934 research outputs found
Modeling Heat Dissipation at the Nanoscale: An Embedding Approach for Chemical Reaction Dynamics on Metal Surfaces
We present an embedding technique for metallic systems that makes it possible
to model energy dissipation into substrate phonons during surface chemical
reactions from first principles. The separation of chemical and elastic
contributions to the interaction potential provides a quantitative description
of both electronic and phononic band structure. Application to the dissociation
of O at Pd(100) predicts translationally "hot" oxygen adsorbates as a
consequence of the released adsorption energy (ca. 2.6 eV). This finding
questions the instant thermalization of reaction enthalpies generally assumed
in models of heterogeneous catalysis.Comment: 6 pages, 2 figure
Non-Adiabatic Vibrational Damping of Molecular Adsorbates: Insights into Electronic Friction and the Role of Electronic Coherence
We present a perturbation approach rooted in time-dependent
density-functional theory to calculate electron hole (eh)-pair excitation
spectra during the non-adiabatic vibrational damping of adsorbates on metal
surfaces. Our analysis for the benchmark systems CO on Cu(100) and Pt(111)
elucidates the surprisingly strong influence of rather short electronic
coherence times. We demonstrate how in the limit of short electronic coherence
times, as implicitly assumed in prevalent quantum nuclear theories for the
vibrational lifetimes as well as electronic friction, band structure effects
are washed out. Our results suggest that more accurate lifetime or
chemicurrent-like experimental measurements could characterize the electronic
coherence.Comment: Article as accepted for publication in Physical Review Letter
Stabilizing a Molecular Switch at Solid Surfaces: A Density-Functional Theory Study of Azobenzene at Cu(111), Ag(111), and Au(111)
We present a density-functional theory trend study addressing the binding of
the trans-cis conformational switch azobenzene (C6H5-N=N-C6H5) at three coinage
metal surfaces. From the reported detailed energetic, geometric, and electronic
structure data we conclude that the governing factor for the molecule-surface
interaction is a competition between covalent bonding of the central azo
(-N=N-) bridge on the one hand and the surface interaction of the two
closed-shell phenyl (-C6H5) rings on the other. With respect to this factor the
cis conformer exhibits a more favorable gas-phase geometric structure and is
thus more stabilized at the studied surfaces. With the overall binding still
rather weak the relative stability of the two isomers is thereby reduced at
Ag(111) and Au(111). This is significantly different at Cu(111), where the cis
bonding is strong enough to even reverse the gas-phase energetic order at the
level of the employed semi-local electronic exchange and correlation (xc)
functional. While this actual reversal may well be affected by the deficiencies
due to the approximate xc treatment, we critically discuss that the
rationalization of the general effect of the surface on the meta-stable
molecular states is quite robust. This should equally hold for the presented
analysis of recent tip-manipulation and photo-excitation isomerization
experiments from the view point of the derived bonding mechanism.Comment: 10 pages including 4 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.htm
Safety Relevant Positioning Applications in Rail Traffic using the European Satellite System "Galileo"
Die Ortung im Eisenbahnverkehr hat eine hohe sicherheitstechnische Relevanz. Eine falsch detektierte Position eines Fahrzeugs kann zu einer erheblichen GefĂ€hrdung fĂŒhren, da die ermittelte Ortsinformation fĂŒr die Freigabe und das Wiederbesetzen von Gleisabschnitten genutzt wird. Daraus abgeleitet, mĂŒssen Ortungssysteme bei der Zulassung unter anderem die folgenden sicherheitskritischen Anforderungen erfĂŒllen Genauigkeit, ZuverlĂ€ssigkeit, IntegritĂ€t und VerfĂŒgbarkeit der Ortungsinformation, die gemÀà SIL 4 nachzuweisen sind
Electronic friction-based vibrational lifetimes of molecular adsorbates: Beyond the independent atom approximation
We assess the accuracy of vibrational damping rates of diatomic adsorbates on
metal surfaces as calculated within the local-density friction approximation
(LDFA). An atoms-in-molecules (AIM) type charge partitioning scheme accounts
for intra-molecular contributions and overcomes the systematic underestimation
of the non-adiabatic losses obtained within the prevalent independent atom
approximation. The quantitative agreement obtained with theoretical and
experimental benchmark data suggests the LDFA-AIM as an efficient and reliable
approach to account for electronic dissipation in ab initio molecular dynamics
simulations of surface chemical reactions.Comment: 5 pages including 2 figure
Connecting [4Feâ4S] Clusters and Hemes â Towards Modeling the Active Site of Sulfite Reductase
In this paper, we present a new design for biologically inspired models for the active site of assimilatory sulfite and nitrite reductases (aSIR and aNIR), which consists of a siroheme that is directly linked to a [4Feâ4S] cubane cluster. The individual components used here to construct this model are a siteâdifferentiated [4Feâ4S] cluster, a bifunctional bridging ligand, and a metalloporphyrin. We have prepared two new siteâdifferentiated clusters, [Fe 4 S 4 (TriS)(SPy)] and [Fe 4 S 4 (TriS)(SEtIm)], which contain pyridine and imidazole linkers for the binding to a metalloporphyrin, and characterized these compounds, using UV/Vis , IR, and 1 HâNMR spectroscopy, cyclic voltammetry (CV), and mass spectrometry. Titration experiments where then performed by using [Zn(TPP)] (TPP 2â = meso âtetraphenylporphyrin dianion) and corresponding fluorinated derivatives to find the best [4Feâ4S]âheme combination for an optimal binding of the two components in solution. Excitingly, our results demonstrate the formation of the desired [4Feâ4S]âheme catalytic arrays in solution with high specificity . The best combination of cubane cluster and metalloporphyrin for future catalyst development corresponds to the complex (Bu 4 N) 2 [M(T o âF 2 PP)â{Fe 4 S 4 (TriS)(SEtIm)}] {T o âF 2 PP 2â = meso âtetra( ortho âdifluorophenyl)porphyrin dianion}. The binding between these components with M = Zn 2+ was further confirmed by CV. Thus, we have created a new type of biologically inspired model system for the aSIR and aNIR active site that leads to a robust attachment of the individual components in solution. The binding between fluorinated zinc porphyrins and siteâdifferentiated [4Feâ4S] clusters through pyridylâ and imidazolylthiolate bifunctional bridging ligands generates catalytic arrays inspired by the active site of sulfite reductase. The properties of the unique [4Feâ4S] clusters applied here are described, and binding constants are reported for the two components of the resulting macromolecular scaffold.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/99598/1/3883_ftp.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/99598/2/ejic_201300308_sm_miscellaneous_information.pd
Azobenzene versus 3,3',5,5'-tetra-tert-butyl-azobenzene (TBA) at Au(111): Characterizing the role of spacer groups
We present large-scale density-functional theory (DFT) calculations and
temperature programmed desorption measurements to characterize the structural,
energetic and vibrational properties of the functionalized molecular switch
3,3',5,5'-tetra-tert-butyl-azobenzene (TBA) adsorbed at Au(111). Particular
emphasis is placed on exploring the accuracy of the semi-empirical dispersion
correction approach to semi-local DFT (DFT-D) in accounting for the substantial
van der Waals component in the surface chemical bond. In line with previous
findings for benzene and pure azobenzene at coinage metal surfaces, DFT-D
significantly overbinds the molecule, but seems to yield an accurate adsorption
geometry as far as can be judged from the experimental data. Comparing the
trans adsorption geometry of TBA and azobenzene at Au(111) reveals a remarkable
insensitivity of the structural and vibrational properties of the -N=N- moiety.
This questions the established view of the role of the bulky tert-butyl-spacer
groups for the switching of TBA in terms of a mere geometric decoupling of the
photochemically active diazo-bridge from the gold substrate.Comment: 9 pages including 6 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.htm
Herstellung und Zusammensetzung von asiatischen Fischsaucen
Abstract
Fish sauce belongs to the most important condiments in Southeast Asian cuisine. It is a clear, amber to reddish liquid with an intensive smell. Fish sauce is used instead of salt for nearly each meal.
Asian fish sauce is made from anchovies and other small fish. For the traditional process whole fresh fish are mixed with salt in the ratio 1:1 to 6:1 in wooden, clay or concrete tanks at tropical temperatures for 6 to 18 months. The liquefaction of the fish tissue is due to the action of endogenous enzymes in fish and exogenous enzymes from bacteria. During the fermentation amino acids, peptides and a lot of other substances are built, which are responsible for the characteristic aroma and flavour of these sauces.
You can buy pure fish sauce, diluted fish sauce and fish sauce made from other types of animals like mussels, prawns and squids. In single Asian countries there are different national standards for the quality of fish sauces. In order to get a general idea of these products we have bought 16 fish and two oyster sauces from the retail trade in Hamburg and analyzed them with physical, chemical, sensory and microbiological methods.
Kurzfassung
Fischsauce gehört zu den wichtigsten WĂŒrzsaucen in der sĂŒdostasiatischen KĂŒche. Es ist eine klare, bernsteinfarbene bis rötlichbraune, sehr intensiv riechende FlĂŒssigkeit. Sie wird anstelle von Salz verwendet und daher fast zu jedem Essen gereicht.
Zur Herstellung von Fischsaucen werden hauptsĂ€chlich Anchovis und Ă€hnliche kleine Fische verwendet. Bei der traditionellen Herstellung werden die ganzen Fische mit Meersalz in einem Holzfass, TongefÀà oder Betontank im VerhĂ€ltnis 1:1 bis 6:1 gemischt. WĂ€hrend der anschlieĂenden 6 â 18 Monate dauernden Lagerung bei tropischen Temperaturen bauen sich die Gewebeproteine durch fischeigene Enzyme und Mikroorganismen ab. Bei diesem mehrmonatigen Fermentationsprozess
entstehen die fĂŒr den Geschmack wichtigen AminosĂ€uren, Peptide und Aromastoffe. Es gibt neben reiner Fischsauce, auch verdĂŒnnte Fischsauce und Fischsaucen aus anderen Tieren wie Muscheln, Garnelen und Tintenfische.
In den einzelnen asiatischen LÀndern gibt es unterschiedliche nationale QualitÀtsstandards. Um diese Produktgruppe nÀher kennen zu lernen, haben wir 16 Fisch- und 2 Austernsaucen aus dem Einzelhandel (Hamburg) mit physikalischen, chemischen, sensorischen und mikrobiologischen Verfahren untersucht
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