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1.
19F and (119)Sn MAS NMR spectroscopy have been used to investigate the fluoride ion conductor, BaSnF(4), a member of the MSnF(4) family of fluorite-related anionic conductors containing double layers of Sn(2+) and M(2+) cations. Two fluorine sublattices were observed by (19)F MAS NMR, which could be assigned to specific sites in the lattice. The first sublattice is due to fluorine atoms located in Ba(2+) double layers and is rigid on the MAS NMR time scale at room temperature. The second sublattice comprises the fluoride ions between the Ba(2+) and Sn(2+) layers, and the few fluorine atoms that inhabit the Sn(2+)-Sn(2+) double layers. These ions are in rapid exchange with each other, and an extremely short correlation time tau(C) for the motion of these ions of <3 x 10(-)(5) s is obtained at -100 degrees C. T(1) measurements indicate that tau(C) approaches 10(-)(8) s at room temperature. (19)F-to-(119)Sn cross-polarization (CP) experiments confirmed the assignments of the resonances, and that the fluorine atoms located next to the tin atoms are extremely mobile at room temperature (and thus do not contribute to the CP process). Two-dimensional (19)F exchange experiments showed that exchange between the rigid and mobile lattice does occur, but at a much slower rate (tau(C) approximately 10 ms at 250 degrees C). Low-temperature (19)F MAS and (19)F-to-(119)Sn CP NMR spectra demonstrate that the motion of the fluoride ions has almost completely frozen out by -150 degrees C. The results are consistent with rapid two-dimensional (anisotropic) conductivity involving the fluoride ions between the Ba and Sn layers. Conductivity in three dimensions requires hops between the ions in the BaF(2)-like layers and the mobile ions. This process does occur, but with exchange rates that are at least 6-7 orders of magnitude slower. 相似文献
2.
Garcia Ruano JL Alcudia A del Prado M Barros D Maestro MC Fernandez I 《The Journal of organic chemistry》2000,65(10):2856-2862
The addition of the lithium anions derived from (R)- and (S)-methyl and -ethyl p-tolyl sulfoxides to (S)-N-benzylidene-p-toluenesulfinamide provides an easy access route to enantiomerically pure beta-(N-sulfinyl)amino sulfoxides. Stereoselectivity can be achieved when the configurations at the sulfur atoms of the two reagents are opposite (matched pair), thus resulting in only one diastereoisomer, even for the case in which two new chiral centers are created. The N-sulfinyl group primarily controls the configuration of the carbon bonded to the nitrogen, whereas the configuration of the alpha-sulfinyl carbanion seems to be responsible for the level of asymmetric induction, as well as for the configuration of the new stereogenic C-SO carbon in the reactions with ethyl p-tolyl sulfoxides. An efficient method for transforming the obtained beta-(N-sulfinyl)amino sulfoxides into optically pure beta-amino alcohols, based on the stereoselective non-oxidative Pummerer reaction, is also reported. 相似文献
3.
Pressure-dependent luminescence spectra of trans-dioxo complexes of rhenium(V) with ancillary ethylenediamine ligands exhibit resolved vibronic structure in the O=Re=O symmetric stretching mode at room temperature. The intensity distribution within the vibronic progression changes with pressure, leading to band shapes that are also pressure-dependent. These spectroscopic features arise from coupled electronic states and depend on the energy differences between ground and excited states, which vary by 2500 cm(-1) for the three complexes with ethylenediamine, tetramethylethylenediamine, and tetraethylethylenediamine ancillary ligands. We describe the pressure-dependent vibronic structure and band shapes with anharmonic adiabatic potential energy surfaces for the ground states of all complexes. The calculated spectra reveal the pressure dependence of the energies of electronic origins, luminescence band maximums, offsets between ground- and emitting-state potential minimums, and vibrational frequencies. The largest pressure effects are observed where the coupled electronic states are close in energy. 相似文献
4.
5.
Mössbauer spectra and X-ray diffraction data have been recorded for compounds in the solid solution series FexTi3?xO5, 0 ? x ? 1. Compounds with x > 0.35 have the orthorhombic pseudobrookite structure, and those with x < 0.35 a monoclinic distortion of it. Ferrous ions in the fourfold and eightfold sites give rise to separate quadrupole doublets in the Mössbauer spectra, and thus it has been possible to determine the site occupancy as a function of composition by a least-square analysis of the spectra. The Mössbauer method has also proved to be a sensitive tool for the investigation of the orthorhombic-monoclinic distortion. 相似文献
6.
7.
Peter D. Battle Jonathan C. Burley Clare P. Grey 《Journal of solid state chemistry》2004,177(1):119-125
Polycrystalline samples of the n=2 Ruddlesden-Popper phase La3LiMnO7 have been prepared and characterized. X-ray and neutron diffraction suggest that the structure is tetragonal with a disordered distribution of Li and Mn cations over the octahedral sites, but 6Li MAS NMR shows that the Li and Mn are 1:1 ordered locally. Electron microscopy shows that the stacking of the cation-ordered, perovskite-like bilayers along the crystallographic z-axis is disordered on the distance scale sampled by X-ray and neutron diffraction. Magnetometry data and neutron diffraction data collected at 2 K together suggest that the Mn cations within each structural domain order antiferromagnetically at 14 K, but that the disorder along z prevents the establishment of long-range magnetic order. 相似文献
8.
Garcia Ruano JL Alemparte C Martin Castro AM Adams H Rodriguez Ramos JH 《The Journal of organic chemistry》2000,65(23):7938-7943
The behavior of (Z)-3-p-tolylsulfinylacrylonitrile (1) as a chiral dienophile has been evaluated from its reactions with furan and acyclic dienes. Electrostatic interactions of the cyano group with the sulfinyl one restrict the conformational mobility around the C-S bond, thus controlling the pi-facial selectivity, which is almost complete in all cases, the approach of the diene from the less-hindered face of the dienophile (that bearing the lone electron pair) in the predominant rotamer being the favored one. The regioselectivity is also completely controlled by the cyano group. Additionally, the reactivity of compound 1 as well as its endo-selectivity are both higher than those observed for the corresponding (Z)-3-sulfinylacrylates, thus proving the potential of sulfinylnitriles as chiral dienophiles. 相似文献
9.
The perovskite-related phase Ca3Nb2O8, when grown as single crystals from a calcium vanadate flux, incorporates a small amount of vanadium from the flux to form the composition Ca3Nb2−xVxO8 with x=0.025. The crystals have pseudo-cubic symmetry with a=6×ac(perovskite). The actual symmetry is rhombohedral, space group R3, with ah=16.910(1) Å, ch=41.500(2) Å. The structure was solved using a combination of single-crystal methods together with constrained refinements of powder X-ray and neutron powder data. The unit-cell composition is [Ca138□24]A [Ca42Nb117V3]B[O480□6], with vacancies in both the anion sites and A-cation sites. The Ca and Nb atoms are fully ordered in the B-sites such that (001) layers containing only Nb-centered octahedra alternate with layers containing both Nb-centered and Ca-centered octahedra. At the origin B-site, ordered oxygen vacancies result in the octahedron being transformed to a tetrahedron, which, in the single crystals, is occupied by vanadium. The structure displays a new type of octahedral tilt system in which 3×3×3 blocks of (a+a+a+) tilts are periodically twinned on the pseudo-cubic {1 0 0}c planes. 相似文献
10.
New weberite-type Ca2Ta2O7 and zirconolite-type CaZrTi2O7 polytypes have been prepared by doping with Nd/Zr and Th/Al, respectively, and their structures have been refined using single-crystal X-ray diffraction intensity data. The 3T zirconolite polytype, Ca0.8Ti1.35Zr1.3Th0.15Al0.4O7, has a=7.228(1), c=16.805(1) Å. The 3T weberite-type polytype, Ca1.92Ta1.92Nd0.08Zr0.08O7, has a=7.356(1), c=18.116(1) Å. Both 3T polytypes have space group P3121, Z=6. The 4M Ca2Ta2O7 polytype has the same composition, from electron microprobe analyses, as the 3T polytype, and has cell parameters: a=12.761(1), b=7.358(1), c=24.565(1) Å, β=100.17(1)°, space group C2, Z=16. The structural relationships between the different zirconolite and weberite polytypes are discussed. A consideration of the structures from the viewpoint of anion-centered tetrahedral arrays shows that zirconolite can be considered as an anion-deficient fluorite derivative phase. However, the fluorite-type topology of edge-shared OM4 tetrahedra is not maintained in the Ca2Ta2O7 weberite-type polytypes, even though they have a fluorite-like fcc packing of metal atoms. One of the oxygen atoms moves from a tetrahedral Ta3Ca interstice to an adjacent Ta2Ca4 octahedral interstice in the weberite polytypes. 相似文献