排序方式: 共有56条查询结果,搜索用时 15 毫秒
31.
32.
M Carravetta M Edén O G Johannessen H Luthman P J Verdegem J Lugtenburg A Sebald M H Levitt 《Journal of the American Chemical Society》2001,123(43):10628-10638
We describe magic-angle-spinning NMR methods for the accurate determination of internuclear dipole-dipole couplings between homonuclear spins-(1/2) in the solid state. The new sequences use symmetry principles to treat the effect of magic-angle sample-rotation and resonant radio frequency fields. The pulse-sequence symmetries generate selection rules which reduce the interference of undesirable interactions and improve the robustness of the pulse sequences with respect to chemical shift anisotropies. We show that the pulse sequences may be used to estimate distances between 13C spins in organic solids, including bond lengths in systems with large chemical shift anisotropies, such as conjugated systems. For bond-length measurements, the precision of the method is +/-2 pm with a systematic overestimate of the internuclear distance by 3 +/- 1 pm. The method is expected to be a useful tool for investigating structural changes in macromolecules. 相似文献
33.
Thomas Riis‐Johannessen Dr. Natalia Dalla Favera Tanya K. Todorova Dr. Stefan M. Huber Dr. Laura Gagliardi Prof. Dr. Claude Piguet Prof. Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(46):12702-12718
Deviations from statistical binding, that is cooperativity, in self‐assembled polynuclear complexes partly result from intermetallic interactions ΔEM,M, whose magnitudes in solution depend on a balance between electrostatic repulsion and solvation energies. These two factors have been reconciled in a simple point‐charge model, which suggests severe and counter‐intuitive deviations from predictions based solely on the Coulomb law when considering the variation of ΔEM,M with metallic charge and intermetallic separation in linear polynuclear helicates. To demonstrate this intriguing behaviour, the ten microscopic interactions that define the thermodynamic formation constants of some twenty‐nine homometallic and heterometallic polynuclear triple‐stranded helicates obtained from the coordination of the segmental ligands L1 – L11 with Zn2+ (a spherical d‐block cation) and Lu3+ (a spherical 4f‐block cation), have been extracted by using the site binding model. As predicted, but in contrast with the simplistic coulombic approach, the apparent intramolecular intermetallic interactions in solution are found to be i) more repulsive at long distance ( > ), ii) of larger magnitude when Zn2+ replaces Lu3+ ( > ) and iii) attractive between two triply charged cations held at some specific distance ( <0). The consequences of these trends are discussed for the design of polynuclear complexes in solution. 相似文献
34.
Madhu PK Johannessen OG Pike KJ Dupree R Smith ME Levitt MH 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2003,163(2):310-317
We report pulse sequences for the sensitivity enhancement of magic-angle spinning and multiple-quantum magic-angle spinning spectra of spin-72 systems. Sensitivity enhancement is obtained with the use of fast amplitude-modulated (FAM) radiofrequency pulses. In one-dimensional magic-angle spinning experiments, signal enhancement of 3 is obtained by a FAM pulse followed by a soft 90 degrees pulse. In two-dimensional multiple-quantum magic-angle spinning experiments, FAM pulses are used for both the excitation of multiple-quantum coherences and for their conversion into observable single-quantum coherences. The observed signal enhancements are 2.2 in 3Q experiments, 3.1 in 5Q experiments, and 4.1 in 7Q experiments, compared to the conventional two-pulse scheme. The pulse schemes are demonstrated on the 45Sc NMR of Sc2(SO4)3 x 5H2O and the 139La NMR of LaAlO3. We also demonstrate the generation of FAM pulses by double-frequency irradiation. 相似文献
35.
36.
Gemeng Liang Zhibin Wu Christophe Didier Wenchao Zhang Jing Cuan Baohua Li Kuan‐Yu Ko Po‐Yang Hung Cheng‐Zhang Lu Yuanzhen Chen Grzegorz Leniec Sawomir Maksymilian Kaczmarek Bernt Johannessen Lars Thomsen Vanessa K. Peterson Wei Kong Pang Zaiping Guo 《Angewandte Chemie (International ed. in English)》2020,59(26):10594-10602
Spinel LiNi0.5Mn1.5O4 (LNMO) is a promising cathode candidate for the next‐generation high energy‐density lithium‐ion batteries (LIBs). Unfortunately, the application of LNMO is hindered by its poor cycle stability. Now, site‐selectively doped LNMO electrode is prepared with exceptional durability. In this work, Mg is selectively doped onto both tetrahedral (8a) and octahedral (16c) sites in the Fd m structure. This site‐selective doping not only suppresses unfavorable two‐phase reactions and stabilizes the LNMO structure against structural deformation, but also mitigates the dissolution of Mn during cycling. Mg‐doped LNMOs exhibit extraordinarily stable electrochemical performance in both half‐cells and prototype full‐batteries with novel TiNb2O7 counter‐electrodes. This work pioneers an atomic‐doping engineering strategy for electrode materials that could be extended to other energy materials to create high‐performance devices. 相似文献
37.
Levitt MH Johannessen OG 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2000,142(1):190-194
This article analyzes the influence of the radiofrequency mixing scheme on the sign of phase shifts experienced by the nuclear spins. It is an addendum to a previous article on the signs of phases and frequencies in NMR (M. H. Levitt, J. Magn. Reson. 126, 164 (1997)). 相似文献
38.
Concistrè M Gansmüller A McLean N Johannessen OG Marín Montesinos I Bovee-Geurts PH Verdegem P Lugtenburg J Brown RC DeGrip WJ Levitt MH 《Journal of the American Chemical Society》2008,130(32):10490-10491
The 13C chemical shifts of the primary visual photointermediate bathorhodopsin have been observed by performing double-quantum magic-angle-spinning NMR at low temperature in the presence of illumination. Strong isomerization shifts have been observed upon the conversion of rhodopsin into bathorhodopsin. 相似文献
39.
Yameng Fan Dr. Emilia Olsson Dr. Gemeng Liang Dr. Zhijie Wang Dr. Anita M. D'Angelo Dr. Bernt Johannessen Dr. Lars Thomsen Dr. Bruce Cowie Jingxi Li Fangli Zhang Dr. Yunlong Zhao Dr. Wei Kong Pang Dr. Qiong Cai Prof. Zaiping Guo 《Angewandte Chemie (International ed. in English)》2023,62(5):e202213806
The application of Li-rich layered oxides is hindered by their dramatic capacity and voltage decay on cycling. This work comprehensively studies the mechanistic behaviour of cobalt-free Li1.2Ni0.2Mn0.6O2 and demonstrates the positive impact of two-phase Ru doping. A mechanistic transition from the monoclinic to the hexagonal behaviour is found for the structural evolution of Li1.2Ni0.2Mn0.6O2, and the improvement mechanism of Ru doping is understood using the combination of in operando and post-mortem synchrotron analyses. The two-phase Ru doping improves the structural reversibility in the first cycle and restrains structural degradation during cycling by stabilizing oxygen (O2−) redox and reducing Mn reduction, thus enabling high structural stability, an extraordinarily stable voltage (decay rate <0.45 mV per cycle), and a high capacity-retention rate during long-term cycling. The understanding of the structure-function relationship of Li1.2Ni0.2Mn0.6O2 sheds light on the selective doping strategy and rational materials design for better-performance Li-rich layered oxides. 相似文献
40.
The Raman optical activity (ROA) spectra of proteins show distinct patterns arising from the secondary structure. It is generally believed that the spectral contributions of the side-chains largely cancel out because of their flexibility and the occurrence of many side-chains with different conformations. Yet, the influence of the side-chains on the ROA patterns assigned to different secondary structures is unknown. Here, the first systematic study of the influence of all amino acid side-chains on the ROA patterns is presented based on density functional theory (DFT) calculations of an extensive collection of peptide models that include many different side-chain and secondary structure conformations. It was shown that the contributions of the side-chains to a large extent average out with conformational flexibility. However, specific side-chain conformations can have significant contributions to the ROA patterns. It was also shown that α-helical structure is very sensitive to both the exact backbone conformation and the side-chain conformation. Side-chains with χ1≈−60° generate ROA patterns alike those in experiment. Aromatic side-chains strongly influence the amide III ROA patterns. Because of the huge structural sensitivity of ROA, the spectral patterns of proteins arise from extensive conformational averaging of both the backbone and the side-chains. The averaging results in the fine spectral details and relative intensity differences observed in experimental spectra. 相似文献