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排序方式: 共有219条查询结果,搜索用时 15 毫秒
211.
Jakobsen HJ Hove AR Bildsøe H Skibsted J Brorson M 《Chemical communications (Cambridge, England)》2007,(16):1629-1631
We report the first (33)S chemical shift anisotropy (CSA) data as obtained from a combined determination of (33)S CSA and quadrupole coupling parameters utilizing the observation of both the (33)S (I = 3/2) central and satellite transitions in a natural abundance (33)S MAS NMR study aimed at characterizing the two important tetrathiometallates (NH4)(2)MoS(4) and (NH4)(2)WS(4). 相似文献
212.
213.
Chavan S Vitillo JG Gianolio D Zavorotynska O Civalleri B Jakobsen S Nilsen MH Valenzano L Lamberti C Lillerud KP Bordiga S 《Physical chemistry chemical physics : PCCP》2012,14(5):1614-1626
The recently discovered UiO-66/67/68 class of isostructural metallorganic frameworks (MOFs) [J. H. Cavka et al. J. Am. Chem. Soc., 2008, 130, 13850] has attracted great interest because of its remarkable stability at high temperatures, high pressures and in the presence of different solvents, acids and bases [L. Valenzano et al. Chem. Mater., 2011, 23, 1700]. UiO-66 is obtained by connecting Zr(6)O(4)(OH)(4) inorganic cornerstones with 1,4-benzene-dicarboxylate (BDC) as linker resulting in a cubic MOF, which has already been successfully reproduced in several laboratories. Here we report the first complete structural, vibrational and electronic characterization of the isostructural UiO-67 material, obtained using the longer 4,4'-biphenyl-dicarboxylate (BPDC) linker, by combining laboratory XRPD, Zr K-edge EXAFS, TGA, FTIR, and UV-Vis studies. Comparison between experimental and periodic calculations performed at the B3LYP level of theory allows a full understanding of the structural, vibrational and electronic properties of the material. Both materials have been tested for molecular hydrogen storage at high pressures and at liquid nitrogen temperature. In this regard, the use of a longer ligand has a double benefit: (i) it reduces the density of the material and (ii) it increases the Langmuir surface area from 1281 to 2483 m(2) g(-1) and the micropore volume from 0.43 to 0.85 cm(3) g(-1). As a consequence, the H(2) uptake at 38 bar and 77 K increases from 2.4 mass% for UiO-66 up to 4.6 mass% for the new UiO-67 material. This value is among the highest values reported so far but is lower than those reported for MIL-101, IRMOF-20 and MOF-177 under similar pressure and temperature conditions (6.1, 6.2 and 7.0 mass%, respectively) [A. G. Wong-Foy et al. J. Am. Chem. Soc., 2006, 128, 3494; M. Dinca and J. R. Long. Angew. Chem., Int. Ed., 2008, 47, 6766]. Nevertheless the remarkable chemical and thermal stability of UiO-67 and the absence of Cr in its structure would make this material competitive. 相似文献
214.
Jakobsen HJ Hove AR Bildsøe H Skibsted J 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2006,180(2):170-177
Experiences obtained from recent improvements in the performance of solid-state (14)N MAS NMR spectroscopy have been used in a natural abundance (33)S MAS NMR investigation of the satellite transitions for this interesting spin I=3/2 isotope. This study reports the first observation of manifolds of spinning sidebands for these transitions in (33)S MAS NMR as observed for the two alums XAl(SO(4))(2) x 12H(2)O with X=NH(4) and K. For the NH(4)-alum a variable temperature (33)S MAS NMR study, employing the satellite transitions, shows that the (33)S quadrupole coupling constant (C(Q)) exhibits a linear temperature dependence (in the range -35 degrees C to 70 degrees C) with a temperature gradient of 3.1 kHz/ degrees C and undergoes a sign change with zero-crossing for C(Q) at 4 degrees C (277 K). For the isostructural K-alum a quite similar increase in the magnitude of C(Q) with increasing temperature is observed, and with a temperature gradient of 2.3 kHz/ degrees C. Finally, for optimization purposes, a study on the effect of the applied pulse widths at constant rf field strength on the intensity and variation in second-order quadrupolar lineshape for the central (1/2<-->-1/2) transition of the K-alum has been performed. 相似文献
215.
Researchers at the University of Michigan have developed sets of items that can be used to analyze teachers’ mathematical knowledge for teaching (MKT). In this paper, we consider what is required in the adaptation of a set of these items for use in a Norwegian context. We discuss how analysis of item difficulty and point–biserial correlation can be applied in combination with qualitative approaches to ensure a high-quality process of piloting adapted MKT items. Findings indicate that researchers who attempt to adapt MKT items for use in cultural contexts other than those for which they were designed need to use different methods to analyze all aspects of the adaptation process. The results from the different analyses conducted might then be used to inform other parts of the process, and this will mean that the process of adapting and piloting items becomes cyclic and iterative. 相似文献
216.
We present a new multimode dispersion measurement technique based on the time-of-flight method. The modal delay and group velocity dispersion of all excited modes in a few-mode fiber can be measured simultaneously by a tunable pulsed laser and a high speed sampling oscilloscope. A newly designed higher-order-mode fiber with large anomalous dispersion in the LP(02) mode has been characterized using this method, and experimental results are in good agreement with the designed dispersion values. The demonstrated technique is significantly simpler to implement than the existing frequency-domain or interferometry-based methods. 相似文献
217.
We obtain nonsymmetric upper and lower bounds on the rate of convergence of general monotone approximation/numerical schemes for parabolic Hamilton-Jacobi-Bellman equations by introducing a new notion of consistency. Our results are robust and general - they improve and extend earlier results by Krylov, Barles, and Jakobsen. We apply our general results to various schemes including Crank-Nicholson type finite difference schemes, splitting methods, and the classical approximation by piecewise constant controls. In the first two cases our results are new, and in the last two cases the results are obtained by a new method which we develop here.
218.
Bearden IG Beavis D Besliu C Blyakhman Y Brzychczyk J Budick B Bøggild H Chasman C Christensen CH Christiansen P Cibor J Debbe R Gaardhøje JJ Grotowski K Hagel K Hansen O Holm A Holme AK Ito H Jakobsen E Jipa A Jørdre JI Jundt F Jørgensen CE Keutgen T Kim EJ Kozik T Larsen TM Lee JH Lee YK Løvhøiden GL Majka Z Makeev A McBreen B Murray M Natowitz J Nielsen BS Olchanski K Olness J Ouerdane D Planeta R Rami F Röhrich D Samset BH Sanders SJ Sheetz RA Sosin Z Staszel P Thorsteinsen TF Tveter TS 《Physical review letters》2001,87(11):112305
Measurements, with the BRAHMS detector, of the antiproton-to-proton ratio at midrapidities and forward rapidities, are presented for Au+Au reactions at square root of [s(NN)] = 130 GeV, and for three different collision centralities. For collisions in the 0%-40% centrality range, we find N(&pmacr;)/N(p) = 0.64+/-0.04((stat))+/-0.06((syst)) at y approximately 0, 0.66+/-0.03+/-0.06 at y approximately 0.7, and 0.41+/-0.04+/-0.06 at y approximately 2. The ratios are found to be nearly independent of collision centrality and transverse momentum. The antiproton and proton rapidity densities vary differently with rapidity, and indicate a significant degree of collision transparency, although a net-baryon free midrapidity plateau (Bjorken limit) is not yet reached. 相似文献
219.
ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option. 相似文献