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排序方式: 共有199条查询结果,搜索用时 15 毫秒
91.
92.
Studies of quadrupole interactions and relaxation phenomena in Dy2Ti2O7 at various temperatures up to 750 K have been performed using the Mössbauer effect of the 25.6 keV transition in 161Dy. The source used was 161Tb in 160Gd2Ti2O7 at 150 K, which emits a very narrow 25.6 keV γ line. Quadrupole interaction parameters and relaxation times, as function of temperature, were deduced from the measurements. 相似文献
93.
Mössbauer-effect studies yield the following nuclear parameters: In 155Gd, Q(86)/Q(0) = 0.087 ± 0.006, Q(105)/Q(0) = 1.00 ± 0.03. In . The value of g(89) is in very good agreement with the theoretical value. 相似文献
94.
95.
Recoilless absorption measurements of the 74.5 keV γ-rays of 161Dy were carried out using absorbers. The analysis of the absorption spectrum obtained with the nitrate absorber yielded a value of 1.38 ± 0.02 for the ratio of the g-factor of the 74.5 keV and the ground levels of 161Dy and a value of 0.58 ± 0.04 for the corresponding ratio of the quadrupole moments. Relying on previous measurements of the ground level moments, the values derived for the magnetic and quadrupole moments of the 74.5 level are −0.377 ± 0.012 n.m. and 1.5 ± 0.2 b, respectively. These values are compared with the values predicted by nuclear theory. The isomeric shift between DyNi2 and
absorbers was found to be 0.9 ± 0.2 mm/sec. 相似文献
96.
97.
Yusen Qiao Haolin Yin Liane M. Moreau Rulin Feng Robert F. Higgins Brian C. Manor Patrick J. Carroll Corwin H. Booth Jochen Autschbach Eric J. Schelter 《Chemical science》2021,12(10):3558
A series of cerium(iv) mixed-ligand guanidinate–amide complexes, {[(Me3Si)2NC(NiPr)2]xCeIV[N(SiMe3)2]3−x}+ (x = 0–3), was prepared by chemical oxidation of the corresponding cerium(iii) complexes, where x = 1 and 2 represent novel complexes. The Ce(iv) complexes exhibited a range of intense colors, including red, black, cyan, and green. Notably, increasing the number of the guanidinate ligands from zero to three resulted in significant redshift of the absorption bands from 503 nm (2.48 eV) to 785 nm (1.58 eV) in THF. X-ray absorption near edge structure (XANES) spectra indicated increasing f occupancy (nf) with more guanidinate ligands, and revealed the multiconfigurational ground states for all Ce(iv) complexes. Cyclic voltammetry experiments demonstrated less stabilization of the Ce(iv) oxidation state with more guanidinate ligands. Moreover, the Ce(iv) tris(guanidinate) complex exhibited temperature independent paramagnetism (TIP) arising from the small energy gap between the ground- and excited states with considerable magnetic moments. Computational analysis suggested that the origin of the low energy absorption bands was a charge transfer between guanidinate π orbitals that were close in energy to the unoccupied Ce 4f orbitals. However, the incorporation of sterically hindered guanidinate ligands inhibited optimal overlaps between Ce 5d and ligand N 2p orbitals. As a result, there was an overall decrease of ligand-to-metal donation and a less stabilized Ce(iv) oxidation state, while at the same time, more of the donated electron density ended up in the 4f shell. The results indicate that incorporating guanidinate ligands into Ce(iv) complexes gives rise to intense charge transfer bands and noteworthy electronic structures, providing insights into the stabilization of tetravalent lanthanide oxidation states.A series of cerium(iv) mixed-ligand guanidinate-amide complexes, {[(Me3Si)2NC(NiPr)2]xCeIV[N(SiMe3)2]3−x}+ (x = 0−3), was prepared by chemical oxidation and studied spectroscopically and computationally, revealing trends in 4f/5d orbital occupancies. 相似文献
98.
Castoldi D Caggiano L Panigada L Sharon O Costa AM Gennari C 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,12(1):51-62
Asymmetric oxyallylation reactions and ring-closing metathesis have been used to synthesize compound 3, a key advanced intermediate used in the total synthesis of eleutherobin reported by Danishefsky and co-workers. The aldehyde 6, which is readily prepared from commercially available R-(-)-carvone in six steps in 30 % overall yield on multigram quantities, was converted into the diene 5 utilizing two stereoselective titanium-mediated Hafner-Duthaler oxyallylation reactions. The reactions gave the desired products (8 and 12) in high yields (73 and 83 %, respectively) as single diastereoisomers, with the allylic alcohol already protected as the p-methoxyphenyl (PMP) ether, which previous work has demonstrated actually aids ring-closing metathesis compared to other protective groups and the corresponding free alcohol. Cyclization under forcing conditions, using Grubbs' second-generation catalyst 13, gave the ten-membered carbocycle (E)-14 in 64 % yield. This result is in sharp contrast to similar, but less functionalized, dienes, which have all undergone cyclization to give the Z stereoisomers exclusively. A detailed investigation of this unusual cyclization stereochemistry by computational methods has shown that the E isomer of the ten-membered carbocycle is indeed less thermodynamically stable than the corresponding Z isomer. In fact, the selectivity is believed to be due to the dense functionality around the ruthenacyclobutane intermediate that favors the trans-ruthenacycle, which ultimately leads to the less stable E isomer of the ten-membered carbocycle under kinetic control. During the final synthetic manipulations the double bond of enedione (E)-16 isomerized to the more thermodynamically stable enedione (Z)-4, giving access to the advanced key-intermediate 3, which was spectroscopically and analytically identical to the data reported by Danishefsky and co-workers, and thereby completing the formal synthesis of eleutherobin. 相似文献
99.
We investigated the nature of the freezing in the geometrically frustrated Heisenberg spin glass Y2(Mo2)O(7) by measuring the temperature dependence of the static internal magnetic field distribution above the spin-glass temperature, Tg, using the muon spin relaxation technique. The evolution of the field distribution cannot be explained by changes in the spin susceptibility alone and suggests a lattice deformation. This possibility is addressed by numerical simulations of the Heisenberg Hamiltonian with magnetoelastic coupling at T > 0. 相似文献
100.
Brillouin zone spectroscopy of nonlinear photonic lattices 总被引:1,自引:0,他引:1
We present a novel, real-time, experimental technique for linear and nonlinear Brillouin zone spectroscopy of photonic lattices. The method relies on excitation with random-phase waves and far-field visualization of the spatial spectrum of the light exiting the lattice. Our technique facilitates mapping the borders of the extended Brillouin zones and the areas of normal and anomalous dispersion within each zone. For photonic lattices with defects (e.g., photonic crystal fibers), our technique enables far-field visualization of the defect mode overlaid on the extended Brillouin zone structure of the lattice. The technique is general and can be used for photonic crystal fibers as well as for periodic structures in areas beyond optics. 相似文献