34,782 research outputs found
Studies on Bis(imido) molybdenum complexes containing unsaturated hydrocarbon ligands
No full textThis thesis describes the synthesis and characterisation of molybdenum bis(imido) complexes containing unsaturated hydrocarbon ligands. A principal objective of the work was to examine the effect of various imido substituents on the coordination number of the complex and the orientations adopted by olefin and acetylene ligands. Chapter One highlights areas of transition metal chemistry relevant to the thesis, with particular emphasis on the psuedo-isolobal analogy between cyclopentadienyl and imido ligands, A convenient one-pot synthesis of molybdenum bis(imido) complexes of the type Mo(NR)(NR')Cl(_2).DME (R=R'=l-adamantyl, 2-t- BUC(_6)H(_4); R=2,6-i-Pr(_2)C(_6)H(_3), R'=t-Bu) is described in Chapter Two. Mo(N-l- adamantyl)(O)Cl(_2).DME has been synthesised, and its structure determined by single crystal X-ray diffraction.- The preparation of olefin complexes Mo(NR)(NR')(C(_2)H(_4))(PMe(_3))n (R=R'=l-adamantyl, n=l; R=R'=2-t-BuC(_6)H(_4), n=2; R=2,6-i-Pr(_2)C(_6)H(_3), R'=t- Bu, n=l) is outlined in Chapter Three. Structural information derived from NMR data has allowed comparison with metallocene-like olefin adducts. Chapter Four describes the synthesis of complexes containing σ-bound phenyl ligands (Mo(NR)(NR')(σ-C(_6)H(_5))(PMe(_3)) (R=R'=l-adamantyl, 2-t-BuC(_6)H(_4); R=2,6-i-Pr(_2)C(_6)H(_3), R'=t-Bu)) as potential precursors to benzyne complexes. Chapter Five describes the preparation of diphenylacetylene complexes Mo(NR)(NR')(PhC=CPh)(PMe(_3)), structural information derived from NMR data allows comparison with previously known metallocene-like acetylene complexes. Full experimental details for Chapters Two to Five are given in Chapter Six
Enhanced flux pinning and formation of Ba4Y2CuMoOy in top-seeded melt growth processed YBa2Cu3O7-d superconductors with Mo additions
No full textThe effect of Mo addition (0-10 wt%) on the superconductivity of top-seeded melt growth (TSMG) processed YBa2Cu3O7-y (Y123) superconductors was studied. The low level Mo addition (<= 1 wt%) led to a small decrease of the superconducting transition temperature (T-c) and increase of the critical current density (J(c)). The J(c) improvement induced by the low level Mo additions appeared as a peak effect at the intermediated magnetic fields and peak position shift to the lower magnetic fields with increasing Mo content. The enhanced flux pinning caused by Mo additions seems to be attributed to the partial Cu substitution by Mo, YBa2(Cu1-xMox)(3)O7-d. The high level Mo additions (2-10 wt%), however, led to a large J(c) decrease and broad superconducting transition due to the formation of low-T-c phases and the increased volume of the non-superconducting Mo-containing phase. The second particle phase formed by the high level Mo additions was identified as Ba4Y2CuMoOy (Mo4211) by x-ray diffraction (XRD) and scanning electron microscopy energy dispersive x-ray (SEM EDX) analysis
Bishop H. P. Porter, J. H. Bray, J. H. Moore, May 10-11, 1934
No full textBishop H. P. Porter, J. H. Bray, and J. H. Moore are shown posing for a picture at a CME Church Conference in St. Louis, MO., which took place May 10-11, 1934
Modelling the IGM and the Ly alpha forest at high redshift from the dark matter distribution
No full textA variety of approximate schemes for modelling the low-density intergalactic medium (IGM) in the high-redshift Universe are compared with the results of a large high-resolution hydrodynamical simulation. These schemes use either an analytical description of the dark matter distribution and the IGM or numerical simulations of the dark matter (DM) distributions combined with different approximate relations between dark matter field and the gas distribution. Schemes based on a filtering of the dark matter distribution with a global Jeans scale result in a rather poor description of the gas distribution. An adaptive filtering which takes into account the density/temperature dependence of the Jeans scale is required. A reasonable description of the gas distribution can be achieved using a fit of the mean relation between the dark matter and gas densities in the hydrodynamical simulation to relate dark matter and gas distribution. In the hydrodynamical simulations deviations from this mean relation are correlated with gradients in the dark matter peculiar velocity field indicative of shocks in the gas component. A scheme which takes into account this correlation results in a further improved gas distribution. Such adaptive filtering schemes applied to dark matter simulations will be very well suited for studies of statistical properties of the Lyalpha forest which investigate the IGM and the underlying dark matter distribution and require a large dynamic range and/or an extensive parameter study
Surface dynamics of Mo(110)-H and Mo(110)-Li
No full textWe compare the surface dynamics of the adsorbate systems Mo(110)-H and Mo(110)-Li. In both cases electron energy loss spectroscopy measurements revealed strong substrate surface phonon anomalies. Whereas the phonon anomaly of the hydrogen-covered surface was unequivocally assigned to be of the Kohn type, the anomalous behavior of the surface phonons of the lithium-covered surface remained obscure. In this paper we develop an experimental criterion based on the dispersion of adsorbate phonons, which allows to decide whether the observed substrate surface phonon anomaly is of the Kohn type or not. Employing this criterion we now definitely rule out that the anomaly on Mo(110)-Li is due to the Kohn effect. (C) 2003 Elsevier Science B.V. All rights reserved
The first layered analogue of Sr<sub>2</sub>FeMoO<sub>6</sub>; the structure and electronic properties of Sr<sub>4</sub>FeMoO<sub>8</sub>
No full textThe n = 1 Ruddlesden–Popper phase Sr4FeMoO8 has been synthesised by high temperature ceramic methods under reducing conditions. Rietveld refinements of the structure against X-ray and neutron powder diffraction data shows that this phase adopts the space group I4/mmm (a = 3.92962(5), c = 12.6707(2) Å) and contains a crystallographically disordered arrangement of Fe and Mo on the single octahedral site in the structure. Mössbauer spectroscopy data show that iron is in the trivalent state and that the transition metals are fully disordered at a local scale. SQUID magnetometry measurements and low temperature neutron diffraction experiments have been used to examine the magnetic properties of this material. The magnetic susceptibility shows Curie–Weiss paramagnetism above a magnetic transition at 16(1) K. Below this temperature the sample shows magnetic hysteresis, but the neutron diffraction data show no evidence of the additional Bragg intensity associated with the formation of a magnetically ordered phase. The magnetic transition at 16 K is therefore assigned to the formation of a spin-glass phase. The absence of magnetic order in this material is due to the chemical disorder in the Fe/Mo occupancy of the octahedral site and thus the randomisation of the magnetic exchange interactions
Synthesis and structure of the tetrameric [Cp V(mu-F)(2)](4) (Cp = C5Me5); Preparation of the imido molybdenum fluoride [(2,6-i-Pr2C6H3N)(2)MoF2]center dot THF and the structural investigation of [(2,6-i-Pr2C6H3N)(6)Mo-4(mu(3)-F)(2)Me-2(mu-O)(4)]
No full textTreating [Cp V(mu-Cl)(2)](3) (Cp = C5Me5) and [(2,6-i-Pr2C6H3N)(2)MoMe2], respectively, with Me3SnF afforded the title compounds [Cp V(mu-F)(2)](4) (1) and [(2,6-i-Pr2C6H3N)(2)MoF2] THF (2). 1 has a tetrameric structure, in which four V atoms can be regarded as being arranged at the vertices of a distorted tetrahedron, with four long edges bridged by one F atom and each of the other two short edges bridged by two F atoms with a mean V-F bond length of 2.00 Angstrom. A hydrolyzed product of 2, [(2,6-i-Pr2C6H3N)(6)Mo-4(mu(3)-F)(2)Me-2(mu-O)(4)] (3) was characterized by elemental analyses and X-ray single crystal study. The X-ray diffraction analysis reveals that 3 has a unique tetranuclear structure, containing two five and two six coordinated Mo atoms connecting each other by four mu-O and two mu(3)-F atoms. The geometries around the two Mo atoms can be described having distorted trigonal bipyramidal and distorted octahedral coordination spheres. respectively. The Mo-(mu-O) bond lengths are 1.813 Angstrom (average) for Eve coordinated Mo atoms and 2.030 Angstrom (average) for those of six coordinated, respectively, indicating an additional pi bonding between five coordinated Mo atoms and the mu-O atoms. The MO-(mu(3)-F) distances range from 2.291 to 2.352 Angstrom
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