Net Electron Transfer Research Articles

DFT Studies on the Reduction of Dinitrogen to Ammonia by a Thiolate-Bridged Diiron Complex as a Nitrogenase Mimic

We have recently reported that the binuclear iron complexes Cp*Fe(μ-SR1)2(μ,η2-R2N═NH)FeCp* (R1 = Me, Et; R2 = Me, Ph; Cp* = η5-C5Me5) as novel models of nitrogenase could effectively catalyze the N–N bond cleavage of hydrazines, including NH2NH2 (J. Am. Chem. Soc. 2008, 130, 15250−15251). However, the mechanistic aspects involved in the catalytic cycle and the possibility of reducing N2 by these complexes have remained unexplored. In the present study, DFT has been applied for modeling the binding of Cp*Fe(μ-SEt)2FeCp* with N2 and the reduction of the N2 to two NH3 molecules at the diiron centers. The calculations of model system indicate that the hydrogenation (H+ + e–) energetically prefers to occur at the N atoms rather than the Fe or S atoms. The rate-determining step of the reduction of HN═NH could be the isomerization of a μ,η2-HN–NH2 moiety to a μ-HN–NH2 form with a NH-bridging feature, which occurred at the diiron centers. Such a transformation of the binding mode of HN–NH2 might be driven by unequal charge populations on the two Fe atoms. The results show an event of net electron transfer from the ancillary ligands to the Fe atoms and the NHNH2 moiety during the rate-determining step. In view of the experimental observations reported previously, the current computations suggest that the diiron complex Cp*Fe(μ-SEt)2FeCp* is possible to bind N2 and reduce it to NH3 via protonation/reduction. Such a reduction of N2 to NH3 at the diiron centers favorably occurs through the HNNH and HNNH2 forms rather than via the H2NNH2 unit.

First-principle investigation of Jahn–Teller distortion and topological analysis of chemical bonds in LiNiO 2

First-principle GGA+U calculations were performed on the undistorted rhombohedral R3̄ m model, the collinear Jahn–Teller distorted monoclinic C2/ m model, and six non-collinear Jahn–Teller distortion ordering models of LiNiO 2. The zigzag and C2/ m models are found to be the most stable and the next most stable structural models, respectively. An energy gap appears for the C2/ m and zigzag structures, whereas no energy gap appears for the R3̄ m structure. Topological analyses were performed on the R3̄ m, C2/ m and zigzag models using the atoms-in-molecules theory and the electron localization function. The results show that the Ni–O interaction is the transit closed-shell interaction, in which the net electron transfer occurs from the Ni ion to the ligand O ions. The Ni–O bond possesses the σ dative bond character and is polarized toward the O ions. In the distorted structures, the bonding electrons around the oxygen atom are strongly polarized toward the long Ni–O bond.

Identification of magnetic minerals related to hydrocarbon authigenesis in venezuelan oil fields using an alternative decomposition of isothermal remanence curves

In this work we try to better characterise the shallow magnetic signature of hydrocarbon microseepage in oil fields from eastern and western Venezuela. To get a better insight of the processes involved, we attempt to find out the main magnetic phases responsible for the observed oil-related shallow magnetic anomalies. In this way, a new and alternative numerical approach to decompose Isothermal Remagnetization (IRM) curves is introduced. The method is based on a Direct Signal Analysis (DSA) of the IRM curve in order to identify the number and type of magnetic components. Representative wells from western (La Victoria) and eastern (San Joaquin) Venezuelan fields are studied. The DSA approach, together with rock magnetic experimental results, indicates that in the well from western Venezuela the main magnetic mineralogy associated to hydrocarbon microseepage is magnetite. Conversely, in the well from eastern Venezuela, these MS anomalies are mainly caused by the presence of Fe-sulphides (i.e. greigite). These results support the hypothesis of two different authigenic processes. For the well at the western field, we propose that a net electron transfer from the organic matter, degraded by hydrocarbon gas leakage, should occur precipitating Fe(II) magnetic minerals (e.g. magnetite). On the other hand, in the well of the eastern field, high concentrations of H2S at shallow levels, might allow the formation of secondary Fe-sulphides.

External field control of charge transmission through single molecules: Switching effects and transient currents

Net electron transfer through a single molecule attached to nano electrodes can undergo a pronounced change by resonant optical excitation. Caused by the population of excited electronic states of the molecule new transmission channels are opened which alter the current–voltage characteristics of the junction. This recent suggestion is further investigated here by studying the transient behavior of the current if the external laser pulse excitation is switched-on and -off within a certain time-interval. The computations concentrate on the case of weak and intermediate molecule–lead coupling. They are carried out in using rate equations which govern the populations of the different electron-vibrational states of the molecule being in its neutral or charged state. The interrelation of the switching-time and the time of charging and discharge of the molecule as well as of the time of vibrational relaxation is demonstrated by working in a switching-time range of some hundreds fs up to about 50ps.

Electronic Quantum Fluxes during Pericyclic Reactions Exemplified for the Cope Rearrangement of Semibullvalene

The outcome of a pericyclic reaction is typically represented by arrows in the Lewis structure of the reactant, symbolizing the net electron transfer. Quantum simulations can be used to interpret these arrows in terms of electronic fluxes between neighboring bonds. The fluxes are decomposed into contributions from electrons in so-called pericyclic orbitals, which account for the mutation of the Lewis structure for the reactant into that for the product, in other valence and in core orbitals. Series of time-integrated fluxes of pericyclic electrons can be assigned to the arrows, for example 0.09-0.23 electrons for Cope rearrangement of semibullvalene, with hysteresis-type time evolutions for 27.3 fs. This means asynchronous electronic fluxes during synchronous rearrangement of all the nuclei. These predictions should become observable by emerging techniques of femto- to attosecond time-resolved spectroscopy.

Preparation and characterization of silica-supported, group IB–Pd bimetallic catalysts prepared by electroless deposition methods

Electroless deposition has been used to synthesize a series of Au–, Ag–, and Cu–Pd/SiO 2 bimetallic catalysts having incremental surface coverages and compositions of each group IB metal. Thermodynamically unstable, yet kinetically stable, electroless bath(s) were developed using metal bis-cyano salts of the group 1B metal and N 2H 4 (for Au and Ag) or DMAB (for Cu) as reducing agents. The times (∼1–2 h) and profiles (1st order in group 1B metal concentration) observed for complete deposition indicate good kinetic control of the electroless deposition process. The bimetallic catalysts have been characterized using selective chemisorption, atomic absorption spectroscopy (AAS), Fourier transform infrared spectroscopy (FTIR) of adsorbed CO, and X-ray photoelectron spectroscopy (XPS) techniques. Decreases in Pd surface sites with addition of IB metals confirm deposition onto the supported Pd nanoparticle surfaces. FTIR studies suggest that deposition of Cu and Ag are selective towards Pd(1 1 1) sites, while Au deposits non-discriminately on all Pd sites. Finally, XPS measurements for each family of bimetallic catalysts suggest a net electron transfer from the Pd to the deposited metal.

The Lycopene Cyclase CrtY from Pantoea ananatis (Formerly Erwinia uredovora) Catalyzes an FADred-dependent Non-redox Reaction

The cyclization of lycopene generates provitamin A carotenoids such as beta-carotene and paves the way toward the formation of cyclic xanthophylls playing distinct roles in photosynthesis and as precursors for regulatory molecules in plants and animals. The biochemistry of lycopene cyclization has been enigmatic, as the previously proposed acid-base catalysis conflicted with the possibility of redox catalysis as predicted by the presence of a dinucleotide binding site. We show that reduced FAD is the essential lycopene cyclase (CrtY) cofactor. Using flavin analogs, mass spectrometry, and mutagenesis, evidence was obtained based on which a catalytic mechanism relying on cryptic (net) electron transfer can be refuted. The role of reduced FAD is proposed to reside in the stabilization of a transition state carrying a (partial) positive charge or of a positively charged intermediate via a charge transfer interaction, acid-base catalysis serving as the underlying catalytic principle. Lycopene cyclase, thus, ranks among the novel class of non-redox flavoproteins, such as isopentenyl diphosphate:dimethylallyl diphosphate isomerase type 2 (IDI-2) that requires the reduced form of the cofactor.

Synthesis and characterization of Au–Pd/SiO 2 bimetallic catalysts prepared by electroless deposition

A series of Au–Pd/SiO 2 catalysts have been prepared by the electroless deposition of Au onto a Pd/SiO 2 catalyst. A kinetically stable, electroless bath consisting of Au ( CN ) 2 - and N 2H 4 (i.e., Au source and reducing agent) was developed and optimized, allowing for incremental coverages of Au on Pd to be obtained, while avoiding deposition onto the SiO 2 support. The structural and electronic properties of the catalysts were characterized using hydrogen titration of oxygen-precovered Pd, scanning transmission electron microscopy with energy dispersed X-ray spectroscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The results suggest that Au was deposited on all types of Pd surface sites (e.g., planes, steps, kinks, edges) in a non-discriminatory fashion, with a net electron transfer from Pd to Au. The catalysts were evaluated for propylene hydrogenation, revealing significantly enhanced turnover frequencies at elevated fractional coverage of Au on Pd ( θ Au ⩾ 0.60). The enhanced catalytic activity can be explained by the disruption of continuous Pd ensembles by Au deposition, which prevents formation of the multiply bonded and less reactive propylidyne, while permitting formation of highly reactive and weakly π-bonded propylene.

Nonperturbative, quantum-mechanical approach to ion collisions from molecular targets

A quantum-mechanical approach to ion-molecule collisions is presented. It involves a separation of molecular geometry and collision dynamics and enables the use of the basis generator method developed for ion-atom collisions with relatively minor modifications. As a first application, we consider the $p{\text{-H}}_{2}\text{O}$ collision system in the impact energy range of 20--5000 keV, and we report total cross sections for net electron transfer and ionization. They are in remarkably good agreement with experimental data.

Molecular-Beam Scattering Experiments and Theoretical Calculations Probing Charge Transfer in Weakly Bound Complexes of Water

We describe and analyze in depth a series of molecular beam scattering experiments, first reported by Aquilanti et al. (Angew. Chemie Int. Ed. 2005, 44, 2356.), proving that a measurable bond stabilization component beyond the van-der-Waals forces is present in the prototypal hydrophobic interaction of water with the noble gases (Ng). The experimental integral cross-section data, exhibiting a fully resolved "glory" interference pattern in the velocity dependence, are here quantitatively analyzed and characterized employing a recently proposed model potential. The stabilization component of the water-Ng bond has recently been attributed, through very accurate theoretical calculations and an unambiguous, model-free analysis of the electron density displacement, to a net electron transfer taking place from Ng to H(2)O. We review the theoretical analysis and discuss additional computational results, comparing them to experiment, that clarify the effect of charge transfer on the interaction energies.

Electrification of granular systems of identical insulators

Insulating particles can become highly electrified during powder handling, volcanic eruptions, and the wind-blown transport of dust, sand, and snow. Measurements in these granular systems have found that smaller particles generally charge negatively, while larger particles charge positively. These observations are puzzling since particles in these systems are generally chemically identical and thus have no contact potential difference. We show here that simple geometry leads to a net transfer of electrons from larger to smaller particles, in agreement with these observations. We integrate this charging mechanism into the first quantitative charging scheme for a granular system of identical insulators and show that its predictions are in agreement with measurements. Our theory thus seems to provide an explanation for the hitherto puzzling phenomenon of the size-dependent charging of granular systems of identical insulators.

Adsorption of Naphthalene and Quinoline on Pt, Pd and Rh: A DFT Study

The adsorption of naphthalene and quinoline on Pt(111), Pd(111) and Rh(111) surfaces is studied using density functional theory. The metal surfaces are simulated by means of large confined clusters and for Pt by means of a slab with periodic boundary conditions (PBC). Calculation parameters such as basis set convergence, basis set superposition error and effects of cluster relaxation and size are analyzed in order to assess the aptness of the cluster model. For all the metals, the preferred sites of adsorption are analyzed, thus revealing their different behaviors concerning structure and stability of adsorption modes. On Pt, the molecules have the richest theoretical configurational variety. Naphthalene and quinoline are found to adsorb preferentially on di-bridge[7] sites on the three metals, and Rh exhibits higher adsorption energies than Pt and Pd. Structural features of the adsorbed molecules are correlated to the calculated adsorption energies. The di-bridge[7] adsorption modes are studied in deeper detail decomposing the adsorption energies in two terms arising from molecular distortion and binding interaction to the metal. Molecular distortion is correlated to the HOMO-LUMO energy gap. The larger adsorption energies found for interactions with Rh result from the lower contribution of the distortion term. Binding interactions are described by analyzing the wave functions of naphthalene and quinoline adsorbed on a subunit of the large clusters in order to reduce the complexity of the analysis. Molecular orbitals are studied using concepts of Frontier Molecular Orbitals theory. This approach reveals that in the adsorption of naphthalene and quinoline on Pt and Pd, an antibonding state lies below the Fermi energy, while on Rh all antibonding states are empty, in agreement with the larger interaction energies. In addition, further insight is gained by projecting the density of states on the d band of the clean surfaces and of the adsorbed systems. This results in the rationalization of the structural features in terms of the concepts of electronic structure theory. The distributions of electronic density are described by means of Hirshfeld charges and isosurfaces of differential electron density. The net electron transfer from the metals to the molecules for most of the sites correlates with the trends of the adsorption energies.

Particle dynamics simulations of triboelectric charging in granular insulator systems

Particle dynamics simulations are carried out to study triboelectric charging in granular systems composed of a single insulating material. The simulations implement a model in which electrons trapped in localized high energy states can be transferred during collisions to low energy states in the other particle. It is shown that this effect alone can generate electrostatic charging in the system, and cause net electron transfer from larger particles to smaller particles. The magnitude of charging is small for systems of a single particle size but becomes much greater for a system with polydispersal particle sizes, due to the net electron transfer from larger to smaller particles. The negative charge of smaller particles, and positive charge of larger particles has been observed in field studies and laboratory experiments of granular systems.

Observations at geosynchronous orbit of a persistent Pc5 geomagnetic pulsation and energetic electron flux modulations

Abstract. A long lasting narrow-band (4–7 mHz) Pc5 fluctuation event at geosynchronous orbit is presented through measurements from GOES-8 and GOES-10 and the response of energetic electrons with drift frequencies close to the narrow-band pulsation frequency is monitored through a spectral analysis of flux data from the LANL-SOPA energetic electron instrument. This analysis shows electron flux modulations at the magnetospheric pulsation's frequency as well as at various other frequencies in the Pc5 range, related to the particles' drift-frequencies and their harmonics. A drift resonance effect can be seen, with electron flux modulation becoming more evident in the energy channels of electrons with drift frequencies closer to the wave frequency; however no net increase or decrease in energetic electron flux is observed, indicating that the net energy transfer and transport of electrons is not significant. This Pc5 event has a long duration, being observed for more than a couple of days at geosynchronous orbit over several traversals of the two GOES satellites, and is localized in azimuthal extent. Spectral analysis shows that most of the power is in the transverse components. The frequency of the narrow-band event, as observed at geosynchronous orbit shifts during the time of the event from 7±0.5 mHz to about 4±0.5 mHz. On the ground, CARISMA magnetometers record no distinct narrow-band fluctuation in the magnetic field, and neither does Geotail, which is traversing the outer magnetosphere a few RE further out from geosynchronous orbit, at the same UT and LT that GOES-8 and -10 observe the pulsations, suggesting that that there is no connection to external fluctuations originating in the solar wind. An internal generation mechanism is suggested, such as could be provided by energetic ring current particles, even though conclusive evidence could not be provided for this particular event. Through a statistical study, it is found that this event belongs to a class of similar events, occurring predominantly in the post-noon region in the inner magnetosphere.

Silver Nanocluster Redox‐Couple‐Promoted Nonclassical Electron Transfer: An Efficient Electrochemical Wolff Rearrangement of α‐Diazoketones

In this work we report the unique electrocatalytic role of benzoic acid protected silver nanoclusters (Ag(n), mean core diameter 2.5 nm) in the Wolff rearrangement (Scheme 1) of alpha-diazoketones. More specifically, the presence of a Ag(n) (0)/Ag(n) (+) redox couple facilitates a nonclassical electron-transfer process, involving chemical reaction(s) interposed between two electron-transfer steps occurring in opposite directions. Consequently, the net electron transfer between the electron mediator (Ag(n)) and alpha-diazoketone is zero. In-situ UV-visible studies using pyridine as a nucleophilic probe indicate the participation of alpha-ketocarbene/ketene as important reaction intermediates. Controlled potential coulometry of alpha-diazoketones using Ag(n) as the anode results in the formation of Wolff rearranged carboxylic acids in excellent yield, without sacrificing the electrocatalyst.

Photoregulated Transmembrane Charge Separation by Linked Spiropyran−Anthraquinone Molecules

Amide-linked spiropyran-anthraquinone (SP-AQ) conjugates were shown to mediate ZnTPPS(4-)-photosensitized transmembrane reduction of occluded Co(bpy)3(3+) within unilamellar phosphatidylcholine vesicles by external EDTA. Overall quantum yields for these reactions were dependent upon the isomeric state of the dye; specifically, 30-35% photoconversion of the closed-ring spiropyran (SP) moiety to the open-ring merocyanine (MC) form caused the quantum yield to decrease by 6-fold in the simple conjugate and 3-fold for an analogue containing a lipophilic 4-dodecylphenoxy substituent on the anthraquinone moiety. Transient spectroscopic and fluorescence quenching measurements revealed that two factors contributed to these photoisomerization-induced changes in quantum yields: increased efficiencies of fluorescence quenching of 1ZnTPPS4- by the merocyanine group and lowered transmembrane diffusion rates of the merocyanine-containing redox carriers. Transient spectrophotometry also revealed the sequential formation and decay of two reaction intermediates, identified as 3ZnTPPS4- and a species with the optical properties of a semiquinone radical. Kinetic profiles for Co(bpy)3(3+) reduction under continuous photolysis in the presence and absence of added ionophores indicated that transmembrane redox mediated by SP-AQ was electroneutral, but reaction by the other quinone-containing mediators was electrogenic. The minimal reaction mechanism suggested from the combined studies is oxidative quenching of vesicle-bound 3ZnTPPS4- by the anthraquinone unit, followed by either H+/e- cotransport by transmembrane diffusion of SP-AQH* or, for the other redox mediators, semiquinone anion-quinone electron exchange leading to net transmembrane electron transfer, with subsequent one-electron reduction of the internal Co(bpy)3(3+). Thermal one-electron reduction of Co(bpy)3(3+) by EDTA is energetically unfavorable; the photosensitized reaction therefore occurs with partial conversion of photonic energy to chemical and transmembrane electrochemical potentials.

Electronic structures of Au–Al thin-film alloys by high-energy XPS and XANES

The Auger parameter analysis in a conjunction with Au M 3-edge XANES has been used to obtain the electronic configurations for Al 2Au and Au 2Al alloys. The net electron transfer from Al to Au (0.13 e − for Al 2Au) has been estimated using the modified final state Auger parameter shifts in combination with the Thomas and Weightman model. The magnitude and direction of charge transfer appear to be in agreement with the differences in electronegativity values of the elements. Further, the opposite sign of the Auger parameter shift for Au and Al indicates that the Au core holes in the alloys are in a better screening environment than the core holes in pure Au while Al in the alloy environment is less screened. Au M 3-edge XANES has been used to probe the unoccupied densities of Au 5d states. The appearance of the white-line features in Al 2Au and Au 2Al indicates the loss of 5d electrons at the Au site due to the charge redistribution caused by intra- [Au(5d)→Au(6s, 6p)] and extra-atomic [Au(5d)→Al(3s, 3p)] orbital hybridization. Al 2Au exhibits an increased transfer of charge away from the gold compared to Au 2Al due to the greater availability of empty states in the Al valence band. In general, the net charge transfer from Al to Au was substantially compensated by the 5d electron depletion, which is responsible for the shifts of core-level and valence band to higher binding energy. Changes in observed plasmon loss structures could be related to the strength of Au–Al bonding resulting from the localization of valence electrons. This further gives a clear picture of the charge rearrangement between Au and Al during alloying.

Ab initio density functional simulation of structural and electronic properties of MgO ultra-thin adlayers on the (001) Ag surface

To simulate the properties of ultrathin layers of magnesium oxide epitaxially grown on silver (001), we have adopted a periodic slab model, consisting of six layers of Ag covered on both sides with an MgO monolayer. All calculations have been performed with the crystal98 program. Several DFT functionals were tried and a rich basis set was adopted. The electronic and structural properties of the two bulk materials (Ag and MgO) were quite accurately reproduced. The presence of the metallic substrate was found to have an appreciable influence on the structural and electronic features of the oxide surface. In the most stable configuration (O ions directly above Ag atoms, Mg ions in the hollow sites), the surface is corrugated, and there is a net transfer of electrons from the overlayer to the metal, leading to a substantial reduction of the work function of the metal and to a decrease of the electrostatic field at the surface. The reactivity properties of the supported oxide surface have been investigated by studying the interaction of the composite material with water molecules.

Molecular and isotopic analysis of anaerobic methane-oxidizing communities in marine sediments

Convergent lines of molecular, carbon-isotopic, and phylogenetic evidence have previously indicated (Hinrichs, K.-U., Hayes, J.M., Sylva, S.P., Brewer, P.G., DeLong, E.F., 1999. Methane-consuming archaebacteria in marine sediments. Nature 398, 802–805.) that archaea are involved in the anaerobic oxidation of methane in sediments from the Eel River Basin, offshore northern California. Now, further studies of those same sediments and of sediments from a methane seep in the Santa Barbara Basin have confirmed and extended those results. Mass spectrometric and chromatographic analyses of an authentic standard of sn-2-hydroxyarchaeol (hydroxylated at C-3 in the sn-2 phytanyl moiety) have confirmed our previous, tentative identification of this compound but shown that the previously examined product was the mono-TMS, rather than di-TMS, derivative. Further analyses of 13C-depleted lipids, appreciably more abundant in samples from the Santa Barbara Basin, have shown that the archaeal lipids are accompanied by two sets of products that are only slightly less depleted in 13C. These are additional glycerol ethers and fatty acids. The alkyl substituents in the ethers (mostly monoethers, with some diethers) are non-isoprenoidal. The carbon-number distributions and isotopic compositions of the alkyl substituents and of the fatty acids are similar, suggesting strongly that they are produced by the same organisms. Their structures, n-alkyl and methyl-branched n-alkyl, require a bacterial rather than archaeal source. The non-isoprenoidal glycerol ethers are novel constituents in marine sediments but have been previously reported in thermophilic, sulfate- and nitrate-reducing organisms which lie near the base of the rRNA-based phylogenetic tree. Based on previous observations that the anaerobic oxidation of methane involves a net transfer of electrons from methane to sulfate, it appears likely that the non-archaeal, 13C-depleted lipids are products of one or more previously unknown sulfate-reducing bacteria which grow syntrophically with the methane-utilizing archaea. Their products account for 50% of the fatty acids in the sample from the Santa Barbara Basin. At all methane-seep sites examined, the preservation of aquatic products is apparently enhanced because the methane-oxidizing consortium utilizes much of the sulfate that would otherwise be available for remineralization of materials from the water column.

Electronic and structural properties of carbon nanohorns

We use parametrized linear combination of atomic orbitals calculations to determine the stability, optimum geometry and electronic properties of nanometer-sized capped graphitic cones, called ``nano-horns''. Different nano-horn morphologies are considered, which differ in the relative location of the five terminating pentagons. Simulated scanning tunneling microscopy images of the various structures at different bias voltages reflect a net electron transfer towards the pentagon vertex sites. We find that the local density of states at the tip, observable by scanning tunneling spectroscopy, can be used to discriminate between different tip structures. Our molecular dynamics simulations indicate that disintegration of nano-horns at high temperatures starts in the highest-strain region near the tip.