamics at the Copper(II) phthalocyanine (CuPc)/Au(111) interface. It was found that the strong electronic coupling between the first layer of CuPc molecules and Au(111) substrate favors ultrafast electron transfer from the lowest unoccupied molecular orbital of the CuPc molecules to the conduction band of Au(111) in the time scale of {approx}6 fs. In contrast, the intermolecular electron transfer within multilayers of CuPc molecules via the weak van der Waals interaction was much slower.

  10. Enhanced electron transfer kinetics through hybrid graphene-carbon nanotube films

    PubMed Central

    Henry, Philémon A.; Raut, Akshay S.; Ubnoske, Stephen M.; Parker, Charles B.; Glass, Jeffrey T.

    2014-01-01

    We report the first study of the electrochemical reactivity of a graphenated carbon nanotube (g-CNT) film. The electron transfer kinetics of the ferri-ferrocyanide couple were examined for a g-CNT film and compared to the kinetics to standard carbon nanotubes (CNTs). The g-CNT film exhibited much higher catalytic activity, with a heterogeneous electron-transfer rate constant, k0, approximately two orders of magnitude higher than for standard CNTs. Scanning electron microscopy and Raman spectroscopy were used to correlate the higher electron transfer kinetics with the higher edge-density of the g-CNT film. PMID:25309121

  11. Coherent control of long-range photoinduced electron transfer by stimulated X-ray Raman processes.

    PubMed

    Dorfman, Konstantin E; Zhang, Yu; Mukamel, Shaul

    2016-09-01

    We show that X-ray pulses resonant with selected core transitions can manipulate electron transfer (ET) in molecules with ultrafast and atomic selectivity. We present possible protocols for coherently controlling ET dynamics in donor-bridge-acceptor (DBA) systems by stimulated X-ray resonant Raman processes involving various transitions between the D, B, and A sites. Simulations presented for a Ru(II)-Co(III) model complex demonstrate how the shapes, phases and amplitudes of the X-ray pulses can be optimized to create charge on demand at selected atoms, by opening up otherwise blocked ET pathways. PMID:27559082

  12. Electronic Energy Transfer on CaO Surfaces

    SciTech Connect

    Joly, Alan G.; Beck, Kenneth M.; Hess, Wayne P.

    2008-09-28

    We excite low-coordinated surface sites of nanostructured CaO samples using tunable UV laser pulses and observe hyperthermal O-atom emission indicative of an electronic excited-state desorption mechanism. The O-atom yield increases dramatically with photon energy, between 3.75 and 5.4 eV, below the bulk absorption threshold. The peak of the kinetic energy distribution does not increase with photon energy in the range 3.9 to 5.15 eV. These results are analyzed in the context of a laser desorption model developed previously for nanostructured MgO samples. The data are consistent with desorption induced by exciton localization at corner-hole trapped surface sites following either direct corner excitation or diffusion and localization of excitons from higher coordinated surface sites.

  13. Charge transfer and electronic doping in nitrogen-doped graphene

    PubMed Central

    Joucken, Frédéric; Tison, Yann; Le Fèvre, Patrick; Tejeda, Antonio; Taleb-Ibrahimi, Amina; Conrad, Edward; Repain, Vincent; Chacon, Cyril; Bellec, Amandine; Girard, Yann; Rousset, Sylvie; Ghijsen, Jacques; Sporken, Robert; Amara, Hakim; Ducastelle, François; Lagoute, Jérôme

    2015-01-01

    Understanding the modification of the graphene’s electronic structure upon doping is crucial for enlarging its potential applications. We present a study of nitrogen-doped graphene samples on SiC(000) combining angle-resolved photoelectron spectroscopy, scanning tunneling microscopy and spectroscopy and X-ray photoelectron spectroscopy (XPS). The comparison between tunneling and angle-resolved photoelectron spectra reveals the spatial inhomogeneity of the Dirac energy shift and that a phonon correction has to be applied to the tunneling measurements. XPS data demonstrate the dependence of the N 1s binding energy of graphitic nitrogen on the nitrogen concentration. The measure of the Dirac energy for different nitrogen concentrations reveals that the ratio usually computed between the excess charge brought by the dopants and the dopants’ concentration depends on the latter. This is supported by a tight-binding model considering different values for the potentials on the nitrogen site and on its first neighbors. PMID:26411651

  14. Lifetimes of Heavy-Rydberg Ion-Pair States Formed through Rydberg Electron Transfer

    SciTech Connect

    Cannon, M.; Wang, C. H.; Dunning, F. B.; Reinhold, Carlos O

    2010-01-01

    The lifetimes of K{sup +}-Cl{sup -}, K{sup +}-CN{sup -}, and K{sup +}-SF{sub 6}{sup -} heavy-Rydberg ion-pair states produced through Rydberg electron transfer reactions are measured directly as a function of binding energy using electric field induced detachment and the ion-pair decay channels discussed. The data are interpreted using a Monte Carlo collision code that models the detailed kinematics of electron transfer reactions. The lifetimes of K{sup +}-Cl{sup -} ion-pair states are observed to be very long, >100 {micro}s, and independent of binding energy. The lifetimes of strongly bound (>30 meV) K{sup +}-CN{sup -} ion pairs are found to be similarly long but begin to decrease markedly as the binding energy is reduced below this value. This behavior is attributed to conversion of rotational energy in the CN{sup -} ion into translational energy of the ion pair. No long-lived K{sup +}-SF{sub 6}{sup -} ion pairs are observed, their lifetimes decreasing with increasing binding energy. This behavior suggests that ion-pair loss is associated with mutual neutralization as a result of charge transfer.

  15. Peroxidase activity stabilization of cytochrome P450(BM3) by rational analysis of intramolecular electron transfer.

    PubMed

    Vidal-Limón, Abraham; Águila, Sergio; Ayala, Marcela; Batista, Cesar V; Vazquez-Duhalt, Rafael

    2013-05-01

    Combined quantum mechanical and molecular mechanical (QM/MM) calculations were used to explore the electron pathway involved in the suicide inactivation of cytochrome P450BM3 from Bacillus megaterium. The suicide inactivation is a common phenomenon observed for heme peroxidases, in which the enzyme is inactivated as a result of self-oxidation mediated by highly oxidizing enzyme intermediates formed during the catalytic cycle. The selected model was a mutant comprising only the heme domain (CYPBM3 21B3) that had been previously evolved to efficiently catalyze hydroxylation reactions with hydrogen peroxide (H2O2) as electron acceptor. An extensive mapping of residues involved in electron transfer routes was obtained from density functional calculations on activated heme (i.e. Compound I) and selected amino acid residues. Identification of oxidizable residues (electron donors) was performed by selectively activating/deactivating different quantum regions. This method allowed a rational identification of key oxidizable targets in order to replace them for less oxidizable residues by site-directed mutagenesis. The residues W96 and F405 were consistently predicted by the QM/MM electron pathway to hold high spin density; single and double mutants of P450BM3 on these positions (W96A, F405L, W96A/F405L) resulted in a more stable variants in the presence of hydrogen peroxide, displaying a similar reaction rate than P450BM3 21B3. Furthermore, mass spectrometry confirmed these oxidation sites and corroborated the possible routes described by QM/MM electron transfer (ET) pathways. PMID:23425936

  16. Electron transfer in tetranuclear mixed-valence iron clusters. Role of the topology on the magnetic properties

    NASA Astrophysics Data System (ADS)

    Borras-Almenar, J. J.; Coronado, E.; Georges, R.; Gomez-Garcia, C. J.

    1992-10-01

    The energy levels and magnetic properties of mixed-valence d 5d 5d 5d 6 clusters are studied from a model that takes into account the electron transfers between orbitally nondegenerate orbitals of the metal ions, and superexchange interactions. The influence of the electron transfer on the spin state energies and magnetic properties is examined as a function of the geometry of the tetranuclear cluster, including the limiting cases with T d and D 4h symmetries. For geometries close to the tetrahedral, effects of antiferromagnetic superexchange interactions are also discussed.

  17. Electron transfer in a two-level system within a Cole-Davidson vitreous bath

    NASA Astrophysics Data System (ADS)

    Ratner, Mark; Zarea, Mehdi; Wasielewski, Michael

    2014-03-01

    We study electron transfer (ET) in a two level quantum system coupled to a glassy viscous bath. The bath is modeled by the Cole-Davidson (CD) spectral density. The ET in this model is compared to the ET in a normal Drude-Debye (DD) model. It is shown that at low temperatures and when the coupling to the bath is weak, the viscous bath preserves the quantum coherence for a longer time. However in the strong coupling regime, the tunneling rate is higher in the CD. In the classical high temperature limit the difference between the CD and DD models is negligible. This work was supported by NSF grant no. CHE-/266201 and by DARPA-QUBE Program: N66001-10-1-4066/P00001.

  18. Direct simulation of proton-coupled electron transfer across multiple regimes

    NASA Astrophysics Data System (ADS)

    Kretchmer, Joshua S.; Miller, Thomas F.

    2013-04-01

    The coupled transfer of electrons and protons is a central feature of biological and molecular catalysis, yet fundamental aspects of these reactions remain poorly understood. In this study, we extend the ring polymer molecular dynamics (RPMD) method to enable direct simulation of proton-coupled electron transfer (PCET) reactions across a wide range of physically relevant regimes. In a system-bath model for symmetric, co-linear PCET in the condensed phase, RPMD trajectories reveal distinct kinetic pathways associated with sequential and concerted PCET reaction mechanisms, and it is demonstrated that concerted PCET proceeds by a solvent-gating mechanism in which the reorganization energy is mitigated by charge cancellation among the transferring particles. We further employ RPMD to study the kinetics and mechanistic features of concerted PCET reactions across multiple coupling regimes, including the fully non-adiabatic (both electronically and vibrationally non-adiabatic), partially adiabatic (electronically adiabatic, but vibrationally non-adiabatic), and fully adiabatic (both electronically and vibrationally adiabatic) limits. Comparison of RPMD with the results of PCET rate theories demonstrates the applicability of the direct simulation method over a broad range of conditions; it is particularly notable that RPMD accurately predicts the crossover in the thermal reaction rates between different coupling regimes while avoiding a priori assumptions about the PCET reaction mechanism. Finally, by utilizing the connections between RPMD rate theory and semiclassical instanton theory, we show that analysis of ring-polymer configurations in the RPMD transition path ensemble enables the a posteriori determination of the coupling regime for the PCET reaction. This analysis reveals an intriguing and distinct "transient-proton-bridge" mechanism for concerted PCET that emerges in the transition between the proton-mediated electron superexchange mechanism for fully non

  19. Direct simulation of proton-coupled electron transfer across multiple regimes

    SciTech Connect

    Kretchmer, Joshua S.; Miller, Thomas F. III

    2013-04-07

    The coupled transfer of electrons and protons is a central feature of biological and molecular catalysis, yet fundamental aspects of these reactions remain poorly understood. In this study, we extend the ring polymer molecular dynamics (RPMD) method to enable direct simulation of proton-coupled electron transfer (PCET) reactions across a wide range of physically relevant regimes. In a system-bath model for symmetric, co-linear PCET in the condensed phase, RPMD trajectories reveal distinct kinetic pathways associated with sequential and concerted PCET reaction mechanisms, and it is demonstrated that concerted PCET proceeds by a solvent-gating mechanism in which the reorganization energy is mitigated by charge cancellation among the transferring particles. We further employ RPMD to study the kinetics and mechanistic features of concerted PCET reactions across multiple coupling regimes, including the fully non-adiabatic (both electronically and vibrationally non-adiabatic), partially adiabatic (electronically adiabatic, but vibrationally non-adiabatic), and fully adiabatic (both electronically and vibrationally adiabatic) limits. Comparison of RPMD with the results of PCET rate theories demonstrates the applicability of the direct simulation method over a broad range of conditions; it is particularly notable that RPMD accurately predicts the crossover in the thermal reaction rates between different coupling regimes while avoiding a priori assumptions about the PCET reaction mechanism. Finally, by utilizing the connections between RPMD rate theory and semiclassical instanton theory, we show that analysis of ring-polymer configurations in the RPMD transition path ensemble enables the a posteriori determination of the coupling regime for the PCET reaction. This analysis reveals an intriguing and distinct 'transient-proton-bridge' mechanism for concerted PCET that emerges in the transition between the proton-mediated electron superexchange mechanism for fully non

  20. Charge Transfer Models of Zinc and Magnesium in Water.

    PubMed

    Soniat, Marielle; Hartman, Lisa; Rick, Steven W

    2015-04-14

    Quantum mechanical studies point to the importance of polarization and charge transfer (CT) in zinc binding. A new CT force field is used to study these effects in ion-water dimers and in aqueous solution. Quantum mechanics calculations are carried out to determine amounts of CT. Models for zinc and magnesium are parametrized to reproduce solvation structure, hydration free energy, and CT properties. The new models are subjected to energy decomposition, in which the effects of polarization and CT are investigated. The importance of these multibody interactions in the liquid is also considered. We find that, for divalent cations, polarization and charge transfer both strongly affect binding to water. Though polarization increases the internal (self) energy of water and ions, this is more than compensated for by a stronger ion-water interaction energy. The direction of the charge transfer from the water to the cation weakens the ion-water interaction; this increase in energy is counteracted by a decrease in the system energy due to electron delocalization. PMID:26574375

  1. Study of intermediates from transition metal excited-state electron- transfer reactions. Progress report, January 1, 1992--March 31, 1993

    SciTech Connect

    Hoffman, M.Z.

    1993-03-31

    Progress on 6 projects is reported: excited state absorption spectrum of Ru(bpy){sub 3}{sup 2+}, solvent cage model for electron transfer quenching, reductive quenching of {sup *}Cr(III) complexes, solution medium effects in oxidative quenching of {sup *}Ru(II) complexes, photosensitized oxidation of phenol in aqueous solution, and quenching of Ru(II) complexes by oxygen.

  2. Bimolecular photoinduced electron transfer beyond the diffusion limit: the Rehm-Weller experiment revisited with femtosecond time resolution.

    PubMed

    Rosspeintner, Arnulf; Angulo, Gonzalo; Vauthey, Eric

    2014-02-01

    To access the intrinsic, diffusion free, rate constant of bimolecular photoinduced electron transfer reactions, fluorescence quenching experiments have been performed with 14 donor/acceptor pairs, covering a driving-force range going from 0.6 to 2.4 eV, using steady-state and femtosecond time-resolved emission, and applying a diffusion-reaction model that accounts for the static and transient stages of the quenching for the analysis. The intrinsic electron transfer rate constants are up to 2 orders of magnitude larger than the diffusion rate constant in acetonitrile. Above ∼1.5 eV, a slight decrease of the rate constant is observed, pointing to a much weaker Marcus inverted region than those reported for other types of electron transfer reactions, such as charge recombination. Despite this, the driving force dependence can be rationalized in terms of Marcus theory. PMID:24400958

  3. Stepwise versus pseudo-concerted two-electron-transfer in a triarylamine-iridium dipyrrin-naphthalene diimide triad.

    PubMed

    Klein, Johannes H; Sunderland, Travis L; Kaufmann, Christina; Holzapfel, Marco; Schmiedel, Alexander; Lambert, Christoph

    2013-10-14

    A triad based on triarylamine donors, an iridium dipyrrin sensitiser and a naphthalene diimide acceptor is investigated using fs-pump-probe spectroscopy at two different pump wavelengths. Excitation of the naphthalene diimide induces a stepwise electron transfer process that yields within ca. 100 ps a charge separated state with 50-60% quantum yield in which one triarylamine is oxidised and the imide is reduced. Although being in the Marcus normal region, this state has a lifetime of 580 ns in MeCN. Excitation at the dipyrrin ligand centred absorption directly produces the charge separated state within ca. 1 ns however without formation of an intermediate. This pseudo-concerted two-electron transfer step was analysed with the help of a model dyad and a modified triad and is interpreted to be caused by inverted electron transfer rates. Although being much slower the latter process has a quantum yield of practically 100%. PMID:23959428

  4. Symmetrical windowing for quantum states in quasi-classical trajectory simulations: Application to electron transfer

    SciTech Connect

    Cotton, Stephen J.; Igumenshchev, Kirill; Miller, William H.

    2014-08-28

    It has recently been shown [S. J. Cotton and W. H. Miller, J. Chem. Phys. 139, 234112 (2013)] that a symmetrical windowing quasi-classical (SQC) approach [S. J. Cotton and W. H. Miller, J. Phys. Chem. A 117, 7190 (2013)] applied to the Meyer-Miller model [H.-D. Meyer and W. H. Miller, J. Chem. Phys. 70, 3214 (1979)] for the electronic degrees of freedom in electronically non-adiabatic dynamics is capable of quantitatively reproducing quantum mechanical results for a variety of test applications, including cases where “quantum” coherence effects are significant. Here we apply this same SQC methodology, within a flux-side correlation function framework, to calculate thermal rate constants corresponding to several proposed models of electron transfer processes [P. Huo, T. F. Miller III, and D. F. Coker, J. Chem. Phys. 139, 151103 (2013); A. R. Menzeleev, N. Ananth, and T. F. Miller III, J. Chem. Phys. 135, 074106 (2011)]. Good quantitative agreement with Marcus Theory is obtained over several orders of magnitude variation in non-adiabatic coupling. Moreover, the “inverted regime” in thermal rate constants (with increasing bias) known from Marcus Theory is also reproduced with good accuracy by this very simple classical approach. The SQC treatment is also applied to a recent model of photoinduced proton coupled electron transfer [C. Venkataraman, A. V. Soudackov, and S. Hammes-Schiffer, J. Chem. Phys. 131, 154502 (2009)] and population decay of the photoexcited donor state is found to be in reasonable agreement with results calculated via reduced density matrix theory.

  5. Excess-Electron Transfer in DNA by a Fluctuation-Assisted Hopping Mechanism.

    PubMed

    Lin, Shih-Hsun; Fujitsuka, Mamoru; Majima, Tetsuro

    2016-02-01

    The dynamics of excess-electron transfer in DNA has attracted the attention of scientists from all kinds of research fields because of its importance in biological processes. To date, several studies on excess-electron transfer in consecutive adenine (A):thymine (T) sequences in donor-DNA-acceptor systems have been published. However, the reported excess-electron transfer rate constants for consecutive T's are in the range of 10(10)-10(11) s(-1) depending on the photosensitizing electron donor, which provides various driving forces for excess-electron injection into DNA. In this study, we employed a strongly electron-donating photosensitizer, a dimer of 3,4-ethylenedioxythiophene (2E), and an electron acceptor, diphenylacetylene (DPA), to synthesize a series of modified DNA oligomers (2-Tn, n = 3-6) in order to investigate the excess-electron transfer dynamics in these donor-DNA-acceptor systems using femtosecond laser flash photolysis. The relation between the free energy change for charge injection and the excess-electron transfer rate among consecutive T's provided an intrinsic excess-electron hopping rate constant of (3.8 ± 1.5) × 10(10) s(-1) in the DNA, which is consistent with the fluctuation frequency of the DNA sugar backbone and bases (3.3 × 10(10) s(-1)). Thus, we discuss the effect of structural fluctuations on the excess-electron hopping in DNA. PMID:26741048

  6. Redox potential of the terminal quinone electron acceptor QB in photosystem II reveals the mechanism of electron transfer regulation

    PubMed Central

    Kato, Yuki; Nagao, Ryo; Noguchi, Takumi

    2016-01-01

    Photosystem II (PSII) extracts electrons from water at a Mn4CaO5 cluster using light energy and then transfers them to two plastoquinones, the primary quinone electron acceptor QA and the secondary quinone electron acceptor QB. This forward electron transfer is an essential process in light energy conversion. Meanwhile, backward electron transfer is also significant in photoprotection of PSII proteins. Modulation of the redox potential (Em) gap of QA and QB mainly regulates the forward and backward electron transfers in PSII. However, the full scheme of electron transfer regulation remains unresolved due to the unknown Em value of QB. Here, for the first time (to our knowledge), the Em value of QB reduction was measured directly using spectroelectrochemistry in combination with light-induced Fourier transform infrared difference spectroscopy. The Em(QB−/QB) was determined to be approximately +90 mV and was virtually unaffected by depletion of the Mn4CaO5 cluster. This insensitivity of Em(QB−/QB), in combination with the known large upshift of Em(QA−/QA), explains the mechanism of PSII photoprotection with an impaired Mn4CaO5 cluster, in which a large decrease in the Em gap between QA and QB promotes rapid charge recombination via QA−. PMID:26715751

  7. Protein Dynamics Control of Electron Transfer in Photosynthetic Reaction Centers from Rps. Sulfoviridis

    PubMed Central

    Medvedev, E. S.; Kotelnikov, A. I.; Barinov, A. V.; Psikha, B. L.; Ortega, J. M.; Popović, D. M.; Stuchebrukhov, A. A.

    2009-01-01

    In the cycle of photosynthetic reaction centers, the initially oxidized special pair of bacteriochlorophyll molecules is subsequently reduced by an electron transferred over a chain of four hemes of the complex. Here, we examine the kinetics of electron transfer between the proximal heme c-559 of the chain and the oxidized special pair in the reaction center from Rps. sulfoviridis in the range of temperatures from 294 to 40 K. The experimental data were obtained for three redox states of the reaction center, in which one, two, or three nearest hemes of the chain are reduced prior to special pair oxidation. The experimental kinetic data are analyzed in terms of a Sumi–Marcus-type model developed in our previous paper,1 in which similar measurements were reported on the reaction centers from Rps. viridis. The model allows us to establish a connection between the observed nonexponential electron-transfer kinetics and the local structural relaxation dynamics of the reaction center protein on the microsecond time scale. The activation energy for relaxation dynamics of the protein medium has been found to be around 0.1 eV for all three redox states, which is in contrast to a value around 0.4–0.6 eV in Rps. viridis.1 The possible nature of the difference between the reaction centers from Rps. viridis and Rps. sulfoviridis, which are believed to be very similar, is discussed. The role of the protein glass transition at low temperatures and that of internal water molecules in the process are analyzed. PMID:18284231

  8. Protein dynamics control of electron transfer in photosynthetic reaction centers from Rps. sulfoviridis.

    PubMed

    Medvedev, E S; Kotelnikov, A I; Barinov, A V; Psikha, B L; Ortega, J M; Popović, D M; Stuchebrukhov, A A

    2008-03-13

    In the cycle of photosynthetic reaction centers, the initially oxidized special pair of bacteriochlorophyll molecules is subsequently reduced by an electron transferred over a chain of four hemes of the complex. Here, we examine the kinetics of electron transfer between the proximal heme c-559 of the chain and the oxidized special pair in the reaction center from Rps. sulfoviridis in the range of temperatures from 294 to 40 K. The experimental data were obtained for three redox states of the reaction center, in which one, two, or three nearest hemes of the chain are reduced prior to special pair oxidation. The experimental kinetic data are analyzed in terms of a Sumi-Marcus-type model developed in our previous paper,1 in which similar measurements were reported on the reaction centers from Rps. viridis. The model allows us to establish a connection between the observed nonexponential electron-transfer kinetics and the local structural relaxation dynamics of the reaction center protein on the microsecond time scale. The activation energy for relaxation dynamics of the protein medium has been found to be around 0.1 eV for all three redox states, which is in contrast to a value around 0.4-0.6 eV in Rps. viridis.1 The possible nature of the difference between the reaction centers from Rps. viridis and Rps. sulfoviridis, which are believed to be very similar, is discussed. The role of the protein glass transition at low temperatures and that of internal water molecules in the process are analyzed. PMID:18284231

  9. Transferable pseudoclassical electrons for aufbau of atomic ions.

    PubMed

    Ekesan, Solen; Kale, Seyit; Herzfeld, Judith

    2014-06-01

    Generalizing the LEWIS reactive force field from electron pairs to single electrons, we present LEWIS• in which explicit valence electrons interact with each other and with nuclear cores via pairwise interactions. The valence electrons are independently mobile particles, following classical equations of motion according to potentials modified from Coulombic as required to capture quantum characteristics. As proof of principle, the aufbau of atomic ions is described for diverse main group elements from the first three rows of the periodic table, using a single potential for interactions between electrons of like spin and another for electrons of unlike spin. The electrons of each spin are found to distribute themselves in a fashion akin to the major lobes of the hybrid atomic orbitals, suggesting a pointillist description of the electron density. The broader validity of the LEWIS• force field is illustrated by predicting the vibrational frequencies of diatomic and triatomic hydrogen species. PMID:24752384

  10. Curly arrows meet electron density transfers in chemical reaction mechanisms: from electron localization function (ELF) analysis to valence-shell electron-pair repulsion (VSEPR) inspired interpretation.

    PubMed

    Andrés, Juan; Berski, Sławomir; Silvi, Bernard

    2016-07-01

    Probing the electron density transfers during a chemical reaction can provide important insights, making possible to understand and control chemical reactions. This aim has required extensions of the relationships between the traditional chemical concepts and the quantum mechanical ones. The present work examines the detailed chemical insights that have been generated through 100 years of work worldwide on G. N. Lewis's ground breaking paper on The Atom and the Molecule (Lewis, G. N. The Atom and the Molecule, J. Am. Chem. Soc. 1916, 38, 762-785), with a focus on how the determination of reaction mechanisms can be reached applying the bonding evolution theory (BET), emphasizing how curly arrows meet electron density transfers in chemical reaction mechanisms and how the Lewis structure can be recovered. BET that combines the topological analysis of the electron localization function (ELF) and Thom's catastrophe theory (CT) provides a powerful tool providing insight into molecular mechanisms of chemical rearrangements. In agreement with physical laws and quantum theoretical insights, BET can be considered as an appropriate tool to tackle chemical reactivity with a wide range of possible applications. Likewise, the present approach retrieves the classical curly arrows used to describe the rearrangements of chemical bonds for a given reaction mechanism, providing detailed physical grounds for this type of representation. The ideas underlying the valence-shell-electron pair-repulsion (VSEPR) model applied to non-equilibrium geometries provide simple chemical explanations of density transfers. For a given geometry around a central atom, the arrangement of the electronic domain may comply or not with the VSEPR rules according with the valence shell population of the considered atom. A deformation yields arrangements which are either VSEPR defective (at least a domain is missing to match the VSEPR arrangement corresponding to the geometry of the ligands), VSEPR compliant

  11. Spin Chains and Electron Transfer at Stepped Silicon Surfaces.

    PubMed

    Aulbach, J; Erwin, S C; Claessen, R; Schäfer, J

    2016-04-13

    High-index surfaces of silicon with adsorbed gold can reconstruct to form highly ordered linear step arrays. These steps take the form of a narrow strip of graphitic silicon. In some cases--specifically, for Si(553)-Au and Si(557)-Au--a large fraction of the silicon atoms at the exposed edge of this strip are known to be spin-polarized and charge-ordered along the edge. The periodicity of this charge ordering is always commensurate with the structural periodicity along the step edge and hence leads to highly ordered arrays of local magnetic moments that can be regarded as "spin chains." Here, we demonstrate theoretically as well as experimentally that the closely related Si(775)-Au surface has--despite its very similar overall structure--zero spin polarization at its step edge. Using a combination of density-functional theory and scanning tunneling microscopy, we propose an electron-counting model that accounts for these differences. The model also predicts that unintentional defects and intentional dopants can create local spin moments at Si(hhk)-Au step edges. We analyze in detail one of these predictions and verify it experimentally. This finding opens the door to using techniques of surface chemistry and atom manipulation to create and control silicon spin chains. PMID:26974012

  12. How fast is optically induced electron transfer in organic mixed valence systems?

    PubMed

    Lambert, C; Moos, M; Schmiedel, A; Holzapfel, M; Schäfer, J; Kess, M; Engel, V

    2016-07-28

    The rate of thermally induced electron transfer in organic mixed valence compounds has thoroughly been investigated by e.g. temperature dependent ESR spectroscopy. However, almost nothing is known about the dynamics of optically induced electron transfer processes in such systems. Therefore, we investigated these processes in mixed valence compounds based on triphenylamine redox centres bridged by conjugated spacers by NIR transient absorption spectroscopy with fs-time resolution. These experiments revealed an internal conversion (IC) process to be on the order of 50-200 fs which is equivalent to the back electron transfer after optical excitation into the intervalence charge transfer band. This IC is followed by ultrafast cooling to the ground state within 1 ps. Thus, in the systems investigated optically induced electron transfer is about 3-4 orders of magnitude faster than thermally induced ET. PMID:27376572

  13. Objective evaluation of the relative modulation transfer function of densitometers for digitisation of electron micrographs.

    PubMed

    Roseman, A M; Neumann, K

    2003-08-01

    Digitisation of images recorded on film is a crucial part of data acquisition in electron microscopy, particularly for electron cryo-microscopy of biological specimens where the contrast and signal-to-noise ratio are low. A quantitative method to evaluate and compare the quality of densitometers, as measured by the modulation transfer function (MTF), is described here. The densitometer is modelled as a linear system, the output being the convolution of the input image and a point spread function. The MTF is the magnitude of the Fourier transform of the point spread function. The relative MTF describes the quality of signal transfer with spatial frequency. It is important that fine structural details in the micrograph are digitised with a high value for the MTF which does not vary with direction. A test pattern has been generated by projecting an electron image of a grid pattern onto film. The film is scanned and a computer program measures the intensities of the diffraction orders of the repeating pattern. Three different scanners are compared, one is a point scanner and the other two are line scanners. The test can be used to check if a scanner is set up optimally, and how it compares with another scanner. PMID:12672572

  14. Direct simulation of proton-coupled electron transfer reaction dynamics and mechanisms

    NASA Astrophysics Data System (ADS)

    Kretchmer, Joshua S.; Miller, Thomas F., III

    2014-03-01

    Proton-coupled electron transfer (PCET) reactions, in which both an electron and an associated proton undergo reactive transfer, play an important role in many chemical and biological systems. Due to the complexity of this class of reactions, a variety of different mechanisms fall under the umbrella of PCET. However, the physical driving forces that determine the preferred mechanism in a given system still remain poorly understood. Towards this end, we extend ring polymer molecular dynamics (RPMD), a path-integral quantum dynamics method, to enable the direct simulation and characterization of PCET reaction dynamics in both fully atomistic and system-bath models of organometallic catalysts. In addition to providing validation for the simulation method via extensive comparison with existing PCET rate theories, we analyze the RPMD trajectories to investigate the competition between the concerted and sequential reaction mechanisms for PCET, elucidating the large role of the solvent in controlling the preferred mechanism. We further employ RPMD to determine the kinetics and mechanistic features of concerted PCET reactions across different regimes of electronic and vibrational coupling, providing evidence for a new and distinct PCET reaction mechanism.

  15. Polar firn layering in radiative transfer models

    NASA Astrophysics Data System (ADS)

    Linow, Stefanie; Hoerhold, Maria

    2016-04-01

    For many applications in the geosciences, remote sensing is the only feasible method of obtaining data from large areas with limited accessibility. This is especially true for the cryosphere, where light conditions and cloud coverage additionally limit the use of optical sensors. Here, instruments operating at microwave frequencies become important, for instance in polar snow parameters / SWE (snow water equivalent) mapping. However, the interaction between snow and microwave radiation is a complex process and still not fully understood. RT (radiative transfer) models to simulate snow-microwave interaction are available, but they require a number of input parameters such as microstructure and density, which are partly ill-constrained. The layering of snow and firn introduces an additional degree of complexity, as all snow parameters show a strong variability with depth. Many studies on RT modeling of polar firn deal with layer variability by using statistical properties derived from previous measurements, such as the standard deviations of density and microstructure, to configure model input. Here, the variability of microstructure parameters, such as density and particle size, are usually assumed to be independent of each other. However, in the case of the firn pack of the polar ice sheets, we observe that microstructure evolution depends on environmental parameters, such as temperature and snow deposition. Accordingly, density and microstructure evolve together within the snow and firn. Based on CT (computer tomography) microstructure measurements of antarctic firn, we can show that: first, the variability of density and effective grain size are linked and can thus be implemented in the RT models as a coupled set of parameters. Second, the magnitude of layering is captured by the measured standard deviation. Based on high-resolution density measurements of an Antarctic firn core, we study the effect of firn layering at different microwave wavelengths. By means of

  16. Coherent transfer of light polarization to electron spins in a semiconductor.

    PubMed

    Kosaka, Hideo; Shigyou, Hideki; Mitsumori, Yasuyoshi; Rikitake, Yoshiaki; Imamura, Hiroshi; Kutsuwa, Takeshi; Arai, Koichiro; Edamatsu, Keiichi

    2008-03-01

    We demonstrate that the superposition of light polarization states is coherently transferred to electron spins in a semiconductor quantum well. By using time-resolved Kerr rotation, we observe the initial phase of Larmor precession of electron spins whose coherence is transferred from light. To break the electron-hole spin entanglement, we utilized the big discrepancy between the transverse g factors of electrons and light-holes. The result encourages us to make a quantum media converter between flying photon qubits and stationary electron-spin qubits in semiconductors. PMID:18352739

  17. Sequential energy and electron transfer in a three-component system aligned on a clay nanosheet.

    PubMed

    Fujimura, Takuya; Ramasamy, Elamparuthi; Ishida, Yohei; Shimada, Tetsuya; Takagi, Shinsuke; Ramamurthy, Vaidhyanathan

    2016-02-21

    To achieve the goal of energy transfer and subsequent electron transfer across three molecules, a phenomenon often utilized in artificial light harvesting systems, we have assembled a light absorber (that also serves as an energy donor), an energy acceptor (that also serves as an electron donor) and an electron acceptor on the surface of an anionic clay nanosheet. Since neutral organic molecules have no tendency to adsorb onto the anionic surface of clay, a positively charged water-soluble organic capsule was used to hold neutral light absorbers on the above surface. A three-component assembly was prepared by the co-adsorption of a cationic bipyridinium derivative, cationic zinc porphyrin and cationic octaamine encapsulated 2-acetylanthracene on an exfoliated anionic clay surface in water. Energy and electron transfer phenomena were monitored by steady state fluorescence and picosecond time resolved fluorescence decay. The excitation of 2-acetylanthracene in the three-component system resulted in energy transfer from 2-acetylanthracene to zinc porphyrin with 71% efficiency. Very little loss due to electron transfer from 2-acetylanthracene in the cavitand to the bipyridinium derivative was noticed. Energy transfer was followed by electron transfer from the zinc porphyrin to the cationic bipyridinium derivative with 81% efficiency. Analyses of fluorescence decay profiles confirmed the occurrence of energy transfer and subsequent electron transfer. Merging the concepts of supramolecular chemistry and surface chemistry we realized sequential energy and electron transfer between three hydrophobic molecules in water. Exfoliated transparent saponite clay served as a matrix to align the three photoactive molecules at a close distance in aqueous solutions. PMID:26820105

  18. Mixed Quantum-Classical Liouville Approach for Calculating Proton-Coupled Electron-Transfer Rate Constants.

    PubMed

    Shakib, Farnaz; Hanna, Gabriel

    2016-07-12

    In this work, we derive a general mixed quantum-classical formula for calculating thermal proton-coupled electron-transfer (PCET) rate constants, starting from the time integral of the quantum flux-flux correlation function. This formula allows for the direct simulation of PCET reaction dynamics via the mixed quantum-classical Liouville approach. Owing to the general nature of the derivation, this formula does not rely on any prior mechanistic assumptions and can be applied across a wide range of electronic and protonic coupling regimes. To test the validity of this formula, we applied it to a reduced model of a condensed-phase PCET reaction. Good agreement with the numerically exact rate constant is obtained, demonstrating the accuracy of our formalism. We believe that this approach constitutes a solid foundation for future investigations of the rates and mechanisms of a wide range of PCET reactions. PMID:27232936

  19. Ultrafast electron transfer at organic semiconductor interfaces: Importance of molecular orientation

    DOE PAGESBeta

    Ayzner, Alexander L.; Nordlund, Dennis; Kim, Do -Hwan; Bao, Zhenan; Toney, Michael F.

    2014-12-04

    Much is known about the rate of photoexcited charge generation in at organic donor/acceptor (D/A) heterojunctions overaged over all relative arrangements. However, there has been very little experimental work investigating how the photoexcited electron transfer (ET) rate depends on the precise relative molecular orientation between D and A in thin solid films. This is the question that we address in this work. We find that the ET rate depends strongly on the relative molecular arrangement: The interface where the model donor compound copper phthalocyanine is oriented face-on with respect to the fullerene C60 acceptor yields a rate that is approximatelymore » 4 times faster than that of the edge-on oriented interface. Our results suggest that the D/A electronic coupling is significantly enhanced in the face-on case, which agrees well with theoretical predictions, underscoring the importance of controlling the relative interfacial molecular orientation.« less

  20. Ultrafast electron transfer at organic semiconductor interfaces: Importance of molecular orientation

    SciTech Connect

    Ayzner, Alexander L.; Nordlund, Dennis; Kim, Do -Hwan; Bao, Zhenan; Toney, Michael F.

    2014-12-04

    Much is known about the rate of photoexcited charge generation in at organic donor/acceptor (D/A) heterojunctions overaged over all relative arrangements. However, there has been very little experimental work investigating how the photoexcited electron transfer (ET) rate depends on the precise relative molecular orientation between D and A in thin solid films. This is the question that we address in this work. We find that the ET rate depends strongly on the relative molecular arrangement: The interface where the model donor compound copper phthalocyanine is oriented face-on with respect to the fullerene C60 acceptor yields a rate that is approximately 4 times faster than that of the edge-on oriented interface. Our results suggest that the D/A electronic coupling is significantly enhanced in the face-on case, which agrees well with theoretical predictions, underscoring the importance of controlling the relative interfacial molecular orientation.