Limits on an energy dependence of the speed of light from a flare of the active galaxy PKS 2155-304

In the past few decades, several models have predicted an energy dependence of the speed of light in the context of quantum gravity. For cosmological sources such as active galaxies, this minuscule effect can add up to measurable photon-energy dependent time lags. In this Letter a search for such time lags during the High Energy Stereoscopic System observations of the exceptional very high energy flare of the active galaxy PKS 2155-304 on 28 July 2006 is presented. Since no significant time lag is found, lower limits on the energy scale of speed of light modifications are derived.     

Optimized FIB silicon samples suitable for lattice parameters measurements by convergent beam electron diffraction

The aim of this paper is to check the effect of artefacts introduced by focused ion beam (FIB) milling on the strain measurement by convergent beam electron diffraction (CBED). We show that on optimized silicon FIB samples, the strain measurement can be performed with a sensitivity of about 2.5 × 10⁻⁴ which is very close to the theoretical one and we conclude that FIB preparation can be suitable for such measurements in microelectronic devices.

To achieve this, we first used CBED and electron energy loss spectroscopy (EELS) which provide a procedure permitting an exact knowledge of the sample geometry, i.e. the thickness of both amorphous and crystalline layers. This procedure was used in order to measure the FIB-amorphized sidewall layer. It was found that if the FIB preparation is optimized one can reduce this amorphous layer down to around 7 nm on each side. Secondly different preparation techniques (cleavage, Tripod™ and FIB) permit to check if the surface damaged layer introduced by FIB influences the strain state of the sample. Finally, it was found that the damaged layer does not introduce measurable strain in pure silicon but reduces appreciably the quality of the CBED patterns.     

Atom equivalents for converting DFT energies calculated on molecular mechanics structures to formation enthalpies

New atom equivalents are introduced to convert BP/DN**//MMFF energies into formation enthalpies. As a result of using molecular mechanics structures, poor results are obtained for compounds outside the scope of the force field, such as those bearing  NF₂ groups or some nitrogenous systems. Notwithstanding these limitations, present procedures compare well with the results of previous atom equivalents schemes. Indeed, rms deviations from experiment are below 9 kJ/mol for hydrocarbons, and close to 16 kJ/mol for a variety of compounds reasonably well described by MMFF. The explicit inclusion of thermal and vibrational contributions, using calculated frequencies, does not improve the results. This study demonstrates that cost‐effective approaches to formation enthalpies may be developed on the basis of a combination of DFT with a suitable molecular mechanics force field. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 367–379, 2000     

Functionalization and Rearrangement of Spirocyclohexadienyl Oxindoles: Experimental and Theoretical Investigations

The preparation and functionalization of spirocyclohexa‐2,5‐diene oxindoles is described. The spirocyclic core of the title compounds was installed by using a SmI₂‐mediated cyclization of aryl iodobenzamides. Epoxidation with CF₃CO₃H was then carried out and was shown to occur with a high level of diastereocontrol: the reagent approaches the diene moiety syn to the amide group, which is likely to be as a consequence of hydrogen bonding between the amide C?O bond and the peracid hydrogen. Carbanionic functionalization of the spirocyclohexa‐2,5‐diene oxindoles was then examined, leading to an unprecedented rearrangement of the strained spiro system into dearomatized phenanthridinones. Upon treatment with lithium diisopropylamide (LDA) at ?40 °C, the dienes rearranged to provide a phenanthridinone lithium enolate intermediate that was trapped by electrophiles including alkyl halides and aldehydes. Interestingly, alkylation and hydroxyalkylation occurred with different regiocontrol. DFT calculations were performed that rationalize the observed skeleton rearrangement, emphasizing the role of LDA/diisopropylamine in this rearrangement. The proposed mechanism thus relies on a thermodynamically driven diisopropylamine‐mediated proton transfer with the cleavage of the diene–amide C?O bond as the key step.     

On a variational approach to the Soret coefficient

In this paper, a variational approach to the Soret coefficient initiated by Kempers [J. Chem. Phys. 90, 6541 (1989)] is critically revisited. We show that the physical coherence of the whole procedure leads to a very peculiar choice of the type of constraint one can select to mimic the nonequilibrium stationary state. However, we demonstrate that its precise definition would require a statistical evaluation of the heat of transfer, or a variational approach based on more microscopic ingredients.     

Optimization of the anisotropic united atoms intermolecular potential for n-alkanes: improvement of transport properties

The parameters of the anisotropic united atom (AUA) intermolecular potential for n-alkanes originally proposed by Toxvaerd [J. Chem. Phys. 93, 4290 (1990)] [AUA(3)] was optimized by Ungerer et al. [J. Chem. Phys. 112, 5499 (2000)] [AUA(4)] on the basis of equilibrium properties (vapor pressures, vaporization enthalpies, and liquid densities). In this work we analyze the influence of the torsion potential in the internal and collective dynamics of the AUA model. The modified potential [AUA(4m)] preserves all the intermolecular parameters and only explores an increment in the trans-gauche and gauche(+)-gauche(-) transition barrier of the torsion potential. This modification better reproduce different transport properties (shear viscosity, self-diffusion coefficient, and internal relaxation times), keeping the accuracy achieved in our previous work for equilibrium properties. An extensive investigation of the shear viscosity of ethane, n-pentane, n-dodecane, and n-eicosane in a wide range of pressures and temperatures shows that the AUA(4m) improves the accuracy of the original AUA(4), reducing the absolute average deviation from 30% to 14.5%. Finally, the self-diffusion coefficient of n-hexane computed with the new model in the range of 223-333 K and from 0.1 to 295 MPa is in better agreement with respect to the experimental data than the original model.     

The role of pendant amines in the breaking and forming of molecular hydrogen catalyzed by nickel complexes

We present the results of a comprehensive theoretical investigation of the role of pendant amine ligands in the oxidation of H(2) and formation of H(2) by [Ni(P(R)(2)N(R')(2))(2)](2+) electrocatalysts (P(R)(2)N(R')(2) is the 1,5-R'-3,7-R derivative of 1,5-diaza-3,7-diphosphacyclooctane, in which R and R' are aryl or alkyl groups). We focus our analysis on the thermal steps of the catalytic cycle, as they are known to be rate-determining for both H(2) oxidation and production. We find that the presence of pendant amine functional groups greatly facilitates the heterolytic H(2) bond cleavage, resulting in a protonated amine and a Ni hydride. Only one single positioned pendant amine is required to serve this function. The pendant amine can also effectively shuttle protons to the active site, making the redistribution of protons and the H(2) evolution a very facile process. An important requirement for the overall catalytic process is the positioning of at least one amine in close proximity to the metal center. Indeed, only protonation of the pendant amines on the metal center side (endo position) leads to catalytically active intermediates, whereas protonation on the opposite side of the metal center (exo position) leads to a variety of isomers, which are detrimental to catalysis.     

Synthesis and functional pharmacological effects on human bronchi of 20-hydroxyeicosatetraenoic acid

We have synthesized 20-hydroxyeicosatetraenoic acid (20-HETE) following a new route and delineated its functional effects in human bronchi. Isometric tension measurements were performed, and they demonstrated that synthetic 20-HETE induced a concentration-dependent relaxant effect in ASM on resting tone and on bronchi pre-contracted with methacholine.     

A study of the gold/aqueous electrolyte interphase by modulated reflection spectroscopy

The admittance and differential reflectivity of an electrode of polycrystalline gold in contact with aqueous 0.02 M NaF, KClO₄, 0.01 M Na₂SO₄ and KCl have been measured in the range of potential where the electrode is ideally polarized (?0.7 V to 0.8 V NHE). In NaF and KClO₄ evidence was obtained for the compression of the adsorbed layer of water. The increase of the number of water molecules, with respect to the number in the least original state (at?4μC cm^?2), is 3% at +8 μC cm^?2 and at?17 μC cm^?2. At the potential of zero charge the water dipoles are oriented preferentially with the oxygen towards the metal. In KCl the optical measurements confirm the existence of two types of Au?Cl^? interaction and the covalent character of the bond at the most positive charges.