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21.
Paul Gesche Franois Klinger Andreas Riesen Thophile Tschamber Margareta Zehnder Jacques Streith 《Helvetica chimica acta》1987,70(8):2087-2097
Saponification of homodiazepine 1a and 1b , in the absence of any proton donors, led to the formation of the 6π electron anionic species A which, by virtue of a 1,5-electrocyclization, is in equilibrium with the allylic anion B . This latter tricyclic species is thermodynamically less favoured than its bicyclic isomer A . Nevertheless, B could be trapped by acylation and led tupe- 2 compounds which are the major reaction products. This is due to the fact that B is more nucleophilic and, therefore, much more reactive than A . The transoid topology of the tricyclic products 2 was demonstrated by 1H-NMR and by an X-ray diagram of 2d . The transoid geometry of 2 is a consequence of a torquospecific 1,5-electrocyclization (of A ), which is due to a steric, and possibly even to an electronic factor. 相似文献
22.
Experimental Mechanics - Hydrogels are a class of soft, highly deformable materials formed by swelling a network of polymer chains in water. With mechanical properties that mimic biological... 相似文献
23.
Ralph A. Zehnder Robert A. Renn Matthias Zeller Jason A. Carr Nathan C. McMullen 《Journal of Molecular Structure》2011,985(1):109-119
Various lanthanide open framework materials incorporating the terephthalate (TP) entity were prepared using hydrothermal synthesis methods at a moderate temperature of 170 °C. The compounds Nd2(TP)3(H2O)4(1), Er2(TP)3(H2O)4(2), Yb2(TP)3(H2O)2(3), Yb2(TP)3(H2O)6(4), and Yb2(TP)3(H2O)8·2H2O (5), were characterized by single crystal structural analysis and FT-IR spectroscopy. While compounds 1 and 2 have been reported before on the basis of powder X-ray diffraction, the structural characterization of any ytterbium terephthalate species is unprecedented. Compounds 1-5 crystallize in triclinic settings with space group P-1. The compounds are compared with their previously reported Er and Tb-counterparts and the reduction of the dimensionality of the resulting networks from 3D over 2D to 1D with increasing level of hydration is discussed. Compounds 1, 2, and 3 with the lowest water content assemble in three-dimensional network lattices. Compounds 4 and 5, however, form 2D layered systems and 1D rod like chains, respectively, which are held together by hydrogen bonds originating from coordinating H2O. The crystal lattices of the 3D networks experience higher levels of tension as can be seen by increasing out-of-plane torsion with regard to the terephthalate carboxylate groups. Moreover, there seems to be a correlation between the level of strain on the aromatic ligands and the reduction of the number of carboxylate oxygen atoms that are part of the coordination polyhedra. 相似文献
24.
Advances in wide bandgap SiC for optoelectronics 总被引:1,自引:0,他引:1
Haiyan Ou Yiyu Ou Aikaterini Argyraki Saskia Schimmel Michl Kaiser Peter Wellmann Margareta K. Linnarsson Valdas Jokubavicius Jianwu Sun Rickard Liljedahl Mikael Syväjärvi 《The European Physical Journal B - Condensed Matter and Complex Systems》2014,87(3):1-16
Silicon carbide (SiC) has played a key role in power electronics thanks to its unique physical properties like wide bandgap, high breakdown field, etc. During the past decade, SiC is also becoming more and more active in optoelectronics thanks to the progress in materials growth and nanofabrication. This paper will review the advances in fluorescent SiC for white light-emitting diodes, covering the poly-crystalline doped SiC source material growth, single crystalline epitaxy growth of fluorescent SiC, and nanofabrication of SiC to enhance the extraction efficiency for fluorescent SiC based white LEDs. 相似文献
25.
KAM theory in configuration space 总被引:6,自引:0,他引:6
A new approach to the Kolmogorov-Arnold-Moser theory concerning the existence of invariant tori having prescribed frequencies
is presented. It is based on the Lagrangian formalism in configuration space instead of the Hamiltonian formalism in phase
space used in earlier approaches. In particular, the construction of the invariant tori avoids the composition of infinitely
many coordinate transformations. The regularity results obtained are applied to invariant curves of monotone twist maps. The
Lagrangian approach has been prompted by a recent study of minimal foliations for variational problems on a torus by J. Moser.
This research has been supported by the Nuffields Foundation under grant SCI/180/173/G and by the Stiftung Volkswagenwerk. 相似文献
26.
Nazaretski E Merithew RD Kostroun VO Zehnder AT Pohl RO Parpia JM 《Physical review letters》2004,92(24):245502
We report on the mechanical behavior of an a-SiO2 84 kHz torsional oscillator operated between 100> or =T> or =1.0 mK. Below 10 mK we observed well-differentiated transient responses which we attribute to the interaction with low-level background radiation (gamma quanta and cosmic ray micro) and which can be modeled in terms of a change in the spring constant. 相似文献
27.
Peak compression in CEC is a phenomenon that can generate very narrow peaks with extremely high efficiencies that defy current chromatographic theory. This review article summarises the content of publications in this area up to this date. Two main types of peak compression effects have been observed in the literature. First, an irreproducible and hard to control focusing effect of unclear origin, observed on strong cation exchangers. Second, a reproducible continuous stacking effect caused by sample composition induced system zones demonstrated on several types of stationary phases. 相似文献
28.
Almqvist F Manner S Thornqvist V Berg U Wallin M Frejd T 《Organic & biomolecular chemistry》2004,2(21):3085-3090
The formylated spirobyclic alcohol was computer modeled to be a mimetic of paclitaxel. In this model, the formyl group was used as a truncated paclitaxel side chain in order to reduce the computational work. Compound , carrying the paclitaxel side chain, was synthesized in six steps from optically active 1,3-diketone . Microtubule stabilization was not observed for , indicating that the model needs to be adjusted. 相似文献
29.
Irena Blute Martin Svensson Krister Holmberg Margareta Bergh Ann-Therese Karlberg 《Colloids and surfaces. A, Physicochemical and engineering aspects》1999,150(1-3):105-113
It has previously been shown that alcohol ethoxylates readily undergo autoxidation and that one of the major oxidation products is the surfactant aldehyde, i.e. an ethoxylate carrying a –CH2CHO group at the terminal end of the polyoxyethylene chain. In this work the cloud point, phase behavior and aggregation characteristics of the surfactant aldehyde produced by oxidation of C12H25(OCH2CH2)5OH (C12E5) are determined and compared with the values obtained with the parent surfactant. It was found that the physico–chemical behavior of the two species was very similar, which indicates that a considerable portion of the aldehyde group is in hydrated state, i.e. the surfactant aldehyde consists of a mixture of aldehyde in carbonyl form and the corresponding geminal diol. The cloud point of the surfactant aldehyde decreased rapidly with time, even when it was stored at low temperature. Also the parent surfactant and its homologue C12E6 exhibited a decrease in cloud point during storage. For instance, a 1% aqueous solution of C12E6 showed a cloud point decrease from 62 to 32°C after 4 months storage at 40°C. Such a change in solution behavior can have important practical implications. 相似文献
30.
J. Joly R. Brauns P. Raikow N. Prodanow L. Zehnder und J. L. Andreae 《Fresenius' Journal of Analytical Chemistry》1887,26(1):619-621
Ohne Zusammenfassung 相似文献