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991.
Kramer F Jung A Brunner E Glaser SJ 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2004,169(1):49-59
Based on the sign and amplitude of TOCSY transfer functions, it is possible to determine the relative sign and size of scalar and residual dipolar couplings in homonuclear spin systems consisting of two spins 1/2. The efficiency of different mixing sequences and different transfer functions is examined both theoretically and experimentally. 相似文献
992.
In response to the requirements of the "Global Approach" in Germany, a national accreditation system has been established
based on traditional experiences in the recognition of the competence of laboratories and certification bodies. In 1991, the
German Accreditation Council (DAR) was founded as a coordinating body supported by the Federal and State Ministries and German
industry. In this way, it was possible to organise coordinated representation in international bodies, to offer a forum for
cooperation between authorities and industry, and to provide for a structure subdivided into sectors which better fits the
needs of the industrial sectors and gives them the necessary transparency and acceptance. The DAR itself does not carry out
accreditations. This is done by the accreditation bodies represented in the DAR and specialised in for specific subjects,
thereby having a close connection with expert groups of a high technical competence in their specific technical fields. At
the end of this paper, the procedure for the assessment of the technical competence of a laboratory, an important part of
the accreditation, is described. 相似文献
993.
Hänke T Sykora S Schlegel R Baumann D Harnagea L Wurmehl S Daghofer M Büchner B van den Brink J Hess C 《Physical review letters》2012,108(12):127001
A crucial step in revealing the nature of unconventional superconductivity is to investigate the symmetry of the superconducting order parameter. Scanning tunneling spectroscopy has proven a powerful technique to probe this symmetry by measuring the quasiparticle interference (QPI) which sensitively depends on the superconducting pairing mechanism. A particularly well-suited material to apply this technique is the stoichiometric superconductor LiFeAs as it features clean, charge neutral cleaved surfaces without surface states and a relatively high T(c)~18 K. Our data reveal that in LiFeAs the quasiparticle scattering is governed by a van Hove singularity at the center of the Brillouin zone which is in stark contrast to other pnictide superconductors where nesting is crucial for both scattering and s(±) superconductivity. Indeed, within a minimal model and using the most elementary order parameters, calculations of the QPI suggest a dominating role of the holelike bands for the quasiparticle scattering. Our theoretical findings do not support the elementary singlet pairing symmetries s(++), s(±), and d wave. This brings to mind that the superconducting pairing mechanism in LiFeAs is based on an unusual pairing symmetry such as an elementary p wave (which provides optimal agreement between the experimental data and QPI simulations) or a more complex order parameter (e.g., s+id wave symmetry). 相似文献
994.
Steffen Fleischer Svenja Werkmeister Dr. Shaolin Zhou Dr. Kathrin Junge Prof. Dr. Matthias Beller 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(29):9005-9010
Enantiomerically pure chiral amines are of increasing importance and commercial value in the fine chemical, pharmaceutical, and agrochemical industries. Here, we describe the straightforward synthesis of chiral amines by combining the atom‐economic and environmentally friendly hydroamination of alkynes with an enantioselective hydrogenation of in situ generated imines by using inexpensive hydrogen. By following this novel approach, a wide range of terminal alkynes can be reductively hydroaminated with primary amines including alkyl‐, and arylalkynes as well as aryl and heteroaryl amines. Excellent yields and selectivities up to 94 % ee and 96 % isolated yield were obtained. 相似文献
995.
Koch N Gerlach A Duhm S Glowatzki H Heimel G Vollmer A Sakamoto Y Suzuki T Zegenhagen J Rabe JP Schreiber F 《Journal of the American Chemical Society》2008,130(23):7300-7304
The interfaces formed between pentacene (PEN) and perfluoropentacene (PFP) molecules and Cu(111) were studied using photoelectron spectroscopy, X-ray standing wave (XSW), and scanning tunneling microscopy measurements, in conjunction with theoretical modeling. The average carbon bonding distances for PEN and PFP differ strongly, that is, 2.34 A for PEN versus 2.98 A for PFP. An adsorption-induced nonplanar conformation of PFP is suggested by XSW (F atoms 0.1 A above the carbon plane), which causes an intramolecular dipole of approximately 0.5 D. These observations explain why the hole injection barriers at both molecule/metal interfaces are comparable (1.10 eV for PEN and 1.35 eV for PFP) whereas the molecular ionization energies differ significantly (5.00 eV for PEN and 5.85 eV for PFP). Our results show that the hypothesis of charge injection barrier tuning at organic/metal interfaces by adjusting the ionization energy of molecules is not always readily applicable. 相似文献
996.
997.
Star chromatic numbers of graphs 总被引:10,自引:0,他引:10
We investigate the relation between the star-chromatic number (G) and the chromatic number (G) of a graphG. First we give a sufficient condition for graphs under which their starchromatic numbers are equal to their ordinary chromatic numbers. As a corollary we show that for any two positive integersk, g, there exists ak-chromatic graph of girth at leastg whose star-chromatic number is alsok. The special case of this corollary withg=4 answers a question of Abbott and Zhou. We also present an infinite family of triangle-free planar graphs whose star-chromatic number equals their chromatic number. We then study the star-chromatic number of An infinite family of graphs is constructed to show that for each >0 and eachm2 there is anm-connected (m+1)-critical graph with star chromatic number at mostm+. This answers another question asked by Abbott and Zhou. 相似文献
998.
Laskoski M Steffen W Morton JG Smith MD Bunz UH 《Journal of the American Chemical Society》2002,124(46):13814-13818
The synthesis of eight new cyclobutadiene or ferrocene-fused organometallic dehydroannulenes is reported. Cadiot-type coupling of a 1-bromoethynyl-2-silylethynylbenzene derivative to an organometallic diyne (1,2-diethynyl-3,4-bis(trimethylsilyl)cyclobutadiene(cyclopentadienyl )cobalt or 1,2-diethynylferrocene) is followed by deprotection and Cu(OAc)(2)-promoted ring closure. Five of the organometallic dehydroannulenes were structurally characterized. Three of the novel cycles explode at temperatures from 196 to 293 degrees C and form insoluble carbon materials. The soot produced from 13a shows a high abundance of onion-like carbon nanostructures. The nanostructures were characterized by high-resolution transmission electron microscopy. 相似文献
999.
1000.
Chenghao Wan Zhen Zhang David Woolf Colin M. Hessel Jura Rensberg Joel M. Hensley Yuzhe Xiao Alireza Shahsafi Jad Salman Steffen Richter Yifei Sun M. Mumtaz Qazilbash Rüdiger Schmidt‐Grund Carsten Ronning Shriram Ramanathan Mikhail A. Kats 《Annalen der Physik》2019,531(10)
The insulator‐to‐metal transition (IMT) in vanadium dioxide (VO2) can enable a variety of optics applications, including switching and modulation, optical limiting, and tuning of optical resonators. Despite the widespread interest in VO2 for optics, the wavelength‐dependent optical properties across its IMT are scattered throughout the literature, are sometimes contradictory, and are not available at all in some wavelength regions. Here, the complex refractive index of VO2 thin films across the IMT is characterized for free‐space wavelengths from 300 nm to 30 µm, using broadband spectroscopic ellipsometry, reflection spectroscopy, and the application of effective‐medium theory. VO2 films of different thicknesses are studied, on two different substrates (silicon and sapphire), and grown using different synthesis methods (sputtering and sol–gel). While there are differences in the optical properties of VO2 synthesized under different conditions, these differences are surprisingly small in the ≈2–11 µm range where the insulating phase of VO2 also has relatively low optical loss. It is anticipated that the refractive‐index datasets from this article will be broadly useful for modeling and design of VO2‐based optical and optoelectronic components, especially in the mid‐wave and long‐wave infrared. 相似文献