Formation constants for the complexes of levulinate and acetate with manganese(II), cobalt(II), nickel(II), copper(II), zinc(II) and hydrogen ions

Formation constants are reported for the levulinate complexes of manganese(II), cobalt(II), nickel(II), copper(II) and zinc(II) at 25 degrees in 0.1M chloride medium. In addition, results are presented for the corresponding acetate complexes for comparison. Protonation constants for the two ligands are also reported.     

The photoreaction of vacuum-dried rhodopsin at low temperature: evidence for charge stabilization by water

The photoreaction of vacuum-dried rhodopsin was monitored by UV-visible absorption spectroscopy. The results indicate that in dry rhodopsin, isorhodopsin and lumirhodopsin a protonation equilibrium exists between the protonated and the non-protonated Schiff base. On hydration the water stabilizes the protonated forms. In metarhodopsin-I the protein itself is able to stabilize the protonated Schiff base. The direct involvement of water in the retinal binding site was demonstrated by measuring the rhodopsin-bathorhodopsin FTIR difference spectra of rhodopsin hydrated with H2O and H2(18)O. The results are discussed with respect to the problem of charge stabilization and energy storage.     

Derivatized vancomycin stationary phases for LC chiral separations

In this work, synthetic and natural chiral selectors were combined to form two different chiral stationary phases (CSPs). These were made by bonding R- or S-(1-naphthylethyl) carbamate (R-NEC or S-NEC)-derivatized vancomycin molecules to a silica gel support. The two CSPs were evaluated using a set of 60 enantiomeric pairs. The results were compared to the ones obtained with the commercial underivatized vancomycin CSP. Three Chromatographic modes were used: (i) the normal-phase mode using a nonpolar mobile phase with different ratios of hexane and ethanol; (ii) the reversed-phase mode with hydro-organic mobile phases; and (iii) the polar aprotic organic mode with nonaqueous acetonitrile plus small amounts of methanol and an acid and/or base to control retention and selectivity. It is shown that the polarity of the underivatized vancomycin phase is higher than that of the two R- and S-NEC-derivatized CSPs. In the pH range 4-7, there is no ionization change of the chiral selector for the three CSPs. 43% of the studied compounds were resolved by the NEC-derivatized phases when they could not be resolved by the vancomycin CSP. However, the enantiorecognition for 12% of the compounds on the native vancomycin CSP was lost upon NEC derivatization. 45% of the studied compounds were resolved by the NEC-derivatized and native CSPs. The NEC derivatization procedure may block some useful active sites on the vancomycin molecule. Also, the R- and S-NEC moieties are chiral themselves and can contribute additional interaction sites not available on the native vancomycin molecule.     

Base-induced heterochiral dimerization of an oxiranyl carbaldimine: stereoselective synthesis of a highly functionalized aziridine

[reaction: see text] Treatment of the oxiranyl carbaldimine with base (LDA or LDA/KOtBu) leads in an one-step procedure to the polyfunctionalized aziridine. This highly diastereoselective reaction is explained by a new type of an Aza-Darzens reaction, in which one enantiomer of the starting material is deprotonated to form an oxiranyl anion, which attacks the imine carbon atom of the other enantiomer (mutual kinetic resolution by double diastereofacial selection).     

Stratospheric chemical ionization mass spectrometry: nitric acid detection by different ion molecule reaction schemes

Detailed height profiles of stratospheric nitric acid mixing ratios have been derived with a baloon borne chemical ionization mass spectrometer by applying several ion molecule reaction schemes, each associated to a specific and selective ion source. These ions (CO₃⁻, Cl_n⁻, CF₃O⁻, and CF₃O⁻H₂O) give rise to specific product ions (mainly CO₃⁻HNO₃, NO₃⁻HCl, NO₃⁻HF, and CF₃O⁻HNO₃) upon reaction with ambient nitric acid molecules. This paper reports on the instrumental details as well as on the results obtained during two balloon flights with the instrument. Within the accuracy of the measurements, the nitric acid height profiles obtained with the three different ion sources are in good agreement with one another as well as with literature data.     

Reaction Mechanisms of Strontium Ferrites Synthesis

The reactions between strontium and iron nitrates have been studied in an open atmosphere system using three different molar ratios, 1:1 (I), 1:2 (II) and 2:1 (III) at different temperatures as pointed out from the DTA data. The reaction mechanism was discussed based on the chemical composition characterized by means of thermal analysis, X‐ray diffraction patterns, infrared spectra and magnetic susceptibility. It was found that the reaction products depend on both temperature of reaction and the ratio between reactants. The reaction products were found to be composed of a variety of iron compounds that possess different valences: SrFeO_2.86, SrFeO_2.97, SrFe₂O₄, SrFe₁₂O₁₉, Sr₂Fe₂O₅ and Sr₇Fe₁₀O₂₂ in addition to some accessory reaction products namely α‐Fe₂O₃ and FeO(OH).     

Determination of Retention Factors of s-Triazines Homologous Series in Water Using a Numerical Method Basing on Ościk's Equation

The retention factors in pure water for a homologous series of s-triazines were calculated by a numerical method basing on Ościk's equation and were correlated with log k _w values obtained by linear and parabolic extrapolation. Chromatographic data (log k _w ) were compared with the software-calculated partition coefficients in the n-octanol/water system (Alog P, IAlog P, clog P, log P _Kowin , xlog P, log P _ACD and log P _Chem.Off.) as alternative hydrophobicity indices. The effect of organic modifier (methanol and acetonitrile) and its concentration in the mobile phase used for log k _w evaluation were investigated. Very good linear correlations were found between log k _w values calculated by the numerical method and log P _ACD , log P _Chem.Off . and clog P values, independent of organic modifier type.

     

Koordinationschemie funktioneller Phosphorylide. VII. Pentacarbonylmetall-Komplexe des Cyanmethylen-triphenyl-phosphorans, [Ph3PCH(CN)] · M(CO)5 (M = Cr,Mo, W)

Coordination Chemistry of Functional Phosphorus Ylides. VII. Pentacarbonylmetal Complexes of Cyanomethylene Triphenylphosphorane, [Ph₃P?CH(CN)] · M(CO)₅ (M = Cr, Mo, W) The pentacarbonyl complexes [Ph₃P?CH(CN)] · M(CO)₅ ( 1a –c) are obtained by reaction of the irradiated metal hexacarbonyls M(CO)₆ (M = Cr, Mo, W) with cyanomethylene triphenylphosphorane under exclusion of light. The IR and NMR spectra indicate N-coordination of the ligand, that means a nitrile complex. The chromium complex 1a crystallizes triclinic (space group P1 , Z = 2) with the lattice constant a = 1126.5(2), b = 1153.6(3), c = 951.4(3) pm; α = 103.47(3), β = 102.04(3), γ = 84.00(2)°. The linear array of the ligand atoms C7, C6, N forms an angle of 168.1(5)° with the metal-nitrogen bond. Significant bond distances are Cr? N = 206.2(6), N? C6 = 115.3(7), C6? C7 = 137.2(8) and P? C7 = 170.9(5) pm.     

Zur Reaktion von Phosphor(Arsen)-chalkogeniden mit Phosphor(Arsen)-triiodid

Reactions of Phosphorus(Arsenic)chalcogenides with Phosphorus(Arsenic)triiodide The specific heats and enthalpies of melting of the tetraphosphorustrithio(seleno)-diiodides have been determined. For the preparation of β-P₄S₃I₂ a new mthod by reacting P₄S₃ with PI₃ in CS₂ solution was found. Experiments to prepare compounds of the type As₄S₃I₂ by the classical methods for the preparation of the resp. phosphorus compounds failed. The reaction of As₄S₃ with AsI₃ leads to the formation of As₄S₄. The other sulphides of the As? S system As₂S₃ and As₄S₄ react with AsI₃ to AsSI, however, in the reaction of As₄S₄ the addition of S is necessary.     

Tunneling experiments on amorphous and crystalline superconducting tantalum films

The energy gap and the transition temperature of amorphous and crystalline superconducting Ta films are measured by tunneling experiments. The following values are obtained: 2(0)=0,65 meV,T _c=2,11 K for the amorphous state after quenched condensation and 2(0)=1,24 meV,T _c=4,06 K for the crystalline state after annealing at room temperature. The reduced energy gap 2(0)/kT _c=3,58 demonstrates that amorphous Ta films are weak-coupling superconductors. The crystallization of the amorphous Ta films takes place at 250 K.