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11.
The dispersion mechanism in flow-injection extraction systems has been investigated. The phase with the highest affinity for the tubing material forms a thin film on the wall (e.g., 0.055 mm in a 0.7-mm i.d. PTFE tube with pentanol/water at a flow velocity of 11 cm s?1). The film thickness increases linearly with increasing flow velocity and can be related to the viscosity/interfacial tension ratio in such a way that a low ratio indicates a thin film. The analyte is extracted into the film and into the adjacent segments. The film is stationary relative to the moving segments and this results in a backward transport of analyte molecules giving rise to dispersion. The thicker the film, the larger the dispersion. By decreasing the tube diameter and the flow velocity, lower dispersion results. Minimum dispersion is obtained for systems in which the phase carrying the analyte does not form the film. 相似文献
12.
13.
We analyze various processes where particles are added irreversibly and sequentially at the sites of infinite ladders or broader strips (i.e., on terraces) of adsorption sites. For sufficiently narrow strips or ladders, exact solution in closed form is possible for a variety of processes. Often this is most naturally achieved by mapping the process onto an equivalent one-dimensional process typically involvingcompetitive adsorption. We demonstrate this procedure for sequential adsorption with nearest-neighbor exclusion on a 2× square ladder. For other select processes on strips slightly too broad for exact solution, almost exact analysis is possible exploiting an empty-site shielding property. In this way, we determine a jamming coverage of 0.91556671 for random sequential adsorption of dimers on a 2× square ladder. For broader strips, we note that the complexity of these problems quickly approaches that for × lattices. 相似文献
14.
Wilhelmsson LM Westerlund F Lincoln P Nordén B 《Journal of the American Chemical Society》2002,124(41):12092-12093
We here report a remarkably slow rearrangement of binding modes for a binuclear ruthenium(II) complex upon interaction with DNA. It has been previously shown that Delta,Delta-[mu-(11,11'-bidppz)(phen)4Ru2]4+ binds to DNA in one of the grooves. However, we find that this is only an initial, metastable, binding mode, which is extremely slowly reorganized into an intercalative binding geometry. The slow rearrangement and dissociation, revealed by flow linear dichroism and fluorescence spectroscopy, are concluded to be a result from the complex being threaded through the DNA, with one of the bridging aromatic dppz ligands intercalated between the base pairs of the DNA, placing one metal center in the minor groove and one in the major groove. A negative LD, a high luminescence quantum yield, and long luminescence lifetimes, similar to the intercalating complex Delta-[Ru(phen)2dppz]2+, indicate intercalation of the bidppz moiety. The unique slow dissociation of the complex in its final DNA-binding mode suggests that this class of threading, partially intercalated binuclear complexes may be interesting in the context of cancer therapy. Also, their unique optical and photophysical properties could make such complexes, either alone or scaffolded by DNA structures, of interest for the development of nanometer-sized molecular optoelectronic devices. 相似文献
15.
Bengt Nordén 《光谱学快报》2013,46(6):483-488
Attention is called to an artifact in linear dichroism (LD), so called absorption statistics, which leads to reduced LD intensity in regions of high absorption. The reduction factor is larger than that governing the corresponding reduction in absorption, when LD is measured by using a Legrand-Grosjean circular dichroism spectrophotometer. In the case of infinite fibres at perfect order it coincides with the absorption statistical reduction factor of optical activity. The artifact was observed for β-carothene solubilized in lecithin liposomes oriented at flow. 相似文献
16.
Gregory B. Kharas Xue Tian Whitney K. Castle Marie B. Cuison Maria R. Fath Bridget Nord 《高分子科学杂志,A辑:纯化学与应用化学》2013,50(4):355-358
Electrophilic trisubstituted ethylene monomers, ring‐substituted 2‐cyano‐N,N‐dimethyl‐3‐phenyl‐2‐propenamides, RC6H4CH?C(CN)CON(CH3)2 (where R is 4‐(CH3)2N, 4‐CH3CO2, 4‐CH3CONH, 2‐CN, 3‐CN, 4‐CN, 4‐(C2H5)2N) were synthesized by potassium hydroxide catalyzed Knoevenagel condensation of ring‐substituted benzaldehydes and N,N‐dimethyl cyanoacetamide, and characterized by CHN elemental analysis, IR, 1H‐ and 13C‐NMR. Novel copolymers of the ethylenes and styrene were prepared at equimolar monomer feed composition by solution copolymerization in the presence of a radical initiator, ABCN at 70°C. The composition of the copolymers was calculated from nitrogen analysis, and the structures were analyzed by IR, 1H and 13C NMR, GPC, DSC, and TGA. High Tg of the copolymers in comparison with that of polystyrene indicates a substantial decrease in chain mobility of the copolymer due to the high dipolar character of the trisubstituted ethylene monomer unit. The gravimetric analysis indicated that the copolymers decompose in the 300–450°C range. 相似文献
17.
A semi-empirical SCF-MO method, the PEEL method, has been applied in an investigation of the electronic structure and excited states of two iron compounds, [Fe(II)-(GMI)3]++ and [Fe(II)-(bipy)3]++.The electronic absorption spectra have been recorded. The calculations show that it is necessary to account for the trigonal distortion and the covalency in order to explain these spectra quantitatively. Mössbauer measurements have also been performed. The calculated electronic population of the iron is in accord with Mössbauer isomer shift data, indicating that a realistic electron distribution has been obtained by the PEEL method.
Zusammenfassung Eine semiempirische SCF-MO-Methode, die PEEL-Methode, wurde zur Untersuchung der Elektronenstruktur sowie von angeregten Zuständen der beiden Eisenverbindungen [Fe(II)-(GMI)3]++ und [Fe(II)-(bipy)3]++ angewendet.Die elektronischen Absorptionsspektren wurden aufgenommen. Die Berechnungen zeigen, daß die trigonale Verzerrung und die Kovalenz berücksichtigt werden müssen, um die Spektren quantitativ zu erklären. Mössbauer-Messungen wurden ebenfalls durchgeführt. Die berechnete Elektronenverteilung am Eisenatom ist in Übereinstimmung mit den Daten der Isomerenverschiebung der Mössbauer-Messungen, wodurch gezeigt wird, daß mit der PEEL-Methode eine realistische Elektronen-Verteilung erhalten wurde.
Résumé Une méthode SCF-MO semi-empirique, la méthode PEEL, a été appliquée à une étude de la structure électronique et des états excités de deux composés ferreux: [Fe(II)-(bipy)3]++ et [Fe(II)-(GMI)3]++. Les spectres d'absorption électronique ont été enregistrés. Les calculs montrent qu'il est nécessaire de rendre compte de la distorsion trigonale et de la covalence pour expliquer ces spectres quantitativement.Des mesures de l'effet Mössbauer ont aussi été effectuées. La population électronique calculée du fer est en bon accord avec les données sur le déplacement isomérique de Mössbauer, ce qui indique que la méthode PEEL fournit une distribution électronique réaliste.相似文献
18.
Couturier M Ménard F Ragan JA Riou M Dauphin E Andresen BM Ghosh A Dupont-Gaudet K Girardin M 《Organic letters》2004,6(11):1857-1860
Lewis-base-catalyzed cycloisomerization of bis(enones) to decalins has been demonstrated as an alternative to the traditional Lewis acid catalyzed Diels-Alder cycloaddition. In this process, a trialkylphosphine mediates both bond formation steps in two distinct catalytic cycles. The single-pot operation generates two carbon-carbon bonds and up to five contiguous stereocenters in one step, starting from achiral, aliphatic substrates; eight examples are provided. [reaction: see text] 相似文献
19.
The rotational spectra of o-fluorotoluene and its seven 13C isotopic species were recorded in the frequency range from 4 to 20 GHz with employment of pulsed molecular beam Fourier-transform microwave (MB-FTMW) spectrometers. The analysis of the spectra in the two lowest states of methyl internal rotation (torsional ground state, A and E species) was based on a asymmetric frame-rigid symmetric top Hamiltonian with inclusion of centrifugal distortion terms, yielding structural rotational constants, as well as the threefold barrier V
3 to internal rotation and the angle(a,i) between the principal moment of inertia a axis and the internal rotor axis i. The rotational constants of all eight isotopomeres were used to derive the seven 13C r
s
coordinates of the molecule. 相似文献
20.