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31.
Spin-lattice relaxation in multiple-pulse experiments as contrast parameter in NMR imaging of solids
The molecular motion contrast parameter for NMR imaging of solids and quasi-solids based on the spin-lattice relaxation (T leff) in multiple-pulse experiments is discussed. For Ostroff-Waugh multiple-pulse sequence theT leff contrast parameter is evaluated in slow and fast molecular motion regime and compared with spin-lattice relaxation in the rotating frame contrast parameter. It is shown thatT leff is offering a good molecular motion contrast in NMR imaging of polymer systems. The radio-frequency pulse scheme forT leff-imaging using magic-echo phase-encoding procedure for recording spatial distribution in solids is introduced. A method forT leff-weighted imaging using gradient spin-echo valid for weak dipolar solids is also discussed. The one-dimensional protonT leff image using Ostroff-Waugh pulse sequence in combination with frequency-encoding imaging procedure is presented for a phantom of poly(ethyleneoxide) and poly(methylmethacrylate). The distribution of mechanical stresses in a acrylate film on glass is investigated by protonT leff-imaging. A proton spin-density image weighted byT leff, for a mixture of two elastomers with different crosslink density is also shown. 相似文献
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33.
Applied Biochemistry and Biotechnology - Two aqueous two-phased sytems were investigated to determine their ability to reduce product inhibition in the acetone-butanol-ethanol (A-B-E) fermentation.... 相似文献
34.
P. Weigand C. Nagl M. Schmid P. Varga 《Fresenius' Journal of Analytical Chemistry》1993,346(1-3):281-283
Summary Ion scattering spectroscopy results show Pt enrichment in the topmost atomic layers of a Pt25Ni75(111) single crystal due to preferential sputtering. The increased lattice constant caused thereby leads to subsurface lattice mismatch dislocations, which have been studied by scanning tunneling microscopy. Agreement is found for the Pt concentration estimated from the density of dislocations and the results obtained by ISS. Thermal annealing induces further segregation of Pt in the topmost atomic layer. The composition of the subsurface layers has been studied and a strong dependence on the annealing temperature has been found. The observed Pt enrichment in the first monolayer for the thermodynamic equilibrium state agrees well with a thermodynamic theory. 相似文献
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36.
von Nussbaum F Brands M Hinzen B Weigand S Häbich D 《Angewandte Chemie (International ed. in English)》2006,45(31):5072-5129
To create a drug, nature's blueprints often have to be improved through semisynthesis or total synthesis (chemical postevolution). Selected contributions from industrial and academic groups highlight the arduous but rewarding path from natural products to drugs. Principle modification types for natural products are discussed herein, such as decoration, substitution, and degradation. The biological, chemical, and socioeconomic environments of antibacterial research are dealt with in context. Natural products, many from soil organisms, have provided the majority of lead structures for marketed anti-infectives. Surprisingly, numerous "old" classes of antibacterial natural products have never been intensively explored by medicinal chemists. Nevertheless, research on antibacterial natural products is flagging. Apparently, the "old fashioned" natural products no longer fit into modern drug discovery. The handling of natural products is cumbersome, requiring nonstandardized workflows and extended timelines. Revisiting natural products with modern chemistry and target-finding tools from biology (reversed genomics) is one option for their revival. 相似文献
37.
We construct the first globally consistent semirealistic type I string vacua on an elliptically fibered manifold where the zero modes of the Euclidean D1-instanton sector allow for the generation of nonperturbative Majorana masses of an intermediate scale. In another class of global models, a D1-brane instanton can generate a Polonyi-type superpotential breaking supersymmetry at an exponentially suppressed scale. 相似文献
38.
Fuchs O Zharnikov M Weinhardt L Blum M Weigand M Zubavichus Y Bär M Maier F Denlinger JD Heske C Grunze M Umbach E 《Physical review letters》2008,100(2):027801
High-resolution x-ray absorption and emission spectra of liquid water exhibit a strong isotope effect. Further, the emission spectra show a splitting of the 1b1 emission line, a weak temperature effect, and a pronounced excitation-energy dependence. They can be described as a superposition of two independent contributions. By comparing with gas phase, ice, and NaOH/NaOD, we propose that the two components are governed by the initial state hydrogen bonding configuration and ultrafast dissociation on the time scale of the O 1s core hole decay. 相似文献
39.
Mohammad El-khateeb Hassan Abul-Futouh Hadil Alshurafa Helmar Görls Wolfgang Weigand 《应用有机金属化学》2020,34(11):e5940
Substitution of carbonyl ligands of the hydrogenase model complex [Fe2(μ-SeCH2CH(Me)CH2Se-μ)(CO)6] ( A ), by 1,1′-bis (diphenylphosphino)ferrocene (dppf), 1,2-bis (diphenylphosphino)benzene (dppbz) or 1,2-bis (diphenylphosphino)acetylene (dppac) is investigated. It is found that the reaction product depends on the diphosphine used. In the case of dppf, the product is an intramolecular bridged disubstituted complex [Fe2{μ-SeCH2CH(Me)CH2Se-μ}(CO)4{μ,κ1,κ1(P,P)-dppf}] ( 1 ), while the dppac-reaction produces an intermolecular bridged tetra-iron model [Fe2{μ-SeCH2CH(Me)CH2Se-μ}(CO)5]2{μ,κ1,κ1(P,P)-dppac} ( 2 ). However, the dppbz-reaction gives [Fe2{μ-SeCH2CH(Me)CH2Se-μ}(CO)4{κ2(P,P)-dppbz}] ( 3 ) in which the dppbz ligand is bonded to one Fe atom in a chelated manner. The newly prepared complexes ( 1 – 3 ) have been characterized by elemental analysis, IR, 1H-, 13C{H}-, 31P{H}-, 77Se{H}-NMR spectroscopy and X-ray structure determination. The electrochemical behavior of 2 and 3 , in absence and presence of acid, is described by cyclic voltammetric measurements in CH2Cl2. 相似文献
40.
Synthesis and Characterization of GaIII,YIII, and LuIII Complexes with Etifenin and Analogues 下载免费PDF全文
Michel Pfeifer‐Leeg Gergely Szabó Zsolt Baranyai Tobias Niksch Wolfgang Weigand Martin Freesmeyer 《无机化学与普通化学杂志》2016,642(6):486-491
Complexes of etifenin ( 1a ), disofenin ( 1b ), and mebrofenin ( 1c ) with GaIII, YIII, and LuIII were synthesized and characterized by NMR spectroscopy and UV/Vis spectroscopy. Stability and protonation constants of the complexes of 1a were determined by potentiometry and distribution diagrams were calculated. It was shown that the YIII and LuIII species are most stable as bis chelates at higher pH, whereas the GaIII complex degrades at pH < 4.5. 相似文献