PM3 semi-empirical study of stereoelectronic effects in the Baeyer-Villiger reaction

The Baeyer-Villiger reaction of polymethoxybenzaldehydes with peroxysuccinic acid (PSA) is studied theoretically. The semi-empirical SCF-MO/PM3 method has been used to study the reaction and to calculate the energy profiles. In addition, heat of formation computations and bond order analyses of the transition species along the reaction coordinates have been performed and are interpreted qualitatively in terms of electron flow through the system during aryl migration. Our results support the hypothesis that the second step is a concerted reaction. Our data support a definite effect of the substituent on the aryl migration. The theoretical predictions are in good agreement with experimental results.     

Surface plasmon near-field imaging of very thin microstructured polymer layers

We report on the near-field imaging of microstructured polymer layers deposited on an homogeneous metal thin film on which a surface plasmon mode is excited. The microstructures in the polymer layers are designed by electron beam lithography, and the near-field imaging is performed with a photon scanning tunneling microscope (PSTM). We show that, despite their very small height, the microstructures can be conveniently imaged with a PSTM thanks to the field enhancement at the surface of the metal thin film supporting the surface plasmon. The influence of the illumination conditions on the contrast of the PSTM images is discussed. In particular, we show that both the field enhancement and the near-field intensity distribution around the microstructures depend dramatically upon the illumination conditions, leading to the conclusion that the PSTM is well suited for spatially resolved near-field surface plasmon sensing purposes.     

The first fluorogenic assay for detecting a Baeyer-Villigerase activity in microbial cells

The first fluorogenic assay allowing for detection of microbial enzymes able to perform Baeyer-Villiger oxidation is described. This is based on the use of 4-oxopentyl umbelliferyl ether 1 as a fluorogenic substrate. When Baeyer-Villigerases active against this test ketone are present in the selected whole cells, 1 is transformed into 3-hydroxypropyl umbelliferyl ether 3, which, in a subsequent step, releases the fluorescent product umbelliferone. Different microorganisms, known to be endowed with Baeyer-Villigerase activity, were assayed.     

Dithiadiazolium salts as initiators for the cationic ring-opening polymerization of tetrahydrofuran

Mono-, bis- and tris-(1,3,2,4-dithiadiazolium) salts [R-(CNSNS ⁺)_n]_n+[AsF^-₆]_n (R = aryl, n = 1, 2, 3) were found to initiate the cationic ring-opening polymerization of tetrahydrofuran (THF) at room temperature to give clear gels from which the pure polymer was precipitated. 1,3,2,4-Dithiadiazolium cations associated with the hard [AsF₆]^- anion thus constitute a new class of cationic polymerization initiators. The poly(THF) formed by initiation with 1,3,2,4-dithiadiazolium cation was characterized by gel permeation chromatography, infrared spectrophotometry, and ¹³C-NMR spectroscopy. Number-average molecular weights of 198 700 g mol^-1 (polydispersity 1.96) and 190 000 g mol^-1 (polydispersity 1.61) were obtained using [PhCNSNS ] [AsF₆] and [C₆H₃-1,3,5-(CNSNS )₃][AsF₆]₃, respectively, as initiators. The use of multifunctional dithiadiazolium salts as initiators suggests that they may be useful in the preparation of starburst and dendritic polymers. © 1992 John Wiley & Sons, Inc.     

Radiation: Waves or particles? A quantized approach to time

A fundamental difficulty in theoretical physics is the dual and apparently incompatible interpretations of radiation as showing both continuous and extensive wave properties but also those of discrete atomic or smaller individual particles. Some of these contradictions are outlined. The explanation offered is of a quantized nature of time; units t_o=h/m_oc² for a particle at rest, and of similar interval unit s_o when in relative motion, with conventional relativistic corrections.

For many purposes this form of quantization replaces the need for a wave concept which then appears as a mathematical approach, chosen to avoid the physical concept of an intrinsicc time for any particle, just as we have for its intrinsic mass, spin and electrical charge. t_o and s_o are directly related to its frequency, energy and mass. The uncertainty principle and interference relations follow directly from this model, without any physical wave concept.     

High performance carbon-supported catalysts for fuel cells via phosphonation

Carbon-supported catalysts were phosphonated using 2-aminoethylphosphonic acid, and the resulting catalysts with largely enhanced proton conductivity performed substantially better than the untreated counterparts in proton-exchange membrane fuel cells.     

Investigation of the luminescent properties of terbium-anthranilate complexes and application to the determination of anthranilic acid derivatives in aqueous solutions

The luminescent properties of terbium complexes with furosemide (FR), flufenamic (FF) acid, tolfenamic (TF) acid and mefenamic (MF) acid have been investigated in aqueous solutions. For all four compounds, complexation occurs when the carboxylic acid of the aminobenzoic group is dissociated and is greatly favoured in the presence of trioctylphosphine oxide as co-ligand and Triton X-100 as surfactant. Under optimum conditions, luminescence of the lanthanide ion is efficiently sensitised and the lifetime of the resonance level of terbium in the complex is ranging between 1 and 1.9 ms, against 0.4 ms for the aqua ion. The sensitivity of the method for the determination of anthranilic acid derivatives is improved by one to two orders of magnitude with respect to that achieved using native fluorescence or terbium-sensitised luminescence in methanol. The limits of detection are 2×10⁻¹⁰, 5×10⁻¹⁰ and 2×10⁻⁹ mol l⁻¹ for flufenamic acid, furosemide and tolfenamic acid, and mefenamic acid, respectively, with within-run RSD values of less than 1%. The method has been applied to the determination of flufenamic acid in spiked calf sera with and without sample pretreatment. Depending on the method and the analyte concentration, the recovery was ranging between 83 and 113% and the lowest concentration attainable in serum samples was close to 1×10⁻⁷ mol l⁻¹.     

Tricarbonyliron complexes of some blocked cyclohexadienes

Complexation commencing with some substituted 1,3- or 1,4-cyclohexadienes with Fe(CO)₅ indicates that the steric direction is controlled by classical hindrance with groups such as alkyl, but probably involves transmission of the entering group through intermediate complexation with CO₂Me. The results help to define methods of obtaining desired stereoisomers in the series.     

The selenium-mercury interaction: synthesis,spectroscopic and x-ray structural studies of methylmercury-selenourea complexes

The synthesis of selenium bound selenourea complexes of methylmercury, [CH₃HgSeC(NH₂)₂]X (X=Cl, Br, NO₃, ClO₄) are described. A single crystal X-Ray analysis of the nitrate salt has provided for the first time bond length data pertinent to the biologically important mercury-selenium interaction. Crystals of [CH₃HgSeC(NH₂)₂] NO₃ belong to the space group PnZic with a = 7.524(1), b = 11.204(2), c = 9.738(2)Å, and z = 4. The structure was solved and refined using 561 observed reflections measured on a Syntex P2₁ diffractometer to a final R value of 0.037. The mercury atom is linearly coordinated to the methyl group and the selenium atom of the selenourea with Hg-Se of 2.477(3)Å. Strong Hg-Se bonding is indicated by the X-Ray data and by the ¹H - ¹⁹⁹Hg nmr coupling constants. A comparison of spectroscopic data for analogous thio and selenourea complexes is presented.     

Brightness Reversion of Mechanical Pulps. Part XVII: Diffuse Reflectance Study on Brightness Stabilization by Additives Under Various Atmospheres

Brightness reversion of softwood (SW) and hardwood (HW) bleached chemithermomechanical pulps (BCTMP) with a 300–400nm light source under argon, oxygen, and air were studied in the absence and presence of UV-screens using UV–Vis reflectance difference spectroscopy. The UV-Vis difference absorption spectra of control SW and HW BCTMP testsheets indicate that as the concentration of atmospheric oxygen is increased, the absorption at max 360nm increases and a blue shift occurs. In all of the cases studied, brightness reversion was observed to occur under an argon atmosphere. The addition of benzophenone and benzotriazole additives to BCTMP testsheets resulted in a significant decrease in formation of chromophores absorbed at 400nm and 360nm. The possible mechanisms contributing to these effects are discussed in terms of the photoformation of lignin chromophoric structures.