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271.
Frank Lindenberg Joachim Sieler Evamarie Hey-Hawkins Ulrich Müller Astrid Pilz 《无机化学与普通化学杂志》1996,622(4):683-688
Synthesis and Insertion Reactions of Cp2′HfCl{As(SiMe3)2} (Cp′ = C5H4Me) The reaction of Cp2′HfCl2 (Cp′ = C5H4Me) with Li(THF)2,5As(SiMe3)2 (1 : 1) at room temperature gives the terminal hafnocene arsenido complex Cp2′HfCl{As(SiMe3)2} ( 1 ) in high yield. 1 inserts CS2 and PhNC into the Hf? As bond yielding Cp2′HfCl{η2-S2CAs(SiMe3)2} ( 2 ) and Cp2′HfCl{η2-N(Ph)CAs(SiMe3)2} ( 3 ). The thermally sensitive complexes 1–3 were characterised spectroscopically and crystal structure determinations were carried out on 1 and 3 which shows the η2 bonding mode of the N(Ph)CAs(SiMe3)2 ligand in the latter. 相似文献
272.
Astrid Drechsler Nicole Petong Junfeng Zhang Daniel Y. Kwok Karina Grundke 《Colloids and surfaces. A, Physicochemical and engineering aspects》2004,250(1-3):357-366
The interaction force between a very hydrophobic polymer surface and colloidal silica particles with a roughness of 10–15 nm has been measured in aqueous solutions of KOH and KCl using an atomic force microscope. The interaction can be described according to the DLVO theory by an electrical double-layer force that is repulsive at long distances and attractive at short distances and an attractive van der Waals force. The electrical double-layer potentials are compared to the zeta potentials of Teflon AF and the silica spheres. The roughness of the silica particles leads to an underestimation of the short-range attraction and the surface potential. Both KCl and KOH solutions affect the potential of the interacting surfaces. OH− ions that adsorb preferentially to the Teflon AF surface create higher potentials than Cl− ions. Range and strength of the attractive interaction are not affected by KCl solutions but reduced by addition of KOH. This can be explained by decreasing potential differences between the silica sphere and Teflon AF with increasing KOH concentration. In addition, the preferential adsorption of OH− ions may lead to a reduction of the van der Waals interaction. The presence of nanobubbles, too, might play a role. 相似文献
273.
Percio Augusto Mardini Farias Arnaldo Aguiar Castro Angela de Luca Rebello Wagener Astrid Altamirano Junqueira 《Electroanalysis》2007,19(11):1207-1212
A stripping method for the determination of single‐stranded DNA in presence of copper at the submicromolar concentration levels is described. The method is based on controlled adsorptive accumulation of adenine (from acid‐treated DNA) at thin‐film mercury electrode followed by linear scan voltammetry measurement of the surface species. Optimum experimental conditions were found to be the use of a 5.0×10?3 M NaOH solution, an accumulation potential of ?0.40 V and a scan rate of 200 mV s?1. The response of adenine–copper is linear over the concentration range 50–250 ppb. For an accumulation time of 15 minutes, the detection limit was found to be 4 ppb. The more convenient relation to measuring the ssDNA in presence of metals and nitrogenated bases were also investigated. The utility of the method is demonstrated by the presence of adenosine‐triphosphate (ATP) and amino acids. 相似文献
274.
Astrid Momberg V. María E. Carrera B. Dietrich von Baer Carlos Bruhn F. Malcolm R. Smyth 《Analytica chimica acta》1984
The voltammetric behaviour of sulphanilic acid and eight sulphonamides at the glassy carbon electrode is discussed. These compounds were found to undergo a 2e? process which appeared to involve the formation of iminobenzoquinone as the final product. The pKa values relating to the deprotonation of the p-amino substituent on the phenyl ring, evaluated from the voltammetric data, were found to be in good agreement with those obtained from ultraviolet spectroscopic data. Voltammetric methods were applied to the analysis of selected mixtures, to the determination of sulphaguanidine in a drug formulation, and as a method for the detection of sulphonamides after high-performance liquid chromatography. 相似文献
275.
The different stationary points on the potential energy surface relative to the title reaction have been reinvestigated at the B3LYP/aug-cc-pVDZ level with relative energies computed at the CCSD(T)/aug-cc-pVTZ level with B3LYP/aug-cc-pVDZ optimized geometries and by using the G3B3 composite method. Two entrance channels have been identified. The first one corresponds to boron addition at one of the oxygen atoms of the CO 2 molecule leading to trans-BOCO, which is found to be about 27 kcal/mol exothermic with a potential energy barrier of 16.4 kcal/mol (G3B3). The second channel, which has not been identified in previous theoretical works, corresponds to a direct insertion of the boron atom into a CO bond and leads to OBCO. The B + CO 2 --> OBCO step is found to be about 84 kcal/mol exothermic and needs to overcome a potential energy barrier of only 3.6 kcal/mol (G3B3). The rate constant at 300 K of the insertion step, calculated by using TST theory with G3B3 calculated activation energy value, is 5.4 10 (-14) cm (3) molecule (-1) s (-1), in very good agreement with the experimental data ((7.0 +/- 2.8) 10 (-14) cm (3) molecule (-1) s (-1), DiGiuseppe, T. G.; Davidovits, P. J. Chem. Phys. 1981, 74, 3287). The one corresponding to the addition process is found to be several orders of magnitude smaller because of a much higher potential energy barrier. The addition channel would not contribute to the title reaction even at high temperature. A modified Arrhenius equation has been fitted in the 300-1000 K temperature range, which might be useful for chemical models. 相似文献
276.
277.
Nanophase cobalt, nickel and zinc ferrites, in which the crystallites are in the size range 4-25 nm, were synthesised by coprecipitation and subsequent annealing. X-Ray absorption spectroscopy using synchrotron radiation (supported by X-ray powder diffraction) was used to study the effects of particle size on the distributions of the metal atoms over the tetrahedral and octahedral sites of the spinel structure. Deviations from the bulk structure were found which are attributed to the significant influence of the surface on very small particles. Like the bulk material, nickel ferrite is an inverse spinel in the nanoregime, although the population of metals on the octahedral sites increases with decreasing particle size. Cobalt ferrite and zinc ferrite take the inverse and normal forms of the spinel structure respectively, but within the nanoregime both systems show similar trends in being partially inverted. Further, in zinc ferrite, unlike the normal bulk structure, the nanophase system involves mixed coordinations of zinc(ii) and iron(iii) consistent with increasing partial inversion with size. 相似文献
278.
Bacteria have a great influence on the migration behaviour of heavy metals in the environment. Lipopolysaccharides form the main part of the outer membrane of Gram-negative bacteria. We investigated the interaction of the uranyl cation (UO2(2+)) with lipopolysaccharide (LPS) from Pseudomonas aeruginosa by using potentiometric titration and time-resolved laser-induced fluorescence spectroscopy (TRLFS) over a wide pH and concentration range. Generally, LPS consists of a high density of different functionalities for metal binding such as carboxyl, phosphoryl, amino and hydroxyl groups. The dissociation constants and corresponding site densities of these functional groups were determined using potentiometric titration. The combination of both methods, potentiometry and TRLFS, show that at an excess of LPS uranyl phosphoryl coordination dominates, whereas at a slight deficit on LPS compared to uranyl, carboxyl groups also become important for uranyl coordination. The stability constants of one uranyl carboxyl complex and three different uranyl phosphoryl complexes and the luminescence properties of the phosphoryl complexes are reported. 相似文献
279.
Middelthon-Bruer TM Gjelstad A Rasmussen KE Pedersen-Bjergaard S 《Journal of separation science》2008,31(4):753-759
Thirty-five different basic drugs were extracted by electro membrane extraction (EME), from acidified samples containing HCl as the BGE, through an organic solvent immobilized in the pores in the wall of a porous hollow fiber (supported liquid membrane, SLM), and into an acidified acceptor solution (HCl) in the lumen of the hollow fiber by the application of an electrical potential difference of 50 V. With 2-nitrophenyl pentyl ether (NPPE) as the SLM, and with 10 mM HCl as BGE in the sample and acceptor solution, singly charged basic drugs with log P >2 were extracted with recoveries in the range 30-81% within 5 min. For doubly charged basic drugs, extraction was effectively enhanced by decreasing the concentration of HCl in the sample from 10 to 0.1 mM, reducing the ionization of the analytes. For medium polar analytes (1 < log P < 2), an ion balance of 0.01 was combined with addition of tris-(2-ethylhexyl) phosphate (TEHP) to the SLM, and this provided recoveries in the range 36-70%. The ion balance was defined as the concentration ratio of BGE between the sample and the acceptor solution. For the most polar drugs (log P <1), EME was accomplished with an ion balance of 0.01 and with di-(2-ethylhexyl) phosphate (DEHP) added to the SLM, but in spite of this, recoveries were in the range of only 4-17%. 相似文献
280.
Stadler A Brenner J Pauschitz A Wendrinsky J Schindel A Nauer GE 《Analytical and bioanalytical chemistry》2008,390(6):1527-1535
The aim of this study was to gain knowledge of tribological layers on a steel disc. This steel disc has been used in a transmission
system in the field. ATF-oil (automatic transmission fluid), which contains different surface-adhering additives, serves as
a lubricant and as a cooling fluid in the tribological system. Over time, the tribological characteristics of the system changed.
Special interest in this study lies in the modifications of the steel surface. The field-used disc was therefore compared
with model tested discs. In a disc-on-disc tribometer, all samples are tested for a short period of time to evaluate the actual
condition of the system regarding friction behaviour. Analysis and characterisation of surfaces and layer formation were carried
out with, among other techniques, 3D topography, SEM / EDX, AFM, XPS, and ToF-SIMS. Results indicate that in some regions
a complex composed tribological layer is formed. Differences were detected between the steel discs used in the field and the
model discs. This indicates the problem of evaluation of long-term behaviour exclusively by use of short or accelerated model
experiments. A combination of carefully selected and sophisticated analytical methods is necessary to trace small changes
of the system.
Figure Atomic force measurement of field used and model tested “new” friction discs 相似文献