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541.
Raquel B.R. Mesquita M. Teresa S.O.B. Ferreira Ildikó V. Tóth Adriano A. Bordalo Ian D. McKelvie António O.S.S. Rangel 《Analytica chimica acta》2011,701(1):15
A sequential injection system with dual analytical line was developed and applied in the comparison of two different detection systems viz; a conventional spectrophotometer with a commercial flow cell, and a multi-reflective flow cell coupled with a photometric detector under the same experimental conditions. The study was based on the spectrophotometric determination of phosphate using the molybdenum-blue chemistry. The two alternative flow cells were compared in terms of their response to variation of sample salinity, susceptibility to interferences and to refractive index changes. The developed method was applied to the determination of phosphate in natural waters (estuarine, river, well and ground waters). The achieved detection limit (0.007 μM PO43−) is consistent with the requirement of the target water samples, and a wide quantification range (0.024–9.5 μM) was achieved using both detection systems. 相似文献
542.
Sandra Gomez‐Esteban Marco Pezella Angel Domingo Dr. Gunther Hennrich Dr. Berta Gómez‐Lor 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(47):16080-16086
Three truxene derivatives functionalized with alkyl chains, either attached directly or distanced by linking phenyl or ethynyl groups, self‐assemble in solution and induce the gelation of different solvents in spite of not being endowed with groups able to establish strong directional interactions. A 1H NMR study points to face‐to‐face alternating π‐stacked motifs at the origin of nucleation. Solvents play an important role in modulating the aggregation of these derivatives giving rise to fibrous or spherical superstructures. Analysis of the influence of different solvents on the morphology of the aggregates provides a better understanding of the various stages of the hierarchical self‐assembly. The way in which alkyl chains are attached to the central core also strongly affects the self‐assembling properties and gelation ability of this series. Phenyl spacers present the highest association constants in solution and give rise to gelation in a broader range of solvents. This behavior has been rationalized by means of 1H NMR spectroscopy, X‐ray powder diffraction, SEM, and photophysical measurements. Interestingly, it was found that these compounds in the gel state exhibit unusual emission properties most likely arising from the formation of excimers, which evidences that π–π interactions also occur in the excited state. 相似文献
543.
Arlette Vega-González Pascale Subra-Paternault Ana M. López-Periago Carlos A. García-González Concepción Domingo 《European Polymer Journal》2008,44(4):1081-1094
A supercritical carbon dioxide (SCCO2) antisolvent technique was used for the precipitation of several biopolymers into fibers organized in a three-dimensional network. The work was first focused on separately processing either a biodegradable (polycaprolactone or polylactic acid) or a non-biodegradable (polymethylmetacrylate) homopolymer. Second, we established that the antisolvent supercritical technique can be also used to make fibrous networks of blends constitute by non-biodegradable and biodegradable polymers (polymethylmetacrylate/polycaprolactone). The influence of several operating variables (e.g., liquid solution concentration or flow rate and nozzle design) on the polymers morphology and properties was evaluated. For all studied systems, fibers with a rough textured surface and an extremely high surface area in the order of 100–400 m2 g?1 were precipitated. Prepared materials have potential applications in tissue engineering, since they have intrinsic advantages from a biomimetic approach. 相似文献
544.
Many faces of dipyrrins: from hydrogen-bonded networks to homo- and heteronuclear metallamacrocycles
The amphoteric 5-(4-cyanophenyl)dipyrrin ligand, offering three distinct states, i.e., cationic, neutral, and anionic, has been exploited for the formation of a 1-D hydrogen-bonded network in its protonated form and both homo- and heterobinuclear metallamacrocycles, in its neutral and deprotonated states, respectively, with a variety of coordination modes. 相似文献
545.
The formal [2sigma + 2sigma + 2pi] cycloaddition of quadricyclane, 1, with dimethyl azodicarboxylate, 2, in water has been studied using DFT methods at the B3LYP/6-31G** and MPWB1K/6-31G** levels. In the gas phase, the reaction of 1 with 2 has a two-stage mechanism with a large polar character and an activation barrier of 23.2 kcal/mol. Inclusion of water through a combined discrete-continuum model changes the mechanism to a two-step model where the first nucleophilic attack of 1 to 2 is the rate-limiting step with an activation barrier of 14.7 kcal/mol. Analysis of the electronic structure of the transition state structures points out the large zwitterionic character of these species. A DFT analysis of the global electrophilicity and nucleophilicity of the reagents provides a sound explanation about the participation of 1 as a nucleophile in these cycloadditions. This behavior is reinforced by a further study of the reaction of 1 with 1,1-dicyanoethylene. 相似文献
546.
The purpose of this paper is to study pointwise upper bounds for integral operators whenever some bound for the input function is known. Applications to the cases of Hardy, Riemann–Liouville or Volterra type operators and the Abel transform, among others, are given. The underlying techniques are closely related to Yano's extrapolation theory. 相似文献
547.
548.
Let G be a finite (additive written) abelian group of order n. Let w1,…,wn be integers coprime to n such that w1+w2+?+wn≡0 (mod n). Let I be a set of cardinality 2n-1 and let ξ={xi:i∈I} be a sequence of elements of G. Suppose that for every subgroup H of G and every a∈G, ξ contains at most terms in a+H.Then, for every y∈G, there is a subsequence {y1,…,yn} of ξ such that y=w1y1+?+wnyn.Our result implies some known generalizations of the Erd?s-Ginzburg-Ziv Theorem. 相似文献
549.
Neus Domingo Alberto M. Testa Dino Fiorani Chris Binns Stephen Baker Javier Tejada 《Journal of magnetism and magnetic materials》2007
Magnetic properties of Co nanoparticles of 1.8 nm diameter embedded in Mn and Ag matrices have been studied as a function of the volume fraction (VFF). While the Co nanoparticles in the Ag matrix show superparamagnetic behavior with TB=9.5 K (1.5% VFF) and TB=18.5 K (8.9% VFF), the Co nanoparticles in the antiferromagnetic Mn matrix show a transition peak at ∼65 K in the ZFC/FC susceptibility measurements, and an increase of the coercive fields at low temperature with respect to the Ag matrix. Exchange bias due to the interface exchange coupling between Co particles and the antiferromagnetic Mn matrix has also been studied. The exchange bias field (Heb), observed for all Co/Mn samples below 40 K, decreases with decreasing volume fraction and with increasing temperature and depends on the field of cooling (Hfc). Exchange bias is accompanied by an increase of coercivity. 相似文献
550.
A local reactivity difference index R(k) is shown to be able to predict the local electrophilic and/or nucleophilic activation within an organic molecule. Together with the electrophilic and/or nucleophilic behavior of the center k given by the sign, the magnitude of the R(k) index accounts for the extent of the electronic activation, a behavior that allows for the use of the R(k) index as a measure of the molecular reactivity especially in polar processes. 相似文献