222 research outputs found
Vinylene-Linked Covalent Organic Frameworks by Base-Catalyzed Aldol Condensation
Two 2D covalent organic frameworks (COFs) linked by vinylene (âCH=CHâ) groups (VâCOFâ1 and VâCOFâ2) are synthesized by exploiting the electron deficient nature of the aromatic sâtriazine unit of C3âsymmetric 2,4,6âtrimethylâsâtriazine (TMT). The acidic terminal methyl hydrogens of TMT can easily be abstracted by a base, resulting in a stabilized carbanion, which further undergoes aldol condensation with multitopic aryl aldehydes to be reticulated into extended crystalline frameworks (VâCOFs). Both VâCOFâ1 (with terepthalaldehyde (TA)) and VâCOFâ2 (with 1,3,5âtris(pâformylphenyl)benzene (TFPB)) are polycrystalline and exhibit permanent porosity and BET surface areas of 1341â
m2âgâ1 and 627â
m2âgâ1, respectively. Owing to the close proximity (3.52â
Ă
) of the preâorganized vinylene linkages within adjacent 2D layers stacked in eclipsed fashion, [2+2] photoâcycloadditon in VâCOFâ1 formed covalent crosslinks between the COF layers.TU Berlin, Open-Access-Mittel - 2019DFG, 390540038, EXC 2008: UniSysCa
Lake Kinneret (Israel): New insights into Holocene regional palaeoclimate variability based on high-resolution multi-proxy analysis
The southern Levant is a Mediterranean climate zone of complex variability in which uncertainty remains in regional palaeoclimate reconstruction. In spite of the proven value of diatoms in circum-Mediterranean palaeoenvironmental research, their potential remains largely unexplored in the southern Levant region. In this study, we generate a new, high-resolution multi-proxy record for the last ca. 9000 cal. yr BP, supported by diatom data and key biological, mineralogical and geochemical indicators preserved in a 17.8-m-long sediment sequence recovered from Lake Kinneret (the Sea of Galilee), Israel. During the Holocene, well-correlated shifts in the diatom, minero-geochemical and palynological data indicate marked lake-level variation over time as well as changes in the trophic state of Lake Kinneret. Our results are particularly important in improving the reconstruction of Holocene lake-level variation, and thus past moisture availability. Diatom-inferred lake-level oscillations correlate well with the output from climatic models from the Levantine region and clarify previous uncertainty concerning regional variation in moisture availability. The Early Holocene (from ca. 9000 to 7400 cal. yr BP) was characterized by lake-level shifts due to fluctuating dry-wet climate conditions. During the mid-Holocene (from 7400 to 2200 cal. yr BP), a stable, deep lake-level phase persisted due to high humidity. The lake level of modern Lake Kinneret not only fluctuates seasonally with available moisture, but has also been influenced for ca. 2000 years by the impacts of water abstraction for human consumption and agriculture. Over the last 9000 cal. yr BP, the trophic state of Lake Kinneret has changed from an oligotrophic to a meso- to eutrophic environment, mainly triggered by increased human impact from around 2200 cal. yr BP onwards. The lakeâs ecosystem status was not strongly affected by the documented major changes in human occupation patterns during the mid-Holocene, when a relatively stable environment persisted
A polymer analogous reaction for the formation of imidazolium and NHC based porous polymer networks
Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugÀnglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.A polymer analogous reaction was carried out to generate a porous polymeric network with N-heterocyclic carbenes (NHC) in the polymer backbone. Using a stepwise approach, first a polyimine network is formed by polymerization of the tetrafunctional amine tetrakis(4-aminophenyl)methane. This polyimine network is converted in the second step into polyimidazolium chloride and finally to a polyNHC network. Furthermore a porous Cu(II)-coordinated polyNHC network can be generated. Supercritical drying generates polymer networks with high permanent surface areas and porosities which can be applied for different catalytic reactions. The catalytic properties were demonstrated for example in the activation of CO2 or in the deoxygenation of sulfoxides to the corresponding sulfides.BMBF, 01RC0901F, Dream Reactions - Stoffliche CO2-VerwertungDFG, EXC 314, Unifying Concepts in Catalysi
Identification of Active Magnetic Bearing Systems Utilizing a Modulating Function Technique
An application of modulating function (MF) technique to the direct closed-loop identification for the estimation of a continuous-time model of multi-input-multi-output magnetic bearing system from sampled data are presented in this paper. The plant description given as a differential matrix polynomial is transformed into a matrix of difference equations using the modulating function approach, which allows to avoid numerical problems by aproximating time derivatives. Additionally, the initial conditions can be neglected as a characteristic of the MF technique. The system parameter estimation represents a solution of a least-squares problem derived from the difference system model with experimental data from an industrial magentic bearing system
Acridine functionalized covalent organic frameworks (COFs) as photocatalysts for metallaphotocatalytic CâN cross-coupling
Covalent organic frameworks (COFs) are structurally tuneable, porous and crystalline polymers constructed through the covalent attachment of small organic building blocks as elementary units. Using the myriad of such building blocks, a broad spectrum of functionalities has been applied for COF syntheses for broad applications, including heterogeneous catalysis. Herein, we report the synthesis of a new family of porous and crystalline COFs using a novel acridine linker and benzene-1,3,5-tricarbaldehyde derivatives bearing a variable number of hydroxy groups. With the broad absorption in the visible light region the COFs were applied as photocatalysts in metallaphotocatalytic CâN cross coupling. The fully ÎČ-ketoenamine linked COF showed the highest activity, due to the increased charge separation upon irradiation. The COF showed good to excellent yields for several aryl bromides, good recyclability and even catalysed the organic transformation in presence of green light as energy source
Programmable Photocatalytic Activity of Multicomponent Covalent Organic Frameworks Used as Metallaphotocatalysts
The multicomponent approach allows to incorporate several functionalities into a single covalent organic framework (COF) and consequently allows the construction of bifunctional materials for cooperative catalysis. The well-defined structure of such multicomponent COFs is furthermore ideally suited for structure-activity relationship studies. We report a series of multicomponent COFs that contain acridine- and 2,2â-bipyridine linkers connected through 1,3,5-benzenetrialdehyde derivatives. The acridine motif is responsible for broad light absorption, while the bipyridine unit enables complexation of nickel catalysts. These features enable the usage of the framework materials as catalysts for light-mediated carbonâheteroatom cross-couplings. Variation of the node units shows that the catalytic activity correlates to the keto-enamine tautomer isomerism. This allows switching between high charge-carrier mobility and persistent, localized charge-separated species depending on the nodes, a tool to tailor the materials for specific reactions. Moreover, nickel-loaded COFs are recyclable and catalyze cross-couplings even using red light irradiation
Astrometry with the Hubble Space Telescope: Trigonometric Parallaxes of Selected Hyads
We present absolute parallaxes and proper motions for seven members of the
Hyades open cluster, pre-selected to lie in the core of the cluster. Our data
come from archival astrometric data from FGS 3, and newer data for 3 Hyads from
FGS 1R, both white-light interferometers on the Hubble Space Telescope (HST).
We obtain member parallaxes from six individual Fine Guidance Sensor (FGS)
fields and use the field containing van Altena 622 and van Altena 627 (= HIP
21138) as an example. Proper motions, spectral classifications and VJHK
photometry of the stars comprising the astrometric refer- ence frames provide
spectrophotometric estimates of reference star absolute parallaxes. Introducing
these into our model as observations with error, we determine absolute
parallaxes for each Hyad. The parallax of vA 627 is significantly improved by
including a perturbation orbit for this previously known spectroscopic binary,
now an astrometric binary. Compared to our original (1997) determina- tions, a
combination of new data, updated calibration, and improved analysis lowered the
individual parallax errors by an average factor of 4.5. Comparing parallaxes of
the four stars contained in the Hipparcos catalog, we obtain an average factor
of 11 times improvement with the HST . With these new results, we also have
better agreement with Hipparcos for the four stars in common. These new
parallaxes provide an average distance for these seven members, = 47.5
pc, for the core a \pm 1 - {\sigma} dispersion depth of 3.6 pc, and a minimum
depth from individual components of 16.0 \pm 0.9 pc. Absolute magnitudes for
each member are compared to established main sequences, with excellent
agreement. We obtain a weighted average distance modulus for the core of the
Hyades of m-M=3.376 \pm 0.01, a value close to the previous Hipparcos values,
m-M=3.33\pm 0.02.Comment: 21 pages, 7 figures, Astronomical Journal, accepted 2011-3-
A Catalog of Chandra X-ray Sources in the Carina Nebula
We present a catalog of ~14,000 X-ray sources observed by the ACIS instrument
on the Chandra X-ray Observatory within a 1.42 square degree survey of the
Great Nebula in Carina, known as the Chandra Carina Complex Project (CCCP).
This study appears in a Special Issue of the ApJS devoted to the CCCP. Here, we
describe the data reduction and analysis procedures performed on the X-ray
observations, including calibration and cleaning of the X-ray event data, point
source detection, and source extraction. The catalog appears to be complete
across most of the field to an absorption-corrected total-band luminosity of
~10^{30.7} erg/s for a typical low-mass pre-main sequence star. Counterparts to
the X-ray sources are identified in a variety of visual, near-infrared, and
mid-infrared surveys. The X-ray and infrared source properties presented here
form the basis of many CCCP studies of the young stellar populations in Carina.Comment: Accepted for the ApJS Special Issue on the Chandra Carina Complex
Project (CCCP), scheduled for publication in May 2011. All 16 CCCP Special
Issue papers are available at
http://cochise.astro.psu.edu/Carina_public/special_issue.html through 2011 at
least. 29 pages, 11 figure
Submillimeter and Far-InfraRed Experiment (SAFIRE): A PI class instrument for SOFIA
SAFIRE is a versatile imaging Fabry-Perot spectrograph covering 145 to 655
microns, with spectral resolving powers ranging over 5-10,000. Selected as a
"PI" instrument for the airborne Stratospheric Observatory for Infrared
Astronomy (SOFIA), SAFIRE will apply two-dimensional pop-up bolometer arrays to
provide background-limited imaging spectrometry. Superconducting transition
edge bolometers and SQUID multiplexers are being developed for these detectors.
SAFIRE is expected to be a "First Light" instrument, useable during the initial
SOFIA operations. Although a PI instrument rather than a "Facility Class"
science instrument, it will be highly integrated with the standard SOFIA
planning, observation, and data analysis tools.Comment: 11 page
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