Hydrogen Bond Cooperativity and the Three‐Dimensional Structures of Water Nonamers and Decamers

Broadband rotational spectroscopy of water clusters produced in a pulsed molecular jet expansion has been used to determine the oxygen atom geometry in three isomers of the nonamer and two isomers of the decamer. The isomers for each cluster size have the same nominal geometry but differ in the arrangement of their hydrogen bond networks. The nearest neighbor O? O distances show a characteristic pattern for each hydrogen bond network isomer that is caused by three‐body effects that produce cooperative hydrogen bonding. The observed structures are the lowest energy cluster geometries identified by quantum chemistry and the experimental and theoretical O? O distances are in good agreement. The cooperativity effects revealed by the hydrogen bond O? O distance variations are shown to be consistent with a simple model for hydrogen bonding in water that takes into account the cooperative and anticooperative bonding effects of nearby water molecules.     

Conformational Panorama and Chirality Controlled Structure–Energy Relationship in a Chiral Carboxylic Acid Dimer

Chirality recognition in dimers of tetrahydro‐2‐furoic acid (THFA) was studied in a conformer‐specific manner using rotational spectroscopy and theoretical approaches. THFA shows a strong preference for the trans‐ over the cis‐COOH configuration. Two drastically different scenarios are possible for the detectable (THFA)₂: a kinetically preferred dimer bound by feeble interactions between two trans THFAs or a thermodynamically favored dimer with a double hydrogen‐bonded ring structure between two cis subunits. To identify the conformers responsible for the extremely dense rotational spectra observed, it was essential not only to locate several hundred homo/heterochiral (THFA)₂ minima in ab initio calculations but also to evaluate the energetic connectivities among the minima. The study further reveals an interesting chirality dependent structure–energy ordering relationship. A method for enantiomeric excess (ee) determination of THFA is presented using a recently proposed chiral self‐tag approach.     

Calculations of molecules,clusters, and solids with a simplified LCAO-DFT-LDA scheme

A simplified LCAO-DFT-LDA scheme for calculations of structure and electronic structure of large molecules, clusters, and solids is presented. Forces on the atoms are calculated in a semiempirical way considering the electronic states. The small computational effort of this treatment allows one to perform molecular dynamics (MD ) simulations of molecules and clusters up to a few hundred atoms as well as corresponding simulations of condensed systems within the Born-Oppenheimer approximation. The accuracy of the method is illustrated by the results of calculations for a series of small molecules and clusters. © 1996 John Wiley & Sons, Inc.     

Interactions between a dsDNA Oligonucleotide and Imidazolium Chloride Ionic Liquids: Effect of Alkyl Chain Length,Part I

Ionic liquids (ILs) have become nearly ubiquitous solvents and their interactions with biomolecules has been a focus of study. Here, we used the fluorescence emission of DAPI, a groove binding fluorophore, coupled with molecular dynamics (MD) simulations to report on interactions between imidazolium chloride ([Im_n,1]⁺) ionic liquids and a synthetic DNA oligonucleotide composed entirely of T/A bases (7(TA)) to elucidate the effects ILs on a model DNA duplex. Spectral shifts on the order of 500–1000 cm⁻¹, spectral broadening (~1000 cm⁻¹), and excitation and emission intensity ratio changes combine to give evidence of an increased DAPI environment heterogeneity on added IL. Fluorescence lifetimes for DAPI/IL solutions yielded two time constants 0.15 ns (~80% to 60% contribution) and 2.36–2.71 ns for IL up to 250 mM. With DNA, three time constants were required that varied with added IL (0.33–0.15 ns (1–58% contribution), ~1.7–1.0 ns (~5% contribution), and 3.8–3.6 ns (94–39% contribution)). MD radial distribution functions revealed that π-π stacking interactions between the imidazolium ring were dominant at lower IL concentration and that electrostatic and hydrophobic interactions become more prominent as IL concentration increased. Alkyl chain alignment with DNA and IL-IL interactions also varied with IL. Collectively, our data showed that, at low IL concentration, IL was primarily bound to the DNA minor groove and with increased IL concentration the phosphate regions and major groove binding sites were also important contributors to the complete set of IL-DNA duplex interactions.     

Influence of Predeposition on the Properties of GaN

Growth experiments with GaN in the system Ga/HCl/NH₃/He or H₂ were carried out in a reactor with two deposition zones. The extent of the reaction in the first zone, called the predeposition zone, is controlled by the NH₃ flow rate. In the second zone GaN is deposited epitaxially onto sapphire substrates. The variation of the carrier concentration of these epilayers indicates the dominating effect of impurities especially oxygen in opposition to the widely accepted vacancy model. Due to the high incorporation ratio of donor impurities into the solid a predeposition reduces the impurity content in the vapour phase. A reduction of the free carrier concentration in the epitaxial layers could be achieved. Different behaviour in the two carrier gas systems could be established.     

Problems and results of diffusion length investigations by energy-dependent EBIC measurements

The fundamentals of the method are described. From applications of this measuring method the following results were obtained: The determination of the diffusion length on glass-plate bevels with evaporated Schottky contacts is frequently quite difficult and sometimes even impossible. Besides a reduction of the diffusion length with increasing microdefect density surprisingly also the opposite behaviour can be found. Comparative PEM and EBIC investigations yield differences between both diffusion lengths (L_EBIC < < L_PEM) depending on the sample pre-history.     

Trim-to-Coherence Fourier Transform

We introduce a discrete Fourier transform technique which extracts more spectral information from a given time series data set than conventional discrete Fourier transform (DFT). Valid information is obtained between the spectral bins of conventional DFT, scalloping error is greatly reduced, and amplitude and phase of Fourier components are more true to the process under study as with conventional DFT. We call the general idea Trim-to-Coherence Fourier Transform, and its particular embodiment ‘Phase-Rotation Fourier Transform’. Treatment of the raw data is minimally invasive; e.g. there is no zero padding.     

High Pressure — High Temperature — Synthesis and Properties of Chromium Kyanite, (Al,Cr)2SiO5

At pressures up to 30 kb and temperatures of 1000–1500°C up to 25% of the Al₃₊ in the Al₂SiO₂-polymorph kyanite could be replaced by Cr₃₊ (composition (Al_1.5Cr_0.5)SiO₅). The lattice constants a₀, b₀, c₀ increase by this substitution, whereas the triclinic angles α, β, γ remain virtually unaffected. Cr₃₊ substitutes for Al in the chains of (AlO₃₊)-octahedra parallel to c₀. The reflectance spectrum (240-1000 nm) exhibits the spin-forbidden ruby lines in addition to the spin-allowed transitions in Cr₃₊ in octahedral coordination. The half width (～3.8 kK), position, and shape of the 10 Dq band indicates a symmetry not higher than D_4h for the Cr₃₊-containing octahedra which is in accordance with structure refinement data for pure Kyanite (Burnham 1963).     

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