Note: This page contains sample records for the topic electron transfer catalyst from Science.gov.
While these samples are representative of the content of Science.gov,
they are not comprehensive nor are they the most current set.
We encourage you to perform a real-time search of Science.gov
to obtain the most current and comprehensive results.
Last update: August 15, 2014.
1

Au25 Clusters as Electron-Transfer Catalysts Induced the Intramolecular Cascade Reaction of 2-nitrobenzonitrile  

PubMed Central

Design of atomically precise metal nanocluster catalysts is of great importance in understanding the essence of the catalytic reactions at the atomic level. Here, for the first time, Au25z nanoslusters were employed as electron transfer catalysts to induce an intramolecular cascade reaction at ambient conditions and gave rise to high conversion (87%) and selectivity (96%). Electron spin-resonance spectra indeed confirmed the consecutive electron transfer process and the formation of N radical. UV-vis absorption spectra also verified Au25z was intact after the catalytic circle. Our research may open up wide opportunities for extensive organic reactions catalyzed by Au25z.

Chong, Hanbao; Li, Peng; Wang, Shuxin; Fu, Fangyu; Xiang, Ji; Zhu, Manzhou; Li, Yadong

2013-01-01

2

The study of electron transfer in advanced materials (electrodes for Li-ion batteries or catalysts)  

NASA Astrophysics Data System (ADS)

Mössbauer Spectroscopy (MS) is really suitable to study local electronic structures. Its hyperfine parameters, isomer shift (IS) and quadrupole splitting (QS) allow to characterize the oxidation state and coordination of the probed element. So, the capabilities of this powerful technique have been highlighted for the study of electron transfer occurring during electrochemical or catalytic processes. Several examples illustrate how MS can be used for the determination of reaction mechanisms in new electrode materials of Li-ion batteries (Sb or Sn-based, Ti oxides) or reforming catalysts (supported bimetallic PtSn catalysts). Deeper insight into the mechanisms determining the electrochemical or catalytic performances can be expected.

Jumas, Jean-Claude; Womes, Manfred; Lippens, Pierre-Emmanuel; Olivier-Fourcade, Josette

2008-02-01

3

Arsenate stabilized Cu?O nanoparticle catalyst for one-electron transfer reversible reaction.  

PubMed

The befitting capping capabilities of AsO4(3-) provide a stable Cu2O nanocatalyst from a galvanic reaction between a Cu(II) precursor salt and As(0) nanoparticles. This stable Cu2O hydrosol appears to be a suitable catalyst for the one-electron transfer reversible redox reaction between Eosin Y and NaBH4. The progress of the reaction relates to three different kinetic stages. In the presence of the new catalyst the reversible redox reaction of Eosin Y in air shows a periodic color change providing a new crowd-pleasing demonstration, i.e. a "clock reaction". PMID:24626596

Sahoo, Ramkrishna; Dutta, Soumen; Pradhan, Mukul; Ray, Chaiti; Roy, Anindita; Pal, Tarasankar; Pal, Anjali

2014-05-14

4

Photoinduced electron transfer in a chromophore-catalyst assembly anchored to TiO2.  

PubMed

Photoinduced formation, separation, and buildup of multiple redox equivalents are an integral part of cycles for producing solar fuels in dye-sensitized photoelectrosynthesis cells (DSPECs). Excitation wavelength-dependent electron injection, intra-assembly electron transfer, and pH-dependent back electron transfer on TiO(2) were investigated for the molecular assembly [((PO(3)H(2)-CH(2))-bpy)(2)Ru(a)(bpy-NH-CO-trpy)Ru(b)(bpy)(OH(2))](4+) ([TiO(2)-Ru(a)(II)-Ru(b)(II)-OH(2)](4+); ((PO(3)H(2)-CH(2))(2)-bpy = ([2,2'-bipyridine]-4,4'-diylbis(methylene))diphosphonic acid); bpy-ph-NH-CO-trpy = 4-([2,2':6',2?-terpyridin]-4'-yl)-N-((4'-methyl-[2,2'-bipyridin]-4-yl)methyl) benzamide); bpy = 2,2'-bipyridine). This assembly combines a light-harvesting chromophore and a water oxidation catalyst linked by a synthetically flexible saturated bridge designed to enable long-lived charge-separated states. Following excitation of the chromophore, rapid electron injection into TiO(2) and intra-assembly electron transfer occur on the subnanosecond time scale followed by microsecond-millisecond back electron transfer from the semiconductor to the oxidized catalyst, [TiO(2)(e(-))-Ru(a)(II)-Ru(b)(III)-OH(2)](4+)?[TiO(2)-Ru(a)(II)-Ru(b)(II)-OH(2)](4+). PMID:23101955

Ashford, Dennis L; Song, Wenjing; Concepcion, Javier J; Glasson, Christopher R K; Brennaman, M Kyle; Norris, Michael R; Fang, Zhen; Templeton, Joseph L; Meyer, Thomas J

2012-11-21

5

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

6

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

PubMed

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

2012-09-25

7

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

PubMed

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 [Cp(R)Cr(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

2014-07-01

8

The study of electron transfer in advanced materials (electrodes for Li-ion batteries or catalysts)  

Microsoft Academic Search

Mössbauer Spectroscopy (MS) is really suitable to study local electronic structures. Its hyperfine parameters, isomer shift\\u000a (IS) and quadrupole splitting (QS) allow to characterize the oxidation state and coordination of the probed element. So, the\\u000a capabilities of this powerful technique have been highlighted for the study of electron transfer occurring during electrochemical\\u000a or catalytic processes. Several examples illustrate how MS

Jean-Claude Jumas; Manfred Womes; Pierre-Emmanuel Lippens; Josette Olivier-Fourcade

2008-01-01

9

The study of electron transfer in advanced materials (electrodes for Li-ion batteries or catalysts)  

Microsoft Academic Search

Mössbauer Spectroscopy (MS) is really suitable to study local electronic structures. Its hyperfine parameters, isomer shift\\u000a (IS) and quadrupole splitting (QS) allow to characterize the oxidation state and coordination of the probed element. So, the\\u000a capabilities of this powerful technique have been highlighted for the study of electron transfer occurring during electrochemical\\u000a or catalytic processes. Several examples illustrate how MS

Jean-Claude Jumas; Manfred Womes; Pierre-Emmanuel Lippens; Josette Olivier-Fourcade

10

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

11

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

12

The development of aqueous transfer hydrogenation catalysts.  

PubMed

This review discusses the development of aqueous phase, homogeneous, transfer hydrogenation catalysis. Transfer hydrogenation catalysts, based on Ru, Ir and Rh, reduce organic substrates in water by assisting the transfer of hydrogen from simple donor species. These catalysts are expected to have significant benefits when compared with organic phase catalysts, including greater activity, greater selectivity and smaller environmental impact. They will therefore be expected to make a significant contribution to homogeneous catalysis and 'green chemistry'. Here, we comprehensively examine these catalysts, paying special attention to structural features. PMID:21709918

Robertson, Andrew; Matsumoto, Takahiro; Ogo, Seiji

2011-10-28

13

Proton-electron transport and transfer in electrocatalytic films. Application to a cobalt-based O2-evolution catalyst.  

PubMed

Solar-driven electrochemical transformations of small molecules, such as water splitting and CO2 reduction, pertinent to modern energy challenges, require the assistance of catalysts preferably deposited on conducting or semiconducting surfaces. Understanding mechanisms and identifying the factors that control the functioning of such systems are required for rational catalyst optimization and improved performance. A methodology is proposed, in the framework of rotating disk electrode voltammetry, to analyze the current responses expected in the case of a semigeneral reaction scheme involving a proton-coupled catalytic reaction associated with proton-coupled electron hopping through the film as rate controlling factors in the case where there is no limitation by substrate diffusion. The predictions concern the current density vs overpotential (Tafel) plots and their dependence on buffer concentration (including absence of buffer), film thickness and rotation rate. The Tafel plots may have a variety of slopes (e.g., F/RT?ln?10, F/2RT?ln?10, 0) that may even coexist within the overpotential range of a single plot. We show that an optimal film thickness exists beyond which the activity of the film plateaus. Application to water oxidation by films of a cobalt-based oxidic catalyst provides a successful test of the applicability of the proposed methodology, which also provides further insight into the mechanism by which these cobalt-based films catalyze the oxidation of water. The exact nature of the kinetic and thermodynamic characteristics that have been derived from the analysis is discussed as well as their use in catalyst benchmarking. PMID:23822172

Bediako, D Kwabena; Costentin, Cyrille; Jones, Evan C; Nocera, Daniel G; Savéant, Jean-Michel

2013-07-17

14

Vectorial electron transfer in spatially ordered arrays  

SciTech Connect

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

Fox, M.A.

1993-02-01

15

Electrochemistry of Oxygenation Catalysts. 3.(1) Thermodynamic Characterization of Electron Transfer and Solvent Exchange Reactions of Co(salen)/[Co(salen)](+) in DMF, Pyridine, and Their Mixtures.  

PubMed

Redox and ligand exchange reactions of the oxygenation catalyst (N,N '-bis(salicylidene)ethylenediaminato)cobalt(II), Co(salen), and its one-electron oxidation product, Co(salen)(+), are investigated in DMF, pyridine, and mixtures of these solvents. Electron transfers and solvent exchange reactions involving three neutral Co(II) and three cationic Co(III) complexes with different axially bound solvent molecules (two DMF, one DMF and one pyridine, or two pyridine molecules) form a three-rung ladder scheme. All formal potentials E(0) and equilibrium constants K in this scheme are determined from electrochemical or spectrophotometric experiments or the construction of thermodynamic cycles. The latter are also used to prove consistency of the results. Values for the E(0) and K are discussed in terms of the Co coordination geometry, solvent effects on the potentials, the thermodynamics of cross reactions, and the distribution of Co(II) and Co(III) species as a function of the solvent composition. Some peculiarities found in the oxygenation of flavonols and indoles are explained. PMID:11669996

Eichhorn, Emerich; Rieker, Anton; Speiser, Bernd; Stahl, Hartmut

1997-07-16

16

Photophysical studies of chromium sensitizers designed for excited state hole transfer to semiconductors and sequential hole/electron transfers from photoexcited cadmium sulfide nanorods to mononuclear ruthenium water-oxidation catalysts  

NASA Astrophysics Data System (ADS)

This dissertation describes three research projects related to solar cells and solar water splitting with a goal of utilizing solar energy, a renewable energy source. The first project is focused on photophysical studies of four newly-synthesized Cr(III) tris-bipyridyl complexes featuring the 4-dmcbpy (dimethyl 2,2'-bipyridine-4,4'-dicarboxylate) ligand. Static and time-resolved emission results suggest that the complexes store ˜1.7 eV of energy for multiple microseconds. Using cyclic voltammetry, it is found that the inclusion of 4-dmcbpy shifts the E1/2 of CrIII/II by +0.2 V from the homoleptic parent complexes without 4-dmcbpy. All four complexes have excited state potentials of CrIII*/II between +1.8 and +2.0 V vs. NHE, placing them among the most powerful photooxidants reported and making them candidates for hole-injection sensitizers. The second project continues with Cr(III) complexes, but using iminopyridine Schiff base ligands. Two complexes feature hexadentate ligands and the other two are their tris-bidentate analogues. One of each pair contains methyl ester groups for attachment to semiconductors. Cyclic voltammograms show that the hexadentate and tris-bidentate analogues have almost identical reduction potentials, but the addition of ester substituents shifts the reduction potentials by +0.2 V. The absorption spectra of the hexadentate complexes show improved absorption of visible light compared to the tris-bidentate analogues. For freshly prepared sample solutions in CH3CN, time-resolved emission and transient absorption measurements for the Cr(III) tris-bidentate ester complex show a doublet excited state with a 17-19 microsecond lifetime at room temperature, while no emission or transient absorption signals from the doublet states are observed for the hexadentate analogue under the same conditions. The dramatic difference is due to the presence of a nonligated bridgehead nitrogen atom. The third project features charge transfer interactions between a photoexcited cadmium sulfide nanorod and [Ru(diethyl 2,2'-bipyridine-4,4'-dicarboxylate)(2,2':6',2"-terpyridine)Cl] +, a mononuclear water-oxidation catalyst. Upon photoexcitation, hole transfer from the cadmium sulfide nanorod oxidizes the catalyst (Ru 2+ ? Ru3+) on a 100 ps to 1 ns timescale. This is followed by electron transfer (10-100 ns) from the nanorod to reduce the Ru3+ center. The relatively slow electron transfer dynamics may provide opportunities for the accumulation of multiple holes at the catalyst, which is required for water oxidation.

Tseng, Huan-Wei

17

Puzzles of electron transfer.  

National Technical Information Service (NTIS)

Major progress has been made to dramatically change our concepts of electron transfer reactions over the last several years by defining the distances, energetics, and even orientation of electron donor and acceptors groups. Despite this progress, several ...

J. R. Miller

1989-01-01

18

Electron transfer in biology  

Microsoft Academic Search

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

R. J. P. Williams

1989-01-01

19

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

20

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

21

High-field pulsed EPR spectroscopy for the speciation of the reduced [PV(2)Mo(10)O(40)](6-) polyoxometalate catalyst used in electron-transfer oxidations.  

PubMed

An in-depth spectroscopic EPR investigation of a key intermediate, formally notated as [PV(IV)V(V)Mo(10)O(40)](6-) and formed in known electron-transfer and electron-transfer/oxygen-transfer reactions catalyzed by H(5)PV(2)Mo(10)O(40), has been carried out. Pulsed EPR spectroscopy have been utilized: specifically, W-band electron-electron double resonance (ELDOR)-detected NMR and two-dimensional (2D) hyperfine sub-level correlation (HYSCORE) measurements, which resolved (95)Mo and (17)O hyperfine interactions, and electron-nuclear double resonance (ENDOR), which gave the weak (51)V and (31)P interactions. In this way, two paramagnetic species related to [PV(IV)V(V)Mo(10)O(40)](6-) were identified. The first species (30-35 %) has a vanadyl (VO(2+))-like EPR spectrum and is not situated within the polyoxometalate cluster. Here the VO(2+) was suggested to be supported on the Keggin cluster and can be represented as an ion pair, [PV(V)Mo(10)O(39)](8-)[V(IV)O(2+)]. This species originates from the parent H(5)PV(2)Mo(10)O(40) in which the vanadium atoms are nearest neighbors and it is suggested that this isomer is more likely to be reactive in electron-transfer/oxygen-transfer reaction oxidation reactions. In the second (70-65 %) species, the V(IV) remains embedded within the polyoxometalate framework and originates from reduction of distal H(5)PV(2)Mo(10)O(40) isomers to yield an intact cluster, [PV(IV)V(V)Mo(10)O(40)](6-). PMID:20645349

Kaminker, Ilia; Goldberg, Hila; Neumann, Ronny; Goldfarb, Daniella

2010-09-01

22

Manifestations of sequential electron transfer.  

National Technical Information Service (NTIS)

An essential feature of efficient photo-initiated charge separation is sequential electron transfer. Charge separation is initiated by photoexcitation of an electron donor followed by rapid electron transfer steps from the excited donor through a series o...

M. C. Thurnauer J. Tang

1996-01-01

23

Nonadiabatic anharmonic electron transfer.  

PubMed

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 ? = -?G. The temperature insensitivity of the transfer rate is clearly seen when the exoergicity equals the collective reorganization energy (? = ?(s)) along a maximum ln?(w) vs. ? ridge of the surface. The surface also reveals additional regions for ? where ln?(w) appears to be insensitive to temperature, or effectively activationless, for some kinds of inner sphere contributions. PMID:23556710

Schmidt, P P

2013-03-28

24

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.

25

Intramolecular electron transfer rates  

NASA Astrophysics Data System (ADS)

The initial goals of this project were: (1) to construct pulsed-accelerated-flow and pulsed-laser (transient absorbance) instruments for intramolecular electron-transfer rate measurements, (2) to design and synthesize appropriate molecules and perform such measurements, (3) to develop further an electrochemical method for gauging site-to-site electronic coupling, and (4) to apply time-dependent Raman scattering theory to the problem of inner-shell reorganization in charge-transfer reactions. Although all four goals were met, we also found it necessary to pursue studies in some unforeseen directions. For example, early on we discovered that medium effects (aggregation and ion pairing) could play a very large, and previously unrecognized, role in some optical intervalence reactions. Given the importance of the effects to the areas above, we chose to map them in a fairly complete fashion. Also, in anticipation of possible renewal we initiated studies in a new area: bimolecular photoredox kinetics in supercritical media. Finally, in a small project carried out largely by undergraduates we examined solvent tuning effects upon lifetimes of photo-excited ruthenium am(m)ine bipyridine complexes. The key new findings and other highlights of these studies are outlined.

Hupp, Joseph T.

26

Charge transfer in metal catalysts supported on doped TiO{sub 2}: A theoretical approach based on metal - semiconductor contact theory  

SciTech Connect

The authors developed mathematical models for charge transfer in supported metal catalysts. The electronic structure of supported metal crystallites under the conditions of charge transfer is presented. 53 refs., 5 figs., 3 tabs.

Ioannides, T.; Verykios, X.E. [Univ. of Patras (Greece)] [Univ. of Patras (Greece)

1996-07-01

27

Ultrafast Studies on Electron Transfer.  

National Technical Information Service (NTIS)

Excited state intramolecular charge separation in organic electron donor/acceptor (D-A) compounds has become an important model reaction for studying the mechanism of small barrier electron transfer (et) reactions. In this paper we present new ultrafast f...

G. C. Walker K. Tominaga P. F. Barbara

1991-01-01

28

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

29

Coherent electron transfer in polyacetylene  

NASA Astrophysics Data System (ADS)

We examine, using mixed classical-quantum electron-ion dynamics, electron transfer in a donor-acceptor-like molecular junction system based on polyacetylene. We identify two qualitatively-different transfer regimes: hopping and tunnelling. We discuss the criteria for achieving each one and for minimising inelastic scattering and decoherence arising from the coupling to the ions, and we connect our main results to quantities derived from electron dynamics involving simpler, three-state model systems. We identify the requirements to have near-ballistic transfer.

Psiachos, D.

2014-06-01

30

Heterogenized rhodium complexes as hydrogen transfer catalysts  

Microsoft Academic Search

Cationic rhodium(I) norbornadiene complexes with polystyrene-immobilized imidazole, tetramethylbiimidazole or phosphine ligands in the presence of potassium hydroxide catalyze hydrogen transfer from isopropanol to acetophenone and 1-hexene. [Rh(NBD) (poly-PPh2)2] ClO4 complexes are particularly efficient for the reduction of acetophenone, but slow decomposition to rhodium metal is observed.

L. A. Oro; R. Sariego

1982-01-01

31

Rational development of iron catalysts for asymmetric transfer hydrogenation.  

PubMed

The asymmetric reduction of ketones and imines by transfer of hydrogen from isopropanol as the solvent catalyzed by metal complexes is a very useful method for preparing valuable enantioenriched alcohols and amines. Described here is the development of three generations of progressively more active iron catalysts for this transformation. Key features of this process of discovery involved the realization that one carbonyl ligand was needed (as in hydrogenases), the synthesis of modular ligands templated by iron, the elucidation of the mechanisms of catalyst activation and action, as well as the rational synthesis of precursors that lead directly and easily to the species in the catalytic cycle. The discovery that iron, an abundant element that is essential to life, can form catalysts of these hydrogenation reactions is a contribution to green chemistry. PMID:24763699

Sues, Peter E; Demmans, Karl Z; Morris, Robert H

2014-06-01

32

Mechanisms of photoinduced electron transfer  

NASA Astrophysics Data System (ADS)

Work on photoinduced electron transfer reactions in organic compounds conducted under Department of Energy sponsorship over the past ten years at Boston University is described. Early studies in the series were directed to the development of new mechanisms for driving isomerization processes which store light energy as latent heat. Other investigations were devoted to an understanding of the dynamics of charge separation for photoexcited (charge-transfer) complexes or ion-pairs. Recent studies focused on the development of charge relays such as dithioethers and viologen or pyridinium ions, the electron transfer photochemistry of high potential quinones, and the photophysical properties of organic dyes bound to water-soluble polymers.

Jones, Guilford, II

1987-12-01

33

Electron beam irradiation effect on nanostructured molecular sieve catalysts  

Microsoft Academic Search

Electron impact can induce chemical changes on particle surfaces of zeolites and molecular sieve catalysts. Some experimental observations of electron irradiation effect on molecular sieve catalysts are presented, e.g., electron-beam-induced growth of bare silver nanowires from zeolite crystallites, formation of vesicles in calcium phosphate, migration of microdomains in iron-oxide doped mesoporous silicas, structural transformation from mesostructured MCM-41 to microporous ZSM-5,

Zhong-Yong Yuan; Wuzong Zhou; Viorica Parvulescu; Bao-Lian Su

2003-01-01

34

Characterisation of Catalysts by Transmission Electron Microscopy  

Microsoft Academic Search

Catalysts are one of the most important and the most widely used materials. Microstructures in heterogeneous catalysts are\\u000a closely related to the catalytic properties. TEM and related microanalytic techniques are powerful tools in characterising\\u000a catalysts at atomic level. The obtained structural information is essential to the understanding of correlations between microstructures\\u000a and catalytic properties. In this lecture note, the general

Di Wang

35

Vectorial electron transfer in spatially ordered arrays. Progress report, January 1992--January 1993  

SciTech Connect

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

Fox, M.A.

1993-02-01

36

The electronic couplings in electron transfer and excitation energy transfer.  

PubMed

The transport of charge via electrons and the transport of excitation energy via excitons are two processes of fundamental importance in diverse areas of research. Characterization of electron transfer (ET) and excitation energy transfer (EET) rates are essential for a full understanding of, for instance, biological systems (such as respiration and photosynthesis) and opto-electronic devices (which interconvert electric and light energy). In this Account, we examine one of the parameters, the electronic coupling factor, for which reliable values are critical in determining transfer rates. Although ET and EET are different processes, many strategies for calculating the couplings share common themes. We emphasize the similarities in basic assumptions between the computational methods for the ET and EET couplings, examine the differences, and summarize the properties, advantages, and limits of the different computational methods. The electronic coupling factor is an off-diagonal Hamiltonian matrix element between the initial and final diabatic states in the transport processes. ET coupling is essentially the interaction of the two molecular orbitals (MOs) where the electron occupancy is changed. Singlet excitation energy transfer (SEET), however, contains a Frster dipole-dipole coupling as its most important constituent. Triplet excitation energy transfer (TEET) involves an exchange of two electrons of different spin and energy; thus, it is like an overlap interaction of two pairs of MOs. Strategies for calculating ET and EET couplings can be classified as (1) energy-gap-based approaches, (2) direct calculation of the off-diagonal matrix elements, or (3) use of an additional operator to describe the extent of charge or excitation localization and to calculate the coupling value. Some of the difficulties in calculating the couplings were recently resolved. Methods were developed to remove the nondynamical correlation problem from the highly precise coupled cluster models for ET coupling. It is now possible to obtain reliable ET couplings from entry-level excited-state Hamiltonians. A scheme to calculate the EET coupling in a general class of systems, regardless of the contributing terms, was also developed. In the past, empirically derived parameters were heavily invoked in model description of charge and excitation energy drifts in a solid-state device. Recent advances, including the methods described in this Account, permit the first-principle quantum mechanical characterization of one class of the parameters in such descriptions, enhancing the predictive power and allowing a deeper understanding of the systems involved. PMID:19215069

Hsu, Chao-Ping

2009-04-21

37

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.

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

2012-01-01

38

Electron Energy Loss Spectroscopy (EELS) of Iron Fischer Tropsch Catalysts  

Microsoft Academic Search

Electron energy loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy have been used to study iron catalysts for Fischer Tropsch synthesis. When silica-containing iron oxide precursors are activated in flowing CO, the iron phase segregates into iron carbide crystallites, leaving behind some unreduced iron oxide in an amorphous state coexisting with the silica binder. The iron carbide

Aming Jin; Huifang Xu; Abhaya K. Datye

2006-01-01

39

Theory of Electron Transfer in Complex Systems  

SciTech Connect

This research grant is the most recent of a continuous series of grants from the Department of Energy supporting our theoretical efforts to understand various electron transfer processes. In part, my research on this topic has focused on distributions of electric fields, as it is these fields that drive electron transfer. Additional focus has been concerned with kinetics, as the rates of electron transfer are central to the utility of electron transfer. Finally, we have found that the theoretical tools we originally developed to treat electron transfer can be suitably applied to understand the workings of molecular motors.

Chandler, David

2004-10-20

40

Ultrafast studies on electron transfer  

NASA Astrophysics Data System (ADS)

Excited state intramolecular charge separation in organic electron donor/acceptor (D-A) compounds has become an important model reaction for studying the mechanism of small barrier electron transfer (et) reactions. In this paper we present new ultrafast fluorescence measurements on the excited states dynamics of 4-(9-anthryl)-N, N'-dimethylaniline (ADMA), especially on the excited state et process (D-A* = D+ -A-, conventionally denoted by LE = CT). The variation on the static absorption and emission spectra of ADMA, as a function of solvent, is primary due to simple solvent coordinate effects, rather than large intramolecular structural changes. In polar solvents the excited state et of ADMA is in the Marcus inverted regime. There are two distinct et kinetic components, a faster component which is not solvent controlled and probably involves intramolecular bath states: and a slower solvent controlled component with a limiting rate constant.

Barbara, Paul F.; Tominaga, Keisuke; Walker, Gilbert C.

1991-05-01

41

Epoxides as reducing agents for low-catalyst-concentration atom transfer radical polymerization.  

PubMed

Activators regenerated by electron transfer atom transfer radical polymerization (ARGET ATRP) conditions utilizing a low concentration of catalyst are successfully applied for the preparation of well-defined poly(glycidyl methacrylate) without the addition of external reducing agents. The living character of polymerization is evidenced by successful chain extensions with methyl methacrylate and methyl acrylate, again, in the absence of additional reducing agents, yielding block copolymers. The epoxide groups in glycidyl methacrylate or the corresponding polymer can serve as an intrinsic reducing agent to continuously regenerate the Cu(I) -based ATRP activator from the Cu(II) halide complex present in the systems. The reactivity of various epoxides in the reduction of the Cu(II) Br2 complex of tris(2-pyridylmethyl)amine is compared. PMID:24214162

Woodruff, Shannon R; Davis, Brad J; Tsarevsky, Nicolay V

2014-01-01

42

Symmetry and electron transfer in biomolecules  

Microsoft Academic Search

We present the theory of finite temperature electron transfer between biomolecules with mirror symmetry, such as DNA base pairs. The symmetry requirement alters the coupling between the nuclear coordinates and the electronic degrees of freedom. We show that for this new coupling the classical theory of Garg, Onuchic and Ambegaokar [1] of electron transfer breaks down. We present a modified

Maria R. D'Orsogna; Robijn Bruinsma

2002-01-01

43

Homogeneous catalytic reduction of dioxygen using transfer hydrogenation catalysts.  

PubMed

Solutions of Cp*IrH(rac-TsDPEN) (TsDPEN = H2NCHPhCHPhN(SO2C6H4CH3)-) (1H(H)) with O2 generate Cp*Ir(TsDPEN-H) (1) and 1 equiv of H2O. Kinetic analysis indicates a third-order rate law (second order in [1H(H)] and first order in [O2]), resulting in an overall rate constant of 0.024 +/- 0.013 M(-2) s(-1). Isotopic labeling revealed that the rate of the reaction of 1H(H) + O2 was strongly affected by deuteration at the hydride position (k(HH2)/k(DH2) = 6.0 +/- 1.3) but insensitive to deuteration of the amine (k(HH2)/k(HD2) = 1.2 +/- 0.2); these values are more disparate than for conventional transfer hydrogenation (Casey, C. P.; Johnson, J. B. J. Org. Chem. 2003, 68, 1998-2001). The temperature dependence of the reaction rate indicated DeltaH = 82.2 kJ/mol, DeltaS = 13.2 J/mol K, and a reaction barrier of 85.0 kJ/mol. A CH2Cl2 solution under 0.30 atm of H2 and 0.13 atm of O2 converted to H2O in the presence of 1 and 10 mol % of H(OEt2)2BAr(F)4 (BAr(F)4- = B(C6H3-3,5-(CF3)2)4-). The formation of water from H2 was verified by 2H NMR for the reaction of D2 + O2. Solutions of 1 slowly catalyze the oxidation of amyl alcohol to pentanal; using 1,4-benzoquinone as a cocatalyst, the conversion was faster. Complex 1 also catalyzes the reaction of O2 with RNH2BH3 (R = H, t-Bu), resulting in the formation of water and H2. The deactivation of the catalyst 1 in its reactions with O2 was traced to degradation of the Cp* ligand to a fulvene derivative. This pathway is not observed in the presence of amine-boranes, which were shown to reduce fulvenes back to Cp*. This work suggests the potential of transfer hydrogenation catalysts in reactions involving O2. PMID:17958423

Heiden, Zachariah M; Rauchfuss, Thomas B

2007-11-21

44

Probing Electron Transfer Mechanisms in Shewanella oneidensis MR-1 using a Nanoelectrode Platform and Single-Cell Imaging.  

National Technical Information Service (NTIS)

Microbial fuel cells (MFCs) represent a promising approach for sustainable energy production as they generate electricity directly from metabolism of organic substrates without the need for catalysts. However, the mechanisms of electron transfer between m...

J. Hu J. C. Biffinger, L. A. Fitzgerald, P. Xie, X. Jiang

2010-01-01

45

Modelling of hydrogen transfer in coal hydroliquefaction. 8. Influence of catalyst on the whole hydroliquefaction process  

Microsoft Academic Search

The mechanisms of the rehydrogenation of 1,2-dihydronaphthalene and naphthalene, formed during the tetralin H-transfer process for stabilizing free radicals stemming from thermal decomposition of model radical precursors such as benzyl phenyl ether (BPE), were studied using natural catalysts (pyrite, pyrrhotite) or catalysts originating from in situ sulfidation of iron oxide. These catalysts had a significant influence on the rate of

A. P. Oviawe; D. Nicole; R. Gerardin

1995-01-01

46

Development of a Method for the Extraction of Oil from Clay by Friendly Phase Transfer Catalyst  

Microsoft Academic Search

The separation of waste oil from clay was studied using benzyl triethanol ammonium chloride as phase transfer catalyst. The study showed that the surface tension has an effect on oil recovery. An induced air flotation process was used and polymeric decyl phenol formaldehyde ethoxylate was used as the collector. The effects of various parameters including concentration of catalyst, collector, time

A. K. El Morsi; A. M. A. Omar

2010-01-01

47

Superexchange Coupling Mechanisms for Electron Transfer Processes.  

National Technical Information Service (NTIS)

Electron transfer matrix elements for electron exchange between various pairs of transition metal complexes in close contact have been calculated and analyzed for a variety of approach geometries for the two reactants. The coupling between the nominal met...

M. D. Newton

1989-01-01

48

Electron Energy Loss Spectroscopy (EELS) of Iron Fischer Tropsch Catalysts  

NASA Astrophysics Data System (ADS)

Electron energy loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy have been used to study iron catalysts for Fischer Tropsch synthesis. When silica-containing iron oxide precursors are activated in flowing CO, the iron phase segregates into iron carbide crystallites, leaving behind some unreduced iron oxide in an amorphous state coexisting with the silica binder. The iron carbide crystallites are found covered by characteristic amorphous carbonaceous surface layers. These amorphous species are difficult to analyze by traditional catalyst characterization techniques, which lack spatial resolution. Even a surface-sensitive technique such as XPS shows only broad carbon or iron peaks in these catalysts. As we show in this work, EELS allows us to distinguish three different carbonaceous species: reactive amorphous carbon, graphitic carbon, and carbidic carbon in the bulk of the iron carbide particles. The carbidic carbon K edge shows an intense “[pi]*” peak with an edge shift of about 1 eV to higher energy loss compared to that of the [pi]* of amorphous carbon film or graphitic carbon. EELS analysis of the oxygen K edge allows us to distinguish the amorphous unreduced iron phase from the silica binder, indicating these are two separate phases. These results shed light onto the complex phase transformations that accompany the activation of iron catalysts for Fischer Tropsch synthesis.

Jin, Aming; Xu, Huifang; Datye, Abhaya K.

2006-04-01

49

Electron Transfer and Multi-Electron Accumulation in ExBox(4+).  

PubMed

Molecules capable of accepting and storing multiple electrons are crucial components of artificial photosynthetic systems designed to drive catalysts, such as those used to reduce protons to hydrogen. ExBox(4+) , a boxlike cyclophane comprising two ?-electron-poor extended viologen units tethered at both ends by two p-xylylene linkers, has been shown previously to accept an electron through space from a photoexcited guest. Herein is an investigation of an alternate, through-bond intramolecular electron-transfer pathway involving ExBox(4+) using a combination of transient absorption and femtosecond stimulated Raman spectroscopy (FSRS). Upon photoexcitation of ExBox(4+) , an electron is transferred from one of the p-xylylene linkers to one of the extended viologen units in ca. 240?ps and recombines in ca. 4?ns. A crystal structure of the doubly reduced species ExBox(2+) was obtained. PMID:24828229

Dyar, Scott M; Barnes, Jonathan C; Jurí?ek, Michal; Stoddart, J Fraser; Co, Dick T; Young, Ryan M; Wasielewski, Michael R

2014-05-19

50

Kinetics of dichlorocyclopropanation of vinylcyclohexane catalyzed by a new multi-site phase transfer catalyst  

Microsoft Academic Search

In this work, a novel soluble multi-site phase transfer catalyst viz., 1,4-bis-(ethylmethyleneammounium chloride)-2,5-dimethoxybenzene (BEMACDB) was synthesized and its catalytic efficiency was assessed by observing the kinetics of dichlorocarbene addition to vinylcyclohexane. The formation of the product was monitored by GLC. Dichlorocyclopropanation of vinylcyclohexane catalyzed by the new phase transfer catalyst was carried out in two-phase medium under pseudo-first-order conditions by

P. A. Vivekanand; T. Balakrishnan

2009-01-01

51

Evaluation of catalyst acidity and substrate electronic effects in a hydrogen bond-catalyzed enantioselective reaction.  

PubMed

A modular catalyst structure was applied to evaluate the effects of catalyst acidity in a hydrogen bond-catalyzed hetero Diels-Alder reaction. Linear free energy relationships between catalyst acidity and both rate and enantioselectivity were observed, where greater catalyst acidity leads to increased activity and enantioselectivity. A relationship between reactant electronic nature and rate was also observed, although there is no such correlation to enantioselectivity, indicating the system is under catalyst control. PMID:20919686

Jensen, Katrina H; Sigman, Matthew S

2010-11-01

52

Effect of mass transfer and catalyst layer thickness on photocatalytic reaction  

SciTech Connect

Semiconductor photocatalytic processes have been studied for nearly 20 years due to their intriguing advantages in environmental remediation. A rational approach in determining the effect of mass transfer and catalyst layer thickness during photocatalytic reactions is proposed. The reaction occurs at the liquid-catalyst interface, and therefore when the catalyst is immobilized, both external and internal mass transfer plays significant roles in overall photocatalytic processes. Several model parameters--external mass-transfer coefficient, dynamic adsorption equilibrium constant, adsorption rate constant, internal mass-transfer coefficient, and effective diffusivity--were determined either experimentally or by fitting realistic models to experimental results using benzoic acid as a model component. Even though all these parameters are critical to the design and development of photocatalytic processes, they are not available in the literature. The effect of the internal mass transfer on the photocatalytic degradation rate over different catalyst layer thicknesses under two different operating configurations was analyzed theoretically and experimentally verified. It was observed that an optimal catalyst layer thickness exists for substrate-to-catalyst illumination.

Chen, D.; Li, F.; Ray, A.K.

2000-05-01

53

On the Nature of Reaction-Controlled Phase Transfer Catalysts for Epoxidation of Olefin: A 31 P NMR Investigation  

Microsoft Academic Search

[p-C5H5NC16H33]3[PW4O16] was reported to be an excellent epoxidation catalyst which exhibited a unique reaction-controlled phase transfer behavior. In the paper, the composition and structural changes of the reaction-controlled phase transfer catalyst during and after reaction have been investigated by 31P NMR spectroscopy. The 31P MAS NMR confirmed that the original catalyst was a mixture of heteropoly tungstophosphates. When the catalyst

Yangying Chen; Jianqin Zhuang; Xiumei Liu; Jinbo Gao; Xiuwen Han; Xinhe Bao; Ning Zhou; Shuang Gao; Zuwei Xi

2004-01-01

54

Extracellular electron transfer via microbial nanowires  

Microsoft Academic Search

Microbes that can transfer electrons to extracellular electron acceptors, such as Fe(III) oxides, are important in organic matter degradation and nutrient cycling in soils and sediments. Previous investigations on electron transfer to Fe(III) have focused on the role of outer-membrane c-type cytochromes. However, some Fe(III) reducers lack c-cytochromes. Geobacter species, which are the predominant Fe(III) reducers in many environments, must

Gemma Reguera; Kevin D. McCarthy; Teena Mehta; Julie S. Nicoll; Mark T. Tuominen; Derek R. Lovley

2005-01-01

55

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

56

Quantitative structural assessment of heterogeneous catalysts by electron tomography.  

PubMed

We present transmission electron microscope (TEM) tomography investigations of ruthenium-based fuel cell catalyst materials as employed in direct methanol fuel cells (DMFC). The digital three-dimensional representation of the samples not only enables detailed studies on number, size, and shape but also on the local orientation of the ruthenium particles to their support and their freely accessible surface area. The shape analysis shows the ruthenium particles deviate significantly from spherical symmetry which increases their surface to volume ratio. The morphological studies help to understand the structure formation mechanisms during the fabrication as well as the high effectiveness of these catalysts in the oxygen reduction reaction at the cathode side of fuel cells. PMID:21916435

Grothausmann, Roman; Zehl, Gerald; Manke, Ingo; Fiechter, Sebastian; Bogdanoff, Peter; Dorbandt, Iris; Kupsch, Andreas; Lange, Axel; Hentschel, Manfred P; Schumacher, Gerhard; Banhart, John

2011-11-16

57

Electron Transfer Pathways in Cytochrome c Oxidase  

PubMed Central

Mixed quantum mechanical/molecular mechanics calculations, were used to explore the electron pathway of the terminal electron transfer enzyme, Cytochrome c Oxidase. This enzyme catalyzes the reduction of molecular oxygen to water in a multiple step process. Density functional calculations on the three redox centers allowed for the characterization of the electron transfer mechanism, following the sequence CuA ? heme a ? heme a3. This process is largely affected by the presence of positive charges, confirming the possibility of a proton coupled electron transfer. An extensive mapping of all residues involved in the electron transfer, between the CuA center (donor) and the O2 reduction site heme a3-CuB (receptor), was obtained by selectively activating/deactivating different quantum regions. The method employed, called QM/MM e-pathway, allowed the identification of key residues along the possible electron transfer paths, consistent with experimental data. In particular, the role of arginines 481 and 482 appears crucial in the CuA ? heme a and in the heme a ? heme a3 electron transfer processes.

Lucas, M. Fatima; Rousseau, Denis L.; Guallar, Victor

2011-01-01

58

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

59

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

60

Electronic Funds Transfer and Monetary Policy.  

National Technical Information Service (NTIS)

A comprehensive analysis of the institutions, the instruments, and the channels of influence of monetary policy in the national economy is presented. It includes an examination of major points of impact of electronic funds transfer on monetary policy base...

1976-01-01

61

3-D Heterogeneous Electronics by Transfer Printing  

Microsoft Academic Search

Here we describe an approach, called transfer printing, to allow the combination of broad classes of materials into three-dimensional (3-D) heterogeneously integrated electronic devices. The process involves fabrication of source wafers that contain high performance single crystal devices from materials including, but not limited to, silicon, gallium arsenide and gallium nitride. These devices are then delineated and transferred to a

C. A. Bower; E. Menard; J. Carr; J. A. Rogers

2007-01-01

62

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

63

Quantum effects in biological electron transfer.  

PubMed

Over recent decades, quantum effects such as coherent electronic energy transfers, electron and hydrogen tunneling have been uncovered in biological processes. In this Perspective, we highlight some of the main conceptual and methodological tools employed in the field to investigate electron tunneling in proteins, with a particular emphasis on the methodologies we are currently developing. In particular, we describe our recent contributions to the development of a mixed quantum-classical framework aimed at describing physical systems lying at the border between the quantum and semi-classical worlds. We present original results obtained by combining our approach with constrained Density Functional Theory calculations. Moving to coarser levels of description, we summarize our latest findings on electron transfer between two redox proteins, thereby showing the stabilization of inter-protein, water-mediated, electron-transfer pathways. PMID:22434318

de la Lande, Aurélien; Babcock, Nathan S; Rezá?, Jan; Lévy, Bernard; Sanders, Barry C; Salahub, Dennis R

2012-05-01

64

Reaction of electron-transfer flavoprotein with electron-transfer flavoprotein-ubiquinone oxidoreductase  

Microsoft Academic Search

The oxidative half-reaction of electron-transfer flavoprotein (ETF), electron transfer from ETF to electron-transfer flavoprotein-ubiquinone oxidoreductase (ETF-QO), is dependent on complementary surface charges on the two proteins. ETF is the positively charged member of the redox pair. The evidence is based on the pH and ionic strength dependencies of the comproportionation of oxidized ETF and ETF hydroquinone catalyzed by ETF-QO and

Joe D. Beckmann; Frank E. Frerman

1985-01-01

65

Tunnelling hot electron transfer amplifiers  

NASA Astrophysics Data System (ADS)

Significant advances were achieved in tunneling hot electron amplifiers and in the understanding of hot electron transport in the work funded by the contract and carried out by M. Heiblum and his co-workers. The results of this work are described in detail in nine papers that have been published in the open literature. (These papers are listed below as references 1-9 and copies of them are attached.) This report summarizes the principal results. There are three aspects of the work in which the results are particularly noteworthy: the demonstration of the first hot hole tunneling transistor and its use to study hot hole transport in GaAs; the successful fabrication of a hot electron 'THETA' transistor with a pseudomorphic InGaAs base which allowed high transistor gains (beta approx. = 30) to be achieved; and the conception and successful fabrication of a lateral hot electron device and its use to investigate ballistic, hot electron transport in a 2-dimensional electron gas in GaAs.

Kircher, C. J.

1993-10-01

66

Tracking Students Who Transfer: Electronic Transcript Exchange.  

ERIC Educational Resources Information Center

A discussion of standardized statewide electronic transcript formats and national standardization of electronic data exchange for college student records looks at implications for tracking transfer students' progress. Relative ease of use of the formats and the information contained in the records are examined, technical requirements are noted,…

Green, Michael J.

1995-01-01

67

Electron transfer reactions within zeolites: Radical cation from benzonorbornadiene  

SciTech Connect

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

Pitchumani, K.; Ramamurthy, V. [Tulane Univ., New Orleans, LA (United States)] [Tulane Univ., New Orleans, LA (United States); Corbin, D.R. [The Du Pont Company, Wilmington, DE (United States)] [The Du Pont Company, Wilmington, DE (United States)

1996-08-28

68

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.

Schertl, Peter; Braun, Hans-Peter

2014-01-01

69

Mechanisms of metal ion-coupled electron transfer.  

PubMed

Redox inactive metal ions acting as Lewis acids can control electron transfer from electron donors (D) to electron acceptors (A) by binding to radical anions of electron acceptors which act as Lewis bases. Such electron transfer is defined as metal ion-coupled electron transfer (MCET). Mechanisms of metal ion-coupled electron transfer are classified mainly into two pathways, i.e., metal ion binding to electron acceptors followed by electron transfer (MB/ET) and electron transfer followed by metal ion binding to the resulting radical anions of electron acceptors (ET/MB). In the former case, electron transfer and the stronger binding of metal ions to the radical anions occur in a concerted manner. Examples are shown in each case to clarify the factors to control MCET reactions in both thermal and photoinduced electron-transfer reactions including back electron-transfer reactions. PMID:22596095

Fukuzumi, Shunichi; Ohkubo, Kei; Morimoto, Yuma

2012-05-17

70

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

71

Protein electron transfer: Dynamics and statistics.  

PubMed

Electron transfer between redox proteins participating in energy chains of biology is required to proceed with high energetic efficiency, minimizing losses of redox energy to heat. Within the standard models of electron transfer, this requirement, combined with the need for unidirectional (preferably activationless) transitions, is translated into the need to minimize the reorganization energy of electron transfer. This design program is, however, unrealistic for proteins whose active sites are typically positioned close to the polar and flexible protein-water interface to allow inter-protein electron tunneling. The high flexibility of the interfacial region makes both the hydration water and the surface protein layer act as highly polar solvents. The reorganization energy, as measured by fluctuations, is not minimized, but rather maximized in this region. Natural systems in fact utilize the broad breadth of interfacial electrostatic fluctuations, but in the ways not anticipated by the standard models based on equilibrium thermodynamics. The combination of the broad spectrum of static fluctuations with their dispersive dynamics offers the mechanism of dynamical freezing (ergodicity breaking) of subsets of nuclear modes on the time of reaction/residence of the electron at a redox cofactor. The separation of time-scales of nuclear modes coupled to electron transfer allows dynamical freezing. In particular, the separation between the relaxation time of electro-elastic fluctuations of the interface and the time of conformational transitions of the protein caused by changing redox state results in dynamical freezing of the latter for sufficiently fast electron transfer. The observable consequence of this dynamical freezing is significantly different reorganization energies describing the curvature at the bottom of electron-transfer free energy surfaces (large) and the distance between their minima (Stokes shift, small). The ratio of the two reorganization energies establishes the parameter by which the energetic efficiency of protein electron transfer is increased relative to the standard expectations, thus minimizing losses of energy to heat. Energetically efficient electron transfer occurs in a chain of conformationally quenched cofactors and is characterized by flattened free energy surfaces, reminiscent of the flat and rugged landscape at the stability basin of a folded protein. PMID:23862967

Matyushov, Dmitry V

2013-07-14

72

Protein electron transfer: Dynamics and statistics  

NASA Astrophysics Data System (ADS)

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

Matyushov, Dmitry V.

2013-07-01

73

Syntheses of new dimeric Cinchona alkaloid as a chiral phase transfer catalysts for the alkylation of Schiff base  

Microsoft Academic Search

New dimeric cinchona quaternary ammonium salts have been synthesized and used as efficient chiral phase transfer catalysts for enantioselective alkylation of N-(diphenylmethylene)glycine tert-butyl ester giving very good chemical yield and up to 99% enantiomeric excess. The catalytic efficiency was compared with the previously reported single site chiral phase transfer catalysts.

Ayyanar Siva; Eagambaram Murugan

2005-01-01

74

Structural and electronic factors in supported silver catalysts for the oxidation of ethylene  

Microsoft Academic Search

Structural and electronic factors in supported silver catalysts for the oxidation of ethylene structural and electronic factors in supported silver catalysts for the oxidation of ethylene were studied by impregnating ..cap alpha..-alumina supports with silver nitrate solution and characterizing the catalysts in terms of total surface area (0.02-3.03 sq m\\/g) by the BET method, silver crystallite size (300-7400 A.) by

Verykios

1979-01-01

75

Primary amines by transfer hydrogenative reductive amination of ketones by using cyclometalated Ir(III) catalysts.  

PubMed

Cyclometalated iridium complexes are found to be versatile catalysts for the direct reductive amination (DRA) of carbonyls to give primary amines under transfer-hydrogenation conditions with ammonium formate as both the nitrogen and hydrogen source. These complexes are easy to synthesise and their ligands can be easily tuned. The activity and chemoselectivity of the catalyst towards primary amines is excellent, with a substrate to catalyst ratio (S/C) of 1000 being feasible. Both aromatic and aliphatic primary amines were obtained in high yields. Moreover, a first example of homogeneously catalysed transfer-hydrogenative DRA has been realised for ?-keto ethers, leading to the corresponding ?-amino ethers. In addition, non-natural ?-amino acids could also be obtained in excellent yields with this method. PMID:24516890

Talwar, Dinesh; Poyatos Salguero, Noemí; Robertson, Craig M; Xiao, Jianliang

2014-01-01

76

Radical Polymerization of n-BMA Using a Di-Site Phase Transfer Catalyst  

Microsoft Academic Search

The di-site phase transfer catalyzed radical polymerization of butyl methacrylate with the phase transfer catalyst N,N?-diheptyl-N,N,N?,N?-tetramethyl-1,2-ethanediammonium dibromide (DHTMEDADB) was carried out in an aqueous-organic biphase system at 60 ± 1°C under nitrogeneous atmosphere at fixed pH and ionic strength. A suitable kinetic scheme has been proposed and its significance was discussed. The polymers obtained were investigated and characterized (FTIR, H-NMR and XRD

J. Usha; M. J. Umapathy

2009-01-01

77

Mechanism of cobalt self-exchange electron transfer.  

PubMed

A heptanuclear cobalt cluster was synthesized in two different oxidation states, Co(II)7 and a mixed valence Co(III)Co(II)6, as a soluble model of a cobalt-phosphate/borate (Co-OEC) water splitting catalyst. Crystallographic characterization indicates similar cluster cores, distinguished primarily at the central Co atom. An anion associates to the cluster cores via hydrogen bonding. Using an isotope exchange method, an anomalously slow self-exchange electron transfer rate constant (k(obs) = 1.53 × 10(-3) M(-1) s(-1) at 40 °C and 38 mM [OTf] in MeCN), as compared to that predicted from semiclassical Marcus theory, supports a charge transfer process that is accelerated by dissociation of the anion from the oxidized cluster. This mechanism sheds light on the inverse dependence of anions in the self-repair mechanism of Co-OECs. Moreover, because H2O cannot directly bridge cobalt centers, owing to the encapsulation of the central Co within the cluster core, the observed results address a long-standing controversy surrounding the Co(2+/3+) self-exchange electron transfer reaction of the hexaaqua complex. PMID:23987247

Ullman, Andrew M; Nocera, Daniel G

2013-10-01

78

Superexchange coupling mechanisms for electron transfer processes  

NASA Astrophysics Data System (ADS)

Electron transfer matrix elements for electron exchange between various pairs of transition metal complexes in close contact have been calculated and analyzed for a variety of approach geometries for the two reactants. The coupling between the nominal metal ion donor/acceptor sites is achieved by superexchange of the hole type arising from ligand-to-metal charge transfer (LMCT), the dominant ligand-field interaction for the electron-donor ligands considered (H sub 2 O, NH sub 3, the cyclopentadienide anion). The pronounced variations of H sub if with geometry are not correlated with the separation distance of the metal ions (between which the direct overlap is negligible) and span the range from non-adiabatic to strongly adiabatic electronic coupling. The values for metallocene/metallocinium redox pairs bracket recently reported experimental values. Analysis of the results using the method of corresponding orbitals demonstrates the validity of an effective 1-electron model for the electron transfer process to within about 10 percent for the class of systems considered. A higher-order superexchange mechanism was encountered for the Co(NH sub 3) sub 6 (2+/3+) exchange process, in which the LMCT-driven hole-transport mechanism couples excited local states of the metal ions, which in turn are connected to the corresponding ground states by spin-orbit mixing. This mechanism yields on electronic transmission factor within two orders of magnitude of unity.

Newton, M. D.

79

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...

80

Extracellular electron transfer via microbial nanowires.  

PubMed

Microbes that can transfer electrons to extracellular electron acceptors, such as Fe(iii) oxides, are important in organic matter degradation and nutrient cycling in soils and sediments. Previous investigations on electron transfer to Fe(iii) have focused on the role of outer-membrane c-type cytochromes. However, some Fe(iii) reducers lack c-cytochromes. Geobacter species, which are the predominant Fe(iii) reducers in many environments, must directly contact Fe(iii) oxides to reduce them, and produce monolateral pili that were proposed, on the basis of the role of pili in other organisms, to aid in establishing contact with the Fe(iii) oxides. Here we report that a pilus-deficient mutant of Geobacter sulfurreducens could not reduce Fe(iii) oxides but could attach to them. Conducting-probe atomic force microscopy revealed that the pili were highly conductive. These results indicate that the pili of G. sulfurreducens might serve as biological nanowires, transferring electrons from the cell surface to the surface of Fe(iii) oxides. Electron transfer through pili indicates possibilities for other unique cell-surface and cell-cell interactions, and for bioengineering of novel conductive materials. PMID:15973408

Reguera, Gemma; McCarthy, Kevin D; Mehta, Teena; Nicoll, Julie S; Tuominen, Mark T; Lovley, Derek R

2005-06-23

81

Extracellular electron transfer via microbial nanowires  

NASA Astrophysics Data System (ADS)

Microbes that can transfer electrons to extracellular electron acceptors, such as Fe(III) oxides, are important in organic matter degradation and nutrient cycling in soils and sediments. Previous investigations on electron transfer to Fe(III) have focused on the role of outer-membrane c-type cytochromes. However, some Fe(III) reducers lack c-cytochromes. Geobacter species, which are the predominant Fe(III) reducers in many environments, must directly contact Fe(III) oxides to reduce them, and produce monolateral pili that were proposed, on the basis of the role of pili in other organisms, to aid in establishing contact with the Fe(III) oxides. Here we report that a pilus-deficient mutant of Geobacter sulfurreducens could not reduce Fe(III) oxides but could attach to them. Conducting-probe atomic force microscopy revealed that the pili were highly conductive. These results indicate that the pili of G. sulfurreducens might serve as biological nanowires, transferring electrons from the cell surface to the surface of Fe(III) oxides. Electron transfer through pili indicates possibilities for other unique cell-surface and cell-cell interactions, and for bioengineering of novel conductive materials.

Reguera, Gemma; McCarthy, Kevin D.; Mehta, Teena; Nicoll, Julie S.; Tuominen, Mark T.; Lovley, Derek R.

2005-06-01

82

Dynamics in electron transfer protein complexes  

Microsoft Academic Search

Recent studies have provided experimental evidence for the existence of an encounter complex, a transient intermediate in the formation of protein complexes. We have used paramagnetic relaxation enhancement NMR spectroscopy in combination with Monte Carlo simulations to characterize and visualize the ensemble of encounter orientations in the short-lived electron transfer complex of yeast Cc and CcP. The complete conformational space

Qamar Bashir

2010-01-01

83

Reversible electron transfer in photochemistry and electrochemistry  

Microsoft Academic Search

A complete set of integral equations is used to describe the kinetics of reversible photoionization after instantaneous excitation, including geminate and bimolecular charge recombination, to either the ground or excited states of neutral products. The normalized distribution of ionization products, calculated taking into account reverse electron transfer, differs from that for irreversible photoionization. At low reorganization energy and slow diffusion,

A. I. Burshtein; A. A. Neufeld; K. L. Ivanov

2001-01-01

84

Electron Transfer at Sensitized Semiconductor Electrodes.  

National Technical Information Service (NTIS)

Electron transfer from the excited state of sensitizing dyes to the conduction band of semiconductors has been studied through photoelectrochemical techniques. Two systems were analyzed in detail: rhodamine B on ZnO and rose bengal on TiO sub 2 . Prior to...

M. T. Spitler

1977-01-01

85

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.

Consortium, The C.

2011-12-11

86

Catalysts  

Microsoft Academic Search

A hydrotreating catalyst which is resistant to carbonaceous deposits and is particularly useful for hydrotreating coal derived liquids, is molybdenum or tungsten disulphide in an amount of 0.1 to 10% by weight, substantially completely on the outer surface of an active carbon support having a surface area in excess of 800 m2\\/g. The catalyst can be made by absorbing molybdenum

D. G. Gavin; M. A. Jones

1982-01-01

87

Quinoline transfer hydrogenation by a rhodium bipyridine catalyst  

Microsoft Academic Search

The catalytic activity of the rhodium complex cis-[Rh(bipy)2Cl2]Cl·2H2O in the transfer hydrogenation of different unsaturated substrates is reported. This complex, if pre-activated, is very active in the transfer hydrogenation of ketones (i.e., cyclohexanone is reduced with a 38.1% conversion at 283K and 100% at 313K) while in the case of hex-1-ene, a 36.8% conversion was reached at 293K. A cyclic

Piero Frediani; Luca Rosi; Lorenza Cetarini; Marco Frediani

2006-01-01

88

Horizontal gene transfer in nematodes: a catalyst for plant parasitism?  

PubMed

The origin of plant parasitism within the phylum Nematoda is intriguing. The ability to parasitize plants has originated independently at least three times during nematode evolution and, as more molecular data has emerged, it has become clear that multiple instances of horizontal gene transfer (HGT) from bacteria and fungi have played a crucial role in the nematode's adaptation to this new lifestyle. The first reported HGT cases in plant-parasitic nematodes were genes encoding plant cell wall-degrading enzymes. Other putative examples of HGT were subsequently described, including genes that may be involved in the modulation of the plant's defense system, the establishment of a nematode feeding site, and the synthesis or processing of nutrients. Although, in many cases, it is difficult to pinpoint the donor organism, candidate donors are usually soil dwelling and are either plant-pathogenic or plant-associated microorganisms, hence occupying the same ecological niche as the nematodes. The exact mechanisms of transfer are unknown, although close contacts with donor microorganisms, such as symbiotic or trophic interactions, are a possibility. The widespread occurrence of horizontally transferred genes in evolutionarily independent plant-parasitic nematode lineages suggests that HGT may be a prerequisite for successful plant parasitism in nematodes. PMID:21539433

Haegeman, Annelies; Jones, John T; Danchin, Etienne G J

2011-08-01

89

Catalysts  

SciTech Connect

A hydrotreating catalyst which is resistant to carbonaceous deposits and is particularly useful for hydrotreating coal derived liquids, is molybdenum or tungsten disulphide in an amount of 0.1 to 10% by weight, substantially completely on the outer surface of an active carbon support having a surface area in excess of 800 m2/g. The catalyst can be made by absorbing molybdenum or tungsten trisulphide onto the active carbon support and reducing the trisulphide to the disulphide.

Gavin, D.G.; Jones, M.A.

1982-02-02

90

CATALYST EFFECTIVENESS FACTORS UNDER GAS-TO-LIQUID MASS TRANSFER LIMITING REGIME IN LIQUID PHASE METHANOL SYNTHESIS PROCESS  

Microsoft Academic Search

The effectiveness factors of methanol synthesis catalyst were experimentally measured under condition of gas-to-liquid mass transfer limiting regime in the liquid phase methanol synthesis process, where the synthesis catalyst is slurried in an inert liquid phase. The experimental measurements of effectiveness factors were based on an intrinsic methanol synthesis rate per unit mass of catalyst (gmol\\/kg cat.h) which is not

Makarand Gogate; Sunggyu Lee

1994-01-01

91

Bacterial Nanowires Facilitate Extracellular Electron Transfer  

NASA Astrophysics Data System (ADS)

Dissimilatory metal reducing bacteria, including Shewanella oneidensis and Geobacter sulfurreducens, produce electrically conductive nanowires that facilitate electron transfer to solid phase iron oxides. Nanowires produced by S. oneidensis strain MR-1 are functionalized by decaheme cytochromes MtrC and OmcA that are distributed along the length of the nanowires, as confirmed by immunolocalization experiments using peptide specific antibodies. Mutants lacking MtrC and OmcA produce nanowires that were poorly conductive, are unable to reduce solid phase iron oxides, and do not produce electric current in microbial fuel cells. Although less completely characterized, nanowires are also produced by organisms throughout a broad metabolic spectrum, from sulfate reducing bacteria to oxygenic, phototrophic cyanobacteria. Our research suggests that electrically conductive nanowires may be common throughout the microbial world and may serve as structures for efficient electron transfer and energy dissemination in complex communities such as microbial mats and biofilms.

Gorby, Y.

2005-12-01

92

Bacterial Nanowires Facilitate Extracellular Electron Transfer  

Microsoft Academic Search

Dissimilatory metal reducing bacteria, including Shewanella oneidensis and Geobacter sulfurreducens, produce electrically conductive nanowires that facilitate electron transfer to solid phase iron oxides. Nanowires produced by S. oneidensis strain MR-1 are functionalized by decaheme cytochromes MtrC and OmcA that are distributed along the length of the nanowires, as confirmed by immunolocalization experiments using peptide specific antibodies. Mutants lacking MtrC and

Y. Gorby

2005-01-01

93

Quantum coherent contributions in biological electron transfer  

Microsoft Academic Search

Many biological electron transfer (ET) reactions are mediated by metal centres in proteins. NADH:ubiquinone oxidoreductase (complex I) contains an intramolecular chain of seven iron-sulphur (FeS) clusters, one of the longest chains of metal centres in biology and a test case for physical models of intramolecular ET. In biology, intramolecular ET is commonly described as a diffusive hopping process, according to

Ross Dorner; John Goold; Libby Heaney; Tristan Farrow; Philippa G. Roberts; Judy Hirst; Vlatko Vedral

2011-01-01

94

Electron transfer control in soluble methane monooxygenase.  

PubMed

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

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

2014-07-01

95

Hydrolytic enantioselective protonation of cyclic dienyl esters and a ?-diketone with chiral phase-transfer catalysts.  

PubMed

Hydrolytic enantioselective protonation of dienyl esters and a ?-diketone catalyzed by phase-transfer catalysts are described. The latter reaction is the first example of an enantio-convergent retro-Claisen condensation. Corresponding various optically active ?,?-unsaturated ketones having tertiary chiral centers adjacent to carbonyl groups were obtained in good to excellent yields and enantiomeric ratios (83-99%, up to 97.5:2.5 er). PMID:23205886

Yamamoto, Eiji; Gokuden, Daichi; Nagai, Ayano; Kamachi, Takashi; Yoshizawa, Kazunari; Hamasaki, Akiyuki; Ishida, Tamao; Tokunaga, Makoto

2012-12-21

96

Efficient Electronic Energy Transfer in Polymer Nanocomposite Assemblies.  

National Technical Information Service (NTIS)

Electronic energy transfer plays a critical role in biological photosynthesis, artificial photosynthesis, photovoltaic devices, photocatalysis, and other multichromophore systems and photoinduced processes. Our studies of electronic energy transfer in nov...

S. A. Jenekhe C. J. Yang

1996-01-01

97

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

Federal Register 2010, 2011, 2012, 2013

...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

98

Non-Nernstian two-electron transfer photocatalysis at metalloporphyrin-TiO2 interfaces.  

PubMed

A long-standing question in the photochemical sciences concerns how to integrate single-electron transfers to catalytic multielectron transfer reactions that produce useful chemical fuels. Here we provide a strategy for the two-electron formation of C-C bonds with molecular catalysts anchored to semiconductor nanocrystallites. The blue portion of the solar spectrum provides band gap excitation of the semiconductor while longer wavelengths of light initiate homolytic cleavage of metal-carbon bonds that, after interfacial charge transfer, restore the catalyst. The semiconductor utilized was the anatase polymorph of TiO(2) present as a nanocrystalline, mesoporous thin film. The catalyst was cobalt meso-5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin chloride, Co(TCPP)Cl. For this catalyst and iron protoporphyrin IX chloride, Fe(PPIX)Cl, two distinct and sequential metal-based M(III/II) and M(II/I) reductions were observed under band gap illumination. Spectroelectrochemical characterization indicated that both reductions were non-Nernstian, behavior attributed to an environmentally dependent potential drop across the molecule-semiconductor interface. Reaction of Co(I)(TCPP)/TiO(2) with organobromides (RBr = 1-Br-hexane or benzyl bromide) resulted in the formation of Co(III)-R(TCPP)/TiO(2). Visible light excitation induced homolytic cleavage of the Co-C bond and the formation of C-C-bonded products. The reactions were catalytic when band gap excitation or an electrochemical bias provided TiO(2) electrons to the oxidized catalyst. Sustained photocurrents were quantified in photoelectrosynthetic solar cells under forward bias. PMID:21888402

Ardo, Shane; Achey, Darren; Morris, Amanda J; Abrahamsson, Maria; Meyer, Gerald J

2011-10-19

99

Promoting interspecies electron transfer with biochar.  

PubMed

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

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

2014-01-01

100

Promoting Interspecies Electron Transfer with Biochar  

PubMed Central

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

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

2014-01-01

101

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. The knowledge gained from the study of synthetic model systems which abstract features of the natural photosynthetic apparatus can be used to design artificial photosynthetic systems which employ the basic physics and chemistry of photosynthesis to help meet mankind's energy needs. More specifically, the proposed models 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.

Gust, J.D. Jr.; Moore, T.A.

1988-04-12

102

Fast photoinduced electron transfer through DNA intercalation.  

PubMed Central

We report evidence for fast photoinduced electron transfer mediated by the DNA helix that requires metal complexes that are avid intercalators of DNA. Here the donor bis(phenanthroline)(dipyridophenazine)ruthenium(II) [Ru(phen)2dppz2+] and acceptor bis(9,10-phenanthrenequinone diimine)(phenanthroline)rhodium(III) [Rh(phi)2phen3+] intercalate into DNA with Kb > 10(6) M-1. Luminescence quenching experiments in the presence of two different lengths of DNA yield upward-curving Stern-Volmer plots and the loss of luminescence intensity far exceeds the change in emission lifetimes. In the presence of a nonintercalative electron acceptor, Ru(NH3)3+(6), Ru(phen)2dppz2+ luminescence is quenched much less efficiently compared to that found for the intercalative Rh(phi)2phen3+ quencher and follows linear Stern-Volmer kinetics; steady-state and time-resolved Stern-Volmer plots are comparable in scale. These experiments are consistent with a model involving fast long-range electron transfer between intercalators through the DNA helix. Images

Murphy, C J; Arkin, M R; Ghatlia, N D; Bossmann, S; Turro, N J; Barton, J K

1994-01-01

103

Stabilization of non-productive conformations underpins rapid electron transfer to electron-transferring flavoprotein.  

PubMed

Crystal structures of protein complexes with electron-transferring flavoprotein (ETF) have revealed a dual protein-protein interface with one region serving as anchor while the ETF FAD domain samples available space within the complex. We show that mutation of the conserved Glu-165beta in human ETF leads to drastically modulated rates of interprotein electron transfer with both medium chain acyl-CoA dehydrogenase and dimethylglycine dehydrogenase. The crystal structure of free E165betaA ETF is essentially identical to that of wild-type ETF, but the crystal structure of the E165betaA ETF.medium chain acyl-CoA dehydrogenase complex reveals clear electron density for the FAD domain in a position optimal for fast interprotein electron transfer. Based on our observations, we present a dynamic multistate model for conformational sampling that for the wild-type ETF. medium chain acyl-CoA dehydrogenase complex involves random motion between three distinct positions for the ETF FAD domain. ETF Glu-165beta plays a key role in stabilizing positions incompatible with fast interprotein electron transfer, thus ensuring high rates of complex dissociation. PMID:15975918

Toogood, Helen S; van Thiel, Adam; Scrutton, Nigel S; Leys, David

2005-08-26

104

Analytical and computational studies of intramolecular electron transfer pertinent to electron transfer and electron capture dissociation mass spectrometry.  

PubMed

Earlier work from this group has suggested that, in electron capture and electron-transfer mass spectrometry experiments on positively charged gas-phase samples of polypeptides, the initial electron attachment event most likely occurs at one of the peptide's positively charged sites (e.g., protonated side chains), although electron attachment can occur at a disulfide or amide site ca. 1-10% of the time. Focusing on the 90-99% dominant channel in which initial electron attachment occurs at a positive site, this paper addresses to what extent and over what distances electron transfer can take place from a positively charged site to a disulfide sigma* or amide pi* orbital, because it is thought that it is through such orbitals that disulfide or N-C(alpha) backbone bond cleavage occurs. Ab initio electronic structure calculations show that, as long as an SS sigma* (or OCN pi*) orbital experiences sufficient Coulomb stabilization from proximal positively charged groups, there are a myriad of excited Rydberg states located on positive sites that are able to induce such intrapeptide electron transfer. Computational data show that the transfer rates decay exponentially with distance for a given Rydberg orbital. An analytical model is developed that allows us to estimate the rates of Rydberg-to-valence and Rydberg-to-Rydberg electron transfers as functions of the Rydberg orbitals' n quantum numbers. This model suggests that transfer can occur over very long distances at rates that are more than competitive with the rates of radiationless relaxation within the manifold of Rydberg states (the latter processes eventually terminate the electron-transfer process an thus the disulfide or N-C(alpha) bond cleavages), and it gives formulas for how these rates depend on n (and thus the radial span of the Rydberg orbitals). PMID:19731901

Neff, Diane; Simons, Jack

2010-01-28

105

Heterogeneities of the nanostructure of platinum/zeolite y catalysts revealed by electron tomography.  

PubMed

To develop structure-performance relationships for important catalysts, a detailed characterization of their morphology is essential. Using electron tomography, we determined in three dimensions the structure of Pt/zeolite Y bifunctional catalysts. Optimum experimental conditions enabled for the first time high-resolution 3D imaging of Pt particles as small as 1 nm located inside zeolite micropores. Semiautomated image analysis of 3D reconstructions provided an efficient study of numbers, size distributions, and interparticle distances of thousands of Pt particles within individual zeolite crystals. Upon extending this approach to a number of zeolite crystals of one batch of Pt/zeolite Y catalyst, heterogeneities were revealed. The Pt loading, an important parameter for catalyst performance, varied between zeolite crystals up to a factor of 35. This discovery calls for re-evaluation of catalyst preparation methods and suggests potential for lowering the nominal loading with noble metals. PMID:23521107

Ze?evi?, Jovana; van der Eerden, Ad M J; Friedrich, Heiner; de Jongh, Petra E; de Jong, Krijn P

2013-04-23

106

NOx removal with multiple pulsed electron beam free of catalysts or reagents.  

PubMed

A catalyst free approach for nitrogen oxides (NOx) removal has been developed at the United States Naval Research Laboratory. Our goals were to assess the ability of pulsed electron beam to enhance NOx removal at potential lower capital cost with greater efficiency than other large scale NOx removal methods. Removal efficiency over 95% has been attained for NOx concentrations of 1000 parts per million (ppm), 500 ppm and 200 ppm in nitrogen atmosphere. The NOx concentration dropped from 204 ppm to below 4.8 ppm after 10 shots supplying a total dose of 65 kGy. The resultant chemicals after catalyst free pulsed electron beam processing of NOx are nitrogen and oxygen, same as components of air. Pulsed electron beams in a catalyst free approach remove a larger percentage of NOx than continuous wave electron beam with a catalyst. Catalyst free approach removes issues of handling, collecting, transporting and efficiently distributing chemical byproducts. Pulsed electron beams are as efficient as continuous wave electron beams for small removal percentages and have a significant advantage at higher fractional removal percentages of NOx. Preferential destruction of NO species relative to the removal of NO2 species is observed in the pulsed electron beam reaction chamber. The energy required to remove a kilogram of NOx is nearly the same at pressures of 1.16 atmospheres and 1.02 atmospheres. PMID:23417142

Wolford, Matthew F; Myers, Matthew C; Hegeler, Frank; Sethian, John D

2013-03-28

107

Ultrafast Interfacial Proton-Coupled Electron Transfer  

SciTech Connect

Interfaces between metallic or semiconducting solids and protic solvent adsorbates or liquids represent one of the most important, and yet hardly explored material environments for proton coupled electron transfer (PCET) processes. PCET mediated dynamical phenomena driven by light, electron, and chemical potentials are central in energy transduction processes of vast economic and environmental importance including the photocatalytic splitting of H?O, the photo and electrochemical reduction of CO?, and the conversion of chemical to electrical energy in fuel cells. Experimental and theoretical investigations of the dynamical aspects of PCET at solid surfaces are particularly challenging because relatively localized charges within a solvent couple in the presence of strong interfacial potentials to delocalized states of electronic continua of semiconductor or metal electrodes. Moreover, the localized charges are never the bare protons and electrons that balance chemical equations, but rather are dressed particles with associated polarization clouds inhomogeneously distributed and comprised variously of free electrons, lattice ions, and solvent molecules. The polarization clouds screen the Coulomb potential on the medium specific time scales and impose energetic costs associated with transport through the inhomogeneous region of interfaces between the solid and molecular environments. We introduce some recent theoretical studies aimed at providing an atomistic description on metal-protic solvent interface and modeling of simple processes such as the discharge of H? at a metal interface. Because of the paucity of experimental research and embryonic stage of theory, our goal is to present some key theoretical concepts and early experimental efforts based primarily on a surface science approach to ultrafast electron induced dynamics. In order to introduce some key features of interfacial PCET in the strong and intermediate coupling regimes, we discuss specific examples of photoinduced dissociation of alkanes on metals and photoinduced PCET dynamics of methanol covered TiO? surfaces.

Petek, Hrvoje; Zhao, Jin

2010-12-08

108

Electronic excitation transfer in concentrated micelle solutions  

SciTech Connect

Electronic excitation transport among interacting clusters of chromophores is investigated as a function of chromophores is investigated as a function of chromophore and cluster concentration. The technique of time-correlated single photon counting is employed to obtain time-resolved fluorescence depolarization data on aqueous octadecylrhodamine B/triton X-100 micelle solutions. The time-dependent fluorescence anisotropy, the energy transport observable, is directly compared to a theory developed to model this system. The theory is based on a first-order cumulant approximation to the solution of the transport master equation. The model depicts the micelles as monodisperse hard spheres with chromophores (octadecylrhodamine B) distributed about their surfaces. At low micelle concentration, the dynamics of excitation transfer depend only on internal micelle structure. At high micelle concentration excitation transfer occurs among chromophores on different micelles in addition to intramicelle transfer. The theoretical treatment provides nearly quantitative descriptions of the time and concentration dependence of the excitation transport. It correctly predicts the concentration at which intermicelle transfer becomes significant. In the low micelle concentration limit (energy transport confined to isolated micelles) the model having a Poisson distribution of chromophores works well for small {nu} ([chromophores]/[micelle]), but progressively worse as {nu} is increased. Following the literature, a chromophere interaction parameter (in the form of a two dimensional second virial coefficient) is used to skew the probe distribution. This enables the transport theory to reproduce the data for all the values of {nu} investigated and provides a determination of the second virial coefficient. 34 refs., 4 figs., 2 tabs.

Marcus, A.H.; Diachun, N.A.; Fayer, M.D. [Stanford Univ., CA (United States)

1992-10-29

109

NEGATIVE ELECTRON TRANSFER DISSOCIATION OF GLYCOSAMINOGLYCANS  

PubMed Central

Structural characterization of glycosaminoglycans (GAGs) has been a challenge in the field of mass spectrometry, and the application of electron detachment dissociation (EDD) Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) has shown great promise to GAG oligosaccharide characterization in a single tandem mass spectrometry experiment. In this work, we apply the technique of negative electron transfer dissociation (NETD) to GAGs on a commercial ion trap mass spectrometer. NETD of GAGs, using fluoranthene or xenon as the reagent gas, produces fragmentation very similar to previously observed EDD fragmentation. Using fluoranthene or xenon, both glycosidic and cross-ring cleavages are observed, as well as even- and odd-electron products. The loss of SO3 can be minimized and an increase in cross-ring cleavages is observed if a negatively-charged carboxylate is present during NETD, which can be controlled by the charge state or the addition of sodium. NETD effectively dissociates GAGs up to eight saccharides in length, but the low resolution of the ion trap makes assigning product ions difficult. Similar to EDD, NETD is also able to distinguish the epimers iduronic acid from glucuronic acid in heparan sulfate tetrasaccharides and suggests that a radical intermediate plays an important role in distinguishing these epimers. These results demonstrate that NETD is effective at characterizing GAG oligosaccharides in a single tandem mass spectrometry experiment on a widely available mass spectrometry platform.

Wolff, Jeremy J.; Leach, Franklin E.; Laremore, Tatiana N.; Kaplan, Desmond A.; Easterling, Michael L.; Linhardt, Robert J.; Amster, I. Jonathan

2010-01-01

110

Negative electron transfer dissociation of glycosaminoglycans.  

PubMed

Structural characterization of glycosaminoglycans (GAGs) has been a challenge in the field of mass spectrometry, and the application of electron detachment dissociation (EDD) Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) has shown great promise to GAG oligosaccharide characterization in a single tandem mass spectrometry experiment. In this work, we apply the technique of negative electron transfer dissociation (NETD) to GAGs on a commercial ion trap mass spectrometer. NETD of GAGs, using fluoranthene or xenon as the reagent gas, produces fragmentation very similar to previously observed EDD fragmentation. Using fluoranthene or xenon, both glycosidic and cross-ring cleavages are observed, as well as even- and odd-electron products. The loss of SO(3) can be minimized and an increase in cross-ring cleavages is observed if a negatively charged carboxylate is present during NETD, which can be controlled by the charge state or the addition of sodium. NETD effectively dissociates GAGs up to eight saccharides in length, but the low resolution of the ion trap makes assigning product ions difficult. Similar to EDD, NETD is also able to distinguish the epimers iduronic acid from glucuronic acid in heparan sulfate tetrasaccharides and suggests that a radical intermediate plays an important role in distinguishing these epimers. These results demonstrate that NETD is effective at characterizing GAG oligosaccharides in a single tandem mass spectrometry experiment on a widely available mass spectrometry platform. PMID:20380445

Wolff, Jeremy J; Leach, Franklin E; Laremore, Tatiana N; Kaplan, Desmond A; Easterling, Michael L; Linhardt, Robert J; Amster, I Jonathan

2010-05-01

111

Vectorial electron transfer in spatially ordered arrays. Progress report, January 1991--January 1994  

SciTech Connect

Objective was to find methods for rapid, controlled placement of light absorbers, relays, and multi-electron catalysts at defined sites with respect to a semiconductor or metal surface and thus to develop methods for preparing chemically modified photoactive surfaces as artificial photosynthetic units. Progress has been made in four areas: synthesis of new materials for directional electron transfer, preparation and characterization of anisotropic composites containing organic and inorganic components, elaboration of mechanisms of electrocatalysis, and development of new methods for surface modification of metals and semiconductors.

Fox, M.A.

1994-01-01

112

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

PubMed Central

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

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

2005-01-01

113

Synthesis of calix[4]arene alkylamine derivatives as new phase-transfer catalysts for esterification reaction  

Microsoft Academic Search

This study reports the synthesis of calix[4]arene-based phase-transfer catalysts derived from the reaction of 5,17-di-tert-butyl-25,27,26,28-tetrahydroxycalix[4]arene with N-ethylpiperazine, diallylamine or 4-benzylpiperidine. The catalytic efficiency of the calix[4]arenes alkylamine derivatives was evaluated by carrying out the ester-forming reaction of alkali metal carboxylates (sodium butyrate or sodium caprylate) with p-nitrobenzyl bromide. It has been observed that the ester-forming reaction of alkali metal carboxylates

Ezgi Akceylan; Mustafa Yilmaz

2011-01-01

114

SCANNING ELECTRON MICROSCOPY STUDY OF CO-PRECIPITATED IRON OXIDE CATALYSTS  

Microsoft Academic Search

Iron oxide Fischer-Tropsch catalysts were prepared using a sol-gel method to incorporate varying amounts of silica (SiO2) and a constant amount of potassium oxide (K2O) promoters. The sol-gel slurries containing different Fe\\/Si ratios were dried using a spray dryer. The spray-dried materials were calcined at 300°C prior to the use as catalysts for the Fischer-Tropsch reaction. Scanning electron microscopy (SEM),

Ram Srinivasan; Robert L Spicer; Bob OBrien; Rolando Gonzalez; Fred Tungate; Burtron H. Davis

1997-01-01

115

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

PubMed

The design of molecular electrocatalysts for H(2) 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 H(2) 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

2012-09-25

116

Selective 4e-/4H+ O2 reduction by an iron(tetraferrocenyl)porphyrin complex: from proton transfer followed by electron transfer in organic solvent to proton coupled electron transfer in aqueous medium.  

PubMed

An iron porphyrin catalyst bearing four ferrocenes and a hydrogen bonding distal pocket is found to catalyze 4e(-)/4H(+) oxygen reduction reaction (ORR) in organic solvent under homogeneous conditions in the presence of 2-3 equiv of Trifluoromethanesulphonic acid. Absorption spectroscopy, electron paramagnetic resonance (EPR), and resonance Raman data along with H2O2 assay indicate that one out of the four electrons necessary to reduce O2 to H2O is donated by the ferrous porphyrin while three are donated by the distal ferrocene residues. The same catalyst shows 4e(-)/4H(+) reduction of O2 in an aqueous medium, under heterogeneous conditions, over a wide range of pH. Both the selectivity and the rate of ORR are found to be pH independent in an aqueous medium. The ORR proceeds via a proton transfer followed by electron transfer (PET) step in an organic medium and while a 2e(-)/1H(+) proton coupled electron transfer (PCET) step determines the electrochemical potential of ORR in an aqueous medium. PMID:24304224

Mittra, Kaustuv; Chatterjee, Sudipta; Samanta, Subhra; Dey, Abhishek

2013-12-16

117

Cinchona alkaloid-based polymer-bound phase-transfer catalysts: Efficient enantioselective alkylation of benzophenone imine of glycine esters  

Microsoft Academic Search

Cross-linked polystyrene-bound and poly(ethylene glycol)-bound phase-transfer catalysts as well as homopolymers of cinchona alkaloid derivatives have been synthesised. Both soluble and insoluble polymers have been investigated. The enantioselective alkylation of N-diphenyl methylene glycine t-butyl ester has been successfully carried out in heterogeneous and homogeneous systems. High enantioselectivities (up to 96%) have been obtained. The polymer-bound catalysts have been easily recovered

Baptiste Thierry; Jean-Christophe Plaquevent; Dominique Cahard

2005-01-01

118

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.

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

2011-01-01

119

Hydrogen peroxide oxidation of mustard-model sulfides catalyzed by iron and manganese tetraarylporphyrines. Oxygen transfer to sulfides versus H(2)O(2) dismutation and catalyst breakdown.  

PubMed

Fe(III)- and Mn(III)-meso-tetraarylporphyrin catalysis of H(2)O(2) oxidation of dibenzyl and phenyl-2-chloroethyl sulfides, 1, is investigated in ethanol with the aim of designing catalytic systems for mustard decontamination. The sulfide conversion, the sulfoxide and sulfone yields, the oxygen transfer from H(2)O(2) to the sulfide, and the catalyst stability depend markedly on the metal, on the substituents of its ligand, and on the presence or the absence of a cocatalyst, imidazole or ammonium acetate. With Fe, sulfones, the only oxidation products, are readily obtained whatever the ligand (TPP, F(20)TPP, or TDCPP) and the cocatalyst; the oxygen transfer is fairly good, up to 95% when the catalyst concentration is small ([1]/[Cat] = 420); the catalyst breakdown is insignificant only in the absence of any cocatalyst. With Mn, the sulfide conversion is achieved completely when the ligand is TDCPP or TSO(3)PP, but not F(20)TPP or TPP; a mixture of sulfoxide, 2, and sulfone, 3, is always obtained with [2]/[3] = 3.5-0.85 depending on the ligand and the cocatalyst (electron withdrawing substituents favor 3 and NH(4)OAc, 2). The catalyst stability is very good, but the oxygen transfer is poor whatever the ligand and the cocatalyst. These results are discussed in terms of a scheme in which sulfide oxygenation, H(2)O(2) dismutation, and oxidative ligand breaking compete. It is shown that the efficiency of the oxygen transfer is related not only to the rate constant of the dismutation route but also to the concentration of the active metal-oxo intermediate, most likely a perferryl or permanganyl species, i.e., to the rate of its formation. PMID:11701009

Marques, A; Marin, M; Ruasse, M F

2001-11-16

120

Desulfurization of coal: enhanced selectivity using phase transfer catalysts. Quarterly report, March 1 - May 31, 1996  

SciTech Connect

Due to environmental problems related to the combustion of high sulfur Illinois coal, there continues to be interest in the development in viable pre-combustion desulfurization processes. Recent studies by the authors have obtained very good sulfur removals but the reagents that are used are too expensive. Use of cheaper reagents leads to a loss of desired coal properties. This study investigated the application phase transfer catalysts to the selective oxidation of sulfur in coal using air and oxygen as oxidants. The phase transfer catalyst is expected to function as a selectivity moderator by permitting the use of milder reaction conditions that otherwise necessary. This would enhance the sulfur selectivity and help retain the heating value of the coal. The use of certain coal combustion wastes for desulfurization, and the application of cerium (IV) catalyzed air oxidation for selective sulfur oxidation are also being studied. If successful, this project could lead to the rapid development of a commercially viable desulfurization process. This would significantly improve the marketability of Illinois coal.

Palmer, S.R.; Hippo, E.J. [Southern Illinois Univ., Carbondale, IL (United States)

1996-12-31

121

Single-element Electron-transfer Optical Detector System  

NASA Technical Reports Server (NTRS)

An optical detector system includes an electrically resistive screen that is substantially transparent to radiation energy having a wavelength of interest. An electron transfer element (e.g., a low work function photoactive material or a carbon nanotube (CNT)-based element) has a first end and a second end with its first end spaced apart from the screen by an evacuated gap. When radiation energy passes through the screen with a bias voltage being applied thereto, transfer of electrons through the electron transfer element is induced from its first to its second end such that a quantity indicative of the electrons transferred can be detected.

Jordan, Jeffrey D. (Inventor)

2004-01-01

122

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

SciTech Connect

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

Not Available

1989-01-01

123

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

SciTech Connect

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

Not Available

1989-12-31

124

Well-defined iron complexes as efficient catalysts for "green" atom-transfer radical polymerization of styrene, methyl methacrylate, and butyl acrylate with low catalyst loadings and catalyst recycling.  

PubMed

Environmentally friendly iron(II) catalysts for atom-transfer radical polymerization (ATRP) were synthesized by careful selection of the nitrogen substituents of N,N,N-trialkylated-1,4,9-triazacyclononane (R3 TACN) ligands. Two types of structures were confirmed by crystallography: "[(R3 TACN)FeX2 ]" complexes with relatively small R groups have ionic and dinuclear structures including a [(R3 TACN)Fe(?-X)3 Fe(R3 TACN)](+) moiety, whereas those with more bulky R groups are neutral and mononuclear. The twelve [(R3 TACN)FeX2 ]n complexes that were synthesized were subjected to bulk ATRP of styrene, methyl methacrylate (MMA), and butyl acrylate (BA). Among the iron complexes examined, [{(cyclopentyl)3 TACN}FeBr2 ] (4?b) was the best catalyst for the well-controlled ATRP of all three monomers. This species allowed easy catalyst separation and recycling, a lowering of the catalyst concentration needed for the reaction, and the absence of additional reducing reagents. The lowest catalyst loading was accomplished in the ATRP of MMA with 4?b (59?ppm of Fe based on the charged monomer). Catalyst recycling in ATRP with low catalyst loadings was also successful. The ATRP of styrene with 4?b (117?ppm Fe atom) was followed by precipitation from methanol to give polystyrene that contained residual iron below the calculated detection limit (0.28?ppm). Mechanisms that involve equilibria between the multinuclear and mononuclear species were also examined. PMID:24664500

Nakanishi, So-Ichiro; Kawamura, Mitsunobu; Kai, Hidetomo; Jin, Ren-Hua; Sunada, Yusuke; Nagashima, Hideo

2014-05-01

125

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

126

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

127

Electron Transfer Dissociation of Milk Oligosaccharides  

NASA Astrophysics Data System (ADS)

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.

Han, Liang; Costello, Catherine E.

2011-06-01

128

Time-resolved EPR identifies unexpected electron transfer in cryptochrome**  

PubMed Central

Tuning photoinduced electron transfer: Subtle differences in local sequence and conformation can produce diversity and specificity in electron transfer (ET) in proteins, despite high structural conservation of redox partners. For individual ET steps, distance is not necessarily the decisive parameter; orientation and solvent accessibility of ET partners, and therefore, stabilization of charge-separated states contribute substantially.

Biskup, Till; Hitomi, Kenichi; Getzoff, Elizabeth D.; Krapf, Sebastian; Koslowski, Thorsten; Schleicher, Erik

2012-01-01

129

Review of Industrial Applications of Heat Transfer to Electronics  

Microsoft Academic Search

Applications of heat transfer to electronic components and devices are presented and discussed. The major objectives are to review the state of the art in such applications and to indicate the need for a better grasp of the science of heat transfer in aiding the creative engineering of new electronic devices operating at extreme conditions of temperature, heat flux, air

Joseph Kaye

1956-01-01

130

Investigation of electron transfer between hexaarylbiimidazole and visible sensitizer  

NASA Astrophysics Data System (ADS)

Photoinduced electron transfer between the hexaarylbiimidazole (HABI) and visible sensitizer, 2,5-bis [(2,3,6,7-tetrahydro- 1H,5H-benzo [i,j] quinolizin- l-yl ) methylene] cyclopentanone (JAW), in liquid solution was investigated experimentally. The relative fluorescence quantum yield of the sensitizer molecule was measured as a function of HABI concentration. An exponential dependence on HABI concentration was observed for the sensitizer fluorescence yield, from which the critical distance for the electron transfer interaction was determined to be approximately 12 A. The time-dependent fluorescence quenching was studied with the time-correlated single-photon counting technique. The dynamics of the electron transfer process was determined by comparing the experimental data to existing models of electron transfer in liquids, The inclusion of the relative diffusion between the electron donor and acceptor molecules is necessary to fit all the observations and to obtain accurate transfer parameters.

Lin, Yi; Liu, Andong; Trifunac, Alexander D.; Krongauz, Vadim V.

1992-10-01

131

Probing active electron transfer branch in photosystem I reaction center.  

NASA Astrophysics Data System (ADS)

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

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

2003-03-01

132

Identification and optimization of short helical peptides with novel reactive functionality as catalysts for acyl transfer by reactive tagging.  

PubMed

Herein we describe the screening and subsequent optimization of peptide catalysts for ester activation. A combinatorial methodology using dye-tagged substrate analogs is described for determining which components of a His-containing helical library display acyl transfer activity. We found that helical peptides display high activity, and amino acids that reinforce this propensity are advantaged. Through this approach two new structural motifs have been discovered that are capable of activating esters in organic solvents. Unlike most acyl transfer catalysts functioning in organic solvents, these catalysts are histidine- rather than N-alkyl histidine-based. Longer peptides with localization of reactive groups on the C-terminal end of the peptide were found to further enhance catalytic activity up to ?2800-fold over background. PMID:24448664

Bezer, Silvia; Matsumoto, Masaomi; Lodewyk, Michael W; Lee, Stephen J; Tantillo, Dean J; Gagné, Michel R; Waters, Marcey L

2014-03-01

133

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.

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

2012-01-01

134

Desulfurization of coal: Enhanced selectivity using phase transfer catalysts. Technical report, September 1--November 30, 1995  

SciTech Connect

Due to environmental problems related to the combustion of high sulfur Illinois coal, there continues to be interest in the development of viable pre-combustion desulfurization processes. Recent studies by the authors have obtained very good sulfur removals but the reagents that are used are too expensive. Use of cheaper reagents leads to a loss of desired coal properties. This study investigates the application of phase transfer catalysts to the selective oxidation of sulfur in coal using air and oxygen as oxidants. The phase transfer catalyst is expected to function as a selectivity moderator by permitting the use of milder reaction conditions than otherwise necessary. This would enhance the sulfur selectivity and help retain the heating value of the coal. The use of certain coal combustion wastes for desulfurization, and the application of cerium (IV) catalyzed air oxidations for selective sulfur oxidation are also being studied. If successful this project could lead to the rapid development of a commercially viable desulfurization process. This would significantly improve the marketability of Illinois coal. During this quarter aliquots of the IBC-101 coal have been ground to various particle sizes in an attempt to find the optimum physical pretreatment for mineral, especially pyrite, removal. Analysis of these various aliquots shows them to be representative of the original coal. In addition, preliminary desulfurization reactions using fly ash and scrubber sludges have been performed on an unoxidized IBC-101 sample. Results will be available next quarter. Also, SEM-EDAX analysis of the fly ash indicates that it contains oxides that have shown activity in base desulfurization reactions.

Palmer, S.R.; Hippo, E.J.

1995-12-31

135

Variable Electron Transfer Pathways in an Amphibian Cryptochrome  

PubMed Central

Electron transfer reactions play vital roles in many biological processes. Very often the transfer of charge(s) proceeds stepwise over large distances involving several amino acid residues. By using time-resolved electron paramagnetic resonance and optical spectroscopy, we have studied the mechanism of light-induced reduction of the FAD cofactor of cryptochrome/photolyase family proteins. In this study, we demonstrate that electron abstraction from a nearby amino acid by the excited FAD triggers further electron transfer steps even if the conserved chain of three tryptophans, known to be an effective electron transfer pathway in these proteins, is blocked. Furthermore, we were able to characterize this secondary electron transfer pathway and identify the amino acid partner of the resulting flavin-amino acid radical pair as a tyrosine located at the protein surface. This alternative electron transfer pathway could explain why interrupting the conserved tryptophan triad does not necessarily alter photoreactions of cryptochromes in vivo. Taken together, our results demonstrate that light-induced electron transfer is a robust property of cryptochromes and more intricate than commonly anticipated.

Biskup, Till; Paulus, Bernd; Okafuji, Asako; Hitomi, Kenichi; Getzoff, Elizabeth D.; Weber, Stefan; Schleicher, Erik

2013-01-01

136

Studies in catalytic transfer hydrogenation of soybean oil using ammonium formate as donor over 5% Pd\\/C catalyst  

Microsoft Academic Search

The catalytic transfer hydrogenation (CTH) is a novel, alternative process for the selective hydrogenation of soybean oil due to the lower rate of formation of stearic acid and trans-isomer, which have an adverse health effects. Ruthenium and palladium supported catalysts were screened along with hydrogen donors during CTH process. The CTH of soybean oil was studied using aqueous ammonium formate

Manoj A. Tike; Vijaykumar V. Mahajani

2006-01-01

137

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.

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

2009-01-01

138

A molecular shift register based on electron transfer  

NASA Technical Reports Server (NTRS)

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

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

1988-01-01

139

Ultrafast proton-coupled electron transfer in heterogenous photocatalysis  

NASA Astrophysics Data System (ADS)

At metal-oxide/protic-solvent interfaces, partially hydrated or "wet electron" states represent the lowest energy pathway for electron transfer. Here we study the photoinduced charge transfer at the H IIO/TiO II(110) interface by means of timeresolved two-photon photoemission spectroscopy and electronic structure theory. At ~1 monolayer coverage of H IIO on partially hydroxylated TiO II surfaces we find an unoccupied electronic state 2.4+/-0.1 eV above the Fermi level. Density functional theory shows this to be a two-dimensional "wet electron" state, which is distinct from hydrated electrons observed on water-covered metal surfaces. The decay of electrons from the wet electron state by the resonant charge transfer to the conduction band of TiO II occurs in <=15 femtoseconds. Similar unoccupied electronic structure is observed for CH 3OH covered TiO II(110) surfaces; however, the electron dynamics are considerably more complex. The wet electron state dynamics of CH 3OH/TiO II exhibit both energy and population decay. The excited state lifetime is strongly coverage dependent increasing to >100 fs range above 1 ML CH 3OH coverage. Significantly, a pronounced deuterium isotope effect (CH 3OD) indicates a strong correlation between the interfacial electron transfer and the motion of protons in the molecular overlayer.

Zhao, Jin; Onda, Ken; Li, Bin; Petek, Hrvoje

2006-09-01

140

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

141

Engineering of an alternative electron transfer path in photosystem II.  

PubMed

The initial steps of oxygenic photosynthetic electron transfer occur within photosystem II, an intricate pigment/protein transmembrane complex. Light-driven electron transfer occurs within a multistep pathway that is efficiently insulated from competing electron transfer pathways. The heart of the electron transfer system, composed of six linearly coupled redox active cofactors that enable electron transfer from water to the secondary quinone acceptor Q(B), is mainly embedded within two proteins called D1 and D2. We have identified a site in silico, poised in the vicinity of the Q(A) intermediate quinone acceptor, which could serve as a potential binding site for redox active proteins. Here we show that modification of Lysine 238 of the D1 protein to glutamic acid (Glu) in the cyanobacterium Synechocystis sp. PCC 6803, results in a strain that grows photautotrophically. The Glu thylakoid membranes are able to perform light-dependent reduction of exogenous cytochrome c with water as the electron donor. Cytochrome c photoreduction by the Glu mutant was also shown to significantly protect the D1 protein from photodamage when isolated thylakoid membranes were illuminated. We have therefore engineered a novel electron transfer pathway from water to a soluble protein electron carrier without harming the normal function of photosystem II. PMID:20457933

Larom, Shirley; Salama, Faris; Schuster, Gadi; Adir, Noam

2010-05-25

142

Evolution of Gold Structure During Thermal Treatment of Au\\/FeOx Catalysts Revealed by Aberration-Corrected Electron Microscopy  

Microsoft Academic Search

High-resolution aberration-corrected electron microscopy was performed on a series of catalysts derived from a parent material, 2 at.% Au\\/FeO (WGC ref. no. 60C), prepared by co-precipitation and calcined in air at 400 C, and a catalyst prepared by leaching surface gold from the parent catalyst and exposed to various treatments, including use in the water-gas shift reaction at 250 C.

Steven H Overbury; Lawrence Frederick Allard Jr; Albina Y Borisevich; Weiling Deng; Rui Si; Maria Flytzani-Stephanopoulos

2009-01-01

143

Photoinduced electron transfers with carbon dots  

PubMed Central

The photoluminescence in carbon dots (surface-passivated small carbon nanoparticles) could be quenched efficiently by electron acceptor or donor molecules in solution, namely that photo-excited carbon dots are both excellent electron donors and excellent electron acceptors, thus offering new opportunities for their potential uses in light energy conversion and related applications.

Wang, Xin; Cao, Li; Lu, Fushen; Meziani, Mohammed J.; Li, Heting; Qi, Gang; Zhou, Bing; Harruff, Barbara A.; Kermarrec, Fabien; Sun, Ya-Ping

2009-01-01

144

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

PubMed

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

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

2013-05-01

145

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.

2013-01-01

146

Single-molecule spectroscopy of interfacial electron transfer.  

PubMed

It is widely appreciated that single-molecule spectroscopy (SMS) can be used to measure properties of individual molecules which would normally be obscured in an ensemble-averaged measurement. In this report we show how SMS can be used to measure photoinduced interfacial electron transfer (IET) and back electron transfer rates in a prototypical chromophore-bridge-electrode nonadiabatic electron transfer system. N-(1-hexylheptyl)-N'-(12-carboxylicdodecyl)perylene-3,4,9,10-tetracarboxylbisimide was synthesized and incorporated into mixed self-assembled monolayers (SAMs) on an ITO (tin-doped indium oxide, a p-type semiconductor) electrode. Single-molecule fluorescence time trajectories from this system reveals "blinks", momentary losses in fluorescence (>20 ms to seconds in duration), which are attributed to discrete electron transfer events: electron injection from the perylene chromophore into the conduction band of the ITO leads to the loss of fluorescence, and charge recombination (back electron transfer) leads to the return of fluorescence. Such blinks are not observed when an electrode is not present. The fluorescence trajectories were analyzed to obtain the forward and back electron rates; the measured rates are found to lie in the millisecond to second regime. Different rates are observed for different molecules, but the lifetime distributions for the forward or back electron transfer for any given molecule are well fit by single exponential kinetics. The methodology used is applicable to a wide variety of systems and can be used to study the effects of distance, orientation, linker, environment, etc. on electron transfer rates. The results and methodology have implications for molecular electronics, where understanding and controlling the range of possible behaviors inherent to molecular systems will likely be as important as understanding the individual behavior of any given molecule. PMID:14531709

Holman, Michael W; Liu, Ruchuan; Adams, David M

2003-10-15

147

Solvational Barriers to Interfacial Electron Transfer: Minimization via Valence Delocalization.  

National Technical Information Service (NTIS)

Standard rate constants (k (sub s)) for interfacial electron transfer (ET) have been obtained for several redox couples featuring very small internal activation barriers. To render these ordinarily fast rates measurable, we have employed low-defect-densit...

J. T. Hupp X. L. Zhang

1993-01-01

148

Quality assurance and data collection -- Electronic Data Transfer.  

National Technical Information Service (NTIS)

The Radiological Environmental Monitoring (REM) group at the Fernald Environmental Management Project is involved in an Electronic Data Transfer practice that will result in the improved quality assurance of collected data. This practice focuses on electr...

L. M. Tomczak W. G. Lohner E. C. Ray J. A. Salesky H. B. Spitz

1993-01-01

149

MONT3E: A Monte Carlo electron heat transfer code.  

National Technical Information Service (NTIS)

A Monte Carlo code, MONT3E, was written to analyze heat transfer due to electron backscattering. The availability of supercomputers has made large-scale applications of this analysis technique possible. Statistically valid solutions of complicated enginee...

J. D. Maltby B. T. Kornblum

1990-01-01

150

Solvation and Dielectric Dispersion in Optical Electron Transfer.  

National Technical Information Service (NTIS)

The effect of dielectric dispersion of the solvent on the energetics of optical electron transfer is determined quantitatively by variations of the free energy of solvation of the species being photoionized. The solvation free energy varies because the so...

P. Delahay A. Dziedzic

1984-01-01

151

Intramolecular electron transfer rates. Three year progress report.  

National Technical Information Service (NTIS)

The initial goals of this project were: (1) to construct pulsed-accelerated-flow and pulsed-laser (transient absorbance) instruments for intramolecular electron-transfer rate measurements, (2) to design and synthesize appropriate molecules and perform suc...

J. T. Hupp

1990-01-01

152

Some Perspectives on Solution-Phase Electron-Transfer Processes.  

National Technical Information Service (NTIS)

This article presents a selective summary of pertinent issues and a review of experimental inquiries into solution-phase electron transfer processes, with illustrations taken in part from our own laboratories. Appreciable emphasis is placed on solvent eff...

J. T. Hupp M. J. Weaver

1994-01-01

153

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

154

Photoinduced electron transfer from DABCO to trans-nitrostilbenes  

NASA Astrophysics Data System (ADS)

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

Görner, Helmut; Schulte-Frohlinde, Dietrich

155

45 CFR 162.1601 - Health care electronic funds transfers (EFT) and remittance advice transaction.  

Code of Federal Regulations, 2013 CFR

...2013-10-01 2013-10-01 false Health care electronic funds transfers (EFT... ADMINISTRATIVE REQUIREMENTS Health Care Electronic Funds Transfers (EFT...Remittance Advice § 162.1601 Health care electronic funds transfers...

2013-10-01

156

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

Federal Register 2010, 2011, 2012, 2013

...Electronic Funds Transfer (EFT) Market Research Study AGENCY: Financial Management...Electronic Funds Transfer (EFT) Market Research Study.'' DATES: Written comments...Electronic Funds Transfer (EFT) Market Research Study. OMB Number:...

2012-11-28

157

Characterization of durable nanostructured thin film catalysts tested under transient conditions using analytical aberration-corrected electron microscopy  

SciTech Connect

The stability of Ru0.1Ir0.9 oxidation evolution reaction (OER) catalysts deposited on Pt-coated nanostructured thin films (NSTFs) has been investigated by aberration-corrected electron microscopy. Accelerated stress tests showed that the OER catalysts significantly improved the durability of the Pt under cell reversal conditions. High-resolution images of the end-of-life NSTFs showed significant Ir loss from the whisker surfaces, while no Pt loss was observed, indicating that the OER catalysts had protected the catalyst coated whisker surfaces from degradation.

Cullen, David A [ORNL; More, Karren Leslie [ORNL; Reeves, Kimberly Shawn [ORNL; Vernstrom, George [3M Industrial Mineral Products; Atanasoska, Liliana [3M Industrial Mineral Products; Haugen, Gregory [3M Industrial Mineral Products; Atanasoski, Radoslav [3M Industrial Mineral Products

2011-01-01

158

Electron transfer to iron(III) oxide colloids  

SciTech Connect

The electron transfer reaction from viologen radicals to colloidal iron(III) oxide and oxyhydroxide particles was investigated by pulse and steady-state radiolysis techniques. The rate of electron injection to oxyhydroxide particles was clearly controlled by their surface charge and in the region near the isoelectric point, the electron transfer rate is slower than the diffusion-controlled rate by at least two orders of magnitude. Conductivity measurements clearly reveal the occurrence of protonation reactions immediately following the initial electron transfer step. On the basis of the experimentally measured ratio of protons consumed to electrons injected, a mechanistic pathway describing the sequence of protonation reactions is proposed. The competition between surface trapping of charge and the migration of charge into the interior of the particles and the factors controlling these two events are also discussed. 17 refs., 3 figs.

Mulvaney, P.; Grieser, F.; Swayambunathan, V.; Meisel, D.

1987-01-01

159

Static and dynamic order of cooperative multi-electron transfer  

NASA Astrophysics Data System (ADS)

We report on high correlation and ordering of multi-electron transfer at neutral-ionic transformation in quasi one-dimensional prototype crystal of tetrathiafulvalene-chloranil (TTF-CA). We present experimental evidences that the ordering of lattice-relaxed charge-transfer strings, which are nano-scale objects, occurs under static thermal equilibrium condition and non-equilibrium one achieved by femto-second laser irradiation. We suggest that the transformation is a profound example of self-organized, cooperative electron transfer with tendency for phase separation driven by energetics of inter-chain interaction and ferroelectric ordering.

Luty, T.; Cailleau, H.; Koshihara, S.; Collet, E.; Takesada, M.; Lemée-Cailleau, M. H.; Buron-LeCointe, M.; Nagaosa, N.; Tokura, Y.; Zienkiewicz, E.; Ouladdiaf, B.

2002-08-01

160

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...

161

Nuclear interlevel transfer driven by electronic transitions  

SciTech Connect

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

Solem, J.C.; Rinker, G.

1985-01-01

162

Small reorganization energies of photoinduced electron transfer between spherical fullerenes.  

PubMed

Rate constants of photoinduced electron transfer between spherical fullerenes were determined using triscandium nitride encapsulated C80 fullerene (Sc3N@C80) as an electron donor and the triplet excited state of lithium ion-encapsulated C60 fullerene (Li(+)@C60) as an electron acceptor in polar and less polar solvents by laser flash photolysis measurements. Upon nanosecond laser excitation at 355 nm of a benzonitrile (PhCN) solution of Li(+)@C60 and Sc3N@C80, electron transfer from Sc3N@C80 to the triplet excited state [(3)(Li(+)@C60)*] occurred to produce Sc3N@C80(•+) and Li(+)@C60(•-) (?(max) = 1035 nm). The rates of the photoinduced electron transfer were monitored by the decay of absorption at ?(max) = 750 nm due to (3)(Li(+)@C60)*. The second-order rate constant of electron transfer from Sc3N@C80 to (3)(Li(+)@C60)* was determined to be k(et) = 1.5 × 10(9) M(-1) s(-1) from dependence of decay rate constant of (3)(Li(+)@C60)* on the Sc3N@C80 concentration. The rate constant of back electron transfer from Li(+)@C60(•-) to Sc3N@C80(•+) was also determined to be k(bet) = 1.9 × 10(9) M(-1) s(-1), which is close to be the diffusion limited value in PhCN. Similarly, the rate constants of photoinduced electron transfer from C60 to (3)(Li(+)@C60)* and from Sc3N@C80 to (3)C60* were determined together with the back electron-transfer reactions. The driving force dependence of log k(et) and log k(bet) was well fitted by using the Marcus theory of outer-sphere electron transfer, in which the internal (bond) reorganization energy (?i) was estimated by DFT calculations and the solvent reorganization energy (?s) was calculated by the Marcus equation. When PhCN was replaced by o-dichlorobenzene (o-DCB), the ? value was decreased because of the smaller solvation changes of highly spherical fullerenes upon electron transfer in a less polar solvent. PMID:23862971

Kawashima, Yuki; Ohkubo, Kei; Fukuzumi, Shunichi

2013-08-01

163

InP Transferred Electron Cathodes: Basic to Manufacturing Methods.  

National Technical Information Service (NTIS)

InP has gained an emerging importance as a negative electron affinity (NEA) transferred electron photocathode (TEP) material in imaging technologies. InP provides itself as a substrate to grow these small band gap materials, and also serves as an efficien...

D. Lee P. Pianetta S. Sun W. E. Spicer Z. Liu

2007-01-01

164

Electron transfer between distant quantum dots by surface acoustic waves  

Microsoft Academic Search

Quantum dots (QDs) provide a useful system for manipulating and storing quantum information. Tunneling of electrons between double dots has been demonstrated but over larger distances a tunnel barrier is inadequate. We show such long range transfer of single electrons between QDs through a depleted 1D channel using a surface acoustic wave (SAW) pulse. Surface gates define two QDs (RQD

R. P. G. McNeil; M. Kataoka; C. J. B. Ford; C. H. W. Barnes; D. Anderson; G. A. C. Jones; I. Farrer; D. A. Ritchie

2010-01-01

165

Numerical prediction of electronic component heat transfer: an industry perspective  

Microsoft Academic Search

This study aims to provide a perspective on the current capabilities of computational fluids dynamics (CFD) as a design tool to predict component operating temperature in electronic systems. A systematic assessment of predictive accuracy is presented for printed circuit board (PCB) mounted component heat transfer, using a CFD code dedicated to the thermal analysis of electronic systems. Component operating temperature

Valtrie Eveloy; P. Rodgers; M. S. J. Hashmi

2003-01-01

166

1,3-Diene-Containing Phase Transfer Catalysts and Their Removal From Solution by a Diels-Alder Reaction with an Immobilized Triazolinedione Dienophile  

Microsoft Academic Search

8E,10E-Dodecadienyl-tri-n-butylamonium and -tri-n-butylphosphonium methanesulfonates were prepared and shown to be effective phase transfer catalysts. After reaction the catalysts can be removed quantitatively from the organic phase by addition of a silica gelimmobilized triazolinedione dienophile followed by filtration.

John F. W. Keana; David D. Ward

1983-01-01

167

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.

168

Heat transfer between adsorbate and laser-heated hot electrons  

NASA Astrophysics Data System (ADS)

Strong short laser pulses can give rise to a strong increase in the electronic temperature at metal surfaces. Energy transfer from the hot electrons to adsorbed molecules may result in adsorbate reactions, e.g. desorption or diffusion. We point out the limitations of an often used equation to describe the heat transfer process in terms of a friction coupling. We propose a simple theory for the energy transfer between the adsorbate and hot electrons using a newly introduced heat transfer coefficient, which depends on the adsorbate temperature. We calculate the transient adsorbate temperature and the reaction yield for a Morse potential as a function of the laser fluency. The results are compared to those obtained using a conventional heat transfer equation with temperature-independent friction. It is found that our equation of energy (heat) transfer gives a significantly lower adsorbate peak temperature, which results in a large modification of the reaction yield. We also consider the heat transfer between different vibrational modes excited by hot electrons. This mode coupling provides indirect heating of the vibrational temperature in addition to the direct heating by hot electrons. The formula of heat transfer through linear mode-mode coupling of two harmonic oscillators is applied to the recent time-resolved study of carbon monoxide and atomic oxygen hopping on an ultrafast laser-heated Pt(111) surface. It is found that the maximum temperature of the frustrated translation mode can reach high temperatures for hopping, even when direct friction coupling to the hot electrons is not strong enough.

Ueba, H.; Persson, B. N. J.

2008-06-01

169

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

PubMed Central

Recent advances in the theoretical treatment of proton-coupled electron transfer (PCET) reactions are reviewed. These reactions play an important role in a wide range of biological processes, as well as in fuel cells, solar cells, chemical sensors, and electrochemical devices. A unified theoretical framework has been developed to describe both sequential and concerted PCET, as well as hydrogen atom transfer (HAT). A quantitative diagnostic has been proposed to differentiate between HAT and PCET in terms of the degree of electronic nonadiabaticity, where HAT corresponds to electronically adiabatic proton transfer and PCET corresponds to electronically nonadiabatic proton transfer. In both cases, the overall reaction is typically vibronically nonadiabatic. A series of rate constant expressions have been derived in various limits by describing the PCET reactions in terms of nonadiabatic transitions between electron-proton vibronic states. These expressions account for the solvent response to both electron and proton transfer and the effects of the proton donor-acceptor vibrational motion. The solvent and protein environment can be represented by a dielectric continuum or described with explicit molecular dynamics. These theoretical treatments have been applied to numerous PCET reactions in solution and proteins. Expressions for heterogeneous rate constants and current densities for electrochemical PCET have also been derived and applied to model systems.

Hammes-Schiffer, Sharon; Soudackov, Alexander V.

2009-01-01

170

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

171

The application of aberration-corrected electron microscopy to the characterization of gold-based catalysts  

NASA Astrophysics Data System (ADS)

Electron microscopy has long been used to study the morphology of heterogeneous catalysts. Recent advances in electron optics now allow for the correction of the inherent spherical aberration (Cs) produced by the objective lens in the scanning transmission electron microscope (STEM, resulting in a significantly improved spatial resolution as well as the ability to use a much larger probe-current than was previously possible. In this thesis, the combination of high-angle annular dark-field (HAADF) imaging and microanalysis by x-ray energy dispersive spectroscopy (XEDS) in an aberration-corrected STEM has been applied for the first time to the characterization of gold-based heterogeneous catalysts. Multi-variate statistical analysis (MSA) has been employed in order to further improve the STEM-XEDS spectrum image data acquired with this technique. In addition, supplemental analysis using electron-energy loss spectroscopy (EELS) and energy-filtered transmission electron microscopy (EFTEM) in an aberration-corrected instrument has also been attempted. These techniques have proven extremely valuable in providing complimentary information to more traditional catalyst characterization techniques such as x-ray photoelectron spectroscopy and x-ray diffraction in four specific problems relating to catalysis. Firstly, the atomic-scale resolution of Cs-corrected HAADF imaging has been utilized to study Au/FeOx catalysts in order to determine the size and structure of the Au clusters present on the support surface. It was discovered that, while both inactive and active catalysts for low-temperature CO oxidation contained large Au particles (> 5 nm) and individual Au atoms, the active catalyst also contained sub-nm clusters comprised of only a few Au atoms. Secondly, novel CeO2 support materials for Au and Au-Pd catalysts were synthesized by precipitation with supercritical CO2. These supports were found to produce significantly more active catalysts than those based on CeO2 prepared using more traditional methods. The combination of STEM-HAADF imaging and XEDS mapping has been used to characterize these catalysts and a strong correlation between the catalytic activity and the enhanced degree of metal dispersion over the support is demonstrated. Thirdly, a systematic series of Au-Pd/Al2O3 catalysts has been studied in order to characterize the effects of various heat treatments on the development of core-shell morphologies within the bi-metallic particles and its subsequent effect on their catalytic performance for H2O 2 synthesis. STEM-XEDS spectrum imaging was employed in order to determine the degree of alloying and segregation behavior within the individual Au-Pd particles as a function of calcination/reduction temperature. It was found that the as prepared catalyst contained homogeneous Au-Pd alloy particles and that a Pd-rich shell/Au-rich core morphology gradually developed upon calcination. Subsequent reduction of the catalyst caused a large fraction of the particles to invert and form Pd-rich core/Au-rich shell structures. These changes are related to both the activity and stability of the catalyst. Finally, the washing of activated carbon support materials in acid was found to be extremely beneficial for producing Au-Pd catalysts for the direct synthesis of H2O2. STEM-HAADF imaging revealed that the acid-washing treatment increased the dispersion of the metal on the carbon supports. Aberration-corrected STEM-XEDS spectrum imaging demonstrated a strong size dependence of the Au-Pd particle composition. Crucially, the acid-washing pre-treatment enhanced the alloying of Au and Pd by suppressing the formation of large (> 25 nm) Au-rich particles. In summary, the application of aberration-corrected HAADF imaging and STEM-XEDS spectrum imaging to the characterization of Au-based catalysts has enhanced the understanding of the structural and chemical features that determine their catalytic behavior. Specifically, they have allowed us to achieve the following: (a) image individual metal atoms and clusters of just a few atoms

Herzing, Andrew A.

172

Electron Transfer on Impurity doped Graphene Nanoribbon  

NASA Astrophysics Data System (ADS)

Electronic transport properties in armchair shaped edges graphene nanoribbons (AGNRs) doped various impurities have been simulated by the non-equilibrium Green's function approach combined with the first principle calculation based on the density functional theory. We have observed that impurity levels appear in electronic structures, and that the quantization of transmission function is moderated for doped AGNRs. The I-V characteristic can be computed from the transmission function. Our simulation results show that AGNRs doped impurities have higher conductance than the non-doped one.

Tsuyuki, Hiroyoshi; Sakamoto, Shoichi; Tomiya, Mitsuyoshi

2012-12-01

173

Desulfurization of coal: Enhanced selectivity using phase transfer catalysts. Final technical report, September 1, 1995--August 31, 1996  

SciTech Connect

Due to environmental problems related to the combustion of high sulfur Illinois coal, there continues to be interest in the development of viable pre-combustion desulfurization processes. Recent studies by the authors have obtained very good sulfur removals but the reagents that are used are too expensive. Use of cheaper reagents leads to a loss of desired coal properties. This study investigated the application of phase transfer catalysts to the selective oxidation of sulfur in coal using air and oxygen as oxidants. The phase transfer catalyst was expected to function as a selectivity moderator by permitting the use of milder reaction conditions than otherwise necessary. This would enhance the sulfur selectivity and help retain the heating value of the coal. The use of certain coal combustion wastes for desulfurization, and the application of cerium (IV) catalyzed air oxidations for selective sulfur oxidation were also studied. If successful this project would have lead to the rapid development of a commercially viable desulfurization process. This would have significantly improved the marketability of Illinois coal. However, the phase transfer catalysts, the cerium and the scrubber sledge did not catalize the sulfur removal significantly.

Palmer, S.R.; Hippo, E.J.

1997-05-01

174

Vectorial electron transfer in spatially ordered arrays  

SciTech Connect

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

Fox, M.A.

1992-01-01

175

Fluoroponytailed crown ethers and quaternary ammonium salts as solid-liquid phase transfer catalysts in organic synthesis.  

PubMed

Fluorous derivatives of dibenzo-18-crown-6 ether were prepared, and then successfully applied in representative solid-liquid phase transfer catalysis reactions, which were performed in standard organic solvents, such as chlorobenzene and toluene, as well as in fluorous solvents, such as perfluoro-1,3-dimethylcyclohexane. It was clearly shown that properly designed fluoroponytailed crown ethers could promote the disintegration of the crystal lattice of alkali salts, and transfer anions from the solid surface into an apolar, non-coordinating perfluorocarbon phase, for phase transfer catalysis reactions in organic synthesis. Furthermore, 3,5-bis(perfluorooctyl)benzyl bromide and triethylamine were reacted under mild conditions to provide an analogue of the versatile phase transfer catalyst, benzyltriethylammonium chloride, containing two fluoroponytails. This fluoroponytailed quaternary ammonium salt was also successfully employed as a catalyst in a variety of organic reactions conducted under solid-liquid phase transfer catalysis conditions, without a perfluorocarbon phase. Thus, being both hydrophobic and lipophobic, fluorous crown ethers and ammonium salts, could be rapidly recovered in quantitative yields, and reused without loss of activity, over several reaction cycles. PMID:21928010

Pozzi, Gianluca; Fish, Richard H

2012-01-01

176

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

177

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

178

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.

Shrestha, Pravin Malla; Rotaru, Amelia-Elena

2014-01-01

179

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.

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

180

Real-time electron transfer in respiratory complex I  

PubMed Central

Electron transfer in complex I from Escherichia coli was investigated by an ultrafast freeze-quench approach. The reaction of complex I with NADH was stopped in the time domain from 90 ?s to 8 ms and analyzed by electron paramagnetic resonance (EPR) spectroscopy at low temperatures. The data show that after binding of the first molecule of NADH, two electrons move via the FMN cofactor to the iron–sulfur (Fe/S) centers N1a and N2 with an apparent time constant of ?90 ?s, implying that these two centers should have the highest redox potential in the enzyme. The rate of reduction of center N2 (the last center in the electron transfer sequence) is close to that predicted by electron transfer theory, which argues for the absence of coupled proton transfer or conformational changes during electron transfer from FMN to N2. After fast reduction of N1a and N2, we observe a slow, ?1-ms component of reduction of other Fe/S clusters. Because all elementary electron transfer rates between clusters are several orders of magnitude higher than this observed rate, we conclude that the millisecond component is limited by a single process corresponding to dissociation of the oxidized NAD+ molecule from its binding site, where it prevents entry of the next NADH molecule. Despite the presence of approximately one ubiquinone per enzyme molecule, no transient semiquinone formation was observed, which has mechanistic implications, suggesting a high thermodynamic barrier for ubiquinone reduction to the semiquinone radical. Possible consequences of these findings for the proton translocation mechanism are discussed.

Verkhovskaya, Marina L.; Belevich, Nikolai; Euro, Liliya; Wikstrom, Marten; Verkhovsky, Michael I.

2008-01-01

181

[Electron transfer, ionization, and excitation in atomic collisions]. Progress report  

SciTech Connect

Fundamental processes of electron transfer, ionization, and excitation in ion-atom and ion-ion collisions are studied. Attention is focussed on one- and two-electron systems and, more recently, quasi-one-electron systems whose electron-target-ion core can be accurately modeled by one-electron potentials. The basic computational approaches can then be taken with few, if any, approximations, and the underlying collisional mechanisms can be more clearly revealed. At intermediate collision energies (e.g., proton energies for p-He{sup +} collisions on the order of 100 kilo-electron volts), many electronic states are strongly coupled during the collision, a coupled-state approach, such as a coupled-Sturmian-pseudostate approach, is appropriate. At higher collision energies (million electron-volt energies) the coupling is weaker with, however, many more states being coupled together, so that high-order perturbation theory is essential.

Not Available

1992-12-31

182

Heat transfer and temperature distribution in a catalyst oven of 250mm diameter  

Microsoft Academic Search

The catalyst oven tested was a tube, which was surrounded by a steam-heated jacket, had air passing upward through it, and was filled with cylindrical pills of catalysts. Measurement of heat flow, temperature, and pressure drop was taken at various places in the tube and at various flow rates of air. Higher flow rates of air produced a quicker reaching

1942-01-01

183

Photocurrent generation by direct electron transfer using photosynthetic reaction centres  

NASA Astrophysics Data System (ADS)

Photosynthetic reaction centres (RCs) convert light into separated charges with nearly perfect quantum efficiency, and have been used to generate photocurrent. Previous work has shown that electron tunnelling rates between redox centres in proteins depend exponentially on the tunnelling distance. In this work the RC from Rhodobacter sphaeroides was genetically modified with the aim of achieving the shortest tunnelling distances yet demonstrated between the RC's electron-accepting P site and underlying graphite and gold electrodes, and between the electron donor Q site and graphite electrodes. Opposite charges are carried to counter electrodes using mobile mediators, as in dye-sensitised solar cells. Native RCs are bound to graphite surfaces through N-(1-pyrene)iodoacetamide. Although the linker's length is only 4 Å, the electron transfer pathway between the Q electron donor site on the RC and the electrode surface is still too large for current to be significant. A mutant version with the electron acceptor P side close to the graphite surface produced currents of 15 nA cm-2 upon illumination. Direct binding of RCs to a gold surface is shown, resulting in currents of 5 nA cm-2. In both cases the current was unaffected by mediator concentration but increased with illumination, suggesting that direct electron transfer was achieved. The engineering of an RC to achieve direct electron transfer will help with long term efforts to demonstrate RC-based photovoltaic devices.

Mahmoudzadeh, A.; Saer, R.; Jun, D.; Mirvakili, S. M.; Takshi, A.; Iranpour, B.; Ouellet, E.; Lagally, E. T.; Madden, J. D. W.; Beatty, J. T.

2011-09-01

184

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

185

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.

Guallar, Victor; Wallrapp, Frank

2008-01-01

186

Electron Transfer Between Colloidal ZnO Nanocrystals  

PubMed Central

Colloidal ZnO nanocrystals, capped with dodecylamine and dissolved in toluene, can be charged photochemically to give stable solutions in which electrons are present in the conduction bands of the nanocrystals. These conduction band electrons are readily monitored by EPR spectroscopy, with g* values that correlate with the nanocrystal sizes. Mixing a solution of charged small nanocrystals with a solution of uncharged large nanocrystals, e-CB:ZnO–S + ZnO–L, causes changes in the EPR spectrum indicative of quantitative electron transfer from small to large nanocrystals. EPR spectra of the reverse reaction, e-CB:ZnO–L + ZnO–S, show that electrons do not transfer from large to small nanocrystals. Stopped-flow kinetic studies monitoring the change in the UV band edge absorption show that reactions of 50 ?M nanocrystals are complete within the 5 ms mixing time of the instrument. Similar results are obtained for the reaction of charged nanocrystals with methyl viologen (MV2+). These and related results indicate that the electron transfer reactions of these colloidal nanocrystals are quantitative and very rapid, despite the presence of ~1.5 nm long dodecylamine capping ligands. These soluble ZnO nanocrystals are thus well-defined redox reagents suitable for electron transfer studies involving semiconductor nanostructures.

Hayoun, Rebecca; Whitaker, Kelly M.; Gamelin, Daniel R.; Mayer, James M.

2011-01-01

187

Photochemical electron transfer in liquid/liquid solvent systems  

SciTech Connect

A photoinduced charge transfer at the water/1,2-dichloroethane and water/benzonitrile interfaces was studied, using the photoredox couple ruthenium-trisbipyridine complex, Ru(byp){sub 3}{sup 2+}, dissolved in the organic phase and methyl viologen, MV{sup 2+}, in the aqueous phase. It was concluded that the photocurrent observed can be ascribed to an electron transfer between the excited ruthenium complex and methyl viologen.

Marecek, V.; De Armond, A.H.; De Armond, M.K. (New Mexico State Univ., Las Cruces (USA))

1989-03-29

188

Prediction of electronic component-board transient conjugate heat transfer  

Microsoft Academic Search

Numerical analysis of electronic component transient heat transfer has generally been confined to nonconjugate methods. This study discusses the need for conjugate (conduction\\/convection) analysis, both for component temperature and thermo-mechanical behavior prediction in operational, assembly and reliability qualification environments. The capability of computational fluid dynamics (CFD) analysis to predict component transient conjugate heat transfer is investigated using an industry-standard CFD

Valérie Eveloy; Peter Rodgers

2005-01-01

189

Multi-Element Electron-Transfer Optical Detector System  

NASA Technical Reports Server (NTRS)

A multi-element optical detector system includes an electrically resistive screen that is substantially transparent to radiation energy having a wavelength of interest. A plurality of electron transfer elements (e.g., a low work function photoactive material or a carbon nanotube (CNT)-based element) are provided with each having a first end and a second end. The first end of each element is spaced apart from the screen by an evacuated gap. When the radiation energy passes through the screen with a bias voltage applied thereto, transfer of electrons through each element is induced from the first end to the second end such that a quantity indicative of the electrons transferred through each element can be detected.

Jordan, Jeffrey D. (Inventor)

2004-01-01

190

Photo-induced electron-transfer reactions in heterogeneous media  

NASA Astrophysics Data System (ADS)

The conversion of solar energy into chemical energy was pursued by two approaches. One is the photo-induced electron transfer reactions in heterogeneous media, and the other is the photo-decomposition of water with liquid-junction solar cells. Photo-induced electron-transfer reactions in heterogeneous media with colloidal silica or poly-acrylate were studied by flash photolysis. In an effort to illustrate that small band-gap semiconductors can be protected from photo-corrosion through surface modification, the surface of polycrystalline ZnO was chemically coated with zinc phthalocyanine and the electron-transfer process across the coated ZnO-electrolyte interface was studied by photo-electrochemical techniques.

Yang, J. M.

1981-11-01

191

Auger-assisted electron transfer from photoexcited semiconductor quantum dots.  

PubMed

Although quantum confined nanomaterials, such as quantum dots (QDs) have emerged as a new class of light harvesting and charge separation materials for solar energy conversion, theoretical models for describing photoinduced charge transfer from these materials remain unclear. In this paper, we show that the rate of photoinduced electron transfer from QDs (CdS, CdSe, and CdTe) to molecular acceptors (anthraquinone, methylviologen, and methylene blue) increases at decreasing QD size (and increasing driving force), showing a lack of Marcus inverted regime behavior over an apparent driving force range of ?0-1.3 V. We account for this unusual driving force dependence by proposing an Auger-assisted electron transfer model in which the transfer of the electron can be coupled to the excitation of the hole, circumventing the unfavorable Franck-Condon overlap in the Marcus inverted regime. This model is supported by computational studies of electron transfer and trapping processes in model QD-acceptor complexes. PMID:24359156

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

2014-03-12

192

Gemini PI Electronic Transfer and Distribution (PIETD)  

NASA Astrophysics Data System (ADS)

The Gemini Science Archive (GSA) has been developed and is operated by the Canadian Astronomy Data Centre (CADC) to provide the worldwide scientific community with tools for effective on-line access to data collected by the Gemini telescopes. In the past year, the CADC has developed tools to allow Gemini staff to automatically create and distribute Science Program data packages to Principal Investigators (PI). The tools operate in the GSA databases to create linkages between science files, calibration files, weather files, log files and science programs and assemble them upon user request. All data distribution is performed electronically via data retrieval from the CADC data stores. This automation results in significant manpower savings for Gemini relieving staff from the manual assembly of files and production of hard media. The complete version of PIETD, including the distribution of processed data products, was released in the fall of 2005.

Gaudet, S.; Bohlender, D.; Damian, A.; Goliath, S.; Hill, N.; Melnychuk, G.; Aspin, C.

2006-07-01

193

Reactant-product quantum coherence in electron transfer reactions.  

PubMed

We investigate the physical meaning of quantum superposition states between reactants and products in electron transfer reactions. We show that such superpositions are strongly suppressed and, to leading orders of perturbation theory, do not pertain in electron transfer reactions. This is because of the intermediate manifold of states separating the reactants from the products. We provide an intuitive description of these considerations with Feynman diagrams. We also discuss the relation of such quantum coherences to understanding the fundamental quantum dynamics of spin-selective radical-ion-pair reactions. PMID:23005829

Kominis, I K

2012-08-01

194

A role for excreted quinones in extracellular electron transfer.  

PubMed

Respiratory processes in bacteria are remarkable because of their ability to use a variety of compounds, including insoluble minerals, as terminal electron acceptors. Although much is known about microbial electron transport to soluble electron acceptors, little is understood about electron transport to insoluble compounds such as ferric oxides. In anaerobic environments, humic substances can serve as electron acceptors and also as electron shuttles to ferric oxides. To explore this process, we identified mutants in Shewanella putrefaciens that are unable to respire on humic substances. Here we show that these mutants contain disruptions in a gene that is involved in the biosynthesis of menaquinone. During growth, the wild type releases a menaquinone-related redox-active small molecule into the medium that complements the mutants. This finding raises the possibility that electron transfer to a variety of oxidants, including poorly soluble minerals, may be mediated by microbially excreted quinones that have yet to be identified. PMID:10811225

Newman, D K; Kolter, R

2000-05-01

195

Intramolecular electron transfer in pentaammineruthenium(III)-modified cobaltocytochrome c  

SciTech Connect

The iron in the heme group of horse-heart cytochrome c was replaced by cobalt according to established methods. The resulting cobalticytochrome c was subsequently modified at histidine-33 with a pentaammineruthenium group. Proof of correct derivatization was obtained by atomic absorption analysis of cobalt and ruthenium, differential pulse voltammetry, and enzymatic proteolysis analyzed by diode-array HPLC. Cobalt(II)-to-ruthenium(III) intramolecular electron transfer rates were measured as a function of temperature by electron pulse radiolysis. The azide radical (N{sub 3}{sup {sm_bullet}}) was used to oxidize the fully reduced form in order to generate the desired electron transfer precursor. The intramolecular electron transfer rate is 1.28 {plus_minus} 0.04 s{sup {minus}1} at 25 {degrees}C ({Delta}H{sup {double_dagger}} = 5.7 {plus_minus} 0.2 kcal/mol, {Delta}S{sup {double_dagger}} = -38.7 {plus_minus} 0.5 cal/(deg mol)) for a driving force of 0.28 {plus_minus} 0.02 eV. The results are compared with those for analogous pentaammineruthenium-modified, native iron, and zinc-substituted cytochromes c. The 0.4 eV increase in driving force for intramolecular electron transfer when iron is replaced by cobalt is largely compensated by an increase in reorganization energy.

Sun, Ji; Su, Chang; Wishart, J.F. [Brookhaven National Lab., Upton, NY (United States)] [Brookhaven National Lab., Upton, NY (United States)

1996-09-25

196

Study of ring influence and electronic response to proton transfer reactions. Reaction electronic flux analysis.  

PubMed

In this article, a theoretical study of 1-5 proton transfers is presented. Two model systems which represent 1-5 proton transfer, 3-hidroxy-2-propenimine and salicyldenaniline have been studied as shown in Fig. 1. For this purpose, a DFT/B3LYP/6-311+G**, reaction force and reaction electronic flux analysis is made. The obtained results indicate that both proton transfers exhibit energetic and electronic differences emphasizing the role of the neighbor ring and the impact of conjugation on electronic properties. PMID:20658304

Herrera, Barbara

2011-05-01

197

Electron-donating, acid-base, and magnetic properties of samaria catalyst  

Microsoft Academic Search

The electron-donor properties of Sm2O3 activated at 300, 500, and 800°C are reported from studies on the adsorption of electron acceptors of various electron affinities (electron affinity values in eV are given in parentheses): 7,7,8,8-tetracyanoquino-dimethane (2.84), 2,3,5,6-tetrachloro-1,4-benzoquinone (2.40), p-dinitrobenzene (1.77), and m-dinitrobenzene (1.26) in acetonitrile and 1,4-dioxane. The extent of electron transfer during the adsorption was determined from magnetic measurements.

S. Sugunan; J. J. Malayan

1995-01-01

198

Energy and electron transfer processes in polymethine dyes  

NASA Astrophysics Data System (ADS)

Polymethine dyes and its derivatives are attractive for their interesting optical and photo-electric properties. They are used as very efficient spectral sensitizers and laser dyes. Due to the high rate constant of deactivation channels of such dyes the primary processes of bimolecular processes as energy or electron transfer proceed within not more than some picoseconds or even shorter. In the case of a polymethine which does not isomerize we were able to show by means of time-resolved absorption spectroscopy that the singlet state photoelectron transfer to methyl- and benzylviologen had an efficiency of 0.15 with rate constants of 6.7.109 and 4.6.109 l/mole.s, respectively, yielding the polymethine dication radical. The photoreduction with tetraphenylborate and potassium rhodanide is also very efficient with an efficiency of about 0.10 with rate constants of 2.4.1010 and 1.6.1010 l/mole.s, respectively, yielding the polymethine neutral radical. The spectral differences of the observed radical spectra are small. The investigation of the temperature dependence of the photo induced electron transfer of the investigated polymethine to methylviologen results in an activation energy ?G*=24 kJ/mole and a value of the frequency factor of A=4.7.1014 l/mole.s. Strong deviation from a linear Arrhenius plot was observed at low temperatures which can be explained by solvent-solute interaction decreasing the electron transfer rate constant at lower temperatures. The calculated electron transfer rate constants agree with the assumption of the investigated process as a diffusion-controlled one. Energy transfer occurs as a efficient competitive deactivation channel from photo excited polymethine dyes to other chromophore systems with a strong overlapping of the fluorescence and the absorption bands of the donor and the acceptor, respectively. We have investigated the time and spectral evolution of the energy transfer process from a polymethine dye to different energy acceptor dyes in solution. The general question within this respect was the involvement of an intermediate electron transfer as competitive process in the energy transfer process. Whereas the Förster energy transfer radius calculated from the time-resolved data exceeds the value received from the overlap integral by 15%, indicating deviation from a normal Förster decay type the semilogarithmic plot of the ground state recovery kinetics vs. square root of time results in an ideal straight line dependence. No intermediate spectra as well as intermediate time behaviour was found in these complexes.

Feller, Karl-Heinz; Gadonas, Roaldas

1996-04-01

199

Energy and electron transfer processes in polymethine dyes  

NASA Astrophysics Data System (ADS)

Polymethine dyes and its derivatives are attractive for their interesting optical and photo- electric properties. In the case of a polymethine which does not isomerize we were able to show by means of time-resolved absorption spectroscopy that the singlet state photoelectron transfer to methyl- and benzylviologen had an efficiency of 0.15 with rate constants of 6.7 (DOT) 109 and 4.6 (DOT) 109 l/mole(DOT)s, respectively, yielding the polymethine dication radical. The photo-reduction with tetraphenylborate and potassium rhodanide is also very efficient with an efficiency of about 0.10 with rate constants of 2.4 (DOT) 1010 and 1.6 (DOT) 1010 l/mole(DOT)s, respectively, yielding the polymethine neutral radical. The spectral differences of the observed radical spectra are small. The investigation of the temperature dependence of the photo induced electron transfer of the investigated polymethine to methylviologen results in an activation energy (Delta) G* equals 24 kJ/mole and a value of the frequency factor of A equals 4.7 (DOT) 1014 l/mole(DOT)s. Strong deviation from a linear Arrhenius plot was observed at low temperatures which can be explained by solvent-solute interaction decreasing the electron transfer rate constant at lower temperatures. The calculated electron transfer rate constants agree with the assumption of the investigated process as a diffusion-controlled one. We have investigated the time and spectral evolution of the energy transfer process from a polymethine dye to different energy acceptor dyes in solution as well as in Langmuir-Blodgett layers. The general question within this respect was the involvement of an intermediate electron transfer as competitive process in the energy transfer process.

Feller, Karl-Heinz

1995-01-01

200

Electron Transfer from Highly Excited Atoms to Molecules  

Microsoft Academic Search

It is theoretically shown that the rate constant for reaction of electron transfer from highly excited atoms A** to polyatomic molecules B, A**+B-->A++B-, (1) at thermal velocities is equal to that of non-dissociative thermal electron capture by the molecules B, e+B-->B-, (2) based on the assumption that the negative ion formed in reaction (1) is the same species as that

Michio Matsuzawa

1972-01-01

201

Magnetic resonance studies of photo-induced electron transfer reactions  

SciTech Connect

Fourier Transform Electron Paramagnetic Resonance (FT EPR) is useful in study of photochemical reactions: a microwave pulse rotates the electron spin magnetization vector from z (magnetic field) into xy plane ([pi]/2 pulse); the time evolution of magnetization in xy plane, the free induction decay (FID), is sampled. Fourier transform of FID gives the frequency domain EPR spectrum of the free radicals, and the method is ideal for time-resolved studies of free radicals produced by pulsed-laser excitation. Investigations of electron transfer reactions focused on porphyrin (donor) - quinone (acceptor) systems. First, two hydrogen abstraction reactions were studied with FT EPR: photoreduction of acetone with 2-propanol, yielding the acetone ketyl radical, and the reaction of 2-propanol with t-butoxy radicals. Then, the FT EPR study of benzoquinone or duroquinone anion radicals generated by pulsed-laser induced electron transfer from zinc tetraphenylporphyrin (ZnTPP) or tetrasulfonated Zn(TPP), was carried out in homogeneous solution, micellar solutions, and silica gel. Finally, FT EPR was used to study electron transfer quenching of triplet C[sub 60] by electron donors.

van Willigen, H.

1992-11-01

202

Fluctuations in Biological and Bioinspired Electron-Transfer Reactions  

PubMed Central

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

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

2010-01-01

203

Intramolecular electron scattering and electron transfer following autoionization in dissociating molecules.  

PubMed

Resonant Auger decay of core-excited molecules during ultrafast dissociation leads to a Doppler shift of the emitted electrons depending on the direction of the electron emission relative to the dissociation axis. We have investigated this process by angle-resolved electron-fragment ion coincidence spectroscopy. Electron energy spectra for selected emission angles for the electron relative to the molecular axis reveal the occurrence of intermolecular electron scattering and electron transfer following the primary emission. These processes amount to approximately 25% of the resonant atomic Auger intensity emitted in the studied transition. PMID:15323819

Kugeler, O; Prümper, G; Hentges, R; Viefhaus, J; Rolles, D; Becker, U; Marburger, S; Hergenhahn, U

2004-07-16

204

Intramolecular Electron Scattering and Electron Transfer Following Autoionization in Dissociating Molecules  

NASA Astrophysics Data System (ADS)

Resonant Auger decay of core-excited molecules during ultrafast dissociation leads to a Doppler shift of the emitted electrons depending on the direction of the electron emission relative to the dissociation axis. We have investigated this process by angle-resolved electron-fragment ion coincidence spectroscopy. Electron energy spectra for selected emission angles for the electron relative to the molecular axis reveal the occurrence of intermolecular electron scattering and electron transfer following the primary emission. These processes amount to approximately 25% of the resonant atomic Auger intensity emitted in the studied transition.

Kugeler, O.; Prümper, G.; Hentges, R.; Viefhaus, J.; Rolles, D.; Becker, U.; Marburger, S.; Hergenhahn, U.

2004-07-01

205

Energy and photoinduced electron transfer in porphyrin-fullerene dyads  

SciTech Connect

Time-resolved fluorescence and absorption techniques have been used to investigate energy and photoinduced electron transfer in a covalently linked free-base porphyrin-fullerene dyad and its zinc analogue. In toluene, the porphyrin first excited singlet states decay in about 20 ps by singlet-singlet energy transfer to the fullerene. The fullerene first excited singlet state is not quenched and undergoes intersystem crossing to the triplet, which exists in equilibrium with the porphyrin triplet state. In benzonitrile, photoinduced electron transfer from the porphyrin first excited singlet state to the fullerene competes with energy transfer. The fullerene excited singlet state is also quenched by electron transfer from the porphyrin. Overall, the charge-separated state is produced with a quantum yield approaching unity. This state lives for 290 ps in the free-base dyad and 50 ps in the zinc analog. These long lifetimes suggest that such dyads may be useful as components of more complex light-harvesting systems. 32 refs., 12 figs., 1 tab.

Kuciauskas, D.; Lin, S.; Seely, G.R.; Moore, A.L.; Moore, T.A.; Gust, D. [Arizona State Univ., Tempe, AZ (United States)] [Arizona State Univ., Tempe, AZ (United States); Drovetskaya, T.; Reed, C.A. [Univ. of Southern California, Los Angeles, CA (United States)] [Univ. of Southern California, Los Angeles, CA (United States); Boyd, P.D.W. [Univ. of Auckland (New Zealand)] [Univ. of Auckland (New Zealand)

1996-09-26

206

Polydentate phosphine oxides as external electron donors for titanium-magnesium catalysts for propylene polymerization  

Microsoft Academic Search

The possibility of using R\\u000a n\\u000a P(O)(CH2OR?)3—n\\u000a (R = alkyl, R? = methyl or acyl, n = 0–2) polydentate phosphine oxides as external electron donors for the titanium-magnesium catalysts for isotactic polypropylene\\u000a synthesis is demonstrated for the first time. The kinetics of propylene polymerization in liquid monomer at 70°C and the isotacticity\\u000a and molecular-weight characteristics of the resulting polypropylene are

O. I. Kudinova; T. E. Kron; T. A. Ladygina; A. N. Kozhevnikov; E. S. Petrov; L. A. Novokshonova

2010-01-01

207

Characterization of durable nanostructured thin film catalysts tested under transient conditions using analytical aberration-corrected electron microscopy  

Microsoft Academic Search

The stability of Ru0.1Ir0.9 oxidation evolution reaction (OER) catalysts deposited on Pt-coated nanostructured thin films (NSTFs) has been investigated by aberration-corrected electron microscopy. Accelerated stress tests showed that the OER catalysts significantly improved the durability of the Pt under cell reversal conditions. High-resolution images of the end-of-life NSTFs showed significant Ir loss from the whisker surfaces, while no Pt loss

David A Cullen; Karren Leslie More; Kimberly Shawn Reeves; George Vernstrom; Liliana Atanasoska; Gregory Haugen; Radoslav Atanasoski

2011-01-01

208

Photoinduced Electron Transfer in Ordered Polymers: Progress Report, May 1, 1988-December 31, 1988.  

National Technical Information Service (NTIS)

Long-range photoinduced electron transfer between potential electron donors and acceptors is of considerable current interest in terms of strategies for artificial photosynthesis and studies regarding the redox properties of proteins. Electron transfer ov...

G. Jones

1988-01-01

209

Efficient Multistep Photoinitiated Electron Transfer in a Molecular Pentad  

Microsoft Academic Search

A synthetic five-part molecular device has been prepared that uses a multistep electron transfer strategy similar to that of photosynthetic organisms to capture light energy and convert it to chemical potential in the form of long-lived charge separation. It consists of two covalently linked porphyrin moieties, one containing a zinc ion (PZn) and the other present as the free base

Devens Gust; Thomas A. Moore; Ana L. Moore; Seung-Joo Lee; Edith Bittersmann; David K. Luttrull; Aden A. Rehms; Janice M. Degraziano; Xiaochun C. Ma; Feng Gao; Robert E. Belford; Todd T. Trier

1990-01-01

210

Electronic Structure, Charge Transfer Excitations and High Temperature Superconductivity.  

National Technical Information Service (NTIS)

High precision local density electronic band structure results (for YBa sub 2 Cu sub 3 O sub 7 , YBa sub 2 Cu sub 3 O sub 6 and GdBa sub 2 Cu sub 3 O sub 7 ) lead to the possibly important role of charge transfer excitations as the mechanism of high T/sub...

A. J. Freeman J. Yu S. Massidda D. D. Koelling

1987-01-01

211

Energy and electron transfer processes in polymethine dyes  

Microsoft Academic Search

Polymethine dyes and its derivatives are attractive for their interesting optical and photo-electric properties. They are used as very efficient spectral sensitizers and laser dyes. Due to the high rate constant of deactivation channels of such dyes the primary processes of bimolecular processes as energy or electron transfer proceed within not more than some picoseconds or even shorter. In the

Karl-Heinz Feller; Roaldas Gadonas

1996-01-01

212

Light Induced Electron Transfer Reactions of Metal Complexes.  

National Technical Information Service (NTIS)

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 excite...

N. Sutin C. Creutz

1980-01-01

213

Solvation and dielectric dispersion in optical electron transfer  

Microsoft Academic Search

The effect of dielectric dispersion of the solvent on the energetics of optical electron transfer is determined quantitatively by variations of the free energy of solvation of the species being photoionized. The solvation free energy varies because the solvent polarizability in the inner-sphere region of the photon absorbing species changes with photon energy on account of dispersion. The solvation free

Paul Delahay; Andrew Dziedzic

1984-01-01

214

Conformational dependence of electron transfer across de novo designed metalloproteins.  

PubMed Central

Flash photolysis and pulse radiolysis measurements demonstrate a conformational dependence of electron transfer rates across a 16-mer helical bundle (three-helix metalloprotein) modified with a capping CoIII(bipyridine)3 electron acceptor at the N terminus and a 1-ethyl-1'-ethyl-4,4'- bipyridinium donor at the C terminus. For the CoIII(peptide)3-1-ethyl-1'-ethyl-4,4'-bipyridinium maquettes, the observed transfer is a first order, intramolecular process, independent of peptide concentration or laser pulse energy. In the presence of 6 M urea, the random coil bundle (approximately 0% helicity) has an observed electron transfer rate constant of kobs = 900 +/- 100 s-1. In the presence of 25% trifluoroethanol (TFE), the helicity of the peptide is 80% and the kobs increases to 2000 +/- 200 s-1. Moreover, the increase in the rate constant in TFE is consistent with the observed decrease in donor-acceptor distance in this solvent. Such bifunctional systems provide a class of molecules for testing the effects of conformation on electron transfer in proteins and peptides. Images Fig. 1

Mutz, M W; McLendon, G L; Wishart, J F; Gaillard, E R; Corin, A F

1996-01-01

215

Electron transfer in voltage tunable two-color infrared photodetectors  

NASA Astrophysics Data System (ADS)

Two-color quantum-well infrared photodetectors (QWIPs) that are based on electron transfer between coupled QWs suffer from the presence of the shorter wavelength peak at all bias voltages. We investigate this problem in such detectors with 50 or 200 Å AlGaAs barriers between the QW pair. We deduce the absorption coefficient ? and photoconductive gain g of the detectors with 50 Å barriers using corrugated QWIPs with different corrugation periods. We find that ? has a number of small peaks in its spectrum but its value remains almost constant between 0.1 and 0.2 ?m-1 in the 6-12 ?m range for most experimental conditions. The wavelength dependence of g, which always has a pronounced peak at the shorter detection wavelength, determines the responsivity line shape. These results are attributed to insufficient electron transfer between the coupled QWs and to low tunneling probability of the longer wavelength photoelectrons. A comparison of measured responsivity and calculated absorption spectrum of the detectors with 200 Å barriers indicates that there is significant electron transfer between the coupled wells. Despite efficient electron transfer, these detectors have a shorter wavelength detection peak at all bias voltages because of significant short wavelength absorption in both the QWs.

Majumdar, Amlan; Choi, K. K.; Rokhinson, L. P.; Reno, J. L.; Tsui, D. C.

2002-04-01

216

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

SciTech Connect

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

Not Available

1990-02-14

217

Electron solvation in water clusters following charge transfer from iodide.  

PubMed

The dynamics following charge transfer to solvent from iodide to a water cluster are studied using time-resolved photoelectron imaging of I-(H2O)n and I-(D2O)n clusters with n< or =28. The results show spontaneous conversion, on a time scale of approximately 1 ps, from water cluster anions with surface-bound electrons to structures in which the excess electron is more strongly bound and possibly more internalized within the solvent network. The resulting dynamics provide valuable insight into the electron solvation dynamics in water clusters and the relative stabilities between recently observed isomers of water cluster anions. PMID:16392904

Verlet, Jan R R; Kammrath, Aster; Griffin, Graham B; Neumark, Daniel M

2005-12-15

218

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

219

Facile stereoslective synthesis of spiro[indole-oxiranes] by combination of phase transfer catalyst and ultrasound irradiation and their bioassay.  

PubMed

An alternative and environmentally benign pathway for diastereoslective synthesis of fluorinated spiro[indole-3,2'-oxirane]-3'-benzoyl-2(1H)-ones (2a-g) is reported. The spiro[indole-3,2'-oxiranes] derivatives were obtained in 90-97% yield exclusively via the epoxidation of 3-aroylmethylene indole-2-ones with 30% aqueous hydrogen peroxide using cetyltrimethyl ammonium bromide as a phase transfer catalyst under ultrasound irradiation. The lead compounds have been tested for their antimicrobial activity and antioxidant properties. PMID:21292529

Dandia, Anshu; Singh, Ruby; Bhaskaran, Sumit

2011-09-01

220

Microstructural and electron spectroscopic characterization of carbon nanostructures and nanotubes produced using multimetal catalysts  

NASA Astrophysics Data System (ADS)

The microstructure of graphite arc discharge soot catalysed with Fe, Ni, Co and a mixture of Fe-Ni is investigated. It is shown that as well as single shell nanotubes and nanotube bundles, particles containing random networks of tubules and large diameter short tubules are found. Furthermore, the presence of fullerene nanocrystals directly formed during arcing is reported for the first time. The addition of a supplementary inert metallic element (Ag) to the catalysts, introduced with the aim of filling the tubules, reduces the nanotube yield significantly. Particles of Ag of spherical shape are found to be aligned and encapsulated in a carbon envelope. Energy loss spectroscopy carried out in an electron microscope reveals significant variations in the carbon-carbon bonding of the various soot morphologies which can be related to the formation site during arc discharge. Spectra from single shell nanotube bundles show an increase in the density of states on the antibonding ? bands, in agreement with electronic structure calculation predicting metallicity. Spectra from spherical soot particles indicate a similarity in the carbon-carbon bond with the C60/C70 fullerenes. Important variations of the carbon bonding in areas of the nanotube bundles where pentagons are introduced during the growth are also detected. Considering the observed morphologies and yield variations due to various catalysts and the presence of Ag, mechanisms contributing to the growth of the observed microstructures are proposed.

Botton, G. A.; Burnell, G.; Humphreys, C. J.; Yadav, T.; Withers, J. C.

1997-07-01

221

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

PubMed Central

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

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

2012-01-01

222

DNA-mediated Electron Transfer in Naphthalene-modified Oligonucleotides  

PubMed Central

Novel naphthalene-modified oligonucleotides have been synthesized and characterized with respect to electron transfer chemistry. Using the Sonogashira-coupling reaction, naphthalene can be covalently anchored onto a modified uridine through an ethynyl linkage. This tethering allows for effective electronic coupling with the DNA bases, resulting in a significant red shift of the absorption bands of the naphthalenic chromophore. Modification with this chromophore does not appear to affect the overall stability and structure of the DNA. Upon selective irradiation of the naphthalene moiety at 340 nm, photoreduction of a distal electron trap, 5-bromouridine, embedded in the DNA base stack occurs. This DNA-mediated reduction from a distance was found to be significantly more efficient with substitution of 5-bromouridine towards the 5?-end than towards the 3?-end. These results support a general preference for electron transfer through DNA towards the 5?-end, irrespective of the donor. In addition, differences in efficiency of photoreduction through intrastrand and interstrand pathways are observed. For DNA-mediated reduction, as with DNA-mediated oxidation, significant differences in the charge transfer reaction are apparent that depend upon subtle differences in coupling into the DNA base stack.

Tanaka, Makiko; Elias, Benjamin; Barton, Jacqueline K.

2010-01-01

223

A Dynamical Theory of Electron Transfer: Crossover from Weak to Strong Electronic Coupling  

Microsoft Academic Search

We present a real-time path integral theory for the rate of electron transfer reactions. Using graph theoretic techniques, the dynamics is expressed in a formally exact way as a set of integral equations. With a simple approximation for the self-energy, the rate can then be computed analytically to all orders in the electronic coupling matrix element. We present results for

Juergen T. Stockburger; C. H. Mak

1996-01-01

224

A dynamical theory of electron transfer: Crossover from weak to strong electronic coupling  

Microsoft Academic Search

We present a real-time path integral theory for the rate of electron transfer reactions. Using graph theoretic techniques, the dynamics is expressed in a formally exact way as a set of integral equations. With a simple approximation for the self-energy, the rate can then be computed analytically to all orders in the electronic coupling matrix element. We present results for

C. H. Mak

1996-01-01

225

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

226

Alternating electron and proton transfer steps in photosynthetic water oxidation  

PubMed Central

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 S2 ? S3 transition of the classical S-state cycle, an intermediate is formed by deprotonation clearly before electron transfer to the oxidant (). 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 (Ea = 0.46 ± 0.05 eV) and strong H/D kinetic isotope effect (approximately 6). The characteristics of a proton transfer step during the S0 ? S1 transition are similar (?, approximately 100 µs; Ea = 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.

Klauss, Andre; Haumann, Michael; Dau, Holger

2012-01-01

227

Au-to-CO electron transfer evidenced by surface-enhanced Raman scattering of 2,6-dimethylphenyl isocyanide  

NASA Astrophysics Data System (ADS)

Au nanoparticles are known to be efficient CO oxidation catalysts under ambient conditions. Investigation of the interaction of CO with Au is difficult particularly using Raman spectroscopy. Although indirect, we demonstrate the transfer of electrons from Au to CO by means of surface-enhanced Raman scattering of 2,6-dimethylphenyl isocyanide, exploiting the susceptibility of the NC stretching band of isocyanide to the variation of the surface potential of Au nanoparticles. The back-bonding from Au to the CO 2?b orbital must be dominant over the 5? donation of CO to Au.

Kim, Kwan; Lee, Seung Hun; Kim, Kyung Lock; Shin, Kuan Soo

2013-10-01

228

Hydrodechlorination of carbon tetrachloride to chloroform in the liquid phase with a Pd\\/carbon catalyst. Study of the mass transfer steps  

Microsoft Academic Search

The effect of the resistance to interface mass transfer on the overall rate of the hydrodechlorination of carbon tetrachloride has been analyzed with a 1% Pd\\/carbon catalyst, in a bubble column slurry reactor. The reaction was carried out at 393K and 3MPa. The effect of catalyst concentration on the external diffusion was analyzed for 7.96,19.09,39.77 and 63.64glTTCM?1 (0.5,1.2,2.5 and 4wt%,

Luisa Ma. Gómez-Sainero; Xosé L. Seoane; Emilio Tijero; Adolfo Arcoya

2002-01-01

229

Synthesis and characterization of a novel multi-site phase transfer catalyst and a kinetic study of the intramolecular cyclopentanation of indene  

Microsoft Academic Search

An easy access to a hitherto unknown novel multi-site catalyst viz., 1,3,5-tris(benzyltriethylammonium bromide)benzene (TBTABB) is delineated from low cost starting materials. The structures of the synthesized MPTC's and spiro indene derivative were evidenced by 1H NMR, 13CNMR, IR and elemental analysis. The potentiality of the new multi-site phase transfer catalyst was demonstrated by following the kinetics of the cyclopentanation of

P. A. Vivekanand; T. Balakrishnan

2009-01-01

230

Modeling of ultrafast electron-transfer processes: Validity of multilevel Redfield theory  

Microsoft Academic Search

The capability of multilevel Redfield theory to describe ultrafast photoinduced electron-transfer reactions is investigated. Adopting a standard model of photoinduced electron transfer in a condensed-phase environment, we consider electron-transfer reactions in the normal and inverted regimes, as well as for different values of the electron-transfer parameters, such as reorganization energy, electronic coupling, and temperature. Based on the comparison with numerically

Dassia Egorova; Michael Thoss; Wolfgang Domcke; Haobin Wang

2003-01-01

231

Ultrasound assisted free radical polymerization of glycidyl methacrylate by a new disite phase-transfer catalyst system: A kinetic study.  

PubMed

The kinetics of multi-site phase-transfer catalyzed free radical polymerisation of glycidyl methacrylate (GMA) using potassium peroxy disulphate (PDS) as water soluble initiator and newly synthesized 1,4-dihexadecylpyrazine-1,4-diium dibromide as multi-site phase-transfer catalyst (MPTC) has been investigated in ethyl acetate/water two phase system at constant temperature 65±1°C under nitrogen atmosphere and ultrasound irradiation conditions. The rate of polymerization increases with an increase in concentrations of GMA, PDS and MPTC. The order with respect to monomer, initiator and MPTC were found to be 1.0, 0.5 and 1.0, respectively. The comparative study reveals that the Rp of GMA determined in the presence of PTC combined with ultrasound has shown more enhancements in the activity than PTC alone. Based on the observed results a suitable mechanism has been proposed to account for the experimental observations and its significance was discussed. PMID:22592149

Sankar, Kavitha; Rajendran, Venugopal

2012-11-01

232

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

233

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.

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

2014-01-01

234

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

235

Polyethylene glycol immobilized on silica gel as a new solid–liquid phase-transfer catalyst for regioselective azidolysis of epoxides in water: An efficient route to 1,2-azido alcohols  

Microsoft Academic Search

Polyethylene glycol, a polymer widely used for biomedical applications and in supported synthesis, was easily immobilized on silica gel and used as a new solid–liquid phase-transfer catalyst. It is proved to be an efficient heterogeneous catalyst for regioselective ring opening of epoxide by azide ion in water to give azidohydrine in excellent yield under mild reaction conditions. The polymeric catalyst

Ali-Reza Kiasat; Maryam Zayadi

2008-01-01

236

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

237

SF4: electron affinity determination by charge-transfer reactions  

NASA Astrophysics Data System (ADS)

A selected-ion flow drift tube was used to conduct an extensive study of the negative ion/molecule reaction of SF4 and SF-4. Thirteen reactions proceed by electron transfer. Data from these reactions, and information from systems that do not react at all, are used to determine the electron affinity of SF4, EA(SF4) = 1.5 ± 0.2 eV. Additional thermochemical data are used to determine the fluoride affinity of SF3, D(SF3-F-) = 1.8 ± 0.3 eV.

Viggiano, A. A.; Miller, Thomas M.; Stevens Miller, Amy E.; Morris, Robert A.; van Doren, Jane M.; Paulson, John F.

1991-11-01

238

Application scope and limitations of TADDOL-derived chiral ammonium salt phase-transfer catalysts.  

PubMed

We have recently introduced a new class of chiral ammonium salt catalysts derived from easily available TADDOLs. To get a full picture of the scope of application and limitations of our catalysts we tested them in a variety of different important transformations. We found that, although these compounds have recently shown their good potential in the asymmetric ?-alkylation of glycine Schiff bases, they clearly failed when we attempted to control more reactive nucleophiles like b-keto esters. On the other hand, when using them to catalyse the addition of glycine Schiff bases to different Michael acceptors it was found necessary to carefully optimize the reaction conditions for every single substrate class, as seemingly small structural changes sometimes required the use of totally different reaction conditions. Under carefully optimized conditions enantiomeric ratios up to 91:9 could be achieved in the addition of glycine Schiff bases to acrylates, whereas acrylamides and methyl vinyl ketone gave slightly lower selectivities (up to e.r. 77:23 in these cases). Thus, together with additional studies towards the syntheses of these catalysts we have now a very detailed understanding about the scope and limitations of the synthesis sequence to access our PTCs and about the application scope of these catalysts in asymmetric transformations. PMID:23584056

Gururaja, Guddeangadi N; Herchl, Richard; Pichler, Antonia; Gratzer, Katharina; Waser, Mario

2013-01-01

239

The electron-transfer site of spinach plastocyanin.  

PubMed

Two sites for electron transfer have been proposed for plastocyanin: one near the copper ion and the other close to the acid patch formed by residues 42-45. Calculations of electrostatic properties of spinach plastocyanin and ionic strength dependences of electron-transfer reactions of this protein have been used to distinguish between these two sites. Calculations show that the electric potential field of spinach plastocyanin is highly asymmetric and that the protein has a dipole moment of 360 D. The negative end of the dipole axis emerges between the negative patches formed by residues 42-45, which is though to be the cation binding site, and residues 59-61. The angles between the dipole vector and vectors from the center of mass to the copper ion and to the acid patch are 90 degrees and 30 degrees, respectively. The angle between the dipole vector and a line from the center of mass to the site of electron transfer is evaluated from the ionic strength dependence of electron-transfer rates at pH 7.8 with the help of equations developed by Van Leeuwen et al. [van Leeuwen, J.W., Mofers, F.J.M., & Veerman, E.C.I. (1981) Biochim. Biophys. Acta 635, 434] and Van Leeuwen [van Leeuwen, J.W. (1983) Biochim. Biophys. Acta 743, 408]. The angles found are 85 degrees, 110 degrees, and 75 +/- 15 degrees for reactions with tris(1,10-phenanthroline)cobalt(III), hexacyanoferrate(III), and ferrocytochrome c, respectively. The electric potential field calculations suggest that the hexacyanoferrate(III) interaction angle corresponds to a unique site of minimum repulsion at the hydrophobic region of the protein surface, close to the copper ion.(ABSTRACT TRUNCATED AT 250 WORDS) PMID:2847776

Rush, J D; Levine, F; Koppenol, W H

1988-08-01

240

Marcus wins nobel prize in chemistry for electron transfer theory  

SciTech Connect

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

Levi, B.G.

1993-01-01

241

Photoinduced electron transfer in a distyryl BODIPY-fullerene dyad.  

PubMed

A novel distyryl BODIPY-fullerene dyad is prepared. Upon excitation at the distyryl BODIPY moiety, the dyad undergoes photoinduced electron transfer to give a charge-separated state with lifetimes of 476 ps and 730 ps in polar (benzonitrile) and nonpolar (toluene) solvents, respectively. Transient absorption measurements show the formation of the triplet excited state of distyryl BODIPY in the dyad, which is populated from charge-recombination processes in both solvents. PMID:20960495

Liu, Jian-Yong; El-Khouly, Mohamed E; Fukuzumi, Shunichi; Ng, Dennis K P

2011-01-01

242

Direct electron transfer reactions between human ceruloplasmin and electrodes  

Microsoft Academic Search

In an effort to find conditions favouring bioelectrocatalytic reduction of oxygen by surface-immobilised human ceruloplasmin (Cp), direct electron transfer (DET) reactions between Cp and an extended range of surfaces were considered. Exploiting advances in surface nanotechnology, bare and carbon-nanotube-modified spectrographic graphite electrodes as well as bare, thiol- and gold-nanoparticle-modified gold electrodes were considered, and ellipsometry provided clues as to the

Karolina Haberska; Cristina Vaz-Domínguez; Antonio L. De Lacey; Marius Dagys; Curt T. Reimann; Sergey Shleev

2009-01-01

243

Electron transfer mechanism of cytochrome c at graphene electrode  

Microsoft Academic Search

We report the direct electron transfer of cytochrome c (Cyt c) observed at graphene electrodes. Graphene nanosheets were chemically synthesized and immobilized on to a glassy carbon electrode. Cyclic voltammetry of Cyt c in phosphate buffered saline was performed at these electrodes. Results indicated a pair of reversible redox waves with a peak-to-peak separation value of 0.07 V in a

Subbiah Alwarappan; Rakesh K. Joshi; Manoj K. Ram; Ashok Kumar

2010-01-01

244

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

245

Electron transfer and decay processes of highly charged iodine ions  

Microsoft Academic Search

In the present experimental work we have investigated multi-electron transfer processes in Iq+ (q = 10, 15, 20 and 25) + Ne, Ar, Kr and Xe collisions at 1.5q keV energy. Using the coincidence technique between charge-selected projectile and recoil ions, the branching ratios between Auger and radiative decay channels have been measured in decay processes of multiply excited states

Hiroyuki A. Sakaue; Atsunori Danjo; Kazumoto Hosaka; Daiji Kato; Masahiro Kimura; Atsushi Matsumoto; Nobuyuki Nakamura; Shunsuke Ohtani; Makoto Sakurai; Hiroyuki Tawara; Ichihiro Yamada; Masuhiro Yoshino

2004-01-01

246

Intramolecular electron transfer in pentaammineruthenium(III)-modified cobaltocytochrome c  

Microsoft Academic Search

The iron in the heme group of horse-heart cytochrome c was replaced by cobalt according to established methods. The resulting cobalticytochrome c was subsequently modified at histidine-33 with a pentaammineruthenium group. Proof of correct derivatization was obtained by atomic absorption analysis of cobalt and ruthenium, differential pulse voltammetry, and enzymatic proteolysis analyzed by diode-array HPLC. Cobalt(II)-to-ruthenium(III) intramolecular electron transfer rates

Ji Sun; Chang Su; James F. Wishart

1996-01-01

247

Mitochondrial copper(I) transfer from Cox17 to Sco1 is coupled to electron transfer.  

PubMed

The human protein Cox17 contains three pairs of cysteines. In the mitochondrial intermembrane space (IMS) it exists in a partially oxidized form with two S-S bonds and two reduced cysteines (HCox17(2S-S)). HCox17(2S-S) is involved in copper transfer to the human cochaperones Sco1 and Cox11, which are implicated in the assembly of cytochrome c oxidase. We show here that Cu(I)HCox17(2S-S), i.e., the copper-loaded form of the protein, can transfer simultaneously copper(I) and two electrons to the human cochaperone Sco1 (HSco1) in the oxidized state, i.e., with its metal-binding cysteines forming a disulfide bond. The result is Cu(I)HSco1 and the fully oxidized apoHCox17(3S-S), which can be then reduced by glutathione to apoHCox17(2S-S). The HSco1/HCox17(2S-S) redox reaction is thermodynamically driven by copper transfer. These reactions may occur in vivo because HSco1 can be found in the partially oxidized state within the IMS, consistent with the variable redox properties of the latter compartment. The electron transfer-coupled metallation of HSco1 can be a mechanism within the IMS for an efficient specific transfer of the metal to proteins, where metal-binding thiols are oxidized. The same reaction of copper-electron-coupled transfer does not occur with the human homolog of Sco1, HSco2, for kinetic reasons that may be ascribed to the lack of a specific metal-bridged protein-protein complex, which is instead observed in the Cu(I)HCox17(2S-S)/HSco1 interaction. PMID:18458339

Banci, Lucia; Bertini, Ivano; Ciofi-Baffoni, Simone; Hadjiloi, Theodoros; Martinelli, Manuele; Palumaa, Peep

2008-05-13

248

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

249

Electron transfer to covalently immobilized Keggin polyoxotungstates on gold.  

PubMed

Spontaneously adsorbed monolayers have been formed on gold electrodes using a Keggin polyoxotungstate with covalently attached alkanethiol linkers of two different lengths. Films of both polyoxotungstates show two well-defined reduction processes associated with the polyoxotungstate centers where the ionic liquid, [BMIM][BF4], acts as supporting electrolyte. The surface coverages are both less than that expected for a close-packed monolayer. For the short and long linkers, the voltammetric response can be described in terms of the Butler-Volmer response involving a surface confined species using standard heterogeneous electron transfer rate constants of 170 and 140 s(-1) for the first reduction and 150 and 100 s(-1) for the second reduction processes, respectively. The rate of electron transfer to a solution phase redox probe, ferrocyanide, is significantly more sensitive to the length of the linker than the rate of electron transfer to the tungstate centers. This behavior probably arises due to potential-induced changes in the film structure. PMID:24670101

Yaqub, Mustansara; Walsh, James J; Keyes, Tia E; Proust, Anna; Rinfray, Corentin; Izzet, Guillaume; McCormac, Timothy; Forster, Robert J

2014-04-22

250

Correlated Single Quantum Dot Blinking and Interfacial Electron Transfer Dynamics  

PubMed Central

The electron transfer (ET) dynamics from core/multi-shell (CdSe/CdS3MLZnCdS2MLZnS2ML) quantum dots (QDs) to adsorbed Fluorescein (F27) molecules have been studied by single particle spectroscopy to probe the relationship between single QD interfacial electron transfer and blinking dynamics. Electron transfer from the QD to F27 and the subsequent recombination were directly observed by ensemble-averaged transient absorption spectroscopy. Single QD-F27 complexes show correlated fluctuation of fluorescence intensity and lifetime, similar to those observed in free QDs. With increasing ET rate (controlled by F27-to-QD ratio), the lifetime of on states decreases and relative contribution of off states increases. It was shown that ET is active for QDs in on states, the excited state lifetime of which reflects the ET rate, whereas in the off state QD excitons decay by Auger relaxation and ET is not a competitive quenching pathway. Thus, the blinking dynamics of single QDs modulate their interfacial ET activity. Furthermore, interfacial ET provides an additional pathway for generating off states, leading to correlated single QD interfacial ET and blinking dynamics in QD-acceptor complexes. Because blinking is a general phenomenon of single QDs, it appears that the correlated interfacial ET and blinking and the resulting intermittent ET activity are general phenomena for single QDs.

Jin, Shengye; Hsiang, Jung-Cheng; Zhu, Haiming; Song, Nianhui; Dickson, Robert M.; Lian, Tianquan

2011-01-01

251

Correlated Single Quantum Dot Blinking and Interfacial Electron Transfer Dynamics.  

PubMed

The electron transfer (ET) dynamics from core/multi-shell (CdSe/CdS(3ML)ZnCdS(2ML)ZnS(2ML)) quantum dots (QDs) to adsorbed Fluorescein (F27) molecules have been studied by single particle spectroscopy to probe the relationship between single QD interfacial electron transfer and blinking dynamics. Electron transfer from the QD to F27 and the subsequent recombination were directly observed by ensemble-averaged transient absorption spectroscopy. Single QD-F27 complexes show correlated fluctuation of fluorescence intensity and lifetime, similar to those observed in free QDs. With increasing ET rate (controlled by F27-to-QD ratio), the lifetime of on states decreases and relative contribution of off states increases. It was shown that ET is active for QDs in on states, the excited state lifetime of which reflects the ET rate, whereas in the off state QD excitons decay by Auger relaxation and ET is not a competitive quenching pathway. Thus, the blinking dynamics of single QDs modulate their interfacial ET activity. Furthermore, interfacial ET provides an additional pathway for generating off states, leading to correlated single QD interfacial ET and blinking dynamics in QD-acceptor complexes. Because blinking is a general phenomenon of single QDs, it appears that the correlated interfacial ET and blinking and the resulting intermittent ET activity are general phenomena for single QDs. PMID:21915369

Jin, Shengye; Hsiang, Jung-Cheng; Zhu, Haiming; Song, Nianhui; Dickson, Robert M; Lian, Tianquan

2010-08-31

252

The Electron Transfer System of Syntrophically Grown Desulfovibrio vulgaris? †  

PubMed Central

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

2009-01-01

253

77 FR 10373 - Greenhouse Gas Reporting Program: Electronics Manufacturing: Revisions to Heat Transfer Fluid...  

Federal Register 2010, 2011, 2012, 2013

...Electronics Manufacturing: Revisions to Heat Transfer Fluid Provisions AGENCY: Environmental...Reporting Rule related to fluorinated heat transfer fluids. More specifically, EPA...the definition of fluorinated heat transfer fluids and to the provisions...

2012-02-22

254

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...

255

New method for analysis of nanoparticle geometry in supported fcc metal catalysts with scanning transmission electron microscopy.  

PubMed

To apply the knowledge of reaction mechanisms of heterogeneously catalyzed reactions on the atomic scale to supported catalyst systems, a detailed description of the structure of active particles on the atomic scale is required. In this article, a method is developed to construct atomic-scale geometric models for supported active fcc metal nanoparticles, based on a measurement of particle sizes and particle volumes by Scanning Transmission Electron Microscopy (STEM) and the M-M coordination number determined from EXAFS. The method is applied to supported Au/TiO(2), Au/MgAl(2)O(4)(-), and Au/Al(2)O(3) catalysts. These geometric models allow for estimation of geometric properties, such as specific Au surface area, metal-support contact perimeter, metal-support contact surface area, edge length, and number of Au atoms located at the corners of the particles, with an error on the order of 20%. In the three catalysts studied here we find that the Au particles in the Al(2)O(3) supported catalyst are small. The Au particles in the Au/TiO(2) catalyst are smaller in diameter than those for the Au/MgAl(2)O(4), but also thicker. The differences in particle size and shape seem to reflect the differences in the metal-support interface energy in the three catalyst systems. PMID:16539459

Carlsson, Anna; Puig-Molina, Anna; Janssens, Ton V W

2006-03-23

256

Synthesis, Characterization and Application of a Multi-Site Phase Transfer Catalyst in Radical Polymerization of n-Butyl Methacrylate—A Kinetic Study  

Microsoft Academic Search

The multi-site phase transfer catalyzed radical polymerization of n-butyl methacrylate (n-BMA) using newly synthesized and characterized 1,1,2,2-tetramethyl-1-benzyl-2-n-propylethylene-1,2-diammonium bromide chloride (TMBPEDBC) as a multi-site phase transfer catalyst was investigated in an aqueous-organic two-phase system at 60 ± 1°C under nitrogen circumstances. The kinetics and effects of various operating variables (monomer, initiator, catalyst, temperature, acid, and ionic strength) on the rate of polymerization (Rp)

Murugesan Vajjiravel; M. J. Umapathy

2010-01-01

257

Preparation of a novel soluble multi-site phase transfer catalyst and the kinetic study for the C-alkylation of ?-pinene  

Microsoft Academic Search

Novel soluble “multi-site” phase transfer catalyst viz.1,3,5-tris(4-(2,2?-bis(N-triethylammoniummethylene chloride)eth-1-ene)phenoxymethyl)benzene (TBTEAPB) was synthesized and characterized by different spectral techniques such as FT-IR, 1H NMR, 13C NMR, MALDI-TOF mass and chloride ion analysis. The catalytic potential of this new multi-site TBTEAPB phase transfer catalyst was demonstrated by the C-alkylation of ?-pinene with epichlorohydrin using low concentration of base (20% NaOH) at 40°C. The

E. Murugan; A. Siva

2005-01-01

258

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

259

Interfacial Electron Transfer into Functionalized Crystalline Polyoxotitanate Nanoclusters  

PubMed Central

Interfacial electron transfer (IET) between a chromophore and a semi-conductor nanoparticle is one of the key processes in a dye sensitized solar cell. Theoretical simulations of the electron transfer in polyoxotitanate nanoclusters Ti17O24(OPri)20 (Ti17) functionalized with four para-nitrophenyl acetylacetone (NPA-H) adsorbates, of which the atomic structure has been fully established by X-ray diffraction measurements, are presented. Complementary experimental information showing IET has been obtained by EPR spectroscopy. Evolution of the time-dependent photoexcited electron during the initial 5 fs after instantaneous excitation to the NPA LUMO+1 has been evaluated. Evidence for delocalization of the excitation over multiple chromophoresafter excitation to the NPA LUMO+2 state on a 15 fs timescale is also obtained. While chromophores are generally considered electronically isolated with respect to neighboring sensitizers, our calculations show that this is not necessarily the case. The present work is the most comprehensive study to date of a sensitized semiconductor nanoparticle in which the structure of the surface and the mode of molecular adsorption are precisely defined.

Snoeberger, Robert C.; Young, Karin J.; Tang, Jiji; Allen, Laura J.

2014-01-01

260

Effect of protein dynamics on biological electron transfer  

PubMed Central

Computer simulations of the effect of protein dynamics on the long distance tunneling mediated by the protein matrix have been carried out for a Ru-modified (His 126) azurin molecule. We find that the tunneling matrix element is a sensitive function of the atomic configuration of the part of the protein matrix in which tunneling currents (pathways) are localized. Molecular dynamics simulations show that fluctuations of the matrix element can occur on a time scale as short as 10 fs. These short time fluctuations are an indication of a strong dynamic coupling of a tunneling electron to vibrational motions of the protein nuclear coordinates. The latter results in a modification of the conventional Marcus picture of electron transfer in proteins. The new element in the modified theory is that the tunneling electron is capable of emitting or absorbing vibrational energy (phonons) from the medium. As a result, some biological reactions may occur in an activationless fashion. An analytical theoretical model is proposed to account for thermal fluctuations of the medium in long distance electron transfer reactions. The model shows that, at long distances, the phonon-modified inelastic tunneling always dominates over the conventional elastic tunneling.

Daizadeh, Iraj; Medvedev, Emile S.; Stuchebrukhov, Alexei A.

1997-01-01

261

Electron Transfer Dissociation (ETD) of Peptides Containing Intrachain Disulfide Bonds  

NASA Astrophysics Data System (ADS)

The fragmentation chemistry of peptides containing intrachain disulfide bonds was investigated under electron transfer dissociation (ETD) conditions. Fragments within the cyclic region of the peptide backbone due to intrachain disulfide bond formation were observed, including: c (odd electron), z (even electron), c-33 Da, z + 33 Da, c + 32 Da, and z-32 Da types of ions. The presence of these ions indicated cleavages both at the disulfide bond and the N-C? backbone from a single electron transfer event. Mechanistic studies supported a mechanism whereby the N-C? bond was cleaved first, and radical-driven reactions caused cleavage at either an S-S bond or an S-C bond within cysteinyl residues. Direct ETD at the disulfide linkage was also observed, correlating with signature loss of 33 Da (SH) from the charge-reduced peptide ions. Initial ETD cleavage at the disulfide bond was found to be promoted amongst peptides ions of lower charge states, while backbone fragmentation was more abundant for higher charge states. The capability of inducing both backbone and disulfide bond cleavages from ETD could be particularly useful for sequencing peptides containing intact intrachain disulfide bonds. ETD of the 13 peptides studied herein all showed substantial sequence coverage, accounting for 75%-100% of possible backbone fragmentation.

Cole, Scott R.; Ma, Xiaoxiao; Zhang, Xinrong; Xia, Yu

2012-02-01

262

Catalysts supported on polymer colloids  

Microsoft Academic Search

Polymer colloids containing tetraakylphosphonium ions as phase transfer catalysts and cobalt ions as autoxidation catalysts have been prepared by emulsion polymerization and subsequent functional group transformations on the colloids. The 0.27 pm colloidal phase transfer catalysts are more active for reaction of cyanide ion with benzyl bromide than are the analogous 20 pm spherical bead catalysts, but the high electrolyte

Warren T. Ford; Rama Chandran; Hayrettin Turk

1988-01-01

263

Molecular mimicry of photosynthetic energy and electron transfer  

SciTech Connect

Proper application of reaction design considerations can yield artificial photosynthetic devices which credibility mimic the three natural photochemical processes. One approach is to use pigments and electron donors and acceptors related to those found in natural photosynthesis (and thus presumably optimal for that system), but to replace the protein with covalent bonds as an organizing precept. Molecular pentads described herein exemplify the success of this approach. At the heart of these molecules, are two covalently linked synthetic porphyrin moieties (P-P). One of these models for chlorophyll is attached to a carotenoid polyene (C), whereas the other is linked to a rigid diquinone (Q-Q). As discussed later in this paper, excitation of such a pentad is followed by photoinitiated electron transfer steps which ultimately give a C[sup [center dot]+]-P-P-Q-Q[sup [center dot]-] charge-separated state. Depending upon the structure of the pentad and the conditions, these states are formed with quantum yields of up to 0.83, have lifetimes approaching 0.5 ms, and store about one-half of the energy of the exciting singlet state. Related photosynthesis mimics display singlet-singlet energy transfer from carotenoid polyenes to porphyrins and among porphyrin chromophores, and rapid quenching of porphyrin triplet states by attached carotenoids. How have the structures of these and other successful artificial reaction centers evolved, and what will be the next steps in their development The authors will address these questions from the point of view of photoinitiated electron transfer, and then singlet and triplet energy transfer will briefly be considered. 37 refs., 4 figs.

Gust, D.; Moore, T.A.; Moore, A.L. (Arizona State Univ., Tempe (United States))

1993-04-01

264

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

2014-02-24

265

Electron Transfer Reactions in Colloidal Quantum Dot-Ligand Complexes  

NASA Astrophysics Data System (ADS)

This thesis describes a quantitative analysis of the chemical composition of colloidal II-VI quantum dot (QD)-ligand complexes and transient absorption experiments analyzing the rates of electron transfer reactions in these complexes functionalized with redox active ligands. Chemical analysis reveals that phosphonate impurities in the surfactants used to synthesize CdSe QDs are the dominant ligands on the surface of the QDs, and these phosphonate impurities cause size-dependent Cd-enrichment of the QD surface. A study of the adsorption equilibrium of solution-phase CdS quantum dots and acid-derivatized viologen ligands (V2+) reveals that the structure of the surfaces of the QDs depends on the concentration of the QDs. A new model based on the Langmuir isotherm that treats both the number of adsorbed ligands per QD and the number of available binding sites per QD as binomially-distributed quantities is described. Transient absorption spectroscopy of solution-phase mixtures of colloidal CdS QDs and V2+ indicates electron transfer occurs from the conduction band of the QD to the LUMO of V2+. The rate constant for photoinduced electron transfer (PET) is independent of the number of methylene groups in the alkyl chain on the acid-derivatized viologen. The insensitivity of the electron transfer rate constant to the length of the functional groups on the viologen suggests a van der Waals (vdW) pathway for PET, where the electron bypasses the alkylcarboxylate and tunnels through the orbitals of the QD and of the bipyridinium core. The rate of PET from colloidal CdSe quantum dots (QDs) to oxo-centered triruthenium clusters (Ru 3O) depends on the structure of the chemical headgroup by which the Ru3O clusters adsorb to the QDs. Complexes comprising QDs and Ru 3O clusters adsorbed through a pyridine-4-carboxylic acid ligand have a PET rate constant of (4.9 ± 0.9)×109 s -1 whereas complexes comprising QDs and Ru3O clusters adsorbed through a 4-mercaptopyridine ligand have an intrinsic PET rate constant of (36 ± 7)×109 s-1. The difference in the observed rates of PET for the two complexes is attributed to a difference in donor-acceptor electronic coupling.

Morris-Cohen, Adam Joshua

266

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

SciTech Connect

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, Leon [Groupe de Recherche en Sciences des Radiations, Faculte de Medecine, Universite de Sherbrooke, Sherbrooke, QC J1H 5N4 (Canada)

2006-05-26

267

Investigation of a traditional catalyst by contemporary methods: Parallel electron spectroscopic and catalytic studies on Pt black  

NASA Astrophysics Data System (ADS)

Results of electron spectroscopy (XPS and UPS) of platinum black catalyst measured in various states of the catalyst have been summarized. XPS showed up to almost 50% carbon and up to 20% oxygen on a sample stored in air. These, however, had almost no influence on the chemical state of Pt, except for the appearance of minor surface oxide. A Pt purity of ˜90% could be reached by regeneration with O 2 and H 2. The C 1s peak contained several components from individual C atoms to graphitic and polymeric hydrocarbon layers. Thus, the active catalyst was not clean Pt but metallic Pt; the impurities exerting little influence on catalytic activity. Regeneration and deactivation led also to slight structural rearrangement, as detected by XRD. Intentional deactivation with hydrocarbon-hydrogen mixtures was monitored by XPS, UPS and catalytic tests. Correlation was found between catalytic activity and selectivity in hexane reaction and the amount - and also the chemical state - of carbon accumulated during deactivating runs. A short summary of electron spectroscopy of supported Pt catalysts is also given. The main underlying idea regards solid catalyst and reactants as a dynamic system, including also solid-state changes of the former.

Paál, Zoltán; Schlögl, Robert

2009-06-01

268

Photoinduced Electron Transfer in a Dynamic Supramolecular System with Curved ?-Structures.  

PubMed

Photoinduced electron-transfer processes in a carbonaceous supramolecular combination of a tubular host and a C60 guest were investigated with time-resolved transient absorption spectra upon laser flash photolysis. Following the formation of triplet charge-separated species via electron transfer from the host to the guest, a rapid back electron transfer proceeded to afford triplet C60. PMID:24918189

Hitosugi, Shunpei; Ohkubo, Kei; Iizuka, Ryosuke; Kawashima, Yuki; Nakamura, Kosuke; Sato, Sota; Kono, Hirohiko; Fukuzumi, Shunichi; Isobe, Hiroyuki

2014-06-20

269

Reduced density matrix hybrid approach: Application to electronic energy transfer  

NASA Astrophysics Data System (ADS)

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.; Markland, Thomas E.; Reichman, David R.

2012-02-01

270

Electron and hydrogen transfer in small hydrogen fluoride anion clusters.  

PubMed

A new stable structure has been found for the anion clusters of hydrogen fluoride. The ab initio method was used to optimize the structures of the (HF)(3)(-), (HF)(4)(-), (HF)(5)(-), and (HF)(6)(-) anion clusters with an excess "solvated" electron. Instead of the well-known "zig-zag" (HF)(n)(-) structure, a new form, (HF)(n-1)F(-)···H, was found with lower energy. In this new form, the terminal hydrogen atom in the (HF)(n)(-) chain is separated from the other part of the cluster and the inner hydrogens transfer along the hydrogen bonds toward the outside fluoride. The negative charge also transfers from the terminal HF molecule of the chain to the center fluoride atoms. The (HF)(n)(-) clusters for n = 4, 5, and 6 have not yet been observed experimentally. These results should assist in the search for these systems and also provide a possible way to study the proton and electron transfer in some large hydrogen bonding systems. PMID:21854014

Bai, Xin; Ning, Ming; Brown, Richard E

2011-09-29

271

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

272

Hetero-cycloreversions mediated by photoinduced electron transfer.  

PubMed

Discovered more than eight decades ago, the Diels-Alder (DA) cycloaddition (CA) remains one of the most versatile tools in synthetic organic chemistry. Hetero-DA processes are powerful methods for the synthesis of densely functionalized six-membered heterocycles, ubiquitous substructures found in natural products and bioactive compounds. These reactions frequently employ azadienes and oxadienes, but only a few groups have reported DA processes with thiadienes. The electron transfer (ET) version of the DA reaction, though less investigated, has emerged as a subject of increasing interest. In the last two decades, researchers have paid closer attention to radical ionic hetero-cycloreversions, mainly in connection with their possible involvement in the repair of pyrimidine(6-4)pyrimidone photolesions in DNA by photolyases. In biological systems, these reactions likely occur through a reductive photosensitization mechanism. In addition, photooxidation can lead to cycloreversion (CR) reactions, and researchers can exploit this strategy for DNA repair therapies. In this Account, we discuss electron-transfer (ET) mediated hetero-CR reactions. We focus on the oxidative and reductive ET splitting of oxetanes, azetidines, and thietanes. Photoinduced electron transfer facilitates the splitting of a variety of four-membered heterocycles. In this context, researchers have commonly examined oxetanes, both experimentally and theoretically. Although a few studies have reported the cycloreversion of azetidines and thietanes carried out under electron transfer conditions, the number of examples remains limited. In general, the cleavage of the ionized four-membered rings appears to occur via a nonconcerted two-step mechanism. The trapping of the intermediate 1,4-radical ions and transient absorption spectroscopy data support this hypothesis, and it explains the observed loss of stereochemistry in the products. In the initial step, either C-C or C-X bond breaking may occur, and the preferred route depends on the substitution pattern of the ring, the type of heteroatom, and various experimental conditions. To better accommodate spin and charge, C-X cleavage happens more frequently, especially in the radical anionic version of the reaction. The addition or withdrawal of a single electron provides a new complementary synthetic strategy to activate hetero-cycloreversions. Despite its potential, this strategy remains largely unexplored. However, it offers a useful method to achieve C?X/olefin metathesis or, upon ring expansion, to construct six-membered heterocyclic rings. PMID:24702062

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

2014-04-15

273

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 ?.

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

2012-01-01

274

Advances in Enhanced Boiling Heat Transfer From Electronic Components  

NASA Astrophysics Data System (ADS)

This paper reviews recent advances in enhancing boiling heat transfer from electronic components immersed in dielectric liquids by use of surface microstructures. The microstructures developed include rough surfaces produced by sanding, vapor blasting hard particles, sputtering of SiO2 followed by wet etching of the surface, chemical vapor deposition of SiO2 film etc., laser-drilled cavities, a brush-like structure (dendritic structure), reentrant and micro-reentrant cavities, microfins, and porous structures fabricated by alumina particle spraying and painting of silver flakes, diamond particles, aluminum particles and copper particles. Heat sink studs with drilled holes, microfins, multi-layered micro-channels and pores, and pin fins with and without microporous coating have also been developed. The height of microstructure ranges from 0 to 12mm. The primary issues discussed are the mitigation of temperature overshoot at boiling incipience, enhancement of nucleate boiling heat transfer and increasing the critical heat flux.

Honda, Hiroshi; Wei, Jinjia

275

Photoinduced electron transfer from dialkyl nitroxides to halogenated solvents  

SciTech Connect

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

Chateauneuf, J. (National Research Council of Canada, Ottawa, Ontario (Canada)); Lusztyk, J.; Ingold, K.U. (Univ. of Notre Dame, IN (United States))

1990-02-02

276

Electron irradiation of single crystal thorium dioxide and electron transfer reactions  

Microsoft Academic Search

This first study of electron irradiation (3 MeV) of single crystals of thoria has revealed that the crystals turn blue, as seen with neutron irradiation, but the absorption profiles in the 12?000–17?260 cm?1 (1.49–2.14 eV) region were similar but not identical. Calculations showed that the electrons transferred energy sufficient to displace lattice oxygen atoms, but not thorium atoms. The crystals

Trevor R Griffiths; James Dixon

2000-01-01

277

Microbial interspecies electron transfer via electric currents through conductive minerals  

PubMed Central

In anaerobic biota, reducing equivalents (electrons) are transferred between different species of microbes [interspecies electron transfer (IET)], establishing the basis of cooperative behaviors and community functions. IET mechanisms described so far are based on diffusion of redox chemical species and/or direct contact in cell aggregates. Here, we show another possibility that IET also occurs via electric currents through natural conductive minerals. Our investigation revealed that electrically conductive magnetite nanoparticles facilitated IET from Geobacter sulfurreducens to Thiobacillus denitrificans, accomplishing acetate oxidation coupled to nitrate reduction. This two-species cooperative catabolism also occurred, albeit one order of magnitude slower, in the presence of Fe ions that worked as diffusive redox species. Semiconductive and insulating iron-oxide nanoparticles did not accelerate the cooperative catabolism. Our results suggest that microbes use conductive mineral particles as conduits of electrons, resulting in efficient IET and cooperative catabolism. Furthermore, such natural mineral conduits are considered to provide ecological advantages for users, because their investments in IET can be reduced. Given that conductive minerals are ubiquitously and abundantly present in nature, electric interactions between microbes and conductive minerals may contribute greatly to the coupling of biogeochemical reactions.

Kato, Souichiro; Hashimoto, Kazuhito; Watanabe, Kazuya

2012-01-01

278

How Much Is Transferred from Training to the Job? The 10% Delusion as a Catalyst for Thinking about Transfer  

ERIC Educational Resources Information Center

This article explores the common belief that only a small amount of what is taught in a training program is actually transferred to the job. After providing evidence of the source of the generalization and the acceptance of the notion despite the lack of empirical, behavioral evidence, we take the opportunity to examine the likely reasons for that…

Ford, J. Kevin; Yelon, Stephen L.; Billington, Abigail Q.

2011-01-01

279

Suppression of Electron Transfer to Dioxygen by Charge Transfer and Electron Transfer Complexes in the FAD-dependent Reductase Component of Toluene Dioxygenase*  

PubMed Central

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

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

2012-01-01

280

45 CFR 162.1602 - Standards for health care electronic funds transfers (EFT) and remittance advice transaction.  

Code of Federal Regulations, 2013 CFR

...2013-10-01 false Standards for health care electronic funds transfers (EFT... ADMINISTRATIVE REQUIREMENTS Health Care Electronic Funds Transfers (EFT... § 162.1602 Standards for health care electronic funds transfers...

2013-10-01

281

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

Code of Federal Regulations, 2013 CFR

...2013-10-01 false Operating rules for health care electronic funds transfers (EFT... ADMINISTRATIVE REQUIREMENTS Health Care Electronic Funds Transfers (EFT...162.1603 Operating rules for health care electronic funds transfers...

2013-10-01

282

48 CFR 52.232-35 - Designation of Office for Government Receipt of Electronic Funds Transfer Information.  

Code of Federal Regulations, 2013 CFR

...Designation of Office for Government Receipt of Electronic Funds Transfer Information. 52...Designation of Office for Government Receipt of Electronic Funds Transfer Information. As prescribed...of Office for Government Receipt of Electronic Funds Transfer Information (JUL...

2013-10-01

283

Self-regeneration of Pd-LaFeO3 catalysts: new insight from atomic-resolution electron microscopy.  

PubMed

Aberration-corrected transmission electron microscopy was used to study atomic-scale processes in Pd-LaFeO(3) catalysts. Clear evidence for diffusion of Pd into LaFeO(3) and out of LaFe(0.95)Pd(0.05)O(3-?) under high-temperature oxidizing and reducing conditions, respectively, was found, but the extent to which these processes occurred was quite limited. These observations cast doubt that such phenomena play a significant role in a postulated mechanism of self-regeneration of this system as an automotive exhaust-gas catalyst. PMID:22007950

Katz, Michael B; Graham, George W; Duan, Yingwen; Liu, Hong; Adamo, Carolina; Schlom, Darrell G; Pan, Xiaoqing

2011-11-16

284

New XAFS beamline for structural and electronic dynamics of nanoparticle catalysts in fuel cells under operating conditions  

NASA Astrophysics Data System (ADS)

We are currently constructing a new X-ray absorption fine structure (XAFS) beamline BL36XU at SPring-8 dedicated for the structural and electronic analysis of the dynamic events on polymer electrolyte fuel cell (PEFC) cathode catalysts for the development of next-generation PEFCs. To investigate the cathode catalyst nanoparticles in PEFCs under the operating conditions, the beamline is designed to provide time- and spatially resolved XAFS techniques having a time resolution of 100 ?s, spatial resolution of 200 nm, and depth resolution of 1 ?m. We report the outline and design of the new beamline.

Sekizawa, O.; Uruga, T.; Tada, M.; Nitta, K.; Kato, K.; Tanida, H.; Takeshita, K.; Takahashi, S.; Sano, M.; Aoyagi, H.; Watanabe, A.; Nariyama, N.; Ohashi, H.; Yumoto, H.; Koyama, T.; Senba, Y.; Takeuchi, T.; Furukawa, Y.; Ohata, T.; Matsushita, T.; Ishizawa, Y.; Kudo, T.; Kimura, H.; Yamazaki, H.; Tanaka, T.; Bizen, T.; Seike, T.; Goto, S.; Ohno, H.; Takata, M.; Kitamura, H.; Ishikawa, T.; Yokoyama, T.; Iwasawa, Y.

2013-04-01

285

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.

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

2013-01-01

286

Phonon-assisted near-field activation of electron transfer  

NASA Astrophysics Data System (ADS)

An optical near field should promote phonon-assisted multiple excitation in nanoscale structures. With the phonon-assisted process, greater catalytic activity is expected without heating. To confirm this effect, photo-induced current generation using platinum black electrodes in ferricyanide solution (an absorption band-edge wavelength of 470 nm) under visible light irradiation continuous wave [(CW), ?=532 nm] was observed. Higher order dependence of the generated current density on the incident light power was observed, indicating two-step activation of electron transfer, which originated from the phonon-assisted near-field effect on the nanostructured surface of the electrode.

Yatsui, Takashi; Iijima, Kazaunori; Imoto, Tsubasa; Kitamura, Kokoro; Kawazoe, Tadashi

2013-01-01

287

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

288

Stimulations of Oxygen Uptake by Electron Transfer Inhibitors 1  

PubMed Central

The stimulation of oxygen uptake induced in avocado tissue slices by amytal, azide and cyanide has been studied. The effects of these inhibitors on O2 uptake and on phosphorylation suggest the coexistence of phosphorylating and non-phosphorylating electron transfer systems in the fruit. The reason for the stimulations of O2 uptake is believed to be the result of an increased supply of a limiting cofactor to the phosphorylating sites. The increased availability of cofactor per site is due to the inhibition of part of the cytochrome chain and the consequent reduction in the number of active phosphorylating sites.

Lips, S. Herman; Biale, Jacob B.

1966-01-01

289

Phase-Transfer Catalysis: Free-Radical Polymerization of Methyl Methacrylate using K2S2O8Quaternary Ammonium Salt Catalyst System. A Kinetic Study  

Microsoft Academic Search

The kinetics of phase-transfer-agent-assisted free-radical polymerization of methyl methacrylate using K2S2O8 as the water-soluble initiator and triethylbenzylammonium chloride (TEBA) as the phase-transfer catalyst (PTC) was investigated in toluene-water biphase media at 60°C. The effect of varying [MMA], [K2S2O8], [TEBA], [H], the ionic strength of the medium, and the temperature on the rate of polymerization (Rp) was studied. Rp was found

T. Balakrishnan; N. Jayachandramani

1994-01-01

290

Synergetic Effect of Tetrabutylammonium Bromide and Polyethylene Glycol as Phase Transfer Catalysts in Third Liquid Phase for Benzyl n Butyl Ether Synthesis  

Microsoft Academic Search

This study intends to clarify the forming characteristics of third liquid phase in phase transfer catalytic system in the presence of n-butanol and potassium hydroxide when tetrabutylammonium bromide, polyethylene glycol and their mixture serve as phase transfer catalyst, respectively. At 323 K, the three catalytic systems were applied to yield benzyl-n-butyl ether from benzyl chloride and n-butanol, and they performed distinct

Gong Jin; Cuifang Zhang; Tadaatsu Ido; Shigeo Goto

2004-01-01

291

Copper(I)/ABNO-catalyzed aerobic alcohol oxidation: alleviating steric and electronic constraints of Cu/TEMPO catalyst systems.  

PubMed

Cu/TEMPO catalyst systems promote efficient aerobic oxidation of sterically unhindered primary alcohols and electronically activated substrates, but they show reduced reactivity with aliphatic and secondary alcohols. Here, we report a catalyst system, consisting of ((MeO)bpy)Cu(I)(OTf) and ABNO ((MeO)bpy = 4,4'-dimethoxy-2,2'-bipyridine; ABNO = 9-azabicyclo[3.3.1]nonane N-oxyl), that mediates aerobic oxidation of all classes of alcohols, including primary and secondary allylic, benzylic, and aliphatic alcohols with nearly equal efficiency. The catalyst exhibits broad functional group compatibility, and most reactions are complete within 1 h at room temperature using ambient air as the source of oxidant. PMID:24128057

Steves, Janelle E; Stahl, Shannon S

2013-10-23

292

A novel N-acetophenone cinchona ammonium salts as chiral phase transfer catalysts for the alkylation of Schiff base in water  

Microsoft Academic Search

Novel N-acetophenone cinchona ammonium salts have been successfully synthesized and used as chiral phase transfer catalysts for the asymmetric alkylation of tert-butyl benzophenone Schiff base derivatives in aqueous media at room temperature with the highest ee and yield up to 96 and 98%, respectively. We have also studied the influence of substituted acetophenone groups in quaternary ammonium salts derived from

Xin Wang; Jian Lv; Lei Liu; Yongmei Wang; Yang Wu

2007-01-01

293

New trimeric Cinchona alkaloid-based quaternary ammonium salts as efficient chiral phase transfer catalysts for enantioselective synthesis of ?-amino acids  

Microsoft Academic Search

New trimeric quaternary ammonium salts derived from cinchonine or cinchonidine bridging N,N?-bis(ethyl)-4-(methyl)-phenyl-amine moiety are used as a chiral phase transfer catalysts for the asymmetric alkylation of a N-(diphenylmethylene)glycine tert-butyl ester with very good chemical yield (up to 97%) and ee's (up to 98%).

Ayyanar Siva; Eagambaram Murugan

2006-01-01

294

Synthesis of allyl and alkyl vinyl ethers using an in situ prepared air-stable palladium catalyst. Efficient transfer vinylation of primary, secondary, and tertiary alcohols.  

PubMed

An air-stable palladium catalyst formed in situ from commercially available components efficiently catalyzed the transfer vinylation between butyl vinyl ether and various allyl and alkyl alcohols to give the corresponding allyl and alkyl vinyl ethers in 61-98% yield in a single step. PMID:12816481

Bosch, Martin; Schlaf, Marcel

2003-06-27

295

Direct one-pot conversion of acylated carbohydrates into their alkylated derivatives under heterogeneous reaction conditions using solid NaOH and a phase transfer catalyst  

Microsoft Academic Search

A convenient one-pot protocol for the direct conversion of acyl-protected carbohydrates into their alkylated counterparts has been developed by using alkyl halides in the presence of solid sodium hydroxide and a phase transfer catalyst. These economically convenient, mild, two-phase reaction conditions allow the preparation of a variety of monosaccharide intermediates for use in the synthesis of complex oligosaccharides.

Soni Kamlesh Madhusudan; Geetanjali Agnihotri; Devendra S. Negi; Anup Kumar Misra

2005-01-01

296

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

297

The electronic transfer of information and aerospace knowledge diffusion  

NASA Technical Reports Server (NTRS)

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

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

1992-01-01

298

Electron-Mediated Vibration-Electronic (V-E) Energy Transfer in Optically Pumped Plasmas  

NASA Astrophysics Data System (ADS)

Energy transfer from high vibrational states of the CO electronic ground state to electronically excited states is investigated in optically pumped CO-Ar or CO-N2 plasmas, with and without O2 and NO additives. Ionization in these strongly nonequilibrium plasmas occurs by an associative ionization mechanism in collisions of two highly vibrationally excited CO molecules. Removal of the electrons from the optically pumped plasmas using a saturated Thomson discharge results in substantial reduction of UV/visible radiation from the plasma (CO 4th positive bands, NO ? bands, CN violet bands, and C2 Swan bands). On the other hand, deliberate electron density increase by adding small amounts of O2 or NO to the optically pumped CO-Ar plasmas produces substantial increase of the UV/visible radiation intensity, which correlates with the electron density. The present experiments strongly suggest that the V-E energy transfer process CO(X^1?arrowA^1?), and, possibly, analogous processes populating radiating excited electronic states of NO, CN, and C_2, in optically pumped plasmas, may be mediated by the presence of electrons. Detailed kinetics of the processes will be presented.

Ploenjes, Elke; Palm, Peter; Rich, J. William; Adamovich, Igor V.

2001-10-01

299

Electron transfer across ?-helical peptide monolayers: importance of interchain coupling.  

PubMed

Four helical peptides with the general formula (Boc)-Cys-(S-Acm)-(Ala-Leu)(n)-NH-(CH(2))(2)-SH (n = 4-7) were synthesized and further used for the preparation of self-assembled monolayers (SAMs) on gold substrates. The electron-transfer behavior of these systems was probed using current-sensing atomic force microscopy (CS-AFM). It was found that the electron transmission through SAMs of helical peptides trapped between an AFM conductive tip and a gold substrate occurs very efficiently and that the distance dependence obeys the exponential trend with a decay constant of 4.6 nm(-1). This result indicates that the tunneling mechanism is operative in this case. Conductance measurements under mechanical stress show that peptide-mediated electron transmission occurs with the possible contribution of intermolecular electron tunneling between adjacent helices. It was also demonstrated that an external electric field applied between metallic contacts can affect the structure of the peptide SAM by changing its thickness. This explains the asymmetry of the current-voltage response of metal-monolayer-metal junction. PMID:23181704

Pawlowski, Jan; Juhaniewicz, Joanna; Tymecka, Dagmara; Sek, Slawomir

2012-12-18

300

Dissociative electron transfer to organic chlorides: electrocatalysis at metal cathodes.  

PubMed

The reductive cleavage of a series of organic chlorides, including chloroaromatics, benzyl chlorides, activated chloroalkanes and polychloromethanes, was investigated at Ag, Cu, Pd and glassy carbon (GC) electrodes in CH(3)CN + 0.1 M (C(2)H(5))(4)NClO(4). The silver cathode was either a 2-mm diameter disc, fabricated from Ag wire, or nanoclusters of average diameter d = 304 nm, prepared by electrodeposition on GC. Ag, Cu and Pd electrodes have shown remarkable electrocatalytic properties for the reduction of several compounds. The peak potentials recorded at these electrodes, for example, at upsilon = 0.1 V s(-1) are positively shifted by 0.3-0.8 V with respect to the reduction potentials measured at a non catalytic electrode such as GC. Electrocatalysis is strictly related to the concerted nature of the dissociative electron transfer to the carbon-chlorine bond. No catalysis is observed when the dissociative electron transfer to RCl occurs according to a stepwise mechanism involving the intermediate formation of a radical anion. The catalytic surfaces affect the reaction scheme, offering a more favourable route possibly through the formation of strongly adsorbed activated complexes. PMID:18414732

Isse, Abdirisak A; Gottardello, Silvia; Durante, Christian; Gennaro, Armando

2008-05-01

301

Electron transfer in native and mutated photosystem I reaction centers  

NASA Astrophysics Data System (ADS)

Femtosecond time-resolved absorption difference studies were performed on photosystem I complexes from the cyanobacterium Synechocystis sp. PCC 6803. The overal electron transfer from the special pair P700 to the secondary acceptor A1 has been shown to be 10 ps, twice shorter than the previously estimated value. Similar studies were performed on more than 10 genetically engineered species, where protein structure was altered in the visinity of the reaction center (RC). The functioning of the PS I complex was found to be extremelly sensitive to the protein sequence in the immediate proximity of the RC: less than half of the studied mutations resulted in photosynthetically active complexes, and all of the latter had electron transfer dynamics indistinguishable from that of the wild type. Most of the mutations in the other areas of the PS I, including antenna, did not affect the photosynthetic function of this complex radically. These results confirm the extreme importance of the precise RC structure and demonstrate why millions of years of evolution resulted in only two types of topologically similar RC's shared by all photosynthetic organisms.

Savikhin, Sergei; Xu, Wu; Chitnis, Parag; Struve, Walter

2002-03-01

302

Photoinduced electron transfer modeling to simulate flavoprotein fluorescence decay.  

PubMed

A method of analysis is described on the photoinduced electron transfer (PET) from aromatic amino acids as tryptophans (Trp) and tyrosines (Tyr) to the excited isoalloxazine (Iso*) in FMN-binding proteins (FBP) from Desulfovibrio vulgaris (strain, Miyazaki F). Time-dependent geometrical factors as the donor-acceptor distances are determined by means of a molecular dynamics simulation (MDS) of the proteins. Fluorescence decays of the single mutated isoforms of FBP are used as experimental data. The electrostatic (ES) energy between the photoproducts and ionic groups in the proteins is introduced into the Kakitani and Mataga (KM) model, which is modeled for an electron transfer process in solution. The PET parameters contained in the KM rate are determined by means of a nonlinear least square method, according to the Marquardt algorithm. The agreement between the observed and calculated decays is quite good, but not optimal. Characteristics on PET in flavoproteins, obtained by the present method, are described. Possible improvements of the method are discussed. PMID:24108633

Nunthaboot, Nadtanet; Lugsanangarm, Kiattisak; Nueangaudom, Arthit; Pianwanit, Somsak; Kokpol, Sirirat; Tanaka, Fumio

2014-01-01

303

Electron emission and electron transfer processes in proton-naphthalene collisions at intermediate velocities  

NASA Astrophysics Data System (ADS)

We investigate the fragmentation and ionization of naphthalene by protons at intermediate velocities (between 1.41 and 2.68 a.u.). Relative cross sections for electron capture (EC), electron emission (EE), and capture ionization are measured. The EC cross sections decrease rapidly over the energy range under consideration (50-150 keV) and are lower than EE cross sections. The EE cross sections, on the other hand, change very slowly in this energy range. The energetics of interactions is quantified by comparing the mass spectra with the photodissociation breakdown curves from literature. In the case of single capture, resonant electron transfer to n = 1 state in H+ is seen to dominate the interaction but is shown to be accompanied by a small amount of electronic energy loss. In the EE mode, two mechanisms are shown to be active in the collision process: large impact parameter plasmon excitation mode, and closer encounters with higher amounts of electronic energy loss.

Mishra, P. M.; Rajput, J.; Safvan, C. P.; Vig, S.; Kadhane, U.

2013-11-01

304

Collisional electron transfer to photoexcited acceptor radical anions  

NASA Astrophysics Data System (ADS)

In this article, we show that photoexcitation of radical anions facilitates electron transfer from sodium atoms in femtosecond encounters. Thus, excitation of 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) and fluorinated TCNQ (TCNQ-F4) anions to the second optically active state at 478 nm led to increases in the yields of dianions of about 20% and 10%, respectively. Photoexcitation with a nanosecond-long laser pulse was done a few microseconds before the ions entered the sodium collision cell so that none of the ions would be in any of the initially reached doublet-excited states. We suggest an explanation for the higher electron capture cross section based on the formation of long-lived quartet state anions. Excitation of TCNQ anions within the lowest-energy absorption band, where there are no accessible quartet states, led instead to a lower yield of dianions. There are at least three explanations for the lower dianion yields: (1) Depletion of the monoanion beam due to photodetachment after the absorption of minimum two photons; (2) Formation of short-lived vibrationally excited dianions that decay by electron autodetachment prior to identification; and (3) Lower electron capture cross sections of vibrationally excited monoanions. Similar losses in dianion signal can occur at 478 nm so the actual yield of dianions at this wavelength due to the population of quartet states is therefore greater than that observed. Our methodology devises a more efficient route for the production of molecular dianions, and at the same time it may provide information on long-lived electronic states.

Wyer, Jean Ann; Støchkel, Kristian; Nielsen, Steen Brøndsted

2012-02-01

305

Electronic excitation transfer in clustered chromophore systems: Calculation of time-resolved observables for intercluster transfer  

SciTech Connect

A theoretical description is given for electronic excitation transport among interacting clusters of chromophores. Each cluster is a finite volume system with a limited number of chromophores. At high cluster concentration, intercluster transfer will become significant. The theory is based on a first-order cumulant approximation of the solution to the transport master equation. {ital G}{sup {ital s}}({ital t}) the probability of finding the excitation on the initially excited chromophore is calculated. The problem is first solved for two clusters at fixed separations. This result is extended to many clusters and then to the thermodynamic limit of an infinite number of clusters in an infinite volume. An example calculation is performed of excitation transport among chromophores on the surfaces of interacting micelles. For realistic parameters characterizing the system octadecylrhodamine B (chromophores) in Triton X-100 micelles, it is found that intermicelle excitation transfer can compete with intramicelle transfer. For an isolated micelle--chromophore system (chromophores on the surface of a sphere), a new time domain expression for {ital G}{sup {ital s}}({ital t}) is obtained.

Marcus, A.H.; Fayer, M.D. (Department of Chemistry, Stanford University, Stanford, California 94305 (US))

1991-04-15

306

O-Acylation of Substituted Phenols with Various Alkanoyl Chlorides Under Phase-Transfer Catalyst Conditions  

Microsoft Academic Search

Esterification of several types of mono- and disubstituted phenols with various mono- and dialkanoyl chlorides was performed in phase-transfer catalysis conditions, using tetrabutylammonium chloride in a mixture of aqueous NaOH and dichloromethane. The process is particularly efficient (almost quantitative yields) as well as rapid (only 5 min reaction time, at a temperature of 0 °C).

Alina Marieta Simion; Iwao Hashimoto; Yoshiharu Mitoma; Naoyoshi Egashira; Cristian Simion

2012-01-01

307

O-Acylation of Substituted Phenols with Various Alkanoyl Chlorides Under Phase Transfer Catalyst Conditions  

Microsoft Academic Search

Esterification of several types of mono- and di-substituted phenols with various mono-and di-alkanoyl chlorides was performed in phase transfer catalysis conditions, using tetrabutylammonium chloride in a mixture of aqueous NaOH and dichloromethane. The process is particularly efficient (almost quantitative yields) as well as rapid (only 5 minutes reaction time, at a temperature of 0 °C).

Alina Marieta Simion; Iwao Hashimoto; Yoshiharu Mitoma; Naoyoshi Egashira; Cristian Simion

2011-01-01

308

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...

309

Photoelectric conversion based on proton-coupled electron transfer reactions.  

PubMed

Making efficient use of solar energy is one of the biggest challenges of our time. In nature, solar energy can be harvested by photosynthesis where proton-coupled electron transfer (PCET) plays a critical role. Here, PCET is utilized for the first time to directly convert light energy to electrical energy. Quinone/hydroquinone PCET redox couples were used to produce a photovoltage along with spiropyrans, photoswitchable compounds that undergo reversible transformation between a ring-closed (Sp) and ring-opened form (Mc). The Mc form is more basic than the Sp form, and the open-circuit voltage (Voc) is related to the proton concentration and that of the Sp/Mc ratio controlled by light. Voc values from 100 to 140 mV were produced. In addition to direct current (Jsc ca. 9 ?A cm(-2)), alternating current in the range of 0.1 to 200 Hz was also produced by manipulating the input light. PMID:24835106

Xie, Xiaojiang; Bakker, Eric

2014-06-01

310

Light-induced electron transfer vs. energy transfer in molecular thin-film systems  

SciTech Connect

Quenching of fluoranthene (FA) singlets by tetrabromo-o-benzoquinone (TBBQ) and N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) was studied both in xylene solutions and in spin-cast polystyrene (PS) films. Emphasis was placed on time-resolved fluorescence transients resulting from pulsed excitation. Linear Stern-Volmer plots were obtained for quenching in solution and gave diffusion-controlled rate constants, of 1.45 x 10/sup 10/ M/sup -1/ sec/sup -1/ and 1.53 x 10/sup 10/ M/sup -1/ sec/sup -1/ for TBBQ and TMPD, respectively. TBBQ was found to quench FA singlets in PS over the studied concentration range 12 mM < (TBBQ) < 48 mM, but in its presence FA singlets decayed nonexponentially. The results were interpreted quantitatively in terms of pure Foerster's transfer from FA to TBBQ without diffusion of excitons. The critical transfer radius R/sub 0/ was experimentally determined to be 24.3 A, which is in good agreement with the theoretical value of 23 A calculated from spectral data. Quenching of FA singlets in PS films was found to be independent of FA concentration over a 300 mM to 1200 mM FA concentration range for a constant TBBQ concentration of 24.0 mM. TMPD was only slightly effective as a quencher of FA singlets in PS because it apparently behaves strictly as a contact quencher based on reversible charge transfer. The implications of these results for the design of systems intended to exploit light-induced electron transfer are discussed.

Renschler, C. L.; Faulkner, L. R.

1980-01-01

311

Theoretical studies of electronic properties of magnesium oxide catalysts and metal-intercalated single-wall carbon nanotubes  

Microsoft Academic Search

This dissertation presents a theoretical study of electronic properties for MgO surfaces and metal-intercalated single-wall carbon nanotubes (SWNT's). The ab initio embedded cluster model and ab initio periodic calculation have been introduced and applied to the MgO surface and metal-intercalated SWNT systems, respectively. For the MgO catalyst, the catalytic properties of the perfect and oxygen vacancy MgO(100) surfaces for water

Yan Wang

2006-01-01

312

Electron-deficient tin(IV)tetraphenylporphyrin perchlorate: A highly efficient catalyst for chemical fixation of carbon dioxide  

Microsoft Academic Search

Electron-deficient tin(IV)tetraphenylporphyrin perchlorate, [SnIV(TPP)(ClO4)2] was used as a highly efficient catalyst for chemical fixation of carbon dioxide. The bifunctional catalytic system [Sn(TPP)(ClO4)2]\\/tetrabutylphosphonium bromide (TBPB) was applied for preparation of cyclic carbonates from epoxides and carbon dioxide. The effect of reaction parameters was also investigated.

Fatemeh Ahmadi; Shahram Tangestaninejad; Majid Moghadam; Valiollah Mirkhani; Iraj Mohammadpoor-Baltork; Ahmed Reza Khosropour

313

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

314

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

315

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

316

The Golden Rule. Application for fun and profit in electron transfer, energy transfer, and excited-state decay.  

PubMed

Time-dependent perturbation theory and application of the Golden Rule have been shown to be quantitatively applicable to electron transfer in the inverted region, energy transfer, and excited-state decay based on spectroscopic measurements on d?(6) polypyridyl complexes of Ru(II), Os(II), and Re(I). PMID:22842806

Ito, Akitaka; Meyer, Thomas J

2012-10-28

317

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

318

Ions interacting with planar aromatic molecules: Modeling electron transfer reactions  

NASA Astrophysics Data System (ADS)

We present theoretical absolute charge exchange cross sections for multiply charged cations interacting with the Polycyclic Aromatic Hydrocarbon (PAH) molecules pyrene C14H10, coronene C24H12, or circumcoronene C54H18. 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.

2013-02-01

319

Modulating electron transfer properties of gold nanoparticles for efficient biosensing.  

PubMed

Present study concerns modulating the electron transfer properties of gold nanoparticles through amino acid induced coupling among them. In addition to conductivity, the amino functionalization of the nanoparticles results in enhanced activity and operational stability of the biosensor fabricated using the same. Nanoparticles synthesized using amino acid as reducing agent (average diameter-20 nm), incorporate the natural coupling property of amino acids and are seen to align in a chain-like arrangement. The coupling of the individual nanoparticles to form chain like structure was confirmed by both absorption spectroscopy as well as transmission electron microscopy. The glucose biosensor developed by adsorption of glucose oxidase (GOx) enzyme onto these coupled gold nanoparticles showed enhanced efficiency as compared to the one with glucose oxidase immobilized onto gold nanoparticles synthesized using the conventional method (trisodium citrate as reducing agent). The fabricated biosensor demonstrated a wide linear concentration range from 1 ?M-5mM and a high sensitivity of 47.2 ?A mM(-1) cm(-2). Also, an enhanced selectivity to glucose was observed with negligible interference in the physiological range, from easily oxidizable biospecies, e.g. uric acid and ascorbic acid. Furthermore, the electrochemical biosensor has excellent long term stability- retaining greater than 85% of the biosensor activity up to 60 days. PMID:22608576

Sharma, Shikha; Gupta, Nidhi; Srivastava, Sudha

2012-01-01

320

Ions interacting with planar aromatic molecules: modeling electron transfer reactions.  

PubMed

We present theoretical absolute charge exchange cross sections for multiply charged cations interacting with the Polycyclic Aromatic Hydrocarbon (PAH) molecules pyrene C(14)H(10), coronene C(24)H(12), or circumcoronene C(54)H(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. PMID:23406118

Forsberg, B O; Alexander, J D; Chen, T; Pettersson, A T; Gatchell, M; Cederquist, H; Zettergren, H

2013-02-01

321

Direct Electron Transfer Reactivity of Glucose Oxidase on Electrodes Modified With Zirconium Dioxide Nanoparticles  

Microsoft Academic Search

The direct electron transfer between electrodes and glucose oxidase (GOD) immobilized in a matrix containing zirconium dioxide nanoparticles (ZrO2) is described. The protein-nanoparticle assembly is stabilized by charged and uncharged compounds and the direct electron transfer is enhanced. The effects of different compositions on the electrochemical parameters, formal potential, surface loading, and constant heterogeneous electron transfer rate are characterized with

Xiaodi Yang; Qianqian Zhang; Yuming Sun; Songqin Liu

2007-01-01

322

Double and single-electron transfer in H+ +K collisions from 0.3 to 4keV :Separation of direct double transfer and two-step successive single-electron transfer  

Microsoft Academic Search

Double-electron transfer and two-step single-electron transfer in collisions of proton with potassium-metal target are measured in the collision energy from 0.3to4keV by using a charge-inversion mass spectrometry. Two prominent H- ion peaks are observed with different values of the energy loss and show different target density dependences. The peak with larger energy loss is identified as double-electron transfer and the

Shigeo Hayakawa; Kazunori Kadomura; Mineo Kimura; C. M. Dutta

2004-01-01

323

Synergistic contributions by decreasing overpotential and enhancing charge-transfer in ?-Fe2O3/Mn3O4/graphene catalysts with heterostructures for photocatalytic water oxidation.  

PubMed

A novel nanocomposite consisting of ?-Fe2O3, Mn3O4 and reduced graphene oxide (r-GO) has been facilely synthesized through a two-step method: solvothermal reaction for Mn3O4-modified ?-Fe2O3 (?-Fe2O3/Mn3O4) and self-assembly process for combining ?-Fe2O3/Mn3O4 with r-GO (?-Fe2O3/Mn3O4/r-GO). The morphology and structure of the nanocomposite were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS). The results demonstrated that rod-like hematite was modified by Mn3O4 and dispersed on the surface of r-GO. Raman and Fourier transform infrared spectra (FTIR) showed superior interfacial contacts between ?-Fe2O3/Mn3O4 and r-GO. Ultraviolet-visible diffuse reflectance spectroscopy (DRS) and photoelectrochemical characterization revealed a high light-harvesting efficiency, a lowered overpotential for water oxidation and an excellent charge transfer performance of ?-Fe2O3/Mn3O4/r-GO nanocomposite with heterostructures. The photocatalytic oxygen evolution from the optimized photocatalyst was up to 1406.2 ?mol g(-1) in 10 h of UV-vis light irradiation and the quantum yield was ca. 4.35% at 365 nm. Our investigation suggests that constructing a catalyst with heterostructures is a promising method to enhance photocatalytic activity. PMID:24781029

Yin, Shunli; Wang, Xiaomei; Mou, Zhigang; Wu, Yijie; Huang, Hui; Zhu, Mingshan; Du, Yukou; Yang, Ping

2014-06-21

324

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

325

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

326

Structural factors influencing the intramolecular charge transfer and photoinduced electron transfer in tetrapyrazinoporphyrazines.  

PubMed

A series of unsymmetrical tetrapyrazinoporphyrazines (TPyzPzs) from the group of azaphthalocyanines with one peripherally attached amino substituent (donor) were synthesized, and their photophysical properties (fluorescence quantum yield and singlet oxygen quantum yield) were determined. The synthesized TPyzPzs were expected to undergo intramolecular charge transfer (ICT) as the main pathway for deactivating their excited states. Several structural factors were found to play a critical role in ICT efficiency. The substituent in the ortho position to the donor center significantly influences the ICT, with tert-butylsulfanyl and butoxy substituents inducing the strongest ICTs, whereas chloro, methyl, phenyl, and hydrogen substituents in this position reduce the efficiency. The strength of the donor positively influences the ICT efficiency and correlates well with the oxidation potential of the amines used as the substituents on the TPyzPz as follows: n-butylamine < N,N-diethylamine < aniline < phenothiazine. The ICT (with conjugated donors and acceptors) in the TPyzPz also proved to be much stronger than a photoinduced electron transfer in which the donor and the acceptor are connected through an aliphatic linker. PMID:24509513

Novakova, Veronika; Hladík, Petr; Filandrová, Tereza; Zajícová, Ivana; Krepsová, Veronika; Miletin, Miroslav; Len?o, Juraj; Zimcik, Petr

2014-03-21

327

Theoretical study on the electronic structure of triphenyl sulfonium salts: Electronic excitation and electron transfer processes  

NASA Astrophysics Data System (ADS)

Density functional theory (DFT) and Time Dependent DFT calculations on triphenyl sulfonium cation (TPS) and the salts of TPS with triflate, nonaflate, perfluoro-1-octanesulfonate and hexafluoro antimonate anions are presented. These systems are widely used as cationic photoinitiators and as electron ejection layer for polymer light-emitting diodes. While some differences exist in the electronic structure of the different salts, their lowest energy intense absorption maxima are calculated at nearly the same energy for all systems. The first excited state of TPS and of the TPS salts is dissociating. Electron addition to the TPS salts lowers their energy by 1.0-1.33 eV.

Petsalakis, Ioannis D.; Theodorakopoulos, Giannoula; Lathiotakis, Nektarios N.; Georgiadou, Dimitra G.; Vasilopoulou, Maria; Argitis, Panagiotis

2014-05-01

328

Scandium ion-promoted reduction of heterocyclic N=N double bond. Hydride transfer vs electron transfer.  

PubMed

Hydride transfer from 10-methyl-9,10-dihydroacridine (AcrH(2)) to 3,6-diphenyl-1,2,4,5-tetrazine (Ph(2)Tz), which contains a N=N double bond, occurs efficiently in the presence of Sc(OTf)(3) (OTf = OSO(2)CF(3)) in deaerated acetonitrile (MeCN) at 298 K, whereas no reaction occurs in the absence of Sc(3+). The observed second-order rate constant (k(obs)) increases with increasing Sc(3+) concentration to approach a limited value. When AcrH(2) is replaced by the dideuterated compound (AcrD(2)), the rate of Sc(3+)-promoted hydride transfer exhibits the same primary kinetic isotope effect (k(H)/k(D) = 5.2+/-0.2), irrespective of Sc(3+) concentration. Scandium ion also promotes an electron transfer from CoTPP (TPP(2)(-) = tetraphenylporphyrin dianion) and 10,10'-dimethyl-9,9'-biacridine [(AcrH)(2)] to Ph(2)Tz, whereas no electron transfer from CoTPP or (AcrH)(2) to Ph(2)Tz occurs in the absence of Sc(3+). In each case, the observed second-order rate constant of electron transfer (k(et)) shows a first-order dependence on [Sc(3+)] at low concentrations and a second-order dependence at higher concentrations. Such dependence of k(et) on [Sc(3+)] is ascribed to formation of 1:1 and 1:2 complexes between Ph(2)Tz(*)(-) and Sc(3+) at the low and high concentrations of Sc(3+), respectively, which results in acceleration of the rate of electron transfer. The formation of 1:2 complex has been confirmed by the ESR spectrum in which the hyperfine structure is different from that of free Ph(2)Tz(*)(-). The 1:2 complex formation results in the saturated kinetic dependence of k(obs) on [Sc(3+)] for the Sc(3+)-promoted hydride transfer, which proceeds via Sc(3+)-promoted electron transfer from AcrH(2) to Ph(2)Tz, followed by proton transfer from AcrH(2)(*)(+) to the 1:1 Ph(2)Tz(*)(-)-Sc(3+) complex and the subsequent facile electron transfer from AcrH(*) to Ph(2)TzH(*). The effects of counteranions on the Sc(3+)-promoted electron transfer and hydride transfer reactions are also reported. PMID:12381201

Fukuzumi, Shunichi; Yuasa, Junpei; Suenobu, Tomoyoshi

2002-10-23

329

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.

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

2013-01-01

330

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

PubMed

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

2013-01-22

331

Potassium surface stability and electronic promotion in K-NbN 0.9O 0.1 catalysts  

NASA Astrophysics Data System (ADS)

Potassium promoted niobium oxynitride catalysts were investigated by species resolved thermal alkali desorption (SR-TAD) method. Energetic barriers for potassium ions and atoms desorbing from the catalyst surface were determined. They are in the range of 1.0-2.3 and 2.3-3.0 eV for K and K +, respectively, depending on the way of the promoter introduction (via nascent- or post-doping). The work function (3.0-3.8 eV) was evaluated from the Schottky energy cycle. The shifts of the Fermi edge upon K doping were confirmed by parallel XPS studies. The surface stability of the promoter and its electronic action were found to be inimical. The lowering of the work function was greater for the post-doped sample, but the stability of potassium was distinctly lower than that observed in the case of the nascent one.

Kotarba, Andrzej; Adamski, Grzegorz; Sojka, Zbigniew; Djega-Mariadassou, Gerald

2000-07-01

332

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

SciTech Connect

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

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

2012-01-01

333

Type IV pili of Acidithiobacillus ferrooxidans can transfer electrons from extracellular electron donors.  

PubMed

Studies on Acidithiobacillus ferrooxidans accepting electrons from Fe(II) have previously focused on cytochrome c. However, we have discovered that, besides cytochrome c, type IV pili (Tfp) can transfer electrons. Here, we report conduction by Tfp of A. ferrooxidans analyzed with a conducting-probe atomic force microscope (AFM). The results indicate that the Tfp of A. ferrooxidans are highly conductive. The genome sequence of A. ferrooxidans ATCC 23270 contains two genes, pilV and pilW, which code for pilin domain proteins with the conserved amino acids characteristic of Tfp. Multiple alignment analysis of the PilV and PilW (pilin) proteins indicated that pilV is the adhesin gene while pilW codes for the major protein element of Tfp. The likely function of Tfp is to complete the circuit between the cell surface and Fe(II) oxides. These results indicate that Tfp of A. ferrooxidans might serve as biological nanowires transferring electrons from the surface of Fe(II) oxides to the cell surface. PMID:23440740

Li, Yongquan; Li, Hongyu

2014-03-01

334

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

335

A high-resolution electron microscopy investigation of TiO sub 2 (B)-supported vanadium oxide catalysts  

SciTech Connect

TiO{sub 2}(B) crystals were found to be isomorphic with those of its precursor, K{sub 2}TI{sub 4}O{sub 9}. The former crystals had a large number of facetted voids, 3-30 nm, formed as a result of the removal of potassium and water in preceding hydrolysis and calcination steps, respectively. TiO{sub 2}(B)-supported vanadium oxide catalysts with loadings in the range 0.25-10 theoretical layers were prepared by impregnation of the support with an oxalic acid solution of NH{sub 4}VO{sub 3} followed by calcination in air. HREM micrographs of catalysts with a low vanadium loading, recorded using a low electron-dose imaging technique, showed that the surfaces, in the initial stage, were without any anomalous surface structure. This observation may be due to a similar structure of the support and the deposited vanadia phase. At high vanadium loadings, both amorphous and crystalline particles were seen, in agreement with the features revealed by the use of IR spectroscopy. For catalysts with low vanadium loadings, the IR difference spectra showed the presence of tetrahedrally coordinated V{sup 4+} and V{sub 5+} species. In a fully converged electron beam, reduction of the support and the vanadia phases occurred, resulting in the formation of small crystallites. The catalysts were used for the oxidation of toluene to benzaldehyde. However, contrary to what has been observed for the ammoxidation producing benzonitrile, no enhanced catalytic properties, in comparison with those of crystalline V{sub 2}O{sub 5}, were obtained using TiO{sub 2}(B) as support.

Wallenberg, L.R.; Sanati, M.; Andersson, A. (Lund Institute of Technology (Sweden))

1990-11-01

336

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°.

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

2008-01-01

337

Intermolecular interactions in electron transfer through stretched helical peptides.  

PubMed

The helical peptide Cys-Ala-Lys-(Glu-Ala-Ala-Ala-Lys)(2)-Ala-NH-(CH(2))(2)-SH has been organized forming a self-assembled monolayer on gold (0.602 peptides per nm(2)), its conductance behavior under stretching conditions being studied using scanning tunnelling microscopy and current sensing atomic force microscopy. The helical conformation of the peptide has been found to play a fundamental role in the conductance. Moreover, variation of the current upon molecular stretching indicates that peptides can be significantly elongated before the conductance drops to zero, the critical elongation being 1.22 ± 0.47 nm. Molecular dynamics simulations of a single peptide in the free state and of a variable number of peptides tethered to a gold surface (i.e. densities ranging from 0.026 to 1.295 peptides per nm(2)) have indicated that the helical conformation is intrinsically favored in solvated environments while in desolvated environments it is retained because of the fundamental role played by peptide-peptide intermolecular interactions. The structure obtained for the system with 24 tethered peptides, with a density of 0.634 peptides per nm(2) closest to the experimental one, is in excellent agreement with experimental observations. On the other hand, simulations in which a single molecule is submitted to different compression and stretching processes while the rest remain in the equilibrium have been used to mimic the variation of the tip-substrate distance in experimental measures. Results allowed us to identify the existence, and in some cases coexistence, of intermolecular and intramolecular ionic ladders, suggesting that peptide-mediated electron transfer occurs through the hopping mechanism. Finally, quantum mechanical calculations have been used to investigate the variation of the electronic structure upon compression and stretching deformations. PMID:22735160

López-Pérez, Daniel E; Revilla-López, Guillermo; Jacquemin, Denis; Zanuy, David; Palys, Barbara; Sek, Slawomir; Alemán, Carlos

2012-08-01

338

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

Federal Register 2010, 2011, 2012, 2013

...and error resolution and cancellation rights, to consumers who send remittance transfers to other consumers or businesses in a foreign...and error resolution and cancellation rights, to consumers who send remittance transfers to other consumers or businesses in a...

2012-07-10

339

77 FR 6310 - Electronic Fund Transfers (Regulation E)  

Federal Register 2010, 2011, 2012, 2013

...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

340

Charge transfer emission in coumarin 343 sensitized TiOâ nanoparticle: A direct measurement of back electron transfer  

Microsoft Academic Search

Electron injection and back electron transfer dynamics in coumarin 343 (C-343) adsorbed on TiOâ 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

Hirendra Nath Ghosh

1999-01-01

341

Photoinduced electron transfer from triplet fullerene, [sup 3]C[sub 60], to tetracyanoethylene. Fourier transform electron paramagnetic resonance study  

SciTech Connect

Fourier transform EPR spectroscopy was employed in studying the electron transfer (ET) and the quenching mechanisms of the photoexcited triplet state of C[sub 60] (electron donor) in the presence of the electron acceptor tetracyanoethylene (TCNE) in a benzonitrile solution. The ET reaction product, which is the stable anion radical TCNE[sup [minus

Michaeli, S.; Meiklyar, V.; Levanon, H. (Hebrew Univ., Jerusalem (Israel)); Schulz, M.; Moebius, K. (Free Univ., Berlin (Germany))

1994-08-04

342

Electron transfer precedes ATP hydrolysis during nitrogenase catalysis  

PubMed Central

The biological reduction of N2 to NH3 catalyzed by Mo-dependent nitrogenase requires at least eight rounds of a complex cycle of events associated with ATP-driven electron transfer (ET) from the Fe protein to the catalytic MoFe protein, with each ET coupled to the hydrolysis of two ATP molecules. Although steps within this cycle have been studied for decades, the nature of the coupling between ATP hydrolysis and ET, in particular the order of ET and ATP hydrolysis, has been elusive. Here, we have measured first-order rate constants for each key step in the reaction sequence, including direct measurement of the ATP hydrolysis rate constant: kATP = 70 s?1, 25 °C. Comparison of the rate constants establishes that the reaction sequence involves four sequential steps: (i) conformationally gated ET (kET = 140 s?1, 25 °C), (ii) ATP hydrolysis (kATP = 70 s?1, 25 °C), (iii) Phosphate release (kPi = 16 s?1, 25 °C), and (iv) Fe protein dissociation from the MoFe protein (kdiss = 6 s?1, 25 °C). These findings allow completion of the thermodynamic cycle undergone by the Fe protein, showing that the energy of ATP binding and protein–protein association drive ET, with subsequent ATP hydrolysis and Pi release causing dissociation of the complex between the Feox(ADP)2 protein and the reduced MoFe protein.

Duval, Simon; Danyal, Karamatullah; Shaw, Sudipta; Lytle, Anna K.; Dean, Dennis R.; Hoffman, Brian M.; Antony, Edwin; Seefeldt, Lance C.

2013-01-01

343

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

344

Electron-transfer studies of a peroxide dianion.  

PubMed

A peroxide dianion (O2(2-)) can be isolated within the cavity of hexacarboxamide cryptand, [(O2)?mBDCA-5t-H6](2-), stabilized by hydrogen bonding but otherwise free of proton or metal-ion association. This feature has allowed the electron-transfer (ET) kinetics of isolated peroxide to be examined chemically and electrochemically. The ET of [(O2)?mBDCA-5t-H6](2-) with a series of seven quinones, with reduction potentials spanning 1 V, has been examined by stopped-flow spectroscopy. The kinetics of the homogeneous ET reaction has been correlated to heterogeneous ET kinetics as measured electrochemically to provide a unified description of ET between the Butler-Volmer and Marcus models. The chemical and electrochemical oxidation kinetics together indicate that the oxidative ET of O2(2-) occurs by an outer-sphere mechanism that exhibits significant nonadiabatic character, suggesting that the highest occupied molecular orbital of O2(2-) within the cryptand is sterically shielded from the oxidizing species. An understanding of the ET chemistry of a free peroxide dianion will be useful in studies of metal-air batteries and the use of [(O2)?mBDCA-5t-H6](2-) as a chemical reagent. PMID:24773522

Ullman, Andrew M; Sun, Xianru; Graham, Daniel J; Lopez, Nazario; Nava, Matthew; De Las Cuevas, Rebecca; Müller, Peter; Rybak-Akimova, Elena V; Cummins, Christopher C; Nocera, Daniel G

2014-05-19

345

Photoinduced excited state electron transfer at liquid/liquid interfaces.  

PubMed

Several aspects of the photoinduced electron transfer (ET) reaction between coumarin 314 (C314) and N,N-dimethylaniline (DMA) at the water/DMA interface are investigated by molecular dynamics simulations. New DMA and water/DMA potential energy surfaces are developed and used to characterize the neat water/DMA interface. The adsorption free energy, the rotational dynamics, and the solvation dynamics of C314 at the liquid/liquid interface are investigated and are generally in reasonable agreement with available experimental data. The solvent-free energy curves for the ET reaction between excited C314 and DMA molecules are calculated and compared with those calculated for a simple point charge model of the solute. It is found that the reorganization free energy is very small when the full molecular description of the solute is taken into account. An estimate of the ET rate constant is in reasonable agreement with experiment. Our calculations suggest that the polarity of the surface "reported" by the solute, as reflected by solvation dynamics and the reorganization free energy, is strongly solute-dependent. PMID:24428359

Cooper, Jason K; Benjamin, Ilan

2014-07-17

346

Electron transfer across ?-helical peptides: Potential influence of molecular dynamics  

NASA Astrophysics Data System (ADS)

Three hydrophobic leucine-rich peptides Fc18L, Ac18L and 18LAc were prepared. These peptides are equipped with a cystein sulfhydryl group which enables the formation of thin films on gold surfaces. Using these peptides, two types of films of ?-helical peptides have been prepared, in which the redox-active peptide Fc18L is diluted by Ac18L ( SAM1) or by a mixture of Ac18L and 18LAc ( SAM2). In SAM1, the dipole moments of the peptides are aligned in the same direction, whereas in SAM2, they are opposite. Reflection absorption infrared spectroscopy (RAIRS) revealed that the peptides are more vertically oriented in SAM2 compared to those in SAM1. The interaction among the macroscopic helix dipoles gives tighter packing of the peptides in SAM2. Importantly, the electron transfer properties in the two films are significantly different, which is rationalized by differences in the molecular dynamics of the two films.

Mandal, Himadri S.; Kraatz, Heinz-Bernhard

2006-07-01

347

Desulfurization of benzonaphthothiophenes and dibenzothiophene with a Raney nickel catalyst and its relationship to the. pi. -electron density  

SciTech Connect

The hydrodesulfurization of heavy petroleum feedstocks and coal-derived liquids requires the conversion of high molecular weight compounds like dibenzothiophene and benzonaphthothiophenes. There are several studies in the literature which deal with the mechanism of the hydrodesulfurization of multi-ring thiophenic compounds on cobalt or nickel molybdenum catalysts at high pressure. However, there are only a few studies which relate the chemical reactivity of these compounds to their electronic structure. The reactivity of a multi-ring sulfur-containing compound is not determined solely by the size of the molecule. In addition, others studied the relationship between the first step in the hydrotreating reaction of benzonaphthothiophene and the Coulombic interaction term of the compounds using the CNDO/S method. Because there is competition between the different processes (hydrogenation and desulfurization) during reaction, it is difficult to understand the relationship between desulfurization and the electronic properties of the compounds under reaction conditions. The calculation of electronic structures necessarily involves many sigma bonds of hydrogenated aromatic rings as well as many electrons of high molecular weight compounds. For this reason, it is best to select a catalyst and reaction conditions under which desulfurization takes place without hydrogenation.

Nagai, M.; Urimoto, H.; Uetake, K.; Sakikawa, N.; Gonzalez, R.D.

1986-09-01

348

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

349

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.

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

2011-01-01

350

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

351

Primary reactions in photosynthetic reaction centers of Rhodobacter sphaeroides - Time constants of the initial electron transfer  

NASA Astrophysics Data System (ADS)

The primary dynamics of reaction centers from Rhodobacter sphaeroides at room temperature are studied at low excitation intensities and low excitation rates. Analysis based on singular value decomposition yields three time constants in the picosecond range (ca. 1.2 ps, 3.5 ps and 220 ps). The spectral and temporal signatures are fully consistent with the step-wise electron transfer model published previously, with a first electron transfer to the bacteriochlorophyll with a time constant of 3.5 ps and a second 1.2 ps transfer to the bacteriopheophytin. No indications for adiabatic electron transfer are found in the time range >0.5 ps.

Dominguez, Pablo Nahuel; Himmelstoss, Matthias; Michelmann, Jeff; Lehner, Florian Thomas; Gardiner, Alastair T.; Cogdell, Richard J.; Zinth, Wolfgang

2014-05-01

352

Photoinduced through-bond electron transfer: Remote activation of unique aroyl azide reactivity  

SciTech Connect

Herein the authors report the first example of unique photochemical reactivity associated with through-bond electron transfer in aroyl azide substituted N-arylpiperidines. In methyl alcohol solution, light absorbed by the biphenylamine chromophore of BPA initiates reaction of an aroyl azide radical anion formed by through-bond electron transfer.

Yong Zhu; Schuster, G.B. (Univ. of Illinois, Urbana (United States))

1990-11-07

353

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

354

Functional LH1 antenna complexes influence electron transfer in bacterial photosynthetic reaction centers  

Microsoft Academic Search

The effect of the light harvesting 1 (LH1) antenna complex on the driving force for light-driven electron transfer in the Rhodobacter sphaeroidesreaction center has been examined. Equilibrium redox titrations show that the presence of the LH1 antenna complex influences the free energy change for the primary electron transfer reaction through an effect on the reduction potential of the primary donor.

Ronald W. Visschers; Simone I. E. Vulto; Michael R. Jones; Rienk van Grondelle; Ruud Kraayenhof

1999-01-01

355

Direct Electron-to-Carbon Polarization Transfer in Homogeneously Doped Polycarbonates.  

National Technical Information Service (NTIS)

A direct electron to carbon DNP solid effect polarization transfer is observed for the aromatic and carbonyl carbons of polycarbonate homogeneously doped with BDPA. The direct electron to carbon polarization transfer operates over a range of 30 to 60 A. C...

J. Schaefer M. Afeworki S. Vega

1991-01-01

356

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

357

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

358

On the role of solvent electronic polarization in charge transfer reactions  

Microsoft Academic Search

The effect of a solvent’s electronic polarization on the rate of a charge transfer reaction is studied in both continuum and discrete solvent models. An effective system Hamiltonian that contains the equilibrium solvation from the solvent electronic polarization is obtained, and leads to an effective matrix element Veff coupling the charge transfer states that is smaller than the gas phase

Jianjun Zhu; R. I. Cukier

1995-01-01

359

Photochemical reactions of electron-deficient olefins with N,N,N?,N?-tetramethylbenzidine via photoinduced electron-transfer  

NASA Astrophysics Data System (ADS)

Photoinduced electron transfer reactions of several electron-deficient olefins with N, N, N', N'-tetramethylbenzidine (TMB) in acetonitrile solution have been studied by using laser flash photolysis technique and steady-state fluorescence quenching method. Laser pulse excitation of TMB yields 3TMB* after rapid intersystem crossing from 1TMB*. The triplet which located at 480 nm is found to undergo fast quenching with the electron acceptors fumaronitrile (FN), dimethyl fumarate (DMF), diethyl fumarate (DEF), cinnamonitrile (CN), ?-acetoxyacrylonitrile (AAN), crotononitrile (CrN) and 3-methoxyacrylonitrile (MAN). Substituents binding to olefin molecule own different electron-donating/withdrawing powers, which determine the electron-deficient property (?-cloud density) of olefin molecule as well as control the electron transfer rate constant directly. The detection of ion radical intermediates in the photolysis reactions confirms the proposed electron transfer mechanism, as expected from thermodynamics. The quenching rate constants of triplet TMB by these olefins have been determined at 510 nm to avoid the disturbance of formed TMB cation radical around 475 nm. All the kqT values approach or reach to the diffusion-controlled limit. In addition, fluorescence quenching rate constants kqS have been also obtained by calculating with Stern-Volmer equation. A correlation between experimental electron transfer rate constants and free energy changes has been explained by Marcus theory of adiabatic outer-sphere electron transfer. Disharmonic kq values for CN and CrN in endergonic region may be the disturbance of exciplexs formation.

Pan, Yang; Zhao, Junshu; Ji, Yuanyuan; Yan, Lei; Yu, Shuqin

2006-01-01

360

The effect of mass transfer on the catalytic combustion of benzene and methane over palladium catalysts supported on porous materials  

Microsoft Academic Search

Catalytic combustion of benzene and methane over palladium catalysts supported on FAU and MOR zeolites and MCM-41 and KIT-1 mesoporous materials were studied to illustrate the effect of pore size and shape of supports on their catalytic activities. The palladium catalysts supported on mesoporous materials showed high activity and a steep increase in the conversion of benzene with rising temperature.

Min-Woong Ryoo; Sang-Gwi Chung; Jong-Ho Kim; Yo Soon Song; Gon Seo

2003-01-01

361

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

362

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

2014-07-01

363

The Hydrogen Catalyst Cobaloxime - a Multifrequency EPR & DFT Study of Cobaloxime's Electronic Structure  

PubMed Central

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 as 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 between structure, surroundings and catalytic activity of these complexes. To assess the strength and nature of ligand cobalt interactions, the BF2-capped cobaloxime, Co(dmgBF2)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 like methanol result in larger g-tensor anisotropy than strongly coordinating ligands like pyridine. Additionally, a coordination number effect is seen for the strongly coordinating ligands with both singly-ligated LCo(dmgBF2)2 and doubly-ligated L2Co(dmgBF2)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(dmgBF2)2 is discussed. Finally, the influence of molecular oxygen and formation of Co(III) superoxide radicals LCo(dmgBF2)2O2• is studied. The experimental results are compared with a comprehensive set of DFT calculations on Co(dmgBF2)2 model systems with various axial ligands. Comparison with experimental values for the “key” magnetic parameters like g-tensor and 59Co hyperfine coupling tensor allows the determination of the conformation of the axially ligated Co(dmgBF2)2 complexes. The data presented here are vital for understanding the influence of solvent and ligand coordination on the catalytic efficiency of cobaloximes.

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

2012-01-01

364

Probing the ultrafast electron transfer at the CuPc/Au(111) interface  

SciTech Connect

Core-hole clock spectroscopy and near-edge x-ray-absorption fine structure measurements have been used to investigate the ultrafast electron transfer dynamics at the Copper(II) phthalocyanine (CuPc)/Au(111) interface. It was found that the strong electronic coupling between the first layer of CuPc molecules and Au(111) substrate favors ultrafast electron transfer from the lowest unoccupied molecular orbital of the CuPc molecules to the conduction band of Au(111) in the time scale of {approx}6 fs. In contrast, the intermolecular electron transfer within multilayers of CuPc molecules via the weak van der Waals interaction was much slower.

Chen Wei; Wang Li; Qi Dongchen; Chen Shi; Gao Xingyu; Wee, Andrew Thye Shen [Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542 (Singapore)

2006-05-01

365

Electron transfer catalysis with monolayer protected Au25 clusters  

NASA Astrophysics Data System (ADS)

Au25L18 (L = S(CH2)2Ph) clusters were prepared and characterized. The resulting monodisperse clusters were reacted with bis(pentafluorobenzoyl) peroxide in dichloromethane to form Au25L18+ quantitatively. The kinetics and thermodynamics of the corresponding electron transfer (ET) reactions were characterized via electrochemistry and thermochemical calculations. Au25L18+ was used in homogeneous redox catalysis experiments with a series of sym-substituted benzoyl peroxides, including the above peroxide, bis(para-cyanobenzoyl) peroxide, dibenzoyl peroxide, and bis(para-methoxybenzoyl) peroxide. Peroxide dissociative ET was catalyzed using both the Au25L18/Au25L18- and the Au25L18+/Au25L18 redox couples as redox mediators. Simulation of the CV curves led to determination of the ET rate constant (kET) values for concerted dissociative ET to the peroxides. The ET free energy ?G° could be estimated for all donor-acceptor combinations, leading to observation of a nice activation-driving force (log kETvs. ?G°) relationship. Comparison with the kET obtained using a ferrocene-type donor with a formal potential similar to that of Au25L18/Au25L18- showed that the presence of the capping monolayer affects the ET rate rather significantly, which is attributed to the intrinsic nonadiabaticity of peroxide acceptors.Au25L18 (L = S(CH2)2Ph) clusters were prepared and characterized. The resulting monodisperse clusters were reacted with bis(pentafluorobenzoyl) peroxide in dichloromethane to form Au25L18+ quantitatively. The kinetics and thermodynamics of the corresponding electron transfer (ET) reactions were characterized via electrochemistry and thermochemical calculations. Au25L18+ was used in homogeneous redox catalysis experiments with a series of sym-substituted benzoyl peroxides, including the above peroxide, bis(para-cyanobenzoyl) peroxide, dibenzoyl peroxide, and bis(para-methoxybenzoyl) peroxide. Peroxide dissociative ET was catalyzed using both the Au25L18/Au25L18- and the Au25L18+/Au25L18 redox couples as redox mediators. Simulation of the CV curves led to determination of the ET rate constant (kET) values for concerted dissociative ET to the peroxides. The ET free energy ?G° could be estimated for all donor-acceptor combinations, leading to observation of a nice activation-driving force (log kETvs. ?G°) relationship. Comparison with the kET obtained using a ferrocene-type donor with a formal potential similar to that of Au25L18/Au25L18- showed that the presence of the capping monolayer affects the ET rate rather significantly, which is attributed to the intrinsic nonadiabaticity of peroxide acceptors. This article was submitted as part of a Themed Issue on metallic clusters. Other papers on this topic can be found in issue 14 of vol. 4 (2012). This issue can be found from the Nanoscale homepage [http://www.rsc.org/nanoscale].

Antonello, Sabrina; Hesari, Mahdi; Polo, Federico; Maran, Flavio

2012-08-01

366

SO 2 adsorption and thermal stability and reducibility of sulfates formed on the magnesium-aluminate spinel sulfur-transfer catalyst  

NASA Astrophysics Data System (ADS)

Magnesium-aluminate spinel used as a sulfur-transfer catalyst in the fluid catalytic cracking units for SO x emission control was prepared by the precipitation method. The crystalline structure, textural property, and surface dehydroxylation of the sample were characterized by thermogravimetry-derivative thermogravimetry (TG-DTG), differential thermal analysis (DTA), X-ray diffraction (XRD), liquid N 2 adsorption-desorption and infrared spectroscopy (IR) measurements. The behavior of SO 2 adsorption and oxidation on the surface of catalyst was evaluated with IR from 50°C to 600°C. Particularly, the thermal stability and H 2-reducibility of the formed sulfite or sulfate during SO 2 adsorption or oxidation were tested under various conditions. In the absence of oxygen in the feed mixture, weak physically adsorbed SO 2 species and surface sulfite were identified. In the case of SO 2 oxidative adsorption, both surface sulfate and bulk-like sulfate were formed. When the sulfated sample was reduced with hydrogen, the surface sulfite and sulfates were completely removed below 550°C in vacuum. The bulk-like sulfate, however, showed a high ability to resist H 2-reduction, which indicates that the reducibility of bulk-like sulfate formed on magnesium-aluminate spinel must be enhanced when it is used as a sulfur-transfer catalyst.

Wang, Jin-an; Li, Cheng-lie

2000-07-01

367

Double and single-electron transfer in H{sup +}+K collisions from 0.3 to 4keV: Separation of direct double transfer and two-step successive single-electron transfer  

Microsoft Academic Search

Double-electron transfer and two-step single-electron transfer in collisions of proton with potassium-metal target are measured in the collision energy from 0.3 to 4 keV by using a charge-inversion mass spectrometry. Two prominent H⁻ ion peaks are observed with different values of the energy loss and show different target density dependences. The peak with larger energy loss is identified as double-electron

Shigeo Hayakawa; Kazunori Kadomura; Mineo Kimura; C. M. Dutta

2004-01-01

368

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.

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

2013-01-01

369

Electron-transfer catalysis by surfactant vesicle stabilized colloidal platinum  

SciTech Connect

Preparation of the polymerized surfactant vesicle matrix and the colloidal platinum catalyst within this matrix is described. Colloidal formation and polymerization were monitored by absorption spectroscopy. The catalytic efficiency of vesicle entrapped colloidal platinum was demonstrated. Bubbling of hydrogen through a solution containing this vesicle resulted in the reduction of methylene blue and 10-methyl-5-deazaisoalloxazine-3-propanesulfonic acid. No reduction occurred in the absence of the colloidal platinum either in vesicles or in homogeneous solutions. The reduced polymer matrix can be reoxidized by the addition of ferric chloride. The feasibility of the use of these systems was demonstrated satisfactorily. 3 figures.

Kurihara, K.; Fendler, J.H.

1983-09-21

370

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

371

Kinetic resolution of racemic alpha-arylalkanoic acids with achiral alcohols via the asymmetric esterification using carboxylic anhydrides and acyl-transfer catalysts.  

PubMed

A variety of optically active carboxylic esters are produced by the kinetic resolution of racemic alpha-substituted carboxylic acids using achiral alcohols, aromatic or aliphatic carboxylic anhydrides, and chiral acyl-transfer catalysts. The combination of 4-methoxybenzoic anhydride (PMBA) or pivalic anhydride with the modified benzotetramisole-type catalyst ((S)-beta-Np-BTM) is the most effective for promotion of the enantioselective coupling reaction between racemic carboxylic acids and a novel nucleophile, bis(alpha-naphthyl)methanol, to give the corresponding esters with high ee's. This protocol was successfully applied to the production of nonracemic nonsteroidal anti-inflammatory drugs from racemic compounds utilizing the transacylation process to generate the mixed anhydrides from the acid components with the suitable carboxylic anhydrides. PMID:20681552

Shiina, Isamu; Nakata, Kenya; Ono, Keisuke; Onda, Yu-suke; Itagaki, Makoto

2010-08-25

372

Acidbase, electron-donating and magnetic properties of Nd 2 O 3 and its mixed oxides with alumina catalysts  

Microsoft Academic Search

The electron donor properties of Nd2O3 activated at 300, 500 and 800°C were investigated through studies on the adsorption of electron acceptors of various electron affinities — 7, 7, 8, 8-tetracyanoquinodimethane (2.84 eV), 2, 3, 5, 6-tetrachloro-1, 4-benzoquinone (2.40 eV), p-dinitrobenzene (1.77 eV), and m-dinitrobenzene (1.26 eV) in solvents acetonitrile and 1, 4-dioxan. The extent of electron transfer during adsorption

S. Sugunan; G. Devika Rani

1993-01-01

373

Chemical electron-transfer reactions in electrospray mass spectrometry: Effective oxidation potentials of electron-transfer reagents in methylene chloride  

SciTech Connect

Cyclic voltammetry (CV), UV/visible absorption spectroscopy, and electrospray mass spectrometry (ES-MS) are used in conjunction to study the mono- and /or dications produced in solution from the reaction of three model compounds ([beta]-carotene, cobalt(II) octaethylporphyrin (Co[sup II]OEP), nickel(II) octaethylporphyrin (Ni[sup II]OEP), in three different solvent/electron-transfer reagent systems (methylene chloride/0.1% trifluoroacetic acid (TFA) (v/v), methylene chloride/0.1% TFA/2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) v/v/200 [mu]M), methylene chloride/0.1% TFA/0.1% antimony pentafluoride (SbF[sub 5]) (v/v/v)). The reactions were carried out on-line with ES-MS by means of flow injection. Correlation of the CV data for these analytes with the ionic species determined to be in the solution on the basis of UV/visible absorption spectra and/or on the basis of the ionic species observed in the gas phase by ES-MS, along with our previously published data on these solvent/reagent systems, allowed an effective oxidation potential range, E, to be assigned to these solvent/reagent systems: methylene chloride/0.1% TFA (v/v), 0.6V [le] E[sub TFA] < 0.7 V; methylene chloride/0.1% TFA/DDQ (v/v/200 [mu]M), 0.8 [le] E[sub TFA/DDQ] < 1.0 V; methylene chloride/0.1% TFA/0.1% SbF[sub 5] (v/v/v), 1.3 [le] E[sub TFA/SbF(5)] < 1.5. 40 refs., 7 figs.

Van Berkel, G.J.; Zhou, F. (Oak Ridge National Lab., TN (United States))

1994-10-15

374

Photoinduced Electron Transfer in the C2H4--Br2 Complex  

NASA Astrophysics Data System (ADS)

We have used a new dual-nozzle late-mixing scheme for the trapping and interrogation of pre-reactive donor-acceptor complexes to examine photoinduced electron transfer in the prototypical Mulliken donor-acceptor (halogen bonded) ?-complex, C2H4--Br2. The charge transfer transition of this band was measured for the first time, and the position and intensity of this band is in excellent agreement with theoretical expectations. Excitation into the intense charge transfer band of the complex leads exclusively to the anti-conformer of the single reaction product, 1,2-dibromoethane, in agreement with the Mulliken theory of electron transfer.

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

2011-06-01

375

Photoinduced interfacial electron transfer within a mesoporous transparent conducting oxide film.  

PubMed

Interfacial electron transfer to and from conductive Sn-doped In2O3 (ITO) nanoparticles (NPs) in mesoporous thin films has been investigated by transient absorption measurements using surface-bound [Ru(II)(bpy)2(dcb)](2+) (bpy is 2,2'-bipyridyl and dcb is 4,4'-(COOH)2-2,2'-bipyridyl). Metal-to-ligand charge transfer excitation in 0.1 M LiClO4 MeCN results in efficient electron injection into the ITO NPs on the picosecond time scale followed by back electron transfer on the nanosecond time scale. Rates of back electron transfer are dependent on thermal annealing conditions with the rate constant increasing from 1.8 × 10(8) s(-1) for oxidizing annealing conditions to 8.0 × 10(8) s(-1) for reducing conditions, presumably due to an enhanced electron concentration in the latter. PMID:24460093

Farnum, Byron H; Morseth, Zachary A; Lapides, Alexander M; Rieth, Adam J; Hoertz, Paul G; Brennaman, M Kyle; Papanikolas, John M; Meyer, Thomas J

2014-02-12

376

The effects of friction on the electron transfer of 9,10-dicyanoanthracene  

NASA Astrophysics Data System (ADS)

We have used the the bond energy?bond order (BEBO) method to approximate the shape of the barrier to electron transfer and combined it with the Kramers relation and with the Grote-Hynes relation to demonstrate a possible role of friction in electron transfer reactions. Using our own data and that of Abdullah and Kemp we have shown that not only can friction be considered in electron transfer reactions, but that this friction might be the non-markovian frequency-dependent friction derivable from the generalized Langevin equation. From these results we have estimated possible limits of the frequency-dependent friction for the electron transfer from 9,10-dicyanoanthracene to electron acceptors.

Statman, David; Kovner, Marcia; Shea, James C.

1987-07-01

377

Dispersion solute-solvent coupling in electron transfer reactions. I. Effective potential  

Microsoft Academic Search

Theories of electron transfer (ET) reactions and optical spectra in condensed phases consider electronic transitions between instantaneous Born-Oppenheimer energies of the intramolecular electronic states which depend on the system nuclear configuration. With the aim of constructing a molecular description of the solvent effect on these phenomena, we consider in the present paper a system composed of a polar polarizable solute

Dmitry V. Matyushov; Branka M. Ladanyi

1998-01-01

378

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

379

Double- and single-electron transfer in H{sup +}+K collisions from 0.3 to 4keV: Separation of direct double transfer and two-step successive single-electron transfer  

SciTech Connect

Double-electron transfer and two-step single-electron transfer in collisions of proton with potassium-metal target are measured in the collision energy from 0.3 to 4 keV by using a charge-inversion mass spectrometry. Two prominent H{sup -} ion peaks are observed with different values of the energy loss and show different target density dependences. The peak with larger energy loss is identified as double-electron transfer and the other as two-step successive single-electron transfer from the analysis of the target density dependence. The two-step single-electron transfer is considered to occur as the process accompanying spontaneous Ly-{alpha} emission, followed by negative H{sup -} formation. A theoretical analysis is also carried out, and the single-electron transfer cross section obtained is found to be in excellent agreement with the present measurement, while the present measurement for double-electron transfer is found to be much smaller than those evaluated earlier and the present theory.

Hayakawa, Shigeo; Kadomura, Kazunori; Kimura, Mineo; Dutta, C.M. [College of Integrated Arts and Sciences, Osaka Prefecture University, Gakuen-cho 1-1, Sakai, Osaka, 599-8531 (Japan); Graduate School of Sciences, Kyushu University, Hakozaki, Fukuoka 812-8581 (Japan); Department of Physics, Rice University, Houston, Texas 77501 (United States)

2004-08-01

380

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. © 2014 Wiley Periodicals, Inc. PMID:24752384

Ekesan, Solen; Kale, Seyit; Herzfeld, Judith

2014-05-01

381

Influence of ionic strength on triplet-state natural organic matter loss by energy transfer and electron transfer pathways.  

PubMed

Triplet state excited natural organic matter chromophores ((3)NOM*) are important reactive intermediates in indirect photochemical processes, yet the impact of salt concentrations relevant to estuarine and marine environments on (3)NOM* is poorly understood. The formation rates, pseudo-first-order loss rate constants, and steady-state concentration of (3)NOM* were monitored using the sorbate probe method in synthetic matrices with increasing ionic strength (IS) to seawater values using seawater halides or other salts. The steady-state concentration of (3)NOM* approximately doubled at seawater IS, regardless of the salt used, due to a decrease in the (3)NOM* decay rate constant. The electron transfer-mediated degradation of 2,4,6-trimethylphenol (TMP) by (3)NOM* was significantly slowed at higher IS. A model is proposed wherein high IS slows intra-organic matter electron transfer pathways, an important (3)NOM* loss pathway, leading to longer (3)NOM* lifetimes. Although IS did not appear to impact energy transfer pathways directly, the higher (3)NOM* steady-state concentrations promote energy transfer interactions. The observed decrease in decay rate constant, increase in steady-state concentration of (3)NOM* at high IS, and the inhibition of electron transfer pathways should be considered when determining the fate of organic pollutants in estuarine and marine environments. PMID:23952218

Parker, Kimberly M; Pignatello, Joseph J; Mitch, William A

2013-10-01

382

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

383

Electron- and hydride-transfer reactivity of an isolable manganese(V)-oxo complex.  

PubMed

The electron-transfer and hydride-transfer properties of an isolated manganese(V)?oxo complex, (TBP8Cz)Mn(V)(O) (1) (TBP8Cz = octa-tert-butylphenylcorrolazinato) were determined by spectroscopic and kinetic methods. The manganese(V)?oxo complex 1 reacts rapidly with a series of ferrocene derivatives ([Fe(C5H4Me)2], [Fe(C5HMe4)2], and ([Fe(C5Me5)2] = Fc*) to give the direct formation of [(TBP8Cz)Mn(III)(OH)]? ([2-OH]?), a two-electron-reduced product. The stoichiometry of these electron-transfer reactions was found to be (Fc derivative)/1 = 2:1 by spectral titration. The rate constants of electron transfer from ferrocene derivatives to 1 at room temperature in benzonitrile were obtained, and the successful application of Marcus theory allowed for the determination of the reorganization energies (?) of electron transfer. The ? values of electron transfer from the ferrocene derivatives to 1 are lower than those reported for a manganese(IV)?oxo porphyrin. The presumed one-electron-reduced intermediate, a Mn(IV) complex, was not observed during the reduction of 1. However, a Mn(IV) complex was successfully generated via one-electron oxidation of the Mn(III) precursor complex 2 to give [(TBP8Cz)Mn(IV)]+ (3). Complex 3 exhibits a characteristic absorption band at ?(max) = 722 nm and an EPR spectrum at 15 K with g(max)? = 4.68, g(mid)? = 3.28, and g(min)? = 1.94, with well-resolved 55Mn hyperfine coupling, indicative of a d3 Mn(IV)S = 3/2 ground state. Although electron transfer from [Fe(C5H4Me)2] to 1 is endergonic (uphill), two-electron reduction of 1 is made possible in the presence of proton donors (e.g., CH3CO2H, CF3CH2OH, and CH3OH). In the case of CH3CO2H, saturation behavior for the rate constants of electron transfer (k(et)) versus acid concentration was observed, providing insight into the critical involvement of H+ in the mechanism of electron transfer. Complex 1 was also shown to be competent to oxidize a series of dihydronicotinamide adenine dinucleotide (NADH) analogues via formal hydride transfer to produce the corresponding NAD+ analogues and [2-OH]?. The logarithms of the observed second-order rate constants of hydride transfer (k(H)) from NADH analogues to 1 are linearly correlated with those of hydride transfer from the same series of NADH analogues to p-chloranil. PMID:21218824

Fukuzumi, Shunichi; Kotani, Hiroaki; Prokop, Katharine A; Goldberg, David P

2011-02-16

384

Two-dimensional free energy surfaces for primary electron transfer in a photosynthetic reaction center  

NASA Astrophysics Data System (ADS)

Two-dimensional free energy surfaces for primary electron transfer in a bacterial photosynthetic reaction center are constructed from the MD simulation results by Warshel et al. [J. Photochem. Photobiol. A: Chem 82 (1994) 123] on the same system by choosing the reaction coordinates appropriately. The activation energies for the direct and sequential electron transfer are calculated using these surfaces. One needs these two-dimensional free energy surfaces to analyze the dynamical solvent effect on the competition between the direct and sequential electron transfer properly, although this effect has so far been analyzed by using one-dimensional free energy curves.

Fushiki, M.; Tachiya, M.

1996-06-01

385

Performance of alumina-supported Pt catalysts in an electron-beam-sustained CO2 laser amplifier  

NASA Technical Reports Server (NTRS)

The performance of an alumina-supported Pt catalyst system used to maintain the gas purity in an electron-beam-sustained (636) isotope CO2 laser amplifier has been tested. The system characteristics using the two-zone, parallel flow reactor were determined for both continuous- and end-of-day reactor operation using on-line mass spectrometric sampling. The laser amplifier was run with an energy loading of typically 110 J-l/atm and an electron-beam current of 4 mA/sq cm. With these conditions and a pulse repetition frequency of 10 Hz for up to 10,000 shots, increases on the order of 100 ppm O2 were observed with the purifier on and 150 ppm with it off. The 1/e time recovery time was found to be approximately 75 minutes.

Cunningham, D. L.; Jones, P. L.; Miyake, C. I.; Moody, S. E.

1990-01-01

386

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

387

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

388

On the connection of semiclassical instanton theory with Marcus theory for electron transfer in solution.  

PubMed

We present a derivation of Marcus theory of electron transfer in solution starting from semiclassical instanton theory. The conventional semiclassical instanton theory provides an inadequate description of the electron transfer process in the inverted Marcus regime. This has been attributed to the lack of backscattering in the product region, which is represented as a semi-infinite continuum of states. For electron transfer processes in condensed phase, the electronic states in the acceptor well are bound, which violates the continuum assumption. We show by detailed analysis of the minimum action path of a model system for electron transfer that the proper tunneling coordinate is a delocalized, "bead-count" mode. The tunneling mode is analytically continued in the complex plane as in the traditional derivation. Unlike the traditional analysis where the method of steepest descent is used, the tunneling coordinate is treated as a quasi-zero mode. This feature allows including the influence of backscattering in the acceptor well and leads to the recovery of the Marcus formula for the rate of electron transfer. The results have implications on the performance of ring polymer molecular dynamics for the study of electron transfer dynamics. PMID:23781778

Shushkov, Philip

2013-06-14

389

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

390

On the connection of semiclassical instanton theory with Marcus theory for electron transfer in solution  

SciTech Connect

We present a derivation of Marcus theory of electron transfer in solution starting from semiclassical instanton theory. The conventional semiclassical instanton theory provides an inadequate description of the electron transfer process in the inverted Marcus regime. This has been attributed to the lack of backscattering in the product region, which is represented as a semi-infinite continuum of states. For electron transfer processes in condensed phase, the electronic states in the acceptor well are bound, which violates the continuum assumption. We show by detailed analysis of the minimum action path of a model system for electron transfer that the proper tunneling coordinate is a delocalized, 'bead-count' mode. The tunneling mode is analytically continued in the complex plane as in the traditional derivation. Unlike the traditional analysis where the method of steepest descent is used, the tunneling coordinate is treated as a quasi-zero mode. This feature allows including the influence of backscattering in the acceptor well and leads to the recovery of the Marcus formula for the rate of electron transfer. The results have implications on the performance of ring polymer molecular dynamics for the study of electron transfer dynamics.

Shushkov, Philip [Department of Chemistry, Yale University, New Haven, Connecticut 06520 (United States)

2013-06-14

391

The Mechanism and Properties of Electron Transfer in the Biological Organism  

NASA Astrophysics Data System (ADS)

The mechanism and properties of electron transfer along protein molecules at finite temperature T ? 0 in the life systems are studied using nonlinear theory of bio-energy transport and Green function method, in which the electrons are transferred from donors to acceptors in virtue of the supersound soliton excited by the energy released in ATP hydrolysis. The electron transfer is, in essence, a process of oxidation-reduction reaction. In this study we first give the Hamiltonian and wavefunction of the system and find out the soliton solution of the dynamical equation in the protein molecules with finite temperature, and obtain the dynamical coefficient of the electron transfer. The results show that the speed of the electron transfer is related to the velocity of motion of the soliton, distribution of electrons in the donor and acceptor as well as the interaction strength among them. We finally concluded the changed rule of electric current, arising from the electron transfer, with increasing time. These results are useful in molecular and chemical biology.

Pang, Xiao-Feng

2013-08-01

392

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

393

Slow electron transfer rates for fluorinated cobalt porphyrins: electronic and conformational factors modulating metalloporphyrin ET.  

PubMed

The electron transfer (ET) properties of a series of closely related cobalt porphyrins, [2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato]cobalt, CoF(28)TPP, [2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetraphenyl)porphyrinato]cobalt, CoF(8)TPP, 5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato]cobalt, CoF(20)TPP, and [5,10,15,20-tetraphenylporphyrinato]cobalt, CoTPP, were investigated by cyclic voltammetry, cyclic voltammetric digital simulation, in situ UV-vis and IR spectroelectrochemistry, kinetic ET studies, bulk electrolysis, (19)F NMR spectroscopy, X-ray crystallography, and molecular modeling. In benzonitrile containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF(6)) as supporting electrolyte, the ET rate constants for the Co(2+/3+) redox couples were found to be strongly substituent dependent; the heterogeneous ET rate constant (k(el)) varied by a factor of 10(4), and the ET self-exchange rate constants (k(ex)) varied over 7 orders of magnitude for the compounds studied. The remaining observed ring oxidation and metal and ring reduction events exhibited nearly identical k(el) values for all compounds. UV-vis and IR spectroelectrochemistry, bulk electrolysis, and (19)F NMR spectroscopic studies support attribution of different ET rates to widely varying inner sphere reorganization energies (lambda(i)) for these closely related compounds. Structural and semiempirical (PM3) studies indicate that the divergent kinetic behavior of CoTPP, CoF(8)TPP, CoF(20)TPP, and CoF(28)TPP first oxidations arises mainly from large nuclear reorganization energies primarily associated with core contraction and dilation. Taken together, these studies provide rational design principles for modulating ET rate constants in cobalt porphyrins over an even larger range and provide strategies for similar manipulation of ET rates in other porphyrin-based systems: substituents that lower C-C, C-N, and N-M vibrational frequencies or minimize porphyrin orbital overlap with the metal-centered orbital undergoing a change in electron population will increase k(ET). The heme ruffling apparent in electron transfer proteins such as cytochrome c is interpreted as nature's exploitation of this design strategy. PMID:12971774

Sun, Haoran; Smirnov, Valeriy V; DiMagno, Stephen G

2003-09-22

394

Design of highly active iron-based catalysts for atom transfer radical polymerization: tridentate salicylaldiminato ligands affording near ideal Nernstian behavior.  

PubMed

Iron(II) complexes bearing monoanionic tridentate salicyladiminato ligands are shown to be highly efficient catalysts for atom transfer radical polymerization (ATRP). Polymerization rates for styrene are among the highest reported for iron-mediated ATRP in nonpolar media, correlating well with E1/2 potentials and DeltaEp values for the complexes. The rigidity of the tridentate ligands, combined with ample space around the metal center to accommodate a halogen atom, we believe to be an important factor in the efficient ATRP behavior of these systems. PMID:12848547

O'Reilly, Rachel K; Gibson, Vernon C; White, Andrew J P; Williams, David J

2003-07-16

395

Kinetic resolution of racemic 2-hydroxy-?-butyrolactones by asymmetric esterification using diphenylacetic acid with pivalic anhydride and a chiral acyl-transfer catalyst.  

PubMed

Various optically active 2-hydroxy-?-butyrolactone derivatives are produced via the kinetic resolution of racemic 2-hydroxy-?-butyrolactones with diphenylacetic acid using pivalic anhydride and (R)-benzotetramisole ((R)-BTM), a chiral acyl-transfer catalyst. Importantly, the substrate scope of this novel protocol is fairly broad (12 examples, s-value; up to over 1000). In addition, we succeeded in disclosing the reaction mechanism to afford high enantioselectivity using theoretical calculations and expounded on the substituent effects at the C-3 positions in 2-hydroxylactones. PMID:23461674

Nakata, Kenya; Gotoh, Kouya; Ono, Keisuke; Futami, Kengo; Shiina, Isamu

2013-03-15

396

Electron localization function study on intramolecular electron transfer in the QTTFQ and DBTTFI radical anions.  

PubMed

The unsymmetrical distribution of the unpaired electron in the ground state of the DBTTFI(•-) radical anion (bi(6-n-butyl-5,7-dioxo-6,7-dihydro-5H-[1,3]dithiolo[4,5-f]isoindole-2-ylidene) is theoretically predicted using the M06-2X/6-31+G(d,p) level of calculations. The results are additionally confirmed by single point calculations at B3LYP/aug-cc-pVTZ, LC-?PBE/aug-cc-pVTZ, and M06-2X/aug-cc-pVTZ levels. DBTTFI, containing the TTF (tetrathiafulvalene) fragment, may be used in the construction of organic microelectronic devices, similarly to the radical anion of QTTFQ. The unsymmetrical distribution of spin density in (QTTFQ)(•-) has been confirmed using M06-2X/aug-cc-pVTZ calculations, with subsequent study using topological analysis of electron localization function (ELF). The reorganization of the chemical bonds during intramolecular electron transfer in (QTTFQ)(•-) and (DBTTFI)(•-) has been analyzed using bonding evolution theory (BET). The reaction path has been simulated by the IRC procedure, and the evolution of valence basins has been described using catastrophe theory. The simple mechanisms: (QTTFQ)(•-): ?-1-3-CC(+)-0: (-•)(QTTFQ) and (DBTTFI)(•-): ?-1-3-[F](4)[F(+)](4)-0: (-•)(DBTTFI), each consisting of three steps, have been observed. Two cusp or 4-fold catastrophes occur immediately after the TS. Our study shows that potential future microelectronic devices, constructed on the basis of the (QTTFQ)(•-) and (DBTTFI)(•-) systems, should exploit the properties of the C?C bond. PMID:22029410

Kalinowski, Jaroslaw; Berski, Slawomir; Gordon, Agnieszka J

2011-11-24

397

Extracellular electron transfer of a highly adhesive and metabolically versatile bacterium.  

PubMed

Bacterial adhesion to a solid plays a predominant role in mediating the extracellular electron transfer for genus Acinetobactor, a metabolically versatile bacterium that can couple toluene degradation and electricity generation. PMID:23813865

Liu, Huan; Ishikawa, Masahito; Matsuda, Shoichi; Kimoto, Yuki; Hori, Katsutoshi; Hashimoto, Kazuhito; Nakanishi, Shuji

2013-08-01

398

Non-Markovian electron transfer reactions with frequency-dependent friction  

SciTech Connect

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

Tang, J.

1993-12-31

399

The Legal Environment for the Sharing of Electronic Fund Transfers (EFT) Systems.  

National Technical Information Service (NTIS)

A number of important antitrust issues have arisen with regard to the development of EFT (Electronic Funds Transfer). This is particularly true when the EFT projects are sponsored by the largest and most powerful institutions in a particular market. Some ...

1977-01-01

400

An adaptive technique to improve wireless power transfer for consumer electronics  

Microsoft Academic Search

This paper presents an adaptive technique to improve an efficiency of wireless power transfer system for future portable consumer electronics. By changing a coupling coefficient between coils only in the transmitting side, the system performance increases significantly.

Huy Hoang; Seunggyu Lee; Youngsu Kim; Yunho Choi; Franklin Bien

2012-01-01

401

Reaction Rates in the Phenomenological Adiabatic Excited State Electron Transfer Theory.  

National Technical Information Service (NTIS)

The recently described phenomenological model strongly adiabatic electron transfer (et) in the lowest excited state S1 of donor/acceptor molecules has been developed in more detail. The model has been generalized to include simultaneously two types of ele...

G. C. Walker K. Tominaga P. F. Barbara T. Fonseca T. J. Kang

1991-01-01

402

Facilitation of Electron Transfer in the Presence of Mitochondria-Targeting Molecule SS31  

NASA Astrophysics Data System (ADS)

Electron transfer (ET) processes in mitochondria are very important for the production of adenosine triphosphate (ATP), the common source of the chemical energy. The inability to transfer electrons efficiently in mitochondrial ET chain plays a major role in age associated diseases, including diabetes and cancer. In this work, we used the time dependent absorption and photoluminescence spectroscopy to study the electron transfer kinetics along the ET chain of mitochondria. Our spectroscopic results suggest that SS31, a small peptide molecule targeting to the mitochondrial inner membrane, can facilitate electron transfer and increase ATP production. We show that SS31 targets cytochrome c to both increase the availability of state and also potentially reduce the energy barrier required to reduce cytochrome c.

Nosach, Tetiana; Ebrahim, Mark; Ren, Yuhang; Darrah, Shaun; Szeto, Hazel

2010-03-01

403

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

PubMed

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. PMID:24059515

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

2013-10-01

404

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

Code of Federal Regulations, 2013 CFR

...maintained by or for a Federal, national, or international standards organization. Acceptable transfer formats include the Geography Markup Language (GML) as defined by the Open GIS Consortium. (d) Textual documents . Electronic textual...

2013-07-01

405

Electron Transfer Experiments and Atomic Magnetism Values. Progress Report, February 1, 1975--September 30, 1975.  

National Technical Information Service (NTIS)

Progress in the first seven months of this new research is described. A new apparatus was constructed, tested and moved to Oak Ridge National Laboratory for studies using the Penning ion source test facility. Preliminary electron transfer cross section re...

J. E. Bayfield V. W. Hughes

1975-01-01

406

Photoinduced electron transfer in ordered polymers. Progress report, May 1, 1988-October 31, 1990.  

National Technical Information Service (NTIS)

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 are...

G. Jones

1990-01-01

407

Double Layer Effects on the Enthalpy of Activation of Heterogeneous Electron Transfer Reactions.  

National Technical Information Service (NTIS)

The contributions to the experimental enthalpy of activation due to temperature variation in the double layer effect and the pre-exponential factor of the rate constant are considered in detail for heterogeneous electron transfer reactions. It is shown th...

W. R. Fawcett Z. Kovacova

1990-01-01

408

Photoinduced electron transfer in GFP\\/viologen\\/TCNQ structured hetero-LB film  

Microsoft Academic Search

The photoinduced electron transfer rate in hetero-Langmuir–Blodgett (LB) films, composed of electron sensitizing protein and electron accepting organics, was investigated. The hetero-LB film was fabricated by sequentially depositing green fluorescence protein (GFP), viologen, and TCNQ, which functioned as an electron sensitizer, a relay, and an acceptor, respectively. To verify the hetero-LB film formation, surface morphologies of the GFP film and

Jeong-Woo Choi; Yun-Suk Nam; Hyun-Goo Choi; Won Hong Lee; Dongho Kim; Masamichi Fujihira

2002-01-01

409

Electronic speckle pattern shearing interferometry for determining free convection heat transfer coefficient  

NASA Astrophysics Data System (ADS)

An electronic speckle-pattern shearing interferometer (ESPSI) is proposed for measuring the free convection heat transfer coefficient in liquids. The heat transfer coefficient may be deduced by a simple manipulation of the speckle patterns. Theory of the method as well as its application are presented. The method is robust and easy to use for non-skilled operators.

Schirripa Spagnolo, G.; Ambrosini, D.; Paoletti, D.

1999-06-01

410

Nobel lecture. A structural basis of light energy and electron transfer in biology.  

PubMed Central

Aspects of intramolecular light energy and electron transfer will be discussed for three protein--cofactor complexes, whose three-dimensional structures have been elucidated by X-ray crystallography: components of light-harvesting cyanobacterial phycobilisomes; the purple bacterial reaction centre; and the blue multi-copper oxidases. A wealth of functional data is available for these systems which allows specific correlations between structure and function and general conclusions about light energy and electron transfer in biological materials to be made. Images

Huber, R

1989-01-01

411

Electron Transfer between the Quinones in the Photosynthetic Reaction Center and Its Coupling to Conformational Changes †  

Microsoft Academic Search

The electron transfer between the two quinones QA and QB in the bacterial photosynthetic reaction center (bRC) is coupled to a conformational rearrangement. Recently, the X-ray structures of the dark-adapted and light-exposed bRC from Rhodobacter sphaeroideswere solved, and the conformational changes were characterized structurally. We computed the reaction free energy for the electron transfer from QA ¥- to QB in

Björn Rabenstein; G. Matthias Ullmann; Ernst-Walter Knapp

2000-01-01

412

Electron-transfer mechanism of the triplet state quenching of aluminium tetrasulfonated phthalocyanine by cytochrome c  

Microsoft Academic Search

The mechanism of electron-transfer from aluminium tetrasulfonated phthalocyanine triplet state to cytochrome c was investigated in this work. This reaction successfully quenches the dye triplet state due to the formation of complexes between the solute and the protein at the active site. The electron-transfer rate constant is around 3×107 s?1, and is in accordance with previous results for the singlet

César A. T. Lai; L. F. Vieira Ferreira

2006-01-01

413

Effects of heterocyclic and tertiary permeant amines on the electron transfer in thylakoid membranes  

Microsoft Academic Search

The effect of low concentrations (up to 50 ?M) of lipophilic permeant amines on the electron transfer in thylakoid membranes\\u000a of pea chloroplasts has been investigated. In the presence of heterocyclic amines (9-aminoacridine and neutral red), the electron\\u000a transfer, initiated from H2O to PS I acceptors, has been shown to be inhibited to a level amounting to less than 50%

Vera Opanasenko; Alexey Agafonov; Raissa Demidova

2002-01-01

414

Charge Transfer in Fullerene-Conducting Polymer Compositex: Electronic and Excitonic Properties  

Microsoft Academic Search

C60 doping into conducting polymer with highly extended ?-electron system in the main chain induces remarkable quenching of photoluminescence in conducting polymer and drastic enhancement of photoconductivity. These results can be explained in terms of photo-induced charge transfer between conducting polymer and C60. That is, photoexcited excitons or exciton-polarons on conducting polymer are effectively dissociated at C60 molecules transferring electrons

Katsumi Yoshino; Kazuya Tada; Akihiko Fujii; Kazuhisa Hosoda; Shin-ichi Kawabe; Hirotake Kajii; Masaharu Hirohata; Rahmat Hidayat; Hisashi Araki; Anvar A. Zakhidov; Ryu-ichi Sugmoto; Masahiko Iyoda; Mitsuo Ishikawa; Toshio Masuda

1997-01-01

415

Double exchange in tetrameric tetrahedral clusters with two-electron transfer: magnetic properties  

Microsoft Academic Search

A theoretical model is worked out to study the magnetic properties of mixed-valence tetrameric tetrahedral clusters dn-dn-dn+1-dn+1 (n = 0–4) with two-electron transfer. To calculate the energy levels of systems a computational procedure based on the second quantization technique and group theory approach is used. The exchange-tunnel states of dn-dn-dn+1-dn+1 (n = 0–4) tetrahedral tetrameric clusters with two-electron transfer are

V. P. Coropceanu; F. G. Paladi; S. I. Boldyrev; V. J. Gamurar

1997-01-01

416

SYNTHESIS AND INTRAMOLECULAR EXCITED STATE ELECTRON TRANSFER OF [Co(NH3)5CEPHALEXIN] COMPLEX  

Microsoft Academic Search

[Co(NH3)5-cephalexin] has been synthesized and characterized by elemental analyses, IR and UV-Visible spectroscopy as well as photochemical and thermal studies. The electronic spectrum of the complex in DMF shows two bands at 358 and 267 nm, which are assigned to intraligand charge transfer and ?-?* transitions, respectively. The fluorescence of the cephalexin is quenched by excited state electron transfer to Co(III).

Ahmed H. Osman

2002-01-01

417

Detergent modulation of electron and proton transfer reactions in bovine cytochrome c oxidase  

Microsoft Academic Search

The effect of detergents on electron and proton transfer in bovine cytochrome c oxidase was studied using steady-state and transient-state methods. Cytochrome c oxidase in lauryl maltoside has high maximal turnover (TNmax=400s?1), whereas activity is low (TNmax=10s?1) in Triton X-100. Single turnover studies of intramolecular electron transfer show similar rates in either detergent. Transient proton uptake experiments show the oxidase

Michael Tarasev; Bruce C Hill

2002-01-01

418

The Roles of Outer Membrane Cytochromes of Shewanella and Geobacter in Extracellular Electron Transfer  

Microsoft Academic Search

As key components of the electron transfer (ET) pathways used for dissimilatory reduction of solid iron [Fe(III)] and manganese [Mn(IV)] (hydr)oxides, outer membrane cytochromes MtrC and OmcA of Shewanella oneidensis MR-1 and OmcE and OmcS of Geobacter sulfurreducens mediate ET reactions extracellularly. Cell surface-exposed MtrC and OmcA can transfer electrons directly to the metal oxides. S. oneidensis MR-1 cells also

Liang Shi; David J. Richardson; Zheming Wang; Sebastien N. Kerisit; Kevin M. Rosso; John M. Zachara; James K. Fredrickson

2009-01-01

419

The electrochemical approach to concerted proton--electron transfers in the oxidation of phenols in water  

PubMed Central

Establishing mechanisms and intrinsic reactivity in the oxidation of phenol with water as the proton acceptor is a fundamental task relevant to many reactions occurring in natural systems. Thanks to the easy measure of the reaction kinetics by the current and the setting of the driving force by the electrode potential, the electrochemical approach is particularly suited to this endeavor. Despite challenging difficulties related to self-inhibition blocking the electrode surface, experimental conditions were established that allowed a reliable analysis of the thermodynamics and mechanisms of the proton-coupled electron-transfer oxidation of phenol to be carried out by means of cyclic voltammetry. The thermodynamic characterization was conducted in buffer media whereas the mechanisms were revealed in unbuffered water. Unambiguous evidence of a concerted proton–electron transfer mechanism, with water as proton acceptor, was thus gathered by simulation of the experimental data with appropriately derived theoretical relationships, leading to the determination of a remarkably large intrinsic rate constant. The same strategy also allowed the quantitative analysis of the competition between the concerted proton–electron transfer pathway and an OH?-triggered stepwise pathway (proton transfer followed by electron transfer) at high pHs. Investigation of the passage between unbuffered and buffered media with the example of the PO4H2?/PO4H2? couple revealed the prevalence of a mechanism involving a proton transfer preceding an electron transfer over a PO4H2?-triggered concerted process.

Costentin, Cyrille; Louault, Cyril; Robert, Marc; Saveant, Jean-Michel

2009-01-01

420