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1

Incorporation of Water-Oxidation Catalysts into Photoinduced Electron Transfer Systems: Toward Solar Fuel Generation via Artificial Photosynthesis  

NASA Astrophysics Data System (ADS)

A key goal of artificial photosynthesis is to mimic the photochemistry of photosystem II and oxidize water using light energy, with the ultimate aim of using the liberated electrons for reductive, fuel-forming reactions. One of the more recent challenges in the field of solar fuels chemistry is the efficient activation of molecular water-oxidation catalysts with photoinduced electron transfer, an effort that would benefit from detailed knowledge of the energetics and kinetics of each electron transfer step in a light-driven catalytic cycle. The focus of this thesis is the synthesis and photophysical characterization of covalent assemblies comprising a redox-active organic chromophore and the iridium(III)-based water-oxidation catalyst Cp*Ir(ppy)Cl (ppy = 2-phenylpyridine), and the rates and pathways for photogeneration of higher-valence states of the catalyst are determined with femtosecond transient absorption spectroscopy and other time-resolved spectroscopic techniques. In linking the photooxidant perylene-3,4:9,10-bis (dicarboximide) (PDI) to the Ir(III) catalyst, fast photoinduced electron transfer from the metal complex to PDI outcompetes heavy-atom quenching of the dye excited state, and the catalytic integrity of the complex is retained, as determined by electrocatalysis experiments. Long-lived higher-valence states of the catalyst are necessary for the accumulation of oxidizing equivalents for oxygen evolution, and the lifetime of photogenerated Ir(IV) has been extended by over two orders of magnitude by catalyst incorporation into a covalent electron acceptor--chromophore--catalyst triad, in which the dye is perylene-3,4-dicarboximide (PMI). Time resolved X-ray absorption studies of the triad confirm the photogeneration of an Ir(IV) metal center, a species that is too unstable to observe with chemical or electrochemical oxidation methods. This approach to preparing higher-valence states of water-oxidation catalysts has great promise for deducing catalytic mechanisms and probing highly-reactive intermediates, and it also establishes a basis in systems design for photodriving catalytic processes. Covalent dye-catalyst assemblies have been gaining recognition as a useful motif for incorporation into dye-sensitized photoanodes for photoelectrochemical water-splitting cells, and the PMI-Ir catalyst unit is well-poised, both in the energetics and kinetics of its electron transfer properties, to improve upon current solar-driven fuel-forming devices.

Vagnini, Michael Thomas

2

Ultrafast photodriven intramolecular electron transfer from an iridium-based water-oxidation catalyst to perylene diimide derivatives  

PubMed Central

Photodriving the activity of water-oxidation catalysts is a critical step toward generating fuel from sunlight. The design of a system with optimal energetics and kinetics requires a mechanistic understanding of the single-electron transfer events in catalyst activation. To this end, we report here the synthesis and photophysical characterization of two covalently bound chromophore-catalyst electron transfer dyads, in which the dyes are derivatives of the strong photooxidant perylene-3,4:9,10-bis(dicarboximide) (PDI) and the molecular catalyst is the Cp?Ir(ppy)Cl metal complex, where ppy = 2-phenylpyridine. Photoexcitation of the PDI in each dyad results in reduction of the chromophore to PDI•- in less than 10 ps, a process that outcompetes any generation of 3?PDI by spin-orbit-induced intersystem crossing. Biexponential charge recombination largely to the PDI-Ir(III) ground state is suggestive of multiple populations of the PDI•--Ir(IV) ion-pair, whose relative abundance varies with solvent polarity. Electrochemical studies of the dyads show strong irreversible oxidation current similar to that seen for model catalysts, indicating that the catalytic integrity of the metal complex is maintained upon attachment to the high molecular weight photosensitizer. PMID:22586073

Vagnini, Michael T.; Smeigh, Amanda L.; Blakemore, James D.; Eaton, Samuel W.; Schley, Nathan D.; D’Souza, Francis; Crabtree, Robert H.; Brudvig, Gary W.; Co, Dick T.; Wasielewski, Michael R.

2012-01-01

3

Hydrogen Photogeneration Promoted by Efficient Electron Transfer from Iridium Sensitizers to Colloidal MoS2 Catalysts  

PubMed Central

We report the utilization of colloidal MoS2 nanoparticles (NPs) for multicomponent photocatalytic water reduction systems in cooperation with a series of cyclometalated Ir(III) sensitizers. The effects of the particle size and particle dispersion of MoS2 NPs catalyst, reaction solvent and the concentration of the components on hydrogen evolution efficiency were investigated. The MoS2 NPs exhibited higher catalytic performance than did other commonly used water reduction catalysts under identical experiment conditions. The introduction of the carboxylate anchoring groups in the iridium complexes allows the species to be favorably chem-adsorbed onto the MoS2 NPs surface to increase the electron transfer, resulting in enhancement of hydrogen evolution relative to the non-attached systems. The highest apparent quantum yield, which was as high as 12.4%, for hydrogen evolution, was obtained (? = 400?nm). PMID:24509729

Yuan, Yong-Jun; Yu, Zhen-Tao; Liu, Xiao-Jie; Cai, Jian-Guang; Guan, Zhong-Jie; Zou, Zhi-Gang

2014-01-01

4

Electron-transfer sensitization of H2 oxidation and CO2 reduction catalysts using a single chromophore  

PubMed Central

Energy-storing artificial-photosynthetic systems for CO2 reduction must derive the reducing equivalents from a renewable source rather than from sacrificial donors. To this end, a homogeneous, integrated chromophore/two-catalyst system is described that is thermodynamically capable of photochemically driving the energy-storing reverse water–gas shift reaction (CO2 + H2 ? CO + H2O), where the reducing equivalents are provided by renewable H2. The system consists of the chromophore zinc tetraphenylporphyrin (ZnTPP), H2 oxidation catalysts of the form [CpRCr(CO)3]–, and CO2 reduction catalysts of the type Re(bpy-4,4?-R2)(CO)3Cl. Using time-resolved spectroscopic methods, a comprehensive mechanistic and kinetic picture of the photoinitiated reactions of mixtures of these compounds has been developed. It has been found that absorption of a single photon by broadly absorbing ZnTPP sensitizes intercatalyst electron transfer to produce the substrate-active forms of each. The initial photochemical step is the heretofore unobserved reductive quenching of the low-energy T1 state of ZnTPP. Under the experimental conditions, the catalytically competent state decays with a second-order half-life of ?15 ?s, which is of the right magnitude for substrate trapping of sensitized catalyst intermediates. PMID:24961370

La Porte, Nathan T.; Moravec, Davis B.; Hopkins, Michael D.

2014-01-01

5

Photoinduced electron transfer pathways in hydrogen-evolving reduced graphene oxide-boosted hybrid nano-bio catalyst.  

PubMed

Photocatalytic production of clean hydrogen fuels using water and sunlight has attracted remarkable attention due to the increasing global energy demand. Natural and synthetic dyes can be utilized to sensitize semiconductors for solar energy transformation using visible light. In this study, reduced graphene oxide (rGO) and a membrane protein bacteriorhodopsin (bR) were employed as building modules to harness visible light by a Pt/TiO2 nanocatalyst. Introduction of the rGO boosts the nano-bio catalyst performance that results in hydrogen production rates of approximately 11.24 mmol of H2 (?mol protein)(-1) h(-1). Photoelectrochemical measurements show a 9-fold increase in photocurrent density when TiO2 electrodes were modified with rGO and bR. Electron paramagnetic resonance and transient absorption spectroscopy demonstrate an interfacial charge transfer from the photoexcited rGO to the semiconductor under visible light. PMID:25050831

Wang, Peng; Dimitrijevic, Nada M; Chang, Angela Y; Schaller, Richard D; Liu, Yuzi; Rajh, Tijana; Rozhkova, Elena A

2014-08-26

6

Anisotropic gold nanoparticle doped mesoporous boehmite films and their use as reusable catalysts in electron transfer reactions.  

PubMed

Anisotropic Au nanoparticle (NP) doped mesoporous and oriented boehmite films of about 2 microm in thickness were prepared and used as reusable catalysts. The films were characterized by grazing incidence X-ray diffraction (GIXRD), field emission scanning electron and transmission electron microscopies, optical absorptions and surface area and pore size measurements. GIXRD of the doped films showed a preferential growth of boehmite crystallites in the (020) plane. The electron microscopy studies revealed existence of dispersed anisotropic Au NPs of approximately 15-40 nm size range and irregular Au aggregates of approximately 200-300 nm inside the mesoporous boehmite films. The optical absorption of the films showed Au-plasmon bands at 605 nm and broad absorption covering the near-infrared (NIR) region due to the anisotropic Au nanostructures. These films showed excellent catalytic activities in both the organic (p-nitrophenol to p-aminophenol by sodium borohydride) and inorganic (ferricyanide to ferrocyanide by thiosulphate) electron transfer (redox) reactions in aqueous solutions with high rate constant values. The films can be easily separated after the reaction and reused several times without any significant degradation of their original catalytic activity. PMID:20557082

Jana, Debrina; Dandapat, Anirban; De, Goutam

2010-07-20

7

A kinetic study of plutonium dioxide dissolution in hydrochloric acid using iron (II) as an electron transfer catalyst  

SciTech Connect

Effective dissolution of plutonium dioxide has traditionally been accomplished by contact with strong nitric acid containing a small amount of fluoride at temperatures of {approximately} 100 C. In spite of these aggressive conditions, PuO{sub 2} dissolution is sometimes incomplete requiring additional contact with the solvent. This work focused on an alternative to conventional dissolution in nitric acid where an electron transfer catalyst, Fe(II), was used in hydrochloric acid. Cyclic voltammetry was employed as an in-situ analytical technique for monitoring the dissolution reaction rate. The plutonium oxide selected for this study was decomposed plutonium oxalate with > 95% of the material having a particle diameter (< 70 {micro}m) as determined by a scanning laser microscopy technique. Attempts to dry sieve the oxide into narrow size fractions prior to dissolution in the HCl-Fe(II) solvent system failed, apparently due to significant interparticle attractive forces. Although sieve splits were obtained, subsequent scanning laser microscopy analysis of the sieve fractions indicated that particle segregation was not accomplished and the individual sieve fractions retained a particle size distribution very similar to the original powder assemblage. This phenomena was confirmed through subsequent dissolution experiments on the various screen fractions which illustrated no difference in kinetic behavior between the original oxide assemblage and the sieve fractions.

Fife, K.W.

1996-09-01

8

The electron is a catalyst  

NASA Astrophysics Data System (ADS)

The electron is an efficient catalyst for conducting various types of radical cascade reaction that proceed by way of radical and radical ion intermediates. But because electrons are omnipresent, catalysis by electrons often passes unnoticed. In this Review, a simple analogy between acid/base catalysis and redox catalysis is presented. Conceptually, the electron is a catalyst in much the same way that a proton is a catalyst. The 'electron is a catalyst' paradigm unifies mechanistically an assortment of synthetic transformations that otherwise have little or no apparent relationship. Diverse radical cascades, including unimolecular radical substitution reactions (SRN1-type chemistry), base-promoted homolytic aromatic substitutions (BHAS), radical Heck-type reactions, radical cross-dehydrogenative couplings (CDC), direct arene trifluoromethylations and radical alkoxycarbonylations, can all be viewed as electron-catalysed reactions.

Studer, Armido; Curran, Dennis P.

2014-09-01

9

Long-range electron transfer over graphene-based catalyst for high-performing oxygen reduction reactions: importance of size, N-doping, and metallic impurities.  

PubMed

N-doped carbon materials are considered as next-generation oxygen reduction reaction (ORR) catalysts for fuel cells due to their prolonged stability and low cost. However, the underlying mechanism of these catalysts has been only insufficiently identified, preventing the rational design of high-performing catalysts. Here, we show that the first electron is transferred into O2 molecules at the outer Helmholtz plane (ET-OHP) over a long range. This is in sharp contrast to the conventional belief that O2 adsorption must precede the ET step and thus that the active site must possess as good an O2 binding character as that which occurs on metallic catalysts. Based on the ET-OHP mechanism, the location of the electrode potential dominantly characterizes the ORR activity. Accordingly, we demonstrate that the electrode potential can be elevated by reducing the graphene size and/or including metal impurities, thereby enhancing the ORR activity, which can be transferred into single-cell operations with superior stability. PMID:24905892

Choi, Chang Hyuck; Lim, Hyung-Kyu; Chung, Min Wook; Park, Jong Cheol; Shin, Hyeyoung; Kim, Hyungjun; Woo, Seong Ihl

2014-06-25

10

The Role of a Dipeptide Outer-Coordination Sphere on H2 -Production Catalysts: Influence on Catalytic Rates and Electron Transfer  

SciTech Connect

The outer-coordination sphere of enzymes acts to fine-tune the active site reactivity and control catalytic rates, suggesting that incorporation of analogous structural elements into molecular catalysts may be necessary to achieve rates comparable to those observed in enzyme systems at low overpotentials. In this work, we evaluate the effect of an amino acid and dipeptide outer-coordination sphere on [Ni(PPh2NPh-R2)2]2+ hydrogen production catalysts. A series of 12 new complexes containing non-natural amino acids or dipeptides were prepared to test the effects of positioning, size, polarity and aromaticity on catalytic activity. The non-natural amino acid was either 3-(meta- or para-aminophenyl)propionic acid terminated as an acid, an ester or an amide. Dipeptides consisted of one of the non-natural amino acids coupled to one of four amino acid esters: alanine, serine, phenylalanine or tyrosine. All of the catalysts are active for hydrogen production, with rates averaging ~1000 s-1, 40% faster than the unmodified catalyst. Structure and polarity of the aliphatic or aromatic side chains of the C-terminal peptide do not strongly influence rates. However, the presence of an amide bond increases rates, suggesting a role for the amide in assisting catalysis. Overpotentials were lower with substituents at the N-phenyl meta position. This is consistent with slower electron transfer in the less compact, para-substituted complexes, as shown in digital simulations of catalyst cyclic voltammograms and computational modeling of the complexes. Combining the current results with insights from previous results, we propose a mechanism for the role of the amino acid and dipeptide based outer-coordination sphere in molecular hydrogen production catalysts.

Reback, Matthew L.; Ginovska-Pangovska, Bojana; Ho, Ming-Hsun; Jain, Avijita; Squier, Thomas C.; Raugei, Simone; Roberts, John A.; Shaw, Wendy J.

2013-02-04

11

Photo-induced electron transfer method  

DOEpatents

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.

Wohlgemuth, R.; Calvin, M.

1984-01-24

12

Photo-induced electron transfer method  

DOEpatents

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.

Wohlgemuth, Roland (2823 Hillegass Ave., Berkeley, CA 94705); Calvin, Melvin (2683 Buena Vista Way, Berkeley, CA 94708)

1984-01-01

13

Nonadiabatic anharmonic electron transfer  

SciTech Connect

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.

Schmidt, P. P. [Molecular Physics Research, 6547 Kristina Ursula Court, Falls Church, Virginia 22044 (United States)

2013-03-28

14

Electron transfer in biology  

NASA Astrophysics Data System (ADS)

Electron transfer is one of the key reactions of biology not just in catalysis of oxidation/reduction reactions but in the conversion of sources of energy such as light to usable form for chemical transformations. There are then two intriguing problems. What is the nature of the matrix in which electrons flow in a biological cell after the initial charge separation due for example to the absorption of light. Here we are examining biological structures similar to man's electronic wires and the construction must be of low resistance in what are apparently insulators - organic polymers. It has been found that the electronic conduction system is largely made from metallo-proteins associated with lipid membranes. We understand much about these biological wires today. The second problem concerns the conversion of the energy captured from the light into usable chemical form. The major synthetic step in the production of biological polymers, including proteins, DNA, RNA, polysaccharides and fats, is condensation, i.e. the removal of water in the formation of amides, esters and so on. Now these condensation reactions are driven in biology by using a drying agent in water, namely the anhydride, pyrophosphate, in a special compound ATP, adenosine triphosphate. The central problem is to discover exactly how the flow of electrons can be related to the synthesis of (bound) pyrophosphate. (In a thermodynamic sense pyrophosphate is a water soluble kinetically stable drying agent comparable with solid P2O5.) In the biological systems the connection between these different classes of reaction, electron transfer and condensation, is known to be via the production of an energized gradient of protons across the biological membrane which arises from the flow of electrons across the same membrane in the electron transport wires of biology. However we do not understand thoroughly the steps which lead from electron flow in a membrane to proton gradients in that membrane, i.e. electron/proton coupling. Again we do not understand thoroughly how subsequently the proton gradient across a membrane makes ATP, pyrophosphate. Today there is good experimental evidence as to the likely answers in principle. These analyse the coupling devices in mechanical terms. In this article I describe at first the 'wires' of biology, uncoupled simple electron flow, and then go on to the ways in which electron flow could be transduced by mechanical devices, also proteins, into proton gradients and then ATP. This will be termed coupled electron flow. The objective of the article is to stimulate participation by physical chemists in the further description of biological energy capture from light or the oxidation of hydrocarbons to a form suitable for driving chemical syntheses in a controlled manner.

Williams, R. J. P.

15

TRANSFER CREDIT GUIDE: ELECTRONIC JOURNALISM  

E-print Network

TRANSFER CREDIT GUIDE: ELECTRONIC JOURNALISM If you are a South Carolina technical college student who plans to transfer to the University's and pursue the electronic journalism major in the School the Office of Undergraduate Admissions. Consult the University Bulletin for specific entrance requirements

Almor, Amit

16

Proton-Coupled Electron Transfer  

SciTech Connect

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} ~ ?. 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{sup -}/2H{sup +} 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 force for long-range proton transfer in Photosysem II and through other biological membranes. In EPT, simultaneous transfer of electrons and protons occurs on time scales short compared to the periods of coupled vibrations and solvent modes. A theory for EPT has been developed which rationalizes rate constants and activation barriers, includes temperature- and driving force (?G)-dependences implicitly, and explains kinetic isotope effects. The distance-dependence of EPT is dominated by the short-range nature of proton transfer, with electron transfer being far less demanding.Changes in external pH do not affect an EPT elementary step. Solvent molecules or buffer components can act as proton donor acceptors, but individual H2O molecules are neither good bases (pK{sub a}(H{sub 3}O{sup +}) = ?1.74) nor good acids (pK{sub a}(H{sub 2}O) = 15.7). There are many examples of mechanisms in chemistry, in biology, on surfaces, and in the gas phase which utilize EPT. PCET and EPT play critical roles in the oxygen evolving complex (OEC) of Photosystem II and other biological reactions by decreasing driving force and avoiding high-energy intermediates.

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

2012-01-01

17

Electron transfer in biological molecules  

SciTech Connect

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.

Gray, H.B. [Beckman Institute, Pasadena, CA (United States)

1995-12-01

18

Electron Transfer in Metalloproteins  

E-print Network

Figure 1 Photosynthesis pathways as found in green plants. Electrons are extracted from water-capture machinery of biology, espe- cially in photosynthesis and in mitochondria, there are chains of redox enzymes

Tuscia, Università Degli Studi Della

19

Characterization of Catalysts for Synthesis of Higher Alcohols using Electron Microscopy  

E-print Network

Characterization of Catalysts for Synthesis of Higher Alcohols using Electron reactions creates demand to find better catalysts for these applications [1]. Transmission including catalysts. However, conventional TEM does not provide dynamic information

Dunin-Borkowski, Rafal E.

20

Electron Transfer in Nitrogenase Catalysis  

PubMed Central

Nitrogenase is a two-component enzyme that catalyzes the nucleotide-dependent reduction of N2 to 2 NH3. This process involves three redox-active metal-containing cofactors including a [4Fe-4S] cluster, an eight-iron P cluster and a seven-iron plus molybdenum FeMo-cofactor, the site of substrate reduction. A deficit-spending model for electron transfer has recently been proposed that incorporates protein conformational gating that favors uni-directional electron transfer among the metalloclusters for activation of the substrate binding site. Also reviewed is a proposal that each of the metal clusters cycles through only two redox states of the metal-sulfur core as the system accumulates the multiple electrons required for substrate binding and reduction. In particular, it was suggested that as FeMo-cofactor acquires the four electrons necessary for optimal binding of N2, each successive pair of electrons is stored as an Fe-H?-Fe bridging hydride, with the FeMo-cofactor metal-ion core retaining its resting redox state. We here broaden the discussion of stable intermediates that might form when FeMo-cofactor receives an odd number of electrons. PMID:22397885

Seefeldt, Lance C.; Hoffman, Brian M.; Dean, Dennis R.

2012-01-01

21

Polymerization catalysts containing electron-withdrawing amide ligands  

DOEpatents

The present invention describes methods of making a series of amine-containing organic compounds which are used as ligands for group 3-10 and lanthanide metal compounds. The ligands have electron-withdrawing groups bonded to them. The metal compounds, when combined with a cocatalyst, are catalysts for the polymerization of olefins.

Watkin, John G. (Los Alamos, NM); Click, Damon R. (Bloomington, IN)

2002-01-01

22

Engineering nanocarbon interfaces for electron transfer  

E-print Network

Electron-transfer reactions at nanometer-scale interfaces, such as those presented by single-walled carbon nanotubes (SWCNTs), are important for emerging optoelectronic and photovoltaic technologies. Electron transfer also ...

Hilmer, Andrew J. (Andrew Joseph)

2013-01-01

23

Concerted proton-coupled electron transfer from a metal-hydride complex  

NASA Astrophysics Data System (ADS)

Metal hydrides are key intermediates in the catalytic reduction of protons and CO2 as well as in the oxidation of H2. In these reactions, electrons and protons are transferred to or from separate acceptors or donors in bidirectional proton-coupled electron transfer (PCET) steps. The mechanistic interpretation of PCET reactions of metal hydrides has focused on the stepwise transfer of electrons and protons. A concerted transfer may, however, occur with a lower reaction barrier and therefore proceed at higher catalytic rates. Here we investigate the feasibility of such a reaction by studying the oxidation–deprotonation reactions of a tungsten hydride complex. The rate dependence on the driving force for both electron transfer and proton transfer—employing different combinations of oxidants and bases—was used to establish experimentally the concerted, bidirectional PCET of a metal-hydride species. Consideration of the findings presented here in future catalyst designs may lead to more-efficient catalysts.

Bourrez, Marc; Steinmetz, Romain; Ott, Sascha; Gloaguen, Frederic; Hammarström, Leif

2015-02-01

24

Oligomer and mixed-metal compounds potential multielectron transfer catalysts  

SciTech Connect

Projects related to the design and characterization of multimetallic complexes has proceeded forward with a number of achievements. First, photoprocesses in hydrogel matrices lead to the conclusion that cationic metallochromophores could be ion exchanged into a hydrogel matrix ({kappa}-carageenan) and substantial photocurrents could be generated. Second, X-ray structures of Ru(bpy){sub 3}{sup 2+}, Ru(bpm){sub 3}{sup 2+} and Ru(bpz){sub 3}{sup 2+}, where bpy is 2,2{prime}-bipyridine, bpm is 2,2{prime}-bipyrimidine and bpz is 2,2{prime}-bipyrizine, were obtained and revealed similar Ru-N bond distances in each complex even though their {sigma}-donor and {pi}-acceptor character differ markedly. The structure parameters are expected to provide theoreticians with the information needed to probe the electronic character of the molecular systems and provide us with direction in our synthetic strategies. Third, a copper(I) complex was synthesized with a dimeric-ethane-bridged, 1,10-phenanthroline ligand that resulted in isolation of a bimetallic species. The copper(I) complex did luminesce weakly, suggesting that the dimer possesses potential electron transfer capability. Fourth, the photophysical properties of (Re(CO){sub 4}(L-L)){sup +}, where L-L = heterocyclic diimine ligands, and Pt(bph)X{sub 2}, where bph = the dianion of biphenyl and X = CH{sub 3}CN, py or ethylendiamine, displayed luminescence at high energy and underwent excited-state electron transfer. Such high energy emitters provide high driving forces for undergoing excited-state electron transfer. Fifth, both energy and electron transfer were observed in mixed-metal complexes bridged by 1,2-bis(2,2{prime}-bipyridyl-4{prime}-yl) ethane.

Rillema, D.P.

1992-03-30

25

Optimum Transfer Guide: Electronic Media & Communications Why Study Electronic Media?  

E-print Network

Optimum Transfer Guide: Electronic Media & Communications Why Study Electronic Media? The Electronic Media program is de- signed to train story-tellers proficient in convergent media forms who can professional courses in electronic media, visual communications, digital media production, photography

Gelfond, Michael

26

Photoinduced electron transfer from phenanthrimidazole to magnetic nanoparticles.  

PubMed

The dynamics of photoinduced electron injection from (E)-1-(4-methoxyphenyl)-2-styryl-1H-phenanthro [9,10-d]imidazole (MPSPI) synthesised using nano TiO2 as catalyst to Fe2O3 nanocrystal has been studied by FT-IR, absorption, fluorescence and lifetime spectroscopic methods. The binding between nanoparticle and MPSPI is confirmed by binding constant and binding site. The distance between MPSPI and nanoparticle as well as the critical energy transfer distance has been obtained. The free energy change (?Get) for electron injection has also been deduced. PMID:25591997

Jayabharathi, J; Arunpandiyan, A; Thanikachalam, V; Ramanathan, P

2015-01-01

27

Characterization of Catalysts for the Synthesis of Higher Alcohols using Transmission Electron Microscopy  

E-print Network

Characterization of Catalysts for the Synthesis of Higher Alcohols using Transmission Electron in such reactions is poor, resulting in a demand for better catalysts [1]. Transmission electron microscopy is a powerful tool for characterizing catalysts. However, conventional TEM does not provide dynamic information

Dunin-Borkowski, Rafal E.

28

Synthesis of Externally Initiated Poly(3-alkylthiophene)s via Kumada Catalyst Transfer Polymerization  

NASA Astrophysics Data System (ADS)

The ability of chemists to design and synthesize pi conjugated organic polymers with precise control over their physical and electronic properties remains the key to technological breakthroughs using polymeric material in electronic and photonic devices. Kumada catalyst transfer polymerization (KCTP) technique and Grignard metathesis (GRIM) method have enabled the synthesis of highly regioregular polymers with controlled molecular weights, narrow polydispersity index and uniform end groups. Applying this technique toward external initiation of polymers would enable the preparation of sophisticated and beneficial polymer architectures such as surface grafted polymers. This work presents an investigation of various mechanistic parameters for external initiation of poly(3-alkylthiophene). The effects of binding ligand variation on the Nickel catalyst were investigated utilizing a novel methodology allowing facile screening of ligands. Poly(3-hexylthiophene) was synthesized with high percentage initiator headgroup incorporation with triphenylphosphine ligand while the use of bidentate ligands such as diphenylphosphinopropane only resulted in quantitative initiation when ligand exchange followed initiation with the more active species. A variety of functionally substituted aryl and thiophene halides were explored for their potential to act as external small molecule initiators and the reaction intermediates were characterized via spectroscopic techniques as well as theoretical calculations. Aryl halides were found to be more stable than thiophene halides and the type and position of the initiator functionality has played a deciding role in the polymerization mechanism. Ortho substitution stabilized the aryl-Ni intermediate complex via favorable orbital overlap and kinetic effects as a result of steric hindrance were demonstrated to affect the success of the external initiation. Surface-grafted poly(3-methylthiophene) thin films were synthesized from indium tin oxide where polymer thin film thicknesses were regulated by the variation of monomer solution and polymerization time. Photoelectron spectroscopy analysis had demonstrated that electrochemical oxidation of surface grafted thin films affords the ability to tune the work function and ionization potential. Such materials with controllable thicknesses and electronic properties have the potential to be useful as components in organic photovoltaic devices.

D'Avanzo, Antonella

29

Molybdenum Imido Alkylidene Metathesis Catalysts That Contain Electron-Withdrawing Biphenolates or Binaphtholates  

E-print Network

Molybdenum Imido Alkylidene Metathesis Catalysts That Contain Electron-Withdrawing Biphenolates(pentafluorophenyl)binaphthol] with a bis(2,5-dimethylpyrrolide) complex. The pyrrolide approach can be employed either to isolate catalysts on a preparative scale or to generate catalysts in situ. Several simple preliminary ring-closing metathesis

Müller, Peter

30

Dynamic studies of catalysts for biofuel synthesis in an Environmental Transmission Electron Microscope  

E-print Network

Dynamic studies of catalysts for biofuel synthesis in an Environmental Transmission Electron@cen.dtu.dk Keywords: Biofuel, catalysis, environmental TEM The development of transportation fuels from sustainable

Dunin-Borkowski, Rafal E.

31

Effects of catalyst loading amount on the synthesis of poly(3-hexylthiophene) via externally initiated Kumada catalyst-transfer polycondensation  

NASA Astrophysics Data System (ADS)

A series of model polymerization are carried out via the one-pot externally initiated Kumada catalyst-transfer polycondensation (KCTP) of 2-bromo-5-chloromagnesium thiophene monomers, and the excess amount of initiators or catalysts are found no need to be isolated during the polycondensation process. Especially, the impacts of the nickel catalyst loading variation on regioregularity (rr), yield, molecular weight ( M n), polydispersity (PDI) and initiation efficiency of poly(3-hexylthiophene) (P3HT) are systematically investigated. The 1H NMR, size-exclusion chromatography (SEC), and MALDI-TOF mass spectroscopy results indicated that an excess amount of catalyst does not influence yield, rr, M n, and PDI of P3HT, nor the initiation efficiency. However, the PDI of the product is broad, and the M n and rr values decreased in the absence of 1,3-bis (diphenylphosphino)propane (dppp). It can be concluded that the in-situ KCTP polymerization of P3HT is a practical and effective process. These results are especially valuable for the synthesis of all-conjugated block copolymers where macroinitiators are used.

Wang, Jin; Higashihara, Tomoya

2014-12-01

32

Polyoxometalate-mediated electron transfer-oxygen transfer oxidation of cellulose and hemicellulose to synthesis gas.  

PubMed

Terrestrial plants contain ~70% hemicellulose and cellulose that are a significant renewable bioresource with potential as an alternative to petroleum feedstock for carbon-based fuels. The efficient and selective deconstruction of carbohydrates to their basic components, carbon monoxide and hydrogen, so called synthesis gas, is an important key step towards the realization of this potential, because the formation of liquid hydrocarbon fuels from synthesis gas are known technologies. Here we show that by using a polyoxometalate as an electron transfer-oxygen transfer catalyst, carbon monoxide is formed by cleavage of all the carbon-carbon bonds through dehydration of initially formed formic acid. In this oxidation-reduction reaction, the hydrogen atoms are stored on the polyoxometalate as protons and electrons, and can be electrochemically released from the polyoxometalate as hydrogen. Together, synthesis gas is formed. In a hydrogen economy scenario, this method can also be used to convert carbon monoxide to hydrogen. PMID:25082188

Sarma, Bidyut Bikash; Neumann, Ronny

2014-01-01

33

Photoinduced electron transfer across molecular bridges: electron- and hole-transfer superexchange pathways.  

PubMed

Photoinduced electron transfer plays key roles in many areas of chemistry. Superexchange is an effective model to rationalize photoinduced electron transfer, particularly when molecular bridges between donor and acceptor subunits are present. In this tutorial review we discuss, within a superexchange framework, the complex role played by the bridge, with an emphasis on differences between thermal and photoinduced electron transfer, oxidative and reductive photoinduced processes, charge separation and charge recombination. Modular bridges are also considered, with specific attention to the distance dependence of donor-acceptor electronic coupling and electron transfer rate constants. The possibility of transition, depending on the bridge energetics, from coherent donor-acceptor electron transfer to incoherent charge injection and hopping through the bridge is also discussed. Finally, conceptual analogies between bridge effects in photoinduced electron transfer and optical intervalence transfer are outlined. Selected experimental examples, instrumental to illustration of the principles, are discussed. PMID:24604096

Natali, Mirco; Campagna, Sebastiano; Scandola, Franco

2014-06-21

34

Enantioselective routes to both enantiomers of aryl alcohols with a single catalyst antipode: Ru and Os transfer hydrogenation catalysts.  

PubMed

[reaction--see text] The kinetic resolution of secondary aryl alcohols has been investigated. When (CyRuCl(2))(2), (1R,2S)-(+)-cis-1-amino-2-indanol, and KOH or (t)BuOK (catalyst 1) were combined in the presence of (+/-)-alcohols, ee's > 90% were generally observed. When applied to the kinetic resolution of (+/-)-indanol and (+/-)-tetralol, ee's = 99% (R) were observed. In addition, the asymmetric transfer hydrogenation of ketones was investigated with a catalyst, 2, generated in situ from (CyOsCl(2))(2), (1R,2S)-(+)-cis-1-amino-2-indanol, and (t)BuOK, yielding ee's of up to 98% (S). PMID:11700117

Faller, J W; Lavoie, A R

2001-11-15

35

Catalysts  

NSDL National Science Digital Library

There are two types of catalysis reactions: homogeneous and heterogeneous. In a homogeneous reaction, the catalyst is in the same phase as the reactants. In a heterogeneous reaction, the catalyst is in a different phase from the reactants. This activity addresses homogeneous catalysis.

The Concord Consortium

2011-12-11

36

Potential technology transfers of research on low-temperature carbon monoxide-oxygen recombination catalysts  

NASA Technical Reports Server (NTRS)

Results from research on catalytic recombination of CO-O2 for stable closed-cycle operation of CO2 lasers hold much promise for a variety of technology transfer. Expansion of CO2 laser remote sensing applications toward chemical detection and pollution monitoring would certainly be expected. However, the catalysts themselves may be especially effective in low-temperature oxidation of a number of chemicals in addition to CO. It is therefore of interest to compare the CO-O2 catalysts with chemical systems designed for chemical sensing, air purification and process catalysis. Success in understanding the catalytic mechanisms of the recombination of CO-O2 could help to shed light on how catalyst systems operate. New directions in low-temperature oxidation catalysts, coatings for chemical sensors and sorbents for air purification could well emerge.

Poziomek, Edward J.

1990-01-01

37

Electron transfer mechanisms Dr. Judy Hirst  

E-print Network

Center Bacteriochlorophylls (BCh, purple) and bacteriophaeophytin (BPh, red). Rate of recombination > 100, rate is retarded. BCh BCh* h, 3 ps BCh+BPh- ET, 1 ps #12;The Rate of Electron Transfer 1. Thermodynamic

Schnaufer, Achim

38

Photoinduced electron transfer in ordered polymers  

SciTech Connect

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.

Jones, G. II.

1990-10-20

39

Improved thermally conducting electron transfer polymers  

NASA Technical Reports Server (NTRS)

Development of polymers with improved heat transfer coefficients for use in encapsulating electronic modules is discussed. Chemical reactions for synthesizing the polymers are described and thermodynamic and physical properties are analyzed.

Jenkins, R. K.; Byrd, N. R.; Lister, J. L.

1972-01-01

40

Theory of electron transfer at electrified interfaces  

Microsoft Academic Search

Some recent achievements in condensed phase molecular charge transfer theory are overviewed, with focus on interfacial electrochemical electron transfer (ET). Elements of available and new formalism are addressed in Sections 2 and 3. New elements considered are, firstly, a new convenient parametric scheme for calculation of the rate constant and electrochemical current. The scheme goes beyond the commonly used quadratic

A. M. Kuznetsov; J. Ulstrup

2000-01-01

41

Electronic information transfer in a transport chain  

Microsoft Academic Search

Purpose – The purpose of this paper is to assess electronic information transfer in logistics organizations. The paper approaches information transfer as an elemental component of contemporary business to business (B2B) processes. Design\\/methodology\\/approach – The empirical material concerns companies that together comprise a logistical chain. The paper uses interview data to show information exchange patterns within one particular logistics chain.

Tommi Inkinen; Ulla Tapaninen; Hennariina Pulli

2009-01-01

42

Electron Transfer for Large Molecules through Delocalization  

SciTech Connect

Electron transfer for large molecules lies in between a Marcus-Theory two-state transfer and a Landauer description. We discuss a delocalization formalism which,through the introduction of artificial electric fields which emulate bulk dipole fields, allows calculation between a pair of identical molecules (A+A- (R)A-+A) with several open states. Dynamical electron polarization effects can be inserted with TDDFT and are crucial for large separations.

Neuhauser, D.; Reslan, R.; Hernandez, S.; Arnsen, C.; Lopata, K.; Govind, N.; Gao, Y.; Tolbert, S.; Schwartz, B.; Rubin, Y.; Nardes, A.; Kopidakis, N.

2012-01-01

43

A monolith immobilised iridium Cp* catalyst for hydrogen transfer reactions under flow conditions.  

PubMed

An immobilised iridium hydrogen transfer catalyst has been developed for use in flow based processing by incorporation of a ligand into a porous polymeric monolithic flow reactor. The monolithic construct has been used for several redox reductions demonstrating excellent recyclability, good turnover numbers and high chemical stability giving negligible metal leaching over extended periods of use. PMID:25500632

Rojo, Maria Victoria; Guetzoyan, Lucie; Baxendale, Ian R

2015-01-27

44

BASE-CATALYZED DESTRUCTION OF PCBS-NEW DONORS, NEW TRANSFER AGENTS/CATALYSTS  

EPA Science Inventory

The use of hydrogen transfer agents and catalysts to improve the base-catalyzed decomposition of polychlorinated biphenyls (PCBs) was investigated. The reaction proceeded only in the presence of base, but the rate of PCB disappearance increased with increasing amount of hydrogen ...

45

Magnetically Recoverable Supported Ruthenium Catalyst for Hydrogenation of Alkynes and Transfer Hydrogenation of Carbonyl Compounds  

EPA Science Inventory

A ruthenium (Ru) catalyst supported on magnetic nanoparticles (NiFe2O4) has been successfully synthesized and used for hydrogenation of alkynes at room temperature as well as transfer hydrogenation of a number of carbonyl compounds under microwave irradiation conditions. The cata...

46

Electronic communication and data transfer productivity enhancements  

Microsoft Academic Search

Transportation projects can be small, local efforts or large, multi-sited ventures. For these large projects, electronic mail and data transfer are not merely productivity aids; they are indispensable components of success. Electronic communication via microcomputers is quick and convenient and provides documented evidence of the communication. The rapid exchange of cost data, routing data, plans and scheduled, and information on

T. Truett; S. Stevens

1989-01-01

47

Respiratory electron transfer pathways in plant mitochondria  

PubMed Central

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

Schertl, Peter; Braun, Hans-Peter

2014-01-01

48

Thermally conducting electron transfer polymers  

NASA Technical Reports Server (NTRS)

New polymeric material exhibits excellent physical shock protection, high electrical resistance, and thermal conductivity. It is especially useful for electronic circuitry, such as subminiaturization of components and modular construction of circuits.

Byrd, N. R.; Jenkins, R. K.; Lister, J. L.

1969-01-01

49

Versatile iridicycle catalysts for highly efficient and chemoselective transfer hydrogenation of carbonyl compounds in water.  

PubMed

Cyclometalated iridium complexes are shown to be highly efficient and chemoselective catalysts for the transfer hydrogenation of a wide range of carbonyl groups with formic acid in water. Examples include ?-substituted ketones (?-ether, ?-halo, ?-hydroxy, ?-amino, ?-nitrile or ?-ester), ?-keto esters, ?-keto esters and ?,?-unsaturated aldehydes. The reduction was carried out at substrate/catalyst ratios of up to 50?000 at pH?4.5 and required no organic solvent. The protocol provides a practical, easy and efficient way for the synthesis of ?-functionalised secondary alcohols, such as ?-hydroxyethers, ?-hydroxyamines and ?-hydroxyhalo compounds, which are valuable intermediates in pharmaceutical, fine chemical, perfume and agrochemical synthesis. PMID:25124283

Talwar, Dinesh; Wu, Xiaofeng; Saidi, Ourida; Salguero, Noemí Poyatos; Xiao, Jianliang

2014-09-26

50

Single Molecule Spectroscopy of Electron Transfer  

SciTech Connect

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.

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

2009-10-20

51

Dynamic environmental transmission electron microscopy observation of platinum electrode catalyst deactivation in a proton-exchange-membrane fuel cell  

NASA Astrophysics Data System (ADS)

Spherical-aberration-corrected environmental transmission electron microscopy (AC-ETEM) was applied to study the catalytic activity of platinum/amorphous carbon electrode catalysts in proton-exchange-membrane fuel cells (PEMFCs). These electrode catalysts were characterized in different atmospheres, such as hydrogen and air, and a conventional high vacuum of 10-5 Pa. A high-speed charge coupled device camera was used to capture real-time movies to dynamically study the diffusion and reconstruction of nanoparticles with an information transfer down to 0.1 nm, a time resolution below 0.2 s and an acceleration voltage of 300 kV. With such high spatial and time resolution, AC-ETEM permits the visualization of surface-atom behaviour that dominates the coalescence and surface-reconstruction processes of the nanoparticles. To contribute to the development of robust PEMFC platinum/amorphous carbon electrode catalysts, the change in the specific surface area of platinum particles was evaluated in hydrogen and air atmospheres. The deactivation of such catalysts during cycle operation is a serious problem that must be resolved for the practical use of PEMFCs in real vehicles. In this paper, the mechanism for the deactivation of platinum/amorphous carbon electrode catalysts is discussed using the decay rate of the specific surface area of platinum particles, measured first in a vacuum and then in hydrogen and air atmospheres for comparison.

Yoshida, Kenta; Xudong, Zhang; Bright, Alexander N.; Saitoh, Koh; Tanaka, Nobuo

2013-02-01

52

Dynamic environmental transmission electron microscopy observation of platinum electrode catalyst deactivation in a proton-exchange-membrane fuel cell.  

PubMed

Spherical-aberration-corrected environmental transmission electron microscopy (AC-ETEM) was applied to study the catalytic activity of platinum/amorphous carbon electrode catalysts in proton-exchange-membrane fuel cells (PEMFCs). These electrode catalysts were characterized in different atmospheres, such as hydrogen and air, and a conventional high vacuum of 10(-5) Pa. A high-speed charge coupled device camera was used to capture real-time movies to dynamically study the diffusion and reconstruction of nanoparticles with an information transfer down to 0.1 nm, a time resolution below 0.2 s and an acceleration voltage of 300 kV. With such high spatial and time resolution, AC-ETEM permits the visualization of surface-atom behaviour that dominates the coalescence and surface-reconstruction processes of the nanoparticles. To contribute to the development of robust PEMFC platinum/amorphous carbon electrode catalysts, the change in the specific surface area of platinum particles was evaluated in hydrogen and air atmospheres. The deactivation of such catalysts during cycle operation is a serious problem that must be resolved for the practical use of PEMFCs in real vehicles. In this paper, the mechanism for the deactivation of platinum/amorphous carbon electrode catalysts is discussed using the decay rate of the specific surface area of platinum particles, measured first in a vacuum and then in hydrogen and air atmospheres for comparison. PMID:23340321

Yoshida, Kenta; Xudong, Zhang; Bright, Alexander N; Saitoh, Koh; Tanaka, Nobuo

2013-02-15

53

Electron transfer reactions in microporous solids  

SciTech Connect

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

Mallouk, T.E.

1993-01-01

54

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

NASA Astrophysics Data System (ADS)

Water-splitting dye-sensitized photoelectrochemical (WS-DSPECs) cells utilize molecular sensitizers absorbed on mesoporous TiO2 electrodes to harvest visible light, inject photoexcited electrons into the conduction band of TiO2, and finally transfer holes across the TiO2 surface to water oxidation catalysts, which in turn oxidize water to give molecular oxygen and four protons. Within the TiO2 layer photoinjected electrons are transported to a transparent conductor back contact and from there to a dark cathode to reduce protons to molecular hydrogen. WS-DSPECs offer several advantages for alternative solar fuels systems: the use of low-cost materials, tunable molecular sensitizers, and relaxed catalytic turnover requirements to name a few. Despite these advantageous features, power conversion efficiencies in WS-DSPECs are generally low. Broadly, this thesis explores the fundamental electron transfer processes that control the efficiency of these cells. Chapter 1 presents a survey of the previous literature and individually considers each component of a WS-DSPEC (water oxidation catalyst, sensitizers, electrode materials, redox mediators, and overall system design). Chapter 2 presents a novel method of preparing a WS-DSPEC that utilizes crystalline IrO2 nanoparticles directly sintered to TiO2 as a water oxidation catalyst and describes a previously unknown electron-scavenging pathway by IrO2. Chapter 3 explores how electron trapping by and proton intercalation into TiO2 controls the photoelectrochemical performance of WS-DSPECs. Chapter 4 characterizes how electron recombination with the oxidized sensitizer and electron scavenging by the IrO 2 catalyst combine to limit the concentration of conduction band electrons and by extension photocurrent in WS-DSPECs. Chapter 5 demonstrates the use of the first totally organic sensitizers for light driven water-splitting and explores how the molecular and electronic structure of a sensitizer affects the electron transfer 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.

Swierk, John R.

55

Photoinduced electron transfer in ordered polymers  

SciTech Connect

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.

Jones, G. II.

1991-12-01

56

Photoinduced gas-phase electron transfer reactions.  

PubMed

The electron transfer quenching process, when a reactive excited state is singlet or triplet, for gas-phase systems (benzophenone and anthraquinone with amines and pyridine as well as carbazole with halomethanes) was systematically investigated using time-resolved fluorescence. Bimolecular rate constants were obtained. Variable-temperature measurements were performed for eight donor-acceptor pairs. It was found that under solvent-free conditions various quenchers differing in photochemical reactivity led to change in quenching rates by almost three orders of magnitude. Positive and negative temperature dependences for the electron transfer rate constants were observed. The data were analyzed in terms of the Marcus-Jortner theory. PMID:15615042

Zalesskaya, G A; Sambor, E G; Bely, N N

2004-03-01

57

Electron transfer in weakly interacting systems  

SciTech Connect

A recently proposed semiclassical model, in which an electronic transmission coefficient and a nuclear tunneling factor are introduced as corrections to the classical activated-complex expression, is described. The nuclear tunneling corrections are shown to be important only at low temperatures or when the electron transfer is very exothermic. By contrast, corrections for nonadiabaticity may be significant for most outer-sphere reactions of metal complexes. The rate constants for the Fe(H/sub 2/O)/sub 6//sup 2 +/-Fe(H/sub 2/O)/sub 6//sup 3 +/, Ru(NH/sub 3/)/sub 6//sup 2 +/-Ru(NH/sub 3/)/sub 6//sup 3 +/ and Ru(bpy)/sub 3//sup 2 +/-Ru(bpy)/sub 3//sup 3 +/ electron exchange reactions predicted by the semiclassical model are in very good agreement with the observed values. The implications of the model for optically-induced electron transfer in mixed-valence systems are noted.

Sutin, N.; Brunschwig, B.S.

1981-01-01

58

Photoinitiated electron transfer in multichromophoric species: Synthetic tetrads and pentads  

SciTech Connect

This research project involves the design, synthesis and study of 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).

Not Available

1989-04-12

59

Solvent gating of intramolecular electron transfer  

SciTech Connect

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.

Miller, R.M. (California State Univ., Chico, CA (United States)); Spears, K.G.; Gong, J.H.; Wach, M. (Northwestern Univ., Evanston, IL (United States))

1994-02-03

60

Non-redox metal ion promoted oxygen transfer by a non-heme manganese catalyst.  

PubMed

This work demonstrates that non-redox metal ions as Lewis acids can sharply improve the oxygen transfer efficiency of a manganese(II) catalyst having a non-heme ligand. In the absence of Lewis acid, oxidation of a manganese(II) complex will generate the known di-?-oxo-bridged dinuclear Mn2(III,IV) core which is very sluggish for olefin epoxidation. Adding non-redox metal ions causes the dissociation of the dinuclear core, leading to sharp improvement in its oxygen transfer efficiency. PMID:25525748

Chen, Zhuqi; Yang, Ling; Choe, Cholho; Lv, Zhanao; Yin, Guochuan

2015-02-01

61

Frontispiece: a simple iridicycle catalyst for efficient transfer hydrogenation of N-heterocycles in water.  

PubMed

Transfer hydrogenation Transfer hydrogenation is a simple safe way for reduction reactions and using water as a solvent has the additional benefit of being "green". In their Full paper on page?5370?ff., J. Xiao et?al. report that a cyclometalated iridium complex catalyzes the transfer hydrogenation of various nitrogen heterocycles in an aqueous solution of formate under mild conditions. The catalyst shows excellent functional-group compatibility and high turnover number, with loadings as low as 0.01?mol?% being feasible. A mechanistic study of quinoline reduction suggests that the reaction proceeds via both 1,2- and 1,4-addition pathways, with the catalytic turnover limited by hydride transfer. PMID:25787753

Talwar, Dinesh; Li, Ho Yin; Durham, Emma; Xiao, Jianliang

2015-03-27

62

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

Federal Register 2010, 2011, 2012, 2013, 2014

...Electronic Funds Transfer of Depository Taxes; Correction AGENCY: Internal Revenue...providing guidance relating to Federal tax deposits (FTDs) by Electronic Funds Transfer...List of Subjects 26 CFR Part 40 Excise taxes, Reporting and recordkeeping...

2011-01-06

63

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

Federal Register 2010, 2011, 2012, 2013, 2014

...CFPB-2011-0009] RIN 3170-AA15 Electronic Fund Transfers (Regulation...to other consumers or businesses in a foreign country...which implements the Electronic Fund Transfer Act...to other consumers or businesses in a foreign...

2012-07-10

64

Kinetic ion thermometers for electron transfer dissociation.  

PubMed

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

Pepin, Robert; Ture?ek, František

2015-02-19

65

Frontier orbital symmetry control of intermolecular electron transfer  

SciTech Connect

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)

Stevens, B.

1990-11-01

66

Enantioselective synthesis of ( S)-?-methylphenylalanine using ( S)-BINOLAMs as new phase-transfer catalysts  

Microsoft Academic Search

A series of (S)-bis(aminomethyl)binaphthols [(S)-BINOLAMs] 4 have been prepared and used as catalysts in the enantioselective C-alkylation reaction of the aldimine Schiff bases of alanine esters 5 under solid–liquid phase-transfer catalysis (PTC) conditions employing NaOH as base in toluene at room temperature. (S)-3,3?-Bis[(diethylamino)methyl]-2,2?-dihydroxy-1,1?-binaphthalene 4a gave the best e.e.s. (S)-?-Methylphenylalanine 7 was isolated, after hydrolysis of the iminoester, in 85% yield

Jesús Casas; Carmen Nájera; José M. Sansano; José González; José M. Saá; Manuel Vega

2001-01-01

67

Transfer Hydrogenation Reaction Of Ketones And Formic Acid Under Hydrothermal Conditions Without A Catalyst  

NASA Astrophysics Data System (ADS)

The hydrothermal experiments with ketones and formic acid showed that the hydrogen transfer reduction of ketones can be conducted with formic acid as a hydride donor in the presence of NaOH at 300° C. The yield of alcohols was considerably higher at a much lower ratio of hydrogen source to ketones than the traditional Meerwein-Ponndorf-Verley reduction, reaching 60% for isopropanol from acetone and 70% for lactic acid from pyruvic acid. Water molecules as a catalyst may directly participate in the transition state by making a hydrogen-bond ring network with the substrate molecules.

Shen, Zheng; Zhang, Yalei; Zhou, Xuefei; Wu, Bing; Cao, Jianglin; Jin, Fangming

2010-11-01

68

Probing conformational dynamics by photoinduced electron transfer  

NASA Astrophysics Data System (ADS)

We demonstrate how photoinduced electron transfer (PET) reactions can be successfully applied to monitor conformational dynamics in individual biopolymers. Single-pair fluorescence resonance energy transfer (FRET) experiments are ideally suited to study conformational dynamics occurring on the nanometer scale, e.g. during protein folding or unfolding. In contrast, conformational dynamics with functional significance, for example occurring in enzymes at work, often appear on much smaller spatial scales of up to several Angströms. Our results demonstrate that selective PET-reactions between fluorophores and amino acids or DNA nucleotides represent a versatile tool to measure small-scale conformational dynamics in biopolymers on a wide range of time scales, extending from nanoseconds to seconds, at the single-molecule level under equilibrium conditions. That is, the monitoring of conformational dynamics of biopolymers with temporal resolutions comparable to those within reach using new techniques of molecular dynamic simulations. We present data about structural changes of single biomolecules like DNA hairpins and peptides by using quenching electron transfer reactions between guanosine or tryptophan residues in close proximity to fluorescent dyes. Furthermore, we demonstrate that the strong distance dependence of charge separation reactions on the sub-nanometer scale can be used to develop conformationally flexible PET-biosensors. These sensors enable the detection of specific target molecules in the sub-picomolar range and allow one to follow their molecular binding dynamics with temporal resolution.

Neuweiler, Hannes; Herten, Dirk P.; Marme, N.; Knemeyer, J. P.; Piestert, Oliver; Tinnefeld, Philip; Sauer, Marcus

2004-07-01

69

A Systematic Investigation of Quaternary Ammonium Ions as Asymmetric Phase Transfer Catalysts. Application of Quantitative Structure Activity/Selectivity Relationships  

PubMed Central

While the synthetic utility of asymmetric phase transfer catalysis continues to expand, the number of proven catalyst types and design criteria remains limited. At the origin of this scarcity is a lack in understanding of how catalyst structural features affect the rate and enantioselectivity of phase transfer catalyzed reactions. Described in this paper is the development of quantitative structure-activity relationships (QSAR) and -selectivity relationships (QSSR) for the alkylation of a protected glycine imine with libraries of quaternary ammonium ion catalysts. Catalyst descriptors including ammonium ion accessibility, interfacial adsorption affinity, and partition coefficient were found to correlate meaningfully with catalyst activity. The physical nature of the descriptors was rationalized through differing contributions of the interfacial and extraction mechanisms to the reaction under study. The variation in the observed enantioselectivity was rationalized employing a comparative molecular field analysis (CoMFA) using both the steric and electrostatic fields of the catalysts. A qualitative analysis of the developed model reveals preferred regions for catalyst binding to afford both configurations of the alkylated product. PMID:21446723

Denmark, Scott E.; Gould, Nathan D.; Wolf, Larry M.

2011-01-01

70

A Systematic Investigation of Quaternary Ammonium Ions as Asymmetric Phase Transfer Catalysts. Synthesis of Catalyst Libraries and Evaluation of Catalyst Activity  

PubMed Central

Despite over three decades of research into asymmetric phase transfer catalysis (APTC), a fundamental understanding of the factors that affect the rate and stereoselectivity of this important process are still obscure. This paper describes the initial stages of a long-term program aimed at elucidating the physical organic foundations of APTC employing a chemoinformatic analysis of the alkylation of a protected glycine imine with a libraries of enantiomerically enriched quaternary ammonium ions. The synthesis of the quaternary ammonium ions follows a diversity oriented approach wherein the tandem inter[4+2]/intra[3+2] cycloaddition of nitroalkenes serves as the key transformation. A two part synthetic strategy comprised of: (1) preparation of enantioenriched scaffolds and (2) development of parallel synthesis procedures is described. The strategy allows for the facile introduction of four variable groups in the vicinity of a stereogenic quaternary ammonium ion. The quaternary ammonium ions exhibited a wide range of activity and to a lesser degree enantioselectivity. Catalyst activity and selectivity are rationalized in a qualitative way based on the effective positive potential of the ammonium ion. PMID:21446721

Denmark, Scott E.; Gould, Nathan D.; Wolf, Larry M.

2011-01-01

71

In-situ environmental (scanning) transmission electron microscopy of catalysts at the atomic level  

NASA Astrophysics Data System (ADS)

Observing reacting single atoms on the solid catalyst surfaces under controlled reaction conditions is a key goal in understanding and controlling heterogeneous catalytic reactions. In-situ real time aberration corrected environmental (scanning) transmission electron microscopy (E(S)TEM permit the direct imaging of dynamic surface and sub-surface structures of reacting catalysts. In this paper in-situ AC ETEM and AC ESTEM studies under controlled reaction environments of oxide catalysts and supported metal nanocatalysts important in chemical industry are presented. They provide the direct evidence of dynamic processes at the oxide catalyst surface at the atomic scale and single atom dynamics in catalytic reactions. The ESTEM studies of single atom dynamics in controlled reaction environments show that nanoparticles act as reservoirs of ad-atoms. The results have important implications in catalysis and nanoparticle studies.

Gai, P. L.; Boyes, E. D.

2014-06-01

72

Mixed Valent Sites in Biological Electron Transfer  

SciTech Connect

Many of the active sites involved in electron transfer (ET) in biology have more than one metal and are mixed valent in at least one redox state. These include Cu(A), and the polynuclear Fe-S clusters which vary in their extent of delocalization. In this tutorial review the relative contributions to delocalization are evaluated using S K-edge X-ray absorption, magnetic circular dichroism and other spectroscopic methods. The role of intra-site delocalization in ET is considered.

Solomon, E.I.; Xie, X.; Dey, A.

2009-05-26

73

Mimicking the electron transfer chain in photosystem II with a molecular triad thermodynamically capable of water oxidation.  

PubMed

In the photosynthetic photosystem II, electrons are transferred from the manganese-containing oxygen evolving complex (OEC) to the oxidized primary electron-donor chlorophyll P680(•+) by a proton-coupled electron transfer process involving a tyrosine-histidine pair. Proton transfer from the tyrosine phenolic group to a histidine nitrogen positions the redox potential of the tyrosine between those of P680(•+) and the OEC. We report the synthesis and time-resolved spectroscopic study of a molecular triad that models this electron transfer. The triad consists of a high-potential porphyrin bearing two pentafluorophenyl groups (PF(10)), a tetracyanoporphyrin electron acceptor (TCNP), and a benzimidazole-phenol secondary electron-donor (Bi-PhOH). Excitation of PF(10) in benzonitrile is followed by singlet energy transfer to TCNP (? = 41 ps), whose excited state decays by photoinduced electron transfer (? = 830 ps) to yield Bi-PhOH-PF(10)(•+)-TCNP(•-). A second electron transfer reaction follows (? < 12 ps), giving a final state postulated as BiH(+)-PhO(•)-PF(10)-TCNP(•-), in which the phenolic proton now resides on benzimidazole. This final state decays with a time constant of 3.8 ?s. The triad thus functionally mimics the electron transfers involving the tyrosine-histidine pair in PSII. The final charge-separated state is thermodynamically capable of water oxidation, and its long lifetime suggests the possibility of coupling systems such as this system to water oxidation catalysts for use in artificial photosynthetic fuel production. PMID:22566659

Megiatto, Jackson D; Antoniuk-Pablant, Antaeres; Sherman, Benjamin D; Kodis, Gerdenis; Gervaldo, Miguel; Moore, Thomas A; Moore, Ana L; Gust, Devens

2012-09-25

74

Insights into proton-coupled electron transfer mechanisms of electrocatalytic H2 oxidation and production  

PubMed Central

The design of molecular electrocatalysts for H2 oxidation and production is important for the development of alternative renewable energy sources that are abundant, inexpensive, and environmentally benign. Recently, nickel-based molecular electrocatalysts with pendant amines that act as proton relays for the nickel center were shown to effectively catalyze H2 oxidation and production. We developed a quantum mechanical approach for studying proton-coupled electron transfer processes in these types of molecular electrocatalysts. This theoretical approach is applied to a nickel-based catalyst in which phosphorous atoms are directly bonded to the nickel center, and nitrogen atoms of the ligand rings act as proton relays. The catalytic step of interest involves electron transfer between the nickel complex and the electrode as well as intramolecular proton transfer between the nickel and nitrogen atoms. This process can occur sequentially, with either the electron or proton transferring first, or concertedly, with the electron and proton transferring simultaneously without a stable intermediate. The electrochemical rate constants are calculated as functions of overpotential for the concerted electron-proton transfer reaction and the two electron transfer reactions in the sequential mechanisms. Our calculations illustrate that the concerted electron-proton transfer standard rate constant will increase as the equilibrium distance between the nickel and nitrogen atoms decreases and as the pendant amines become more flexible to facilitate the contraction of this distance with a lower energy penalty. This approach identifies the favored mechanisms under various experimental conditions and provides insight into the impact of substituents on the nitrogen and phosphorous atoms. PMID:22529352

Horvath, Samantha; Fernandez, Laura E.; Soudackov, Alexander V.; Hammes-Schiffer, Sharon

2012-01-01

75

Quantum tunneling resonant electron transfer process in Lorentzian plasmas  

SciTech Connect

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.

Hong, Woo-Pyo [Department of Electronics Engineering, Catholic University of Daegu, Hayang 712-702 (Korea, Republic of); Jung, Young-Dae, E-mail: ydjung@hanyang.ac.kr [Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, 110 8th Street, Troy, New York 12180-3590 (United States); Department of Applied Physics and Department of Bionanotechnology, Hanyang University, Ansan, Kyunggi-Do 426-791 (Korea, Republic of)

2014-08-15

76

Electron Transfer and Localization in Endohederal Metallofullerenes  

SciTech Connect

Endohedral metallofullerenes constitute an appealing class of nanoscale building blocks for fabrication of a wide range of noval materials. One open question of fundamental importance is the precise nature of charge redistribution with the carbon cages (Cn) upon metal encapsulation. Using ab initio density functional theory, we systematically study the electronic structure of metallofullerenes, focusing on the spatial charge redistribution. For all large metallofullerenes (n >32), the valence electrons of the metal atoms are all transferred to the fullerene states. Surprisingly, the transferred charge is found to be highly localized inside the cage near the metal cations, rather than uniformly distributed on the surfaces of the carbon cage as traditionally belied. This counterintuitive charge localization picture is attributed to the strong metal-cage interactions within the systems. These findings may prove to be instrumental in the design of novel fullerene-based functional nanomaterials. 1Supported by the DMSE program and grant no. DE-FG02-05ER46209 of USDOE, grant no. DMR-0606485 of USNSF, and NSF of China.

Yang, Shenyuan [ORNL; Yoon, Mina [ORNL; Hicke, Christian [ORNL; Zhang, Zhenyu [ORNL

2008-01-01

77

Electronic and Nuclear Factors in Charge and Excitation Transfer  

SciTech Connect

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.

Piotr Piotrowiak

2004-09-28

78

COMMUNICATIONS Electron transfer to oriented molecules: Surprising steric effect  

E-print Network

COMMUNICATIONS Electron transfer to oriented molecules: Surprising steric effect in t-butyl bromide,9 and light absorption.10 We recently examined the effect of molecular orienta- tion on electron transfer, it reverses sign at low energies and the alkyl end becomes more reactive. At low energies the electron

Brooks, Philip R.

79

Photoinduced intermolecular electron transfer in complex liquids: Experiment and theory  

E-print Network

transfer between Rhodamine 3B and N,N-dimethylaniline has been studied in a series of seven liquids to examine photoinduced electron transfer for donors and acceptors in seven liquid solvents. Rhodamine 3B R3B

Fayer, Michael D.

80

GPU-accelerated computation of electron transfer.  

PubMed

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

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

2012-11-01

81

Theory of plasmon enhanced interfacial electron transfer.  

PubMed

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

Wang, Luxia; May, Volkhard

2015-04-10

82

Theory of plasmon enhanced interfacial electron transfer  

NASA Astrophysics Data System (ADS)

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.

Wang, Luxia; May, Volkhard

2015-04-01

83

Transient Ru-methyl formate intermediates generated with bifunctional transfer hydrogenation catalysts  

PubMed Central

Desorption electrospray ionization (DESI) coupled to high-resolution Orbitrap mass spectrometry (MS) was used to study the reactivity of a (?-amino alcohol)(arene)RuCl transfer hydrogenation catalytic precursor in methanol (CH3OH). By placing [(p-cymene)RuCl2]2 on a surface and spraying a solution of ?-amino alcohol in methanol, two unique transient intermediates having lifetimes in the submillisecond to millisecond range were detected. These intermediates were identified as Ru (II) and Ru (IV) complexes incorporating methyl formate (HCOOCH3). The Ru (IV) intermediate is not observed when the DESI spray solution is sparged with Ar gas, indicating that O2 dissolved in the solvent is necessary for oxidizing Ru (II) to Ru (IV). These proposed intermediates are supported by high-resolution and high mass accuracy measurements and by comparing experimental to calculated isotope profiles. Additionally, analyzing the bulk reaction mixture using gas chromatography-MS and nuclear magnetic resonance spectroscopy confirms the formation of HCOOCH3. These results represent an example that species generated from the (?-amino alcohol)(arene)RuCl (II) catalytic precursor can selectively oxidize CH3OH to HCOOCH3. This observation leads us to propose a pathway that can compete with the hydrogen transfer catalytic cycle. Although bifunctional hydrogen transfer with Ru catalysts has been well-studied, the ability of DESI to intercept intermediates formed in the first few milliseconds of a chemical reaction allowed identification of previously unrecognized intermediates and reaction pathways in this catalytic system. PMID:22315417

Perry, Richard H.; Brownell, Kristen R.; Chingin, Konstantin; Cahill, Thomas J.; Waymouth, Robert M.; Zare, Richard N.

2012-01-01

84

Photochemical generation of strong one-electron reductants via light-induced electron transfer with reversible donors followed by cross reaction with sacrificial donors.  

PubMed

This work illustrates a modified approach for employing photoinduced electron transfer reactions coupled to secondary irreversible electron transfer processes for the generation of strongly reducing equivalents in solution. Through irradiation of [Ru(LL)3](2+) (LL= diimine ligands) with tritolylamine (TTA) as quencher and various alkyl amines as sacrificial electron donors, yields in excess of 50% can be achieved for generation of reductants with E(0)(2+/1+) values between -1.0 and -1.2 V vs NHE. The key to the system is the fact that the TTA cation radical, formed in high yield in reaction with the photoexcited [Ru(LL)3](2+) complex, reacts irreversibly with various sacrificial electron donating amines that are kinetically unable to directly react with the photoexcited complex. The electron transfer between the TTA(+) and the sacrificial amine is an energetically uphill process. Kinetic analysis of these parallel competing reactions, consisting of bimolecular and pseudo first-order reactions, allows determination of electron transfer rate constants for the cross electron transfer reaction between the sacrificial donor and the TTA(+). A variety of amines were examined as potential sacrificial electron donors, and it was found that tertiary 1,2-diamines are most efficient among these amines for trapping the intermediate TTA(+). This electron-donating combination is capable of supplying a persistent reducing flux of electrons to catalysts used for hydrogen production. PMID:24882233

Shan, Bing; Schmehl, Russell

2014-11-13

85

Electron microscopy studies of real and model oxide supported gold catalysts  

NASA Astrophysics Data System (ADS)

Oxide supported Au catalysts have been the center of intensive research since being discovered as the most active catalysts for low temperature CO oxidation. However, the origin of the high activity of these catalysts remains unknown. The complexity of this catalytic system prevents a clear identification and characterization of the factors truly affecting its properties. In this thesis research, the attention was focused on certain areas that are truly crucial for the understanding of the Au catalysts, including the preparation and activation of Au catalysts, the properties of the TiO2 surface and the interaction between TiO2 and gold nanoparticles. Electron microscopy was used throughout this research along with other techniques and has been proved to be a powerful and irreplaceable tool and provide an insight into this catalytic system with a unique angle. Among all of the findings of this research, the examination of Au catalysts identified the role of chlorine in accelerating the agglomeration of gold particles and poisoning the active sites. Studies on the activation of Au/Al 2O3 and Au/TiO2 catalysts demonstrated the oxidation state and the size of the gold particles were two competing factors during activation and both were very important. The difference in the mobility of gold species on oxide surfaces affects them. The study of the TiO2 surface described the reoxidation process of the TiO2 surface and a new surface reconstruction, c(2x2), on this surface was reported. Its atomic structure was solved by applying Direct Methods and Density Functional Theory calculations. The study of Au/TiO2 model catalysts revealed no preferred orientation between gold nanoparticles and TiO2 supports with various crystallographic orientations and surface conditions, and this fact was explained by the influence of surface adsorbates. Model catalyst studies also characterized surface induced sintering, and estimated the temperature of local heating during surface induced sintering. Finally, the attempt to measure the catalytic properties of the Au model catalyst was presented and the initial results was described.

Wang, Yingmin

86

Electron transfer reactions in microporous solids. Progress report, September 1990--January 1993  

SciTech Connect

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}}, or H{sub 2} and O{sub 2)} from each other. Spectroscopic and electrochemical methods are used to study the kinetics of electron transfer reactions in these hybrid molecular/solid state assemblies.

Mallouk, T.E.

1993-01-01

87

Effect of electron beam irradiation on CO 2 reforming of methane over Ni/Al 2O 3 catalysts  

NASA Astrophysics Data System (ADS)

The effect of electron beam irradiation on the CO 2 reforming of methane over Ni/Al 2O 3 was investigated. The conversion rate of CO 2 and CH 4 forming H 2 and CO using various catalysts irradiated with an absorbed dose greater than 2 MGy was 5-10% higher than when using an untreated catalyst. The Ni/O ratio on the catalyst surface increased after treatment with an electron beam, and was more prominent for catalysts with a higher Ni content. As such, based on XRD and XPS measurements, electron beam treatment was found to result in either the desorption of oxygen from NiO or the removal of OH groups from the outermost surface layer of the catalyst. In addition, the concentration of active sites, such as Ni 2+ and NiO, or surface defects was also found to increase with the absorbed radiation dose, thereby increasing the conversion rate.

Jun, Jin; Kim, Jo-Chun; Shin, Joong-Hyeok; Lee, Ki-Wan; Baek, Young Soon

2004-12-01

88

Photochemical charge separation in zeolites: Electron transfer dynamics, nanocrystals and zeolitic membranes. Final technical report  

SciTech Connect

Aluminosilicate zeolites provide an excellent host for photochemical charge separation. Because of the constraints provided by the zeolite, the back electron transfer from the reduced acceptor to the oxidized sensitizer is slowed down. This provides the opportunity to separate the charge and use it in a subsequent reaction for water oxidation and reduction. Zeolite-based ruthenium oxide catalysts have been found to be efficient for the water splitting process. This project has demonstrated the usefulness of zeolite hosts for photolytic splitting of water.

Dutta, Prabir K.

2001-09-30

89

Photoinitiated electron transfer in multichromophoric species: Synthetic tetrads and pentads  

SciTech Connect

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).

NONE

1993-03-01

90

Electron transfer and protein engineering studies of the soluble methane monooxygenase from Methylococcus capsulatus (Bath)  

E-print Network

Chapter 1. Introduction: Electron Transfer in Biological Systems In many biological processes, including oxidative phosphorylation and photosynthesis, electron transfer reactions play vital roles. Electrons must be transported ...

Blazyk, Jessica L. (Jessica Lee), 1974-

2003-01-01

91

Electron transfer processes in double lanthanide activated YPO 4  

NASA Astrophysics Data System (ADS)

This work reviews electron transfer processes in double lanthanide doped YPO 4 studied using a diversity of optical techniques. The aim is to identify those transfer processes and to relate them with the location of lanthanide electron donor and lanthanide electron acceptor states within the bandgap of YPO 4. Electron transfer from the valence band to a trivalent lanthanide impurity or to the conduction band was studied by traditional single photon excitation on single lanthanide doped YPO 4. This work reports on YPO 4 doped with the electron donor Ce 3+ and co-doped with an electron acceptor Pr 3+, Er 3+, Nd 3+, Ho 3+, Dy 3+, Tm 3+, or Sm 3+ studied by means of thermoluminescence and optical stimulated luminescence techniques. The same samples were studied by pump and probe two photon spectroscopy utilizing synchrotron photons to transfer electrons from Ce 3+ to Ln 3+ defects and 445 nm laser diode to probe the concentration of created Ln 2+.

Dorenbos, P.; Bos, A. J. J.; Poolton, N. R. J.

2011-05-01

92

In situ observation of water in a fuel cell catalyst using scanning electron microscopy.  

PubMed

To visualize water in the catalyst of polymer electrolyte fuel cells (PEFCs), backscattered electron (BSE) imaging by means of scanning electron microscopy was employed. To confine a wet specimen of catalyst, an environmental wet cell was manufactured with a silicon nitride thin film (?100 nm) as the beam window. By supplying humidified gas into the cell, a change in BSE brightness was detected in the catalyst attached to the silicon nitride window. As humidification proceeded, the BSE image became darker and returned brighter by switching to a dry gas. Monte Carlo simulations were performed to evaluate the energy and number of BSE obtained after passing through water with thickness d. Combining the results of the Monte Carlo simulation successfully converted the change in brightness to the change in thickness from d = 100 nm to d = 3 ?m. This established method of evaluating water with a thickness resolution of the order of ?d = 100 nm can be applied to in situ observations of the catalyst in a PEFC during operation. PMID:25430743

Ueda, Satoru; Kobayashi, Yoshio; Koizumi, Satoshi; Tsutsumi, Yasuyuki

2015-04-01

93

Solvent-controlled acceleration of electron transfer reaction  

Microsoft Academic Search

Summary form only given. Liquid phase electron transfer is one of the key reactions in nature and has been extensively studied. In his pioneering work, Marcus (1985) modelled electron transfer (ET) as thermally activated barrier hopping. Subsequent improvements of the theory that incorporated the influence of solvent dynamics on the ET process, led to the central conclusion that the inverse

M. S. Pshenichnikov; H. P. Den Hartog; A. Baltuska; A. Pugzlys; S. Umapathy; D. A. Wiersma

2000-01-01

94

Photoinduced electron transfer from a conducting polymer to buckminsterfullerene  

Microsoft Academic Search

Evidence for photoinduced electron transfer form the excited state of a conducting polymer onto buckminsterfullerene, Cââ, is reported. After photo-excitation of the conjugated polymer with light of energy greater than the Ï-Ï gap, an electron transfer to the Cââ molecule is initiated. Photoinduced optical absorption studies demonstrate a different excitation spectrum for the composite as compared to the separate components,

N. S. Sariciftci; L. Smilowitz; A. J. Heeger; F. Wudl

1992-01-01

95

REGULAR PAPER The photosynthetic apparatus and photoinduced electron transfer  

E-print Network

phototrophic bacteria, these pigment protein complexes are incapable of light-induced electron (e- ) transfer phototrophic bacteria Roseicyclus mahoneyensis and Porphyrobacter meromictius Christopher Rathgeber · Jean Photosynthetic electron transfer has been examined in whole cells, isolated membranes and in par- tially purified

96

Cross-strand histidine-aromatic interactions enhance acyl-transfer rates in beta-hairpin peptide catalysts.  

PubMed

A reactive tagging methodology was used to select the species most reactive to an acylation reagent from a solid phase library of beta hairpin peptides. Hits bearing an electron-rich aromatic residue across strand from a reactive histidine were found to competitively become N-acylated. In addition to displaying rapid N-acylation rates the hit peptide was additionally deacylated in the presence of a nucleophile, thus closing a putative catalytic cycle. Variants of the hit peptide were studied to elucidate both the magnitude (up to 18,000-fold over background, kcat/kuncat = 94,000,000, or 45-fold over Boc-histidine methyl ester) and mechanism of acyl transfer catalysis. A combination of CH-?, cation-? and HisH(+)-O interactions in the cationic imidazole transition state is implicated in the rate acceleration, in addition to the fidelity of the beta hairpin fold. Moreover, NMR structural data on key intermediates or models thereof suggest that a key feature of this catalyst is the ability to access several different stabilizing conformations along the catalysis reaction coordinate. PMID:25254932

Matsumoto, M; Lee, S J; Gagné, M R; Waters, M L

2014-11-21

97

Iridium(III) hydrido N-heterocyclic carbene-phosphine complexes as catalysts in magnetization transfer reactions.  

PubMed

The hyperpolarization (HP) method signal amplification by reversible exchange (SABRE) uses para-hydrogen to sensitize substrate detection by NMR. The catalyst systems [Ir(H)2(IMes)(MeCN)2(R)]BF4 and [Ir(H)2(IMes)(py)2(R)]BF4 [py = pyridine; R = PCy3 or PPh3; IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene], which contain both an electron-donating N-heterocyclic carbene and a phosphine, are used here to catalyze SABRE. They react with acetonitrile and pyridine to produce [Ir(H)2(NCMe)(py)(IMes)(PPh3)]BF4 and [Ir(H)2(NCMe)(py)(IMes)(PCy3)]BF4, complexes that undergo ligand exchange on a time scale commensurate with observation of the SABRE effect, which is illustrated here by the observation of both pyridine and acetonitrile HP. In this study, the required symmetry breaking that underpins SABRE is provided for by the use of chemical inequivalence rather than the previously reported magnetic inequivalence. As a consequence, we show that the ligand sphere of the polarization transfer catalyst itself becomes hyperpolarized and hence that the high-sensitivity detection of a number of reaction intermediates is possible. These species include [Ir(H)2(NCMe)(py)(IMes)(PPh3)]BF4, [Ir(H)2(MeOH)(py)(IMes)(PPh3)]BF4, and [Ir(H)2(NCMe)(py)2(PPh3)]BF4. Studies are also described that employ the deuterium-labeled substrates CD3CN and C5D5N, and the labeled ligands P(C6D5)3 and IMes-d22, to demonstrate that dramatically improved levels of HP can be achieved as a consequence of reducing proton dilution and hence polarization wastage. By a combination of these studies with experiments in which the magnetic field experienced by the sample at the point of polarization transfer is varied, confirmation of the resonance assignments is achieved. Furthermore, when [Ir(H)2(pyridine-h5)(pyridine-d5)(IMes)(PPh3)]BF4 is examined, its hydride ligand signals are shown to become visible through para-hydrogen-induced polarization rather than SABRE. PMID:24215616

Fekete, Marianna; Bayfield, Oliver W; Bayfield, Oliver; Duckett, Simon B; Hart, Sam; Mewis, Ryan E; Pridmore, Natalie; Rayner, Peter J; Whitwood, Adrian

2013-12-01

98

Iridium(III) Hydrido N-Heterocyclic Carbene–Phosphine Complexes as Catalysts in Magnetization Transfer Reactions  

PubMed Central

The hyperpolarization (HP) method signal amplification by reversible exchange (SABRE) uses para-hydrogen to sensitize substrate detection by NMR. The catalyst systems [Ir(H)2(IMes)(MeCN)2(R)]BF4 and [Ir(H)2(IMes)(py)2(R)]BF4 [py = pyridine; R = PCy3 or PPh3; IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene], which contain both an electron-donating N-heterocyclic carbene and a phosphine, are used here to catalyze SABRE. They react with acetonitrile and pyridine to produce [Ir(H)2(NCMe)(py)(IMes)(PPh3)]BF4 and [Ir(H)2(NCMe)(py)(IMes)(PCy3)]BF4, complexes that undergo ligand exchange on a time scale commensurate with observation of the SABRE effect, which is illustrated here by the observation of both pyridine and acetonitrile HP. In this study, the required symmetry breaking that underpins SABRE is provided for by the use of chemical inequivalence rather than the previously reported magnetic inequivalence. As a consequence, we show that the ligand sphere of the polarization transfer catalyst itself becomes hyperpolarized and hence that the high-sensitivity detection of a number of reaction intermediates is possible. These species include [Ir(H)2(NCMe)(py)(IMes)(PPh3)]BF4, [Ir(H)2(MeOH)(py)(IMes)(PPh3)]BF4, and [Ir(H)2(NCMe)(py)2(PPh3)]BF4. Studies are also described that employ the deuterium-labeled substrates CD3CN and C5D5N, and the labeled ligands P(C6D5)3 and IMes-d22, to demonstrate that dramatically improved levels of HP can be achieved as a consequence of reducing proton dilution and hence polarization wastage. By a combination of these studies with experiments in which the magnetic field experienced by the sample at the point of polarization transfer is varied, confirmation of the resonance assignments is achieved. Furthermore, when [Ir(H)2(pyridine-h5)(pyridine-d5)(IMes)(PPh3)]BF4 is examined, its hydride ligand signals are shown to become visible through para-hydrogen-induced polarization rather than SABRE. PMID:24215616

2013-01-01

99

Diameter-dependent electronic transport properties of Au-catalyst/Ge-nanowire Schottky diodes  

SciTech Connect

We present electronic transport measurements in individual Au-catalyst/Ge-nanowire interfaces demonstrating the presence of a Schottky barrier. Surprisingly, the small-bias conductance density increases with decreasing diameter. Theoretical calculations suggest that this effect arises because electron-hole recombination in the depletion region is the dominant charge transport mechanism, with a diameter dependence of both the depletion width and the electron-hole recombination time. The recombination time is dominated by surface contributions and depends linearly on the nanowire diameter.

Picraux, S Thomas [Los Alamos National Laboratory; Leonard, Francois [SNL; Swartzentruber, Brian S [SNL; Talin, A Alee [SNL

2008-01-01

100

Electronic overfill protection for crude oil transfer  

SciTech Connect

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.

Kilgore, D.R.; Miles, D.C. [CIVACON, Cincinnati, OH (United States)

1995-12-31

101

Electron Transfer Dissociation of Milk Oligosaccharides  

PubMed Central

For structural identification of glycans, the classic collision-induced dissociation (CID) spectra are dominated by product ions that derived from glycosidic cleavages, which provide only sequence information. The peaks from cross-ring fragmentation are often absent or have very low abundances in such spectra. Electron transfer dissociation (ETD) is being applied to structural identification of carbohydrates for the first time, and results in some new and detailed information for glycan structural studies. A series of linear milk sugars was analyzed by a variety of fragmentation techniques such as MS/MS by CID and ETD, and MS3 by sequential CID/CID, CID/ETD, and ETD/CID. In CID spectra, the detected peaks were mainly generated via glycosidic cleavages. By comparison, ETD generated various types of abundant cross-ring cleavage ions. These complementary cross-ring cleavages clarified the different linkage types and branching patterns of the representative milk sugar samples. The utilization of different MS3 techniques made it possible to verify initial assignments and to detect the presence of multiple components in isobaric peaks. Fragment ion structures and pathways could be proposed to facilitate the interpretation of carbohydrate ETD spectra and the main mechanisms were investigated. ETD should contribute substantially to confident structural analysis of a wide variety of oligosaccharides. PMID:21953041

Han, Liang; Costello, Catherine E.

2013-01-01

102

Integrating proton coupled electron transfer (PCET) and excited states  

SciTech Connect

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 CO{sub 2} 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 P{sub 680} and electron transfer quenching to give P{sub 680}{sup +}. The oxidized chlorophyll activates the oxygen evolving complex (OEC), a CaMn4 cluster, through an intervening tyrosine–histidine pair, Y{sub Z}. EPT plays a major role in a series of four activation steps that ultimately result in loss of 4e{sup ?}/4H{sup +} 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.

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

2010-01-01

103

Understanding catalyst behavior during in situ heating through simultaneous secondary and transmitted electron imaging  

NASA Astrophysics Data System (ADS)

By coupling techniques of simultaneous secondary (SE) and transmitted electron (TE) imaging at high resolution in a modern scanning transmission electron microscope (STEM), with the ability to heat specimens using a highly stable MEMS-based heating platform, we obtained synergistic information to clarify the behavior of catalysts during in situ thermal treatments. Au/iron oxide catalyst 'leached' to remove surface Au was heated to temperatures as high as 700°C. The Fe2O3 support particle structure tended to reduce to Fe3O4 and formed surface terraces; the formation, coalescence, and mobility of 1- to 2-nm particles on the terraces were characterized in SE, STEM-ADF, and TEM-BF modes. If combined with simultaneous nanoprobe spectroscopy, this approach will open the door to a new way of studying the kinetics of nano-scaled phenomena.

Howe, Jane Y.; Allard, Lawrence F.; Bigelow, Wilbur C.; Demers, Hendrix; Overbury, Steven H.

2014-11-01

104

Understanding catalyst behavior during in situ heating through simultaneous secondary and transmitted electron imaging.  

PubMed

By coupling techniques of simultaneous secondary (SE) and transmitted electron (TE) imaging at high resolution in a modern scanning transmission electron microscope (STEM), with the ability to heat specimens using a highly stable MEMS-based heating platform, we obtained synergistic information to clarify the behavior of catalysts during in situ thermal treatments. Au/iron oxide catalyst 'leached' to remove surface Au was heated to temperatures as high as 700°C. The Fe2O3 support particle structure tended to reduce to Fe3O4 and formed surface terraces; the formation, coalescence, and mobility of 1- to 2-nm particles on the terraces were characterized in SE, STEM-ADF, and TEM-BF modes. If combined with simultaneous nanoprobe spectroscopy, this approach will open the door to a new way of studying the kinetics of nano-scaled phenomena. PMID:25419195

Howe, Jane Y; Allard, Lawrence F; Bigelow, Wilbur C; Demers, Hendrix; Overbury, Steven H

2014-01-01

105

Understanding catalyst behavior during in situ heating through simultaneous secondary and transmitted electron imaging  

PubMed Central

By coupling techniques of simultaneous secondary (SE) and transmitted electron (TE) imaging at high resolution in a modern scanning transmission electron microscope (STEM), with the ability to heat specimens using a highly stable MEMS-based heating platform, we obtained synergistic information to clarify the behavior of catalysts during in situ thermal treatments. Au/iron oxide catalyst 'leached' to remove surface Au was heated to temperatures as high as 700°C. The Fe2O3 support particle structure tended to reduce to Fe3O4 and formed surface terraces; the formation, coalescence, and mobility of 1- to 2-nm particles on the terraces were characterized in SE, STEM-ADF, and TEM-BF modes. If combined with simultaneous nanoprobe spectroscopy, this approach will open the door to a new way of studying the kinetics of nano-scaled phenomena. PMID:25419195

2014-01-01

106

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

NASA Astrophysics Data System (ADS)

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.

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

2013-03-01

107

Proton-Coupled Electron Transfer Dynamics in the Catalytic Mechanism of a [NiFe]-Hydrogenase.  

PubMed

The movement of protons and electrons is common to the synthesis of all chemical fuels such as H2. Hydrogenases, which catalyze the reversible reduction of protons, necessitate transport and reactivity between protons and electrons, but a detailed mechanism has thus far been elusive. Here, we use a phototriggered chemical potential jump method to rapidly initiate the proton reduction activity of a [NiFe] hydrogenase. Coupling the photochemical initiation approach to nanosecond transient infrared and visible absorbance spectroscopy afforded direct observation of interfacial electron transfer and active site chemistry. Tuning of intramolecular proton transport by pH and isotopic substitution revealed distinct concerted and stepwise proton-coupled electron transfer mechanisms in catalysis. The observed heterogeneity in the two sequential proton-associated reduction processes suggests a highly engineered protein environment modulating catalysis and implicates three new reaction intermediates; Nia-I, Nia-D, and Nia-SR(-). The results establish an elementary mechanistic understanding of catalysis in a [NiFe] hydrogenase with implications in enzymatic proton-coupled electron transfer and biomimetic catalyst design. PMID:25790178

Greene, Brandon L; Wu, Chang-Hao; McTernan, Patrick M; Adams, Michael W W; Dyer, R Brian

2015-04-01

108

Environmental transmission electron microscopy for catalyst materials using a spherical aberration corrector.  

PubMed

Atomic resolution has been obtained using environmental transmission electron microscopy (ETEM) by installing a spherical aberration corrector (Cs-corrector) on the objective lens. Simultaneously, the technology for controlling the environment around a specimen in ETEM has advanced significantly in the past decade. Quantification methodology has recently been established for deriving relevant experimental data in catalyst materials from substantial and systematic ETEM observation at the atomic scale. With this background, this paper summarizes aspects of the evolutional microscopy technique: necessary conditions for atomic resolution in ETEM; reduction of the scattering of electrons by the medium surrounding a specimen; and an environmental cell for structural imaging of a crystalline specimen. The high spatial resolution of a Cs-corrected ETEM is demonstrated for different observation conditions. After statistical analysis combined with numerical image analysis of ETEM data is briefly described, the recent applications of the Cs-corrected ETEM to catalyst materials are reviewed. For gold nanoparticulate catalysts, the structural information on the reaction sites and adsorption sites are deduced. For Pt nanoparticulate catalysts, ETEM studies elucidate the correlation between the catalytic activity and the morphology of the nanoparticles. These studies also reveal oxidation and reduction on the topmost Pt surface layer at the atomic scale. Finally, current issues and the future perspectives of Cs-corrected ETEM are summarized, including the reproducibility of ETEM observation data, the control of environments, the critical evaluation of electron irradiation effects, the full implementation of transmission electron microscopy technology in ETEM, and the safety issues for an ETEM laboratory. PMID:25498142

Takeda, Seiji; Kuwauchi, Yasufumi; Yoshida, Hideto

2015-04-01

109

A porphyrin-inspired iron catalyst for asymmetric epoxidation of electron-deficient olefins.  

PubMed

An in situ formed porphyrin-inspired iron complex that catalyzes asymmetric epoxidation of di- and trisubstituted enones is described. The reaction provides highly enantioenriched ?,?-epoxyketones (up to 99% ee). The practical utility of the new catalyst system is demonstrated by the gram-scale synthesis of optically pure epoxide. Hammett analysis suggests that the transition state of the reaction is electron-demanding and the active oxidant is electrophilic. PMID:25643130

Dai, Wen; Li, Guosong; Chen, Bo; Wang, Lianyue; Gao, Shuang

2015-02-20

110

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

PubMed

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

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

2015-04-01

111

Probing the Mechanism of Electron Capture and Electron Transfer Dissociation Using Tags with Variable Electron Affinity  

PubMed Central

Electron capture dissociation (ECD) and electron transfer dissociation (ETD) of doubly protonated electron affinity (EA)-tuned peptides were studied to further illuminate the mechanism of these processes. The model peptide FQpSEEQQQTEDELQDK, containing a phosphoserine residue, was converted to EA-tuned peptides via ?-elimination and Michael addition of various thiol compounds. These include propanyl, benzyl, 4-cyanobenzyl, perfluorobenzyl, 3,5-dicyanobenzyl, 3-nitrobenzyl and 3,5-dinitrobenzyl structural moieties, having a range of EA from -1.15 to 1.65 eV, excluding the propanyl group. Typical ECD or ETD backbone fragmentations are completely inhibited in peptides with substituent tags having EA over 1.00 eV, which are referred to as electron predators in this work. Nearly identical rates of electron capture by the dications substituted by the benzyl (EA = -1.15 eV) and 3-nitrobenzyl (EA = 1.00 eV) moieties are observed, which indicates the similarity of electron capture cross sections for the two derivatized peptides. This observation leads to the inference that electron capture kinetics are governed by the long range electron-dication interaction and are not affected by side chain derivatives with positive EA. Once an electron is captured to high-n Rydberg states, however, through-space or through-bond electron transfer to the EA-tuning tags or low-n Rydberg states via potential curve crossing occurs in competition with transfer to the amide ?* orbital. The energetics of these processes are evaluated using time-dependent density functional theory with a series of reduced model systems. The intramolecular electron transfer process is modulated by structure-dependent hydrogen bonds and is heavily affected by the presence and type of electron withdrawing groups in the EA-tuning tag. The anion radicals formed by electron predators have high proton affinities (approximately 1400 kJ/mol for the 3-nitrobenzyl anion radical) in comparison to other basic sites in the model peptide dication, facilitating exothermic proton transfer from one of the two sites of protonation. This interrupts the normal sequence of events in ECD or ETD leading to backbone fragmentation by forming a stable radical intermediate. The implications which these results have for previously proposed ECD and ETD mechanisms are discussed. PMID:19331417

Sohn, Chang Ho; Chung, Cheol K.; Yin, Sheng; Ramachandran, Prasanna; Loo, Joseph A.; Beauchamp, J. L.

2009-01-01

112

Catalytic enantioselective reactions driven by photoinduced electron transfer  

Microsoft Academic Search

Photoinduced electron transfer is an essential step in the conversion of solar energy into chemical energy in photosystems I and II (ref. 1), and is also frequently used by chemists to build complex molecules from simple precursors. During this process, light absorption generates molecules in excited electronic states that are susceptible to accepting or donating electrons. But although the excited

Andreas Bauer; Felix Westkämper; Stefan Grimme; Thorsten Bach

2005-01-01

113

Electron transfer-initiated epoxidation and isomerization chain reactions of ?-caryophyllene.  

PubMed

The abundant sesquiterpene ?-caryophyllene can be epoxidized by molecular oxygen in the absence of any catalyst. In polar aprotic solvents, the reaction proceeds smoothly with epoxide selectivities exceeding 70?%. A mechanistic study has been performed and the possible involvement of free radical, spin inversion, and electron transfer mechanisms is evaluated using experimental and computational methods. The experimental data-including a detailed reaction product analysis, studies on reaction parameters, solvent effects, additives and an electrochemical investigation-all support that the spontaneous epoxidation of ?-caryophyllene constitutes a rare case of unsensitized electron transfer from an olefin to triplet oxygen under mild conditions (80?°C, 1?bar?O2 ). As initiation of the oxygenation reaction, the formation of a caryophyllene-derived radical cation via electron transfer is proposed. This radical cation reacts with triplet oxygen to a dioxetane via a chain mechanism with chain lengths exceeding 100 under optimized conditions. The dioxetane then acts as an in?situ-formed epoxidizing agent. Under nitrogen atmosphere, the presence of a one-electron acceptor leads to the selective isomerization of ?-caryophyllene to isocaryophyllene. Observations indicate that this isomerization reaction is a novel and elegant synthetic pathway to isocaryophyllene. PMID:25430783

Steenackers, Bart; Campagnol, Nicolò; Fransaer, Jan; Hermans, Ive; De Vos, Dirk

2015-01-26

114

Proton-coupled electron transfer with photoexcited metal complexes.  

PubMed

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 employed a cyclometalated iridium(III) complex. Our other studies with ruthenium(II) complexes and phenols focused on systematic variations of the reaction free energies for the CPET, ET, and proton transfer (PT) steps to explore their influence on the overall PCET reaction. Still other work with rhenium(I) complexes concentrated on the question of how the electronic structure of the metal-to-ligand charge transfer (MLCT) excited states affects PCET. We used covalent rhenium(I)-phenol dyads to explore the influence of the electron donor-electron acceptor distance on bidirectional PCET. In covalent triarylamine-Ru(bpy)?²?/Os(bpy)?²?-anthraquinone triads (bpy = 2,2'-bipyridine), hydrogen-bond donating solvents significantly lengthened the lifetimes of photogenerated electron/hole pairs because of hydrogen-bonding to the quinone radical anion. Until now, comparatively few researchers have investigated this variation of PCET: the strengthening of H-bonds upon photoreduction. PMID:23402212

Wenger, Oliver S

2013-07-16

115

Steam reforming of n-hexane on pellet and monolithic catalyst beds. A comparative study on improvements due to heat transfer  

NASA Astrophysics Data System (ADS)

Monolithic catalysts with higher available active surface areas and better thermal conductivity than conventional pellets beds, making possible the steam reforming of fuels heavier than naphtha, were examined. Performance comparisons were made between conventional pellet beds and honeycomb monolith catalysts using n-hexane as the fuel. Metal-supported monoliths were examined. These offer higher structural stability and higher thermal conductivity than ceramic supports. Data from two metal monoliths of different nickel catalyst loadings were compared to pellets under the same operating conditions. Improved heat transfer and better conversion efficiencies were obtained with the monolith having higher catalyst loading. Surface-gas interaction was observed throughout the length of the monoliths.

1981-10-01

116

Steam reforming of n-hexane on pellet and monolithic catalyst beds. A comparative study on improvements due to heat transfer  

NASA Technical Reports Server (NTRS)

Monolithic catalysts with higher available active surface areas and better thermal conductivity than conventional pellets beds, making possible the steam reforming of fuels heavier than naphtha, were examined. Performance comparisons were made between conventional pellet beds and honeycomb monolith catalysts using n-hexane as the fuel. Metal-supported monoliths were examined. These offer higher structural stability and higher thermal conductivity than ceramic supports. Data from two metal monoliths of different nickel catalyst loadings were compared to pellets under the same operating conditions. Improved heat transfer and better conversion efficiencies were obtained with the monolith having higher catalyst loading. Surface-gas interaction was observed throughout the length of the monoliths.

1981-01-01

117

77 FR 22066 - Proposed Collection of Information: “Notice of Reclamation Electronic Funds Transfer, Federal...  

Federal Register 2010, 2011, 2012, 2013, 2014

...Collection of Information: ``Notice of Reclamation Electronic Funds Transfer, Federal...concerning forms FMS-I33, ``Notice of Reclamation. Electronic Funds Transfer, Federal...described below: Title: ``Notice of Reclamation, Electronic Funds Transfer,...

2012-04-12

118

Quinone methide generation via photoinduced electron transfer.  

PubMed

Photochemical activation of water-soluble 1,8-naphthalimide derivatives (NIs) as alkylating agents has been achieved by irradiation at 310 and 355 nm in aqueous acetonitrile. Reactivity in aqueous and neat acetonitrile has been extensively investigated by laser flash photolysis (LFP) at 355 nm, as well as by steady-state preparative irradiation at 310 nm in the presence of water, amines, thiols, and ethyl vinyl ether. Product distribution analysis revealed fairly efficient benzylation of the amines, hydration reaction, and 2-ethoxychromane generation, in the presence of ethyl vinyl ether, resulting from a [4 + 2] cycloaddition onto a transient quinone methide. Remarkably, we found that the reactivity was dramatically suppressed under the presence of oxygen and radical scavengers, such as thiols, which was usually associated with side product formation. In order to unravel the mechanism responsible for the photoreactivity of these NI-based molecules, a detailed LFP study has been carried out with the aim to characterize the transient species involved. LFP data suggest a photoinduced electron transfer (PET) involving the NI triplet excited state (?(max) 470 nm) of the NI core and the tethered quinone methide precursor (QMP) generating a radical ions pair NI(•-) (?(max) 410 nm) and QMP(•+). The latter underwent fast deprotonation to generate a detectable phenoxyl radical (?(max) 390 and 700 nm), which was efficiently reduced by the radical anion NI(•-), generating detectable QM. The mechanism proposed has been validated through a LFP investigation at 355 nm exploiting an intermolecular reaction between the photo-oxidant N-pentylnaphthalimide (NI-P) and a quaternary ammonium salt of a Mannich base as QMP (2a), in both neat and aqueous acetonitrile. Remarkably, these experiments revealed the generation of the model o-QM (?(max) 400 nm) as a long living transient mediated by the same reactivity pathway. Negligible QM generation has been observed under the very same conditions by irradiation of the QMP in the absence of the NI. Owing to the NIs redox and recognition properties, these results represent the first step toward new molecular devices capable of both biological target recognition and photoreleasing of QMs as alkylating species, under physiological conditions. PMID:21425810

Percivalle, Claudia; La Rosa, Andrea; Verga, Daniela; Doria, Filippo; Mella, Mariella; Palumbo, Manlio; Di Antonio, Marco; Freccero, Mauro

2011-05-01

119

Balancing the two photosystems: photosynthetic electron transfer governs  

E-print Network

Balancing the two photosystems: photosynthetic electron transfer governs transcription of reaction, specialized and quasi-autonomous genetic systems. In photosynthesis, two energy converting photosystems and gene expression determines the function and composition of the chloroplast's extra-nuclear genetic

Allen, John F.

120

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

Federal Register 2010, 2011, 2012, 2013, 2014

...the Federal Register on Monday, August 23, 2010, relating to Federal tax deposits (FTDs) by Electronic Funds Transfer (EFT). FOR FURTHER INFORMATION CONTACT: Michael E. Hara, (202) 622-4910 (not a toll-free number). SUPPLEMENTARY...

2010-08-26

121

Global electronic funds transfer between small and medium sized companies  

E-print Network

Cross-border electronic funds transfer is a rapidly expanding field for business and consumer payments. Large multi-national corporations have been able to invest the capital necessary to create infrastructures or work ...

Stovall, Shawn Eric

2006-01-01

122

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

SciTech Connect

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.

Stevens, B.

1997-07-01

123

Protein Conformational Dynamics Probed by Single-Molecule Electron Transfer  

Microsoft Academic Search

Electron transfer is used as a probe for angstrom-scale structural changes in single protein molecules. In a flavin reductase, the fluorescence of flavin is quenched by a nearby tyrosine residue by means of photo-induced electron transfer. By probing the fluorescence lifetime of the single flavin on a photon-by-photon basis, we were able to observe the variation of flavin-tyrosine distance over

Haw Yang; Guobin Luo; Pallop Karnchanaphanurach; Tai-Man Louie; Ivan Rech; Sergio Cova; Luying Xun; X. Sunney Xie

2003-01-01

124

Subpicosecond photoinduced electron transfer from conjugated polymers to functionalized fullerenes  

Microsoft Academic Search

We report time-resolved excited state absorption measurements which demonstrate subpicosecond photoinduced electron transfer using soluble derivatives of poly(p-phenylene vinylene) as donors blended with a functionalized fullerene (methanofullerene) as acceptor. The subpicosecond photoinduced absorption spectra of the polymer\\/methanofullerene blends show that electron transfer from the donor to the acceptor occurs within a picosecond of photoexcitation of the conjugated polymer. Precise determination

B. Kraabel; Jan C. Hummelen; D. Vacar; D. Moses; N. S. Sariciftci; A. J. Heeger; F. Wudl

1996-01-01

125

Single-molecule electron transfer reactions in nanomaterials  

SciTech Connect

Here we report the study of single molecule electron transfer dynamics by coupling fluorescence microscopy at a conventional electrochemical cell. The single-molecule fluorescence spectroelectrochemistry of cresyl violet in aqueous solution and on nanoparticle surface were studied. We observed that the single-molecule fluorescence intensity of cresyl violet is modulated synchronously with the cyclic voltammetric potential scanning. We attribute the fluorescence intensity change of single cresyl violet molecules to the electron transfer reaction driven by the electrochemical potential.

Hu, Dehong; Lei, Chenghong; Ackerman, Eric J.

2009-07-26

126

Electron beam induced radiation damage in the catalyst layer of a proton exchange membrane fuel cell.  

PubMed

Electron microscopy is an essential tool for the evaluation of microstructure and properties of the catalyst layer (CL) of proton exchange membrane fuel cells (PEMFCs). However, electron microscopy has one unavoidable drawback, which is radiation damage. Samples suffer temporary or permanent change of the surface or bulk structure under radiation damage, which can cause ambiguity in the characterization of the sample. To better understand the mechanism of radiation damage of CL samples and to be able to separate the morphological features intrinsic to the material from the consequences of electron radiation damage, a series of experiments based on high-angle annular dark-field-scanning transmission scanning microscope (HAADF-STEM), energy filtering transmission scanning microscope (EFTEM), and electron energy loss spectrum (EELS) are conducted. It is observed that for thin samples (0.3-1 times ?), increasing the incident beam energy can mitigate the radiation damage. Platinum nanoparticles in the CL sample facilitate the radiation damage. The radiation damage of the catalyst sample starts from the interface of Pt/C or defective thin edge and primarily occurs in the form of mass loss accompanied by atomic displacement and edge curl. These results provide important insights on the mechanism of CL radiation damage. Possible strategies of mitigating the radiation damage are provided. PMID:23897710

He, Qianping; Chen, Jihua; Keffer, David J; Joy, David C

2014-01-01

127

Improved heterogeneous electron transfer kinetics of fluorinated graphene derivatives  

NASA Astrophysics Data System (ADS)

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 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. Electronic supplementary information (ESI) available. See DOI: 10.1039/c4nr02563f

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

2014-08-01

128

Vectorial electron transfer in spatially ordered arrays. Progress report, August 1994--January 1997  

SciTech Connect

With DOE support from August 1994 to August 1997, this project sought to identify methods for controlled placement of light absorbers, relays, and multielectron catalysts at defined sites from a fixed semiconductor or metal surface and, thus, to develop methods for preparing chemically modified photoactive surfaces as artificial photosynthetic units. These designed materials have been evaluated as efficient light collection devices and as substrates for defining the key features that govern the efficiency of long distance electron transfer and energy migration. The authors have synthesized several different families of integrated chemical systems as soluble arrays, as solid thin films, and as adsorbates on solid electrodes, seeking to establish how spatial definition deriving from covalent attachment to a helical polymer backbone, from self assembly of functionalized tethers on gold or metal oxide surfaces, and from rigid or layered block polymers can lead to controlled electron and energy transfer. The authors have also conducted physical characterization of semiconductor-containing composites active in controlled interfacial electron transfer, with charge transport in these materials having been evaluated by photophysical and electrochemical methods.

Fox, M.A.

1997-01-01

129

Electronic reorganization triggered by electron transfer: The intervalence charge transfer of a Fe(3+) /Fe(2+) bimetallic complex.  

PubMed

The key role of the molecular orbitals in describing electron transfer processes is put in evidence for the intervalence charge transfer (IVCT) of a synthetic nonheme binuclear mixed-valence Fe(3+) /Fe(2+) compound. The electronic reorganization induced by the IVCT can be quantified by controlling the adaptation of the molecular orbitals to the charge transfer process. We evaluate the transition energy and its polarization effects on the molecular orbitals by means of ab initio calculations. The resulting energetic profile of the IVCT shows strong similarities to the Marcus' model, suggesting a response behaviour of the ensemble of electrons analogue to that of the solvent. We quantify the extent of the electronic reorganization induced by the IVCT process to be 11.74 eV, a very large effect that induces the crossing of states reducing the total energy of the transfer to 0.89 eV. © 2015 Wiley Periodicals, Inc. PMID:25739890

Domingo, Alex; Angeli, Celestino; de Graaf, Coen; Robert, Vincent

2015-04-30

130

Photo-induced intermolecular electron transfer from electron donating solvents to Coumarin dyes in bile salt aggregates: Role of diffusion in electron transfer reaction  

NASA Astrophysics Data System (ADS)

The photo-induced electron transfer between Coumarin dyes and aromatic amines has been investigated using steady state and time-resolved fluorescence quenching studies. We have observed a Marcus type inversion in the electron transfer rate in correlation of quenching constant to the free energy change occurred during reaction. To justify the "inverted region" obtained in the correlation of quenching constant versus free energy curve, we have performed anisotropy measurement and estimated the several diffusional parameters. The translational diffusion coefficients exhibit a similar picture like electron transfer rate constant when it is plotted against free energy. Thus we argued that the diffusion has played an important role in the electron transfer kinetics.

Chakraborty, Anjan; Chakrabarty, Debdeep; Seth, Debabrata; Hazra, Partha; Sarkar, Nilmoni

2006-03-01

131

76 FR 29901 - Electronic Fund Transfers  

Federal Register 2010, 2011, 2012, 2013, 2014

...committing to pay.\\20\\ However, one survey indicated that a majority of consumers...1)(ii) defines as an error ``a computational or bookkeeping error made by a remittance...with a remittance transfer; (ii) A computational or bookkeeping error made by the...

2011-05-23

132

Improving the efficiency of water splitting in dye-sensitized solar cells by using a biomimetic electron transfer mediator  

PubMed Central

Photoelectrochemical water splitting directly converts solar energy to chemical energy stored in hydrogen, a high energy density fuel. Although water splitting using semiconductor photoelectrodes has been studied for more than 40 years, it has only recently been demonstrated using dye-sensitized electrodes. The quantum yield for water splitting in these dye-based systems has, so far, been very low because the charge recombination reaction is faster than the catalytic four-electron oxidation of water to oxygen. We show here that the quantum yield is more than doubled by incorporating an electron transfer mediator that is mimetic of the tyrosine-histidine mediator in Photosystem II. The mediator molecule is covalently bound to the water oxidation catalyst, a colloidal iridium oxide particle, and is coadsorbed onto a porous titanium dioxide electrode with a Ruthenium polypyridyl sensitizer. As in the natural photosynthetic system, this molecule mediates electron transfer between a relatively slow metal oxide catalyst that oxidizes water on the millisecond timescale and a dye molecule that is oxidized in a fast light-induced electron transfer reaction. The presence of the mediator molecule in the system results in photoelectrochemical water splitting with an internal quantum efficiency of approximately 2.3% using blue light. PMID:22547794

Zhao, Yixin; Swierk, John R.; Megiatto, Jackson D.; Sherman, Benjamin; Youngblood, W. Justin; Qin, Dongdong; Lentz, Deanna M.; Moore, Ana L.; Moore, Thomas A.; Gust, Devens; Mallouk, Thomas E.

2012-01-01

133

Electronic state of supported Rh catalysts and the selectivity for the hydrogenation of carbon monoxide  

SciTech Connect

The electronic states of Rh crystallites highly dispersed on various oxides (SiO/sub 2/, ZnO, ZrO/sub 2/, and ZrO/sub 2/ or TiO/sub 2/ on SiO/sub 2/) and the catalytic hydrogenation of CO to form oxygenated hydrocarbons were examined. From XPS measurement Rh on Rh/SiO/sub 2/ showed an electronic state similar to that of Rh metal. This catalyst was active in forming hydrocarbons. However, Rh on Rh/ZnO, which was active in forming methanol, was in the Rh/sup +/ state. Rh crystallite supported on ZrO/sub 2/ or TiO/sub 2/ on SiO/sub 2/ were active for producing C/sub 2/ oxygenates. Rh on these oxides was in a state between that supported on pure ZnO and on pure SiO/sub 2/. For the formation of C/sub 2/ oxygenates, the ability first to dissociate CO and subsequently to insert CO are indispensable. It was found in this study that Rh/sup 0/ was active for CO dissociation and that Rh/sup +/ was active for the CO insertion reaction. Further the Rh catalyst active in the formation of C/sub 2/ oxygenates was in the electronic state between Rh/sup 0/ and Rh/sup +/ and was active for both reactions, CO insertion and CO dissociation. 20 references, 2 figures, 3 tables.

Kawai, M.; Uda, M.

1985-04-25

134

Electronic interactions and charge transfers of metal atoms and clusters on oxide surfaces.  

PubMed

Understanding the interaction of small metal clusters and isolated atoms with oxide surfaces is crucial in order to rationalize the properties of heterogeneous catalysts composed of sub-nanometer metal particles dispersed on an oxide support. The interaction with the oxide surface can significantly alter the original properties of the metal deposit. In particular, the occurrence and the direction of charge transfer at the metal/oxide interface determine the chemical activity of the supported catalyst. The charge transfer depends on a number of factors like the nature of the oxide (reducible or non-reducible), the surface exposed, the presence of defects, the nature of the supported metal, etc. In this article we describe the most important conceptual aspects of the electronic metal-support interaction, a phenomenon related to the direct modification of the metal nano-particle determined by the formation of chemical bonds at the interface with the oxide. For metal nano-particles with a size of about 1 nm or below these effects become dominant although difficult to identify experimentally. PMID:23287900

Pacchioni, Gianfranco

2013-02-14

135

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

NASA Astrophysics Data System (ADS)

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 tetradecyltrimethylammonium bromide (TTAB). It was found that the effective coupling is reduced compared to donor/acceptor pairs dissolved in simple liquids. In the 2nd half of thesis we have addressed the question of the dynamics of phase transitions. We have demonstrated the ability to use the fluorescent excitation-transfer technique to study the demixing of liquids specifically, kinetics of demixing water and 2,6-dimethylpyridine. These two liquids possess a low critical temperature point, which allowed us to use a temperature jump from a laser pulse to initiate the process of phase separation. It was found that Coumarin480 laser dye and HPTS (8-Hydroxypyrene-1,3,6-trisulfonic acid) fluorescent dye have significantly different solubilities in the components of the mixture. These dyes undergo excitation transfer from Coumarin480 to HPTS in the uniform state, but not in the phase-separated state. A system with a temperature jump pump and an excitation transfer probe measured the time scale of the initial step of the phase separation.

Goun, Alexei A.

136

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...

137

Plugging in or going wireless: strategies for interspecies electron transfer  

PubMed Central

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

Shrestha, Pravin Malla; Rotaru, Amelia-Elena

2014-01-01

138

Long-range photoinduced electron transfer dynamics in rigid media.  

PubMed

In semi-rigid PEG-DMA550 films with added reductive quenchers, electron transfer quenching of the metal-to-ligand charge transfer excited state(s) of [Ru(bpy)3](2+) (bpy = 2,2'-bipyridine) occurs by both rapid, fixed-site, and slow, diffusional, quenching processes. Stern-Volmer analysis of diffusional quenching reveals diffusion-controlled quenching both in the fluid and film with the latter greatly inhibited by the high viscosity of the medium. The data for fixed-site quenching are consistent with electron tunneling with the expected exponential distance dependence. Based on this analysis long-range electron transfer occurs with a distance attenuation factor ? of ?0.47 Å(-1) with a notable decrease, ? = 0.16 Å(-1), when the quencher is incorporated into the PEG backbone. Fixed-site electron transfer quenching varies with driving force. Back electron transfer is complex, as expected for a distribution of fixed sites, and varies with power law kinetics. PMID:24473124

Ito, Akitaka; Fang, Zhen; Brennaman, M Kyle; Meyer, Thomas J

2014-03-14

139

Theory of reversible electron transfer reactions in a condensed phase.  

PubMed

We have derived an exact analytical expression for the average forward rate of a reversible electron transfer reaction, modeled through a reaction coordinate undergoing diffusive motion in arbitrary potential wells of the reactant and the product in presence of a localized sink of arbitrary location and strength. The dynamics of diffusive motion is described by employing two coupled generalized diffusion reaction (Smoluchowski) equations with coordinate dependent diffusivity and delta sink. The average forward electron transfer rate constant obtained here for the system, with equilibrium or nonequilibrium distributions as initial condition, is determined by the forward and backward rate constants calculated based on the transition state theory and the weighted average rate for the well dynamics. We also discuss various limiting cases for the rate of electron transfer reactions corresponding to the different experimental situations. As an illustrative example, we have considered back electron transfer (ET) reaction and shown that the present theory can explain the non-Marcus free energy gap dependence of the rate of ET reactions. More importantly, the approach presented here can easily be extended to systems describing the dynamics of diffusive motion in coupled multipotential surfaces associated with electron transfer reactions. PMID:20866692

Dhole, Kajal; Modak, Brindaban; Samanta, Alok; Ghosh, Swapan K

2010-07-01

140

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

PubMed

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 ?-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. PMID:25242485

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

2014-10-01

141

Electron transfer through rigid organic molecular wires enhanced by electronic and electron–vibration coupling  

NASA Astrophysics Data System (ADS)

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 ?-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.

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

2014-10-01

142

Kinetic Monte Carlo Simulation of Electron Transfer  

E-print Network

shown that this coupling can be mediated by electric currents carried by unidentified electron, filamentous bacteria. They abounded in sediment zones with electric currents and along their length of molecule i, where k [1, ngb(i)] Example: Max_ngb = 2 #12;· Occupation · Rates (constants

Southern California, University of

143

Activation of molecular catalysts using semiconductor quantum dots  

DOEpatents

Photocatalytic materials based on coupling of semiconductor nanocrystalline quantum dots (NQD) and molecular catalysts. These materials have capability to drive or catalyze non-spontaneous chemical reactions in the presence of visible radiation, ultraviolet radiation, or both. The NQD functions in these materials as a light absorber and charge generator. Following light absorption, the NQD activates a molecular catalyst adsorbed on the surface of the NQD via transfer of one or more charges (either electrons or electron-holes) from the NQD to the molecular catalyst. The activated molecular catalyst can then drive a chemical reaction. A photoelectrolytic device that includes such photocatalytic materials is also described.

Meyer, Thomas J. (Chapel Hill, NC); Sykora, Milan (Los Alamos, NM); Klimov, Victor I. (Los Alamos, NM)

2011-10-04

144

Photoinduced electron transfer processes in homogeneous and microheterogeneous solutions  

SciTech Connect

The studies have focused on rapid, efficient bond-fragmentation reactions initiated through photoinduced electron transfer. Electron transfer induced fragmentation of a number of donors have been examined, especially 1,2 diamines and related compounds. Two of the amines fragment with rate constants of 3 [times] 10[sup 8] to 2 [times] 10[sup 9] M[sup [minus]1]sec[sup [minus]1]. A series of amino-substituted pinacols and related compounds have also been examined; they undergo similar but slower fragmentation processes when converted to their cation radicals by photoinduced electron transfer. The studies with linked and polymeric electron donor- electron acceptor coupled molecules have also progressed. Several polymers containing diamine repeat units and anthraquinone or nitroaromatic acceptors have also been prepared that can be photoactivated by visible irradiation; they fragment efficiently in solution and photodegrade even in the solid state. The studies of singlet oxygen initiated fragmentation reactions of diamines, amino alcohols, and aminoketones have nearly been completed. Attention have been turned to fragmentable electron acceptors such as p- cyanobenzyl bromide; irradiation of electron donors such as methyl- or methoxy-naphthalenes can initiate efficient fragmentation of the electron deficient bromide.

Whitten, D.G.

1992-12-01

145

Photoinduced electron transfer processes in homogeneous and microheterogeneous solutions  

SciTech Connect

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.

Whitten, D.G.

1991-10-01

146

Mapping protein electron transfer pathways with QM/MM methods  

PubMed Central

Mixed quantum mechanics/molecular mechanics (QM/MM) methods offer a valuable computational tool for understanding the electron transfer pathway in protein–substrate interactions and protein–protein complexes. These hybrid methods are capable of solving the Schrödinger equation on a small subset of the protein, the quantum region, describing its electronic structure under the polarization effects of the remainder of the protein. By selectively turning on and off different residues in the quantum region, we are able to obtain the electron pathway for short- and large-range interactions. Here, we summarize recent studies involving the protein–substrate interaction in cytochrome P450 camphor, ascorbate peroxidase and cytochrome c peroxidase, and propose a novel approach for the long-range protein–protein electron transfer. The results on ascorbate peroxidase and cytochrome c peroxidase reveal the importance of the propionate groups in the electron transfer pathway. The long-range protein–protein electron transfer has been studied on the cytochrome c peroxidase–cytochrome c complex. The results indicate the importance of Phe82 and Cys81 on cytochrome c, and of Asn196, Ala194, Ala176 and His175 on cytochrome c peroxidase. PMID:18445553

Guallar, Victor; Wallrapp, Frank

2008-01-01

147

High-resolution transmission electron microscopy study of carbon deposited on the NiO/MgO solid solution catalysts  

SciTech Connect

The carbon deposition due to the CH{sub 4} decomposition at 790 C over NiO/MgO catalysts was investigated by high-resolution transmission electron microscopy. While no deposits could be detected over the catalysts with a NiO content smaller than 9.1 wt%, they were detected over the catalysts with NiO contents of 23 and 50 wt%. The carbon deposits are composed of platelets located at distances of about 0.34 nm, corresponding to the graphitic carbon. Various structures of the deposited carbon were observed: (a) carbon consisting of platelets parallel to the surface of the particle, which covers a catalyst particle, (b) nanotubes composed of platelets parallel to their axis, and (c) carbon vortexes consisting of platelets parallel to their axis.

Hu, Y.H.; Ruckenstein, E. [State Univ. of New York, Buffalo, NY (United States). Dept. of Chemical Engineering] [State Univ. of New York, Buffalo, NY (United States). Dept. of Chemical Engineering

1999-05-15

148

Electron transfer in proteins: theory, applications and future perspectives.  

PubMed

The study of electron transfer (ET) by means of computational techniques has experienced a great development in the last few decades. In particular, understanding the atomic details of its mechanism in complex biological systems is currently possible with a large range of different in silico modelling tools. We review here some theories and representative major contributions to this development. We also underline some of our group's main inputs, focusing on long range and protein-protein electron transfer, and analyse future perspectives. At the end of the article, we emphasize the importance of the basic electron transfer knowledge in the frame of medical and bioengineering applications: mitochondrial therapeutic targets, bioengineering for clean energy, and biosensors. PMID:23936901

Saen-Oon, Suwipa; Lucas, Maria Fatima; Guallar, Victor

2013-10-01

149

Energies of Electronic States of Ni (II) Ion in NiO-Al2O3 Catalyst Prepared by Impregnation  

SciTech Connect

The behavior of NiO-Al2O3 catalysts is strongly dependent on the preparation method, as well as on pretreatment conditions. In the present work we investigated the influences of Ni(II) ion on NiO-Al2O3 catalysts properties due to the preparation by impregnation method. Based on experimental diffuse reflectance spectroscopy (DRS) data of electronic d-d transitions of Ni (II) promoter ion the energies of electronic states in spinel-like structure were calculated, and the most probable scheme of molecular orbital have been proposed.

Obadovic, D. Z. [University of Novi Sad, Faculty of Sciences, Department of Physics, Trg D. Obradovica 4, 21000 Novi Sad (Serbia); Kiurski, J. [University of Novi Sad, Faculty of Technical Sciences, Trg D. Obradovica 6, 21000 Novi Sad (Serbia); Marinkovic-Neducin, R. P. [University of Novi Sad, Faculty of Technology, Bul. Cara Lazara 1, 21000 Novi Sad (Serbia)

2007-04-23

150

Electron transfer statistics and thermal fluctuations in molecular junctions.  

PubMed

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

Goswami, Himangshu Prabal; Harbola, Upendra

2015-02-28

151

Electron Transfer Dissociation Mass Spectrometry of Hemoglobin on Clinical Samples  

NASA Astrophysics Data System (ADS)

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.

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

2012-10-01

152

Long-distance photoinitiated electron transfer through polyene molecular wires  

SciTech Connect

Long-chain polyenes can be used as molecular wires to facilitate electron transfer between a photo-excited donor and an acceptor in an artificial photosynthetic system. The authors present data here on two Zn-porphyrin-polyene-anthraquinone molecules possessing either 5 or 9 all trans double bonds between the donor and acceptor, 1 and 2. The center-to-center distances between the porphyrin and the quinone in these relatively rigid molecules are 25 A for 1 and 35 A for 2. Selective picosecond laser excitation of the Zn-porphyrin in 1 and 2 results in the very rapid transfer of an electron to the anthraquinone in < 2 ps and 10 ps, respectively. The resultant radical ion pairs recombine with tau = 10 ps for 1 and tau = 25 ps for 2. The electron transfer rates remain remarkably rapid over these long distances. The involvement of polyene radical cations in the mechanism of the radical ion pair recombination reaction is clear from the transient absorption spectra of 1 and 2, which show strong absorbances in the near-infrared. The strong electronic coupling between the Zn-porphyrin and the anthraquinone provided by low-lying states of the polyene make it possible to transfer an electron rapidly over very long distances.

Wasielewski, M.R.; Johnson, D.G.; Svec, W.A.; Kersey, K.M.; Cragg, D.E.; Minsek, D.W.

1988-01-01

153

Nanostructural and Chemical Characterization of Complex Oxide Catalysts by Analytical Electron Microscopy  

NASA Astrophysics Data System (ADS)

Complex oxide catalysts are used as heterogeneous catalysts for producing various important organic chemicals. In this thesis, three types of complex oxide catalysts prepared using novel preparation methods have been studied. Each of them has been evaluated for its catalytic performance, namely (i) the selective oxidation of n-butane to maleic anhydride over vanadium phosphate (V-P-O) materials; (ii) the oxidative dehydrogenation (ODH) of ethane to ethylene over niobium phosphate (Nb-P-O) materials, and (iii) the oxidation of methanol to formaldehyde over iron molybdate (Fe-Mo-O) materials. Analytical electron microscopy, X-ray diffraction and other related characterization techniques have been used to provide useful information regarding the morphology, crystallography and chemical composition of these complex oxide catalysts. The underlying aim of this work is to uncover meaningful synthesis-structure-performance relationships for these three complex catalyst systems. Firstly, a standard methodology for generating V-P-O materials, i.e. the VPD route, has been revisited and modified. A variety of alkanes have been added during the alcohol reduction step of VOPO4·2H2O (dihydrate), which were found to have a remarkable influence on the morphology and structure of the V-P-O materials produced. Either VOHPO4·0.5H2O (hemihydrate) or VO(H2PO4)2 material can be produced depending on the precise alcohol:alkane volume ratio used in the reaction. In addition, the specific order in which the alkane and alcohol are added to VOPO 4·2H2O during the VPD route has a dramatic effect on the morphology of the resultant precursor. Through detailed electron microscopy studies we have been able to unveil the epitaxial relationship between the dihyrate and hemihydrate crystalline phases as being [001]dihydrate // [001]hemihydrate and [100]dihydrate // [110]hemihydrate. A two-step mechanism by which the topotactic transformation from dihydrate to hemihydrate occurs has been proposed. Secondly, three different novel synthesis routes have been explored for producing V-P-O catalysts. The first route, involving the addition of various V-P-O 'seeds' during the VPD process, was found to have a profound effect on the morphology of the V-P-O precursor and on inducing certain unexpected phase transformations. Specifically, the V-P-O seed was found to induce the transformation of VO(H2PO4)2 to hemihydrate phase in a 3-octanol solution. The second route, namely the use of a di-block copolymer template in the VPO route, was found to generate a more crystalline hemihydrate precursor with a rhomboidal morphology, which could be activated in a much shorter time period as compared to conventional V-P-O precursors. The third route involved encapsulating the fragile V-P-O rosette-type catalysts within a mechanically protective SiO2 shell. When used in a circulating fluidized bed reactor, these core/shell V-P-O catalysts showed a promising initial catalytic performance, but suffered a severe degradation in performance after two years-on-line. We have been able to attribute this degradation to three contributing factors; namely (i) the generation of inactive V 5+ (e.g. beta-VOPO4) phases, (ii) densification of the SiO2 shell and (iii) loss of core V-P-O materials. In addition, through this latter study, the novel X-ray ultramicroscopy (XuM) technique has been shown to have great potential for the non-destructive study of micron-scale catalyst particles. Thirdly, three different niobium phosphate materials, namely the Nb 2P4O15, NbOPO4 and Nb1.91P 2.82O12 phases, have been synthesized. Each of them was evaluated for the ODH of ethane to ethylene and the oxidation of methanol to formaldehyde, respectively. It was found that the Nb1.91P2.82O 12 phase is the most desirable structure for ethane ODH, whereas the NbOPO4 phase is more effective for methanol oxidation. The morphological and structural changes induced by both reactions on these Nb-P-O catalysts have been monitored, and correlated to the measured changes in their catalytic performance. Finally, a highly

Weng, Weihao

154

Solvent structure and hydrodynamic effects in photoinduced electron transfer  

SciTech Connect

A previously developed statistical mechanical theory describing photo-induced electron transfer and geminate recombination in liquid solutions has been modified to account for realistic finite-volume solvent effects. This work introduces physically important effects caused by the solvent which fundamentally affect the rates and spatial distribution of charge transfer events. The finite volume of solvent molecules gives rise to a nonuniform distribution of particles around an electron donor, which is incorporated into the theory by a two-particle radial distribution function (rdf). The Percus{endash}Yevick solutions for the rdf can give numerically useful values for the solvent structure, {ital g}({ital R}) although any form of {ital g}({ital R}) can be used with the method. The nonuniform particle distribution significantly affects the electron transfer rates and the distribution of ion pairs formed by forward electron transfer, particularly at short times. In addition, finite solvent size affects the rate of relative diffusion between any donor{endash}acceptor pair. These {open_quote}{open_quote}hydrodynamic effects{close_quote}{close_quote} slow down the interparticle diffusion rates when near contact, resulting in a major change in the long time behavior of photoexcited electron transfer systems. This work formally introduces the mathematical modifications to charge transfer theory necessary to account for the solvent structure and hydrodynamic effect and illustrates the results with model calculations. These calculations show that analysis of experiments with theories that do not include the rdf and hydrodynamic effects can result in significant errors in the interpretation of data. {copyright} {ital 1996 American Institute of Physics.}

Swallen, S.F.; Weidemaier, K.; Fayer, M.D. [Department of Chemistry, Stanford University, Stanford, California 94305 (United States)] [Department of Chemistry, Stanford University, Stanford, California 94305 (United States)

1996-02-01

155

Photoinitiated electron transfer in multichromophoric species: Synthetic tetrads and pentads. Technical progress report, 1989  

SciTech Connect

This research project involves the design, synthesis and study of 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).

Not Available

1989-04-12

156

Accumulative electron transfer: multiple charge separation in artificial photosynthesis.  

PubMed

To achieve artificial photosynthesis it is necessary to couple the single-electron event of photoinduced charge separation with the multi-electron reactions of fuel formation and water splitting. Therefore, several rounds of light-induced charge separation are required to accumulate enough redox equivalents at the catalytic sites for the target chemistry to occur, without any sacrificial donors or acceptors other than the catalytic substrates. Herein, we discuss the challenges of such accumulative electron transfer in molecular systems. We present a series of closely related systems base on a Ru(II)-polypyridine photosensitizer with appended triaryl-amine or oligo-triaryl-amine donors, linked to nanoporous TiO2 as the acceptor. One of the systems, based on dye 4, shows efficient accumulative electron transfer in high overall yield resulting in the formation of a two-electron charge-separated state upon successive excitation by two photons. In contrast, the other systems do not show accumulative electron transfer because of different competing reactions. This illustrates the difficulties in designing successful systems for this still largely unexplored type of reaction scheme. PMID:22470977

Karlsson, Susanne; Boixel, Julien; Pellegrin, Yann; Blart, Errol; Becker, Hans-Christian; Odobel, Fabrice; Hammarström, Leif

2012-01-01

157

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

SciTech Connect

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.

Lian, Tianquan

2014-04-22

158

CORRELATING ELECTRONIC AND VIBRATIONAL MOTIONS IN CHARGE TRANSFER SYSTEMS  

SciTech Connect

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.

Khalil, Munira

2014-06-27

159

LASER FORWARD TRANSFER OF ELECTRONIC AND POWER GENERATING MATERIALS  

E-print Network

written 3D pixels or voxels resulting from the laser forward #12;340 Laser Ablation and its Applications/or scanning and modulating the laser beam enables complex pattern formation. Commercially available, computerChapter 14 LASER FORWARD TRANSFER OF ELECTRONIC AND POWER GENERATING MATERIALS Alberto Piqué1

Arnold, Craig B.

160

Understanding the Electronic Structure of 4d Metal Complexes: From Molecular Spinors to L-Edge Spectra of a di-Ru Catalyst  

SciTech Connect

L{sub 2,3}-edge X-ray absorption spectroscopy (XAS) has demonstrated unique capabilities for the analysis of the electronic structure of di-Ru complexes such as the blue dimer cis,cis-[Ru{sub 2}{sup III}O(H{sub 2}O){sub 2}(bpy){sub 4}]{sup 4+} water oxidation catalyst. Spectra of the blue dimer and the monomeric [Ru(NH{sub 3}){sub 6}]{sup 3+} model complex show considerably different splitting of the Ru L{sub 2,3} absorption edge, which reflects changes in the relative energies of the Ru 4d orbitals caused by hybridization with a bridging ligand and spin-orbit coupling effects. To aid the interpretation of spectroscopic data, we developed a new approach, which computes L{sub 2,3}-edges XAS spectra as dipole transitions between molecular spinors of 4d transition metal complexes. This allows for careful inclusion of the spin-orbit coupling effects and the hybridization of the Ru 4d and ligand orbitals. The obtained theoretical Ru L{sub 2,3}-edge spectra are in close agreement with experiment. Critically, existing single-electron methods (FEFF, FDMNES) broadly used to simulate XAS could not reproduce the experimental Ru L-edge spectra for the [Ru(NH{sub 3}){sub 6}]{sup 3+} model complex nor for the blue dimer, while charge transfer multiplet (CTM) calculations were not applicable due to the complexity and low symmetry of the blue dimer water oxidation catalyst. We demonstrated that L-edge spectroscopy is informative for analysis of bridging metal complexes. The developed computational approach enhances L-edge spectroscopy as a tool for analysis of the electronic structures of complexes, materials, catalysts, and reactive intermediates with 4d transition metals.

Alperovich, Igor; Smolentsev, Grigory; Moonshiram, Dooshaye; Jurss, Jonah W.; Concepcion, Javier J.; Meyer, Thomas J.; Soldatov, Alexander; Pushkar, Yulia (SFU-Russia); (Purdue); (UNC); (LIT)

2011-12-09

161

Shewanella secretes flavins that mediate extracellular electron transfer.  

PubMed

Bacteria able to transfer electrons to metals are key agents in biogeochemical metal cycling, subsurface bioremediation, and corrosion processes. More recently, these bacteria have gained attention as the transfer of electrons from the cell surface to conductive materials can be used in multiple applications. In this work, we adapted electrochemical techniques to probe intact biofilms of Shewanella oneidensis MR-1 and Shewanella sp. MR-4 grown by using a poised electrode as an electron acceptor. This approach detected redox-active molecules within biofilms, which were involved in electron transfer to the electrode. A combination of methods identified a mixture of riboflavin and riboflavin-5'-phosphate in supernatants from biofilm reactors, with riboflavin representing the dominant component during sustained incubations (>72 h). Removal of riboflavin from biofilms reduced the rate of electron transfer to electrodes by >70%, consistent with a role as a soluble redox shuttle carrying electrons from the cell surface to external acceptors. Differential pulse voltammetry and cyclic voltammetry revealed a layer of flavins adsorbed to electrodes, even after soluble components were removed, especially in older biofilms. Riboflavin adsorbed quickly to other surfaces of geochemical interest, such as Fe(III) and Mn(IV) oxy(hydr)oxides. This in situ demonstration of flavin production, and sequestration at surfaces, requires the paradigm of soluble redox shuttles in geochemistry to be adjusted to include binding and modification of surfaces. Moreover, the known ability of isoalloxazine rings to act as metal chelators, along with their electron shuttling capacity, suggests that extracellular respiration of minerals by Shewanella is more complex than originally conceived. PMID:18316736

Marsili, Enrico; Baron, Daniel B; Shikhare, Indraneel D; Coursolle, Dan; Gralnick, Jeffrey A; Bond, Daniel R

2008-03-11

162

Excess Electron Transfer in DNA: Effect of Base Sequence and Proton Transfer Zhongli Cai, Xifeng Li, and Michael D. Sevilla*  

E-print Network

Excess Electron Transfer in DNA: Effect of Base Sequence and Proton Transfer Zhongli Cai, Xifeng LiVed: October 18, 2001; In Final Form: December 3, 2001 The effect of base sequence on excess electron transfer (ET) along the DNA "-way" is investigated in this work by use of various polynucleotide duplexes

Simons, Jack

163

Probing Atomic and Electronic Structure of Catalysts by Combination of In Situ and Ex Situ Chemical Imaging  

E-print Network

Probing Atomic and Electronic Structure of Catalysts by Combination of In Situ and Ex Situ Chemical Karim, Robert Dagle, Ja Hun Kwak, Janos Szanyi, Charles Peden Purpose Understand the structural capabilities that enable this understanding Directly relate structural information with the catalytic

164

Protein dynamics modulated electron transfer kinetics in early stage photosynthesis  

NASA Astrophysics Data System (ADS)

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.

Kundu, Prasanta; Dua, Arti

2013-01-01

165

Synthesis of imine and reduced imine compounds containing aromatic sulfonamide: use as catalyst for in situ generation of ruthenium catalysts in transfer hydrogenation of acetophenone derivatives.  

PubMed

Three imine and three reduced imine ligands containing aromatic sulfonamide (2-7) were isolated by a simple method and characterized by FT-IR, NMR, and elemental analysis. Meanwhile, the interaction of 2-7 ligands with [(p-cymene)RuCl2]2 was analyzed in situ by UV-vis spectrophotometer. The in situ generated catalytic system derived from N-(2-(benzylideneamino)phenyl)-2,4,6-trimethyl-benzenesulfonamides and N-(2-(benzylamino)phenyl)-2,4,6-trimethyl-benzenesulfonamides with [(p-cymene)RuCl2]2 was used as a catalyst in the transfer hydrogenation (TH) of p-substituted acetophenone derivatives. The catalytic systems displayed high activities, which increased in the order 7<4<5<6<1<2<3. The best activity for the TH of 4-chloroacetophenone was provided with the [(p-cymene)RuCl2]2/ligand (3) catalytic system (turnover frequency values: 720 h(-1) for 10 min on S/C: 500/1). PMID:24184620

Dayan, Serkan; Arslan, Fatma; Kayac?, Nilgün; Kalaycioglu, Nilgun Ozpozan

2014-01-01

166

Quantum simulation of nuclear rearrangement in electron transfer reactions  

PubMed Central

A quantum simulation scheme based on the path integral molecular dynamics technique has been used to calculate the effective activation energies associated with nuclear rearrangement in the electron transfer reactions Co(NH3)62+ + Co(NH3)63+ ? Co(NH3)63+ + Co(NH3)62+ and Ru(NH3)62+ + Ru(NH3)63+ ? Ru(NH3)63+ + Ru(NH3)62+. Even with a simple Hamiltonian and short time dynamic simulations, the results are in satisfactory agreement with other theoretical calculations. This simulation approach can be used in chemical and biological systems where the reactions are largely controlled by nuclear rearrangements, such as those of electron transfer reactions in some electron carrier proteins. PMID:16594063

Zheng, Chong; McCammon, J. Andrew; Wolynes, Peter G.

1989-01-01

167

Chemical dynamics of the first proton-coupled electron transfer of water oxidation on TiO2 anatase.  

PubMed

Titanium dioxide (TiO2) is a prototype, water-splitting (photo)catalyst, but its performance is limited by the large overpotential for the oxygen evolution reaction (OER). We report here a first-principles density functional theory study of the chemical dynamics of the first proton-coupled electron transfer (PCET), which is considered responsible for the large OER overpotential on TiO2. We use a periodic model of the TiO2/water interface that includes a slab of anatase TiO2 and explicit water molecules, sample the solvent configurations by first principles molecular dynamics, and determine the energy profiles of the two electronic states involved in the electron transfer (ET) by hybrid functional calculations. Our results suggest that the first PCET is sequential, with the ET following the proton transfer. The ET occurs via an inner sphere process, which is facilitated by a state in which one electronic hole is shared by the two oxygen ions involved in the transfer. PMID:24308541

Chen, Jia; Li, Ye-Fei; Sit, Patrick; Selloni, Annabella

2013-12-18

168

Regulating proton-coupled electron transfer for efficient water splitting by manganese oxides at neutral pH  

NASA Astrophysics Data System (ADS)

Manganese oxides have been extensively investigated as model systems for the oxygen-evolving complex of photosystem II. However, most bioinspired catalysts are inefficient at neutral pH and functional similarity to the oxygen-evolving complex has been rarely achieved with manganese. Here we report the regulation of proton-coupled electron transfer involved in water oxidation by manganese oxides. Pyridine and its derivatives, which have pKa values intermediate to the water ligand bound to manganese(II) and manganese(III), are used as proton-coupled electron transfer induction reagents. The induction of concerted proton-coupled electron transfer is demonstrated by the detection of deuterium kinetic isotope effects and compliance of the reactions with the libido rule. Although proton-coupled electron transfer regulation is essential for the facial redox change of manganese in photosystem II, most manganese oxides impair these regulatory mechanisms. Thus, the present findings may provide a new design rationale for functional analogues of the oxygen-evolving complex for efficient water splitting at neutral pH.

Yamaguchi, Akira; Inuzuka, Riko; Takashima, Toshihiro; Hayashi, Toru; Hashimoto, Kazuhito; Nakamura, Ryuhei

2014-06-01

169

Regulating proton-coupled electron transfer for efficient water splitting by manganese oxides at neutral pH  

PubMed Central

Manganese oxides have been extensively investigated as model systems for the oxygen-evolving complex of photosystem II. However, most bioinspired catalysts are inefficient at neutral pH and functional similarity to the oxygen-evolving complex has been rarely achieved with manganese. Here we report the regulation of proton-coupled electron transfer involved in water oxidation by manganese oxides. Pyridine and its derivatives, which have pKa values intermediate to the water ligand bound to manganese(II) and manganese(III), are used as proton-coupled electron transfer induction reagents. The induction of concerted proton-coupled electron transfer is demonstrated by the detection of deuterium kinetic isotope effects and compliance of the reactions with the libido rule. Although proton-coupled electron transfer regulation is essential for the facial redox change of manganese in photosystem II, most manganese oxides impair these regulatory mechanisms. Thus, the present findings may provide a new design rationale for functional analogues of the oxygen-evolving complex for efficient water splitting at neutral pH. PMID:24977746

Yamaguchi, Akira; Inuzuka, Riko; Takashima, Toshihiro; Hayashi, Toru; Hashimoto, Kazuhito; Nakamura, Ryuhei

2014-01-01

170

Syntrophic growth via quinone-mediated interspecies electron transfer.  

PubMed

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:25741332

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

2015-01-01

171

Syntrophic growth via quinone-mediated interspecies electron transfer  

PubMed Central

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:25741332

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

2015-01-01

172

Improving electronic structure methods to predict nano-optoelectronics and nano-catalyst functions.  

SciTech Connect

This report focuses on quantum chemistry and ab initio molecular dynamics (AIMD) calculations applied to elucidate the mechanism of the multi-step, 2-electron, electrochemical reduction of the green house gas molecule carbon dioxide (CO{sub 2}) to carbon monoxide (CO) in aqueous media. When combined with H{sub 2} gas to form synthesis ('syn') gas, CO becomes a key precursor to methane, methanol, and other useful hydrocarbon products. To elucidate the mechanism of this reaction, we apply computational electrochemistry which is a fledgling, important area of basic science critical to energy storage. This report highlights several approaches, including the calculation of redox potentials, the explicit depiction of liquid water environments using AIMD, and free energy methods. While costly, these pioneering calculations reveal the key role of hydration- and protonation-stabilization of reaction intermediates, and may inform the design of CO{sub 2}-capture materials as well as its electrochemical reduction. In the course of this work, we have also dealt with the challenges of identifying and applying electronic structure methods which are sufficiently accurate to deal with transition metal ion complex-based catalyst. Such electronic structure methods are also pertinent to the accurate modeling of actinide materials and therefore to nuclear energy research. Our multi-pronged effort towards achieving this titular goal of the LDRD is discussed.

Nielsen, Ida Marie B.; Marzari, Nicola (Massachusetts Institute of Technology); Shelnutt, John Allen; Kulik, Heather J. (Massachusetts Institute of Technology); Medforth, Craig John (University of New Mexico, Albuquerque, NM); Leung, Kevin

2009-10-01

173

Simulations of charge transfer in Electron Multiplying Charge Coupled Devices  

NASA Astrophysics Data System (ADS)

Electron Multiplying Charge Coupled Devices (EMCCDs) are a variant of traditional CCD technology well suited to applications that demand high speed operation in low light conditions. On-chip signal amplification allows the sensor to effectively suppress the noise introduced by readout electronics, permitting sub-electron read noise at MHz pixel rates. The devices have been the subject of many detailed studies concerning their operation, however there has not been a study into the transfer and multiplication process within the EMCCD gain register. Such an investigation has the potential to explain certain observed performance characteristics, as well as inform further optimisations to their operation. In this study, the results from simulation of charge transfer within an EMCCD gain register element are discussed with a specific focus on the implications for serial charge transfer efficiency (CTE). The effects of operating voltage and readout speed are explored in context with typical operating conditions. It is shown that during transfer, a small portion of signal charge may become trapped at the semiconductor-insulator interface that could act to degrade the serial CTE in certain operating conditions.

Bush, N.; Stefanov, K.; Hall, D.; Jordan, D.; Holland, A.

2014-12-01

174

Spectroscopic investigation of photo-induced proton-coupled electron transfer and Dexter energy transfer in model systems  

E-print Network

Spectroscopic investigations of systems designed to advance the mechanistic interrogation of photo-induced proton coupled electron transfer (PCET) and proton-coupled (through-bond) energy transfer (PCEnT) are presented. ...

Young, Elizabeth R. (Elizabeth Renee), 1980-

2009-01-01

175

Mechanistic studies of photo-induced proton-coupled electron transfer and oxygen atom transfer reactions in model systems  

E-print Network

Time-resolved optical spectroscopy has been employed for mechanistic studies in model systems designed to undergo photo-induced proton-coupled electron transfer (PCET) and oxygen atom transfer (OAT) reactions, both of which ...

Hodgkiss, Justin M. (Justin Mark), 1978-

2007-01-01

176

Electron transfer mechanisms of DNA repair by photolyase.  

PubMed

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. PMID:25830375

Zhong, Dongping

2015-04-01

177

In situ electron microscopy studies of the sintering of palladium nanoparticles on alumina during catalyst regeneration processes.  

PubMed

Sintering of a palladium catalyst supported on alumina (Al2O3) in an oxidizing environment was studied by in situ transmission electron microscopy (TEM). In the case of a fresh catalyst, sintering of Pd particles on an alumina surface in a 500 mTorr steam environment happened via traditional ripening or migration and coalescence mechanisms and was not significant unless heating above 500 degrees C. After the catalyst was used for the hydrogenation of alkynes, TEM coupled with convergent beam electron diffraction and electron energy loss spectroscopy analysis revealed that most of the Pd particles were lifted from the alumina surface by hydrocarbon buildup. This dramatically different morphology totally changed the sintering mechanism of Pd particles during the regeneration process. Catalytic gasification of hydrocarbon around these particles in an oxidizing environment allowed the Pd particles to move around and coalesce with each other at temperatures as low as 350 degrees C. For catalysts heating under 500 mTorr steam at 350 degrees C, steam stripped hydrocarbon catalytically at the beginning, but the reaction stopped after 4 h. Heating in air resulted in both catalytic and noncatalytic stripping of hydrocarbon. PMID:15306069

Liu, Rou-Jane; Crozier, Peter A; Smith, C Michael; Hucul, Dennis A; Blackson, John; Salaita, Ghaleb

2004-02-01

178

Alternating electron and proton transfer steps in photosynthetic water oxidation.  

PubMed

Water oxidation by cyanobacteria, algae, and plants is pivotal in oxygenic photosynthesis, the process that powers life on Earth, and is the paradigm for engineering solar fuel-production systems. Each complete reaction cycle of photosynthetic water oxidation requires the removal of four electrons and four protons from the catalytic site, a manganese-calcium complex and its protein environment in photosystem II. In time-resolved photothermal beam deflection experiments, we monitored apparent volume changes of the photosystem II protein associated with charge creation by light-induced electron transfer (contraction) and charge-compensating proton relocation (expansion). Two previously invisible proton removal steps were detected, thereby filling two gaps in the basic reaction-cycle model of photosynthetic water oxidation. In the S(2) ? S(3) transition of the classical S-state cycle, an intermediate is formed by deprotonation clearly before electron transfer to the oxidant (Y Z OX). The rate-determining elementary step (?, approximately 30 µs at 20?°C) in the long-distance proton relocation toward the protein-water interface is characterized by a high activation energy (E(a) = 0.46 ± 0.05 eV) and strong H/D kinetic isotope effect (approximately 6). The characteristics of a proton transfer step during the S(0) ? S(1) transition are similar (?, approximately 100 µs; E(a) = 0.34 ± 0.08 eV; kinetic isotope effect, approximately 3); however, the proton removal from the Mn complex proceeds after electron transfer to . By discovery of the transient formation of two further intermediate states in the reaction cycle of photosynthetic water oxidation, a temporal sequence of strictly alternating removal of electrons and protons from the catalytic site is established. PMID:22988080

Klauss, André; Haumann, Michael; Dau, Holger

2012-10-01

179

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

PubMed

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 (?) and the driving forces (?G°), 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 ? and ?G°. 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: ?coh and ?FC. The former characterizes how fast the electronic coupling varies with nuclear vibrations (which cause dephasing). The latter reflects the time taken by the system to leave the crossing region. In the framework of open quantum systems, ?FC is a short time approximation of the characteristic decoherence time of the electronic subsystem in interaction with its nuclear environment. The comparison of the respective values of ?coh and ?FC allows us to probe the occurrence of non-Condon effects. We use ab initio MD simulations to analyze how decoherence appears in several biological cofactors. We conclude that we cannot account for its order of magnitude by considering only the atoms or bonds directly concerned with the transfer. Decoherence results from contributions from all atoms of the system appearing with a time delay that increases with the distance from the primarily concerned atoms or bonds. The delay and magnitude of the contributions depend on the chemical nature of the system. Finally, we present recent developments based on constrained DFT for efficient and accurate evaluations of the electronic coupling in ab initio MD simulations. These are promising methods to study the subtle fluctuations of the electronic coupling and the mechanisms of electronic decoherence in biological systems. PMID:25730126

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

2015-04-21

180

Proton-Coupled Electron Transfer in Molecular Electrocatalysis: Theoretical Methods and Design Principles  

SciTech Connect

Molecular electrocatalysts play an essential role in a wide range of energy conversion processes. The objective of electrocatalyst design is to maximize the turnover frequency and minimize the overpotential for the overall catalytic cycle. Typically the catalytic cycle is dominated by key proton-coupled electron transfer (PCET) processes comprised of sequential or concerted electron transfer and proton transfer steps. A variety of theoretical methods have been developed to investigate the mechanisms, thermodynamics, and kinetics of PCET processes in electrocatalytic cycles. Electronic structure methods can be used to calculate the reduction potentials and pKa’s and to generate thermodynamic schemes, free energy reaction pathways, and Pourbaix diagrams, which indicate the most stable species at each pH and potential. These types of calculations have assisted in identifying the thermodynamically favorable mechanisms under specified experimental conditions, such as acid strength and overpotential. Such calculations have also revealed linear correlations among the thermodynamic properties, which can be used to predict the impact of modifying the ligand, substituents, or metal center. The role of non-innocent ligands, namely ligand protonation or reduction, has also been examined theoretically. In addition, the rate constants for electron and proton transfer reactions, as well as concerted PCET reactions, have been calculated to investigate the kinetics of molecular electrocatalysts. The concerted PCET mechanism is thought to lower the overpotential required for catalysis by avoiding high-energy intermediates. Rate constant calculations have revealed that the concerted mechanism involving intramolecular proton transfer will be favored by designing more flexible ligands that facilitate the proton donor-acceptor motion while also maintaining a sufficiently short equilibrium proton donor-acceptor distance. Overall, theoretical methods have assisted in the interpretation of experimental data and the design of more effective molecular electrocatalysts. The research on the Ni(P2N2)2 catalysts was supported as part of the 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.

Solis, Brian H.; Hammes-Schiffer, Sharon

2014-07-07

181

Atomic level study of water-gas shift catalysts via transmission electron microscopy and x-ray spectroscopy  

NASA Astrophysics Data System (ADS)

Water-gas shift (WGS), CO + H2O ? CO2 + H2 (DeltaH° = -41 kJ mol -1), is an industrially important reaction for the production of high purity hydrogen. Commercial Cu/ZnO/Al2O3 catalysts are employed to accelerate this reaction, yet these catalysts suffer from certain drawbacks, including costly regeneration processes and sulfur poisoning. Extensive research is focused on developing new catalysts to replace the current technology. Supported noble metals stand out as promising candidates, yet comprise intricate nanostructures complicating the understanding of their working mechanism. In this study, the structure of the supported Pt catalysts is explored by transmission electron microscopy and X-ray spectroscopy. The effect of the supporting phase and the use of secondary metals on the reaction kinetics is investigated. Structural heterogeneities are quantified and correlated with the kinetic descriptors of the catalysts to develop a fundamental understanding of the catalytic mechanism. The effect of the reaction environment on catalyst structure is examined by in-situ techniques. This study benefitted greatly from the use of model catalysts that provide a convenient medium for the atomic level characterization of nanostructures. Based on these studies, Pt supported on iron oxide nano islands deposited on inert spherical alumina exhibited 48 times higher WGS turnover rate (normalized by the total Pt surface area) than Pt supported on bulk iron oxide. The rate of aqueous phase glycerol reforming reaction of Pt supported on multiwall carbon nanotubes (MWCNT) is promoted by co-impregnating with cobalt. The synthesis resulted in a variety of nanostructures among which Pt-Co bimetallic nanoparticles are found to be responsible for the observed promotion. The unprecedented WGS rate of Pt supported on Mo2C is explored by forming Mo 2C patches on top of MWCNTs and the rate promotion is found to be caused by the Pt-Mo bimetallic entities.

Akatay, Mehmed Cem

182

Activation and Deactivation of a Robust Immobilized Cp*Ir-Transfer Hydrogenation Catalyst: A Multielement in Situ X-ray Absorption Spectroscopy Study.  

PubMed

A highly robust immobilized [Cp*IrCl2]2 precatalyst on Wang resin for transfer hydrogenation, which can be recycled up to 30 times, was studied using a novel combination of X-ray absorption spectroscopy (XAS) at Ir L3-edge, Cl K-edge, and K K-edge. These culminate in in situ XAS experiments that link structural changes of the Ir complex with its catalytic activity and its deactivation. Mercury poisoning and "hot filtration" experiments ruled out leached Ir as the active catalyst. Spectroscopic evidence indicates the exchange of one chloride ligand with an alkoxide to generate the active precatalyst. The exchange of the second chloride ligand, however, leads to a potassium alkoxide-iridate species as the deactivated form of this immobilized catalyst. These findings could be widely applicable to the many homogeneous transfer hydrogenation catalysts with Cp*IrCl substructure. PMID:25768298

Sherborne, Grant J; Chapman, Michael R; Blacker, A John; Bourne, Richard A; Chamberlain, Thomas W; Crossley, Benjamin D; Lucas, Stephanie J; McGowan, Patrick C; Newton, Mark A; Screen, Thomas E O; Thompson, Paul; Willans, Charlotte E; Nguyen, Bao N

2015-04-01

183

Real-time simulations of photoinduced coherent charge transfer and proton-coupled electron transfer.  

PubMed

Photoinduced electron transfer (ET) and proton-coupled electron transfer (PCET) are fundamental processes in natural phenomena, most noticeably in photosynthesis. Time-resolved spectroscopic evidence of coherent oscillatory behavior associated with these processes has been reported both in complex biological environments, as well as in biomimetic models for artificial photosynthesis. Here, we consider a few biomimetic models to investigate these processes in real-time simulations based on ab initio molecular dynamics and Ehrenfest dynamics. This allows for a detailed analysis on how photon-to-charge conversion is promoted by a coupling of the electronic excitation with specific vibrational modes and with proton displacements. The ET process shows a characteristic coherence that is linked to the nuclear motion at the interface between donor and acceptor. We also show real-time evidence of PCET in a benzimidazole-phenol redox relay. PMID:25224924

Eisenmayer, Thomas J; Buda, Francesco

2014-10-20

184

Membrane catalyst layer for fuel cells  

DOEpatents

A gas reaction fuel cell incorporates a thin catalyst layer between a solid polymer electrolyte (SPE) membrane and a porous electrode backing. The catalyst layer is preferably less than about 10 .mu.m in thickness with a carbon supported platinum catalyst loading less than about 0.35 mgPt/cm.sup.2. The film is formed as an ink that is spread and cured on a film release blank. The cured film is then transferred to the SPE membrane and hot pressed into the surface to form a catalyst layer having a controlled thickness and catalyst distribution. Alternatively, the catalyst layer is formed by applying a Na.sup.+ form of a perfluorosulfonate ionomer directly to the membrane, drying the film at a high temperature, and then converting the film back to the protonated form of the ionomer. The layer has adequate gas permeability so that cell performance is not affected and has a density and particle distribution effective to optimize proton access to the catalyst and electronic continuity for electron flow from the half-cell reaction occurring at the catalyst.

Wilson, Mahlon S. (Los Alamos, NM)

1993-01-01

185

Solvation-induced one-dimensional polarons and electron transfer.  

PubMed

When a one-dimensional (1D) semiconductor nanostructure is immersed in a sluggish polar solvent, fluctuations of the medium may result in the appearance of localized electronic levels inside the band gap. An excess charge carrier can occupy such a level and undergo self-localization into a large-radius adiabatic polaron surrounded by a self-consistent medium polarization pattern. Within an appropriately adapted framework of the Marcus theory, we explore the description and qualitative picture of thermally activated electron transfer involving solvation-induced polaroniclike states by considering transfer between small and 1D species as well as between two 1D species. Illustrative calculations are performed for tubular geometries with possible applications to carbon nanotube systems. PMID:20441283

Ussery, G L; Gartstein, Yu N

2010-04-28

186

Nile blue can photosensitize DNA damage through electron transfer.  

PubMed

The mechanism of DNA damage photosensitized by Nile blue (NB) was studied using (32)P-5'-end-labeled DNA fragments. NB bound to the DNA strand was possibly intercalated through an electrostatic interaction. Photoirradiated NB caused DNA cleavage at guanine residues when the DNA fragments were treated with piperidine. Consecutive guanines, the underlined G in 5'-GG and 5'-GGG, were selectively damaged through photoinduced electron transfer. The fluorescence lifetime of NB was decreased by guanine-containing DNA sequence, supporting this mechanism. Single guanines were also slightly damaged by photoexcited NB, and DNA photodamage by NB was slightly enhanced in D2O. These results suggest that the singlet oxygen mechanism also partly contributes to DNA photodamage by NB. DNA damage photosensitized by NB via electron transfer may be an important mechanism in medicinal applications of photosensitizers, such as photodynamic therapy in low oxygen. PMID:24576317

Hirakawa, Kazutaka; Ota, Kazuhiro; Hirayama, Junya; Oikawa, Shinji; Kawanishi, Shosuke

2014-04-21

187

Photophysical and electron transfer studies of a stable carbocation  

NASA Astrophysics Data System (ADS)

Photophysical and electron transfer properties of the stable trioxatriangulenium carbocation ( 1) are reported. Photophysical studies include absorption, fluorescence and phosphorescence spectra, singlet and triplet state quantum yields and lifetimes. Both the singlet and triplet excited states of 1 can accept an electron from donor molecules leading to the formation of the donor radical cation and the radical of 1. In aqueous solution, 1 can photo-oxidize DNA nucleosides such as guanosine and adenosine indicating that 1 may have potential use as a DNA cleaving agent.

Dileesh, S.; Gopidas, K. R.

2000-11-01

188

Polarization-transfer electron-Zeeman resolved EPR  

NASA Astrophysics Data System (ADS)

Polarization-transfer electron-Zeeman resolved (EZ) EPR is introduced. In this two-dimensional pulse EPR method, which is based on the stimulated-echo sequence, the magnetic field is changed during the pulse sequence by applying a sinusoidal field of variable amplitude. EZ-EPR can be used to improve the spectral resolution of disordered systems with g anisotropy or to separate overlapping spectra of sites with different g values in single-crystal EPR studies. A theoretical description of the method is given, and model calculations as well as experiments on disordered systems with anisotropic electron Zeeman interactions are presented.

Eichel, Rüdiger-A.; Schweiger, Arthur

2002-05-01

189

Macroscopic turbulent models for heat and mass transfer in catalyst reactors  

NASA Astrophysics Data System (ADS)

Tthis paper reports the development and validation of a Computational Fluid Dynamics (CFD) up-scaling approach to predict wit a porous media approximation the heat transfer efficiency of arbitrary randomly packed bed tubular reactors. The results demonstrate the feasibility to predict the flow, temperature and global heat transfer with macroscopic simulations with a good accuracy.

Mathey, Fabrice

2012-05-01

190

Intermittent Single-Molecule Interfacial Electron Transfer Dynamics  

SciTech Connect

We report on single molecule studies of photosensitized interfacial electron transfer (ET) processes in Coumarin 343 (C343)-TiO2 nanoparticle (NP) and Cresyl Violet (CV+)-TiO2 NP systems, using time-correlated single photon counting coupled with scanning confocal fluorescence microscopy. Fluorescence intensity trajectories of individual dye molecules adsorbed on a semiconductor NP surface showed fluorescence fluctuations and blinking, with time constrants distributed from sub-milliseconds to several seconds.

Biju, Vasudevan P.; Micic, Miodrag; Hu, Dehong; Lu, H. Peter

2004-08-04

191

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...

192

27 CFR 40.357 - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2010 CFR

...TOBACCO PRODUCTS, CIGARETTE PAPERS AND TUBES...TOBACCO Manufacture of Cigarette Papers and Tubes ...Payment of tax by electronic fund transfer. ...in making payment by electronic fund transfer (EFT...tobacco products, cigarette papers, and...

2010-04-01

193

27 CFR 40.357 - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2012 CFR

...TOBACCO PRODUCTS, CIGARETTE PAPERS AND TUBES...TOBACCO Manufacture of Cigarette Papers and Tubes ...Payment of tax by electronic fund transfer. ...in making payment by electronic fund transfer (EFT...tobacco products, cigarette papers, and...

2012-04-01

194

27 CFR 40.357 - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2014 CFR

...TOBACCO PRODUCTS, CIGARETTE PAPERS AND TUBES...TOBACCO Manufacture of Cigarette Papers and Tubes ...Payment of tax by electronic fund transfer. ...in making payment by electronic fund transfer (EFT...tobacco products, cigarette papers, and...

2014-04-01

195

27 CFR 40.357 - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2011 CFR

...TOBACCO PRODUCTS, CIGARETTE PAPERS AND TUBES...TOBACCO Manufacture of Cigarette Papers and Tubes ...Payment of tax by electronic fund transfer. ...in making payment by electronic fund transfer (EFT...tobacco products, cigarette papers, and...

2011-04-01

196

27 CFR 40.357 - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2013 CFR

...TOBACCO PRODUCTS, CIGARETTE PAPERS AND TUBES...TOBACCO Manufacture of Cigarette Papers and Tubes ...Payment of tax by electronic fund transfer. ...in making payment by electronic fund transfer (EFT...tobacco products, cigarette papers, and...

2013-04-01

197

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

Code of Federal Regulations, 2012 CFR

... A person that provides an electronic fund transfer service to...1) and (2), from two business days to four business days after the consumer learns...and this part with respect to electronic fund transfers initiated...

2012-01-01

198

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

Code of Federal Regulations, 2014 CFR

... A person that provides an electronic fund transfer service to...1) and (2), from two business days to four business days after the consumer learns...and this part with respect to electronic fund transfers initiated...

2014-01-01

199

12 CFR 1005.14 - Electronic fund transfer service provider not holding consumer's account.  

Code of Federal Regulations, 2013 CFR

... A person that provides an electronic fund transfer service to...1) and (2), from two business days to four business days after the consumer learns...and this part with respect to electronic fund transfers initiated...

2013-01-01

200

12 CFR 1005.14 - Electronic fund transfer service provider not holding consumer's account.  

Code of Federal Regulations, 2014 CFR

... A person that provides an electronic fund transfer service to...1) and (2), from two business days to four business days after the consumer learns...and this part with respect to electronic fund transfers initiated...

2014-01-01

201

12 CFR 1005.14 - Electronic fund transfer service provider not holding consumer's account.  

Code of Federal Regulations, 2012 CFR

... A person that provides an electronic fund transfer service to...1) and (2), from two business days to four business days after the consumer learns...and this part with respect to electronic fund transfers initiated...

2012-01-01

202

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

Code of Federal Regulations, 2013 CFR

... A person that provides an electronic fund transfer service to...1) and (2), from two business days to four business days after the consumer learns...and this part with respect to electronic fund transfers initiated...

2013-01-01

203

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

Code of Federal Regulations, 2010 CFR

... A person that provides an electronic fund transfer service to...1) and (2), from two business days to four business days after the consumer learns...and this part with respect to electronic fund transfers initiated...

2010-01-01

204

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

Code of Federal Regulations, 2011 CFR

... A person that provides an electronic fund transfer service to...1) and (2), from two business days to four business days after the consumer learns...and this part with respect to electronic fund transfers initiated...

2011-01-01

205

49 CFR 225.37 - Magnetic media transfer and electronic submission.  

Code of Federal Regulations, 2010 CFR

...2010-10-01 2010-10-01 false Magnetic media transfer and electronic submission. ...INVESTIGATIONS § 225.37 Magnetic media transfer and electronic submission. ...updates, and amendments by way of magnetic media (computer diskette or magnetic...

2010-10-01

206

Nanoparticle facilitated extracellular electron transfer in microbial fuel cells.  

PubMed

Microbial fuel cells (MFCs) have been the focus of substantial research interest due to their potential for long-term, renewable electrical power generation via the metabolism of a broad spectrum of organic substrates, although the low power densities have limited their applications to date. Here, we demonstrate the potential to improve the power extraction by exploiting biogenic inorganic nanoparticles to facilitate extracellular electron transfer in MFCs. Simultaneous short-circuit current recording and optical imaging on a nanotechnology-enabled platform showed substantial current increase from Shewanella PV-4 after the formation of cell/iron sulfide nanoparticle aggregates. Detailed characterization of the structure and composition of the cell/nanoparticle interface revealed crystalline iron sulfide nanoparticles in intimate contact with and uniformly coating the cell membrane. In addition, studies designed to address the fundamental mechanisms of charge transport in this hybrid system showed that charge transport only occurred in the presence of live Shewanella, and moreover demonstrated that the enhanced current output can be attributed to improved electron transfer at cell/electrode interface and through the cellular-networks. Our approach of interconnecting and electrically contacting bacterial cells through biogenic nanoparticles represents a unique and promising direction in MFC research and has the potential to not only advance our fundamental knowledge about electron transfer processes in these biological systems but also overcome a key limitation in MFCs by constructing an electrically connected, three-dimensional cell network from the bottom-up. PMID:25310721

Jiang, Xiaocheng; Hu, Jinsong; Lieber, Alexander M; Jackan, Charles S; Biffinger, Justin C; Fitzgerald, Lisa A; Ringeisen, Bradley R; Lieber, Charles M

2014-11-12

207

The Electron Transfer System of Syntrophically Grown Desulfovibrio vulgaris  

SciTech Connect

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.

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

208

The electron transfer system of syntrophically grown Desulfovibrio vulgaris  

SciTech Connect

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.

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

209

The electron transfer system of synthrophically grown desulfovibrio vulgaris  

SciTech Connect

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.

Walker, Christopher [University of Washington, Seattle; He, Zhili [University of Oklahoma; Yang, Zamin Koo [ORNL; Ringbauer, Joseph [University of Washington, Seattle; HE, Qiang [ORNL; Zhou, Jizhong [University of Oklahoma; Voordouw, Gerrit [University of Calgary, ALberta, Canada; Wall, Judy [University of Missouri, Columbia; Arkin, Adam [Lawrence Berkeley National Laboratory (LBNL); Hazen, Terry [Lawrence Berkeley National Laboratory (LBNL); Stolyar, Sergey [University of Washington; Stahl, David [University of Washington

2009-01-01

210

The role of reaction temperature and cracking catalyst characteristics in determining the relative rates of protolytic cracking, chain propagation, and hydrogen transfer  

SciTech Connect

The cracking of isobutane on USY zeolites with different unit cell size has been studied in the temperature range 400-500[degrees]C, using an experimental apparatus which makes it possible to follow the reaction at very short times on stream. By measuring product initial selectivities it has been found that protolytic cracking and bimolecular reactions take place on Broensted acid sites. In this way the contributions of bimolecular reactions involving hydride transfer have been separated from those responsible for chain transfer and those producing hydrogen transfer. Chain transfer accounts for the chain propagation in paraffin cracking, while hydrogen transfer produces the extra paraffin amounts obtained in these reactions. Hydrogen transfer reactions increase, but chain transfer reactions decrease when the unit cell size increases. From energetic considerations, the influence of the zeolite catalyst and reaction conditions on the controlling step in isobutane cracking can be suggested. 24 refs., 6 figs., 4 tabs.

Corma, A. (Universidad Politecnica de Valencia (Spain)); Miguel, P.J.; Orchilles, A.V. (Universitat de Valencia (Spain))

1994-01-01

211

Distinguishing between Dexter and Rapid Sequential Electron Transfer in Covalently Linked Donor-Acceptor Assemblies  

E-print Network

Distinguishing between Dexter and Rapid Sequential Electron Transfer in Covalently Linked Donor associated with the MnII 2 core allows for a definitive assignment of Dexter transfer as the dominant excited for electron transfer (G0 ET ) -0.1 eV), the increase in probability of Dexter transfer due to the closer

McCusker, James K.

212

27 CFR 24.272 - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2014 CFR

...institution to make an electronic fund transfer in the...later than the close of business on the last day for...institution to effect an electronic fund transfer message...before the close of business on the prescribed last...Payment of Tax by Electronic Fund Transfer....

2014-04-01

213

27 CFR 24.272 - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2012 CFR

...institution to make an electronic fund transfer in the...later than the close of business on the last day for...institution to effect an electronic fund transfer message...before the close of business on the prescribed last...Payment of Tax by Electronic Fund Transfer....

2012-04-01

214

27 CFR 24.272 - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2013 CFR

...institution to make an electronic fund transfer in the...later than the close of business on the last day for...institution to effect an electronic fund transfer message...before the close of business on the prescribed last...Payment of Tax by Electronic Fund Transfer....

2013-04-01

215

27 CFR 24.272 - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2010 CFR

...institution to make an electronic fund transfer in the...later than the close of business on the last day for...institution to effect an electronic fund transfer message...before the close of business on the prescribed last...Payment of Tax by Electronic Fund Transfer....

2010-04-01

216

27 CFR 24.272 - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2011 CFR

...institution to make an electronic fund transfer in the...later than the close of business on the last day for...institution to effect an electronic fund transfer message...before the close of business on the prescribed last...Payment of Tax by Electronic Fund Transfer....

2011-04-01

217

Electron and energy transfer in artificial photosynthesis supermolecules  

NASA Astrophysics Data System (ADS)

Photophysical properties of a variety of organic supermolecules were studied in order to elucidate structural and thermodynamic criteria pertinent to the design of artificial photosynthetic structures and molecular electronic devices. These supermolecules were designed using covalently linked porphyrins, carotenoids, C60 fullerenes, quinones, and aromatic imide chromophores. Time-resolved absorption and fluorescence spectroscopy on a femtosecond to millisecond time scale was used to investigate mechanisms of photoinduced electron transfer, singlet and triplet energy transfer, and radical pair dynamics. Several photophysical processes, previously found only in photosynthetic reaction centers, were observed and characterized for the first time in these artificial systems. Molecular dyads, triads, and a hexad were shown to form long-lived charge separated states with high quantum yields, thus efficiently converting electronic excited state energy into (electro) chemical energy. These systems demonstrate that favorable photophysical properties can be conveniently (and predictably) controlled by changes in the molecular structure and the physical characteristics of the medium, such as the solvent polarity or temperature. Molecules studied in this work could be utilized as components of functional supramolecular assemblies. C60 fullerenes were shown to be preferable electron acceptors in molecular structures intended to function in a non-polar media or at low temperatures. This was attributed to the low intramolecular and solvent reorganization energies for C60 fullerenes. The magnetic field dependence of charge recombination in linked organic biradicals was studied. This effect can be used as the basis for a magnetically controlled molecular optical or optoelectronic switch (AND gate).

Kuciauskas, Darius

1999-12-01

218

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

NASA Astrophysics Data System (ADS)

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.

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

2006-05-01

219

45 CFR 162.1603 - Operating rules for health care electronic funds transfers (EFT) and remittance advice transaction.  

Code of Federal Regulations, 2012 CFR

...REQUIREMENTS Health Care Electronic Funds Transfers (EFT) and...Operating rules for health care electronic funds transfers (EFT) and...following for the health care electronic funds transfers (EFT) and...Combinations for CORE-defined Business Scenarios for the...

2012-10-01

220

45 CFR 162.1603 - Operating rules for health care electronic funds transfers (EFT) and remittance advice transaction.  

Code of Federal Regulations, 2014 CFR

...REQUIREMENTS Health Care Electronic Funds Transfers (EFT) and...Operating rules for health care electronic funds transfers (EFT) and...following for the health care electronic funds transfers (EFT) and...Combinations for CORE-defined Business Scenarios for the...

2014-10-01

221

45 CFR 162.1603 - Operating rules for health care electronic funds transfers (EFT) and remittance advice transaction.  

Code of Federal Regulations, 2013 CFR

...REQUIREMENTS Health Care Electronic Funds Transfers (EFT) and...Operating rules for health care electronic funds transfers (EFT) and...following for the health care electronic funds transfers (EFT) and...Combinations for CORE-defined Business Scenarios for the...

2013-10-01

222

Sensitization of ultra-long-range excited-state electron transfer by energy transfer in a polymerized film  

PubMed Central

Distance-dependent energy transfer occurs from the Metal-to-Ligand Charge Transfer (MLCT) excited state to an anthracene-acrylate derivative (Acr-An) incorporated into the polymer network of a semirigid poly(ethyleneglycol)dimethacrylate monolith. Following excitation, to Acr-An triplet energy transfer occurs followed by long-range, Acr-3An—Acr-An ? Acr-An—Acr-3An, energy migration. With methyl viologen dication (MV2+) added as a trap, Acr-3An + MV2+ ? Acr-An+ + MV+ electron transfer results in sensitized electron transfer quenching over a distance of approximately 90 ?. PMID:22949698

Ito, Akitaka; Stewart, David J.; Fang, Zhen; Brennaman, M. Kyle; Meyer, Thomas J.

2012-01-01

223

Reduced density matrix hybrid approach: Application to electronic energy transfer  

SciTech Connect

Electronic energy transfer in the condensed phase, such as that occurring in photosynthetic complexes, frequently occurs in regimes where the energy scales of the system and environment are similar. This situation provides a challenge to theoretical investigation since most approaches are accurate only when a certain energetic parameter is small compared to others in the problem. Here we show that in these difficult regimes, the Ehrenfest approach provides a good starting point for a dynamical description of the energy transfer process due to its ability to accurately treat coupling to slow environmental modes. To further improve on the accuracy of the Ehrenfest approach, we use our reduced density matrix hybrid framework to treat the faster environmental modes quantum mechanically, at the level of a perturbative master equation. This combined approach is shown to provide an efficient and quantitative description of electronic energy transfer in a model dimer and the Fenna-Matthews-Olson complex and is used to investigate the effect of environmental preparation on the resulting dynamics.

Berkelbach, Timothy C.; Reichman, David R. [Department of Chemistry, Columbia University, 3000 Broadway, New York, New York 10027 (United States); Markland, Thomas E. [Department of Chemistry, Stanford University, 333 Campus Drive, Stanford, California 94305 (United States)

2012-02-28

224

Thin layer of carbon-nano-fibers (CNFs) as catalyst support for fast mass transfer in hydrogenation of nitrite  

Microsoft Academic Search

CNF-foam ‘hairy foam’ and CNF aggregates supported Pd catalysts were studied for the reduction of aqueous nitrite solution and also compared with conventional catalysts. Relatively large Pd particle size and similar Pd particle size distribution on all the catalyst supports excludes any structure sensitive effects on reaction. Intrinsic rates over hairy foam catalysts were independent of CNF-layer thickness (8–28?m) and

Jitendra Kumar Chinthaginjala; Johannes Hendrik Bitter; Leon Lefferts

2010-01-01

225

Electronically conducting proton exchange polymers as catalyst supports for proton exchange membrane fuel cells. Electrocatalysis of oxygen reduction, hydrogen oxidation, and methanol oxidation  

SciTech Connect

A variety of supported catalysts were prepared by the chemical deposition of Pt and Pt-Ru particles on chemically prepared poly(3,4-ethylenedioxythiophene)/poly(styrene-4-sulfonate) (PEDOT/PSS) and PEDOT/polyvinylsulfate (PVS) composites. The polymer particles were designed to provide a porous, proton-conducting and electron-conducting catalyst support for use in fuel cells. These polymer-supported catalysts were characterized by electron microscopy, impedance spectroscopy, cyclic voltammetry, and conductivity measurements. Their catalytic activities toward hydrogen and methanol oxidation and oxygen reduction were evaluated in proton exchange membrane fuel-cell-type gas diffusion electrodes. Activities for oxygen reduction comparable to that obtained with a commercial carbon-supported catalyst were observed, whereas those for hydrogen and methanol oxidation were significantly inferior, although still high for prototype catalysts.

Lefebvre, M.C.; Qi, Z.; Pickup, P.G. [Memorial Univ. of Newfoundland, St. John`s, Newfoundland (Canada). Dept. of Chemistry] [Memorial Univ. of Newfoundland, St. John`s, Newfoundland (Canada). Dept. of Chemistry

1999-06-01

226

Ultrafast intramolecular electron transfer from a ferrocene donor moiety to a nile blue acceptor  

NASA Astrophysics Data System (ADS)

The fastest electron transfer processes between molecular donor/acceptor systems have been observed for solvent to solute type reactions, where the number of solvent molecules participating could be high. This might explain the very short electron transfer times of ?100 fs observed. Here the photo-induced electron transfer in a bridged donor/acceptor system was studied by femtosecond pump-probe spectroscopy with a time resolution of 15 fs. After photoexcitation of the acceptor (nile blue) an electron is transferred from the donor (ferrocene) with a time constant of 90 fs, i.e. close to fastest electron transfer times in the solute/solvent systems.

Baigar, E.; Gilch, P.; Zinth, W.; Stöckl, M.; Härter, P.; von Feilitzsch, T.; Michel-Beyerle, M. E.

2002-01-01

227

Modeling biofilms with dual extracellular electron transfer mechanisms  

SciTech Connect

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.

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

2013-11-28

228

Single Electron Transfer Living Radical Polymerization via a New Initiator  

NASA Astrophysics Data System (ADS)

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. ?-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.

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

2014-08-01

229

Nature of the Energy Landscape for Gated Electron Transfer in a Dynamic Redox Protein  

E-print Network

Nature of the Energy Landscape for Gated Electron Transfer in a Dynamic Redox Protein Sam Hay electron transfer (ET) reactions in biology, but we lack general insight into the extent of conformational electron-electron double resonance (ELDOR) studies of the diradical (disemiquinoid) form of human

230

Photoinduced electron transfer and geminate recombination on a micelle surface: Analytical theory and Monte Carlo simulations  

E-print Network

Photoinduced electron transfer and geminate recombination on a micelle surface: Analytical theory analysis of photoinduced electron transfer and geminate recombination on the surface of a spherical micelle for the electron, and when geminate recombination is included in the dynamical problem. For photoinduced electron

Fayer, Michael D.

231

Mechanisms of bridge-mediated electron transfer: a TDDFT electronic dynamics study.  

PubMed

We present a time-dependent density functional theory approach for probing the dynamics of electron transfer on a donor-bridge-acceptor polyene dye scaffold. Two kinds of mechanisms, namely, the superexchange mechanism and the sequential mechanism, may be involved in the electron transfer process. In this work, we have focused on the crossover between these two charge transfer mechanisms on a series of donor-bridge-acceptor polyene dye systems with varying lengths of conjugated bridges. A number of methods and quantities are used to assist in the analysis, including the phase relationship of charge evolution and frequency domain spectra of the time-dependent dipole. Our simulations show that the superexchange mechanism plays a dominant role in the electron transfer from donor to acceptor when the bridge length is small, and the sequential mechanism becomes more important as the polyene bridge is lengthened. Full Ehrenfest dynamics with nuclear motion show that molecular vibrations play a very small role in such ultrafast charge transfer processes. PMID:23249049

Ding, Feizhi; Chapman, Craig T; Liang, Wenkel; Li, Xiaosong

2012-12-14

232

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

SciTech Connect

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.

Spears, K.G.

1993-09-08

233

Magnetic Silica-Supported Ruthenium Nanoparticles: An Efficient Catalyst for Transfer Hydrogenation of Carbonyl Compounds  

EPA Science Inventory

One-pot synthesis of ruthenium nanoparticles on magnetic silica is described which involve the in situ generation of magnetic silica (Fe3O4@ SiO2) and ruthenium nano particles immobilization; the hydration of nitriles and transfer hydrogenation of carbonyl compounds occurs in hi...

234

Transcriptomic and Genetic Analysis of Direct Interspecies Electron Transfer  

PubMed Central

The possibility that metatranscriptomic analysis could distinguish between direct interspecies electron transfer (DIET) and H2 interspecies transfer (HIT) in anaerobic communities was investigated by comparing gene transcript abundance in cocultures in which Geobacter sulfurreducens was the electron-accepting partner for either Geobacter metallireducens, which performs DIET, or Pelobacter carbinolicus, which relies on HIT. Transcript abundance for G. sulfurreducens uptake hydrogenase genes was 7-fold lower in cocultures with G. metallireducens than in cocultures with P. carbinolicus, consistent with DIET and HIT, respectively, in the two cocultures. Transcript abundance for the pilus-associated cytochrome OmcS, which is essential for DIET but not for HIT, was 240-fold higher in the cocultures with G. metallireducens than in cocultures with P. carbinolicus. The pilin gene pilA was moderately expressed despite a mutation that might be expected to repress pilA expression. Lower transcript abundance for G. sulfurreducens genes associated with acetate metabolism in the cocultures with P. carbinolicus was consistent with the repression of these genes by H2 during HIT. Genes for the biogenesis of pili and flagella and several c-type cytochrome genes were among the most highly expressed in G. metallireducens. Mutant strains that lacked the ability to produce pili, flagella, or the outer surface c-type cytochrome encoded by Gmet_2896 were not able to form cocultures with G. sulfurreducens. These results demonstrate that there are unique gene expression patterns that distinguish DIET from HIT and suggest that metatranscriptomics may be a promising route to investigate interspecies electron transfer pathways in more-complex environments. PMID:23377933

Rotaru, Amelia-Elena; Summers, Zarath M.; Shrestha, Minita; Liu, Fanghua; Lovley, Derek R.

2013-01-01

235

Importance of Charge Transfer Excitations in DNA Electron Spectrum:  

NASA Astrophysics Data System (ADS)

Electron spectra of DNA model compounds, adenosine-thymidine and guanosine-cytidine nucleoside base pairs, as well as the relevant homogeneous stacked base pair steps in A-DNA and B-DNA conformations, were investigated using ZINDO semiempirical quantum-chemical method. This work confirms that, in DNA with intact Watson-Crick hydrogen bonding and base stacking, the highest occupied molecular orbitals (HOMO) are residing on purine base residues, whereas the lowest unoccupied molecular orbitals (LUMO) — on pyrimidine base residues. In general, the present results are satisfactorily comparable with the available experimental data. The role of charge transfer excitations in the polymer DNA 260 nm spectral band is discussed.

Starikov, E. B.

236

Light induced electron transfer reactions of metal complexes  

SciTech Connect

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.

Sutin, N.; Creutz, C.

1980-01-01

237

ATP-induced electron transfer by redox-selective partner recognition  

NASA Astrophysics Data System (ADS)

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.

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

2014-08-01

238

Molecular structures of porphyrin-quinone models for electron transfer  

SciTech Connect

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.

Fajer, J.; Barkigia, K.M.; Melamed, D.; Sweet, R.M. [Brookhaven National Lab., Upton, NY (United States)] [Brookhaven National Lab., Upton, NY (United States); Kurreck, H.; Gersdorff, J. von; Plato, M.; Rohland, H.C.; Elger, G.; Moebius, K. [Free Univ. of Berlin (Germany)] [Free Univ. of Berlin (Germany)

1996-08-15

239

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

PubMed

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. PMID:23181299

Khan, Yaser; Brumer, Paul

2012-11-21

240

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

NASA Astrophysics Data System (ADS)

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.

Khan, Yaser; Brumer, Paul

2012-11-01

241

Biochemical Mechanisms Controlling Terminal Electron Transfer in Geobacter sulfurreducens  

NASA Astrophysics Data System (ADS)

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, it is also possible that differential protein expression occurs when G. sulfurreducens is grown with insoluble iron versus insoluble iron. These initial results indicate that G. sulfurreducens allocates energy to unique cellular functions depending on the type of TEA used, suggesting that novel mechanisms are used to enable use of various metal forms for respiration. Follow-up protein expression studies were then conducted and are now being used to begin to delineate what biochemical mechanisms and cellular pathways are involved in these processes.

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

2009-04-01

242

Tetrathiomolybdate Modified Au Electrodes: Convenient Tuning of the kinetics of Electron Transfer and its application in Electrocatalysis  

E-print Network

Ammonium tetrathiomolybdate (ATM) spontaneously self assembles on Au electrodes forming a hydrophilic, air stable, pH (3-11) tolerant multilayer that is stable over a reasonably large potential window. The ATM functionalized Au electrodes can adsorb iron porphyrin catalysts and act as O2 reducing electrodes. These electrodes are stable enough to perform rotating disk electrochemistry (RDE) as well as rotating ring disk electrochemistry (RRDE) experiments. The X-ray photoelectron spectroscopy (XPS) data indicate that the sulphide atoms of ATM anchors a single ATM layer on to Au and the subsequent layers grow vertically due to the presence of hydrogen bonding NH4+ counter-ions. The formation and growth of these ATM adlayers is investigated using atomic force microscopy (AFM), scanning electron microscopy (SEM) and a series of electrochemical data. The ATM functionalized Au electrodes have double layer capacitance comparable to those reported for Au electrode bearing short chain alkyl thiol self assembled monolayer (SAM). Importantly, the rate of interfacial charge transfer (CT) can be tuned by controlling the thickness of the adlayers by simply adjusting the deposition time. Importantly the kinetics of a catalyst adsorbed on this ATM adlayer can be switched from mass transfer limited to CT limited regime by adjusting the deposition time.

Sudipta Chatterjee; Kushal Sengupta; Abhishek Dey

2014-09-10

243

Nitrate storage behavior of Ba/MnOx-CeO2 catalyst and its activity for soot oxidation with heat transfer limitations.  

PubMed

A BaMnCe ternary catalyst was prepared by impregnating barium acetate on MnO(x)-CeO(2) mixed oxides, with the monoxide supported catalysts and the solid solution support as references. The activities of the catalysts for soot oxidation were evaluated in the presence of NO under an energy transference controlled regime. BaMnCe presented the lowest maximal soot oxidation rate temperature at 393 degrees C among the catalysts investigated. Although BaMnCe experienced a loss in the specific surface area and low-temperature redox property due to blocking of the support pores by barium carbonate, its superior soot oxidation activity highlighted the importance of relatively stable bidentate/monodentate nitrates coordinated to Mn(x+) and Ce(x+) sites and more stable ionic barium nitrate. About half of the nitrates stored on this catalyst decomposed within the temperature interval of 350-450 degrees C, and the ignition temperature of soot decreased significantly with involvement of the nitrates or NO(2) released. PMID:20538410

Wu, Xiaodong; Liu, Shuang; Lin, Fan; Weng, Duan

2010-09-15

244

Catalysts for electrochemical generation of oxygen  

NASA Technical Reports Server (NTRS)

Single crystal surfaces of platinum and gold and transition metal oxides of the spinel type were studied to find more effective catalysts for the electrolytic evolution of oxygen and to understand the mechanism and kinetics for the electrocatalysis in relation to the surface electronic and lattice properties of the catalyst. The single crystal studies involve the use of low energy electron diffraction (LEED) and Auger electron spectroscopy as complementary tools to the electrochemical measurements. Modifications to the transfer system and to the thin-layer electrochemical cell used to facilitate the transfer between the ultrahigh vacuum environment of the electron surface physics equipment and the electrochemical environment with a minimal possibility of changes in the surface structure, are described. The electrosorption underpotential deposition of Pb onto the Au(111), (100) and (110) single crystal surfaces with the thin-layer cell-LEED-Auger system is discussed as well as the synthesis of spinels for oxygen evolution studies.

Hagans, P.; Yeager, E.

1978-01-01

245

Photoinduced electron transfer in rhenium(I)-oligotriarylamine molecules.  

PubMed

Two molecular triads with an oligotriarylamine multielectron donor were synthesized and investigated with a view to obtaining charge-separated states in which the oligotriarylamine is oxidized 2-fold. Such photoinduced accumulation of multiple redox equivalents is of interest for artificial photosynthesis. The first triad was comprised of the oligotriarylamine and two rhenium(I) tricarbonyl diimine photosensitizers each of which can potentially accept one electron. In the second triad the oligotriarylamine was connected to anthraquinone, in principle an acceptor of two electrons, via a rhenium(I) tricarbonyl diimine unit. With nanosecond transient absorption spectroscopy (using an ordinary pump-probe technique) no evidence for the generation of 2-fold oxidized oligotriarylamine or 2-fold reduced anthraquinone was found. The key factors limiting the photochemistry of the new triads to simple charge separation of one electron and one hole are discussed, and the insights gained from this study are useful for further research in the area of charge accumulation in purely molecular (nanoparticle-free) systems. An important problem of the rhenium-based systems considered here is the short wavelength required for photoexcitation. In the second triad, photogenerated anthraquinone monoanion is protonated by organic acids, and the resulting semiquinone species leads to an increase in lifetime of the charge-separated state by about an order of magnitude. This shows that the proton-coupled electron transfer chemistry of quinones could be beneficial for photoinduced charge accumulation. PMID:25271567

Bonn, Annabell G; Neuburger, Markus; Wenger, Oliver S

2014-10-20

246

Between a Rock and a Hard Place: Geomicrobial Electron Transfer  

SciTech Connect

The success of microbial life on Earth can largely be attributed to their collective ability to squeeze energy from a broad range of electron donor and acceptor couples including many where the electrochemical potential between the couples is marginally favorable (i.e, small ?G). Many of the redox couples that microbiologists employ to cultivate their favorite organism(s) involve compounds that are relatively water soluble, such as glucose and O2, and can readily diffuse to and from cells. In contrast, many organic and inorganic substrates, either electron donors or acceptors, exist as solids that are poorly soluble or insoluble. Certain heterotrophic microorganisms have overcome this problem by secreting enzymes outside the cell that can break down insoluble polymers, such as chitin and cellulose, to soluble subunits that are readily accessed by cells. Over the past few decades an increasing number of microorganisms have been isolated and studied that are capable of utilizing transition metal ions such as Fe and Mn as electron acceptors or donors. In their most oxidized form Fe(III) and Mn(IV) exist predominantly as metal oxides of varying morphology and composition but have in common low solubility in neutral pH environments and in the absence of complexing ligands. What clever mechanisms have microorganisms evolved to facilitate electron transfer to and from metal ions that are sequestered in minerals?

Fredrickson, Jim K.

2005-03-10

247

Determination of the electronics transfer function for current transient measurements  

E-print Network

We describe a straight-forward method for determining the transfer function of the readout of a sensor for the situation in which the current transient of the sensor can be precisely simulated. The method relies on the convolution theorem of Fourier transforms. The specific example is a planar silicon pad diode connected with a 50 $\\Omega $ cable to an amplifier followed by a 5 GS/s sampling oscilloscope. The charge carriers in the sensor were produced by picosecond lasers with light of wavelengths of 675 and 1060 nm. The transfer function is determined from the 1060 nm data with the pad diode biased at 1000 V. It is shown that the simulated sensor response convoluted with this transfer function provides an excellent description of the measured transients for the laser light of both wavelengths, at voltages 50 V above the depletion voltage of about 90 V up to the maximum applied voltage of 1000 V. The method has been developed for the precise measurement of the dependence of the drift velocity of electrons and holes in high-ohmic silicon on crystal orientation, electric field and temperature. It can also be applied for the analysis of transient-current measurements of radiation-damaged solid state sensors, as long as sensors properties, like high-frequency capacitance, are not too different.

Christian Scharf; Robert Klanner

2014-10-02

248

Photoinduced electron transfer probes for the observation of enzyme activities  

NASA Astrophysics Data System (ADS)

Enzymes engage key roles in a wide variety of important physical and medical processes, which thus can be altered by manipulating the behavior of enzymes in charge. The capability for manipulation requires an exact understanding of enzymatic operation modes though, which can be increased by employing fluorescence spectroscopy techniques. To date several fluorescence-based assays using labeled substrates have been developed to examine different subclasses of hydrolases. We developed a method that circumvents the unspecific probe enzyme interactions and affinity problems occurring in common probes as those based on fluorescence resonance energy transfer (FRET) by taking advantage of the comparably strong electron donating properties of the naturally occurring nucleic acid guanosine (G). Combined with an appropriate fluorophore this compound shows efficient photoinduced electron transfer (PET) quenching reactions only upon contact formation. Thus, initially quenched enzyme substrates, e.g. specific nucleic acid sequences, can be designed that cause a distinct increase in fluorescence signal upon specific hydrolysis. Here we demonstrate the general validity of PET probes for the observation of various nucleases at the ensemble and single molecule level. The rapid response time of the probes enables real-time monitoring of enzyme activities and provides quantitative data which are compared to those of commonly available and recently published, more complex probes. Additionally the applicability of this method is demonstrated for peptidases via fluorophore tryptophan (Trp) interaction.

Henkenjohann, S.; Sauer, M.

2009-02-01

249

Concerted Proton-Electron Transfers: Fundamentals and Recent Developments  

NASA Astrophysics Data System (ADS)

Proton-coupled electron transfers (PCET) are ubiquitous in natural and synthetic processes. This review focuses on reactions where the two events are concerted. Semiclassical models of such reactions allow their kinetic characterization through activation versus driving force relationships, estimates of reorganization energies, effects of the nature of the proton acceptor, and H/D kinetic isotope effect as well as their discrimination from stepwise pathways. Several homogeneous reactions (through stopped-flow and laser flash-quench techniques) and electrochemical processes are discussed in this framework. Once the way has been rid of the improper notion of pH-dependent driving force, water appears as a remarkable proton acceptor in terms of reorganization energy and pre-exponential factor, thanks to its H-bonded and H-bonding properties, similarly to purposely synthesized “H-bond train” molecules. The most recent developments are in modeling and description of emblematic concerted proton-electron transfer (CPET) reactions associated with the breaking of a heavy-atom bond in an all-concerted process.

Savéant, Jean-Michel

2014-06-01

250

Charge transfer emission in coumarin 343 sensitized TiO{sub 2} nanoparticle: A direct measurement of back electron transfer  

SciTech Connect

Electron injection and back electron transfer dynamics in coumarin 343 (C-343) adsorbed on TiO{sub 2} nanoparticles are studied by picosecond transient absorption and time-resolved fluorescence spectroscopy. The direct detection of electrons in the nanoparticles and the parent cation are monitored using picosecond transient absorption spectroscopy, and the corresponding dynamics of the adsorbate are monitored by time-resolved absorption spectra of the cation radical of C-343 in the visible region. When the electron returns from the nanoparticles to the present cation, a low quantum yield red-shifted charge transfer emission is observed. Measuring the charge transfer emission lifetimes by a picosecond time-resolved fluorimeter, the author gets an exact rate of back electron transfer reaction from the nanoparticle to the parent cation.

Ghosh, H.N.

1999-11-25

251

Pore structure modification of diatomite as sulfuric acid catalyst support by high energy electron beam irradiation and hydrothermal treatment  

NASA Astrophysics Data System (ADS)

High energy electron beam (HEEB) irradiation and hydrothermal treatment (HT), were applied in order to remove the impurities and enlarge the pore size of diatomite, making diatomite more suitable to be a catalyst support. The results demonstrated that, through thermal, charge, impact and etching effects, HEEB irradiation could make the impurities in the pores of diatomite loose and remove some of them. Then HT could remove rest of them from the pores and contribute significantly to the modification of the pore size distribution of diatomite due to thermal expansion, water swelling and thermolysis effects. Moreover, the pore structure modification improved the properties (BET (Brunauer-Emmett-Teller) specific surface area, bulk density and pore volume) of diatomite and the catalytic efficiency of the catalyst prepared from the treated diatomite.

Li, Chong; Zhang, Guilong; Wang, Min; Chen, Jianfeng; Cai, Dongqing; Wu, Zhengyan

2014-08-01

252

Transfer printing methods for the fabrication of flexible organic electronics  

NASA Astrophysics Data System (ADS)

A transfer printing method for fabricating organic electronics onto flexible substrates has been developed. The method relies primarily on differential adhesion for the transfer of a printable layer from a transfer substrate to a device substrate. The works of adhesion and cohesion for successful printing are discussed and developed for a model organic thin-film transistor device consisting of a polyethylene terephthalate (PET) substrate, gold (Au) gate and source/drain electrodes, a polymethylmethacrylate (PMMA) [or poly(4-vinylphenol)] dielectric layer, and a pentacene (Pn) organic semiconductor layer. The device components are sequentially printed onto the PET device substrate with no mixed processing steps performed on the device substrate. Optimum printing conditions for the Pn layer were determined to be 600psi and 120°C for 3min. A set of devices with a PMMA dielectric layer was measured as a function of channel length and exhibited a contact resistance corrected mobility of 0.237cm2/Vs. This is larger than the mobility measured for a control device consisting of Pn thermally deposited onto the thermally oxidized surface of a silicon substrate (SiO2/Si) with e-beam deposited Au top source/drain contacts. The structure of transfer printed Pn films was also investigated using x-ray diffraction. The basal spacing correlation length for a 50nm Pn film printed at 600psi and 120°C for 3min onto a PMMA surface showed a 35% increase as compared to an unprinted film on a thermally oxidized silicon substrate. The crystalline size was seen to correlate with the mobility as a function of printing conditions.

Hines, D. R.; Ballarotto, V. W.; Williams, E. D.; Shao, Y.; Solin, S. A.

2007-01-01

253

Theoretical study on a corrole-azafullerene dyad: Electronic structure, spectra and photoinduced electron transfer  

NASA Astrophysics Data System (ADS)

Density Functional Theory and Time-Dependent Density Functional Theory calculations have been carried out on a recently synthesized amino-corrole and a corrole-azafullerene dyad which exhibits photoinduced electron transfer (PET). Good agreement of the theoretical results with experiment is obtained regarding the absorption and emission spectra of the corrole, the absorption spectra of the corrole-azafullerene dyad and the transient anionic and cationic radicals of azafullerene and corrole respectively. Application of Mulliken's theory for charge-transfer states yields the excitation energy of the charge-separated state of the dyad very close to the S1 excitation of amino-corrole, consistent with a PET process.

Petsalakis, Ioannis D.; Theodorakopoulos, Giannoula

2014-08-01

254

Influence of chain-transfer reactions on the molecular-weight-distribution function of diene rubber on a neodymium-containing catalyst system  

NASA Astrophysics Data System (ADS)

The numerical solution of a system of differential equations for the concentrations of the reaction system’s macromolecules, which describe the synthesis of diene rubber on a neodymium-containing catalyst system, has been obtained. The influence of the chain-transfer reactions on the number distributions of the monomer links of macromolecules with an assigned number of active sites has theoretically been investigated.

Aminova, G. A.; Manuiko, G. V.; Bronskaya, V. V.; Ignashina, T. V.; D'Yakonov, G. S.; Bashkirov, D. V.; Demidova, É. V.

2008-11-01

255

Interfacial Electron Transfer and Transient Photoconductivity Studied with Terahertz Spectroscopy  

NASA Astrophysics Data System (ADS)

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 the porphyrin ring and degree of fluorination of ring substituents were analyzed. Due to the high reduction potentials of these sensitizers, injection into TiO2 was generally not observed. Injection timescales from the porphyrins into SnO2 depended strongly on the identity of the central substituent and were affected by competition with excited-state deactivation processes. The carboxylate anchoring group is commonly used to bind DSSC sensitizers to metal oxide surfaces but is typically not stable under the aqueous and oxidative conditions required for water oxidation. Electron injection efficiency and water stability of several alternative anchoring groups, including phosphonic acid, hydroxamic acid, acerylacetone, and boronic acid, were evaluated. While all of the anchoring groups exhibited water stability superior to carboxylate, the hydroxamate anchor had the best combination of ease of handling and electron injection efficiency. The effects on photoconductivity due to metal oxide morphology and the addition of dopants were also analyzed. Mixtures of anatase and rutile TiO 2 nanoparticles are known to exhibit cooperative effects which increase the efficiency of DSSCs and photocatalysis relative to the pure-phase materials. Through analysis of TRTS measurements, the mechanism of this synergistic effect was found to involve electron transfer from the lower-mobility, higher surface area rutile nanoparticles to anatase particles, resulting in a higher charge collection efficiency. In addition to morphology, doping has been investigated as a means of expanding the spectral range of visible absorption of photocatalysts. Doping ZnO nanowires with manganese(II) was found to significantly decrease the electron mobility, and doping with cobalt(II) increased the timescale for electron trapping. These differences can be understood by considering the changes to the band structure of ZnO effected by the dopants. Preliminary analyses of the solvent and electrolyte dependence on the electron injection rate and efficiency suggest that electron injection can be affected by

Milot, Rebecca Lee

256

Photoinduced Electron Transfer and Geminate Recombination in Liquids Kristin Weidemaier, H. L. Tavernier, S. F. Swallen, and M. D. Fayer*  

E-print Network

Photoinduced Electron Transfer and Geminate Recombination in Liquids Kristin Weidemaier, H. L of intermolecular photoinduced forward electron transfer and geminate recombination between donors (rubrene parameters than those presented previously.1 The geminate recombination (back transfer) data are highly

Fayer, Michael D.

257

27 CFR 25.165 - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2012 CFR

...taxpayer's bank to make an electronic fund transfer in the...later than the close of business on the last day for filing...the bank to effect an electronic fund transfer message...before the close of business on the prescribed last...Payment of Tax by Electronic Fund...

2012-04-01

258

27 CFR 25.165 - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2011 CFR

...taxpayer's bank to make an electronic fund transfer in the...later than the close of business on the last day for filing...the bank to effect an electronic fund transfer message...before the close of business on the prescribed last...Payment of Tax by Electronic Fund...

2011-04-01

259

27 CFR 25.165 - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2013 CFR

...taxpayer's bank to make an electronic fund transfer in the...later than the close of business on the last day for filing...the bank to effect an electronic fund transfer message...before the close of business on the prescribed last...Payment of Tax by Electronic Fund...

2013-04-01

260

27 CFR 25.165 - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2014 CFR

...taxpayer's bank to make an electronic fund transfer in the...later than the close of business on the last day for filing...the bank to effect an electronic fund transfer message...before the close of business on the prescribed last...Payment of Tax by Electronic Fund...

2014-04-01

261

Proton-coupled electron transfer reactions in solution: Molecular dynamics with quantum transitions for model systems  

E-print Network

Proton-coupled electron transfer reactions in solution: Molecular dynamics with quantum transitions A general minimal model for proton-coupled electron transfer PCET reactions in solution is presented. This model consists of three coupled degrees of freedom that represent an electron, a proton, and a solvent

Hammes-Schiffer, Sharon

262

Competition between electron transfer, trapping, and recombination in CdS nanorod-hydrogenase complexes.  

PubMed

Electron transfer from photoexcited CdS nanorods to [FeFe]-hydrogenase is a critical step in photochemical H2 production by CdS-hydrogenase complexes. By accounting for the distributions in the numbers of electron traps and enzymes adsorbed, we determine rate constants and quantum efficiencies for electron transfer from transient absorption measurements. PMID:25623885

Utterback, James K; Wilker, Molly B; Brown, Katherine A; King, Paul W; Eaves, Joel D; Dukovic, Gordana

2015-02-10

263

Primary Electron Transfer in Membrane-Bound Reaction Centers with Mutations at the M210 Position  

E-print Network

Primary Electron Transfer in Membrane-Bound Reaction Centers with Mutations at the M210 Position L, 1995; In Final Form: January 9, 1996X The kinetics of primary electron transfer in membrane state was examined, leading to the conclusion that a distribution in the driving force (G) for electron

van Stokkum, Ivo

264

Gene expression and deletion analysis of mechanisms for electron transfer from electrodes to Geobacter sulfurreducens  

E-print Network

Ã?Ã? Ã? Ã?Ã?Ã? Ã? Ã?Ã? Gene expression and deletion analysis of mechanisms for electron transfer from, Kelly P. Nevin, Derek R. Lovley, Gene expression and deletion analysis of mechanisms for electron MANUSCRIPT 1 Gene Expression and Deletion Analysis of Mechanisms for Electron Transfer from Electrodes

Lovley, Derek

265

Free Electron Transfer with Bifunctional Donors: p-Aminotritylsilanes  

NASA Astrophysics Data System (ADS)

This work provides an in-depth look at the bimolecular free-electron transfer (FET) from bisubstituted (amine and -CR2SiMe3 groups) aromatic molecules to the solvent radical cations of n-BuCl. Because of the low rotational barriers, the substrates obtain all possible arrangements in solution. The electron jump is an unhindered process that does not require a defined encounter complex. The resulting radical cations show great conformer diversity because they directly inherit the geometry of their mobile precursors. One part of the radical cations is unstable and dissociates instantly, but the other one is metastable (microsecond lifetime). The two substituents reduce the barrier of internal rotation, resulting in stabilization of the otherwise good leaving group -SiMe3. The amine group governs the reactivity of the system because it receives most of the electron density of the fluctuating highest MOs: primary and secondary amine groups lead to both instant and delayed formation of aminyl radicals; tertiary amines cause the rapid loss of an ?-H+ to yield ?-aminoalkyl radicals.

Karakostas, Nikolaos; Naumov, Sergej; Brede, Ortwin

2009-10-01

266

CNN pincer ruthenium catalysts for hydrogenation and transfer hydrogenation of ketones: experimental and computational studies.  

PubMed

Reaction of [RuCl(CNN)(dppb)] (1-Cl) (HCNN=2-aminomethyl-6-(4-methylphenyl)pyridine; dppb=Ph2 P(CH2 )4 PPh2 ) with NaOCH2 CF3 leads to the amine-alkoxide [Ru(CNN)(OCH2 CF3 )(dppb)] (1-OCH2 CF3 ), whose neutron diffraction study reveals a short RuO???HN bond length. Treatment of 1-Cl with NaOEt and EtOH affords the alkoxide [Ru(CNN)(OEt)(dppb)]?(EtOH)n (1-OEt?n?EtOH), which equilibrates with the hydride [RuH(CNN)(dppb)] (1-H) and acetaldehyde. Compound 1-OEt?n?EtOH reacts reversibly with H2 leading to 1-H and EtOH through dihydrogen splitting. NMR spectroscopic studies on 1-OEt?n?EtOH and 1-H reveal hydrogen bond interactions and exchange processes. The chloride 1-Cl catalyzes the hydrogenation (5 atm of H2 ) of ketones to alcohols (turnover frequency (TOF) up to 6.5×10(4) h(-1) , 40?°C). DFT calculations were performed on the reaction of [RuH(CNN')(dmpb)] (2-H) (HCNN'=2-aminomethyl-6-(phenyl)pyridine; dmpb=Me2 P(CH2 )4 PMe2 ) with acetone and with one molecule of 2-propanol, in alcohol, with the alkoxide complex being the most stable species. In the first step, the Ru-hydride transfers one hydrogen atom to the carbon of the ketone, whereas the second hydrogen transfer from NH2 is mediated by the alcohol and leads to the key "amide" intermediate. Regeneration of the hydride complex may occur by reaction with 2-propanol or with H2 ; both pathways have low barriers and are alcohol assisted. PMID:25195979

Baratta, Walter; Baldino, Salvatore; Calhorda, Maria José; Costa, Paulo J; Esposito, Gennaro; Herdtweck, Eberhardt; Magnolia, Santo; Mealli, Carlo; Messaoudi, Abdelatif; Mason, Sax A; Veiros, Luis F

2014-10-13

267

Reactivity of Cr(III) ?-oxo compounds: catalyst regeneration and atom transfer processes.  

PubMed

Oxidation of CpCr[(XylNCMe)(2)CH] (Xyl = 2,6-Me(2)C(6)H(3)) with pyridine N-oxide or air generated the ?-oxo dimer, {CpCr[(XylNCMe)(2)CH]}(2)(?-O). The ?-oxo dimer was converted to paramagnetic Cr(III) CpCr[(XylNCMe)(2)CH](X) complexes (X = OH, O(2)CPh, Cl, OTs) via protonolysis reactions. The related Cr(III) alkoxide complexes (X = OCMe(3), OCMe(2)Ph) were prepared by salt metathesis and characterized by single crystal X-ray diffraction. The interconversion of the Cr(III) complexes and their reduction back to Cr(II) with Mn powder were monitored using UV-vis spectroscopy. The related CpCr[(DepNCMe)(2)CH] (Dep = 2,6-Et(2)C(6)H(3)) Cr(II) complex was studied for catalytic oxygen atom transfer reactions with PPh(3) using O(2) or air. Both Cr(II) complexes reacted with pyridine N-oxide and ?-terpinene to give the corresponding Cr(III) hydroxide complexes. When CpCr[(DepNCMe)(2)CH] was treated with pyridine N-oxide in benzene in the absence of hydrogen atom donors, a dimeric Cr(III) hydroxide product was isolated and structurally characterized, apparently resulting from intramolecular hydrogen atom abstraction of a secondary benzylic ligand C-H bond followed by intermolecular C-C bond formation. The use of very bulky hexaisopropylterphenyl ligand substituents did not preclude the formation of the analogous ?-oxo dimer, which was characterized by X-ray diffraction. Attempts to develop a chromium-catalyzed intermolecular hydrogen atom transfer process based on these reactions were unsuccessful. The protonolysis and reduction reactions of the ?-oxo dimer were used to improve the previously reported Cr-catalyzed radical cyclization of a bromoacetal. PMID:22175660

MacLeod, K Cory; Patrick, Brian O; Smith, Kevin M

2012-01-01

268

Protein dynamics and electron transfer: Electronic decoherence and non-Condon effects  

PubMed Central

We compute the autocorrelation function of the donor-acceptor tunneling matrix element ?TDA(t)TDA(0)? for six Ru-azurin derivatives. Comparison of this decay time to the decay time of the time-dependent Franck-Condon factor {computed by Rossky and coworkers [Lockwood, D. M., Cheng, Y.-K. & Rossky, P. J. (2001) Chem. Phys. Lett. 345, 159-165]} reveals the extent to which non-Condon effects influence the electron-transfer rate. ?TDA(t)TDA(0)? is studied as a function of donor-acceptor distance, tunneling pathway structure, tunneling energy, and temperature to explore the structural and dynamical origins of non-Condon effects. For azurin, the correlation function is remarkably insensitive to tunneling pathway structure. The decay time is only slightly shorter than it is for solvent-mediated electron transfer in small organic molecules and originates, largely, from fluctuations of valence angles rather than bond lengths. PMID:15738409

Skourtis, Spiros S.; Balabin, Ilya A.; Kawatsu, Tsutomu; Beratan, David N.

2005-01-01

269

Electronic shift register memory based on molecular electron-transfer reactions  

NASA Technical Reports Server (NTRS)

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.

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

1989-01-01

270

Picosecond Raman Spectroscopy And Electron Transfer In Condensed Systems  

NASA Astrophysics Data System (ADS)

The nature of the electron donor/acceptor (EDA) excited state surface leading to the charge transfer (CT) complex is investigated by picosecond techniques. Irradiation into the CT band of the EDA complexes cyanoanthracene/tetracyanoethylene [CNA, TCNE] and indene/ tetracyanoethylene [IN, TCNE] produces the respective radical ion pairs within 25 psec, which then decay within 60 psec, with the [CNA, TCNE] couple returning directly to the ground state, while the [IN, TCNE] couple passes through a 500 psec lived intermediate before returning to the ground state. To obtain structural information about these complexes, a 5 kHz amplified cavity dumped sync pumped dye laser (-v5 psec) resonance Raman spectrometer using a flowing sample jet has been developed. Preliminary work on trans stilbene shows excellent signal to noise and an S1 decay of ~1000 psec, in agreement with literature values.

Hutchinson, J. A.; DiBenedetto, J.; Hilinski, E. F.; Hopkins, J. B.; Rentzepis, P. M.

1986-06-01

271

Concerted and sequential pathways of proton-coupled electron transfer in hydrogen halide elimination  

NASA Astrophysics Data System (ADS)

Proton-coupled electron transfer (PCET) is a key reaction in a diverse array of chemical and biochemical processes. Concerted PCET, a single step process where no intermediates are expected, is often difficult to distinguish from sequential electron transfer/proton transfer (or proton transfer/electron transfer) events that involve one or more reactive intermediates. Here we demonstrate that in an apparently simple and fundamental organic reaction, hydrogen halide elimination (dehydrohalogenation), both mechanisms occur, with concerted PCET kinetically favored over sequential ET/PT. The latter involves a well-defined reactive intermediate, the corresponding iso-halon.

Kalume, Aimable; George, Lisa; Cunningham, Nicole; Reid, Scott A.

2013-01-01

272

Photoinduced electron transfer reaction in polymer-surfactant aggregates: Photoinduced electron transfer between N,N-dimethylaniline and 7-amino coumarin dyes  

NASA Astrophysics Data System (ADS)

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.

Chakraborty, Anjan; Seth, Debabrata; Setua, Palash; Sarkar, Nilmoni

2008-05-01

273

Photoinduced electron transfer reaction in polymer-surfactant aggregates: Photoinduced electron transfer between N,N-dimethylaniline and 7-amino coumarin dyes  

SciTech Connect

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.

Chakraborty, Anjan; Seth, Debabrata; Setua, Palash; Sarkar, Nilmoni [Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302, West Bengal (India)

2008-05-28

274

A stochastic reorganizational bath model for electronic energy transfer.  

PubMed

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. This simple relationship allows one to understand the physical origin of the Stokes shifts in molecular aggregates. PMID:24985614

Fujita, Takatoshi; Huh, Joonsuk; Aspuru-Guzik, Alán

2014-06-28

275

A stochastic reorganizational bath model for electronic energy transfer  

SciTech Connect

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. This simple relationship allows one to understand the physical origin of the Stokes shifts in molecular aggregates.

Fujita, Takatoshi, E-mail: tfujita@fas.harvard.edu, E-mail: aspuru@chemistry.harvard.edu; Huh, Joonsuk; Aspuru-Guzik, Alán, E-mail: tfujita@fas.harvard.edu, E-mail: aspuru@chemistry.harvard.edu [Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138 (United States)

2014-06-28

276

Electron transfer in systems of well-defined geometry  

SciTech Connect

Two mesopyropheophorbide macrocycles can be joined via two covalent linkages to produce a cyclophane. It is possible to insert one or two Mg atoms into the cyclophane. The Qy transitions of the macrocycles are nearly orthogonal. The visible absorption spectrum of the monometal cyclophane is nearly a superposition of the spectra of the monomers. Emission from the monometal cyclophane arises primarily from the red most absorbing chromophore. The excited state difference spectrum shows that both macrocycles are excited. Fluorescence lifetimes of the monometal cyclophane decrease with increasing dielectric strength. Changes in the fluorescence and the triplet yield parallel the shortening of the singlet lifetime. Thus the radiative rate is solvent independent. This is in contrast to what one would expect if the emitting state had charge transfer character. Since the fluorescence lifetime is dependent on dielectric, the nonradiative relaxation from the singlet state is due to formation of a radical pair. The decay rate of the postulated radical pair was monitored by observing the kinetics of ground state repopulation. For the geometry of this cyclophane, electron transfer proceeds relatively slowly (k = 3 x 10/sup 9/ sec/sup -1/) in the forward direction. Modeling calculations indicate that the rate of annihilation of the radical pair may decrease as the solvent dielectric decreases.

Overfield, R.E.; Kaufmann, K.J.; Wasielewski, M.R.

1980-01-01

277

Metalloporphyrins as energy transfer catalysts: Progress report, August 1, 1984-July 31, 1987  

SciTech Connect

Porphyrin excited state dynamics have been explored initially using a simple stationary sample cw laser technique to pump a steady state; the ground state spectrum is obtained by spinning the sample, and the excited state spectrum is obtained by subtraction. In this way triplet state Resoannce Rama (RR) spectra were obtained for the tetraphenylporphine (TPP) complexes of magnesium, zinc, and palladium, and excited state deligation was studied for pyridine and piperidine complexes of nickel TPP. In connection with the latter study, the systematics of RR frequencies and core size were established for 4- and 6-coordinate nickel porphyrins, and a 5-coordinate high-spin complex was definitively characterized. In addition RR spectra were analyzed for ruthenium porphyrins, as background for future excited state studies, and backbonding charcteristics were established. Charge transfer excited states of a 4,4'-bipyridine complex of ruthenium(II) were studied via the power-dependent photoinduced RR spectra produced with a YAG laser. RR spectra of metallo-OEP (octaethylporphyrin) cation radicals have also been studied.

Sprio, T.G.

1987-01-01

278

Iridium N-Heterocyclic Carbene Complexes as Efficient Catalysts for Magnetization Transfer from para-Hydrogen  

PubMed Central

While the characterization of materials by NMR is hugely important in the physical and biological sciences, it also plays a vital role in medical imaging. This success is all the more impressive because of the inherently low sensitivity of the method. We establish here that [Ir(H)2(IMes)(py)3]Cl undergoes both pyridine (py) loss as well as the reductive elimination of H2. These reversible processes bring para-H2 and py into contact in a magnetically coupled environment, delivering an 8100-fold increase in 1H NMR signal strength relative to non-hyperpolarized py at 3 T. An apparatus that facilitates signal averaging has been built to demonstrate that the efficiency of this process is controlled by the strength of the magnetic field experienced by the complex during the magnetization transfer step. Thermodynamic and kinetic data combined with DFT calculations reveal the involvement of [Ir(H)2(?2-H2)(IMes)(py)2]+, an unlikely yet key intermediate in the reaction. Deuterium labeling yields an additional 60% improvement in signal, an observation that offers insight into strategies for optimizing this approach. PMID:21469642

2011-01-01

279

Accumulative charge separation for solar fuels production: coupling light-induced single electron transfer to multielectron catalysis.  

PubMed

The conversion and storage of solar energy into a fuel holds promise to provide a significant part of the future renewable energy demand of our societies. Solar energy technologies today generate heat or electricity, while the large majority of our energy is used in the form of fuels. Direct conversion of solar energy to a fuel would satisfy our needs for storable energy on a large scale. Solar fuels can be generated by absorbing light and converting its energy to chemical energy by electron transfer leading to separation of electrons and holes. The electrons are used in the catalytic reduction of a cheap substrate with low energy content into a high-energy fuel. The holes are filled by oxidation of water, which is the only electron source available for large scale solar fuel production. Absorption of a single photon typically leads to separation of a single electron-hole pair. In contrast, fuel production and water oxidation are multielectron, multiproton reactions. Therefore, a system for direct solar fuel production must be able to accumulate the electrons and holes provided by the sequential absorption of several photons in order to complete the catalytic reactions. In this Account, the process is termed accumulative charge separation. This is considerably more complicated than charge separation on a single electron level and needs particular attention. Semiconductor materials and molecular dyes have for a long time been optimized for use in photovoltaic devices. Efforts are made to develop new systems for light harvesting and charge separation that are better optimized for solar fuel production than those used in the early devices presented so far. Significant progress has recently been made in the discovery and design of better homogeneous and heterogeneous catalysts for solar fuels and water oxidation. While the heterogeneous ones perform better today, molecular catalysts based on transition metal complexes offer much greater tunability of electronic and structural properties, they are typically more amenable to mechanistic analysis, and they are small and therefore require less material. Therefore, they have arguably greater potential as future efficient catalysts but must be efficiently coupled to accumulative charge separation. This Account discusses accumulative charge separation with focus on molecular and molecule-semiconductor hybrid systems. The coupling between charge separation and catalysis involves many challenges that are often overlooked, and they are not always apparent when studying water oxidation and fuel formation as separate half-reactions with sacrificial agents. Transition metal catalysts, as well as other multielectron donors and acceptors, cycle through many different states that may quench the excited sensitizer by nonproductive pathways. Examples where this has been shown, often with ultrafast rates, are reviewed. Strategies to avoid these competing energy-loss reactions and still obtain efficient coupling of charge separation to catalysis are discussed. This includes recent examples of dye-sensitized semiconductor devices with molecular catalysts and dyes that realize complete water splitting, albeit with limited efficiency. PMID:25675365

Hammarström, Leif

2015-03-17

280

Rapid estimation of catalyst nanoparticle morphology and atomic-coordination by high-resolution Z-contrast electron microscopy.  

PubMed

Heterogeneous nanoparticle catalyst development relies on an understanding of their structure-property relationships, ideally at atomic resolution and in three-dimensions. Current transmission electron microscopy techniques such as discrete tomography can provide this but require multiple images of each nanoparticle and are incompatible with samples that change under electron irradiation or with surveying large numbers of particles to gain significant statistics. Here, we make use of recent advances in quantitative dark-field scanning transmission electron microscopy to count the number atoms in each atomic column of a single image from a platinum nanoparticle. These atom-counts, along with the prior knowledge of the face-centered cubic geometry, are used to create atomistic models. An energy minimization is then used to relax the nanoparticle's 3D structure. This rapid approach enables high-throughput statistical studies or the analysis of dynamic processes such as facet-restructuring or particle damage. PMID:25340541

Jones, Lewys; MacArthur, Katherine E; Fauske, Vidar T; van Helvoort, Antonius T J; Nellist, Peter D

2014-11-12

281

The hydrogen catalyst cobaloxime: a multifrequency EPR and DFT study of cobaloxime's electronic structure.  

PubMed

Solar fuels research aims to mimic photosynthesis and devise integrated systems that can capture, convert, and store solar energy in the form of high-energy molecular bonds. Molecular hydrogen is generally considered an ideal solar fuel because its combustion is essentially pollution-free. Cobaloximes rank among the most promising earth-abundant catalysts for the reduction of protons to molecular hydrogen. We have used multifrequency EPR spectroscopy at X-band, Q-band, and D-band combined with DFT calculations to reveal electronic structure and establish correlations among the structure, surroundings, and catalytic activity of these complexes. To assess the strength and nature of ligand cobalt interactions, the BF(2)-capped cobaloxime, Co(dmgBF(2))(2), was studied in a variety of different solvents with a range of polarities and stoichiometric amounts of potential ligands to the cobalt ion. This allows the differentiation of labile and strongly coordinating axial ligands for the Co(II) complex. Labile, or weakly coordinating, ligands such as methanol result in larger g-tensor anisotropy than strongly coordinating ligands such as pyridine. In addition, a coordination number effect is seen for the strongly coordinating ligands with both singly ligated LCo(dmgBF(2))(2) and doubly ligated L(2)Co(dmgBF(2))(2) . The presence of two strongly coordinating axial ligands leads to the smallest g-tensor anisotropy. The relevance of the strength of the axial ligand(s) to the catalytic efficiency of Co(dmgBF(2))(2) is discussed. Finally, the influence of molecular oxygen and formation of Co(III) superoxide radicals LCo(dmgBF(2))(2)O(2)(•) is studied. The experimental results are compared with a comprehensive set of DFT calculations on Co(dmgBF(2))(2) model systems with various axial ligands. Comparison with experimental values for the "key" magnetic parameters such as g-tensor and (59)Co hyperfine coupling tensor allows the determination of the conformation of the axially ligated Co(dmgBF(2))(2) complexes. The data presented here are vital for understanding the influence of solvent and ligand coordination on the catalytic efficiency of cobaloximes. PMID:22375846

Niklas, Jens; Mardis, Kristy L; Rakhimov, Rakhim R; Mulfort, Karen L; Tiede, David M; Poluektov, Oleg G

2012-03-01

282

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

SciTech Connect

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.

Xiao Tiejun; Song Xueyu [Department of Chemistry, Iowa State University, Ames, Iowa 50011 (United States)

2013-03-21

283

Photoinitiated proton-coupled electron transfer and radical transport kinetics in class la ribonucleotide reductase  

E-print Network

Proton-coupled electron transfer (PCET) is a critical mechanism in biology, underpinning key processes such as radical transport, energy transduction, and enzymatic substrate activation. Ribonucleotide reductases (RNRs) ...

Pizano, Arturo A. (Arturo Alejandro)

2013-01-01

284

Intrinsic Barriers for Electron and Hydrogen Atom Transfer Reactions of Biomimetic Iron Complexes  

E-print Network

Intrinsic Barriers for Electron and Hydrogen Atom Transfer Reactions of Biomimetic Iron Complexes) Meunier, B., Ed. Biomimetic Oxidations Catalyzed by Transition Metal Complexes; Imperial College Press

Roth, Justine P.

285

Photoinduced electron transfer processes in homogeneous and microheterogeneous solutions  

NASA Astrophysics Data System (ADS)

Chemical electron transfer-induced fragmentation of a variety of electron donors was shown to be a quite general process. In previous studies, attention was focused on alpha-Beta aminoalcohols and 1,2-ditertiary amines. In the present year, studies were extended to include pinacols, an richer variety of diamines and amino-ketones. The mechanisms, intermediates involved in these fragmentation reactions, and medium effects were also studied. Studies of pinacols and a variety of diamines show that photooxidative cleavage of 1,2-diheteroatom-containing molecules is a very general process which can occur for several different types of molecules. In order to demonstrate clearly that cation radicals of the heteroatom containing donor can be intermediates in these reactions, the reaction was studied of several different donors with the acceptors, dicyanoanthracene (DCA) and tetracyanoanthracene (TCA) using biphenyl (BP) as a cosensitizer. The second major focus of the work was to look at donors which, undergo reversible redox reactions to give relatively high energy products which are capable of undergoing clean thermal reverse redox reactions but which survive long enough to be potentially useful for other purposes. The reaction explored extensively is the oxidation of tertiary amines to enamines by quinone type molecules such as beta-lapachone. These studies are continued and other reactions are examined which may be more promising in terms of finding a photochemical-thermal redox cycle which can be carried out with little loss of material through competing side reactions.

Whitten, D. G.

1990-10-01

286

Ions interacting with planar aromatic molecules: Modeling electron transfer reactions  

SciTech Connect

We present theoretical absolute charge exchange cross sections for multiply charged cations interacting with the Polycyclic Aromatic Hydrocarbon (PAH) molecules pyrene C{sub 14}H{sub 10}, coronene C{sub 24}H{sub 12}, or circumcoronene C{sub 54}H{sub 18}. These planar, nearly circular, PAHs are modelled as conducting, infinitely thin, and perfectly circular discs, which are randomly oriented with respect to straight line ion trajectories. We present the analytical solution for the potential energy surface experienced by an electron in the field of such a charged disc and a point-charge at an arbitrary position. The location and height of the corresponding potential energy barrier from this simple model are in close agreement with those from much more computationally demanding Density Functional Theory (DFT) calculations in a number of test cases. The model results compare favourably with available experimental data on single- and multiple electron transfer reactions and we demonstrate that it is important to include the orientation dependent polarizabilities of the molecules (model discs) in particular for the larger PAHs. PAH ionization energy sequences from DFT are tabulated and used as model inputs. Absolute cross sections for the ionization of PAH molecules, and PAH ionization energies such as the ones presented here may be useful when considering the roles of PAHs and their ions in, e.g., interstellar chemistry, stellar atmospheres, and in related photoabsorption and photoemission spectroscopies.

Forsberg, B. O.; Alexander, J. D.; Chen, T.; Pettersson, A. T.; Gatchell, M.; Cederquist, H.; Zettergren, H. [Department of Physics, Stockholm University, SE-106 91 Stockholm (Sweden)

2013-02-07

287

Catalyst by Design  

SciTech Connect

The development of new catalytic materials is still dominated by trial and error methods. Although it has been successful, the empirical development of catalytic materials is time consuming and expensive with no guarantee of success. In our laboratories, we are developing a comprehensive catalysts by design that involves state-of-the-art first principle density functional theory calculations, experimental design of catalyst sites, and sub- ngstr m resolution imaging with an aberration-corrected electron microscope to characterize the microstructure. In this chapter, we focus on supported platinum cluster catalyst systems which are one of the most important industrial catalysts and attempt to demonstrate the feasibility of the catalyst by design concept.

Narula, Chaitanya Kumar [ORNL] [ORNL; DeBusk, Melanie Moses [ORNL] [ORNL

2014-01-01

288

Ultrafast photoinduced electron transfer reactions in supramolecular arrays: Studies of electronic coupling and solvation  

SciTech Connect

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.

Wasielewski, M.R.; Wiederrecht, G.P.; Svec, W.A.

1993-05-01

289

Ultrafast photoinduced electron transfer reactions in supramolecular arrays: Studies of electronic coupling and solvation  

SciTech Connect

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.

Wasielewski, M.R.; Wiederrecht, G.P.; Svec, W.A.

1993-01-01

290

Thermal transfer structures coupling electronics card(s) to coolant-cooled structure(s)  

DOEpatents

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.

David, Milnes P; Graybill, David P; Iyengar, Madhusudan K; Kamath, Vinod; Kochuparambil, Bejoy J; Parida, Pritish R; Schmidt, Roger R

2014-12-16

291

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...

292

27 CFR 41.63 - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2013 CFR

...TOBACCO PRODUCTS, CIGARETTE PAPERS AND TUBES...Payment of tax by electronic fund transfer...cigars, cigarettes, cigarette papers, and cigarette tubes combining...making payment by electronic fund...

2013-04-01

293

27 CFR 41.63 - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2010 CFR

...TOBACCO PRODUCTS, CIGARETTE PAPERS AND TUBES...Payment of tax by electronic fund transfer...cigars, cigarettes, cigarette papers, and cigarette tubes combining...making payment by electronic fund...

2010-04-01

294

27 CFR 41.63 - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2011 CFR

...TOBACCO PRODUCTS, CIGARETTE PAPERS AND TUBES...Payment of tax by electronic fund transfer...cigars, cigarettes, cigarette papers, and cigarette tubes combining...making payment by electronic fund...

2011-04-01

295

27 CFR 41.63 - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2012 CFR

...TOBACCO PRODUCTS, CIGARETTE PAPERS AND TUBES...Payment of tax by electronic fund transfer...cigars, cigarettes, cigarette papers, and cigarette tubes combining...making payment by electronic fund...

2012-04-01

296

27 CFR 41.63 - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2014 CFR

...TOBACCO PRODUCTS, CIGARETTE PAPERS AND TUBES...Payment of tax by electronic fund transfer...cigars, cigarettes, cigarette papers, and cigarette tubes combining...making payment by electronic fund...

2014-04-01

297

Photoinduced electron transfer from aliphatic amines to coumarin dyes  

NASA Astrophysics Data System (ADS)

Electron transfer (ET) interactions of a series of 7-aminocoumarin dyes with aliphatic amine donors have been investigated using steady-state (SS) and time-resolved (TR) fluorescence quenching, picosecond laser flash photolysis (LFP) and pulse radiolysis (PR) techniques. For different coumarin-amine pairs, the estimated quenching constants (kq) from SS and TR fluorescence measurements are found to be similar within the experimental error. That ET from amine donors to excited (S1) coumarin dyes takes place has been established from the LFP and the PR results. For different coumarin-amine pairs, the kq values are seen to correlate well with the free energy changes (?G0) for the ET reactions following Marcus' outer-sphere ET theory. The total reorganization energy (?) estimated from this correlation is seen to be just similar to the solvent reorganization energy (?s). The leveled-off kq value under diffusion-controlled condition (kqDC) appears to be much lower (˜2.5 times) for the present systems compared to the corresponding value obtained for the ET reactions in coumarin-aromatic amine systems. The large difference in the kqDC values with aliphatic and aromatic amines as the electron donors has been rationalized on the basis of the shapes of the highest occupied molecular orbitals (HOMO) of the amine donors. For aliphatic amines, since their HOMOs are largely localized on the amino group, a large orientation factor is involved in the encounter complexes for the ET reaction to take place. With ? like HOMOs, such orientational restriction is just nominal with the aromatic amines as the electron donors. Present ET results under diffusive conditions have also been discussed with a comparison to the ET rates observed under nondiffusive conditions, where the amines are directly used as the solvent donors.

Nad, Sanjukta; Pal, Haridas

2002-01-01

298

Electronic resonance with anticorrelated pigment vibrations drives photosynthetic energy transfer outside the adiabatic framework  

PubMed Central

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

Tiwari, Vivek; Peters, William K.; Jonas, David M.

2013-01-01

299

Microscale gradients and their role in electron-transfer mechanisms in biofilms  

PubMed Central

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

Beyenal, Haluk; Babauta, Jerome T.

2014-01-01

300

Simultaneous detection of electronic structure changes from two elements of a bifunctional catalyst using wavelength-dispersive X-ray emission spectroscopy and in situ electrochemistry.  

PubMed

Multielectron catalytic reactions, such as water oxidation, nitrogen reduction, or hydrogen production in enzymes and inorganic catalysts often involve multimetallic clusters. In these systems, the reaction takes place between metals or metals and ligands to facilitate charge transfer, bond formation/breaking, substrate binding, and release of products. In this study, we present a method to detect X-ray emission signals from multiple elements simultaneously, which allows for the study of charge transfer and the sequential chemistry occurring between elements. K? X-ray emission spectroscopy (XES) probes charge and spin states of metals as well as their ligand environment. A wavelength-dispersive spectrometer based on the von Hamos geometry was used to disperse K? signals of multiple elements onto a position detector, enabling an XES spectrum to be measured in a single-shot mode. This overcomes the scanning needs of the scanning spectrometers, providing data free from temporal and normalization errors and therefore ideal to follow sequential chemistry at multiple sites. We have applied this method to study MnOx-based bifunctional electrocatalysts for the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR). In particular, we investigated the effects of adding a secondary element, Ni, to form MnNiOx and its impact on the chemical states and catalytic activity, by tracking the redox characteristics of each element upon sweeping the electrode potential. The detection scheme we describe here is general and can be applied to time-resolved studies of materials consisting of multiple elements, to follow the dynamics of catalytic and electron transfer reactions. PMID:25747045

Gul, Sheraz; Ng, Jia Wei Desmond; Alonso-Mori, Roberto; Kern, Jan; Sokaras, Dimosthenis; Anzenberg, Eitan; Lassalle-Kaiser, Benedikt; Gorlin, Yelena; Weng, Tsu-Chien; Zwart, Petrus H; Zhang, Jin Z; Bergmann, Uwe; Yachandra, Vittal K; Jaramillo, Thomas F; Yano, Junko

2015-04-14

301

Nonspherical Nanoparticles with Controlled Morphologies via Seeded Surface-Initiated Single Electron Transfer Radical Polymerization in Soap-Free Emulsion.  

PubMed

This work reports a facile novel approach to prepare asymmetric poly(vinylidene fluoride)/polystyrene (PVDF/PS) composite latex particles with controllable morphologies using one-step soap-free seeded emulsion polymerization, i.e., surface-initiated single electron transfer radical polymerization (SET-RP) of styrene (St) at the surface of PVDF seed particles. It was observed that the morphology was influenced mainly by the St/PVDF feed ratio, the polymerization temperature, and the length of the catalyst Cu(0) wire (? 1.00 mm). When the feed ratio was St/PVDF = 5.0 g/1.0 g, snowman-like Janus particles were exclusively obtained. Raspberry-like and popcorn-like composite particles were observed at a higher reaction temperature or a shorter length of the catalyst wire. The reaction kinetics plots demonstrated some unique features. The formation of nonspherical composite nanoparticles can be ascribed to the surface nucleation of PS bulges following the SET-RP. PMID:25797695

Yuan, Jinfeng; Wang, Lixia; Zhu, Lei; Pan, Mingwang; Wang, Wenjie; Liu, Ying; Liu, Gang

2015-04-14

302

A stochastic reorganizational bath model for electronic energy transfer  

E-print Network

The fluctuations of optical gap induced by the environment play crucial roles 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 model, in which the energy-gap fluctuation is approximated as a white noise. Recently, several groups have employed molecular dynamics simulations and excited-state calculations in conjunction to take the thermal fluctuation of excitation energies into account. 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 the connection, we propose a novel scheme to correct 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. For this purpose, we introduce a simple relationship that relates the reorganization contribution to the Stokes shifts - the reorganization shift - to three parameters: the monomer reorganization energy, the relaxation time of the optical gap, and the exciton delocalization length. This simple relationship allows one to classify the origin of the Stokes shifts in molecular aggregates.

Takatoshi Fujita; Joonsuk Huh; Alan Aspuru-Guzik

2014-06-06

303

Photoinduced electron transfer from semiconductor quantum dots to metal oxide nanoparticles  

PubMed Central

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

Tvrdy, Kevin; Frantsuzov, Pavel A.; Kamat, Prashant V.

2011-01-01

304

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

PubMed

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

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

2011-12-01

305

ELECTRONICS LETTERS 13th May 1999 Vol. 35 No. 10 Method for nonlinear transfer function  

E-print Network

approximation S.L. Loyka The application of the genetic algorithm to the approximation of nonlinear transfer genetic algorithm to expansion of nonlinear transfer func- tions: The genetic algorithm appearsELECTRONICS LETTERS 13th May 1999 Vol. 35 No. 10 Method for nonlinear transfer function

Loyka, Sergey

306

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

NASA Technical Reports Server (NTRS)

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, the average electron temperature is expected to be between 10,000 and 20,000 K. Thus only data for low energy electrons are relevant to the model.

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

1995-01-01

307

Thermoregulated phase-transfer ligands and catalysis. Part VI. Two-phase hydroformylation of styrene catalyzed by the thermoregulated phase-transfer catalyst OPGPP\\/Rh  

Microsoft Academic Search

A novel nonionic water-soluble octylpolyglycol-phenylene-phosphite (OPGPP) was synthesized and the two-phase hydroformylation of styrene catalyzed by an OPGPP\\/Rh catalyst was investigated fully. The catalyst displayed excellent catalytic activity; high styrene conversion and high aldehyde yield (99.6 and 99.3%, respectively) were obtained at 80°C and 5.0 MPa, and the molar ratio of branched\\/normal aldehyde was 4.8. The experimental results revealed that

Ruifang Chen; Xiaozhong Liu; Zilin Jin

1998-01-01

308

Image State Mediated Electron Transfer at Surfaces Department of Applied Physics, Chalmers University of Technology  

E-print Network

Image State Mediated Electron Transfer at Surfaces Shiwu Gao Department of Applied Physics, Chalmers University of Technology S­412 96 G¨oteborg, Sweden David C. Langreth Department of Physics states provide an important channel for electron transfer between an atom and a surface, especially

Langreth, David C.

309

A COMPUTATIONAL STUDY IN THE THERMODYNAMIC AND DYNAMIC PROPERTIES OF ELECTRON TRANSFER PROTEINS  

E-print Network

A COMPUTATIONAL STUDY IN THE THERMODYNAMIC AND DYNAMIC PROPERTIES OF ELECTRON TRANSFER PROTEINS IN THE THERMODYNAMIC AND DYNAMIC PROPERTIES OF ELECTRON TRANSFER PROTEINS by David N. LeBard has been approved December in plastocyanin. vi #12;TABLE OF CONTENTS Page CHAPTER 1 Overview of previous work

Thorpe, Michael

310

Electron Transfer Catalyzed [2 + 2] Cycloreversion of Benzene G. Devi Reddy and Olaf Wiest*  

E-print Network

Electron Transfer Catalyzed [2 + 2] Cycloreversion of Benzene Dimers G. Devi Reddy and Olaf Wiest, Florida 32611-7200 Received December 8, 1998 The catalysis of the [2 + 2] cycloreversion of the anti-o,o-benzene dimer 1 and the syn-o,o- naphthalene-benzene dimer 2 through thermal and photoinduced electron transfer

Hudlicky, Tomas

311

Diversity of promoter elements in a Geobacter sulfurreducens mutant adapted to disruption in electron transfer  

E-print Network

in electron transfer Julia Krushkal & Ching Leang & Jose F. Barbe & Yanhua Qu & Bin Yan & Marko Puljic and to generate bioenergy, molecular mecha- nisms and biological components involved in their electron transfer to determine its molecular, genetic, and physiolog- ical features; its genome sequence1 and a genetic system

Lovley, Derek

312

27 CFR 53.158 - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2010 CFR

...no later than the close of business on the last day for making...financial institution to effect an electronic fund transfer message as...on or before the close of business on the prescribed last day...entitled Payment of Tax by Electronic Fund Transfer. This...

2010-04-01

313

27 CFR 19.240 - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2013 CFR

...240 Payment of tax by electronic fund transfer. (a) General...by no later than close of business on the last day for filing the...tax payment before close of business on the last day for filing...entitled “Payment of Tax by Electronic Fund Transfer”. This...

2013-04-01

314

27 CFR 53.158 - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2013 CFR

...no later than the close of business on the last day for making...financial institution to effect an electronic fund transfer message as...on or before the close of business on the prescribed last day...entitled Payment of Tax by Electronic Fund Transfer. This...

2013-04-01

315

27 CFR 19.240 - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2011 CFR

...240 Payment of tax by electronic fund transfer. (a) General...by no later than close of business on the last day for filing the...tax payment before close of business on the last day for filing...entitled “Payment of Tax by Electronic Fund Transfer”. This...

2011-04-01

316

27 CFR 53.158 - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2012 CFR

...no later than the close of business on the last day for making...financial institution to effect an electronic fund transfer message as...on or before the close of business on the prescribed last day...entitled Payment of Tax by Electronic Fund Transfer. This...

2012-04-01

317

27 CFR 53.158 - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2011 CFR

...no later than the close of business on the last day for making...financial institution to effect an electronic fund transfer message as...on or before the close of business on the prescribed last day...entitled Payment of Tax by Electronic Fund Transfer. This...

2011-04-01

318

27 CFR 19.240 - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2012 CFR

...240 Payment of tax by electronic fund transfer. (a) General...by no later than close of business on the last day for filing the...tax payment before close of business on the last day for filing...entitled “Payment of Tax by Electronic Fund Transfer”. This...

2012-04-01

319

27 CFR 53.158 - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2014 CFR

...no later than the close of business on the last day for making...financial institution to effect an electronic fund transfer message as...on or before the close of business on the prescribed last day...entitled Payment of Tax by Electronic Fund Transfer. This...

2014-04-01

320

27 CFR 19.240 - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2014 CFR

...240 Payment of tax by electronic fund transfer. (a) General...by no later than close of business on the last day for filing the...tax payment before close of business on the last day for filing...entitled “Payment of Tax by Electronic Fund Transfer”. This...

2014-04-01

321

Theory for electron-transfer reactions involving two Marcus surfaces with a different force constant  

SciTech Connect

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.

Tang, Jau

1994-02-01

322

27 CFR 26.267 - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2010 CFR

...making payment by electronic fund transfer (EFT), as defined in paragraph (c) of...by this section, to make remittances by EFT. For purposes of this section, the...determining who is required to make remittances by EFT. (c) Electronic fund transfer...

2010-04-01

323

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

Code of Federal Regulations, 2010 CFR

...making payment by electronic fund transfer (EFT), as defined in paragraph (c) of this...by this section, to make remittances by EFT. For purposes of this section, the...determining who is required to make remittances by EFT. (c) Electronic fund transfer...

2010-04-01

324

27 CFR 26.267 - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2011 CFR

...making payment by electronic fund transfer (EFT), as defined in paragraph (c) of...by this section, to make remittances by EFT. For purposes of this section, the...determining who is required to make remittances by EFT. (c) Electronic fund transfer...

2011-04-01

325

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

Code of Federal Regulations, 2011 CFR

...making payment by electronic fund transfer (EFT), as defined in paragraph (c) of this...by this section, to make remittances by EFT. For purposes of this section, the...determining who is required to make remittances by EFT. (c) Electronic fund transfer...

2011-04-01

326

Electron-Transfer Properties of Cytochrome c Langmuir-Blodgett Films and Interactions of Cytochrome  

E-print Network

Electron-Transfer Properties of Cytochrome c Langmuir-Blodgett Films and Interactions of Cytochrome and electron-transfer properties of cytochrome c (Cyt c) Langmuir-Blodgett (LB) films have been studied using surface analytical techniques such as scanning probe microscopy.1

Zhang, Yanchao

327

Application of electron-transfer theory to several systems of biological interest  

SciTech Connect

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. 22 refs., 3 tabs.

Marcus, R.A.; Sutin, N.

1985-01-01

328

Bimolecular Electron and Energy Transfer Reactivity of Exchange-Coupled Dinuclear Iron(III) Complexes  

E-print Network

Bimolecular Electron and Energy Transfer Reactivity of Exchange-Coupled Dinuclear Iron and electron transfer from a theoretical perspective. Bertrand and Gayda first examined the effect of exchange to experimentally establish a link between Heisenberg spin exchange and chemical reactivity. The acceptors

McCusker, James K.

329

Electron Transfer from the Rieske Iron-Sulfur Protein (ISP) to Cytochrome f in Vitro  

E-print Network

Electron Transfer from the Rieske Iron-Sulfur Protein (ISP) to Cytochrome f in Vitro IS A GUIDED soluble domains of the Rieske iron-sulfur protein (ISP) and cytochrome f subunits of the cytochrome b6f, and one [2Fe-2S] cluster in the Rieske iron-sulfur protein (1­3). The electron transfer pathway

Cramer, William A.

330

Photoinduced electron transfer and geminate recombination in liquids: Analytical theory and Monte Carlo simulations  

E-print Network

Photoinduced electron transfer and geminate recombination in liquids: Analytical theory and Monte recombination in liquid solution are addressed with analytical theory and Monte Carlo simulations. The time of electron transfer fol- lowed by possible geminate recombination involve a com- plex interplay of distance

Fayer, Michael D.

331

Demonstrating Electron Transfer and Nanotechnology: A Natural Dye-Sensitized Nanocrystalline Energy Converter  

Microsoft Academic Search

A unique solar cell fabrication procedure has been developed using natural anthocyanin dyes extracted from berries. It can be reproduced with a minimum amount of resources in order to provide an interdisciplinary approach for lower-division undergraduate students learning the basic principles of biological extraction, physical chemistry, and spectroscopy as well as environmental science and electron transfer. Electron transfer is the

Greg P. Smestad; Michael Gratzel

1998-01-01

332

49 CFR 225.37 - Optical media transfer and electronic submission.  

Code of Federal Regulations, 2011 CFR

...2011-10-01 2011-10-01 false Optical media transfer and electronic submission...CLASSIFICATION, AND INVESTIGATIONS § 225.37 Optical media transfer and electronic submission...reports, updates, and amendments by way of optical media (CD-ROM), or by means of...

2011-10-01

333

Vibrational coherence probes the mechanism of ultrafast electron transfer in polymer–fullerene blends  

NASA Astrophysics Data System (ADS)

The conversion of photoexcitations into charge carriers in organic solar cells is facilitated by the dissociation of excitons at the donor/acceptor interface. The ultrafast timescale of charge separation demands sophisticated theoretical models and raises questions about the role of coherence in the charge-transfer mechanism. Here, we apply two-dimensional electronic spectroscopy to study the electron transfer process in poly(3-hexylthiophene)/PCBM (P3HT/PCBM) blends. We report dynamics maps showing the pathways of charge transfer that clearly expose the significance of hot electron transfer. During this ultrafast electron transfer, vibrational coherence is directly transferred from the P3HT exciton to the P3HT hole polaron in the crystalline domain. This result reveals that the exciton converts to a hole with a similar spatial extent on a timescale far exceeding other photophysical dynamics including vibrational relaxation.

Song, Yin; Clafton, Scott N.; Pensack, Ryan D.; Kee, Tak W.; Scholes, Gregory D.

2014-09-01

334

Ultrafast intermolecular electron transfer from orthomethoxyaniline to excited coumarin dyes  

NASA Astrophysics Data System (ADS)

Ultrafast intermolecular electron transfer (ET) from orthomethoxyaniline (orthoanisidine, ANS) to a number of excited (S1) 4-trifluoromethyl-1,2-benzopyrones (coumarins) having differently substituted 7-amino group has been investigated by femtosecond fluorescence up-conversion technique. The ET dynamics in the present systems are nonsingle-exponential and occur faster than the diffusive solvation dynamics. The ET rates are largely dependent on the nature of the substituents at the 7-amino group of the coumarins. This dependence is well correlated with the free energy changes (?G0) for the ET reactions. The ET dynamics become slower on using deuterated ANS as the donor, where the amino group hydrogens of ANS are substituted by deuterium. The deuterium isotope effect, however, gradually reduces as the ET dynamics becomes faster. Conventional ET theories can not explain all the observations. The results are explained on the basis of the two-dimensional ET model, which considers the solvent coordinate and the intramolecular coordinate separately to depict the ET process. It is seen that in coumarin-ANS systems the ET occurs much faster than the coumarin-aniline systems. It is indicated that the electronic coupling matrix element, a parameter which determines the extent of interaction between the reactant and the product states in the ET process, is much larger in the present systems than for the coumarin-aniline systems. The deuterium isotope effect on the ET dynamics is explained in terms of the changes in the ?G0 values on isotopic substitution of the solvent donors.

Pal, Haridas; Shirota, Hideaki; Tominaga, Keisuke; Yoshihara, Keitaro

1999-06-01

335

Probing concerted proton–electron transfer in phenol–imidazoles  

PubMed Central

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 ?G°, 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 ?G°. PMID:18212121

Markle, Todd F.; Rhile, Ian J.; DiPasquale, Antonio G.; Mayer, James M.

2008-01-01

336

77 FR 6310 - Electronic Fund Transfers (Regulation E)  

Federal Register 2010, 2011, 2012, 2013, 2014

...described above would create information overload for consumers. Subsequent...transfer that contains accurate information about the transfer. The Bureau...described above would create information overload for consumers. The...

2012-02-07

337

Localization of alkali metal ions in sodium-promoted palladium catalysts as studied by low energy ion scattering and transmission electron microscopy  

SciTech Connect

Three series of palladium-based catalysts have been studied by Low Energy Ion Scattering (LEIS) and Transmission Electron Microscopy (TEM). The first series is comprised of Na-Pd/SiO{sub 2} catalysts, obtained by addition of palladium to a silica support and by further addition of sodium ions with a Na/Pd atomic ratio (R) equal to 0,6.4 and 25.6. The second series consists of palladium catalysts supported on natural pumice, in which, due to a different loading of supported palladium, R{prime}, the (Na+K)/Pd atomic ratio, is equal to 17.0 and 39.4. The third series is represented by two palladium-based catalysts supported on {open_quotes}model pumices,{close_quotes} synthetic silico-aluminates, obtained by sol-gel techniques, with a different amount of sodium, and R equal to 2.1 and 6.1 respectively. LEIS experiments and electron microscopy demonstrate a different location of alkali metal ions in the first two series: in the Na-Pd/SiO{sub 2} catalysts sodium is distributed in a way which is not uniform on the support and on the palladium metal, which is partly decorated with Na ions, whereas in the Pd/natural-pumice series the palladium surface is sodium-free. The results on the third series of catalysts, Pd/model pumice, are not definitive on the basis of the LEIS and TEM analyses, but by FTIR study of CO and CO{sub 2} adsorption, the decoration of palladium by sodium ions could be excluded. The results confirm the importance of the alkali metal ion location in alkali-promoted palladium catalysts and open new possibilities in the design of palladium-supported catalysts by a better control of promoter location. 18 refs., 5 figs., 2 tabs.

Liotta, L.F.; Deganello, G.; Delichere, P. [Istituto di Chimica e Tecnologia dei Prodotti Naturali, Palermo (Italy)] [and others] [Istituto di Chimica e Tecnologia dei Prodotti Naturali, Palermo (Italy); and others

1996-12-01

338

Synthesis, Characterization, Photophysics and Photochemistry of Pyrylogen Electron Transfer Sensitizers  

SciTech Connect

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.

Clennan, Edward L. [University of Wyoming, Laramie] [University of Wyoming, Laramie; Liao, Chen [ORNL] [ORNL

2014-01-01

339

Electron transfer reactivity of type zero Pseudomonas aeruginosa azurin.  

PubMed

Type zero copper is a hard-ligand analogue of the classical type 1 or blue site in copper proteins that function as electron transfer (ET) agents in photosynthesis and other biological processes. The EPR spectroscopic features of type zero Cu(II) are very similar to those of blue copper, although lacking the deep blue color, due to the absence of thiolate ligation. We have measured the rates of intramolecular ET from the pulse radiolytically generated C3-C26 disulfide radical anion to the Cu(II) in both type zero C112D/M121L and type 2 C112D Pseudomonas aeruginosa azurins in pH 7.0 aqueous solutions between 8 and 45 °C. We also have obtained rate/temperature (10-30 °C) profiles for ET reactions between these mutants and the wild-type azurin. Analysis of the rates and activation parameters for both intramolecular and intermolecular ET reactions indicates that the type zero copper reorganization energy falls in a range (0.9-1.1 eV) slightly above that for type 1 (0.7-0.8 eV), but substantially smaller than that for type 2 (>2 eV), consistent with XAS and EXAFS data that reveal minimal type zero site reorientation during redox cycling. PMID:21405124

Lancaster, Kyle M; Farver, Ole; Wherland, Scot; Crane, Edward J; Richards, John H; Pecht, Israel; Gray, Harry B

2011-04-01

340

Electron-transfer acceleration investigated by time resolved infrared spectroscopy.  

PubMed

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 discussed process, back-ET in a porphyrin-Re(I)(CO)3(N,N) dyad, demonstrates that formation of a hot product accelerates highly exergonic ET in the Marcus inverted region. Overall, it follows that ET can be accelerated by enhancing the electronic interaction and by vibrational excitation of the reacting system and its medium, stressing the importance of quantum nuclear dynamics in ET reactivity. These effects are experimentally accessible by time-resolved vibrational spectroscopies (IR, Raman) in combination with quantum chemical calculations. It is suggested that structural dynamics play different mechanistic roles in light-triggered ET involving electronically excited donors or acceptors than in ground-state processes. While TRIR spectroscopy is well suitable to elucidate ET processes on a molecular-level, transient 2D-IR techniques combining optical and two IR (or terahertz) laser pulses present future opportunities for investigating, driving, and controlling ET. PMID:25699661

Vl?ek, Antonín; Kvapilová, Hana; Towrie, Michael; Záliš, Stanislav

2015-03-17

341

Heterogeneous electron transfer at nanoscopic electrodes: importance of electronic structures and electric double layers.  

PubMed

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

Chen, Shengli; Liu, Yuwen; Chen, Junxiang

2014-08-01

342

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

SciTech Connect

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.

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

343

Photoinduced bimolecular electron transfer kinetics in small unilamellar vesicles  

SciTech Connect

Photoinduced electron transfer (ET) from N,N-dimethylaniline to some coumarin derivatives has been studied in small unilamellar vesicles (SUVs) of the phospholipid, DL-{alpha}-dimyristoyl-phosphatidylcholine, using steady-state and time-resolved fluorescence quenching, both below and above the phase transition temperature of the vesicles. The primary interest was to examine whether Marcus inversion [H. Sumi and R. A. Marcus, J. Chem. Phys. 84, 4894 (1986)] could be observed for the present ET systems in these organized assemblies. The influence of the topology of SUVs on the photophysical properties of the reactants and consequently on their ET kinetics has also been investigated. Absorption and fluorescence spectral data of the coumarins in SUVs and the variation of their fluorescence decays with temperature indicate that the dyes are localized in the bilayer of the SUVs. Time-resolved area normalized emission spectra analysis, however, reveals that the dyes are distributed in two different microenvironments in the SUVs, which we attribute to the two leaflets of the bilayer, one toward bulk water and the other toward the inner water pool. The microenvironments in the two leaflets are, however, not indicated to be that significantly different. Time-resolved anisotropy decays were biexponential for all the dyes in SUVs, and this has been interpreted in terms of the compound motion model according to which the dye molecules can experience a fast wobbling-in-cone type of motion as well as a slow overall rotating motion of the cone containing the molecule. The expected bimolecular diffusion-controlled rates in SUVs, as estimated by comparing the microviscosities in SUVs (determined from rotational correlation times) and that in acetonitrile solution, are much slower than the observed fluorescence quenching rates, suggesting that reactant diffusion (translational) does not play any role in the quenching kinetics in the present systems. Accordingly, clear inversions are observed in the correlation of the fluorescence quenching rate constants k{sub q} with the free energy change, {delta}G{sup 0} of the reactions. However, the coumarin dyes, C152 and C481 (cf. Scheme 1), show unusually high k{sub q} values and high activation barriers, which is not expected from Marcus ET theory. This unusual behavior is explained on the basis of participation of the twisted intramolecular charge transfer states of these two dyes in the ET kinetics.

Choudhury, Sharmistha Dutta; Kumbhakar, Manoj; Nath, Sukhendu; Pal, Haridas [Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085 (India)

2007-11-21

344

Evidence against the hopping mechanism as an important electron transfer pathway for conformationally constrained oligopeptides.  

PubMed

The rate constant of intramolecular electron transfer through oligopeptides based on the alpha-aminoisobutyric acid residue was determined as a function of the peptide length and found to depend weakly on the donor-acceptor separation. By measuring the electron-transfer activation energy and estimating the energy gap between donor and bridge, we were able to discard the electron hopping mechanism. PMID:15643851

Polo, Federico; Antonello, Sabrina; Formaggio, Fernando; Toniolo, Claudio; Maran, Flavio

2005-01-19

345

Electron transfer and protein dynamics in the photosynthetic reaction center.  

PubMed Central

We have measured the kinetics of electron transfer (ET) from the primary quinone (Q(A)) to the special pair (P) of the reaction center (RC) complex from Rhodobacter sphaeroides as a function of temperature (5-300 K), illumination protocol (cooled in the dark and under illumination from 110, 160, 180, and 280 K), and warming rate (1.3 and 13 mK/s). The nonexponential kinetics are interpreted with a quantum-mechanical ET model (Fermi's golden rule and the spin-boson model), in which heterogeneity of the protein ensemble, relaxations, and fluctuations are cast into a single coordinate that relaxes monotonically and is sensitive to all types of relaxations caused by ET. Our analysis shows that the structural changes that occur in response to ET decrease the free energy gap between donor and acceptor states by 120 meV and decrease the electronic coupling between donor and acceptor states from 2.7 x 10(-4) cm(-1) to 1.8 x 10(-4) cm(-1). At cryogenic temperatures, conformational changes can be slowed or completely arrested, allowing us to monitor relaxations on the annealing time scale (approximately 10(3)-10(4) s) as well as the time scale of ET (approximately 100 ms). The relaxations occur within four broad tiers of conformational substates with average apparent Arrhenius activation enthalpies of 17, 50, 78, and 110 kJ/mol and preexponential factors of 10(13), 10(15), 10(21), and 10(25) s(-1), respectively. The parameterization provides a prediction of the time course of relaxations at all temperatures. At 300 K, relaxations are expected to occur from 1 ps to 1 ms, whereas at lower temperatures, even broader distributions of relaxation times are expected. The weak dependence of the ET rate on both temperature and protein conformation, together with the possibility of modeling heterogeneity and dynamics with a single conformational coordinate, make RC a useful model system for probing the dynamics of conformational changes in proteins. PMID:9591682

McMahon, B H; Müller, J D; Wraight, C A; Nienhaus, G U

1998-01-01

346

Observation of orientation-dependent electron transfer in molecule–surface collisions  

PubMed Central

Molecules typically must point in specific relative directions to participate efficiently in energy transfer and reactions. For example, Förster energy transfer favors specific relative directions of each molecule’s transition dipole [Förster T (1948) Ann Phys 2(1-2):55–75] and electron transfer between gas-phase molecules often depends on the relative orientation of orbitals [Brooks PR, et al. (2007) J Am Chem Soc 129(50):15572–15580]. Surface chemical reactions can be many orders of magnitude faster than their gas-phase analogs, a fact that underscores the importance of surfaces for catalysis. One reason surface reactions can be so fast is the labile change of oxidation state that commonly takes place upon adsorption, a process involving electron transfer between a solid metal and an approaching molecule. By transferring electrons to or from the adsorbate, the process of bond weakening and/or cleavage is initiated, chemically activating the reactant [Yoon B, et al. (2005) Science 307(5708):403–407]. Here, we show that the vibrational relaxation of NO—an example of electronically nonadiabatic energy transfer that is driven by an electron transfer event [Gadzuk JW (1983) J Chem Phys 79(12):6341–6348]—is dramatically enhanced when the molecule approaches an Au(111) surface with the N atom oriented toward the surface. This represents a rare opportunity to investigate the steric influences on an electron transfer reaction happening at a surface. PMID:24127598

Bartels, Nils; Golibrzuch, Kai; Bartels, Christof; Chen, Li; Auerbach, Daniel J.; Wodtke, Alec M.; Schäfer, Tim

2013-01-01

347

Temperature Invariance of the Nitrogenase Electron Transfer Mechanism  

PubMed Central

Earlier studies on electron transfer (ET) from the nitrogenase Fe protein to the MoFe protein concluded that the mechanism for ET changed during cooling from 25°C to 5°C, based on the observation that the rate constant for Fe protein to MoFe protein ET decreases strongly, with a non-linear Arrhenius plot. They further indicated that the ET was reversible, with complete ET at ambient but with an equilibrium constant near unity at 5°C. These studies were carried out with buffers having a strong temperature coefficient. We have examined the temperature variation in the kinetics of oxidation of the Fe protein by the MoFe protein at constant pH = 7.4 fixed by the buffer MOPS, which has a very small temperature coefficient. Using MOPS, we also observe temperature dependent ET rate constants, with non-linear Arrhenius plots. But, we find that ET is gated across the temperature range by a conformational change that involves the binding of numerous water molecules, consistent with an unchanging ET mechanism. Furthermore, there is no sKIE throughout the temperature range studied, again consistent with an unchanging mechanism In addition, the non-linear Arrhenius plots are explained by the change in heat capacity caused by the binding of waters in an invariant gating ET mechanism. Together, these observations contradict the idea of a change in ET mechanism with cooling. Finally, the extent of ET at constant pH does not change significantly with temperature, in contrast to the previously proposed change in ET equilibrium. PMID:23050654

Mayweather, Diana; Danyal, Karamatullah; Dean, Dennis R.; Seefeldt, Lance C.; Hoffman, Brian M.

2012-01-01

348

High throughput electron transfer from carbon dots to chloroplast: a rationale of enhanced photosynthesis  

NASA Astrophysics Data System (ADS)

A biocompatible amine functionalized fluorescent carbon dots were developed and isolated for gram scale applications. Such carbogenic quantum dots can strongly conjugate over the surface of the chloroplast and due to that strong interaction the former can easily transfer electrons towards the latter by assistance of absorbed light or photons. An exceptionally high electron transfer from carbon dots to the chloroplast can directly effect the whole chain electron transfer pathway in a light reaction of photosynthesis, where electron carriers play an important role in modulating the system. As a result, carbon dots can promote photosynthesis by modulating the electron transfer process as they are capable of fastening the conversion of light energy to the electrical energy and finally to the chemical energy as assimilatory power (ATP and NADPH).A biocompatible amine functionalized fluorescent carbon dots were developed and isolated for gram scale applications. Such carbogenic quantum dots can strongly conjugate over the surface of the chloroplast and due to that strong interaction the former can easily transfer electrons towards the latter by assistance of absorbed light or photons. An exceptionally high electron transfer from carbon dots to the chloroplast can directly effect the whole chain electron transfer pathway in a light reaction of photosynthesis, where electron carriers play an important role in modulating the system. As a result, carbon dots can promote photosynthesis by modulating the electron transfer process as they are capable of fastening the conversion of light energy to the electrical energy and finally to the chemical energy as assimilatory power (ATP and NADPH). Electronic supplementary information (ESI) available. See DOI: 10.1039/c3nr06079a

Chandra, Sourov; Pradhan, Saheli; Mitra, Shouvik; Patra, Prasun; Bhattacharya, Ankita; Pramanik, Panchanan; Goswami, Arunava

2014-03-01

349

Measurement of Electromagnetic Radiation Emitted during Rapid Intramolecular Electron Transfer  

E-print Network

, photosynthesis, artificial photosynthesis, redox chemistry, photography, xerog- raphy, and other processes all the electromagnetic (EM) waveform broadcast by the charge-transfer process itself. Intermolecular and intramo- lecular

350

Persistence of Structure Over Fluctuations in Biological Electron-Transfer Reactions  

NASA Astrophysics Data System (ADS)

In the soft-wet environment of biomolecular electron transfer, it is possible that structural fluctuations could wash out medium-specific electronic effects on electron tunneling rates. We show that beyond a transition distance (2 3Å in water and 6 7Å in proteins), fluctuation contributions to the mean-squared donor-to-acceptor tunneling matrix element are likely to dominate over the average matrix element. Even though fluctuations dominate the tunneling mechanism at larger distances, we find that the protein fold is “remembered” by the electronic coupling, and structure remains a key determinant of electron transfer kinetics.

Balabin, Ilya A.; Beratan, David N.; Skourtis, Spiros S.

2008-10-01

351

Coherent transfer of light polarization to electron spins in a semiconductor.  

PubMed

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

Kosaka, Hideo; Shigyou, Hideki; Mitsumori, Yasuyoshi; Rikitake, Yoshiaki; Imamura, Hiroshi; Kutsuwa, Takeshi; Arai, Koichiro; Edamatsu, Keiichi

2008-03-01

352

Diameter selective electron transfer from encapsulated ferrocenes to single-walled carbon nanotubes  

NASA Astrophysics Data System (ADS)

The diameter selective photoluminescence quenching of single-walled carbon nanotubes (SWCNTs) is observed upon ferrocene encapsulation, which can be attributed to electron transfer from the encapsulated ferrocenes to the SWCNTs. Interestingly, the dependence of the electron transfer process on the nanotube diameter is governed by the molecular orientation of the ferrocenes in the SWCNT rather than the reduction potentials of the SWCNT.The diameter selective photoluminescence quenching of single-walled carbon nanotubes (SWCNTs) is observed upon ferrocene encapsulation, which can be attributed to electron transfer from the encapsulated ferrocenes to the SWCNTs. Interestingly, the dependence of the electron transfer process on the nanotube diameter is governed by the molecular orientation of the ferrocenes in the SWCNT rather than the reduction potentials of the SWCNT. Electronic supplementary information (ESI) available: Calculated binding energies of FeCp2@SWCNTs and additional spectroscopic characterization are described in ESI. See DOI: 10.1039/c4nr04398g

Iizumi, Yoko; Suzuki, Hironori; Tange, Masayoshi; Okazaki, Toshiya

2014-10-01

353

Chemical Bonding: The Classical Description sharing or transferring electrons between atoms  

E-print Network

Chemical Bonding: The Classical Description sharing or transferring electrons between atoms LEWIS Structure (w/o quantum mechanics) Chapter. 3 Two or more atoms approach -> their electrons interact and form new arrangements of electrons with lower total potential energy than isolated atoms covalent ionic

Ihee, Hyotcherl

354

Oxidation catalyst  

DOEpatents

The present invention generally relates to catalyst systems and methods for oxidation of carbon monoxide. The invention involves catalyst compositions which may be advantageously altered by, for example, modification of the catalyst surface to enhance catalyst performance. Catalyst systems of the present invention may be capable of performing the oxidation of carbon monoxide at relatively lower temperatures (e.g., 200 K and below) and at relatively higher reaction rates than known catalysts. Additionally, catalyst systems disclosed herein may be substantially lower in cost than current commercial catalysts. Such catalyst systems may be useful in, for example, catalytic converters, fuel cells, sensors, and the like.

Ceyer, Sylvia T. (Cambridge, MA); Lahr, David L. (Cambridge, MA)

2010-11-09

355

Photoinduced electron transfer for an eosin-tyrosine conjugate. Activity of the tyrosinate anion in long-range electron transfer in a protein-like polymer matrix  

SciTech Connect

The Xanthene dye eosin Y has been modified via a thiohydantoin link to the amine terminus of the amino acid L-tyrosine. Photochemical electron transfer involving the singlet state of the dye and the attached phenol-containing residue led to a reduction in eosin fluorescence quantum yield and lifetime for aqueous solutions at elevated pH. The conjugate provided an electron transfer product of relatively long lifetime (1 {mu}s range) observed by flash photolysis of solutions at pH 12.0, conditions under which the tyrosine moiety is ionized. The effects of binding of the conjugate in the polymer poly(vinylpyrrolidone) (PVP) on the rates of electron transfer of species of different charge type were examined. 30 refs., 5 figs., 1 tab.

Jones, G. II; Feng, Z.; Oh, C. [Boston Univ., MA (United States)

1995-03-23

356

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

SciTech Connect

Conventional and fast-kinetics techniques of photochemistry, photophysics, radiation chemistry, and electrochemistry were used to study the intermediates involved in transition metal excited-state electron-transfer reactions. These intermediates were excited state of Ru(II) and Cr(III) photosensitizers, their reduced forms, and species formed in reactions of redox quenchers and electron-transfer agents. Of particular concern was the back electron-transfer reaction between the geminate pair formed in the redox quenching of the photosensitizers, and the dependence of its rate on solution medium and temperature in competition with transformation and cage escape processes. (DLC)

Hoffman, M.Z.

1992-07-31

357

Ultrasound assisted arylation of benzyl alcohol with 4-nitrochlorobenzene under a new multi-site phase-transfer catalyst in solid-liquid condition.  

PubMed

The ultrasound assisted preparation of 1-(benzyloxy)-4-nitrobenzene from the reaction of 4-chloronitrobenzene (CNB) and benzyl alcohol was carried out successfully using potassium hydroxide and catalyzed by a new multi-site phase-transfer catalyst (MPTC) viz., 1,3,5-triethyl-1,3,5-trihexyl-1,3,5-triazinane-1,3,5-triium trichloride in a solid-liquid reaction condition (SL-MPTC). The advantage of using SL-MPTC is to avoid a serious hydration of potassium salt of benzyl alcohol in the reaction between 4-chloronitrobenzene (CNB) and benzyl alcohol. The reaction is greatly enhanced in the solid-liquid system, catalyzed by multi-site quaternary ammonium salt (MPTC) and ultrasound irradiation (40 kHz, 300 W) in a batch reactor, it shows that the overall reaction greatly enhanced with ultrasound irradiation than without ultrasound. The reaction mechanism is proposed and verified by examining the experimental evidence. A kinetic model is proposed in which a pseudo first-order rate law is sufficient to describe the results, such as the effects of agitation speed, ultrasound, different phase transfer catalysts and the effect of organic solvents, the amount of newly prepared MPTC, the effect of temperature, the amount of water, the concentration of 4-chloronitrobenzene (CNB) and potassium hydroxide concentrations. The apparent rate constant (kapp) were investigated in detail. Rational explanations to account for the phenomena on the results were made. PMID:24830817

Selvaraj, Varathan; Abimannan, Pachaiyappan; Rajendran, Venugopal

2014-09-01

358

Stakeholder perceptions to present electronic transfer of prescription (ETP) models Darren Mundy, David W. Chadwick, Ed Ball  

E-print Network

Stakeholder perceptions to present electronic transfer of prescription (ETP) models Darren Mundy, David W. Chadwick, Ed Ball ISI, University of Salford, Salford, M5 4WT Keywords ELECTRONIC TRANSFER the findings of five focus group sessions discussing the Electronic Transfer of Prescriptions (ETP), held

Kent, University of

359

Full-electron calculation of effective electronic couplings and excitation energies of charge transfer states: Application to hole transfer in DNA ?-stacks  

PubMed Central

In this work I develop and apply a theoretical method for calculating effective electronic couplings (or transfer integrals) between redox sites involved in hole or electron transfer reactions. The resulting methodology is a refinement and a generalization of a recently developed approach for transfer integral evaluation. In fact, it holds for any overlap between the charge-localized states used to represent charge transfer (CT) processes in the two-state model. The presented theoretical and computational analyses show that the prototype approach is recovered for sufficiently small overlaps. The method does not involve any empirical parameter. It allows a complete multielectron description, therefore including electronic relaxation effects. Furthermore, its theoretical formulation holds at any value of the given reaction coordinate and yields a formula for the evaluation of the vertical excitation energy (i.e., the energy difference between the adiabatic ground and first-excited electronic states) that rests on the same physical quantities used in transfer integral calculation. In this paper the theoretical approach is applied to CT in B-DNA base dimers within the framework of Density Functional Theory (DFT), although it can be implemented in other computational schemes. The results of this work, as compared with previous Hartree–Fock (HF) and post-HF evaluations, support the applicability of the current implementation of the method to larger ?-stacked arrays, where post-HF approaches are computationally unfeasible. PMID:19778106

Migliore, Agostino

2009-01-01

360

Scanning electrochemical microscopy determination of organic soluble MPC electron-transfer rates.  

PubMed

In this paper, we describe a novel method for measuring the forward heterogeneous electron-transfer rate constant (kf) through the thiol monolayer of gold monolayer protected clusters (MPCs) in solution using scanning electrochemical microscopy (SECM). Applying the equations for mixed mass-transfer and electron-transfer processes, we develop a new formula using only the diffusion coefficient and the tip radius and use it as part of a new method for evaluating SECM approach curves. This method is applied to determine the electron-transfer rates from a series of SECM approach curves for monodisperse hexanethiol MPCs and for polydisperse hexanethiol, octanethiol, decanethiol, dodecanethiol, and 2-phenyethylthiol gold MPCs. Our results show that as the alkanethiol length increases the rate of electron transfer decreases in a manner consistent with the previously proposed tunneling mechanism for the electron transfer in MPCs. However, the effective tunneling coefficient, Beta, is found to be only 0.41 A-1 for alkanethiol passivated MPCs compared to typical values of 1.1 A-1 for alkanethiols as self-assembled monolayers on two-dimensional gold substrates. Similar SECM approach curve results for Pt and Au MPCs indicate that the electron-transfer rate is dependent mostly on the composition of the thiol layer and not on differences in the core metal. PMID:17128997

Peterson, Rachel R; Cliffel, David E

2006-12-01

361

Proton-coupled electron transfer drives the proton pump of cytochrome c oxidase.  

PubMed

Electron transfer in cell respiration is coupled to proton translocation across mitochondrial and bacterial membranes, which is a primary event of biological energy transduction. The resulting electrochemical proton gradient is used to power energy-requiring reactions, such as ATP synthesis. Cytochrome c oxidase is a key component of the respiratory chain, which harnesses dioxygen as a sink for electrons and links O2 reduction to proton pumping. Electrons from cytochrome c are transferred sequentially to the O2 reduction site of cytochrome c oxidase via two other metal centres, Cu(A) and haem a, and this is coupled to vectorial proton transfer across the membrane by a hitherto unknown mechanism. On the basis of the kinetics of proton uptake and release on the two aqueous sides of the membrane, it was recently suggested that proton pumping by cytochrome c oxidase is not mechanistically coupled to internal electron transfer. Here we have monitored translocation of electrical charge equivalents as well as electron transfer within cytochrome c oxidase in real time. The results show that electron transfer from haem a to the O2 reduction site initiates the proton pump mechanism by being kinetically linked to an internal vectorial proton transfer. This reaction drives the proton pump and occurs before relaxation steps in which protons are taken up from the aqueous space on one side of the membrane and released on the other. PMID:16598262

Belevich, Ilya; Verkhovsky, Michael I; Wikström, Mårten

2006-04-01

362

On the involvement of electron transfer reactions in the fluorescence decay kinetics heterogeneity of proteins  

PubMed Central

Time-resolved fluorescence study of single tryptophan-containing proteins, nuclease, ribonuclease T1, protein G, glucagon, and mastoparan, has been carried out. Three different methods were used for the analysis of fluorescence decays: the iterative reconvolution method, as reviewed and developed in our laboratory, the maximum entropy method, and the recent method that we called "energy transfer" method. All the proteins show heterogeneous fluorescence kinetics (multiexponential decay). The origin of this heterogeneity is interpreted in terms of current theories of electron transfer process, which treat the electron transfer process as a radiationless transition. The theoretical electron transfer rate was calculated assuming the peptide bond carbonyl as the acceptor site. The good agreement between experimental and theoretical electron-transfer rates leads us to suggest that the electron-transfer process is the principal quenching mechanism of Trp fluorescence in proteins, resulting in heterogeneous fluorescence kinetics. Furthermore, the origin of apparent homogeneous fluorescence kinetics (monoexponential decay) in some proteins also can be explained on the basis of electron-transfer mechanism. PMID:11567101

Ababou, Abdessamad; Bombarda, Elisa

2001-01-01

363

On the involvement of electron transfer reactions in the fluorescence decay kinetics heterogeneity of proteins.  

PubMed

Time-resolved fluorescence study of single tryptophan-containing proteins, nuclease, ribonuclease T1, protein G, glucagon, and mastoparan, has been carried out. Three different methods were used for the analysis of fluorescence decays: the iterative reconvolution method, as reviewed and developed in our laboratory, the maximum entropy method, and the recent method that we called "energy transfer" method. All the proteins show heterogeneous fluorescence kinetics (multiexponential decay). The origin of this heterogeneity is interpreted in terms of current theories of electron transfer process, which treat the electron transfer process as a radiationless transition. The theoretical electron transfer rate was calculated assuming the peptide bond carbonyl as the acceptor site. The good agreement between experimental and theoretical electron-transfer rates leads us to suggest that the electron-transfer process is the principal quenching mechanism of Trp fluorescence in proteins, resulting in heterogeneous fluorescence kinetics. Furthermore, the origin of apparent homogeneous fluorescence kinetics (monoexponential decay) in some proteins also can be explained on the basis of electron-transfer mechanism. PMID:11567101

Ababou, A; Bombarda, E

2001-10-01

364

Unraveling the charge transfer/electron transport in mesoporous semiconductive TiO2 films by voltabsorptometry.  

PubMed

In this work, we demonstrate that chronoabsorptometry and more specifically cyclic voltabsorptometry are particularly well suited techniques for acquiring a comprehensive understanding of the dynamics of electron transfer/charge transport within a transparent mesoporous semiconductive metal oxide film loaded with a redox-active dye. This is illustrated with the quantitative analysis of the spectroelectrochemical responses of two distinct heme-based redox probes adsorbed in highly-ordered mesoporous TiO2 thin films (prepared from evaporation-induced self-assembly, EISA). On the basis of a finite linear diffusion-reaction model as well as the establishment of the analytical expressions governing the limiting cases, it was possible to quantitatively analyse, predict and interpret the unusual voltabsorptometric responses of the adsorbed redox species as a function of the potential applied to the semiconductive film (i.e., as a function of the transition from an insulating to a conductive state or vice versa). In particular, we were able to accurately determine the interfacial charge transfer rates between the adsorbed redox species and the porous semiconductor. Another important and unexpected finding, inferred from the voltabsorptograms, is an interfacial electron transfer process predominantly governed by the extended conduction band states of the EISA TiO2 film and not by the localized traps in the bandgap. This is a significant result that contrasts those previously observed for dye-sensitized solar cells formed of randomly sintered TiO2 nanoparticles, a behaviour that was ascribed to a particularly low density of localized surface states in EISA TiO2. The present methodology also provides a unique and straightforward access to an activation-driving force relationship according to the Marcus theory, thus opening new opportunities not only to investigate the driving-force effects on electron recombination dynamics in dye-sensitized solar cells but also to study the electron transfer/transport mechanisms in heterogeneous photoelectrocatalytic systems combining nanostructured semiconductor electrodes and heterogeneous redox-active catalysts. PMID:25804293

Renault, Christophe; Nicole, Lionel; Sanchez, Clément; Costentin, Cyrille; Balland, Véronique; Limoges, Benoît

2015-04-01

365

Electron donor-acceptor quenching and photoinduced electron transfer for coumarin dyes. Technical report, 1 January-31 October 1982  

SciTech Connect

The fluorescence of 7-aminocoumarins is quenched by a variety of organic electron donors or acceptors in acetonitrile. In general, donors with half-wave oxidation potentials less positive than 1.0 V vs SCE and acceptors with reduction potentials less negative than -1.5 V vs SCE are candidates for diffusion limited quenching of coumarin singlet states. Profiles of quenching rates are consistent with calculated free energies for electron transfer between excited coumarins and donors or acceptors. In flash photolysis experiments electron transfer for several dyes and quenchers (e.g., methyl viologen) is demonstrated. Relatively low yields of net electron transfer are consistently obtained due to inefficient ionic photodissociation via singlet quenching or a low yield of more photoactive coumarin triplets. Electrochemical properties of the coumarins have been investigated by cyclic voltammetry with the indications of reversible oxidation and irreversible reduction as important processes.

Jones, G. II; Griffin, S.F.; Choi, C.; Bergmark, W.R.

1983-10-31

366

The Role of Protein Fluctuation Correlations in Electron Transfer in Photosynthetic Complexes  

E-print Network

We consider the dependence of the electron transfer in photosynthetic complexes on correlation properties of random fluctuations of the protein environment. The electron subsystem is modeled by a finite network of connected electron (exciton) sites. The fluctuations of the protein environment are modeled by random telegraph processes, which act either collectively (correlated) or independently (uncorrelated) on the electron sites. We derived an exact closed system of first-order linear differential equations with constant coefficients, for the average density matrix elements and for their first moments. Under some conditions, we obtain analytic expressions for the electron transfer rates. We compare the correlated and uncorrelated regimes, and demonstrated numerically that the uncorrelated fluctuations of the protein environment can, under some conditions, either increase or decrease the electron transfer rates.

Alexander I. Nesterov; Gennady P. Berman

2014-11-28

367

Transferable pseudoclassical electrons for aufbau of atomic ions.  

PubMed

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

Ekesan, Solen; Kale, Seyit; Herzfeld, Judith

2014-06-01

368

Ligand and Tetrathiometalate Effects in Induced Internal Electron Transfer Reactions.  

PubMed

New rhenium sulfide complexes, [Re(IV)(2)(&mgr;-S)(2)(&mgr;-S(2))(&mgr;-S(2)COR)(S(2)COR)(2)](-) and [Re(IV)(2)(&mgr;-S)(2)(S(2)COR)(4)], and a new tungsten sulfide complex, [WS(S(2))(S(2)CC(6)H(5))(2)], have been synthesized and isolated via induced internal redox reactions involving the appropriate tetrathiometalate and 1,1-dithiolate disulfide. The red complex [Re(IV)(2)(&mgr;-S)(2)(&mgr;-S(2))(&mgr;-S(2)COR)(S(2)COR)(2)](-), 1, was isolated from the reaction of dialkylxanthogen disulfide, [(ROCS(2))(2)], and tetraethylammonium tetrathioperrhenate, [Et(4)N][Re(VII)S(4)]. Crystal structure analysis of 1 reveals an edge-sharing (&mgr;-S)(2) bioctahedron containing both bridging disulfide and xanthate ligands. This reaction is compared to the known reaction between tetraalkylthiuram disulfide, [(R(2)NCS(2))(2)], and [Et(4)N][ReS(4)], which produces the green complex [Re(IV)(2)(&mgr;-S)(2)(S(2)CNR(2))(4)]. The corresponding green alkyl xanthate analogue, [Re(IV)(2)(&mgr;-S)(2)(S(2)COR)(4)], 2, was synthesized by a simple redox reaction between rhenium pentachloride, ReCl(5), and potassium alkyl xanthate, [K(S(2)COR)]. Comparing 1 with other known [ReS(4)](-)/1,1-dithiolate disulfide reaction products, such as [Re(IV)(2)(&mgr;-S)(2)(S(2)CNR(2))(4)] and [Re(III)(S(2)CC(6)H(5))(S(3)CC(6)H(5))(2)], shows a correlation between the electron-donating ability of the ligand and the nature of the reaction product. Reactions of [Et(4)N](2)[Mo(VI)S(4)], [Et(4)N][Re(VII)S(4)], or [Et(4)N](2)[W(VI)S(4)] with dithiobenzoate disulfide, [(S(2)CC(6)H(5))(2)], reveal a correlation between the ligand-to-metal charge transfer energy band (LMCT(1)) of the tetrathiometalate and the reaction product. The known purple complex [Mo(IV)(S(2)CC(6)H(5))(4)] and two new green complexes, [Re(III)(S(2)CC(6)H(5))(S(3)CC(6)H(5))(2)] (recently communicated) and [W(VI)S(S(2))(S(2)CC(6)H(5))(2)], were isolated from related reactions. PMID:11670869

McConnachie, C. A.; Stiefel, E. I.

1999-03-01

369

Photoinduced electron transfer in zinc naphthalocyanine-naphthalenediimide supramolecular dyads.  

PubMed

Photoinduced electron transfer was studied in self-assembled donor-acceptor dyads, formed by axial coordination of pyridine appended with naphthalenediimide (NDI) to zinc naphthalocyanine (ZnNc). The NDI-py:ZnNc (1) and NDI(CH(2))(2)-py:ZnNc (2) self-assembled dyads absorb light over a wide region of the UV/Vis/near infrared (NIR) spectrum. The formation constants of the dyads 1 and 2 in toluene were found to be 2.5×10(4) and 2.2×10(4) M(-1), respectively, from the steady-state absorption and emission measurements, suggesting moderately stable complex formation. Fluorescence quenching was observed upon the coordination of the pyridine-appended NDI to ZnNc in toluene. The energy-level diagram derived from electrochemical and optical data suggests that exergonic charge separation through the singlet state of ZnNc ((1)ZnNc*) provides the main quenching pathway. Clear evidence for charge separation from the singlet state of ZnNc to NDI was provided by femtosecond laser photolysis measurements of the characteristic absorption bands of the ZnNc radical cation in the NIR region at 960 nm and the NDI radical anion in the visible region. The rates of charge-separation of 1 and 2 were found to be 2.2×10(10) and 4.4×10(9) s(-1), respectively, indicating fast and efficient charge separation (CS). The rates of charge recombination (CR) and the lifetimes of the charge-separated states were found to be 8.50×10(8) s(-1) (1.2 ns) for 1 and 1.90×10(8) s(-1) (5.3 ns) for 2. These values indicate that the rates of the CS and CR processes decrease as the length of the spacer increases. Their absorption over a wide portion of the solar spectrum and the high ratio of the CS/CR rates suggests that the self-assembled NDI-py:ZnNc and NDI(CH(2))(2)-py:ZnNc dyads are useful as photosynthetic models. PMID:22290735

El-Khouly, Mohamed E; Moiseev, Andrey G; van der Est, Art; Fukuzumi, Shunichi

2012-04-10

370

Pulse radiolytic studies of electron transfer processes and applications to solar photochemistry. Progress report  

SciTech Connect

The pulse radiolysis technique is applied to the study of electron transfer processes in a variety of chemical systems. Reactive intermediates are produced in solution by electron pulse irradiation and the kinetics of their reactions are followed by time resolved absorption spectrophotometry. Complementary experiments are carried out with excimer laser flash photolysis. These studies are concerned with mechanisms, kinetics, and thermodynamics of reactions of organic and inorganic radicals and unstable oxidation states of metal ions. Reactions are studied in both aqueous and non-aqueous solutions. The studies focus on the unique ability of pulse radiolysis to provide absolute rate constants for reactions of many inorganic radicals and organic peroxyl radicals, species that are key intermediates in many chemical processes. A special concern of this work is the study of electron transfer reactions of metalloporphyrins, which permits evaluation of these molecules as intermediates in solar energy conversion. Metalloporphyrins react with free radicals via electron transfer, involving the ligand or the metal center, or via bonding to the metal, leading to a variety of chemical species whose behavior is also investigated. The highlights of the results during the past three years are summarized below under the following sections: (a) electron transfer reactions of peroxyl radicals, concentrating on the characterization of new peroxyl radicals derived from vinyl, phenyl, other aryl, and pyridyl; (b) solvent effects on electron transfer reactions of inorganic and organic peroxyl radicals, including reactions with porphyrins, and (c) electron transfer and alkylation reactions of metalloporphyrins and other complexes.

Neta, P.

1995-02-01

371

urated alkyl linkages, suggesting that electron transfer occurs through the aqueous solvent  

E-print Network

with much faster electron transfer rates than the basal plane sites that predominate on carbon nanotubes of these materials. Landis and Hamers report that copper click chemistry can be used to attach groups

West, Stuart

372

Electrode assemblies composed of redox cascades from microbial respiratory electron transfer chains  

SciTech Connect

Respiratory and photosynthetic electron transfer chains are dependent on vectorial electron transfer through a series of redox proteins. Examples include electron transfer from NapC to NapAB nitrate reductase in Paracoccus denitrificans and from CymA to Fcc3 (flavocytochrome c3) fumarate reductase in Shewanella oneidensis MR-1. In the present article, we demonstrate that graphite electrodes can serve as surfaces for the stepwise adsorption of NapC and NapAB, and the stepwise adsorption of CymA and Fcc3. Aspects of the catalytic properties of these assemblies are different from those of NapAB and Fcc3 adsorbed in isolation. We propose that this is due to the formation of NapC-NapAB and of CymA-Fcc3 complexes that are capable of supporting vectorial electron transfer.

Gates, Andrew J.; Marritt, Sophie; Bradley, Justin; Shi, Liang; McMillan, Duncan G.; Jeuken, Lars J.; Richardson, David; Butt, Julea N.

2013-10-01

373

27 CFR 41.115a - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2013 CFR

...in taxes on tobacco products, cigarette papers, and cigarette tubes combining tax liabilities...commercial bank in making payment by electronic fund transfer (EFT) of taxes on tobacco products, cigarette papers, and cigarette...

2013-04-01

374

27 CFR 41.115a - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2014 CFR

...in taxes on tobacco products, cigarette papers, and cigarette tubes combining tax liabilities...commercial bank in making payment by electronic fund transfer (EFT) of taxes on tobacco products, cigarette papers, and cigarette...

2014-04-01

375

27 CFR 40.165a - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2012 CFR

...in taxes on tobacco products, cigarette papers, and cigarette tubes combining tax liabilities...commercial bank in making payment by electronic fund transfer (EFT) of taxes on tobacco products, cigarette papers, and cigarette...

2012-04-01

376

27 CFR 40.165a - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2014 CFR

...in taxes on tobacco products, cigarette papers, and cigarette tubes combining tax liabilities...commercial bank in making payment by electronic fund transfer (EFT) of taxes on tobacco products, cigarette papers, and cigarette...

2014-04-01

377

27 CFR 40.165a - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2013 CFR

...in taxes on tobacco products, cigarette papers, and cigarette tubes combining tax liabilities...commercial bank in making payment by electronic fund transfer (EFT) of taxes on tobacco products, cigarette papers, and cigarette...

2013-04-01

378

27 CFR 41.115a - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2012 CFR

...in taxes on tobacco products, cigarette papers, and cigarette tubes combining tax liabilities...commercial bank in making payment by electronic fund transfer (EFT) of taxes on tobacco products, cigarette papers, and cigarette...

2012-04-01

379

27 CFR 40.165a - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2010 CFR

...in taxes on tobacco products, cigarette papers, and cigarette tubes combining tax liabilities...commercial bank in making payment by electronic fund transfer (EFT) of taxes on tobacco products, cigarette papers, and cigarette...

2010-04-01

380

27 CFR 40.165a - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2011 CFR

...in taxes on tobacco products, cigarette papers, and cigarette tubes combining tax liabilities...commercial bank in making payment by electronic fund transfer (EFT) of taxes on tobacco products, cigarette papers, and cigarette...

2011-04-01

381

Proton-coupled electron transfer : from basic principles to small molecule activation  

E-print Network

Proton-coupled electron transfer (PCET) is the basic mechanism for bioenergetic conversion. Hallmark examples of such reactivities include water oxidation which is coupled to photosynthesis and oxygen reduction which is ...

Rosenthal, Joel, 1979-

2007-01-01

382

75 FR 59172 - Electronic Funds Transfer of Depository Taxes; Hearing Cancellation  

Federal Register 2010, 2011, 2012, 2013, 2014

...document cancels a public hearing on proposed regulation relating to Federal tax deposits (FTDs) by Electronic Funds Transfer (EFT). The proposed regulations affect all taxpayers that currently use FTD coupons. DATES: The public hearing, originally...

2010-09-27

383

76 FR 67153 - Federal Acquisition Regulation; Submission for OMB Review; Payment by Electronic Fund Transfer  

Federal Register 2010, 2011, 2012, 2013, 2014

...under the contract by electronic fund transfer (EFT). The information necessary to make the EFT transaction is specified in clause 52.232-33...Central Contractor Registration, which requires EFT information to be provided as specified by...

2011-10-31

384

48 CFR 52.232-38 - Submission of Electronic Funds Transfer Information with Offer.  

Code of Federal Regulations, 2011 CFR

...that is required to make payment by electronic funds transfer (EFT) under any contract that results from this solicitation. This submission satisfies the requirement to provide EFT information under paragraphs (b)(1) and (j) of the...

2011-10-01

385

Photochemical ribonucleotide reductase for the study of proton-coupled electron transfer  

E-print Network

Charge transport and catalysis in enzymes often rely on amino acid radicals as intermediates. The generation and transport of these radicals are synonymous with proton-coupled electron transfer (PCET), which intrinsically ...

Reece, Steven Y., 1980-

2007-01-01

386

77 FR 1555 - Administrative Simplification: Adoption of Standards for Health Care Electronic Funds Transfers...  

Federal Register 2010, 2011, 2012, 2013, 2014

...January 10, 2012 Part II Department of Health and Human Services...Simplification: Adoption of Standards for Health Care Electronic Funds Transfers (EFTs...DEPARTMENT OF HEALTH AND HUMAN SERVICES Office of the...

2012-01-10

387

27 CFR 41.115a - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2011 CFR

...2011-04-01 false Payment of tax by electronic fund transfer. ...and Firearms ALCOHOL AND TOBACCO TAX AND TRADE BUREAU, DEPARTMENT OF...Brought Into the United States Deferred Payment of Tax in Puerto Rico on Tobacco...

2011-04-01

388

27 CFR 41.115a - Payment of tax by electronic fund transfer.  

Code of Federal Regulations, 2010 CFR

...2010-04-01 false Payment of tax by electronic fund transfer. ...and Firearms ALCOHOL AND TOBACCO TAX AND TRADE BUREAU, DEPARTMENT OF...Brought Into the United States Deferred Payment of Tax in Puerto Rico on Tobacco...

2010-04-01

389

Design of a Molecular Memory Device: The Electron Transfer Shift Register Memory  

NASA Technical Reports Server (NTRS)

A molecular shift register memory at the molecular level is described. The memory elements consist of molecules can exit in either an oxidized or reduced state and the bits are shifted between the cells with photoinduced electron transfer reactions.

Beratan, D.

1993-01-01

390

36 CFR 1235.50 - What specifications and standards for transfer apply to electronic records?  

Code of Federal Regulations, 2010 CFR

...RECORDS ADMINISTRATION RECORDS MANAGEMENT TRANSFER OF RECORDS TO THE NATIONAL...discussions. (b) Data files and databases . Data files and databases must...on the NARA Electronic Records Management Initiative Web page at...

2010-07-01

391

Primary electron-transfer dynamics in 2-phenylindole–9-cyanoanthracene system  

Microsoft Academic Search

Electrochemical measurements by cyclic voltammetry predict the possibility of occurrence of photoinduced electron-transfer (PET) reactions between the ground state of 2-phenylindole (2PI) (electron donor) and the excited singlet of 9-cyanoanthracene (9CNA) molecule acting as an electron acceptor. However, 2PI should be expected to behave as a relatively weaker electron donating agent than the structurally related donor 2-methylindole (2MI) as it

T. Bhattacharya; S. K Pal; T Misra; R. D Saini; T Ganguly

2004-01-01

392

Electron impact excitation of SO2 - Differential, integral, and momentum transfer cross sections  

NASA Technical Reports Server (NTRS)

Electron impact excitation of the electronic states of SO2 was investigated. Differential, integral, and inelastic momentum transfer cross sections were obtained by normalizing the relative measurements to the elastic cross sections. The cross sections are given for seven spectral ranges of the energy-loss spectra extending from the lowest electronic state to near the first ionization limit. Most of the regions represent the overlap of several electronic transitions. No measurements for these cross sections have been reported previously.

Vuskovic, L.; Trajmar, S.

1982-01-01

393

Electron transfer dynamics and excited state branching in a charge-transfer platinum(II) donor-bridge-acceptor assembly.  

PubMed

A linear asymmetric Pt(ii) trans-acetylide donor-bridge-acceptor triad designed for efficient charge separation, NAP[triple bond, length as m-dash]Pt(PBu3)2[triple bond, length as m-dash]Ph-CH2-PTZ (), containing strong electron acceptor and donor groups, 4-ethynyl-N-octyl-1,8-naphthalimide (NAP) and phenothiazine (PTZ) respectively, has been synthesised and its photoinduced charge transfer processes characterised in detail. Excitation with 400 nm, ?50 fs laser pulse initially populates a charge transfer manifold stemming from electron transfer from the Pt-acetylide centre to the NAP acceptor and triggers a cascade of charge and energy transfer events. A combination of ultrafast time-resolved infrared (TRIR) and transient absorption (TA) spectroscopies, supported by UV-Vis/IR spectroelectrochemistry, emission spectroscopy and DFT calculations reveals a self-consistent photophysical picture of the excited state evolution from femto- to milliseconds. The characteristic features of the NAP-anion and PTZ-cation are clearly observed in both the TRIR and TA spectra, confirming the occurrence of electron transfer and allowing the rate constants of individual ET-steps to be obtained. Intriguingly, has three separate ultrafast electron transfer pathways from a non-thermalised charge transfer manifold directly observed by TRIR on timescales ranging from 0.2 to 14 ps: charge recombination to form either the intraligand triplet (3)NAP with 57% yield, or the ground state, and forward electron transfer to form the full charge-separated state (3)CSS ((3)[PTZ(+)-NAP(-)]) with 10% yield as determined by target analysis. The (3)CSS decays by charge-recombination to the ground state with ?1 ns lifetime. The lowest excited state is (3)NAP, which possesses a long lifetime of 190 ?s and efficiently sensitises singlet oxygen. Overall, molecular donor-bridge-acceptor triad demonstrates excited state branching over 3 different pathways, including formation of a long-distant (18 Å) full charge-separated excited state from a directly observed vibrationally hot precursor state. PMID:25361227

Scattergood, Paul A; Delor, Milan; Sazanovich, Igor V; Bouganov, Oleg V; Tikhomirov, Sergei A; Stasheuski, Alexander S; Parker, Anthony W; Greetham, Gregory M; Towrie, Michael; Davies, E Stephen; Meijer, Anthony J H M; Weinstein, Julia A

2014-12-21

394

Electron transfer from humic substances to biogenic and abiogenic Fe(III) oxyhydroxide minerals.  

PubMed

Microbial humic substance (HS) reduction and subsequent abiotic electron transfer from reduced HS to poorly soluble Fe(III) (oxyhydr)oxides, a process named electron shuttling, significantly increases microbial Fe(III) mineral reduction rates. However, the importance of electron shuttling in nature and notably the electron transfer from HS to biogenic Fe(III) (oxyhydr)oxides have thus far not been determined. In this study, we have quantified the rate and extent of electron transfer from reduced and nonreduced Pahokee Peat humic acids (PPHA) and fresh soil organic matter (SOM) extracts to both synthetic and environmentally relevant biogenic Fe(III) (oxyhydr)oxides. We found that biogenic Fe(III) minerals were reduced faster and to an equal or higher degree than their abiogenic counterparts. Differences were attributed to differences in crystallinity and the association of bacterial biomass with biogenic minerals. Compared to purified PPHA, SOM extract transferred fewer electrons per milligram of carbon and electron transfer was observed only to poorly crystalline ferrihydrite but not to more crystalline goethite. This indicates a difference in redox potential distribution of the redox-active functional groups in extracted SOM relative to the purified PPHA. Our results suggest that HS electron shuttling can also contribute to iron redox processes in environments where biogenic Fe(III) minerals are present. PMID:24400782

Piepenbrock, Annette; Schröder, Christian; Kappler, Andreas

2014-02-01

395

Photoinduced electron transfer and geminate recombination in liquids on short time scales: Experiments and theory  

E-print Network

Photoinduced electron transfer and geminate recombination in liquids on short time scales and geminate recombination between the hole donor Rhodamine 3B and hole acceptors N,N- dimethylaniline recombination back transfer is in the inverted region, and semiclassical treatment developed by Jortner J. Chem

Fayer, Michael D.

396

The influence of diffusion on photoinduced electron transfer and geminate recombination  

E-print Network

The influence of diffusion on photoinduced electron transfer and geminate recombination R. C transfer and geminate recombination in solutions of randomly distributed donors and acceptors is explored. The focus is on the effect diffusional motion has on geminate recombination. The reactive state (state

Fayer, Michael D.

397

Tuning electron transfer reactions by selective excitation in porphyrin-acceptor assemblies  

NASA Astrophysics Data System (ADS)

This thesis concerns electron transfer reactions from different excited states in porphyrins, and the effect of changing the energy of the link connecting the donor and acceptor. Photoinduced electron transfer, and subsequent processes were studied using ultrashort laser pulses and nanosecond laser flash photolysis. Excitation of Zn(II)-porphyrins in the Soret band lead to population of the higher lying S2 state. The lifetime and transient absorption spectrum was measured for the S2 state. When an electron acceptor was attached to the Zn(II)-porphyrin, either as an ion pair, or covalently bound through an amide link, electron transfer was found to compete with S 2 to S1 relaxation. In the ion pair, electron transfer was faster than 200 fs, with a lifetime of the charge separated state of 1.3 ps. Further, in the covalently linked dyad, the Zn(II) porphyrin triplet state was repopulated from a charge transfer state. In [2]-rotaxanes, the Zn(II) porphyrin donor (ZnP) and Au(III) porphyrin acceptor (AuP+) are not connected by a direct covalent link. Selective excitation of either the ZnP or the AuP+ resulted in rapid electron transfer from the ZnP to the AuP+. The bis-phenanthroline link connecting the different porphyrins was changed by coordination of Cu(I) or Ag(I). Electron transfer from the 1ZnP singlet was unaffected by coordination of either Ag(I) or Cu(I), while electron transfer to the 3AuP+ triplet was in the Ag(I) link found to occur by an enhanced superexchange, and by a sequential mechanism in the Cu(I) coordinated link.

Andersson, Mikael Anders Robert

398

Cyclic ruthenium-alkylidene catalysts for ring-expansion metathesis polymerization.  

PubMed

A series of cyclic Ru-alkylidene catalysts have been prepared and evaluated for their efficiency in ring-expansion metathesis polymerization (REMP). The catalyst structures feature chelating tethers extending from one N-atom of an N-heterocyclic carbene (NHC) ligand to the Ru metal center. The catalyst design is modular in nature, which provided access to Ru complexes having varying tether lengths, as well as electronically different NHC ligands. Structural impacts of the tether length were unveiled through (1)H NMR spectroscopy as well as single-crystal X-ray analyses. Catalyst activities were evaluated via polymerization of cyclooctene, and key data are provided regarding propagation rates, intramolecular chain transfer, and catalyst stabilities, three areas necessary for the efficient synthesis of cyclic poly(olefin)s via REMP. From these studies, it was determined that while increasing the tether length of the catalyst leads to enhanced rates of polymerization, shorter tethers were found to facilitate intramolecular chain transfer and release of catalyst from the polymer. Electronic modification of the NHC via backbone saturation was found to enhance polymerization rates to a greater extent than did homologation of the tether. Overall, cyclic Ru complexes bearing 5- or 6-carbon tethers and saturated NHC ligands were found to be readily synthesized, bench-stable, and highly active catalysts for REMP. PMID:18729450

Boydston, Andrew J; Xia, Yan; Kornfield, Julia A; Gorodetskaya, Irina A; Grubbs, Robert H

2008-09-24

399

Cyclic Ruthenium-Alkylidene Catalysts for Ring-Expansion Metathesis Polymerization  

PubMed Central

A series of cyclic Ru-alkylidene catalysts have been prepared and evaluated for their efficiency in ring-expansion metathesis polymerization (REMP). The catalyst structures feature chelating tethers extending from one N-atom of an imidazolylidine ligand to the Ru metal center. The catalyst design is modular in nature, which provided access to Ru-complexes having varying tether lengths, as well as electronically different NHC ligands. Structural impacts of the tether length were unveiled through 1H NMR spectroscopy as well as single-crystal X-ray analyses. Catalyst activities were evaluated via polymerization of cyclooctene and key data are provided regarding propagation rates, intramolecular chain-transfer, and catalyst stabilities, three areas necessary for the efficient synthesis of cyclic poly(olefin)s via REMP. From these studies, it was determined that while increasing the tether length of the catalyst leads to enhanced rates of polymerization, shorter tethers were found to facilitate intramolecular chain-transfer and release of catalyst from the polymer. Electronic modification of the NHC via backbone saturation was found to enhance polymerization rates to a greater extent than did homologation of the tether. Overall, cyclic Ru-complexes bearing 5- or 6-carbon tethers and saturated NHC ligands were found to be readily synthesized, bench-stable, and highly active catalysts for REMP. PMID:18729450

Boydston, Andrew J.; Xia, Yan; Kornfield, Julia A.; Gorodetskaya, Irina A.; Grubbs, Robert H.

2009-01-01

400

Amplified electron transfer at poly-ethylene-glycol (PEG) grafted electrodes.  

PubMed

"Amplified" electron transfer is observed purely based on electron transfer kinetic effects at modified carbon surfaces. An anodic attachment methodology is employed to modify the surface of glassy carbon or boron doped diamond electrodes with poly-ethylene glycols (PEGs) for polymerisation degrees of n = 4.5 to 9.1 (PEG200 to PEG400). Voltammetry and impedance data for aqueous Fe(CN)6(3-/4-) suggest systematic PEG structure-dependent effects on the standard rate constant for heterogeneous electron transfer as a function of PEG deposition conditions and average polymer chain length. Tunnel distance coefficients are polymerisation degree dependent and estimated for shorter PEG chains, ? = 0.17 Å(-1) for aqueous Fe(CN)6(3-/4-), consistent with a diffuse water-PEG interface. In contrast, electron transfer to 1,1'-ferrocene-dimethanol (at 1 mM concentration) appears un-impeded by PEG grafts. Mediated or "amplified" electron transfer to Fe(CN)6(3-/4-) based on the 1,1'-ferrocene-dimethanol redox shuttle is observed for both oxidation and reduction with estimated bimolecular rate constants for homogeneous electron transfer of kforward = 4 × 10(5) mol dm(3) s(-1) and kbackward = 1 × 10(5) mol dm(3) s(-1). Digital simulation analysis suggests an additional resistive component within the PEG graft double layer. PMID:25836325

Hotchen, Christopher E; Maybury, Ian J; Nelson, Geoffrey W; Foord, John S; Holdway, Philip; Marken, Frank

2015-04-21

401

Probing the contribution of electronic coupling to the directionality of electron transfer in photosynthetic reaction centers.  

PubMed

Subpicosecond transient absorption studies are reported for a set of Rhodobacter (R.) capsulatus bacterial photosynthetic reaction centers (RCs) designed to probe the origins of the unidirectionality of charge separation via one of two electron transport chains in the native pigment-protein complex. All of the RCs have been engineered to contain a heterodimeric primary electron donor (D) consisting of a bacteriochlorophyll (BChl) and a bacteriopheophytin (BPh). The BPh component of the M heterodimer (Mhd) or L heterodimer (Lhd) is introduced by substituting a Leu for His M200 or His L173, respectively. Previous work on primary charge separation in heterodimer mutants has not included the Lhd RC from R. capsulatus, which we report for the first time. The Lhd and Mhd RCs are used as controls against which we assess RCs that combine the heterodimer mutations with a second mutation (His substituted for Leu at M212) that results in replacement of the native L-side BPh acceptor with a BChl (beta). The transient absorption spectra reveal clear evidence for charge separation to the normally inactive M-side BPh acceptor (H(M)) in Lhd-beta RCs to form D+H(M)- with a yield of approximately 6%. This state also forms in Mhd-beta RCs but with about one-quarter the yield. In both RCs, deactivation to the ground state is the predominant pathway of D decay, as it is in the Mhd and Lhd single mutants. Analysis of the results indicates an upper limit ofV2L/V2m < or = 4 for the contribution of the electronic coupling elements to the relative rates of electron transfer to the L versus M sides of the wild-type RC. In comparison to the L/M rate ratio (kL/kM) approximately 30 for wild-type RCs, our findings indicate that electronic factors contribute approximately 35% at most to directionality with the other 65% deriving from energetic considerations, which includes differences in free energies, reorganization energies, and contributions of one- and two-step mechanisms on the two sides of the RC. PMID:16375408

Kirmaier, Christine; Bautista, James A; Laible, Philip D; Hanson, Deborah K; Holten, Dewey

2005-12-22

402

Catalysis and electron transfer in protein crystals: the binary and ternary complexes of methylamine dehydrogenase with electron acceptors.  

PubMed

Polarized absorption microspectrophotometry has been used to detect catalysis and intermolecular electron transfer in single crystals of two multiprotein complexes: (1) the binary complex between Paracoccus denitrificans methylamine dehydrogenase, which contains tryptophan-tryptophylquinone (TTQ) as a cofactor, and its redox partner, the blue copper protein amicyanin; (2) the ternary complex between the same two proteins and cytochrome c-551i. Continuous wave electron paramagnetic resonance has been used to compare the state of copper in polycrystalline powders of the two systems. While catalysis and intermolecular electron transfer from reduced TTQ to copper are too fast to be accessible to our measurements, heme reduction occurs over a period of several minutes. The observed rate constant is about four orders of magnitude lower than in solution. The analysis of the temperature dependence of this apparent constant provides values for the parameters H(AB), related to electronic coupling between the two centers, and lambda, the reorganizational energy, that are compatible with electron transfer being the rate-determining step. From these parameters and the known distance between copper and heme, it is possible to calculate the parameter beta, which depends on the nature of the intervening medium, obtaining a value typical of electron transfer across a protein matrix. These findings suggest that the ternary complex in solution might achieve a higher efficiency than the rigid crystal structure thanks to an as yet unidentified role of protein dynamics. PMID:12686155

Ferrari, Davide; Merli, Angelo; Peracchi, Alessio; Di Valentin, Marilena; Carbonera, Donatella; Rossi, Gian Luigi

2003-04-11

403

Experimental evidence for electron localization on Au upon photo-activation of Au/anatase catalysts.  

PubMed

Time resolved microwave conductivity (TRMC) measurements show that the presence of Au on anatase Hombikat UV100 significantly reduces the lifetime of mobile electrons formed by photo-excitation of this photocatalyst at 300 nm, providing evidence for the widely acclaimed electron localization effect of Au in promoting TiO(2) photocatalysts. Electron localization efficiency of Au was even higher for Au-promoted Hombikat calcined at 400 degrees C, explained by enlargement of the anatase particle size and the associated, relatively larger fraction of anatase particles in direct contact with Au. PMID:19421529

Carneiro, Joana T; Savenije, Tom J; Mul, Guido

2009-04-21

404

Experimental and theoretical demonstrations for the mechanism behind enhanced microbial electron transfer by CNT network.  

PubMed

Bioelectrochemical systems (BESs) share the principle of the microbially catalyzed anodic substrate oxidation. Creating an electrode interface to promote extracellular electron transfer from microbes to electrode and understanding such mechanisms are crucial for engineering BESs. In this study, significantly promoted electron transfer and a 10-times increase in current generation in a BES were achieved by the utilization of carbon nanotube (CNT) network, compared with carbon paper. The mechanisms for the enhanced current generation with the CNT network were elucidated with both experimental approach and molecular dynamic simulations. The fabricated CNT network was found to be able to substantially enhance the interaction between the c-type cytochromes and solid electron acceptor, indicating that the direct electron transfer from outer-membrane decaheme c-type cytochromes to electrode might occur. The results obtained in this study will benefit for the optimized design of new materials to target the outer membrane proteins for enhanced electron exchanges. PMID:24429552

Liu, Xian-Wei; Chen, Jie-Jie; Huang, Yu-Xi; Sun, Xue-Fei; Sheng, Guo-Ping; Li, Dao-Bo; Xiong, Lu; Zhang, Yuan-Yuan; Zhao, Feng; Yu, Han-Qing

2014-01-01

405

Charge transfer dynamics from adsorbates to surfaces with single active electron and configuration interaction based approaches  

NASA Astrophysics Data System (ADS)

We employ wavepacket simulations based on many-body time-dependent configuration interaction (TD-CI), and single active electron theories, to predict the ultrafast molecule/metal electron transfer time scales, in cyano alkanethiolates bonded to model gold clusters. The initial states represent two excited states where a valence electron is promoted to one of the two virtual ?? molecular orbitals localized on the cyanide fragment. The ratio of the two time scales indicate the efficiency of one charge transfer channel over the other. In both our one-and many-electron simulations, this ratio agree qualitatively with each other as well as with the previously reported experimental time scales (Blobner et al., 2012), measured for a macroscopic metal surface. We study the effect of cluster size and the description of electron correlation on the charge transfer process.

Ramakrishnan, Raghunathan; Nest, Mathias

2015-01-01

406

Experimental and Theoretical Demonstrations for the Mechanism behind Enhanced Microbial Electron Transfer by CNT Network  

PubMed Central

Bioelectrochemical systems (BESs) share the principle of the microbially catalyzed anodic substrate oxidation. Creating an electrode interface to promote extracellular electron transfer from microbes to electrode and understanding such mechanisms are crucial for engineering BESs. In this study, significantly promoted electron transfer and a 10-times increase in current generation in a BES were achieved by the utilization of carbon nanotube (CNT) network, compared with carbon paper. The mechanisms for the enhanced current generation with the CNT network were elucidated with both experimental approach and molecular dynamic simulations. The fabricated CNT network was found to be able to substantially enhance the interaction between the c-type cytochromes and solid electron acceptor, indicating that the direct electron transfer from outer-membrane decaheme c-type cytochromes to electrode might occur. The results obtained in this study will benefit for the optimized design of new materials to target the outer membrane proteins for enhanced electron exchanges. PMID:24429552

Liu, Xian-Wei; Chen, Jie-Jie; Huang, Yu-Xi; Sun, Xue-Fei; Sheng, Guo-Ping; Li, Dao-Bo; Xiong, Lu; Zhang, Yuan-Yuan; Zhao, Feng; Yu, Han-Qing

2014-01-01

407

pH-Dependent Reduction Potentials and Proton-Coupled Electron Transfer Mechanisms in Hydrogen-Producing Nickel Molecular Electrocatalysts  

SciTech Connect

The nickel-based Ph Bz 2 2 P N electrocatalysts, which are comprised of a nickel atom and two 1,5-dibenzyl-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane ligands, have been shown to effectively catalyze H2 production in acetonitrile. Recent electrochemical experiments revealed a linear dependence of the NiII/I reduction potential on pH, suggesting a proton-coupled electron transfer (PCET) reaction. In the proposed mechanism, the catalytic cycle begins with a PCET process involving electrochemical electron transfer to the nickel center and intermolecular proton transfer from an acid to the pendant amine ligand. This paper presents quantum mechanical calculations of this PCET process to examine the thermodynamics of the sequential mechanisms, in which either the electron or the proton transfers first (ET–PT and PT–ET, respectively), and the concerted mechanism (EPT). The favored mechanism depends on a balance among many factors, including the acid strength, association free energy for the acid–catalyst complex, PT free energy barrier, and ET reduction potential. The ET reduction potential is less negative after PT, favoring the PT–ET mechanism, and the association free energy is less positive after reduction, favoring the ET–PT mechanism. The calculations, along with analysis of the experimental data, indicate that the sequential ET–PT mechanism is favored for weak acids because of the substantial decrease in the association free energy after reduction. For strong acids, however, the PT–ET mechanism may be favored because the association free energy is somewhat smaller and PT is more thermodynamically favorable. The concerted mechanism could also occur, particularly for intermediate acid strengths. In the context of the entire catalytic cycle for H2 production, the initial PCET process involving intermolecular PT has a more negative reduction potential than the subsequent PCET process involving intramolecular PT. As a result, the second PCET should occur spontaneously, which is consistent with cyclic voltammogram experiments. This research was supported as part of the 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.

Horvath, Samantha; Fernandez, Laura; Appel, Aaron M.; Hammes-Schiffer, Sharon

2013-04-01

408

Reaction dynamics and proton coupled electron transfer: studies of tyrosine-based charge transfer in natural and biomimetic systems.  

PubMed

In bioenergetic reactions, electrons are transferred long distances via a hopping mechanism. In photosynthesis and DNA synthesis, the aromatic amino acid residue, tyrosine, functions as an intermediate that is transiently oxidized and reduced during long distance electron transfer. At physiological pH values, oxidation of tyrosine is associated with a deprotonation of the phenolic oxygen, giving rise to a proton coupled electron transfer (PCET) reaction. Tyrosine-based PCET reactions are important in photosystem II, which carries out the light-induced oxidation of water, and in ribonucleotide reductase, which reduces ribonucleotides to form deoxynucleotides. Photosystem II contains two redox-active tyrosines, YD (Y160 in the D2 polypeptide) and YZ (Y161 in the D1 polypeptide). YD forms a light-induced stable radical, while YZ functions as an essential charge relay, oxidizing the catalytic Mn?CaO? cluster on each of four photo-oxidation reactions. In Escherichia coli class 1a RNR, the ?2 subunit contains the radical initiator, Y122O•, which is reversibly reduced and oxidized in long range electron transfer with the ?2 subunit. In the isolated E. coli ?2 subunit, Y122O• is a stable radical, but Y122O• is activated for rapid PCET in an ?2?2 substrate/effector complex. Recent results concerning the structure and function of YD, YZ, and Y122 are reviewed here. Comparison is made to recent results derived from bioengineered proteins and biomimetic compounds, in which tyrosine-based charge transfer mechanisms have been investigated. This article is part of a Special Issue entitled: Vibrational spectroscopies and bioenergetic systems. PMID:25260243

Barry, Bridgette A

2015-01-01

409

Building model systems to understand Proton-Coupled Electron Transfer in heme : spectroscopic investigation of charge transfer to axially bound diimide acceptors  

E-print Network

Proton-Coupled Electron Transfer (PCET) is an important mechanistic motif in chemistry, which allows for efficient charge transport in many biological systems. We seek to understand how the proton and electron motions are ...

Hanson, Christina J

2013-01-01

410

Intramolecular, photoinduced electron transfer in ruthenium(II) bipyridine-quinone complexes  

SciTech Connect

Adsorption spectroscopy and transient emission spectroscopy were used to follow intramolecular electron transfer quenching after metal-to-ligand charge transfer (MLCT) excitation of Ru(II)bipyridine-quinone complexes. Studies of various N-((4{prime}methyl-2-2{prime}-bipyridyl)-4-methyl)-9,10-anthraquinone-2-carboxamide (bpy-AQ) complexes, revealed that these complexes quench by rapid electron transfer from ligand radical excited states to the metal center. A donor-chromophore-acceptor complex with the bpy-AQ ligand and the (10-[4({prime}-methyl-2,2{prime}-bipyridin-4-yl)methylphenothiazine)] (bpy-PTZ) ligand displays redox separation on excitation.

Opperman, K.A.; Mecklenburg, S.L.; Meyer, T.J. [Univ. of North Carolina, Chapel Hill, NC (United States)

1994-11-09

411

Use of electron-excitation energy transfer in dye laser active media  

SciTech Connect

A study was made of the spectral luminescence and lasing characteristics of two-component dye mixtures, and of the use of electron-excitation energy transfer to improve the laser emission parameters. A considerable increase in the lasing efficiencyu was found on exciting Trypaflavine by energy transfer from coumarin dyes. The use of electron-excitation energy transfer enabled the laser emission spectrum to be broadened. It was found that by varying the energy donor and acceptor concentrations one could produce laser emission of constant intensity in the spectral range between the donor and acceptor lasing regions.

Rodchenkova, V.V.; Reva, M.G.; Akimov, A.I.; Uzhinov, B.M.

1984-01-01

412

Frontier orbital symmetry control of intermolecular electron transfer  

SciTech Connect

This report contains sections describing the selection of electron donor-acceptor systems, the synthesis and photophysical properties of linked electron-donor-acceptor systems, the estimation of photoinduced charge-separation rate constants from fluorescence quenching data, and radical ion-pair recombination by picosecond transient absorption spectroscopy. 9 refs., 1 fig., 7 tabs.

Stevens, B.

1991-09-01

413

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

NASA Technical Reports Server (NTRS)

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.

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

1980-01-01

414

Cooperative phenomena associated with electron and proton transfer in quinhydrone charge-transfer crystal  

NASA Astrophysics Data System (ADS)

Studies of the IR vibrational spectroscopy of single crystals of the H-bonded quinhydrone charge-transfer complexes show that a new phase transition occurs upon application of hydrostatic pressure, associated with the proton transfer in the 2D proton lattice. The temperature- and pressure-induced changes in the O-H vibrational spectra are accounted for in terms of the melting of the proton lattice due to the proton tunneling effect.

Mitani, T.; Saito, G.; Urayama, H.

1988-05-01

415

A comparison of electron transfer in ribonucleotide reductase and the bacterial photosynthetic reaction center  

NASA Astrophysics Data System (ADS)

The energy requirements for electron transfer in two different proteins are compared, based on quantum chemical calculations. The methods used are the hybrid density functional B3LYP combined with dielectric cavity models to account for the effects of the polarizable protein. The experimental exothermicities for the electron transfer steps from the chlorophyll special pair both to the pheophytin and to the quinone are well reproduced. For ribonucleotide reductase (RNR), the same methods in contrast predict a large endothermicity for the electron transfer from the cysteine at the substrate site to the tyrosyl radical in the interior of the protein. This indicates that another type of process is active in RNR. Previous work has suggested that this process is hydrogen atom transfer.

Siegbahn, Per E. M.; Blomberg, Margareta R. A.; Pavlov, Maria

1998-08-01

416

Sensitizer-catalyst assemblies for water oxidation.  

PubMed

Two molecular assemblies with one Ru(II)-polypyridine photosensitizer covalently linked to one Ru(II)(bda)L2 catalyst (1) (bda = 2,2'-bipyridine-6,6'-dicarboxylate) and two photosensitizers covalently linked to one catalyst (2) have been prepared using a simple C-C bond as the linkage. In the presence of sodium persulfate as a sacrificial electron acceptor, both of them show high activity for catalytic water oxidation driven by visible light, with a turnover number up to 200 for 2. The linked photocatalysts show a lower initial yield for light driven oxygen evolution but a much better photostability compared to the three component system with separate sensitizer, catalyst and acceptor, leading to a much greater turnover number. Photocatalytic experiments and time-resolved spectroscopy were carried out to probe the mechanism of this catalysis. The linked catalyst in its Ru(II) state rapidly quenches the sensitizer, predominantly by energy transfer. However, a higher stability under photocatalytic condition is shown for the linked sensitizer compared to the three component system, which is attributed to kinetic stabilization by rapid photosensitizer regeneration. Strategies for employment of the sensitizer-catalyst molecules in more efficient photocatalytic systems are discussed. PMID:25700086

Wang, Lei; Mirmohades, Mohammad; Brown, Allison; Duan, Lele; Li, Fusheng; Daniel, Quentin; Lomoth, Reiner; Sun, Licheng; Hammarström, Leif

2015-03-16

417

FRONTIERS ARTICLE Dynamics of excited state electron transfer at a liquid interface  

E-print Network

importance in electrochemical exper- iments is that both liquids must contain supporting electrolytes [20 and tech- nological importance [1­7]. The physical and chemical characteris- tics of excited state electron to be important in advancing our understanding of electron transfer pro- cesses [1,18,19]. With the development

Turro, Nicholas J.

418

The transfer matrices and electronic spectrum of the multilayered system in a homogeneous magnetic field  

Microsoft Academic Search

The model of the multilayered media of the Kronig-Penney type with the applied magnetic field oriented parallel to the layer surfaces is investigated. The operator of translation is expressed in terms of the parabolic cylinder functions. The difference equations for the transfer matrix and the electron wavefunction, as well as the rigorous equations on the electron energy spectrum, are obtained.

Yu G. Peisakhovich

1999-01-01

419

Convergent Iterative Constrained Variation Algorithm for Calculation of Electron-Transfer Transition States  

E-print Network

as a function of the elec- trode potential using high-level ab initio quantum computations. Their model, namelyConvergent Iterative Constrained Variation Algorithm for Calculation of Electron the potential-dependent transition states of electron transfer reactions by quantum calculations. This approach

Pitsch, Heinz

420

Electronic Effects in PCP-Pincer Ru(II)Based Hydrogen Transfer Catalysis  

Microsoft Academic Search

The synthesis and characterization of novel cyclometalated ruthenium(II) complexes [RuCl(PCPOMe)(PPh3)] and [RuCl(PCPCF3)(PPh3)] containing monoanionic, tridentate coordinating PCP-pincer ligands [C6H3{CH2P(p-MeOC6H4)2}2-2,6]- (PCPOMe) and [C6H3{CH2P(p-CF3C6H4)2}2-2,6]- (PCPCF3) are reported. These compounds have been tested as catalyst precursors in the hydrogen transfer reaction of cyclohexanone to cyclohexanol in 2-propanol using NaOH as a base. The initial rate of the hydrogen transfer reaction appeared to depend

Marcella Gagliardo; Preston A. Chase; Sander Brouwer; Gerard P. M. van Klink; Gerard van Koten

2007-01-01

421

Rhodium Catalysts in the Oxidation of CO by O2 and NO: Shape, Composition, and Hot Electron Generation  

SciTech Connect

It is well known that the activity, selectivity, and deactivation behavior of heterogeneous catalysts are strongly affected by a wide variety of parameters, including but not limited to nanoparticle size, shape, composition, support, pretreatment conditions, oxidation state, and electronic state. Enormous effort has been expended in an attempt to understand the role of these factors on catalytic behavior, but much still remains to be discovered. In this work, we have focused on deepening the present understanding of the role of nanoparticle shape, nanoparticle composition, and hot electrons on heterogeneous catalysis in the oxidation of carbon monoxide by molecular oxygen and nitric oxide. These reactions were chosen because they are important for environmental applications, such as in the catalytic converter, and because there is a wide range of experimental and theoretical insight from previous single crystal work as well as experimental data on nanoparticles obtained using new state-of-the-art techniques that aid greatly in the interpretation of results on complex nanoparticle systems. In particular, the studies presented in this work involve three types of samples: {approx} 6.5 nm Rh nanoparticles of different shapes, {approx} 15 nm Rh1-xPdx core-shell bimetallic polyhedra nanoparticles, and Rh ultra-thin film ({approx} 5 nm) catalytic nanodiodes. The colloidal nanoparticle samples were synthesized using a co-reduction of metal salts in alcohol and supported on silicon wafers using the Langmuir-Blodgett technique. This synthetic strategy enables tremendous control of nanoparticle size, shape, and composition. Nanoparticle shape was controlled through the use of different organic polymer capping layers. Bimetallic core-shell nanoparticles were synthesized by careful choice of metal salt precursors. Rh/TiO{sub x} and Rh/GaN catalytic nanodiodes were fabricated using a variety of thin film device fabrication techniques, including reactive DC magnetron sputtering, electron beam evaporation, and rapid thermal annealing. The combination of these techniques enabled control of catalytic nanodiode morphology, geometry, and electrical properties.

Renzas, James R.

2010-03-08

422

Distance-dependent electron transfer at passivated electrodes decorated by gold nanoparticles.  

PubMed

The phenomenon of nanoparticles attached to an electrode passivated by an organic layer allowing efficient electron transfer between redox species in solution and the underlying electrode to be restored has resulted in Chazalviel and Allongue proposing a theory [Chazalviel, J.-N.; Allongue, P. J. Am. Chem. Soc.2011, 133, 762-764] to explain this phenomenon. The theory suggests that with electrode-organic layer-nanoparticle constructs, high exchange current densities, compared with when the nanoparticles are absent, results in the rate of electron transfer being independent of the thickness of the organic layer until a threshold thickness is exceeded. Thereafter, the thicker the organic layer, the slower the rate of electron transfer. Herein we provide the first experimental data to support this theory using a single experimental system that can show the transition from thickness independent electron transfer kinetics to distant dependent kinetics. This was achieved using ethylenediamine electrodeposited on a glassy carbon electrode. Different numbers of deposition cycles were applied in order to fabricate different thicknesses of the organic film. The deposited films showed progressively greater blocking abilities toward ruthenium hexamine, as a redox active probe in solution, as the films got thicker. Electron transfer kinetics of nanoparticle-decorated surfaces showed a change from thickness independent to thickness dependent as the organic layer exceeded an average thickness of 20 Å. Electrochemical impedance spectroscopy, cyclic voltammetry, scanning electron microscopy, ellipsometry, and atomic force microscopy were used to characterize the fabricated surfaces. PMID:23215046

Barfidokht, Abbas; Ciampi, Simone; Luais, Erwann; Darwish, Nadim; Gooding, J Justin

2013-01-15

423

Molecular view of an electron transfer process essential for iron–sulfur protein biogenesis  

PubMed Central

Biogenesis of iron–sulfur cluster proteins is a highly regulated process that requires complex protein machineries. In the cytosolic iron–sulfur protein assembly machinery, two human key proteins—NADPH-dependent diflavin oxidoreductase 1 (Ndor1) and anamorsin—form a stable complex in vivo that was proposed to provide electrons for assembling cytosolic iron–sulfur cluster proteins. The Ndor1–anamorsin interaction was also suggested to be implicated in the regulation of cell survival/death mechanisms. In the present work we unravel the molecular basis of recognition between Ndor1 and anamorsin and of the electron transfer process. This is based on the structural characterization of the two partner proteins, the investigation of the electron transfer process, and the identification of those protein regions involved in complex formation and those involved in electron transfer. We found that an unstructured region of anamorsin is essential for the formation of a specific and stable protein complex with Ndor1, whereas the C-terminal region of anamorsin, containing the [2Fe-2S] redox center, transiently interacts through complementary charged residues with the FMN-binding site region of Ndor1 to perform electron transfer. Our results propose a molecular model of the electron transfer process that is crucial for understanding the functional role of this interaction in human cells. PMID:23596212

Banci, Lucia; Bertini, Ivano; Calderone, Vito; Ciofi-Baffoni, Simone; Giachetti, Andrea; Jaiswal, Deepa; Mikolajczyk, Maciej; Piccioli, Mario; Winkelmann, Julia

2013-01-01

424

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

SciTech Connect

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.

Baldacchini, Chiara [Biophysics and Nanoscience Centre, DEB-CNISM, Università della Tuscia, I-01100 Viterbo (Italy); Institute of Agro-environmental and Forest Biology, National Research Council, I-05010 Porano (Italy); Bizzarri, Anna Rita; Cannistraro, Salvatore, E-mail: cannistr@unitus.it [Biophysics and Nanoscience Centre, DEB-CNISM, Università della Tuscia, I-01100 Viterbo (Italy)

2014-03-03

425

Propargylation of indene-1,3-dione under a new phase-transfer catalyst combined with ultrasonication--a kinetic study.  

PubMed

In the present study, kinetics of synthesis of 2,2-di(prop-2-ynyl)-1H-indene-1,3(2H)-dione was successfully carried out by propargylation of indene-1,3-dione with propargyl bromide using aqueous potassium hydroxide and catalyzed by a newly synthesized phase-transfer catalyst viz., N-benzyl-N-ethyl-N-isopropylpropan-2-ammonium bromide, PTC under ultrasonic (40 kHz, 300 W) assisted organic solvent condition. The pseudo first-order kinetic equation was applied to describe the overall reaction. Under ultrasound irradiation (40 kHz, 300 W) in a batch reactor, it shows that the overall reaction rate can be greatly enhanced with ultrasound irradiation than without ultrasound. PMID:24134829

Selvaraj, Varathan; Rajendran, Venugopal

2014-03-01

426

Effect of ultrasound in the free radical polymerization of acrylonitrile under a new multi-site phase-transfer catalyst - a kinetic study.  

PubMed

The kinetics of polymerization of acrylonitrile (AN) was carried out under heterogeneous condition using a new multi-site phase-transfer catalyst (MPTC), viz., N,N'-dihexyl-4,4'-bipyridinium dibromide in the presence of water soluble initiator, potassium peroxydisulphate (PDS) under chlorobenzene/water two phase system assisted by ultrasound irradiation at constant temperature 60+1°C under nitrogen atmosphere. The rate of polymerization increases with an increasing the concentrations of AN, MPTC and PDS. The order with respect to [AN], [MPTC], and [PDS] were found to be 1.01, 1.03 and 0.52, respectively. Based on the observed results a suitable mechanism has been proposed to account for the experimental observations followed by a discussion on its significance. PMID:24880766

Selvaraj, Varathan; Sakthivel, Perumal; Rajendran, Venugopal

2015-01-01

427

Proton-coupled electron transfer: Free radicals under control  

NASA Astrophysics Data System (ADS)

Biological solar energy conversion requires the coordinated and rapid movement of protons and electrons through complex proteins, called reaction centres. Now, an artificial and structurally simple reaction centre has been synthesized that mimics an important, photosynthetic charge relay.

Barry, Bridgette A.

2014-05-01

428

Energy transfer of a relativistic electron beam to a plasma  

Microsoft Academic Search

The energy loss of a relativistic electron beam and the accompanying plasma heating have been measured in an experiment where an electron beam of 500 keV, 0.5 kA, and 20-ns pulse duration was injected into a plasma of density between 100 billion and 1 trillion per cu cm confined in a magnetic mirror trap with 0.2 T in the homogeneous

B. Jurgens; H. J. Hopman; P. de Haan; P. C. de Jagher; J. Kistemaker

1977-01-01

429

Photo-induced electron transfer between a dendritic zinc(II) phthalocyanine and methyl viologen  

NASA Astrophysics Data System (ADS)

The intermolecular electron transfer between the carboxylic dendritic zinc(II) phthalocyanines [G1-ZnPc( and G2-ZnPc(] and methyl viologen (MV) is studied by steady-state fluorescence and UV/Vis absorption spectroscopic method. The effect of dendron generation of this series of dendritic phthalocyanines on intermolecular electron transfer is investigated. The results show that the fluorescence emission of these dendritic phthalocyanines could be greatly quenched by MV upon excitation at 610 nm. The Stern-Volmer constant (KSV) of electron transfer is decreased with increasing dendron generations. Our study suggests that these dendritic phthalocyanines are an effective new electron donor and transmission complex and could be used as a potential artificial photosynthesis system.

Wang, Yuhua; Chen, Jiangxu; Huang, Lishan; Xie, Shusen; Yang, Hongqin; Peng, Yiru

2013-01-01

430

Electron transfer and coupling in graphene-tungsten disulfide van der Waals heterostructures.  

PubMed

The newly discovered two-dimensional materials can be used to form atomically thin and sharp van der Waals heterostructures with nearly perfect interface qualities, which can transform the science and technology of semiconductor heterostructures. Owing to the weak van der Waals interlayer coupling, the electronic states of participating materials remain largely unchanged. Hence, emergent properties of these structures rely on two key elements: electron transfer across the interface and interlayer coupling. Here we show, using graphene-tungsten disulfide heterostructures as an example, evidence of ultrafast and highly efficient interlayer electron transfer and strong interlayer coupling and control. We find that photocarriers injected in tungsten disulfide transfer to graphene in 1?ps and with near-unity efficiency. We also demonstrate that optical properties of tungsten disulfide can be effectively tuned by carriers in graphene. These findings illustrate basic processes required for using van der Waals heterostructures in electronics and photonics. PMID:25421098

He, Jiaqi; Kumar, Nardeep; Bellus, Matthew Z; Chiu, Hsin-Ying; He, Dawei; Wang, Yongsheng; Zhao, Hui

2014-01-01

431

Electron transfer and coupling in graphene–tungsten disulfide van der Waals heterostructures  

NASA Astrophysics Data System (ADS)

The newly discovered two-dimensional materials can be used to form atomically thin and sharp van der Waals heterostructures with nearly perfect interface qualities, which can transform the science and technology of semiconductor heterostructures. Owing to the weak van der Waals interlayer coupling, the electronic states of participating materials remain largely unchanged. Hence, emergent properties of these structures rely on two key elements: electron transfer across the interface and interlayer coupling. Here we show, using graphene–tungsten disulfide heterostructures as an example, evidence of ultrafast and highly efficient interlayer electron transfer and strong interlayer coupling and control. We find that photocarriers injected in tungsten disulfide transfer to graphene in 1?ps and with near-unity efficiency. We also demonstrate that optical properties of tungsten disulfide can be effectively tuned by carriers in graphene. These findings illustrate basic processes required for using van der Waals heterostructures in electronics and photonics.

He, Jiaqi; Kumar, Nardeep; Bellus, Matthew Z.; Chiu, Hsin-Ying; He, Dawei; Wang, Yongsheng; Zhao, Hui

2014-11-01

432

Charge transfer by electronic excitation: Direct measurement by high resolution spectroscopy in the gas phase.  

PubMed

We report a quantitative measurement of the amount of charge that is transferred when the single ammonia complex of the photoacid beta-naphthol (2HNA) is excited by light. The measurement was made by comparing the permanent electric dipole moments of cis-2HNA in its ground (S(0)) and excited (S(1)) states, determined by Stark-effect studies of its fully resolved S(1)<--S(0) electronic spectrum. While the increase in electron transfer from the donor (NH(3)) to the acceptor (2HN) upon excitation is small ( approximately 0.05e), it is sufficient to redshift the electronic spectrum of the complex by approximately 600 cm(-1) ( approximately 0.1 eV). Thereby explored is the incipient motion of the acid-base complex along the excited state (electron-coupled) proton transfer coordinate. PMID:19968329

Fleisher, A J; Morgan, P J; Pratt, D W

2009-12-01

433

Electron transfer from sulfate-reducing becteria biofilm promoted by reduced graphene sheets  

NASA Astrophysics Data System (ADS)

Reduced graphene sheets (RGSs) mediate electron transfer between sulfate-reducing bacteria (SRB) and solid electrodes, and promote the development of microbial fuel cells (MFC). We have investigated RSG-promoted electron transfer between SRB and a glassy carbon (GC) electrode. The RGSs were produced at high yield by a chemical sequence involving graphite oxidation, ultrasonic exfoliation of nanosheets, and N2H4 reduction. Cyclic voltammetric testing showed that the characteristic anodic peaks (around 0.3 V) might arise from the combination of bacterial membrane surface cytochrome c3 and the metabolic products of SRB. After 6 d, another anodic wave gradually increased to a maximum current peak and a third anodic signal became visible at around 0 V. The enhancements of two characteristic anodic peaks suggest that RSGs mediate electron-transfer kinetics between bacteria and the solid electrode. Manipulation of these recently-discovered electron-transport mechanisms will lead to significant advances in MFC engineering.

Wan, Yi; Zhang, Dun; Wang, Yi; Wu, Jiajia

2012-01-01

434

Titanium salan catalysts for the asymmetric epoxidation of alkenes: steric and electronic factors governing the activity and enantioselectivity.  

PubMed

A new insight into the highly enantioselective (up to >99.5?% ee) epoxidation of olefins in the presence of chiral titanium(IV) salan complexes is reported. A series of 14 chiral ligands with varying steric and electronic properties have been designed, and it was found that electronic effects modu