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排序方式: 共有184条查询结果,搜索用时 15 毫秒
1.
Wijdeven MA Botman PN Wijtmans R Schoemaker HE Rutjes FP Blaauw RH 《Organic letters》2005,7(18):4005-4007
The stereoselective total synthesis of the novel quinolizidine alkaloid (+)-epiquinamide is presented, starting from the amino acid l-allysine ethylene acetal. Key steps in the synthesis involved a highly diastereoselective N-acyliminium ion allylation and a ring-closing metathesis reaction to provide the bicyclic skeleton. [reaction: see text] 相似文献
2.
Hennequin Y Evens M Quezada Angulo CM van Duijneveldt JS 《The Journal of chemical physics》2005,123(5):054906
Nearly athermal colloid-polymer mixtures were studied in the "protein limit." A fluid-fluid transition was observed in mixtures of stearyl-alcohol-coated silica particles and large polystyrene coils in toluene. The ratios of the polymer radius of gyration to the particle radii were q=4.1 and q=5.2. The binodal curves and the critical points were determined. Turbidity measurements and analysis for one set of particles allowed the systems to be mapped onto hard sphere-polymer mixtures. A comparison with recent predictions for the miscibility of model mixtures shows that the experimental binodals lie between the two extreme results for ideal and interacting polymers. The critical colloid volume fraction is also found to decrease with increasing size ratios. 相似文献
3.
Yannick Vercammen Jaymes Van Luppen Christiaan Van Roost Roel De Mondt Frank Vangaever Luc Van Vaeck 《Analytical and bioanalytical chemistry》2013,405(6):2053-2064
Molecular depth profiling is needed to develop high-tech materials optimised to the μm or even up to the nm scale. Recent progress in time-of-flight static secondary ion mass spectrometry (ToF-S-SIMS) offers perspectives to molecular depth profiling. However, at this moment, the methodology is not yet capable to deal with a range of materials science applications because of the limited depth range, the loss of intensity in the subsurface and the loss of depth resolution at large distances from the original surface. Therefore, the purpose of this paper is to develop a complementary approach for the molecular 3D analysis at large depth, using a combination of ultra-low angle microtomy (ULAM) and surface analysis of the sectioned material with ToF-S-SIMS. Single inkjet dots with a diameter of 100 μm and height of 22 μm on a PET substrate have been used as a test system for the methodology. It is demonstrated that the use of a diamond knife allows the molecular composition and distribution of components within the microscopic feature to be probed with a lateral resolution of 300 nm. Hence the methodology approaches the physical limit for ion imaging of organic components with local concentrations in the % range. In practice, the achievable depth resolution with ULAM-S-SIMS is ultimately limited by the surface roughness of the section. Careful optimisation of the ULAM step has resulted in a surface roughness within 6 nm (R a value) at a depth of 21 μm. This offers perspective to achieve 3D analysis with a depth resolution as good as 18 nm at such a large distance from the surface. Furthermore, the ULAM-S-SIMS approach is applicable to materials unamenable to ion beam erosion. However, the method is limited to dealing with, for instance, Si or glass substrates that cannot be sectioned with a microtomy knife. Furthermore, sufficient adhesion between stacked layers or between the coating and substrate is required. However, it is found that the approach is applicable to a wide variety of industrially important (multi)layers of polymers on a polymer substrate. 相似文献
4.
5.
Dr. Khurram Saleem Joya Dr. Yasir F. Joya Prof. Kasim Ocakoglu Prof. Roel van de Krol 《Angewandte Chemie (International ed. in English)》2013,52(40):10426-10437
The development of new energy materials that can be utilized to make renewable and clean fuels from abundant and easily accessible resources is among the most challenging and demanding tasks in science today. Solar‐powered catalytic water‐splitting processes can be exploited as a source of electrons and protons to make clean renewable fuels, such as hydrogen, and in the sequestration of CO2 and its conversion into low‐carbon energy carriers. Recently, there have been tremendous efforts to build up a stand‐alone solar‐to‐fuel conversion device, the “artificial leaf”, using light and water as raw materials. An overview of the recent progress in electrochemical and photo‐electrocatalytic water splitting devices is presented, using both molecular water oxidation complexes (WOCs) and nano‐structured assemblies to develop an artificial photosynthetic system. 相似文献
6.
Drent E van Dijk R van Ginkel R van Oort B Pugh RI 《Chemical communications (Cambridge, England)》2002,(7):744-745
Copolymerisation of ethene and alkylacrylates is catalysed by palladium modified with di(2-methoxyphenyl)phosphinobenzene-2-sulfonic acid (DOPPBS); a linear polymer is produced in which acrylate units are incorporated into the polyethylene backbone. 相似文献
7.
This text summarises the PhD thesis that Roel Leus presented to obtain the degree of Doctor in Applied Economics at the Katholieke Universiteit Leuven, in September 2003. The promotor of the thesis was professor Willy Herroelen. The thesis is written in English and is available from the authors website. The goal of the thesis was to provide recommendations for the detailed scheduling of multi-project organisations, when a certain degree of uncertainty exists about a number of characteristics of the project. Up till now, the majority of the literature on project scheduling has consisted of deterministic models for planning a single project: both the uncertainty aspect as well as the intrinsic difficulty of coordination of a portfolio of projects have been largely ignored.Received: October 2003, MSC classification:
09B35, 90B36 相似文献
8.
Smits R Warner N McQueen JM Cutler A 《The Journal of the Acoustical Society of America》2003,113(1):563-574
We present the results of a large-scale study on speech perception, assessing the number and type of perceptual hypotheses which listeners entertain about possible phoneme sequences in their language. Dutch listeners were asked to identify gated fragments of all 1179 diphones of Dutch, providing a total of 488,520 phoneme categorizations. The results manifest orderly uptake of acoustic information in the signal. Differences across phonemes in the rate at which fully correct recognition was achieved arose as a result of whether or not potential confusions could occur with other phonemes of the language (long with short vowels, affricates with their initial components, etc.). These data can be used to improve models of how acoustic-phonetic information is mapped onto the mental lexicon during speech comprehension. 相似文献
9.
Kumar R Spuesens T Mechet P Kumar P Raz O Olivier N Fedeli JM Roelkens G Baets R Van Thourhout D Morthier G 《Optics letters》2011,36(13):2450-2452
Using a 7.5 μm diameter disk fabricated with III-V-on-silicon fabrication technology, we demonstrate bias-free all-optical wavelength conversion for non-return-to-zero on-off keyed pseudorandom bit sequence (PRBS) data at the speed of 10 Gbits/s with an extinction ratio of more than 12 dB. The working principle of such a wavelength converter is based on free-carrier-induced refractive index modulation in a pump-probe configuration. We believe it to be the first bias-free on-chip demonstration of all-optical wavelength conversion using PRBS data. All-optical gating measurements in the pump-probe configuration with the same device have revealed that it is possible to achieve wavelength conversion beyond 20 Gbits/s. 相似文献
10.
Wide electrical tuning of silicon-on-insulator ring resonators is demonstrated using a top cladding layer of nematic liquid crystals. A tuning range of 31?nm is demonstrated for ring resonators guiding the TM mode, covering nearly the entire C-band of optical communications. Ring resonators guiding the TE mode can be tuned over 4.5?nm. The combination of a liquid crystal director calculation and a fully anisotropic mode solver confirms the interpretation of these experimental results. The realization of broad and low-power tuning in silicon-on-insulator opens up new opportunities in the field of tunable lasers, filters, and detectors. 相似文献