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排序方式: 共有106条查询结果,搜索用时 281 毫秒
81.
Kildahl-Andersen G Anthonsen T Liaaen-Jensen S 《Organic & biomolecular chemistry》2007,5(17):2803-2811
The carotene-like polyenes decapreno-beta-carotene (C50), C54-beta-carotene (C54, first synthesis) and dodecapreno-beta-carotene (C60) with 15, 17 and 19 conjugated double bonds, respectively, were synthesized by double Wittig reactions. Introduction of a leaving group in allylic position failed, and cations were obtained by hydride elimination effected by i) triphenylcarbenium tetrafluoroborate-d15, prepared by a new method, or ii) treatment with trifluoroacetic acid-d. Deuterated reagents were employed for product analysis by 1H NMR. Parallel experiments were performed with beta,beta-carotene (C40). NIR spectra at room temperature and at -15 degrees C were employed for characterisation and stability studies of the cationic products. In CH2Cl2lambdamax in the 900-1350 nm region was recorded. NMR data for the cationic product of beta,beta-carotene obtained by the two new preparation methods were consistent with the two monocations previously characterised. The cationic products of the longer polyenes provided downfield-shifted, broadened signals, compatible with C50-monocation, mixed C54-mono- and dication and C60-dication. Combined NIR and NMR data suggest that the extent of charge delocalisation is limited by the maximum soliton width for cations obtained from linear polyenes with more than ca. 20 sp2-hybridized carbon atoms. 相似文献
82.
83.
Matti Knaapila Henrik Høyer Eldrid Svåsand Mark Buchanan Arne T. Skjeltorp Geir Helgesen 《Journal of Polymer Science.Polymer Physics》2011,49(6):399-403
A concept where an alternating electric field (dielectrophoresis) is used to assemble and align carbon nanocone particles (CNCs) into microscopic wires in self‐supporting polymer films is demonstrated. The particle fraction is kept low (one‐tenth of the percolation threshold of isotropic mixture), which allows uniform dispersion and efficient UV curing. The alignment leads to the conductivity enhancement of three to four orders of magnitude (from ~10?7 to ~10?3 S/m) in the alignment direction. It does not require passing current so the material can be isolated from the alignment electrodes. This prevents electrodes attaching to the film, if the film is adhesive in nature. The alignment can be done using either in‐plane or out‐of‐plane geometries. It is proposed that this concept could be applied in areas such as electrostatic discharge applications where inexpensive conductive or dissipative materials and macroscopic uniformity are prerequisites. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011 相似文献
84.
We give a characterisation of an extension of the Kleene-Kreisel continuous functionals to objects of transfinite types using
limit spaces of transfinite types.
Received: 24 August 1999 / Published online: 12 July 2002 相似文献
85.
On the directed hop-constrained shortest path problem 总被引:1,自引:0,他引:1
86.
Geir Dahl 《Operations Research Letters》2004,32(4):345-349
87.
Geir Langli 《Journal of organometallic chemistry》2006,691(13):3089-3097
Synthesis of rigid ansa-zirconocene precatalyst systems with a C2-bridge embedded in a spirane scaffold is described. Fulvenes were key intermediates in regio- and stereoselective preparation of the appropriate spirane ligands. Substitution in the cyclopentadienyl group in the ligand was effected by fulvene methodology. The zirconocenes were active precatalysts for the polymerisation of propene when activated with MAO. The structure of the parent zirconocene dichloride has been verified by X-ray crystal analysis. 相似文献
88.
The article reports acoustic measurements on short corrugated pipes with flow. Such pipes might generate high sound levels associated with length resonances. One of the main objectives of the study was to estimate the location of the effective sources by studying the energy flow through the pipes. It was found that a short section of corrugations will only produce sound effectively when placed at the inflow end, while for fully corrugated pipes, the sound-producing regions are located around the pressure maxima of the observed standing waves. It was further found that the net energy flow is in the upstream direction for nearly the complete length of pipe. 相似文献
89.
Kildahl-Andersen G Naess SN Aslaksen PB Anthonsen T Liaaen-Jensen S 《Organic & biomolecular chemistry》2007,5(18):3027-3033
The reaction of retinoids (retinol, retinyl acetate and anhydroretinol) with Br?nsted acids was studied as a model system for the Carr-Price reaction. The anhydroretinylic cation was characterised by VIS and 2D NMR spectroscopy, including an estimate of the charge distribution and region of bond inversion, observed in a mixture of identified E/Z isomers. Products obtained by quenching with NaOMe-MeOH were identified by HPLC and MS. The classical Carr-Price reaction between retinol (vitamin A) and the Lewis acid SbCl(3) in saturated chloroform solution was reinvestigated by VIS, NMR, EPR, dynamic light scattering and chemical quenching. Whereas product instability and failure to provide informative NMR spectra indicated a radical cation, EPR results excluded free-radical species. Dynamic light scattering experiments, in comparison with model systems, revealed strong aggregation for the Carr-Price complex, rationalizing the low stability, NMR problems and dimerisation observed by chemical quenching. The VIS data support structural similarity of the blue Carr-Price product with the delocalized anhydroretinylic cation, and a detailed structure of the antimony complex is evaluated. 相似文献
90.
This paper presents a systematic approach based on Mixed Integer Linear Programming for finding an optimal singularity-free reconfiguration path of the 5-DOF Gantry-Tau parallel kinematic machine. The results in the paper demonstrate that singularity-free reconfiguration (change of assembly mode) of the machine is possible, which significantly increases the usable workspace. The method has been applied to a full-scale prototype and the singularity-free path has been verified both in simulations and with physical experiments using real-time control of the prototype. The toolpoint positions have been verified by using measurements from a high precision laser tracker. 相似文献