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Intermediate Species (intermediate + species)
Selected AbstractsChemInform Abstract: Crystal Structure of Li2B12H12: A Possible Intermediate Species in the Decomposition of LiBH4.CHEMINFORM, Issue 8 2009Jae-Hyuk Her Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] Mössbauer Investigation of Peroxo Species in the Iron(III),EDTA,H2O2 SystemEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 21 2005Virender K. Sharma Abstract The reaction of a diiron(III),EDTA complex with H2O2 in alkaline medium is studied by Mössbauer spectroscopy in conjunction with the rapid-freeze/quench technique in order to identify possible intermediate species during the formation and decomposition of the purple (EDTA)FeIII(,2 -O2)3, complex ion. Starting from the six-coordinate [FeIIIEDTA], species at acidic pH, it is demonstrated that mononuclear complexes formed at a pH of about 1 are convert into the diiron(III),EDTA complex [(EDTA)FeIII -O-FeIII(EDTA)]4, upon raising the pH to around 10.4. H2O2 reacts with the diiron(III) complex to give peroxide/hydroperoxide related adducts. Initially, the reaction tears apart the dimers to form a peroxo adduct, namely the seven-coordinate mononuclear [(EDTA)FeIII(,2 -O2)]3,, which is stable only at very high pH. The decomposition of this peroxo adduct gives two new species, which are reported for the first time. The Mössbauer parameters of these species suggest a six-coordinate ,-peroxodiiron(III) complex [(EDTA)FeIII -(OO)-FeIII(EDTA)]4, and a seven-coordinate ,-hydroxo-,-peroxodiiron(III) complex [(EDTA)FeIII -(OO)(OH)-FeIII(EDTA)]5,. A badly resolved, extremely broad component is observed in the Mössbauer spectra during the conversion of the monomer to dimeric peroxo species, which may be attributed to the short-lived [(EDTA)FeIII -OO]3, or [(EDTA)FeIII -OOH]2, intermediate species. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Unsupported Copper Nanoparticles in the 1,3-Dipolar Cycloaddition of Terminal Alkynes and Azides,EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 10 2010Francisco Alonso Abstract Readily prepared copper nanoparticles were found to catalyse the 1,3-dipolar cycloaddition of azides and alkynes up to rates comparable to those of microwave chemistry. Both the preparation of the nanoparticles and the click reaction are carried out under mild conditions, in short reaction times and in the absence of any stabilising additive or ligand. A variety of 1,2,3-triazoles were prepared in excellent yields. A reaction mechanism was postulated on the basis of different reactivity studies and deuteration experiments. Copper(I) acetylides were demonstrated to be the real intermediate species. [source] The elusive intermediate on the folding pathway of the prion proteinFEBS JOURNAL, Issue 6 2008David C. Jenkins A key molecular event in prion diseases is the conversion of the cellular conformation of the prion protein (PrPC) to an altered disease-associated form, generally denoted as scrapie isoform (PrPSc). The molecular details of this conformational transition are not fully understood, but it has been suggested that an intermediate on the folding pathway of PrPC may be recruited to form PrPSc. In order to investigate the folding pathway of PrP we designed and expressed two mutants, each possessing a single strategically located tryptophan residue. The secondary structure and folding properties of the mutants were examined. Using conventional analyses of folding transition data determined by fluorescence and CD, and novel phase-diagram analyses, we present compelling evidence for the presence of an intermediate species on the folding pathway of PrP. The potential role of this intermediate in prion conversion is discussed. [source] Assessing biotic integrity in a Mediterranean watershed: development and evaluation of a fish-based indexFISHERIES MANAGEMENT & ECOLOGY, Issue 4 2008M. F. MAGALHÃES Abstract, Biological indicators for Mediterranean rivers are poorly developed. This study evaluates the effectiveness of the Index of Biotic Integrity approach (IBI) with fish assemblages in the Guadiana catchment, a typical Mediterranean watershed in Southern Portugal. Reference sites were selected from a set of 95 sites, using a multivariate approach. Fifty-five candidate metrics were screened for range, responsiveness, precision and redundancy. Final metrics included: proportion of native fish, number of intolerant and intermediate species, number of invertivore native fish, number of phyto-lithophilic and polyphilic species, and catches of exotics. The IBI scores correlated with composite gradients of human impact and differed significantly between reference and non-reference sites. Application of the IBI to an independent validation set with 123 sites produced results congruent with the development set and repeatable assessments at 22 sites showed concordance in IBI scoring. This application highlights the effectiveness of the IBI approach even with fish assemblages of limited diversity and ecological specialisation as in Mediterranean streams. [source] Kinetics of the bromate,bromide reaction at high bromide concentrationsINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 1 2007Guy SchmitzArticle first published online: 30 OCT 200 At bromide concentrations higher than 0.1 M, a second term must be added to the classical rate law of the bromate,bromide reaction that becomes ,d[BrO3,]/dt = [BrO3,][H+]2(k1[Br,] + k2[Br,]2). In perchloric solutions at 25°C, k1 = 2.18 dm3 mol,3 s,1 and k2 = 0.65 dm4 mol,4 s,1 at 1 M ionic strength and k1 = 2.60 dm3 mol3 s,1and k2 = 1.05 dm4 mol,4 s,1 at 2 M ionic strength. A mechanism explaining this rate law, with Br2O2 as key intermediate species, is proposed. Errors that may occur when using the Guggenheim method are discussed. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 39: 17,21, 2007 [source] Role of phenoxy radicals in PCDD/F formationINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 9 2002Sukh Sidhu In this work, the role of phenoxy radicals in polychlorinated dibenzo- p -dioxins and polychlorinated dibenzofurans (PCDD/F) formation was investigated by studying the slow oxidation of 2-chlorophenol (2-CP) and 2-chloroanisole (2-CA) at a gas-phase concentration of 4 ppm (,2.1 × 104 ,g/m3) over a temperature range of 400,800°C. Residence times were maintained at 2.0 ± 0.10 s. PCDD/F reaction products were dibenzofuran, dibenzo- p -dioxin, 4-chlorodibenzofuran, 1-chlorodibenzo- p -dioxin, 4,6-dichlorodibenzofuran, and 1,6-dichlorodibenzo- p -dioxin (1,6-DCDD). Major products observed in these experiments were 2,6-dichlorophenol, 3-phenyl-2-propenal, 1-indanone, 1,3-isobenzofurandione, and 3-phenyl-2-propenoyl chloride. The 2-CP and 2-CA experiments, along with the variable concentration 2-CA experiments, showed that the concentration of radicals present in the oxidation system has a significant effect on the PCDD/F product distribution and ultimately the PCDD/PCDF ratio. Also, the observation of dichlorinated phenoxy phenol and dichlorinated dihydroxybiphenyl, the proposed intermediate species in the radical,radical mechanism, suggests that radical,radical mechanism dominates gas-phase PCDD/F formation. This information will be helpful in constructing a detailed kinetic mechanism of PCDD/F formation/destruction in combustor postcombustion zone. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 531,541, 2002 [source] Numerical study on flame structure and NO formation in CH4,O2,N2 counterflow diffusion flame diluted with H2OINTERNATIONAL JOURNAL OF ENERGY RESEARCH, Issue 14 2004Dong-Jin Hwang Abstract Numerical study on flame structure and NO emission behaviour has been conducted to grasp chemical effects of added H2O on either fuel- or oxidizer-side in CH4,O2,N2 counterflow diffusion flames. An artificial species, which has the same thermodynamic, transport, and radiation properties of added H2O, is introduced to feasibly isolate the chemical effects. Special concern is focused on the important role of remarkably produced OH radicals due to chemical effects of added H2O on flame structure and NO emission. The reason why the difference of behaviours between the principal chain branching reaction rate and flame temperature appear is attributed to the drastic change of reaction step (R120) from the production to the consumption of OH. It is also, however, seen that the most important contribution of produced OH due to chemical effects of added H2O is through reaction step (R127). The importantly contributing reaction steps to NO production are also examined. The production rates of thermal NO and prompt NO are suppressed by chemical effects of added H2O. The contribution of the reaction steps related to HNO intermediate species to the production of prompt NO is also stressed. Copyright © 2004 John Wiley & Sons, Ltd. [source] Deconvolution of femtosecond time-resolved spectroscopy data in multivariate curve resolution.JOURNAL OF CHEMOMETRICS, Issue 7-8 2010Application to the characterization of ultrafast photo-induced intramolecular proton transfer Abstract In femtosecond absorption spectroscopy, deconvolution of the measured kinetic traces is still an important issue as photochemical processes that may possess shorter characteristic times than the time resolution of the experiment are usually considered. In this work, we propose to perform deconvolution of the time-dependent concentration profiles extracted from multivariate curve resolution (MCR) applied to spectrokinetic data. The profiles are fitted with a model function including a description of the instrumental response function (IRF) of the experiment. The method combines the potential benefits of soft-modeling data analysis with the ones of hard-modeling for parameter estimation. The potential of the method is demonstrated first analyzing five synthetic data sets for which IRF of different widths are simulated. It is then successfully applied to resolve femtosecond UV-visible transient absorption spectroscopy data investigating the photodynamics of salicylidene aniline, a photochromic molecule of wide interest. Considering a time resolution of 150,fs for the IRF, a characteristic time of 45,fs is recovered for the first step of the photo-induced process which consists of an ultrafast intramolecular proton transfer. Our results also confirm the existence of an intermediate species with a characteristic time of 240,fs. Copyright © 2010 John Wiley & Sons, Ltd. [source] Kinetic model for noncatalytic supercritical water gasification of cellulose and ligninAICHE JOURNAL, Issue 9 2010Fernando L. P. Resende Abstract This article reports the first kinetics model for Supercritical Water Gasification (SCWG) that describes the formation and interconversion of individual gaseous species. The model comprises 11 reactions, and it uses a lumping scheme to handle the large number of intermediate compounds. We determined numerical values for the rate constants in the model by fitting it to experimental data previously reported for SCWG of cellulose and lignin. We validated the model by showing that it accurately predicts gas yields at biomass loadings and water densities not used in the parameter estimation. Sensitivity analysis and reaction rate analysis indicate that steam-reforming and water,gas shift are the main sources of H2 in SCWG, and intermediate species are the main sources of CO, CO2, and CH4. © 2010 American Institute of Chemical Engineers AIChE J, 2010 [source] Fast and Almost Complete Nitridation of Mesoporous Silica MCM-41 with Ammonia in a Plug-Flow ReactorJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 1 2010Fumitaka Hayashi The title reaction proceeded well to yield silicon (oxy)nitride at 973,1323 K using a plug-flow reactor. The degree of nitridation was studied as a function of temperature and time of nitridation, the sample weight, and the flow rate of ammonia. It was dependent on the reaction temperature and the amount of ammonia supplied per sample weight. The nitridation at 1273 K for 10,25 h yielded the oxynitride with 36,39 wt% nitrogen, which was very close to 40 wt% of Si3N4. Characterization with X-ray diffraction, field-emission scanning electron microscopy and transmission electron microscopy measurements, and nitrogen adsorption revealed the conversion of MCM-41 to the corresponding oxynitride without essential loss of the mesoporous structure, the decrements of the lattice constant and the pore diameter by 20,35%, and the increments of the wall thickness by ca. 45%. Solid-state 29Si nuclear magnetic resonance spectra during the nitridation clearly showed fast decrease in SiO4 species and slow in SiO3(OH). Various intermediate species, SiOxNy(NH2 or NH)z, were observed to be formed and finally, ca. 70% SiN4 species, ca. 20% SiN3(NH2 or NH), and ca. 10% SiON2(NH2 or NH) were produced, being consistent with the results of the above mentioned elemental analysis. [source] Effect of hindered piperidine light stabilizer molecular structure and UV absorber addition on the oxidation of HDPE.JOURNAL OF VINYL & ADDITIVE TECHNOLOGY, Issue 4 2004Part 2: Mechanistic aspects, electron spin resonance spectroscopy study, molecular modeling The first in this series of papers explored the effect of the structural characteristics of 2,2,6,6-tetramethylpiperidine-based hindered amine light stabilizers (HALS) on the thermal and photostabilization of high-density polyethylene. In the second part, the energies (stabilities) of the nitroxyl radicals and various intermediate species have been predicted using AccuModel® and related to stabilization performance and electron spin resonance (ESR) spectral data. Nitroxyl radicals with low predicted stability generally afforded improved thermal and photostabilization. ESR spectra were used to obtain values of nitroxyl radical concentration ([>NO·]) and g -factor as a function of pre-aging time for combinations of pre-aged HDPE and >NH HALS. Demethylation reactions of > N-methyl HALS resulted in uselessly weak ESR spectra. The HALS that afforded poor thermal oxidative stabilization gave rise to pronounced minima in [>NO·] that coincided with a maximum in hydroperoxide concentration. The g -factor values indicated that a predominant nitroxyl canonical form generally promoted superior thermal oxidative stabilization, whereas a predominance of the dipolar N·+O, form promoted superior photo-oxidative stabilization. These trends may be related to greater radical-scavenging and peroxide-trapping effects, respectively. Molecular modeling and ESR spectra can therefore provide valuable insight into the effectiveness of HALS and stabilization mechanisms. J. Vinyl Addit. Technol. 10:159,167, 2004. © 2004 Society of Plastics Engineers. [source] Multi-frequency EPR and Mössbauer spectroscopic studies on freeze-quenched reaction intermediates of nitric oxide synthase ,MAGNETIC RESONANCE IN CHEMISTRY, Issue S1 2005C. Jung Abstract It is believed by analogy to chloroperoxidase (CPO) from Caldariomyces fumago that the electronic structure of the intermediate iron-oxo species in the catalytic cycle of nitric oxide synthase (NOS) corresponds to an iron(IV) porphyrin- , -cation radical. Such species can also be produced by the reaction of ferric NOS with external oxidants within the shunt pathway. We present multi-frequency EPR (9.6, 94, 285 GHz) and Mössbauer spectroscopic studies on freeze-quenched intermediates of the oxygenase domain of nitric oxide synthase which has reacted with peroxy acetic acid within 8,200 ms. The intermediates of the oxygenase domain of both the cytokine inducible NOS (iNOSox) and the neuronal NOS (nNOSox) show an organic radical signal in the 9.6-GHz spectrum overlapping with the spectrum of an unknown species with g -values of 2.24, 2.23 and 1.96. Using 94- and 285-GHz EPR the organic radical signal is assigned to a tyrosine radical on the basis of g -values (i.e. Tyr*562 in nNOSox and Tyr*341 in iNOSox). Mössbauer spectroscopy of 57Fe-labeled unreacted nNOSox shows a ferric low-spin heme-iron (, = 0.38 mms,1, ,EQ = 2.58 mms,1). The reaction of nNOSox with peroxy acetic acid for 8 ms leads to the disappearance of the magnetic background characteristic for native nNOSox and a new species with , = 0.27 mms,1 and ,EQ = 2.41 mms,1 is detected at 4.2 K which does not resemble the parameters typical for a Fe(IV) center. It is proposed that this intermediate species corresponds to a ferric low-spin species which magnetically couples to an amino acid radical (presumably Trp*409). Copyright © 2005 John Wiley & Sons, Ltd. [source] Examination of intermediate species in GaN metal-organic vapor-phase epitaxy by selective-area growthPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7-8 2010Masakazu Sugiyama Abstract The major reactive intermediate species of GaN has been deduced and its surface reaction rate constant has been obtained through the analysis of multi-scale growth-rate profiles both in the reactor-scale and in the micrometer-scale that were obtained by selective-area growth. Usually, it is difficult to explore surface reaction kinetics, especially for metal-organic vapour phase epitaxy (MOVPE), because of mass-transfer-limited kinetics. This multi-scale analysis, however, has clarified that a single precursor, a gas-phase reaction product between (CH3)3Ga and NH3, leads to the growth of GaN with a surface reaction probability of approximately 0.4 at 1400 K which is a typical growth temperature of GaN. Contribution of higher-order polymers was not significant in growth rate, but they seemed to be a cause of degraded surface morphology. A lumped reaction model of GaN MOVPE was proposed that led to reasonable agreement between a simulated growth-rate profile in the reactor-scale and a corresponding measured profile, which would lead to improved design of reactors and growth conditions. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Phosphoenolpyruvate carboxylase genes in C3, crassulacean acid metabolism (CAM) and C3/CAM intermediate species of the genus Clusia: rapid reversible C3/CAM switches are based on the C3 housekeeping genePLANT CELL & ENVIRONMENT, Issue 12 2006ANJA VAASEN ABSTRACT The genus Clusia includes species that exhibit either the C3 or crassulacean acid metabolism (CAM) mode of photosynthesis, or those that are able to switch between both modes according to water availability. In order to screen for species-specific genetic variability, we investigated the key carboxylase for CAM, phosphoenolpyruvate carboxylase (PEPC). Sequence analysis of DNA isolated from the obligate CAM species, Clusia hilariana, the obligate C3 species, Clusia multiflora, and an intermediate species that can switch between C3 and CAM photosynthesis, Clusia minor, revealed three different isoforms for C. hilariana and one each for the other two species. Sequence alignments indicated that PEPC from the intermediate species had high homology with the C3 protein and with one of CAM plant proteins. These were assumed to constitute ,housekeeping' proteins, which can also support CAM in intermediate species. The other two isoforms of the CAM plant C. hilariana were either CAM-specific or showed homologies with PEPC from roots. Phylogenetic trees derived from neighbour-joining analysis of amino acid sequences from 13 different Clusia species resulted in two distinct groups of plants with either ,housekeeping' PEPC only, or additionally CAM-related isoforms. Only C. hilariana showed the third, probably root-specific isoform. The high homology of the PEPC from the intermediate species with the C3 protein indicates that for the reversible transition from the C3 to CAM mode of photosynthesis, the C3 type of PEPC is sufficient. Its expression, however, is strongly increased under CAM-inducing conditions. The use of the C3 isoform could have facilitated the evolution of CAM within the genus, which occurred independently for several times. [source] Spectroelectrochemical and Computational Studies on the Mechanism of Hypoxia Selectivity of Copper RadiopharmaceuticalsCHEMISTRY - A EUROPEAN JOURNAL, Issue 19 2008Jason Abstract Detailed chemical, spectroelectrochemical and computational studies have been used to investigate the mechanism of hypoxia selectivity of a range of copper radiopharmaceuticals. A revised mechanism involving a delicate balance between cellular uptake, intracellular reduction, reoxidation, protonation and ligand dissociation is proposed. This mechanism accounts for observed differences in the reported cellular uptake and washout of related copper bis(thiosemicarbazonato) complexes. Three copper and zinc complexes have been characterised by X-ray crystallography and the redox chemistry of a series of copper complexes has been investigated by using electronic absorption and EPR spectroelectrochemistry. Time-dependent density functional theory (TD-DFT) calculations have also been used to probe the electronic structures of intermediate species and assign the electronic absorption spectra. DFT calculations also show that one-electron oxidation is ligand-based, leading to the formation of cationic triplet species. In the absence of protons, metal-centred one-electron reduction gives the reduced anionic copper(I) species, [CuIATSM],, and for the first time it is shown that molecular oxygen can reoxidise this anion to give the neutral, lipophilic parent complexes, which can wash out of cells. The electrochemistry is pH dependent and in the presence of stronger acids both chemical and electrochemical reduction leads to quantitative and rapid dissociation of copper(I) ions from the mono- or diprotonated complexes, [CuIATSMH] and [CuIATSMH2]+. In addition, a range of protonated intermediate species have been identified at lower acid concentrations. The one-electron reduction potential, rate of reoxidation of the copper(I) anionic species and ease of protonation are dependent on the structure of the ligand, which also governs their observed behaviour in vivo. [source] Chemistry, Urease Inhibition, and Phytotoxic Studies of Binuclear Vanadium(IV) ComplexesCHEMISTRY & BIODIVERSITY, Issue 1 2007Rifat Ara Abstract Vanadium plays an important role in biological systems and exhibits a variety of bioactivities. In an effort to uncover the chemistry and biochemistry of vanadium with nitrogen- and oxygen-containing ligands, we report herein the synthesis and spectroscopic characterization of vanadium(IV) complexes with hydrazide ligands. Substituents on these ligands exhibit systematic variations of electronic and steric factors. Elemental and spectral data indicate the presence of a dimeric unit with two vanadium(IV) ions coordinated with two hydrazide ligands along with two H2O molecules. The stability studies of these complexes over time in coordinating solvent, DMSO, indicates binding of the solvent molecules to give [V2O2L2(H2O)2(DMSO)2]2+ (L=hydrazide ligand) and then conversion of it to a monomeric intermediate species, [VOL(DMSO)3]1+. Hydrazide ligands are inactive against urease, whereas vanadium(IV) complexes of these ligands show significant inhibitory potential against this enzyme and are found to be non-competitive inhibitors. These complexes also show low phytotoxicity indicating their usefulness for soil ureases. Structure,activity relationship studies indicate that the steric and/or electronic effects that may change the geometry of the complexes play an important role in their inhibitory potential and phytotoxicity. [source] | |||||||||||||