1,369 research outputs found
Syntheses of Molybdenum and Tungsten Imido Alkylidene Complexes that Contain a Bidentate Oxo/Thiolato Ligand
3,3′,5,5′-Tetra-tert-butyl-2′-sulfanyl[1,1′-biphenyl]-2-ol (H2[tBu4OS]) was prepared in 24 % yield overall from the analogous biphenol using standard techniques. Addition of H2[tBu4OS] to Mo(NAr)(CHCMe2Ph)(2,5-dimethylpyrrolide)2 led to formation of Mo(NAr)(CHCMe2Ph)[tBu4OS], which was trapped with PMe3 to give Mo(NAr)(CHCMe2Ph)[tBu4OS](PMe3) (1(PMe3)). An X-ray crystallographic study of 1(PMe3) revealed that two structurally distinct square pyramidal molecules are present in which the alkylidene ligand occupies the apical position in each. Both 1(PMe3)A and 1(PMe3)B are disordered. Mo(NAd)(CHCMe2Ph)(tBu4OS)(PMe3) (2(PMe3); Ad=1-adamantyl) and W(NAr)(CHCMe2Ph)(tBu4OS)(PMe3) (3(PMe3)) were prepared using analogous approaches. 1(PMe3) reacts with ethylene (1 atm) in benzene within 45 minutes to give an ethylene complex Mo(NAr)(tBu4OS)(C2H4) (4) that is isolable and relatively stable toward loss of ethylene below 60 °C. An X-ray study shows that the bond distances and angles for the ethylene ligand in 4 are like those found for bisalkoxide ethylene complexes of the same general type. Complex 1(PMe3) in the presence of one equivalent of B(C6F5)3 catalyzes the homocoupling of 1-decene, allyltrimethylsilane, and allylboronic acid pinacol ester at ambient temperature. 1(PMe3), 2(PMe3), and 3(PMe3) all catalyze the ROMP of rac-endo,exo-5,6-dicarbomethoxynorbornene (rac-DCMNBE) in the presence of B(C6F5)3, but the polyDCMNBE that is formed has a random structure
Synthesis of Molybdenum and Tungsten Alkylidene Complexes That Contain Sterically Demanding Arenethiolate Ligands
Imido alkylidene complexes of Mo and W and oxo alkylidene complexes of W that contain thiophenoxide ligands of the type S-2,3,5,6-Ph[subscript 4]C[subscript 6]H (STPP) and S-2,6-(mesityl)[subscript 2]C[subscript 6]H[subscript 3] (SHMT = S-hexamethylterphenyl) have been prepared in order to compare their metathesis activity with that of the analogous phenoxide complexes. All thiolate complexes were significantly slower (up to ∼10× slower) for the metathesis homocoupling of 1-octene or polymerization of 2,3-dicarbomethoxynorbornene, and none of them was Z-selective. The slower rates could be attributed to the greater σ-donating ability of a thiophenoxide versus the analogous phenoxide and consequently a higher electron density at the metal in the thiophenoxide complexes.National Institutes of Health (U.S.) (Grant GM-59426
Analysis of Separation Corridors for Visiting Vehicles from the International Space Station
The International Space Station (ISS) is a very dynamic vehicle with many operational constraints that affect its performance, operations, and vehicle lifetime. Most constraints are designed to alleviate various safety concerns that are a result of dynamic activities between the ISS and various Visiting Vehicles (VVs). One such constraint that has been in place for Russian Vehicle (RV) operations is the limitation placed on Solar Array (SA) positioning in order to prevent collisions during separation and subsequent relative motion of VVs. An unintended consequence of the SA constraint has been the impacts to the operational flexibility of the ISS resulting from the reduced power generation capability as well as from a reduction in the operational lifetime of various SA components. The purpose of this paper is to discuss the technique and the analysis that were applied in order to relax the SA constraints for RV undockings, thereby improving both the ISS operational flexibility and extending its lifetime for many years to come. This analysis focused on the effects of the dynamic motion that occur both prior to and following RV separations. The analysis involved a parametric approach in the conservative application of various initial conditions and assumptions. These included the use of the worst case minimum and maximum vehicle configurations, worst case initial attitudes and attitude rates, and the worst case docking port separation dynamics. Separations were calculated for multiple ISS docking ports, at varied deviations from the nominal undocking attitudes and included the use of two separate attitude control schemes: continuous free-drift and a post separation attitude hold. The analysis required numerical propagation of both the separation motion and the vehicle attitudes using 3-degree-of-freedom (DOF) relative motion equations coupled with rigid body rotational dynamics to generate a large set of separation trajectories
Thermal Characteristics and Systems of Residential Construction in College Station, Texas: 1981-1986
This paper examines the energy efficiency
of residential construction in College
Station from 1981-1986. Housing sizes,
location, and construction activity by
various contractors are described using
data summarized from the College Station
building permit book for this period. Site
visits were made to apartments in complexes
with over 20 percent of the multi-family
units built during 1981-1986 and to
selected single-family houses built by
contractors responsible for over 25 percent
of the single-family construction during
this period. The information obtained on
these visits was used to define the thermal.
characteristics and systems of base cases
representative of below-average and average
multi-family units, and average and above
average single-family units
Syrian Refugees and the Digital Passage to Europe: Smartphone Infrastructures and Affordances
This research examines the role of smartphones in refugees’ journeys. It traces the risks and possibilities afforded by smartphones for facilitating information, communication, and migration flows in the digital passage to Europe. For the Syrian and Iraqi refugee respondents in this France-based qualitative study, smartphones are lifelines, as important as water and food. They afford the planning, navigation, and documentation of journeys, enabling regular contact with family, friends, smugglers, and those who help them. However, refugees are simultaneously exposed to new forms of exploitation and surveillance with smartphones as migrations are financialised by smugglers and criminalized by European policies, and the digital passage is dependent on a contingent range of sociotechnical and material assemblages. Through an infrastructural lens, we capture the dialectical dynamics of opportunity and vulnerability, and the forms of resilience and solidarity, that arise as forced migration and digital connectivity coincide
Search for Radiative Decays of Cosmic Background Neutrino using Cosmic Infrared Background Energy Spectrum
We propose to search for the neutrino radiative decay by fitting a photon
energy spectrum of the cosmic infrared background to a sum of the photon energy
spectrum from the neutrino radiative decay and a continuum. By comparing the
present cosmic infrared background energy spectrum observed by AKARI and
Spitzer to the photon energy spectrum expected from neutrino radiative decay
with a maximum likelihood method, we obatined a lifetime lower limit of to years at 95% confidence level for the
third generation neutrino in the mass range between 50 \mmev
and 150 \mmev under the present constraints by the neutrino oscillation
measurements. In the left-right symmetric model, the minimum lifetime of
is predicted to be years for of 50 \mmev. We
studied the feasibility of the observation of the neutrino radiative decay with
a lifetime of years, by measuring a continuous energy
spectrum of the cosmic infrared background
Vanadium (β-(Dimethylamino)ethyl)cyclopentadienyl Complexes with Diphenylacetylene Ligands
Reduction of the V(III) (β-(dimethylamino)ethyl)cyclopentadienyl dichloride complex [η5:η1-C5H4(CH2)2NMe2]VCl2(PMe3) with 1 equiv of Na/Hg yielded the V(II) dimer {[η5:η1-C5H4(CH2)2NMe2]V(µ-Cl)}2 (2). This compound reacted with diphenylacetylene in THF to give the V(II) alkyne adduct [η5:η1-C5H4(CH2)2NMe2]VCl(η2-PhC≡CPh). Further reduction of 2 with Mg in the presence of diphenylacetylene resulted in oxidative coupling of two diphenylacetylene groups to yield the diamagnetic, formally V(V), bent metallacyclopentatriene complex [η5:η1-C5H4(CH2)2NMe2]V(C4Ph4).
Field-induced Ordering in Critical Antiferromagnets
Transfer-matrix scaling methods have been used to study critical properties
of field-induced phase transitions of two distinct two-dimensional
antiferromagnets with discrete-symmetry order parameters: triangular-lattice
Ising systems (TIAF) and the square-lattice three-state Potts model (SPAF-3).
Our main findings are summarised as follows. For TIAF, we have shown that the
critical line leaves the zero-temperature, zero -field fixed point at a finite
angle. Our best estimate of the slope at the origin is . For SPAF-3 we provided evidence that the zero-field correlation
length diverges as , with , through analysis of the critical curve at plus crossover
arguments. For SPAF-3 we have also ascertained that the conformal anomaly and
decay-of-correlations exponent behave as: (a) H=0: ; (b) .Comment: RevTex, 7 pages, 4 eps figures, to be published in Phys. Rev.
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