2,851 research outputs found
Characterization and Dynamics of Substituted Ruthenacyclobutanes Relevant to the Olefin Cross-Metathesis Reaction
The reaction of the phosphonium alkylidene [(H_(2)IMes)RuCl2āCHP(Cy)_3)]^(+) BF_(4)^ā with propene, 1-butene, and 1-hexene at ā45 Ā°C affords various substituted, metathesis-active ruthenacycles. These metallacycles were found to equilibrate over extended reaction times in response to decreases in ethylene concentrations, which favored increased populations of Ī±-monosubstituted and Ī±,Ī±ā²-disubstituted (both cis and trans) ruthenacycles. On an NMR time scale, rapid chemical exchange was found to preferentially occur between the Ī²-hydrogens of the cis and trans stereoisomers prior to olefin exchange. Exchange on an NMR time scale was also observed between the Ī±- and Ī²-methylene groups of the monosubstituted ruthenacycle (H_(2)IMes)Cl_(2)Ru(CHRCH_(2)CH_(2)) (R = CH_3, CH_(2)CH_3, (CH_2)_)_(3)CH_3). EXSY NMR experiments at ā87 Ā°C were used to determine the activation energies for both of these exchange processes. In addition, new methods have been developed for the direct preparation of metathesis-active ruthenacyclobutanes via the protonolysis of dichloro(1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene)(benzylidene) bis(pyridine)ruthenium(II) and its 3-bromopyridine analogue. Using either trifluoroacetic acid or silica-bound toluenesulfonic acid as the proton source, the ethylene-derived ruthenacyclobutane (H_(2)IMes)Cl_(2)Ru(CH_(2)CH_(2)CH_(2)) was observed in up to 98% yield via NMR at ā40 Ā°C. On the basis of these studies, mechanisms accounting for the positional and stereochemical exchange within ruthenacyclobutanes are proposed, as well as the implications of these dynamics toward olefin metathesis catalyst and reaction design are described
Constraints on the age and dilution of Pacific Exploratory Mission-Tropics biomass burning plumes from the natural radionuclide tracer 210Pb
During the NASA Global Troposphere Experiment Pacific Exploratory Mission-Tropics (PEM-Tropics) airborne sampling campaign we found unexpectedly high concentrations of aerosol-associated 210Pb throughout the free troposphere over the South Pacific. Because of the remoteness of the study region, we expected specific activities to be generally less than 35 Ī¼Bq mā3 but found an average in the free troposphere of 107 Ī¼Bq mā3. This average was elevated by a large number of very active (up to 405 Ī¼Bq mā3) samples that were associated with biomass burning plumes encountered on nearly every PEM-Tropics flight in the southern hemisphere. We use a simple aging and dilution model, which assumes that 222Rn and primary combustion products are pumped into the free troposphere in wet convective systems over fire regions (most likely in Africa), to explain the elevated 210Pb activities. This model reproduces the observed 210Pb activities very well, and predicts the ratios of four hydrocarbon species (emitted by combustion) to CO to better than 20% in most cases. Plume ages calculated by the model depend strongly on the assumed 222Rn activities in the initial plume, but using values plausible for continental boundary layer air yields ages that are consistent with travel times from Africa to the South Pacific calculated with a back trajectory model. The model also shows that despite being easily recognized through the large enhancements of biomass burning tracers, these plumes must have entrained large fractions of the surrounding ambient air during transport
Molecular abundances and low-mass star formation. I: Si- and S-bearing species toward IRAS 16293-2422
Results from millimeter and submillimeter spectral line surveys of the protobinary source IRAS 16293-2422 are presented. Here we outline the abundances of silicon- and sulfur-containing species. A combination of rotation diagram and full statistical equilibrium/radiative transfer calculations is used to constrain the physical conditions toward IRAS 16293 and to construct its beam-averaged chemical composition over a 10-20" (1600-3200 AU) scale. The chemical complexity as judged by species such as SiO, OCS, and H_2S, is mtermedtate between that of dark molecular clouds such as Ll34N and hot molecular cloud cores such as Orion KL. From the richness of the spectra compared to other young stellar objects of similar luminosity, it is clear that molecular abundances do not scale simply with mass; rather, the chemistry is a strong function of evolutionary state, i.e., age
Foreground Subtraction in Intensity Mapping with the SKA
21cm intensity mapping experiments aim to observe the diffuse neutral
hydrogen (HI) distribution on large scales which traces the Cosmic structure.
The Square Kilometre Array (SKA) will have the capacity to measure the 21cm
signal over a large fraction of the sky. However, the redshifted 21cm signal in
the respective frequencies is faint compared to the Galactic foregrounds
produced by synchrotron and free-free electron emission. In this article, we
review selected foreground subtraction methods suggested to effectively
separate the 21cm signal from the foregrounds with intensity mapping
simulations or data. We simulate an intensity mapping experiment feasible with
SKA phase 1 including extragalactic and Galactic foregrounds. We give an
example of the residuals of the foreground subtraction with a independent
component analysis and show that the angular power spectrum is recovered within
the statistical errors on most scales. Additionally, the scale of the Baryon
Acoustic Oscillations is shown to be unaffected by foreground subtraction.Comment: This article is part of the 'SKA Cosmology Chapter, Advancing
Astrophysics with the SKA (AASKA14), Conference, Giardini Naxos (Italy), June
9th-13th 2014
Microoptomechanical pumps assembled and driven by holographic optical vortex arrays
Beams of light with helical wavefronts can be focused into ring-like optical
traps known as optical vortices. The orbital angular momentum carried by
photons in helical modes can be transferred to trapped mesoscopic objects and
thereby coupled to a surrounding fluid. We demonstrate that arrays of optical
vortices created with the holographic optical tweezer technique can assemble
colloidal spheres into dynamically reconfigurable microoptomechanical pumps
assembled by optical gradient forces and actuated by photon orbital angular
momentum.Comment: 4 pages, 3 figures, submitted to Optics Expres
Electrodynamics of Media
Contains research objectives and summary of research on three projects and reports on five research projects.Joint Services Electronics Program (Contract DAAB07-74-C-0630)California Institute of Technology (Contract 953524
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Carbonyl sulfide and carbon disulfide: Large-scale distributions over the western Pacific and emissions from Asia during TRACE-P
An extensive set of carbonyl sulfide (OCS) and carbon disulfide (CS2) observations were made as part of the NASA Transport and Chemical Evolution over the Pacific (TRACE-P) project, which took place in the early spring 2001. TRACE-P sampling focused on the western Pacific region but in total included the geographic region 110Ā°E to 290Ā°E longitude, 5Ā°N to 50Ā°N latitude, and 0ā12 km altitude. Substantial OCS and CS2 enhancements were observed for a great many air masses of Chinese and Japanese origin during TRACE-P. Over the western Pacific, mean mixing ratios of long-lived OCS and shorter-lived CS2 showed a gradual decrease by about 10% and a factor of 5ā10, respectively, from the surface to 8ā10 km altitude, presumably because land-based sources dominated their distribution during February through April 2001. The highest mean OCS and CS2levels (580 and 20 pptv, respectively, based on 2.5Ā° Ć 2.5Ā° latitude bins) were observed below 2 km near the coast of Asia, at latitudes between 25Ā°N and 35Ā°N, where urban Asian outflow was strongest. Ratios of OCS versus CO for continental SE Asia were much lower compared to Chinese and Japanese signatures and were strongly associated with biomass burning/biofuel emissions. We present a new inventory of anthropogenic Asian emissions (including biomass burning) for OCS and CS2 and compare it to emission estimates based on regional relationships of OCS and CS2 to CO and CO2. The OCS and CS2 results for the two methods compare well for continental SE Asia and Japan plus Korea and also for Chinese CS2 emissions. However, it appears that the inventory underestimates Chinese emissions of OCS by about 30ā100%. This difference may be related to the fact that we did not include natural sources such as wetland emissions in our inventory, although the contributions from such sources are believed to be at a seasonal low during the study period. Uncertainties in OCS emissions from Chinese coal burning, which are poorly characterized, likely contribute to the discrepancy
Proteolytic inactivation of human Ī±1 antitrypsin by human stromelysin
AbstractĪ±1Antitrypsin (Ī±1AT) is the main physiological inhibitor of neutrophil elastase, a serine protease which has been implicated in tissue degradation at inflammatory sites. We report here that the connective tissue metalloproteinase, stromelysin, cleaved Ī±1AT (54 kDa), producing fragments of approximately 50 kDa and 4 kDa, as shown by gel electrophoresis. The cleavage of Ī±1AT was accompanied by inactivation of its elastase inhibitory capacity. Isolation of the 4 kDa fragment by reversed-phase HPLC, followed by N-terminal amino acid sequencing, demonstrated that the cleavage of Ī±1AT occurred at the Pro357-Met358 (P2īøP1) peptide bond, one peptide bond to the N-terminal side of the inhibitory site. We suggest that stromelysin may potentiate the activity of neutrophil elastase by proteolytically inactivating Ī±1AT
Impact of Redshift Information on Cosmological Applications with Next-Generation Radio Surveys
In this paper, we explore how the forthcoming generation of large-scale radio
continuum surveys, with the inclusion of some degree of redshift information,
can constrain cosmological parameters. By cross-matching these radio surveys
with shallow optical to near-infrared surveys, we can essentially separate the
source distribution into a low- and a high-redshift sample, thus providing a
constraint on the evolution of cosmological parameters such as those related to
dark energy. We examine two radio surveys, the Evolutionary Map of the Universe
(EMU) and the Westerbork Observations of the Deep APERTIF Northern sky (WODAN).
A crucial advantage is their combined potential to provide a deep, full-sky
survey. The surveys used for the cross-identifications are SkyMapper and SDSS,
for the southern and northern skies, respectively. We concentrate on the galaxy
clustering angular power spectrum as our benchmark observable, and find that
the possibility of including such low redshift information yields major
improvements in the determination of cosmological parameters. With this
approach, and provided a good knowledge of the galaxy bias evolution, we are
able to put strict constraints on the dark energy parameters, i.e.
w_0=-0.9+/-0.041 and w_a=-0.24+/-0.13, with type Ia supernovae and CMB priors
(with a one-parameter bias in this case); this corresponds to a Figure of Merit
(FoM) > 600, which is twice better than what is obtained by using only the
cross-identified sources and greater than four time better than the case
without any redshift information at all.Comment: 12 pages, 6 figures, 6 tables; accepted for publication in MNRA
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