排序方式: 共有38条查询结果,搜索用时 15 毫秒
1.
Battle PD Blundell SJ Brooks ML Hervieu M Kapusta C Lancaster T Nair SP Oates CJ Pratt FL Rosseinsky MJ Ruiz-Bustos R Sikora M Steer CA 《Journal of the American Chemical Society》2004,126(39):12517-12527
The temperature dependence of the crystal structure and electronic properties of brownmillerite-like Ca(2.5)Sr(0.5)GaMn(2)O(8) has been studied by neutron powder diffraction and muSR spectroscopy. The results show that short-range 2D magnetic order begins to develop within the perovskite-like bilayers of MnO(6) octahedra approximately 50 K above the 3D Néel temperature of approximately 150 K. The bilayers show a structural response to the onset of magnetism throughout this temperature range whereas the GaO(4) layers that separate the bilayers only respond below the 3D ordering temperature. XANES spectroscopy shows that the sample contains Mn(3+) and Mn(4+) cations in a 1:1 ratio, and the behavior in the region of the Néel transition is interpreted as a local charge ordering. Electron diffraction and high-resolution electron microscopy have been used to show that the local microstructure is more complex than the average structure revealed by neutron diffraction, and that microdomains exist in which the GaO(4) tetrahedra show different orientations. It is argued that the bonding requirements of diamagnetic gallium control the electronic behavior within the perovskite-like bilayers. 相似文献
2.
The result stated in the title is proved.Research of the second author partially supported by the National Science Foundation. He would also like to thank the Banach Institute, Polish Academy of Sciences and Universität Kaiserslautern for their hospitality. 相似文献
3.
Subsequent treatment of N-crotoyl-(1S,2R)-bornane-10,2-sultam with EtMgCl, recrystallization of the product and saponification, afforded R-(-)-3-methylpenthanoic acid which was used for acylation of (1R,2S)-bornane-10,2-sultam. The product was converted into N-[(2S,3R)-2-amino-3-methylpentanoyl]-(1R,2S)-bornane-10,2-sultam by hydroxyamination with 1-chloro-1-nitrosocyclohexane, followed by reduction of the hydroxylamine grouping. Saponification of the sultam imide provided (+)-alloisoleucine. 相似文献
4.
Baril N Lewa CJ de Certaines JD Canioni P Franconi JM Thiaudière E 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2002,154(1):22-27
A new method for NMR characterization of mechanical waves, based upon radiofrequency field gradient for motion encoding, is proposed. A binomial B1 gradient excitation scheme was used to visualize the mobile spins undergoing a periodic transverse mechanical excitation. A simple model was designed to simulate the NMR signal as a function of the wave frequency excitation and the periodicity of the NMR pulse sequence. The preliminary results were obtained on a gel phantom at low vibration frequencies (0-200 Hz) by using a ladder-shaped coil generating a nearly constant RF field gradient along a specific known direction. For very small displacements and/or B1 gradients, the NMR signal measured on a gel phantom was proportional to the vibration amplitude and the pulse sequence was shown to be selective with respect to the vibration frequency. A good estimation of the direction of vibrations was obtained by varying the angle between the motion direction and the B1 gradient. The method and its use in parallel to more conventional MR elastography techniques are discussed. The presented approach might be of interest for noninvasive investigation of elastic properties of soft tissues and other materials. 相似文献
5.
A method for determination of magnetocrystalline anisotropy constants of arbitrary order is proposed. The method is based on a least squares fitting of a phenomenological anisotropy energy for a given symmetry truncated at an arbitrary order term to a theoretical anisotropy energy computed exactly for a given energy level model. Several applications of the method to cubic systems are considered. The study reveals that the widely used expressions in the literature for the cubic constants K1 and K2 in terms of free energy for the three symmetry direction are of rather limitedvalidity only. The higher-order cubic constants K3, K4 and K5 are determined besides the usual K1 and K2 in temperature range 0 to 300 K. The importance of the higher- order terms with respect to the first term in the cubic anisotropy energy is discussed. The results show that the cubic constants K3 and K4 cannot be neglected for most of the energy level models studied at certain temperatures. 相似文献
6.
Madelin G Baril N Lewa CJ Franconi JM Canioni P Thiaudiére E de Certaines JD 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2003,161(1):108-111
A B(1) field gradient-based method previously described for the detection of mechanical vibrations has been applied to detect oscillatory motions in condensed matter originated from acoustic waves. A ladder-shaped coil generating a quasi-constant RF-field gradient was associated with a motion-encoding NMR sequence consisting in a repetitive binomial 13;31; RF pulse train (stroboscopic acquisition). The NMR response of a gel phantom subject to acoustic wave excitation in the 20-200 Hz range was investigated. Results showed a linear relationship between the NMR signal and the wave amplitude and a spectroscopic selectivity of the NMR sequence with respect to the input acoustic frequency. Spin displacements as short as a few tens of nanometers were able to be detected with this method. 相似文献
7.
8.
Liu H Gaza-Bulseco G Chumsae C Radziejewski CH 《Rapid communications in mass spectrometry : RCM》2008,22(1):1-10
Nitration of a recombinant human monoclonal antibody was carried out in vitro by incubating the antibody with the nitrating reagent tetranitromethane (TNM). The susceptible sites of nitration were identified using high-performance liquid chromatography/mass spectrometry (HPLC/MS). In general, tyrosine residues in the variable domains of the antibody are more susceptible to nitration, while tyrosine residues in the constant domains are relatively resistant to nitration. However, one tyrosine residue in the CH1 domain and one tyrosine residue in the CH2 domain are highly susceptible to nitration. Interestingly, the susceptible tyrosine residue in the CH2 domain is followed by the conserved asparagine residue that is glycosylated. 相似文献
9.
Czeslaw Rudowicz Zi-Yuan Yang Jian Qin 《Journal of Physics and Chemistry of Solids》2003,64(8):1419-1428
A new module has been developed within the CFA/MSH computer package, which is applicable for d2 and d8 ions at sites of trigonal symmetry type I (C3v,D3,D3d) and type II (C3,C3i), including the ‘imaginary’ CF term. For the first time the spin-spin (SS) and spin-other-orbit (SOO) interactions have also been included in the Hamiltonian. This module enables to study the contributions to the energy levels and the spin Hamiltonian parameters, i.e. zero-field splitting D and g-factors: g∥ and g⊥. The contributions arising from the spin-orbit (SO), SS, and SOO interaction as well as those due to the low symmetry CF effects induced by the distortion angle ?, which describes the difference between C3 and C3v symmetry, can be studied. As an application of the new module, calculations have been carried out for V3+(3d2) ions in α-Al2O3 crystal, taking into account for the first time the SS and SOO interactions, and the low symmetry CF effects. The results show that (i) the contributions from the SS and SOO interactions to the energy levels are larger for free V3+ ions than those for V3+ ions in α-Al2O3 crystal, (ii) both the contributions to the SH parameters and the energy levels arising from the SOO interaction are larger than those arising from the SS interaction, (iii) the contributions due to the low symmetry CF effects induced by the distortion angle ? are in general significant, (iv) D and g⊥ are sensitive to the distortion angle ?, whereas g∥ is insensitive to ?, and (v) the influence of the lattice distortions on the spectroscopic properties of V3+ ion in α-Al2O3 is pronounced. It appears important for similar ion-crystal cases to consider the lattice distortions in detailed calculations, which take into account the relevant contributions from the SO, SS and SOO interactions. A good agreement between the theoretical and experimental results has been obtained. 相似文献
10.
A Monte Carlo simulation of the concentration dependence of the fluorescence quantum yield M and emission anisotropyr
M of a system containing dye molecules in the form of monomers M and clusters T (statistical pairs and trimers) playing the role of the imperfect traps for nonradiative excitation energy transfer (NET) has been carried out. The simulation has been made for determined values of Förster critical distancesR
0
MM
andR
0
MT
and for several values ofR
0
TM
andR
0
TT
, assuming that the energy may be transferred from M* to T as well as from T* to M (reverse nonradiative energy transfer, RNET). It was shown that the RNET process in the range of high concentrations may strongly change the values ofr
M as well as those of M. For emission anisotropyr
M an effect of repolarization was observed which decreases rapidly with increasingR
0
TM
andR
0
TT
. A very good agreement between the simulation results of M and the theoretical model with no adjustable parameters was found. In the model, the RNET process and influence of correlation between active molecules on energy migration among monomers were taken into account. 相似文献