9,628 research outputs found
Preparation, Structure, and Reactivity of Nonstabilized Organoiron Compounds. Implications for Iron-Catalyzed Cross Coupling Reactions
A series of unprecedented organoiron complexes of the formal oxidation states â2, 0, +1, +2, and +3 is presented, which are largely devoid of stabilizing ligands and, in part, also electronically unsaturated (14-, 16-, 17- and 18-electron counts). Specifically, it is shown that nucleophiles unable to undergo ÎČ-hydride elimination, such as MeLi, PhLi, or PhMgBr, rapidly reduce Fe(3+) to Fe(2+) and then exhaustively alkylate the metal center. The resulting homoleptic organoferrate complexes [(Me4Fe)(MeLi)][Li(OEt2)]2 (3) and [Ph4Fe][Li(Et2O)2][Li(1,4-dioxane)] (5) could be characterized by X-ray crystal structure analysis. However, these exceptionally sensitive compounds turned out to be only moderately nucleophilic, transferring their organic ligands to activated electrophiles only, while being unable to alkylate (hetero)aryl halides unless they are very electron deficient. In striking contrast, Grignard reagents bearing alkyl residues amenable to ÎČ-hydride elimination reduce FeXn (n = 2, 3) to clusters of the formal composition [Fe(MgX)2]n. The behavior of these intermetallic species can be emulated by structurally well-defined lithium ferrate complexes of the type [Fe(C2H4)4][Li(tmeda)]2 (8), [Fe(cod)2][Li(dme)]2 (9), [CpFe(C2H4)2][Li(tmeda)] (7), [CpFe(cod)][Li(dme)] (11), or [Cp*Fe(C2H4)2][Li(tmeda)] (14). Such electron-rich complexes, which are distinguished by short intermetallic FeâLi bonds, were shown to react with aryl chlorides and allyl halides; the structures and reactivity patterns of the resulting organoiron compounds provide first insights into the elementary steps of low valent iron-catalyzed cross coupling reactions of aryl, alkyl, allyl, benzyl, and propargyl halides with organomagnesium reagents. However, the acquired data suggest that such CâC bond formations can occur, a priori, along different catalytic cycles shuttling between metal centers of the formal oxidation states Fe(+1)/Fe(+3), Fe(0)/Fe(+2), and Fe(â2)/Fe(0). Since these different manifolds are likely interconnected, an unambiguous decision as to which redox cycle dominates in solution remains difficult, even though iron complexes of the lowest accessible formal oxidation states promote the reactions most effectively
Synthetic 26Al emission from galactic-scale superbubble simulations
© 2019 The Author(s).Emission from the radioactive trace element 26Al has been observed throughout the Milky Way with the COMPTEL and INTEGRAL satellites. In particular the Doppler shifts measured with INTEGRAL connect 26Al with superbubbles, which may guide 26Al flows off spiral arms in the direction of Galactic rotation. In order to test this paradigm, we have performed galaxy-scale simulations of superbubbles with 26Al injection in a Milky Way-type galaxy. We produce all-sky synthetic ray emission maps of the simulated galaxies. We find that the 1809keV emission from the radioactive decay of 26Al is highly variable with time and the observer's position. This allows us to estimate an additional systematic variability of 0.2dex for a star formation rate derived from 26Al for different times and measurement locations in Milky Way-type galaxies. High-latitude morphological features indicate nearby emission with correspondingly high integrated gamma-ray intensities. We demonstrate that the 26Al scale height from our simulated galaxies depends on the assumed halo gas density. We present the first synthetic 1809keV longitude-velocity diagrams from 3D hydrodynamic simulations. The line-of-sight velocities for 26Al can be significantly different from the line-of-sight velocities associated with the cold gas. Over time, 26Al velocities consistent with the INTEGRAL observations, within uncertainties, appear at any given longitude, broadly supporting previous suggestions that 26Al injected into expanding superbubbles by massive stars may be responsible for the high velocities found in the INTEGRAL observations. We discuss the effect of systematically varying the location of the superbubbles relative to the spiral arms.Peer reviewedFinal Accepted Versio
Dynamics and Energy Loss in Superbubbles
Martin G. H. Krause, and Roland Diehl, âDynamics and Energy Loss in Superbubbleâ, The Astrophysical Journal Letters, Vol. 794(2), October 2014. © 2014. The American Astronomical Society. All rights reserved. Printed in the U.S.A.Interstellar bubbles appear to be smaller in observations than expected from calculations. Instabilities at the shell boundaries create three-dimensional (3D) effects and are probably responsible for part of this discrepancy. We investigate instabilities and dynamics in superbubbles using 3D hydrodynamics simulations with time-resolved energy input from massive stars, including supernova explosions. We find that the superbubble shells are accelerated by supernova explosions, coincident with substantial brightening in soft X-ray emission. In between the explosions, the superbubbles lose energy efficiently, approaching the momentum-conserving snowplow limit. This and enhanced radiative losses due to instabilities reduce the expansion compared to the corresponding radiative bubbles in pressure-driven snowplow models with constant energy input. We note generally good agreement with observations of superbubbles and some open issues. In particular, there are hints that the shell velocities in the X-ray-bright phases are underpredicted.Peer reviewe
Energetics of positron states trapped at vacancies in solids
We report a computational first-principles study of positron trapping at
vacancy defects in metals and semiconductors. The main emphasis is on the
energetics of the trapping process including the interplay between the positron
state and the defect's ionic structure and on the ensuing annihilation
characteristics of the trapped state. For vacancies in covalent semiconductors
the ion relaxation is a crucial part of the positron trapping process enabling
the localization of the positron state. However, positron trapping does not
strongly affect the characteristic features of the electronic structure, e.g.,
the ionization levels change only moderately. Also in the case of metal
vacancies the positron-induced ion relaxation has a noticeable effect on the
calculated positron lifetime and momentum distribution of annihilating
electron-positron pairs.Comment: Submitted to Physical Review B on 17 April 2007. Revised version
submitted on 6 July 200
Constraining New Forces in the Casimir Regime Using the Isoelectronic Technique
We report the first isoelectronic differential force measurements between a
Au-coated probe and two Au-coated films, made out of Au and Ge. These
measurements, performed at submicron separations using soft
microelectromechanical torsional oscillators, eliminate the need for a detailed
understanding of the probe-film Casimir interaction. The observed differential
signal is directly converted into limits on the parameters and
which characterize Yukawa-like deviations from Newtonian gravity. We
find \alpha \lsim 10^{12} for nm, an improvement of
10 over previous limits.Comment: 10 pages, 4 figure
Kinematic and Thermal Structure at the onset of high-mass star formation
We want to understand the kinematic and thermal properties of young massive
gas clumps prior to and at the earliest evolutionary stages of high-mass star
formation. Do we find signatures of gravitational collapse? Do we find
temperature gradients in the vicinity or absence of infrared emission sources?
Do we find coherent velocity structures toward the center of the dense and cold
gas clumps? To determine kinematics and gas temperatures, we used ammonia,
because it is known to be a good tracer and thermometer of dense gas. We
observed the NH(1,1) and (2,2) lines within seven very young high-mass
star-forming regions with the VLA and the Effelsberg 100m telescope. This
allows us to study velocity structures, linewidths, and gas temperatures at
high spatial resolution of 3-5, corresponding to 0.05 pc. We find on
average cold gas clumps with temperatures in the range between 10 K and 30 K.
The observations do not reveal a clear correlation between infrared emission
peaks and ammonia temperature peaks. We report an upper limit for the linewidth
of 1.3 km s, at the spectral resolution limit of our VLA
observation. This indicates a relatively low level of turbulence on the scale
of the observations. Velocity gradients are present in almost all regions with
typical velocity differences of 1 to 2 km s and gradients of 5 to 10 km
s pc. These velocity gradients are smooth in most cases, but
there is one exceptional source (ISOSS23053), for which we find several
velocity components with a steep velocity gradient toward the clump centers
that is larger than 30 km s pc. This steep velocity gradient is
consistent with recent models of cloud collapse. Furthermore, we report a
spatial correlation of ammonia and cold dust, but we also find decreasing
ammonia emission close to infrared emission sources.Comment: 20 pages, 10 figure
Fragmentation and dynamical collapse of the starless high-mass star-forming region IRDC18310-4
Aims: We study the fragmentation and dynamical properties of a massive
starless gas clump at the onset of high-mass star formation. Methods: Based on
Herschel continuum data we identify a massive gas clump that remains
far-infrared dark up to 100mum wavelengths. The fragmentation and dynamical
properties are investigated by means of Plateau de Bure Interferometer and
Nobeyama 45m single-dish spectral line and continuum observations. Results: The
massive gas reservoir fragments at spatial scales of ~18000AU in four cores.
Comparing the spatial extent of this high-mass region with intermediate- to
low-mass starless cores from the literature, we find that linear sizes do not
vary significantly over the whole mass regime. However, the high-mass regions
squeeze much more gas into these similar volumes and hence have orders of
magnitude larger densities. The fragmentation properties of the presented
low-to high-mass regions are consistent with gravitational instable Jeans
fragmentation. Furthermore, we find multiple velocity components associated
with the resolved cores. Recent radiative transfer hydrodynamic simulations of
the dynamic collapse of massive gas clumps also result in multiple velocity
components along the line of sight because of the clumpy structure of the
regions. This result is supported by a ratio between viral and total gas mass
for the whole region <1. Conclusions: This apparently still starless high-mass
gas clump exhibits clear signatures of early fragmentation and dynamic collapse
prior to the formation of an embedded heating source. A comparison with regions
of lower mass reveals that the linear size of star-forming regions does not
necessarily have to vary much for different masses, however, the mass
reservoirs and gas densities are orders of magnitude enhanced for high-mass
regions compared to their lower-mass siblings.Comment: 11 pages, 10 figures, accepted to Astronomy and Astrophysics,
high-resolution version with all figures included can be found at
http://www.mpia.de/homes/beuther/papers.htm
Liquorproteine bei der Neuromyelitis optica
Es werden zwei FĂ€lle von Neuromyelitis optica beschrieben, die jeweils eine hochgradige Erhöhung der Proteine im Liquor aufwiesen. WĂ€hrend ein Patient nach 6monatigem Krankheitsverlauf starb und pathologisch-anatomisch untersucht werden konnte, ĂŒberlebte der andere Patient mit erheblichen Residuen. Dieser Fall zeichnete sich dadurch aus, daĂ deutlich erhöhte IgA- und IgM-Werte sowohl im Liquor als auch im Serum vorlagen. Es wird diskutiert, ob die gefundenen LiquorverĂ€nderungen möglicherweise eine Abgrenzung entsprechender FĂ€lle gegenĂŒber der multiplen Sklerose zulassen
The Changing Financial Structure of the U.S. Farm Sector
Agricultural Finance,
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