1,474 research outputs found
Letter. On the activation of [CrCl3{R-SN(H)S-R}] catalysts for selective trimerization of ethene: a freeze-quench Cr K-edge XAFS study
Homogeneous chromium catalysts for the selective conversion of ethene to hex-1-ene are formed from Cr(III) reagents, amino-thioether ligands of the type HN(CH2CH2SR)2, and aluminum reagents. In this study the early activation steps are investigated by EPR, UV-visible and Cr K-edge XAFS spectroscopy; rapid stopped-flow mixing and a freeze-quench allows good quality EXAFS analysis of a species formed in ~ 1 second of reaction. This is shown to involve reduction to Cr(II) and deprotonation of a NH group of the auxiliary ligand. This 4-coordinate metal-center may act as precursor for the coordination of ethene and subsequent selective oligomerization
Epidemiology of construction site injuries in Delhi, India: protocol for a retrospective cohort study
Background: Unintentional occupational injuries are a formidable public health challenge. Construction injuries make a significant contribution to occupational injuries. A public health approach to deal with this challenge warrants determination of the magnitude, scope, and characteristics of construction injuries. Objectives: To describe the epidemiology of construction injuries; to investigate trends and risk factors for injuries to construction workers and to thereby generate information vital to mitigating the burden of injuries to construction workers in Delhi, India. Methods: It is a retrospective cohort studybased on the First Information Reports of Delhi Police registered between 1st January 2016 to 31stDecember 2018. Results: injuryrates per 100,000 construction workers per year will be estimated and injury risk factors and trends will be presented. The results of the study will be published in an international, peer-reviewed journal. Findings of the study will also be presented at national and international injury and trauma conferences
Predictive coupled-cluster isomer orderings for some SiC () clusters; A pragmatic comparison between DFT and complete basis limit coupled-cluster benchmarks
The accurate determination of the preferred
isomer is important to guide experimental efforts directed towards synthesizing
SiC nano-wires and related polymer structures which are anticipated to be
highly efficient exciton materials for opto-electronic devices. In order to
definitively identify preferred isomeric structures for silicon carbon
nano-clusters, highly accurate geometries, energies and harmonic zero point
energies have been computed using coupled-cluster theory with systematic
extrapolation to the complete basis limit for set of silicon carbon clusters
ranging in size from SiC to . It is found that
post-MBPT(2) correlation energy plays a significant role in obtaining converged
relative isomer energies, suggesting that predictions using low rung density
functional methods will not have adequate accuracy. Utilizing the best
composite coupled-cluster energy that is still computationally feasible,
entailing a 3-4 SCF and CCSD extrapolation with triple- (T) correlation,
the {\it closo} isomer is identified to be the
preferred isomer in support of previous calculations [J. Chem. Phys. 2015, 142,
034303]. Additionally we have investigated more pragmatic approaches to
obtaining accurate silicon carbide isomer energies, including the use of frozen
natural orbital coupled-cluster theory and several rungs of standard and
double-hybrid density functional theory. Frozen natural orbitals as a way to
compute post MBPT(2) correlation energy is found to be an excellent balance
between efficiency and accuracy
The Solar Neighborhood XIII: Parallax Results from the CTIOPI 0.9-m Program -- Stars with mu >= 1"/year (MOTION Sample)
We present the first set of definitive trigonometric parallaxes and proper
motions from the Cerro Tololo Inter-American Observatory Parallax Investigation
(CTIOPI). Full astrometric reductions for the program are discussed, including
methods of reference stars selection, differential color refraction
corrections, and conversion of relative to absolute parallax. Using data
acquired at the 0.9-m at CTIO, full astrometric solutions and
photometry are presented for 36 red and white dwarf stellar systems with proper
motions faster than 1\farcs0/yr. Of these, thirty three systems have the first
ever trigonometric parallaxes, which comprise 41% of MOTION systems (those with
proper motions greater than 1\farcs0/yr) south of 0 that have no
parallaxes. Four of the systems are new members of the RECONS 10 pc sample for
which the first accurate trigonometric parallaxes are published here: DENIS
J1048-3956 (4.04 0.03 pc), GJ 1128 (LHS 271, 6.53 0.10 pc), GJ 1068
(LHS 22, 6.97 0.09 pc), and GJ 1123 (LHS 263, 9.02 0.16 pc). In
addition, two red subdwarf-white dwarf pairs, LHS 193AB and LHS 300AB, are
identified. The white dwarf secondaries fall in a previously uncharted region
of the HR diagram.Comment: 40 pages, 7 figures, accepted to The Astronomical Journal (scheduled
for April 2005 issue), Re-submit, Table 2 running off the bottom of the page
has been fixe
Recommended from our members
Meeting report: a hard look at the state of enamel research.
The Encouraging Novel Amelogenesis Models and Ex vivo cell Lines (ENAMEL) Development workshop was held on 23 June 2017 at the Bethesda headquarters of the National Institute of Dental and Craniofacial Research (NIDCR). Discussion topics included model organisms, stem cells/cell lines, and tissues/3D cell culture/organoids. Scientists from a number of disciplines, representing institutions from across the United States, gathered to discuss advances in our understanding of enamel, as well as future directions for the field
Relationships between protein and mineral during enamel development in normal and genetically altered mice
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/90093/1/EOS_871_sm_SupportingInformation.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/90093/2/j.1600-0722.2011.00871.x.pd
A light-front coupled-cluster method for the nonperturbative solution of quantum field theories
We propose a new method for the nonperturbative solution of quantum field
theories and illustrate its use in the context of a light-front analog to the
Greenberg--Schweber model. The method is based on light-front quantization and
uses the exponential-operator technique of the many-body coupled-cluster
method. The formulation produces an effective Hamiltonian eigenvalue problem in
the valence Fock sector of the system of interest, combined with nonlinear
integral equations to be solved for the functions that define the effective
Hamiltonian. The method avoids the Fock-space truncations usually used in
nonperturbative light-front Hamiltonian methods and, therefore, does not suffer
from the spectator dependence, Fock-sector dependence, and uncanceled
divergences caused by such truncations.Comment: 11 pages, 4 figures, RevTeX 4.1; expanded description of method and
replaced QED with simpler model for illustratio
Activation of [CrCl3{PPh2N(iPr)PPh2}] for the selective oligomerisation of ethene: a Cr K-edge XAFS study
The activation of the ethene tetramerisation catalyst system based upon [CrCl3(THF)3] and N(iPr)(PPh2)2 has been investigated in situ via the reaction of [CrCl3{PPh2N(R)PPh2}(THF)] 1a (R=iPr) with excess AlMe3 in toluene. The Cr K-edge XAFS spectrum of the solution freeze quenched after 1 min reaction time indicated monomethylation of the metal with the resultant product being [CrClMe(ClAlCl3){PPh2N(R)PPh2}(THF)] 4a (R=iPr). After 5 minutes reaction time the XAFS spectra indicate that ~50% of 4a had been converted to a Cr(II) species, with the central core being high spin [CrCl2{PPh2N(R)PPh2}] 7a (R=iPr); a similar species, [CrClMe{PPh2N(R)PPh2}] 9a (R=iPr) was observed as its adduct with AlMe3 (10a) (R=iPr) when spectra were recorded on samples maintained a room temperature. Detailed analysis (EXAFS and XANES) indicated that 7a and 9a are stabilised by adduct formation of a Cr-Cl bond to the Lewis acids B(C6F5)3 and AlMe3, respectiveley. Modelling with DFT methods indicated that five-coordination was achieved, respectively by Cr-F (11a) and Cr-C (10a) interactions. In the presence of [Ph3C][Al{OC(tBuF)3}4], the Cr XAFS of the room temperature solution was inconsistent with the maintenance of a phosphine complex, but could be modelled with a site like [Cr2Me8]4- {Cr-Cr 2.01(2), Cr-C 2.14(4)}, thus demonstrating considerable variation in the effects of differing Lewis acids
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