Página 1 dos resultados de 19997 itens digitais encontrados em 0.046 segundos

Chemical Modifications in Styrene-Butadiene Rubber after Microwave Devulcanization

Hirayama, Denise; Saron, Clodoaldo
Fonte: AMER CHEMICAL SOC; WASHINGTON Publicador: AMER CHEMICAL SOC; WASHINGTON
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
55.84%
Microwave devulcanization has been studied as a method for elastomer recycling, which is based on the conversion of the reticulated and infusible structure of thermosetting rubbers in free polymeric chains able to be remolded by thermomechanical processing in recycling operations for the manufacture of other products. Elastomeric wastes are often irregularly discarded in nature, producing serious environmental damage, and their mechanical recycling is still considered a challenge. Thus, the development of alternatives for elastomer recycling is directly related to the actions of sustainable development and economic benefits to companies that pay to discard their wastes. The aim of this work is to evaluate the chemical modifications occurring in styrene butadiene rubber (SBR) after microwave devulcanization. Compounds of SBR were vulcanized in the presence of vulcanization agents and variable amounts of carbon black, and then the rubbers were milled and submitted to microwave treatment. Only the SBR with high carbon black content shows some portion of devulcanized material. However, the rubber with lower content of carbon black which was devulcanized by microwave radiation shows an increase in cross-link density. The microwave treatment also causes cross-link breaks mainly in polysulfidic bonds as well as decomposition of chemical groups containing sulfur attached to the chemical structure of SBR...

Formas de uso da noção de representação estrutural no ensino superior de Química; Uses of chemical structure representation in undergraduate courses

Araujo Neto, Waldmir Nascimento de
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Tese de Doutorado Formato: application/pdf
Publicado em 27/03/2009 PT
Relevância na Pesquisa
55.94%
O presente trabalho trata do reconhecimento das formas de uso da noção de representação estrutural no Ensino Superior de Química, interpretando-as à luz de um referencial teórico que relaciona a Mediação Semiótica de Lev Vigotski e a Filosofia das Formas Simbólicas de Ernst Cassirer. O sentido principal dessa relação é oferecer uma função representativa diferenciada para o tema. Procura-se informar sobre a urgência dessa nova função representativa a partir da demarcação do caráter degenerativo que pode ser encontrado em uma revisão das pesquisas sobre educação em química que tratam do tema. A realização combina uma ampla leitura teórica tanto acerca da representação em geral quanto no Ensino de Química, que serve à interpretação de treze episódios obtidos por meio de registros de áudio e vídeo, conseguidos através do acompanhamento de disciplinas em cursos de diferentes universidades e institutos superiores. Os episódios destacam o uso de signos não lingüísticos característicos da representação estrutural em temas da química orgânica e da química inorgânica. Em termos de sua atribuição semiótica, focalizamos nosso olhar na relação entre representante e representado. Destaca-se o caráter inovador de um aporte metodológico que considera o estudo de habilidades espaciais a partir de situações de ensino. A metodologia usada na interpretação dos episódios pressupõe que as diferentes formas de representação estrutural em uso no Ensino Superior de química podem ser qualificadas como ferramentas mediais...

Relationship of chemical structures of textile dyes on the pre-adaptation medium and the potentialities of their biodegradation by Phanerochaete chrysosporium

Martins, M. Adosinda M.; Queiroz, Maria João R. P.; Silvestre, Armando J. D.; Lima, Nelson
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
Publicado em //2002 ENG
Relevância na Pesquisa
55.88%
Azo dye derivatives of azobenzene constitute the largest group of dyes used in the textile industry and possess recalcitrant chemical groups, such as those of azo and sulphonic acid. Some microorganisms are able to degrade these aromatic compounds. In the present work, decolourisation of culture media containing azo dyes by the ligninolytic fungus Phanerochaete chrysosporium was achieved under nitrogenlimited conditions. The dyes used in the study are derivatives of meta- or para-aminosulphonic or aminobenzoic acids and include in their structures groups such as guaiacol or syringol, which are bioaccessible to the lignin degrading fungus P. chrysosporium. The aim of this study was to pre-adapt the microorganism to the structure of the dyes and to establish the relationships of the chemical structure of the dye present in the pre-adaptation medium with the chemical structure of the dye to be degraded. The azo dye used in the pre-adaptation medium that gave the best overall decolourisation performance was a meta-aminosulphonic acid and guaiacol derivative. The azo dye derivative of a meta-aminobenzoic acid and syringol showed a better performance in the decolourisation assays. Preliminary GC-MS studies indicated the formation of a nitroso substituted catechol metabolite...

The nature of the chemical bond

Nascimento,Marco Antonio Chaer
Fonte: Sociedade Brasileira de Química Publicador: Sociedade Brasileira de Química
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/01/2008 EN
Relevância na Pesquisa
45.91%
In this paper we briefly review the basic requirements that must be satisfied by any wave function representing many-electron systems. Following that, we examine the conditions under which the classical concepts of molecular structure, chemical structure and chemical bond can be translated into a quantum-mechanical language. Essential to this aim is the utilization of an independent particle model (IPM) for a many-electron system. In spite of the great popularity of the Hartree-Fock (HF) model only Valence-Bond (VB) type wave functions with optimized, singly occupied and non necessarily orthogonal atomic-like orbitals, can provide a quantum-mechanical translation of the classical concepts of chemical structure and chemical bond, although the HF model can still be useful for translating the concept of molecular structure. Finally, a quantum-dynamical-type of analysis allows us to conclude that, from the quantum mechanical point of view, the chemical bond is a consequence of interference effects. From the energetic point of view, the interference effect responsible for the bond formation manifests itself as a reduction of the kinetic energy of the electrons as the bond is formed.

Chemical Structure of Lipid A Isolated from Flavobacterium meningosepticum Lipopolysaccharide

Kato, Hitomi; Haishima, Yuji; Iida, Takatoshi; Tanaka, Akira; Tanamoto, Ken-ichi
Fonte: American Society for Microbiology Publicador: American Society for Microbiology
Tipo: Artigo de Revista Científica
Publicado em /08/1998 EN
Relevância na Pesquisa
55.74%
The chemical structure of the lipid A of the lipopolysaccharide component isolated from Flavobacterium meningosepticum IFO 12535 was elucidated. Methylation and nuclear magnetic resonance analyses showed that two kinds of hydrophilic backbone exist in the free lipid A: a β (1→6)-linked 2-amino-2-deoxy-d-glucose, which is usually present in enterobacterial lipid A’s, and a 2-amino-6-O-(2,3-diamino-2,3-dideoxy-β-d-glucopyranosyl)-2-deoxy-d-glucose, in a molar ratio of 1.00:0.35. Both backbones were α-glycosidically phosphorylated in position 1, and the hydroxyl groups at positions 4, 4′, and 6′ were unsubstituted. Liquid secondary ion-mass spectrometry revealed a pseudomolecular ion at m/z 1673 [M-H]− as a major monophosphoryl lipid A component carrying five acyl groups. Fatty acid analysis showed that the lipid A contained 1 mol each of amide-linked (R)-3-OH iC17:0, ester-linked (R)-3-OH iC15:0, amide-linked (R)-3-O-(iC15:0)-iC17:0, and both amide- and ester-linked (R)-3-OH C16:0. Fatty acid distribution analyses using several mass spectrometry determinations demonstrated that the former two constituents were distributed on positions 2 and 3 of the reducing terminal unit of the backbones and that the latter two were attached to the 2′ and 3′ positions in the nonreducing terminal residue.

The PubChem chemical structure sketcher

Ihlenfeldt, Wolf D; Bolton, Evan E; Bryant, Stephen H
Fonte: BioMed Central Publicador: BioMed Central
Tipo: Artigo de Revista Científica
Publicado em 17/12/2009 EN
Relevância na Pesquisa
45.9%
PubChem is an important public, Web-based information source for chemical and bioactivity information. In order to provide convenient structure search methods on compounds stored in this database, one mandatory component is a Web-based drawing tool for interactive sketching of chemical query structures. Web-enabled chemical structure sketchers are not new, being in existence for years; however, solutions available rely on complex technology like Java applets or platform-dependent plug-ins. Due to general policy and support incident rate considerations, Java-based or platform-specific sketchers cannot be deployed as a part of public NCBI Web services. Our solution: a chemical structure sketching tool based exclusively on CGI server processing, client-side JavaScript functions, and image sequence streaming. The PubChem structure editor does not require the presence of any specific runtime support libraries or browser configurations on the client. It is completely platform-independent and verified to work on all major Web browsers, including older ones without support for Web2.0 JavaScript objects.

NMR, Mass Spectrometry and Chemical Evidence Reveal a Different Chemical Structure for Methanobactin That Contains Oxazolone Rings

Behling, Lee A.; Hartsel, Scott C.; Lewis, David E.; DiSpirito, Alan A.; Choi, Dong W.; Masterson, Larry R.; Veglia, Gianluigi; Gallagher, Warren H.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
EN
Relevância na Pesquisa
45.92%
Methanobactin (mb) is a small copper-binding peptide produced by methanotrophic bacteria and is intimately involved in both their copper metabolism and their role in the global carbon cycle. The structure for methanobactin comprises seven amino acids plus two chromophoric residues that appear unique to methanobactin. In a previously published structure, both chromophoric residues contain a thiocarbonyl attached to a hydroxyimidazolate ring. In addition, one is attached to a pyrrolidine ring, while the other to an isopropyl ester. A published X-ray determined structure for methanobactin shows these two chromophoric groups forming an N2S2 binding site for a single Cu(I) ion with distorted tetrahedral geometry. In this report we show that NMR, mass spectrometry, and chemical data, reveal a chemical structure that is significantly different than the previously published one. Specifically, the 1H and 13C NMR assignments are inconsistent with an N-terminal isopropyl ester and point instead to a 3-methylbutanoyl group. Our data also indicate that oxazolone rings instead of hydroxyimidazolate rings form the core of the two chromophoric residues. Because these rings are directly involved in the binding of Cu(I) and other metals by methanobactin...

BioSM: A metabolomics tool for identifying endogenous mammalian biochemical structures in chemical structure space

Hamdalla, Mai A.; Mandoiu, Ion I.; Hill, Dennis W.; Rajasekaran, Sanguthevar; Grant, David F.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
EN
Relevância na Pesquisa
45.89%
The structural identification of unknown biochemical compounds in complex biofluids continues to be a major challenge in metabolomics research. Using LC/MS there are currently two major options for solving this problem: searching small biochemical databases, which often do not contain the unknown of interest, or searching large chemical databases which include large numbers of non-biochemical compounds. Searching larger chemical databases (larger chemical space) increases the odds of identifying an unknown biochemical compound, but only if non-biochemical structures can be eliminated from consideration. In this paper we present BioSM; a cheminformatics tool that uses known endogenous mammalian biochemical compounds (as scaffolds) and graph matching methods to identify endogenous mammalian biochemical structures in chemical structure space. The results of a comprehensive set of empirical experiments suggest that BioSM identifies endogenous mammalian biochemical structures with high accuracy. In a leave-one-out cross validation experiment, BioSM correctly predicted 95% of 1,388 Kyoto Encyclopedia of Genes and Genomes (KEGG) compounds as endogenous mammalian biochemicals using 1,565 scaffolds. Analysis of two additional biological datasets containing 2...

Chemical vapor deposition thin films as biopassivation coatings and directly patternable dielectrics

Pryce Lewis, Hilton G. (Hilton Gavin), 1973-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 127 leaves; 6851271 bytes; 6867815 bytes; application/pdf; application/pdf
EN_US
Relevância na Pesquisa
55.91%
Organosilicon thin films deposited by pulsed plasma-enhanced chemical vapor deposition (PPECVD) and hot-filament chemical vapor deposition (HFCVD) were investigated as potential biopassivation coatings for neural probes. It was found that organosilicon films from identical precursors differ in structure according to the method of deposition. For films produced from the cyclic siloxane precursor, hexamethylcyclotrisiloxane, pulsed plasma excitation reduced crosslink density over continuous excitation and produced flexible films resistant to prolonged saline soak testing. Deposition via a thermal process, HFCVD, allowed films of novel organosilicon structure to be formed from both hexamethylcyclotrisiloxane and its eight-membered analog, octamethylcyclotetrasiloxane. Characterization of these films was accomplished, and the effect of filament temperature on the chemical structure was elucidated. Silicon-silicon bonding and the retention of ring structures from the precursor was observed in HFCVD organosilicon films using Micro-Raman spectroscopy. A direct dielectric patterning process was proposed for semiconductor manufacturing. In this process, a dielectric material is patterned directly and developed without the need for a sacrificial photoresist layer. HFCVD fluorocarbon films are under consideration as low-dielectric constant interlayer dielectrics...

First-principles molecular modeling of structure-property relationships and reactivity in the zeolite chabazite

Lo, Cynthia
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 139 p.
ENG
Relevância na Pesquisa
55.86%
Zeolites are crystalline, porous aluminosilicates; while a pure silicate structure is charge-neutral, the substitution of A1³⁺ for Si⁴⁺ creates in the framework a negative charge, which can be compensated by a proton that acts as a strong, acid-donating Bronsted site. Zeolites are widely used in industry, most commonly for catalysis and separations. Unfortunately, they have not yet been able to replace all homogeneous catalysts in industrial processes due to the difficulties in reactant and product diffusion to and from the zeolite surface in the absence of a solvent. However, it is believed that if we had a thorough understanding of how solid acids, especially zeolites, catalyze reactions, then we would be able to design heterogeneous catalysts to overcome these difficulties. The nature of the acid sites in zeolites and the factors contributing to enhanced catalytic activity have been the subject of much study in the literature. In particular, the issue of whether all of the acid sites in a particular zeolite are homogeneous or heterogeneous in acid strength requires the development of a systematic way to quantify acidity. To address this, a detailed density functional theory (DFT) investigation of the reactivity of the acid sites in the zeolite chabazite was performed. Energies of adsorption of bases...

Chemical vapor deposition of antimicrobial polymer coatings

Martin, Tyler Philip, 1977-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 120 p.
ENG
Relevância na Pesquisa
55.75%
There is large and growing interest in making a wide variety of materials and surfaces antimicrobial. Initiated chemical vapor deposition (iCVD), a solventless low-temperature process, is used to form thin films of polymers on fragile substrates. To improve research efficiency, a new combinatorial iCVD system was fabricated and used to efficiently determine the deposition kinetics for two new polymeric thin films, poly(diethylaminoethylacrylate) (PDEAEA) and poly(dimethylaminomethylstyrene) (PDMAMS), both candidates for antimicrobial coatings. Fourier transform infrared (FTIR) spectroscopy shows that functional groups are retained in iCVD of PDMAMS and PDEAEA, whereas essentially all fine chemical structure of the material is destroyed in plasma-enhanced CVD. It was found that the combinatorial system in all cases provided agreement, within experimental certainty, with results of blanket iCVD depositions, thus validating the use of the combinatorial system for future iCVD studies. Finished nylon fabric was subsequently coated with PDMAMS by iCVD with no affect on the color or feel of the fabric. Coatings PDMAMS of up to 540 gg/cm2 were deposited on fabric.; (cont.) A coating of 40 gpg/cm2 of fabric was found to be very effective against gram-negative E. coli...

Assignment of the Qy Absorption Spectrum of Photosystem-I from Thermosynechococcus elongatus Based on the CAM-B3LYP Calculations at the PW91-Optimized Protein Structure

Yin, Shiwe; Dahlbom, Mats; Canfield, Peter; Hush, Noel S; Kobayashi, Rika; Reimers, Jeffrey R
Fonte: American Chemical Society Publicador: American Chemical Society
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.74%
The Qy absorption spectrum of Photosystem-I from Thermosynecochoccus elongatus (formerly Synecochoccus elongatus) is calculated using the CAM-B3LYP density functional and INDO schemes based on a quantummechanically refined structure for the entire photosy

Using a dual plasma process to produce cobalt--polypyrrole catalysts for the oxygen reduction reaction in fuel cells -- part II: analysing the chemical structure of the films

Walter, Christian; Kummer, Kurt; Vyalikh, Denis; Brüser, Volker; Quade, Antje; Weltmann, Klaus-Dieter
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 19/09/2012
Relevância na Pesquisa
55.86%
The chemical structure of cobalt--polypyrrole -- produced by a dual plasma process -- is analysed by means of X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption spectroscopy (NEXAFS), X-ray diffraction (XRD), energy-dispersive X-Ray spectroscopy (EDX) and extended x-ray absorption spectroscopy (EXAFS).It is shown that only nanoparticles of a size of 3\,nm with the low temperature crystal structure of cobalt are present within the compound. Besides that, cobalt--nitrogen and carbon--oxygen structures are observed. Furthermore, more and more cobalt--nitrogen structures are produced when increasing the magnetron power. Linking the information on the chemical structure to the results about the catalytic activity of the films -- which are presented in part I of this contribution -- it is concluded that the cobalt--nitrogen structures are the probable catalytically active sites. The cobalt--nitrogen bond length is calculated as 2.09\,\AA\ and the carbon--nitrogen bond length as 1.38\,\AA.

The influence of impregnation by hydrocarbons on coal structure during its thermal evolution

Iglesias, María José; Cuesta, María José; Laggoun-Défarge, Fatima; Suárez Ruiz, Isabel
Fonte: Elsevier Publicador: Elsevier
Tipo: Artículo Formato: 485442 bytes; application/pdf
ENG
Relevância na Pesquisa
55.87%
The present work analyses the changes in the chemical structure of a perhydrous coal during its thermal evolution at different temperatures in an open-medium pyrolysis system. The results obtained were compared with those described for non-perhydrous coals in order to establish the effect of the substances assimilated by the coal structure (hydrocarbon/oil-like substances) on the thermal evolution of the coal. The transformation ratio at each stage of thermal treatment was determined and the chemical-structural characterisation of the resultant products was performed. Changes in textural and microtextural properties associated with structural modifications during the evolution were also tested. The results obtained show that this perhydrous coal develops a specific evolution pathway different from that followed by non-perhydrous coals with a normal H/C ratio. The substances assimilated by the perhydrous coal cannot be easily and totally released from its structure so that they can be only partially removed after thermal treatment. Thus, the treatment debilitates the interactions between the substances and the coal matrix in addition to weakening and cracking the matrix during the thermal process. The increase in temperature also leads to the conversion of some of the heavy assimilated substances into lighter compounds. However...

Control of the chemical structure of perhydrous coals. FTIR and Py-GC/MS investigations

Iglesias, María José; Río Andrade, José Carlos del; Laggoun-Défarge, Fatima; Cuesta, María José; Suárez Ruiz, Isabel
Fonte: Elsevier Publicador: Elsevier
Tipo: Artículo Formato: 550539 bytes; application/pdf
ENG
Relevância na Pesquisa
65.89%
This work analyses a set of perhydrous coals (mainly composed of huminite/vitrinite maceral group) in order to determine the inter-relations between the hydrogen content and the modifications in the coal structure at a molecular level. The study involves the direct solid state characterisation of the coal combined with the analysis of representative fragments of the coal network obtained through flash-pyrolysis. The perhydrous character of the coals is not reflected either in the aliphatic hydrogen concentration (from FTIR data) or by the presence of straight-chain aliphatic moieties in the pyrolysates. This structural study shows that perhydrous coals contain mainly aromatic structures with 1–2 rings and a very small concentration of aromatic rings of large size. In agreement with this, phenol and alkyl phenols are the most prominent degradation products whereas other aromatic compounds (mainly benzene and naphthalene derivatives) are minor and probably evaporative compounds. The major structural elements in the samples studied are simple phenols with a preponderance of substituted para alkyl. The results obtained show that the processes of hydrogen enrichment affect the reactions of aromatisation and condensation. During the natural evolution of the perhydrous coals the transformations of the oxygenated functionalities in the lignin precursor seem to have taken place without the parallel structural reorganisation of the lignin framework responsible for the formation of polycyclic aromatic systems. As a result...

Microbial synthesis of poly(beta-hydroxyalkanoates) bearing phenyl groups from pseudomonas putida: chemical structure and characterization

Abraham, Gustavo A.; Gallardo Ruiz, Alberto; San Román, Julio; Olivera, Elías R.; Jodra, Ruth; García, Belén; Miñambres Rodríguez, Baltasar; García López, José Luis; Luengo, José M.
Fonte: American Chemical Society Publicador: American Chemical Society
Tipo: Artículo Formato: 94510 bytes; application/pdf
ENG
Relevância na Pesquisa
55.76%
6 pages, 3 tables, 3 figures.-- PMID: 11749221 [PubMed].; New poly(beta-hydroxyalkanoates) having aromatics groups (so-called PHPhAs) from a microbial origin have been characterized. These polymers were produced and accumulated as reserve materials when a beta-oxidation mutant of Pseudomonas putida U, disrupted in the gene that encodes the 3-ketoacyl-CoA thiolase (fadA), was cultured in a chemically defined medium containing different aromatic fatty acids (6-phenylhexanoic acid, 7-phenylheptanoic acid, a mixture of them, or 8-phenyloctanoic acid) as carbon sources. The polymers were extracted from the bacteria, purified and characterized by using (13)C nuclear magnetic resonance spectroscopy (NMR), gel permeation chromatography (GPC), and differential scanning calorimetry (DSC). Structural studies revealed that when 6-phenylhexanoic acid was added to the cultures, an homopolymer (poly-3-hydroxy-6-phenylhexanoate) was accumulated. The feeding with 8-phenyloctanoic acid and 7-phenylheptanoic acid leads to the formation of copolymers of the corresponding units with the n - 2 carbons formed after deacetylation, copoly(3-hydroxy-8-phenyloctanoate-3-hydroxy-6-phenylhexanoate) and copoly(3-hydroxy-7-phenylheptanoate-3-hydroxy-5-phenylvalerate)...

Extraction of chemical structures and reactions from the literature

Lowe, Daniel Mark
Fonte: University of Cambridge; Department of Chemistry; Pembroke College Publicador: University of Cambridge; Department of Chemistry; Pembroke College
Tipo: Thesis; doctoral; PhD
EN
Relevância na Pesquisa
46%
The ever increasing quantity of chemical literature necessitates the creation of automated techniques for extracting relevant information. This work focuses on two aspects: the conversion of chemical names to computer readable structure representations and the extraction of chemical reactions from text. Chemical names are a common way of communicating chemical structure information. OPSIN (Open Parser for Systematic IUPAC Nomenclature), an open source, freely available algorithm for converting chemical names to structures was developed. OPSIN employs a regular grammar to direct tokenisation and parsing leading to the generation of an XML parse tree. Nomenclature operations are applied successively to the tree with many requiring the manipulation of an in-memory connection table representation of the structure under construction. Areas of nomenclature supported are described with attention being drawn to difficulties that may be encountered in name to structure conversion. Results on sets of generated names and names extracted from patents are presented. On generated names, recall of between 96.2% and 99.0% was achieved with a lower bound of 97.9% on precision with all results either being comparable or superior to the tested commercial solutions. On the patent names OPSIN s recall was 2-10% higher than the tested solutions when the patent names were processed as found in the patents. The uses of OPSIN as a web service and as a tool for identifying chemical names in text are shown to demonstrate the direct utility of this algorithm. A software system for extracting chemical reactions from the text of chemical patents was developed. The system relies on the output of ChemicalTagger...

Synthesis, structure, and kinetics and stereochemistry of base-catalyzed hydrolysis of meso - and rac -[Co 2 (tmpdtne)Cl 2 ] 4+ , bis(pentaamine) complexes devoid of deprotonatable NH centers

Jackson, William Gregory; Dickie, Alistair J; McKeon, Josephine A; Spiccia, Leone; Brudenell, Suzanne; Hockless, David; Willis, Anthony
Fonte: American Chemical Society Publicador: American Chemical Society
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.8%
The bis(pentadentate) ligand tmpdtne binds two Co(II) centers, and the entity is readily oxidized to the dicobalt(III) derivative [Co 2(tmpdtne)Cl2]4+ which has been separated into two isomeric forms. NMR studies establish these as meso and rac isomers arising from the different or same absolute configurations for the asym configuration about each Co(III) center. Each dinuclear ion base hydrolyses to the dihydroxo derivative [Co2(tmpdtne)(OH)2]4+ with retained asym configurations about each metal ion and also retained rac or meso configurations. The kinetics for the stepwise loss of the two Cl- ligands is uniphasic, and data are presented to show that the loss of the first chloride is rate determining and is followed by very rapid intramolecular and loss of the second Cl- via a hydroxobridged species to yield the observed dihydroxo derivative. Meso and rac forms of the latter have been crystallized. The X-ray crystal structure of the rac-dihydroxo complex is reported, and it establishes the configurations of all the complexes reported. The 1H NMR spectra for the hydroxo ions show very high field Co-OH resonances (ca. δ -0.5 ppm) not observed previously for such ions, and this result is discussed in the context of published 1H NMR data for bridged Co-OH-Co species. The base hydrolysis kinetics for the dichloro ions are first order in [OH-]...

A Density Functional Theory and Electron Momentum Spectroscopy Study into the Complete Valence Electronic Structure of Cubane

Adcock, W; Brunger, Michael J; McCarthy, I E; Michalewicz, M; Von Niessen, W; Wang, Feng; Weigold, Erich; Winkler, D A
Fonte: American Chemical Society Publicador: American Chemical Society
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.8%
A study of the electronic structure of the complete valence shell of cubane is reported. Results from our many-body Green's function calculation, to the third-order algebraic diagrammatic construction (ADC(3)) level, for the binding energies and spectroscopic factors of the respective valence orbitals of cubane are presented. Binding-energy spectra were measured in the energy regime 6-35 eV over a range of different target electron momenta, so that momentum distributions (MDs) could be determined for each orbital. The corresponding theoretical MDs were calculated using a plane wave impulse approximation (PWIA) model for the reaction mechanism and density functional theory (DFT) for the wave function. Seven basis sets, at the local density approximation (LDA) level and, additionally, incorporating nonlocal correlation functional corrections, were studied. The sensitivity of the level of agreement between the experimental and theoretical MDs to the nonlocal corrections is considered. A critical comparison between the experimental and theoretical MDs allows us to determine the 'optimum' wave function for cubane from the basis sets we studied. This wave function is then used to derive cubane's chemically interesting molecular properties. A summary of these results and a comparison of them with those of other workers is presented with the level of agreement typically being good.

Oxidation State Dependence of the Geometry, Electronic Structure, and Magnetic Coupling in Mixed Oxo- and Carboxylato-Bridged Manganese Dimers

Delfs, Christopher; Stranger, Robert
Fonte: American Chemical Society Publicador: American Chemical Society
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.83%
Approximate density functional theory has been used to investigate changes in the geometry and electronic structure of the mixed oxo- and carboxylato-bridged dimers [Mn2(μ-O)2(O2CH) (NH3)6]n+ and [Mn2(μ-O)(O2CH)2 (NH3)6]n+ in the MnIVMnIV, MnIIIMnIV, an