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991.
Counterions Control Whether Self‐Assembly Leads to Formation of Stable and Well‐Defined Unilamellar Nanotubes or Nanoribbons and Nanorods
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Dr. Dong Shi Christian Schwall George Sfintes Dr. Erling Thyrhaug Dr. Peter Hammershøj Dr. Marite Cárdenas Dr. Jens B. Simonsen Dr. Bo W. Laursen 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(23):6853-6856
Self‐assembly of the amphiphilic π‐conjugated carbenium ion ATOTA‐1+ in aqueous solution selectively leads to discrete and highly stable nanotubes or nanoribbons and nanorods, depending on the nature of the counterion (Cl? vs. PF6?, respectively). The nanotubes formed by the Cl? salt illustrate an exceptional example of a structural well‐defined (29±2 nm in outer diameter) unilamellar tubular morphology featuring π‐conjugated functionality and high stability and flexibility, in aqueous solution. 相似文献
992.
Jens A. Iversen Rolf W. Berg Birgitte K. Ahring 《Analytical and bioanalytical chemistry》2014,406(20):4911-4919
Compared to traditional IR methods, Raman spectroscopy has the advantage of only minimal interference from water when measuring aqueous samples, which makes this method potentially useful for in situ monitoring of important industrial bioprocesses. This study demonstrates real-time monitoring of a Saccharomyces cerevisiae fermentation process using a Raman spectroscopy instrument equipped with a robust sapphire ball probe. A method was developed to correct the Raman signal for the attenuation caused by light scattering cell particulate, hence enabling quantification of reaction components and possibly measurement of yeast cell concentrations. Extinction of Raman intensities to more than 50 % during fermentation was normalized with approximated extinction expressions using Raman signal of water around 1,627 cm?1 as internal standard to correct for the effect of scattering. Complicated standard multi-variant chemometric techniques, such as PLS, were avoided in the quantification model, as an attempt to keep the monitoring method as simple as possible and still get satisfactory estimations. Instead, estimations were made with a two-step approach, where initial scattering correction of attenuated signals was followed by linear regression. In situ quantification measurements of the fermentation resulted in root mean square errors of prediction (RMSEP) of 2.357, 1.611, and 0.633 g/L for glucose, ethanol, and yeast concentrations, respectively. 相似文献
993.
Natalia Quinete Thomas Schettgen Jens Bertram Thomas Kraus 《Analytical and bioanalytical chemistry》2014,406(25):6151-6164
Polychlorinated biphenyls (PCBs) are among the most ubiquitous pollutants in the environment, and their metabolism leads to the formation of hydroxylated PCBs (OH-PCBs) and methyl sulfone PCBs (MeSO2-PCBs). These metabolites are generally more hydrophilic than the parent compound, and therefore are more easily eliminated from the body. However, some congeners have been shown to be strongly retained in human blood, binding to transthyretin with an affinity that is, in general, greater than that of the natural ligand thyroxin itself, which could result in toxicological effects, particularly on the thyroid system. Currently available analytical methods require, in general, extensive sample preparation, which includes a series of time-consuming and low-throughput liquid–liquid and back extractions, evaporations, several cleanup steps, and in some cases, derivatization prior to analysis by gas chromatography (GC) or liquid chromatography (LC) coupled with mass spectrometry (MS). Recent developments in the use of LC coupled with tandem MS (MS/MS) have brought some improvements in terms of sample preparation for the determination of PCB metabolites in blood, although there are still possibilities for continued development. The selected literature has evidenced few studies of LC–MS/MS-based methods, a lack of analytical standards, nonassessment of lower-chlorinated OH-PCBs, and scarce attention to MeSO2-PCBs in blood. This review aims to evaluate critically the currently available analytical methods for determination of OH-PCBs and MeSO2-PCBs in blood. 相似文献
994.
Efficient Access to Substituted Silafluorenes by Nickel‐Catalyzed Reactions of Biphenylenes with Et2SiH2
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Jens Michael Breunig Puneet Gupta Dr. Animesh Das Dr. Samat Tussupbayev Dr. Martin Diefenbach Dr. Michael Bolte Prof. Dr. Matthias Wagner Prof. Dr. Max C. Holthausen Dr. Hans‐Wolfram Lerner 《化学:亚洲杂志》2014,9(11):3163-3173
The reaction of biphenylene ( 1 ) with Et2SiH2 in the presence of [Ni(PPhMe2)4] results in the formation of a mixture of 2‐diethylhydrosilylbiphenyl [ 2 (Et2HSi)] and 9,9,‐diethyl‐9‐silafluorene ( 3 ). Silafluorene 3 was isolated in 37.5 % and 2 (Et2HSi) in 36.9 % yield. The underlying reaction mechanism was elucidated by DFT calculations. 4‐Methyl‐9,9‐diethyl‐9‐silafluorene ( 7 ) was obtained selectively from the [Ni(PPhMe2)4]‐catalyzed reaction of Et2SiH2 and 1‐methylbiphenylene. By contrast, no selectivity could be found in the Ni‐catalyzed reaction between Et2SiH2 and the biphenylene derivative that bears tBu substituents in the 2‐ and 7‐positions. Therefore, two pairs of isomers of tBu‐substituted silafluorenes and of the related diethylhydrosilylbiphenyls were formed in this reaction. However, a subsequent dehydrogenation of the diethylhydrosilylbiphenyls with Wilkinson’s catalyst yielded a mixture of 2,7‐di‐tert‐butyl‐9,9‐diethyl‐9‐silafluorene ( 8 ) and 3,6‐di‐tert‐butyl‐9,9‐diethyl‐9‐silafluorene ( 9 ). Silafluorenes 8 and 9 were separated by column chromatography. 相似文献
995.
Jens E. T. Andersen 《Central European Journal of Chemistry》2014,12(2):194-205
A detailed method validation of graphite-furnace atomic absorption spectrometry (GFAAS) with Zeeman background correction was performed. The aim is to perform a detailed investigation of short-term precision as opposed to long-term precision. It was suggested that release of graphite flakes into the light path during measurement significantly influenced the performance of the method. It was found that significant deviations with respect to the certified values were frequent and an estimate of reliable uncertainties was obtained only after a high number of repetitions. Uncertainty of Interlaboratory testing was evaluated as a method to estimate uncertainties that are comparable to uncertainties that were obtained by Interlaboratory testing and to uncertainties predicted by the Horwitz curve. To a large extent, the uncertainty in measurement that was predicted by pooled calibrations corresponded to the uncertainties that were obtained from multiple determinations of unknowns. It was thus proposed that a large proportion of the difference in uncertainty in measurement between laboratories could be explained by properties of the different detectors. In order to support accuracy, it is suggested that a higher level of uncertainty should be accepted in analytical investigations. 相似文献
996.
Racemic cis-4-amino-1-benzyl-3-phenylpiperidine was prepared by reductive amination of the respective 4-piperidone via its oxime. The resolution of the racemate was accomplished by crystallization as the mandelate. The enantiomeric purity of this material was checked by NMR after derivatization to the corresponding camphorsulfonamide to be 97% ee. The absolute configuration of one enantiomer was confirmed by X-ray single crystal diffraction of the para-bromobenzenesulfonamide derivative. 相似文献
997.
Jens Jägersküpper 《Operations Research Letters》2008,36(3):327-332
Randomized direct-search methods for the optimization of a function f:Rn→R that is given by a black box for f-evaluations are investigated. These iterative methods generate new candidate solutions by adding isotropically distributed vectors to the current candidate solution. Lower bounds on the number of f-evaluations necessary for reducing the approximation error in the search space are proved. 相似文献
998.
Ralph Wagner Jens Gottmann Alexander Horn Ernst Wolfgang Kreutz 《Applied Surface Science》2006,252(24):8576-8579
Subwavelength ripples (<λ/4) are obtained by scanning a tightly focused beam (∼1 μm) of femtosecond laser radiation (λ = 800 nm, tp = 100 fs) over the surface of either bulk fused silica and silicon and Er:BaTiO3. The ripple pattern extends coherently over many overlapping laser pulses parallel and perpendicular to the polarisation. Investigated are the dependence of the ripple spacing on the spacing of successive pulses, the direction of polarisation and the material. The evolution of the ripples is investigated by applying pulse bursts with N = 1 to 20 pulses. The conditions under which these phenomena occur are specified, and some possible mechanisms of ripple growth are discussed. Potential applications are presented. 相似文献
999.
Jens Gustedt 《Order》1998,15(3):203-220
We investigate classes of graphs and posets that admit decompositions to obtain or disprove finiteness results for obstruction sets. To do so we develop a theory of minimal infinite antichains that allows us to characterize such antichains by means of the set of elements below it.In particular we show that the following classes have infinite antichains with respect to the induced subgraph/poset relation: interval graphs and orders, N-free orders, orders with bounded decomposition width. On the other hand for orders with bounded decomposition diameter finiteness of all antichains is shown. As a consequence those classes with infinite antichains have undecidable hereditary properties whereas those with finite antichains have fast algorithms for all such properties. 相似文献
1000.
Jens Ledet Jensen Hans R. Künsch 《Annals of the Institute of Statistical Mathematics》1994,46(3):475-486
We consider point processes defined through a pairwise interaction potential and admitting a two-dimensional sufficient statistic. It is shown that the pseudo maximum likelihood estimate can be stochastically normed so that the limiting distribution is a standard normal distribution. This result is true irrespectively of the possible existence of phase transitions. The work here is an extension of the work Guyon and Künsch (1992,Lecture Notes in Statist.,74, Springer, New York) and is based on viewing a point process interchangeably as a lattice field. 相似文献