Nioboxidfluoride mit Blockstrukturen — Elektronenmikroskopische Durchstrahlungsaufnahmen und ihre Simulation

Niobiumoxidefluorides Nb₅₉O₁₄₇F, Nb₃₁O₇₇F, Nb₆₅O₁₆₁F₃ and Nb₃₄O₈₄F₂ were prepared by reaction of Nb₂O₅ and Nb₃O₇F at 1 270°C. These niobiumoxidefluorides have blockstructures which were examined by high resolution electron microscopy. The observed images of the crystal structures were compared with computer simulated images.

     

An annulene-fused cyclopentadienide. A photochromic cyclopentadienodimethyldihydropyrene where the fused cyclopentadienide group resembles benzene in its effect on the dihydropyrene-metacyclophanediene valence isomerization

The synthesis of the green cyclopentadiene-fused dimethyldihydropyrene 12 was achieved in 36% yield in 7 steps from the parent dihydropyrene 3. Reaction of 12 with KH or LiCH(2)SiMe(3) gave the [14]annulene-fused cyclopentadienide anion quantitatively. In the (1)H NMR spectra, the internal methyl protons of 12 at delta -3.9, change dramatically on formation of anion 5, becoming deshielded to delta -1.82. This is caused by the reduction in diatropicity of the [14]annulene ring on fusion to the 6pi-cyclopentadienide ring. The anion is also a photochromic switch. Irradiation of the closed form 5 with visible light opens it to the open form 5', which reverts to the closed form 5 either with UV light or thermally. The switching behavior is between that of the parent 3/3' and the benzannelated system 4/4' and suggests that in its effect on the photoswitching, cyclopentadienide is behaving chemically similar to benzene.     

Cyclopropane chemistry. Part 5 [1,2]. Hexafluorocyclopropane as a source of difluorocarbene

Thermolysis of hexafluorocyclopropane in the presence of ethylene, propene, vinyl chloride, and vinyl bromide gives good yields of the corresponding 1,1-difluorocyclopropanes, formed by addition of difluorocarbene to the olefin. The tetrafluoroethylene formed dimerises to octafluorocyclobutane, co-dimerises with the olefin, or survives, depending on the reaction conditions. With allene, hexafluorocyclopropane gives 1-(difluoromethylene)cyclopropane, 2,2,3,3-tetrafluorospiropentane, and products derived from tetrafluoroethylene and allene.     

Synthesis and photochromic properties of molecules containing [e]-annelated dihydropyrenes. Two and three way pi-switches based on the dimethyldihydropyrene-metacyclophanediene valence isomerization

The syntheses of several new simple negative, a simple positive, and multiple negative photochromes containing the dihydropyrene-cyclophanediene photochromic system are described. The photo-openings of the negative photochromes, the [e]-annelated benzo (7), naphtho (9), anthro (11), furano (19), and triphenyleno (15) derivatives of the parent 2,7-di-tert-butyl-trans-10b,10c-dimethyl-dihydropyrene (5), as well as its 4,5-dibromo derivative (13), are described to give the corresponding cyclophanedienes, as well as their photoclosures and thermal closures back to the dihydropyrenes. These are compared to the results obtained for the positive photochrome dibenzo[e,l]dihydropyrene (21) and to the bis(dihydropyreno)chrysene (44) and the (dihydropyrenobenzo)(benzo)metacyclophanediene (47) photochromes, which have more than one photochromic switch present and thus have more than a simple "on-off" state. Thermodynamic data are obtained for the thermal closing reactions. The anthrodihydropyrene (12) has the fastest thermal closing (tau(1/2) = 20 min), while the furanodihydropyrene (19') has the slowest (tau(1/2) = 63 h) at 46 degrees C. An electrochemical readout of the state of the switch is demonstrated for the benzodihydropyrene (7).     

DIFFUSION MEASUREMENTS IN MICROEMULSIONS

We have carried out diffusion coefficient measurements in both aqueous micelles and microemulsions using the techniques of palaeography and quasielastic light scattering (QLS) The former method involves the determination of the diffusion coefficient of an electroactive oil soluble probe at a polarizable microelectrode. For high water content microemulsions, both methods yield the same diffusion coefficients, which can be identified as the self diffusion coefficient For cetyltrimethylammonium bromide (CTAB) micelles, both methods yield the same result at the salt (NaBr) concentration at which the QLS measurements are independent of CTAB concentration. In more concentrated microemulsions, QLS data gave diffusion coefficients in agreement with polarography only for a sodium cetyl sulfate (SCS) system at 65-75 wt % water. For the SCS microemulsions at 60% water, and CTAB microemulsion at 60-75% water, the QLS data yielded rapid, nonexponential decays. However, consistent polarographic diffusion coefficients could still be obtained, By using probes of varying chain length (oil solubility), it has been demonstrated that these CTAB and SCS microemulsions containing butanol and pentanol cosurfactants respectively, are not cosolubilized systems but do contain distinct hydrophilic and hydropobic regions.     

A beta-lactone route to chiral gamma-substituted alpha-amino acids: application to the concise synthesis of (S)-alpha-azidobutyro lactone and a natural amino acid

[reaction: see text] Beta-lactones are useful synthetic intermediates allowing access to a number of functional arrays. In this report, enantiomerically pure 4-trichloromethyl-2-oxetanone is shown to be a versatile amino acid synthon leading to a variety of gamma-substituted alpha-amino acid precursors. The utility of this methodology was demonstrated by the concise synthesis of a protected homoserine equivalent, alpha-azidobutyro lactone, and a naturally occurring alpha-amino acid from the seeds of Blighia unijugata.     

Length scales of acceleration for locally isotropic turbulence

Length scales are determined that govern the behavior at small separations of the correlations of fluid-particle acceleration, viscous force, and pressure gradient. The length scales and an associated universal constant are quantified on the basis of published data. The length scale governing pressure spectra at high wave numbers is discussed. Fluid-particle acceleration correlation is governed by two length scales: one arises from the pressure gradient, the other from the viscous force.     

Asymptotic analysis of dielectric coaxial fibers

Using an asymptotic analysis, we analytically calculate the dispersion and the field distribution of guided modes in an all-dielectric coaxial fiber. We compare the analytical results with those obtained from numerical calculations and find excellent agreement between them. We demonstrate that both the Bragg reflection and the total internal reflection play important roles in providing confinement and determining the dispersion characteristics of the coaxial fiber modes.     

Systematic design of single-mode coupled-resonator optical waveguides in photonic crystals

By establishing a direct relation between the dispersion and the field profile of a coupled-resonator optical waveguide (CROW) and those of its constituent cavities, we present a systematic method for the design of a single-mode CROW and for control of its dispersion. The procedure includes the design of a single-mode cavity and control of its frequency by engineering its structure. Then, by chaining these cavities in the proper direction and at an appropriate distance, we achieve the desired dispersion for the CROW.     

Die Adsorption der Halogene durch trockenen,gelöschten Kalk

Ohne ZusammenfassungUebersetzt von J. Matula, Wien.