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Reduction Steps (reduction + step)
Selected AbstractsAdsorptive Stripping Voltammetric Determination of Trace Uranium with a Bismuth-Film Electrode Based on the U(VI),U(V) Reduction Step of the Uranium-Cupferron ComplexELECTROANALYSIS, Issue 3 2006Georgia Kefala Abstract This work reports the use of adsorptive stripping voltammetry (AdSV) for the determination of uranium on a preplated rotating-disk bismuth-film electrode (BiFE). The principle of the method relied on the complexation of U(VI) ions with cupferron and the subsequent adsorptive accumulation of the complex on the surface of the BiFE. The uranium in the accumulated complex was then reduced by means of a cathodic voltammetric scan while the analytically useful U(VI),U(V) reduction signal was monitored. The experimental variables as well as potential interferences were investigated and the figures of merit of the method were established. Using the selected conditions, the 3, limit of detection for uranium was 0.1,,g L,1 at a preconcentration time of 480,s and the relative standard deviation was 4.7% at the 5,,g L,1 level for a preconcentration time of 120,s (n=8). The accuracy of the method was established by analyzing a reference sea water sample. [source] Supramolecular Derivatives of 9,10-Anthraquinone.ELECTROANALYSIS, Issue 5-6 2003Complexing Properties, Electrochemistry at Regular-, Low Ionic Strength Abstract Three newly synthesized polyanthraquinone derivatives: 7,13-bis(9,10-dioxo-1-anthryl)-1,4,10-trioxa-7,13-diazacyclopentadecane, (AQ)A215C5, 7,16-bis(9,10-dioxo-1-anthryl)-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane, (AQ)A218C6, and tris[(9,10-diokso-1-antryl)-aminoethyl]amine, (AQNet)3N, were examined by cyclic and normal pulse voltammetry. All anthraquinone groups in these compounds were electroactive and formed the radicals and dianions similarly to simple anthraquinone. The differences between the voltammograms obtained in the absence and presence of supporting electrolyte are discussed, and the diffusion coefficients of the compounds and the rates of the chemical reactions following the first reduction step were evaluated. (AQNet)3N appears to be a good model compound for multi electron transfers in aprotic solvents. It offers two consecutive nearly reversible 3-electron redox systems: 0/,3 and ,3/,6. The interactions of the radicals and dianions of the above compounds with alkali metal cations were examined. [source] Mixed-Ligand Oxidovanadium(V) Complexes with N, -Salicylidenehydrazides: Synthesis, Structure, and 51V Solid-State MAS NMR Investigation,EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 14 2008Simona Nica Abstract The synthesis and spectroscopic characterization of a series of three oxidovanadium(V) complexes with 8-hydroxyquinoline and Schiff-base ligands derived from salicylaldehyde and ,-hydroxy-functionalized carbohydrazides with different chain lengths are reported. The complex with the hydrazone ligand containing the shortest chain length was crystallographically characterized. This complex crystallizes in the triclinic space group P with two structurally similar but crystallographically independent oxidovanadium(V) complexes. Each vanadium atom is six-coordinate in a distorted-octahedral geometry. The two molecules are assembled through hydrogen-bonding interactions between the hydroxyl groups of the side-chain substituted Schiff-base ligand and the oxido group of one of the two complexes. Electrochemical measurements performed in acetonitrile solution reveal two reversible one-electron reduction steps. The observed pre-wave feature of the second reduction step indicates the presence of dissociation equilibria related to the 8-hydroxyquinoline coligand. Magic-angle spinning solid-state 51V NMR spectroscopy allowed to characterize the full series of complexes with alkyl and hydroxy alkyl-substituted hydrazone ligands that were used. The quadrupolar coupling constants are small with a value of about 4 MHz and show little variation within the series. The asymmetry of the chemical shift tensor indicates a rather axial symmetric environment around the vanadium(V) center. The isotropic chemical shifts observed in the solid state occur at about 30 ppm, which is in the same order of magnitude as the solvent induced variations, about 10 ppm, found for different solvents.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Stereoselective Synthesis of Three Isomers of tert -Butyl 5-Hydroxy-4-methyl-3-oxohexanoate through Alcohol Dehydrogenase-Catalyzed Dynamic Kinetic ResolutionADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 1-2 2009Steffen Lüdeke Abstract Regioselective reduction of the 5-keto group of tert -butyl 4-methyl-3,5-dioxohexanoate (1) leads to a stereodiad of tert -butyl 5-hydroxy-4-methyl-3-oxohexanoate (2). Alcohol dehydrogenases from Lactobacillus brevis (LBADH), Rhodococcus sp. (RS 1-ADH) and Saccharomyces cerevisiae (YGL157w) reduce 1 under dynamic kinetic resolution conditions, thereby establishing two chiral carbons with a single reduction step. While it had been shown previously that LBADH reduction of 1 stereoselectively leads to syn -(4S,5R)- 2, alcohol dehydrogenase-mediated dynamic kinetic resolution now allows easy access to syn -(4R,5S)- 2 (RS 1-ADH; 97.6% ee, syn:anti=92:8, 66% conversion, 37% isolated yield) and anti -(4S,5S)- 2 (YGL157w; 90% ee, anti:syn=93:7, 64% conversion, 42% isolated yield), as well. Thus three out of four possible stereoisomers were formed selectively upon reduction of 1. [source] Effects of non-steady-state iron limitation on nitrogen assimilatory enzymes in the marine diatom thalassiosira weissflogii (BACILLARIOPHYCEAE)JOURNAL OF PHYCOLOGY, Issue 1 2000Allen J. Milligan Since the recognition of iron-limited high nitrate (or nutrient) low chlorophyll (HNLC) regions of the ocean, low iron availability has been hypothesized to limit the assimilation of nitrate by diatoms. To determine the influence of non-steady-state iron availability on nitrogen assimilatory enzymes, cultures of Thalassiosira weissflogii (Grunow) Fryxell et Hasle were grown under iron-limited and iron-replete conditions using artificial seawater medium. Iron-limited cultures suffered from decreased efficiency of PSII as indicated by the DCMU-induced variable fluorescence signal (Fv/Fm). Under iron-replete conditions, in vitro nitrate reductase (NR) activity was rate limiting to nitrogen assimilation and in vitro nitrite reductase (NiR) activity was 50-fold higher. Under iron limitation, cultures excreted up to 100 fmol NO2,·cell,1·d,1 (about 10% of incorporated N) and NiR activities declined by 50-fold while internal NO2, pools remained relatively constant. Activities of both NR and NiR remained in excess of nitrogen incorporation rates throughout iron-limited growth. One possible explanation is that the supply of photosynthetically derived reductant to NiR may be responsible for the limitation of nitrogen assimilation at the NO2, reduction step. Urease activity showed no response to iron limitation. Carbon:nitrogen ratios were equivalent in both iron conditions, indicating that, relative to carbon, nitrogen was assimilated at similar rates whether iron was limiting growth or not. We hypothesize that, diatoms in HNLC regions are not deficient in their ability to assimilate nitrate when they are iron limited. Rather, it appears that diatoms are limited in their ability to process photons within the photosynthetic electron transport chain which results in nitrite reduction becoming the rate-limiting step in nitrogenassimilation. [source] The Formation of Large-Area Conducting Graphene-Like PlateletsCHEMISTRY - A EUROPEAN JOURNAL, Issue 33 2009Riccardo Salvio Abstract The treatment of a suspension of graphite oxide (GO) with sodium azide leads to a material that, after reduction, features amino groups at the top and bottom of the sheets. These groups react through microcontact printing with an isothiocyanate monolayer on a silicon oxide substrate to form covalent bonds that strongly attach to the particles on the surface. With ultrasonication it is possible to obtain exfoliation of the sheets that are not covalently bound to the surface leaving single-layer platelets attached to the substrate. The azido derivative can be also used to functionalize the graphene oxide with long alkylic chains through a click chemistry approach. This functionalization results in the exfoliation of this material in dimethylformamide. The novel materials were fully characterized by different techniques including IR spectroscopy, thermogravimetric analysis (TGA), scanning and transmission electron microscopy (SEM and TEM), X-Ray photoelectron spectroscopy (XPS), and solid state NMR spectroscopy. The material with amino groups, after the reduction step, is conductive with a resistivity only approximately seven times larger than that of unprocessed graphite. This implies that after reduction of the GO, the conjugated sp2 network is largely restored. We consider this to be an important step towards a chemical approach for forming conducting large-area platelet films of single-layer graphene. [source] PREPROCESSING RULES FOR TRIANGULATION OF PROBABILISTIC NETWORKS,COMPUTATIONAL INTELLIGENCE, Issue 3 2005Hans L. Bodlaender Currently, the most efficient algorithm for inference with a probabilistic network builds upon a triangulation of a network's graph. In this paper, we show that pre-processing can help in finding good triangulations for probabilistic networks, that is, triangulations with a maximum clique size as small as possible. We provide a set of rules for stepwise reducing a graph, without losing optimality. This reduction allows us to solve the triangulation problem on a smaller graph. From the smaller graph's triangulation, a triangulation of the original graph is obtained by reversing the reduction steps. Our experimental results show that the graphs of some well-known real-life probabilistic networks can be triangulated optimally just by preprocessing; for other networks, huge reductions in their graph's size are obtained. [source] Mixed-Ligand Oxidovanadium(V) Complexes with N, -Salicylidenehydrazides: Synthesis, Structure, and 51V Solid-State MAS NMR Investigation,EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 14 2008Simona Nica Abstract The synthesis and spectroscopic characterization of a series of three oxidovanadium(V) complexes with 8-hydroxyquinoline and Schiff-base ligands derived from salicylaldehyde and ,-hydroxy-functionalized carbohydrazides with different chain lengths are reported. The complex with the hydrazone ligand containing the shortest chain length was crystallographically characterized. This complex crystallizes in the triclinic space group P with two structurally similar but crystallographically independent oxidovanadium(V) complexes. Each vanadium atom is six-coordinate in a distorted-octahedral geometry. The two molecules are assembled through hydrogen-bonding interactions between the hydroxyl groups of the side-chain substituted Schiff-base ligand and the oxido group of one of the two complexes. Electrochemical measurements performed in acetonitrile solution reveal two reversible one-electron reduction steps. The observed pre-wave feature of the second reduction step indicates the presence of dissociation equilibria related to the 8-hydroxyquinoline coligand. Magic-angle spinning solid-state 51V NMR spectroscopy allowed to characterize the full series of complexes with alkyl and hydroxy alkyl-substituted hydrazone ligands that were used. The quadrupolar coupling constants are small with a value of about 4 MHz and show little variation within the series. The asymmetry of the chemical shift tensor indicates a rather axial symmetric environment around the vanadium(V) center. The isotropic chemical shifts observed in the solid state occur at about 30 ppm, which is in the same order of magnitude as the solvent induced variations, about 10 ppm, found for different solvents.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Synthesis, Characterization, and Electrochemical Properties of Dinuclear Complexes Assembled from Asymmetric CoIII Bis(dioximates) and Boronic AcidsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 19 2005Renata Dreos Abstract Bis(methylphenylglyoximate)cobalt(III) complexes exist both as cis and trans isomers due to the asymmetry of the equatorial ligand, and, when the axial ligands are different, the trans isomer is chiral. The reaction of racemic trans -[CH3Co(mpgH)2py] (1) with either 3- or 4-pyridylboronic acid affords dimeric units arranged on a crystallographic symmetry center such that the pyridyl nitrogen of one moiety coordinates to the Co atom of the symmetry-related unit. In principle, three structurally different dimeric species (two homodimers and one heterodimer) can be obtained. Time-resolved 1H NMR spectra of a 1:1 mixture of racemic 1 and either 3- or 4-pyridylboronic acid in CDCl3/CD3OD show that the reaction does not converge toward a unique species in solution. Nevertheless, X-ray structures show that the heterochiral dimers are the only products that crystallize from the reaction mixture. The nature of the dioximate side groups does not affect the geometry of the dimeric arrangements assembled by 4-pyridylboronic acid ("molecular box"). On the contrary, the geometry of the species assembled by 3-pyridylboronic acid varies from the "molecular parallelogram" obtained from the bis(dimethylglyoximates) to the highly squeezed "molecular box" obtained from bis(methylphenylglyoximates). Cyclic voltammetry studies show that the metal centers in the dimeric species do not interact with each other and undergo a simultaneous redox process. However, depending on the geometry of the systems, the redox process involves a single four-electron reduction for 3 and 5 or two consecutive two-electron reduction steps for 4 and 6. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Glycerol and Glycerol Glycol GlycodendrimersEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 22 2003Mike M. K. Boysen Abstract Non-covalent interactions between structural parts of complex oligosaccharides and saccharide-recognising proteins are of crucial importance for many cell communication phenomena. Specificity of such interactions and stability of these ligand-receptor complexes are achieved through multivalent interactions between multiple copies of a saccharide ligand and a corresponding number of protein receptors. Substances presenting multiple copies of the saccharide ligand on easily accessible scaffold molecules therefore appear to be promising tools for study of multivalent interactions and their possible inhibition. Such multivalent glycomimetics can be prepared by attachment of saccharide residues to the surface functional groups of dendrimers. In the course of our work, we have prepared novel glycodendrimers with glycerol and glycerol glycol polyether scaffolds. Isopropylidene-protected hydroxyethyl mannoside was chosen as the carbohydrate component, with the construction of the dendritic structures proceeding by a convergent approach featuring iterative Williamson etherification and ozonolysis/hydride reduction steps. Deprotected representatives of such structures are potential inhibitors of mannose-binding lectins of E. coli. Three representative compounds were deprotected and their anti-adhesive properties were examined. The route to these glycodendrimers was also evaluated in terms of synthetic chemistry. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] Modeling of partial oxidation in gas,solids downer reactorsAICHE JOURNAL, Issue 8 2010S. Vaishali Abstract Selective partial oxidations represent an important class of reactions in the process industry. Of particular interest is the partial oxidation of n-butane to maleic anhydride (MAN), which is arguably the largest commercialized alkane partial oxidation process. Partial oxidation of n-butane, which uses vanadium phosphorous oxide (VPO) as a heterogeneous catalyst, is believed to operate through a unique mechanism in which lattice oxygen oxidizes n-butane selectively to MAN. Past work has shown that performing partial oxidation reactions in gas,solids riser configuration is realizable and commercially viable, which has lead to commercialization of this technology in the last decade. Though the riser configuration allows optimal and independent control of the oxidation and reduction steps, the riser unit suffers from solid backmixing at walls, which in turn result into lower conversion, nonoptimal selectivity and diminished overall yield of desired product. In recent years, there has been growing interest in downers involving cocurrent downflow of both solids and gas phases, hence offering relatively uniform flow characteristics. In this contribution, we explore through modeling the implications of effecting partial oxidation reactions in a downer (gas,solids cocurrent downflow) compared to that in a conventional riser reactor (gas,solids cocurrent up flow) operated under equivalent operating conditions. Further, we explore the operational space of downers for these reactions, suggesting ways for improving the productivity of downer for partial oxidation applications. © 2009 American Institute of Chemical Engineers AIChE J, 2010 [source] A New Class of Cationic Organoiron Polynorbornenes Containing Azo Dyes,MACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 4 2003Alaa S. Abd-El-Aziz Abstract Ring-opening metathesis polymerization of novel organoiron norbornene monomers functionalized with azobenzene chromophores using the Grubbs catalyst resulted in the production of the corresponding cationic organoiron polynorbornenes. UV-vis analysis of the organometallic polymers in DMF showed maxima from 420,430 nm, and upon acidification a bathochromic shift to 510,520 nm was observed. Cyclic voltammetric studies showed that the iron centers underwent reversible reduction steps between ,1.2 and ,1.4 V. Thermogravimetric analysis of the organoiron polymers showed two weight loss steps from 225 to 255,°C and from 400 to 510,°C. Differential scanning calorimetry showed that the cationic organoiron polynorbornenes had glass transition temperatures (Tg) between 145 and 178,°C, while the Tg values of their organic analogs occurred between 99 and 104,°C. [source] Automatic data reduction and archiving for STELLAASTRONOMISCHE NACHRICHTEN, Issue 6-8 2004Article first published online: 13 OCT 200, M. Weber Abstract The data is collected at the observatory, each data product is registered and a queue is set up to transfer the highest priority observations first. Optionally, lossy compression (e.g. White 1994) can be used to boost transfer speeds for time critical observations. Once the data has been transferred, the reduction process is started at the control-center in Potsdam. The type of reduction steps required can be specified by the user or a default pipeline setup can be used. The users can be notified about the status of their observations in any desired detail. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Mixed-Valence, Mixed-Spin-State, and Heterometallic [2×2] Grid-type Arrays Based on Heteroditopic Hydrazone Ligands: Synthesis and Electrochemical FeaturesCHEMISTRY - A EUROPEAN JOURNAL, Issue 8 2005Lindsay H. Uppadine Dr. Abstract An extended family of heterometallic [M12M22(L,)4]n+ [2×2] grid-type arrays 1,9 has been prepared. The three-tiered synthetic route encompasses regioselective, redox and enantioselective features and is based on the stepwise construction of heteroditopic hydrazone ligands A,C. These ligands contain ionisable NH and nonionisable NMe hydrazone units, which allows the metal redox properties to be controlled according to the charge on the ligand binding pocket. The 2-pyrimidine (R) and 6-pyridine (R,) substituents have a significant effect on complex geometry and influence both the electrochemical and magnetic behaviour of the system. 1H NMR spectroscopic studies show that the FeII ions in the grid can be low spin, high spin or spin crossover depending on the steric effect of substituents R and R,. This steric effect has been manipulated to construct an unusual array possessing two low-spin and two spin-crossover FeII centres (grid 8). Electrochemical studies were performed for the grid-type arrays 1,9 and their respective mononuclear precursor complexes 10,13. The grids function as electron reservoirs and display up to eight monoelectronic, reversible reduction steps. These processes generally occur in pairs and are assigned to ligand-based reductions and to the CoIII/CoII redox couple. Individual metal ions in the heterometallic grid motif can be selectively addressed electrochemically (e.g., either the CoIII or FeII ions can be targeted in grids 2 and 5). The FeII oxidation potential is governed by the charge on the ligand binding unit, rather than the spin state, thus permitting facile electrochemical discrimination between the two types of FeII centre in 7 or in 8. Such multistable heterometallic [2×2] gridlike arrays are of great interest for future supramolecular devices incorporating multilevel redox activity. Une famille de complexes hétérométalliques [M12M22(L,)4]n+du type grille [2×2] 1,9 a été préparée. Les trois approches synthétiques multiétapes des ligands hydrazone hétéroditopiques A,C possèdent chacune des caractéristiques régio-, rédox-, énantio-sélectives. Ces ligands possèdent un groupement NH ionisable et un groupement hydrazone non-ionisable NMe, qui permettent un contrôle des propriétés rédox du métal complexé en fonction de la charge de la cavité coordinante. Les substituants 2-pyrimidine (R) et 6-pyridine (R,) influencent de manière significative la géométrie des complexes formés ainsi que leurs propriétés électrochimiques et magnétiques. Les études par spectroscopie RMN1H montrent que les ions FeII, dans ces complexes, possèdent une configuration électronique soit de bas spin, soit de haut spin ou encore de transition de spin en fonction de l'encombrement stérique du substituant. Cet effet stérique a été utilisé pour construire des grilles originales possédant deux cations FeIIde bas spin et deux cations FeIIà transition de spin (grille 8). Des études électrochimiques détaillées ont été réalisées pour les assemblages du type grille 1,9 et leurs précurseurs mononucléaires 10,13. Ces grilles fonctionnent comme réservoir à électrons. Jusqu'à huit étapes de réduction monoélectronique réversibles peuvent être observées. Ces transferts d'électrons se font généralement par paire et interviennent sur le ligand à l'exception des deux premières réductions correspondant aux couples CoIII/CoII. Chaque cation métallique d'une grille peut être sollicité sélectivement par voie électrochimique (les ions CoIIIou FeIIpeuvent être ciblés dans les grilles 2 et 5). Le potentiel d'oxydation du FeIIest tributaire de la charge des ligands et non de l'état de spin, permettant une discrimination électrochimique parmi les deux types de FeIIdans 7 ou 8. De telles grilles [2×2] hétérométalliques ont des niveaux de stabilité multiples et présentent un grand intérêt pour des systèmes supramoléculaires possédant une multiplicité de niveaux d'oxydo-réduction. [source] Redox Properties of Mixed Lutetium/Yttrium Nitride Clusterfullerenes: Endohedral LuxY3,xN@C80(I) (x=0,3) CompoundsCHEMPHYSCHEM, Issue 7 2009Ján Tarábek Dr. Abstract The complex redox behavior of mixed lutetium/yttrium nitride clusterfullerenes of the series LuxY3,xN§C80(Ih, x=0,3) (see figure) results in up to three irreversible reduction steps. A reduction mechanism is proposed based on fast-scan and square-wave voltammetry experiments combined with simulations of the voltammograms. The redox behavior of mixed lutetium/yttrium nitride clusterfullerenes of the series LuxY3,xN@C80(Ih, x=0,3) is studied for the first time by means of cyclic voltammetry, fast-scan cyclic voltammetry, square-wave voltammetry, and electron paramagnetic resonance (EPR) spectroelectrochemistry. A reversible single-electron-transfer process, which does not result in an EPR signal, is detected during the anodic oxidation sweep of cyclic voltammetry experiments performed at different temperatures (296 and 360 K). The cathodic reduction sweep reveals a rather complex response for all the four clusterfullerenes,with up to three irreversible reduction steps. By correlating the results of fast-scan and square-wave voltammetry and combining them with simulations of the voltammograms, we are able to propose a reduction mechanism for the LuxY3,xN@C80(I) (x=0,3) fullerenes. [source] | ||||||||||