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61.
Ernesto Carmona Jose M. Marin Manuel L. Poveda Robin D. Rogers Jerry L. Atwood 《Journal of organometallic chemistry》1982,238(4):C63-C66
The reduction of [WCl4(PMe3)3] with dispersed sodium, under dinitrogen, gives cis-[W(N2)2(PMe3)4], while under ethylene trans-[W(C2H4)2(PMe3)4] is obtained. The ethylene complex can also be prepared by displacement of the dinitrogen molecules in cis-[W(N2)2(PMe3)4] by ethylene at room temperature and pressure. Interaction of cis-[M(N2)2(PMe3)4] complexes (M = Mo, W), with PMe3, under helium or argon, yields [M(N2)(PMe3)5]. The molybdenum complex crystallizes in the orthorhombic space group Pnma, with a 22.063(6), b 12.106(4), c 9.745(4) Å. The Mo—P distance trans to the dinitrogen ligand (2.483(7) Å) is slightly longer than the average of the other four Mo—P bonds (2.460(5) Å). 相似文献
62.
Analytical fractionation of aquatic humic substances and their metal species by means of multistage ultrafiltration 总被引:2,自引:0,他引:2
Brit Aster P. Burba Jose A. C. A. C. Broekaert 《Fresenius' Journal of Analytical Chemistry》1996,354(5-6):722-728
The molecular-size fractionation of aquatic humic substances (HS) and their metal species by means of a novel sequential-stage ultrafiltration (UF) device equipped with five appropriate ultramembranes (1, 5, 10, 50 and 100 kD) is described. First of all, the concentration dynamics of macromolecules, particulary HS, during five-stage UF and its subsequent washing step has been modelled. Based on these results, the fractionation of aquatic HS (from ground and bog water) by means of multistage UF has been optimized for an analytical scale (10 ml sample, 1 mg/ml HS, 10 ml washing solution, pH 6.0). The molecular size-distribution of selected aquatic HS (BOC 1/2 from the DFG-Versuchsfeld Bocholt, VM 5 from Venner Moor, Germany) studied by five-stage UF exhibited strong systematic influences of the procedure used for their isolation. The molecular-size distribution of HS obtained by on-line UF and gel permeation chromatography (GPC) showed a satisfactory agreement in the range 1–50 kD. Moreover, when interrupting multistage UF for > 48 h a slow transformation in the HS samples has been found as gradually additional HS fractions of < 1 kD have been formed. Besides unloaded HS molecules, the molecular-size distribution of freshly formed metal species of HS (1.0 mg metal/g HS of Al(III), Cd(II), Cu(II), Fe(III), Mn(II), Ni(II), Pb(II), Zn(II), each) has been characterized by multistage UF as a function of pH-value, degree of loading and complexation time. Metal determinations as carried out by flame AAS, showed that considerable metal fractions in HS especially are present in molecules > 50 kD, which seemed to be rather acid-inert. With complexation times of < 2 days a transient shift of the molecular size distribution of both HS and their metal species (e.g., Al(III), Fe(III) to higher values (> 10 kD) has been found. 相似文献
63.
Gallignani M Valero M Ayala C Del Rosario Brunetto M Sánchez A Burguera JL Burguera M 《Talanta》2004,64(5):1290-1298
In this work, the coupling between flow analysis (FA)–vapor phase generation (VPG) and Fourier transform infrared spectrometry (FTIR) has been proposed as a novel and alternative strategy for the determination of nitrite. The analyte was transformed into the gaseous nitric oxide (NO) by on-line reaction with potassium iodide (KI) or ascorbic acid in acidic medium. The gaseous NO generated was transported by means of a N2 gas carrier stream inside the IR gas cell and the corresponding FTIR spectrum was acquired in a continuous mode. The absorbance at 1876 cm−1, corrected by a baseline established between 1879 and 1872 cm−1 at a nominal resolution of 2 cm−1, was selected as a measurement criterion. The effect of different spectroscopic and flow analysis experimental parameters, such as nominal resolution, number of scans, reducing agent and its concentration, acidic medium, reagents and sample flow rates, and the carrier gas flow rate on the analytical signal, and then in the figures of merit were initially evaluated by using a standard short path length (10 cm) IR gas cell. The optimization of the system was carried out by the univariate method. The main aims of this study were: (i) to investigate the on-line generation of gaseous nitric oxide in a continuous flow system, and (ii) the use of Fourier transform infrared spectrometry as an alternative and selective detector for the determination of nitrite. The proposed method was initially tested and applied for the determination of nitrite in samples with very high concentration of nitrite, such as frankfurters. 相似文献
64.
A method is described for the simultaneous determination of organophosphorus and triazine pesticides in olive oil, whereby reversed-phase liquid chromatography (LC) is coupled to gas chromatography by means of an automated through-oven transfer adsorption-desorption (TOTAD) interface. The olive oil needs to be filtered only before it is loaded into the liquid chromatograph, where preseparation of the pesticide residues from the other olive oil components is carried out by using methanol-water as the eluant. The LC fraction containing the pesticides is automatically transferred to the gas chromatograph by using the TOTAD interface, which almost totally eliminates the solvent, so that water-sensitive detectors such as the nitrogen-phosphorus detector can be used. Detection limits range from 0.07 to 0.38 microg/L for organophosphorus pesticides and from 6.0 to 7.0 microg/L for triazines. The results were compared with those obtained by flame ionization detection. 相似文献
65.
Summary Copper(II) and nickel(II) complexes with the Schiff base derived from 2-(2-aminophenyl)benzimidazole and salicylaldehyde, L, have been prepared. They are of the general types ML2X2 (M = Ni or Cu and X = Cl, Br, NO3 or ClO4) and NiL(NCS)2.The compounds have been characterized by elemental analyses, magnetic measurements, e.s.r., electronic and i.r. spectra studies. The i.r. spectra suggest that the molecule, and not the anion, of the Schiff base is coordinated as a bidentate ligand with the metal ion. Possible structures for the complexes have been proposed. 相似文献
66.
Dyke JM Levita G Morris A Ogden JS Dias AA Algarra M Santos JP Costa ML Rodrigues P Andrade MM Barros MT 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(5):1665-1676
The thermal decompositions of methyl azidoformate (N3COOMe), ethyl azidoformate (N3COOEt) and 2-azido-N,N-dimethylacetamide (N3CH2CONMe2) have been studied by matrix isolation infrared spectroscopy and real-time ultraviolet photoelectron spectroscopy. N2 appears as an initial pyrolysis product in all systems, and the principal interest lies in the fate of the accompanying organic fragment. For methyl azidoformate, four accompanying products were observed: HNCO, H2CO, CH2NH and CO2, and these are believed to arise as a result of two competing decomposition routes of a four-membered cyclic intermediate. Ethyl azidoformate pyrolysis yields four corresponding products: HNCO, MeCHO, MeCHNH and CO2, together with the five-membered-ring compound 2-oxazolidone. In contrast, the initial pyrolysis of 2-azido-N,N-dimethyl acetamide, yields the novel imine intermediate Me2NCOCH=NH, which subsequently decomposes into dimethyl formamide (HCONMe2), CO, Me2NH and HCN. This intermediate was detected by matrix isolation IR spectroscopy, and its identity confirmed both by a molecular orbital calculation of its IR spectrum, and by the temperature dependence and distribution of products in the PES and IR studies. Mechanisms are proposed for the formation and decomposition of all the products observed in these three systems, based on the experimental evidence and the results of supporting molecular orbital calculations. 相似文献
67.
Ana Cristina Gmez-Herrero Carlos Snchez-Snchez Frdric Chrioux Jose Ignacio Martínez Jos Abad Luca Floreano Alberto Verdini Albano Cossaro Estelle Mazaleyrat Valrie Guisset Philippe David Simone Lisi Jos Angel Martín Gago Johann Coraux 《Chemical science》2021,12(6):2257
Catechols are ubiquitous substances often acting as antioxidants, thus of importance in a variety of biological processes. The Fenton and Haber–Weiss processes are thought to transform these molecules into aggressive reactive oxygen species (ROS), a source of oxidative stress and possibly inducing degenerative diseases. Here, using model conditions (ultrahigh vacuum and single crystals), we unveil another process capable of converting catechols into ROSs, namely an intramolecular redox reaction catalysed by a Cu surface. We focus on a tri-catechol, the hexahydroxytriphenylene molecule, and show that this antioxidant is thereby transformed into a semiquinone, as an intermediate product, and then into an even stronger oxidant, a quinone, as final product. We argue that the transformations occur via two intramolecular redox reactions: since the Cu surface cannot oxidise the molecules, the starting catechol and the semiquinone forms each are, at the same time, self-oxidised and self-reduced. Thanks to these reactions, the quinone and semiquinone are able to interact with the substrate by readily accepting electrons donated by the substrate. Our combined experimental surface science and ab initio analysis highlights the key role played by metal nanoparticles in the development of degenerative diseases.An antioxidant catechol transforms following intramolecular redox reactions into highly reactive oxygen species, a semiquinone and a quinone, on copper. 相似文献
68.
69.
Jose G. Trujillo‐Ferrara Rosa Luisa Santilln‐Baca Norberto Farfn‐García Itzia I. Padilla‐Martínez Efrn V. García‐Bez 《Acta Crystallographica. Section C, Structural Chemistry》2006,62(3):o125-o128
The molecular and supramolecular structures of 2‐(1,3‐dioxo‐2,3,3a,4,7,7a‐hexahydro‐1H‐isoindol‐2‐yl)phenyl acetate, C16‐H15NO4, (I), and its para isomer, 4‐(1,3‐dioxo‐2,3,3a,4,7,7a‐hexahydro‐1H‐isoindol‐2‐yl)phenyl acetate, (II), are reported. The torsion angle between the succinimide and benzene rings depends on the position of the acetoxy substitution [89.7 (1) and 61.9 (1)° for (I) and (II), respectively]. The twist of the acetoxy group relative to the mean plane of the benzene ring is almost independent of the acetoxy position [66.0 (1) and 70.0 (1)°]. Packing interactions for both compounds include soft C—H⋯X (X = O and Ph) interactions, forming chains of centrosymmetric dimers and interlinked chains for (I) and (II), respectively. In addition, three perpendicular dipole C=O⋯C=O interactions contribute to the supramolecular structure of (II). 相似文献
70.
Cabaleiro-Lago EM Rodríguez-Otero J Varela-Varela SM Peña-Gallego A Hermida-Ramón JM 《The Journal of organic chemistry》2005,70(10):3921-3928
[reactions: see text] Electrocyclization reactions of (3Z)-1,3,5-hexatrienone and nitrogen derivatives were studied by performing density functional theory (DFT) calculations together with the 6-31+G* basis set. Reactants, products, and transition states for each reaction were localized and the IRC connecting reactants and products was also obtained. Magnetic properties were evaluated along the reaction path to elucidate the characteristics of the reactions studied. As obtained from the calculations, electrocyclization of (3Z)-1,3,5-hexatrienone is a pericyclic process, as indicated by a variety of indexes, such as Nucleus Independent Chemical Shift (NICS), anisotropy of the magnetic susceptibility, or anisotropy of the current-induced density (ACID). This reaction presents characteristics of pericyclic reactions despite the activation energy lowering relative to the electrocyclization of (4Z)-1,2,4,6-heptatetraene, and the relatively small NICS values observed in the transition state. Magnetic properties indicate that an enhancement of the aromaticity relative to reactants and products occurs revealing the absence of orbital disconnections on the cyclic loop of interacting orbitals. Only two reactions among those studied exhibit pseudopericyclic character due to the in-plane attack of the lone pair on nitrogen. In these cases, the reactions showed no barrier for the electrocyclization process, and no aromaticity enhancement was observed. 相似文献