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排序方式: 共有273条查询结果,搜索用时 15 毫秒
101.
102.
103.
Haldrup K Harlang T Christensen M Dohn A van Driel TB Kjær KS Harrit N Vibenholt J Guerin L Wulff M Nielsen MM 《Inorganic chemistry》2011,50(19):9329-9336
Ground- and excited-state structures of the bimetallic, ligand-bridged compound Ir2(dimen)4(2+) are investigated in acetonitrile by means of time-resolved X-ray scattering. Following excitation by 2 ps laser pulses at 390 nm, analysis of difference scattering patterns obtained at eight different time delays from 250 ps to 300 ns yields a triplet excited-state distance between the two Ir atoms of 2.90(2) ? and a triplet excited-state lifetime of 410(70) ns. A model incorporating the presence of two ground-state structures differing in Ir–Ir separation is demonstrated to fit the obtained data very well, in agreement with previous spectroscopic investigations. Two ground-state isomers with Ir–Ir separations of 3.60(9) and 4.3(1) ? are found to contribute equally to the difference scattering signal at short time delays. Further studies demonstrate the feasibility of increasing the effective time resolution from the 100 ps probe width down to the 10 ps regime by positioning the laser pump pulse at selected points in the X-ray probe pulse. This approach is used to investigate the structures of both the singlet and the triplet excited states of Ir2(dimen)4(2+). 相似文献
104.
We herein report a simple and convenient one-pot synthesis of highly substituted 2-imidazolines in a regiocontrolled and stereospecific matter through the ring expansion reaction of an imidoyl chloride with an aziridine, analogous to the Heine reaction. 相似文献
105.
Dirk Wulff‐Molder Manfred Meisel 《Acta Crystallographica. Section C, Structural Chemistry》2000,56(1):33-34
In the title complex, tetraphenylphosphonium μ4‐nitrato‐κ4O‐cyclo‐tetrakis(μ‐acetato‐O:O′)tetra‐μ‐oxo‐tetrakis[oxovana‐dium(V)], the anion lies about a twofold axis and consists of the cyclic [V4O8] unit coordinated by four acetato ligands with interatomic V?V distances of 3.269 (1) and 3.273 (1) Å. The double‐bonded O atom [N=O 1.102 (6) and N—O 1.268 (4) Å] of the nitrato ligand links the four V atoms with V—O bond distances of 2.613 (2) and 2.813 (2) Å. The negative charge of the complex is balanced by tetraphenylphosphonium cations occupying the Na positions in the NaCl‐type ionic packing of the structure. 相似文献
106.
(N) . In this article, it is shown that the dynamics near meandering spiral waves or other patterns is determined by a finite-dimensional
vector field that has a certain skew-product structure over the group SESE(N) . This generalizes our earlier work on center-manifold theory near rigidly rotating spiral waves to meandering spirals. In
particular, for meandering spirals, it is much more sophisticated to extract the aforementioned skew-product structure since
spatio-temporal rather than only spatial symmetries have to be accounted for. Another difficulty is that the action of the
Euclidean symmetry group on the underlying function space is not differentiable, and in fact may be discontinuous. Using this
center-manifold reduction, Hopf bifurcations and periodic forcing of spiral waves are then investigated. The results explain
the transitions to patterns with two or more temporal frequencies that have been observed in various experiments and numerical
simulations.
Received December 8, 1997; accepted May 19, 1996 相似文献
107.
108.
Hans R. Kricheldorf Dirk F. Wulff 《Journal of polymer science. Part A, Polymer chemistry》1997,35(5):947-957
Copolycondensations of (S,S)-2,5-bis(2-methylbutyloxy) terephthaloylchloride with 2,5-bis(dodecyloxy)terephthaloylchloride and with 4,4′-bistrimethylsiloxybiphenyl yielded a series of novel chiral thermotropic copolyesters. These polyesters were characterized by elemental analyses, inherent viscosities, 1H-NMR spectroscopy, optical rotations, optical microscopy, DSC measurements, and WAXS powder patterns recorded with synchrotron radiation under variation of the temperature. All homo- and copolyesters formed a solid sanidic layer structure with melting temperatures (Tm) ≥ 200°C. A broad enantiotropic nematic or cholesteric phase is formed above Tm with isotropization temperatures (Tis) in the range of 275–325°C. Yet, the Tm of the chiral homopolyester is so high (378°C) that the melting process is immediately followed by rapid degradation. The cholesteric phases of the copolyesters displayed unusual mobile schlieren textures, but a stable Grandjean texture was never obtained. Cholesteric domains consisting of loose bundles of more or less helical main chains are discussed as supramolecular order responsible for the observed textures and their pronounced temperature dependence. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 947–957, 1997 相似文献
109.
Günter Wulff Stefan Gladow Bernd Kühneweg Stephan Krieger 《Macromolecular Symposia》1996,101(1):355-362
Asymmetric cyclocopolymerization of judiciously designed bifunctional monomers with a number of common comonomers affords optically active vinyl polymers with main chain chirality. Tacticity and sequence of comonomeric units can be controlled and thus complicated structures built up. Under different reaction conditions the same bifunctional monomer 1 can undergo cyclization involving a 19-, a 21-, or an 18-membered ring. With monomer 3 an 11-membered ring is formed. 相似文献
110.
Stefan Kubik Olaf Hller Andreas Steinert Martin Tolksdorf Günter Wulff 《Macromolecular Symposia》1995,99(1):93-102
Inclusion complexes of slightly hydroxypropylated amylose with organic guest molecules have been investigated by means of c.d. spectroscopy and microcalorimetry in aqueous solution. From these results, the structure of the complexes and the mechanism of their fomation are deduced. It is concluded that amylose can adjust its helix conformation to the shape and structure of the guest molecule included. Investigations about intramolecular and intermolecular crosslinking of amylose indicate that the complexing behavior of amylose can be manipulated specifically. 相似文献