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81.
The reaction of the tetranuclear rare earth metal polyhydrido complexes {Cp'Ln(mu-H)2}4(THF) (Cp' = C5Me4SiMe3, Ln = Y (1a), Lu (1b)) with carbon monoxide (1 atm) yielded ethylene and the corresponding tetraoxo cubane complexes (Cp'Ln)4(mu3-O)4 (Ln = Y (5a), Lu (5b)). Stepwise formation of some key reaction intermediates, such as oxymethylene complexes (Cp'Ln)4(mu-OCH2)(mu-H)6(THF) (Ln = Y (2a), Lu (2b)), enolate species (Cp'Y)4(OCH=CH2)(mu-O)(mu-H)5(THF) (3), and dioxo complex (Cp'Y)4(mu3-O)2(mu-H)4(THF) (4), was confirmed. The molecular structures of 2a, 4, and 5b were determined by X-ray diffraction studies. 相似文献
82.
We consider the N-body problem in spaces of constant curvature and study its rotopulsators, i.e. solutions for which the configuration of the bodies rotates and changes size during the motion. Rotopulsators fall naturally into five groups: positive elliptic, positive elliptic–elliptic, negative elliptic, negative hyperbolic, and negative elliptic–hyperbolic, depending on the nature and number of their rotations and on whether they occur in spaces of positive or negative curvature. After obtaining existence criteria for each type of rotopulsator, we derive their conservation laws. We further deal with the existence and uniqueness of some classes of rotopulsators in the 2- and 3-body case and prove two general results about the qualitative behaviour of rotopulsators. More precisely, for positive curvature we show that there is no foliation of the 3-sphere with Clifford tori such that the motion of each body is confined to some Clifford torus. For negative curvature, a similar result is proved relative to foliations of the hyperbolic 3-sphere with hyperbolic cylinders. 相似文献
83.
The second harmonic of a Rayleigh wave passing through a minute surface crack has been numerically analyzed by semi-explicit FEM including special elements which account for a nonlinear stress-strain relation at crack surfaces. Minute cracks perpendicular to a free, flat surface close under compressive stress when width of the crack opening is less than the longitudinal amplitude of the Rayleigh wave. Thereafter, compressive and shear stresses are partially transmitted through the closed cracks, whereas tensile and shear stresses are not transmitted through cracks that remain open. This leads to marked nonlinear ultrasonic response. Calculation was performed for an aluminum block having a surface crack. The transverse component of the Rayleigh wave propagating through the cracks shows distorted waveforms, making the second harmonic amplitude clearly noticeable. In an experiment, the second harmonic component of the leaky Rayleigh wave was detected for a simple crack model consisting of two aluminum blocks, by use of a PVDF line-focused transducer. The results of the experiment show that the second harmonic amplitude is a second-order function of the fundamental wave amplitude, and is more pronounced for low compressive stress applied to close the crack surfaces. 相似文献
84.
Crystal growth and the magnetic properties of bismuth substituted yttrium iron garnet (Bi-YIG) nanoparticles were studied with particular focus on the bismuth composition dependence of the magnetic properties of the particles and the effects of annealing on the garnet phase formation. The Bi-YIG nanoparticles of 47–67 nm in size can be chemically synthesized when they are annealed at 650–850 °C. Both the lattice constant and the magnetization of the garnet nanoparticles linearly increase when the bismuth composition in the Bi-YIG particles increases. We have found that chemically synthesized nanoparticles transform from the amorphous to the garnet phase when annealed at temperatures below 650 °C, while the onset of magnetic moment of iron in the garnet nanoparticles is observed slightly above 650 °C. According to Mössbauer effect measurements, the hyperfine fields of 57Fe at the tetrahedral and octahedral sites in the garnet are 39 and 48 T, respectively. 相似文献
85.
86.
In low-Reynolds-number turbulent flows, the influence of the molecular viscosity is important. The turbulence models which
are applied to those flows should include the low-Reynolds-number effect. In this study, turbulent flow with the molecular
viscosity effect is analyzed theoretically with the aid of a two-scale direct-interaction approximation (TSDIA) and the energy
spectrum and a new low-Reynolds-number-type eddy-viscosity representation are derived. An priori test for the model expression on the basis of the result of direct numerical simulation (DNS) for turbulent Couette flows
is performed.
Received 5 July 2002 and accepted 8 January 2003 Published online 25 March 2003
Communicated by T.B. Gatski 相似文献
87.
The activation of molecular hydrogen is of interest both from a chemical and biological viewpoint. The covalent bond of H(2) is strong (436 kJ mol(-1)). Its cleavage is catalyzed by metals or metal complexes in chemical hydrogenation reactions and by metalloenzymes named hydrogenases in microorganisms. Until recently only two types of hydrogenases are known, the [FeFe[-hydrogenases and [NiFe[-hydrogenases. Both types, which are phylogenetically unrelated, harbor in their active site a dinuclear metal center with intrinsic CO and cyanide ligands and contain iron-sulfur clusters for electron transport as revealed by their crystal structures. Fifteen years ago a third type of phylogenetically unrelated hydrogenase was discovered, which has a mononuclear iron active site and is devoid of iron-sulfur clusters. It was initially referred to as "metal free" hydrogenase, but was later renamed iron-sulfur cluster-free hydrogenase or [Fe[-hydrogenase. In this review, we introduce first the [FeFe[-hydrogenases and [NiFe[-hydrogenases, and then focus on the structure and function of the iron-sulfur cluster-free hydrogenase (Hmd) and show that this enzyme contains an iron-containing cofactor. The low-spin iron is complexed by two intrinsic CO-, one sulfur- and one or two N/O ligands and has one open coordination site, which is proposed to be the location of H(2) binding. 相似文献
88.
Na2S/Al2O3 catalyzed the Knoevenagel condensation between an aldehyde and an active methylene compound to yield various trisubstituted alkenes. 相似文献
89.
Previously we reported that fluorescent light (FL) illumination prior to UV irradiation (FL preillumination) had multiple effects on the repair of cyclobutane pyrim-idine dimers (CPD) and (6-4) photoproducts ([6-4] PD) in cultured goldfish cells (RBCF-1) at the exponentially growing phase. In this study, it is shown that even under the confluent condition of RBCF-1 cells, FL preillumination increased the disappearance of (6-4) PD in the dark. In addition, both at confluence and at the exponentially growing phase, the disappearance of (6-4) PD after PR treatment was increased by FL preillumination to RBCF-1 cells. Moreover, it was found that H2O2 pre-treatment, followed by UV irradiation, of the exponentially growing cells also enhanced the disappearance of (6-4) PD in the dark and by photorepair treatment. The degree of enhancement by H2O2 pretreatment was almost the same as that by FL preillumination. 相似文献
90.
Rüffer T Ohashi M Shima A Mizomoto H Kaneda Y Mashima K 《Journal of the American Chemical Society》2004,126(39):12244-12245
Reaction of Mo2(pyphos)4 (1) with [RhCl(CO)2]2 followed by treatment of excess amounts of tBuNC resulted in the clean formation of [Mo2Rh2(tBuNC)4(pyphos)4](X)2 (4a; X = Cl). The X-ray diffraction study as well as spectroscopic analyses of 4c (X = BPh4) implied that there is no direct sigma-bonding interaction between each Rh(I) atom and the Mo2 core. Each Rh(I) atom in 4 can be oxidized concurrently by 2 equiv of [Cp2Fe]PF6 to afford [Mo2Rh2(Cl)2(tBuNC)4(pyphos)4](PF6)2 (5) along with the formation of two Mo-Rh(II) single bonds and the reduction of the bond orders of the Mo-Mo moiety. 相似文献