Mechanistic study of alcohol oxidation by the Pd(OAc)(2)/O(2)/DMSO catalyst system and implications for the development of improved aerobic oxidation catalysts

The Pd(OAc)2/O2/DMSO catalyst system displays impressive versatility in the aerobic oxidation of organic substrates, ranging from alcohols to olefins. This report details mechanistic insights into these reactions. Dimethyl sulfoxide (DMSO) plays no redox role in the chemistry, and kinetic experiments identify the turnover-limiting step as DMSO-promoted oxidation of palladium(0) by molecular oxygen. The "chemical oxidase" pathway characterized for this catalyst system holds great promise for the design of new aerobic oxidation reactions.     

The mechanism of the loss of H2 and H2O from 1,4-cyclohexanediol anion and related compounds

Losses of H₂ and H₂O from the [M-H]^? alkoxide anions of 1,4-cyclohexanediol formed in OH^? negative chemical ionization were studied using synthetic deuterated analogues and labelling by hydrogen-deuterium exchange between neutrals in the ion source prior to ionization. Simple 1,2-H₂ elimination leading to an enolate ion does not occur, but stereospecific mechanisms are shown to give a ketolate intermediate daughter ion. Collisionally activated dissociation spectra of the resulting daughter ions are the same as those of the corresponding ions generated directly from ketols.     

Copolymerization parameters of methyl methacrylate and acrylic acid

The relative reactivity of acrylic acid is known to be influenced by the polymerization medium. Nonetheless, the more commonly used reactivity ratios do not show this dependence because they were calculated from low-conversion polymerizations. We have studied the copolymerization of acrylic acid and methyl methacrylate in a number of non-hydrogen-bonding and hydrogen-bonding solvents. We found that the acrylic acid fraction in the copolymer was larger when copolymerized in a non-hydrogen-bonding medium and that the methyl methacrylate fraction was larger when copolymerized in a hydrogen-bonding medium. The precise reactivity ratios were reported when toluene, benzene, isopentyl, acetate, ethyl acetate, methyl formate, and tert-butyl alcohol were used as the polymerization medium. The values were obtained by chromatographic analysis of residual monomer, followed by computation based on the nonlinear, least-squares technique of Tidwell and Mortimer.     

Microchemical characterization of paintings — A case study

Summary The fundamental importance of scientific research for the preservation and restoration of works of art is uncontestable. The methods described in this paper, such as light microscopy, microchemical tests, emission spectroscopy, chromatography, scanning electron analysis, electron microprobe analysis and infrared spectroscopy, are of special significance in this filed. Nevertheless a better understanding of the problems that arise out of new special questions of art history can be obtained only by the right interpretation of the data obtained by scientific methods.

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Mikrochemische Charakterisierung von Gemälden

Zusammenfassung Die grundlegende Bedeutung wissenschaftlicher Forschung für die Erhaltung und Restaurierung von Kunstwerken ist unbestritten. Die hier beschriebenen Methoden (Lichtmikroskopie, mikrochemische Reaktionen, Emissionsspektroskopie, Chromatographie, Rasterelektronen-Analyse und Infrarotspektroskopie) sind hierfür von zunehmender Bedeutung. Sie ermöglichen ein besseres Verständnis neuerer kunsthistorischer Probleme nur durch die richtige Interpretation der Ergebnisse wissenschaftlicher Methoden.

     

Isothermal titration calorimetric studies of surfactant interactions with negatively charged, ‘hairy’ latex nanoparticles

Isothermal titration calorimetry (ITC) measurements of the mixture of the cationic surfactant cetyl trimethyl ammonium bromide (CTAB) with negatively charged, hairy copolymer latices (poly-(2,3-epoxypropylmethacrylate-co-methacrylic acid) in different ratio) at high water excess indicate a monomer adsorption mechanism of CTAB by the polymer particles. The number of adsorbed CTAB molecules at saturation corresponds approximately to the number of negative elementary charges bound at the surface of the latices. The mixing enthalpy is the sum of demicellization and sorption enthalpies. At 25 °C for CTAB the demicellization enthalpy amounts to 10 kJ/mol, whereas the adsorption enthalpy varies from –7 kJ/mol (surface charge density of the latices =–0.37 C/m²) to +3 kJ/mol (=–0.085 C/m²). The hydrodynamic radius R_H of the latex particles upon titration of cationic detergent and salt (NaBr) decreases by about 2 nm until the onset of aggregation near the isoelectric point. Titration of nonionic or anionic detergents has much less influence on the hydrodynamic radius and produces no measurable adsorption heat. The results are consistent within a model of latex particles with extended negatively charged polymer chains interacting predominantly via Coulombic forces with detergents.     

On-line coupling of packed capillary electrochromatography with coordination ion spray-mass spectrometry for the separation of enantiomers

Pressure-supported packed capillary electrochromatography (CEC) and packed capillary high-performance liquid chromatography (pHPLC) have been coupled on-line to electrospray ionization-mass spectrometry (ESI-MS) and coordination ion spray-mass spectrometry (CIS-MS). Separation of enantiomers of barbiturates and chlorinated alkyl phenoxypropanoates were performed on a permethylated beta-cyclodextrin stationary phase by pressure-supported CEC. For on-line detection with ESI- and CIS-MS, a modified sheath-liquid interface was used. CIS-MS is a universal, novel ionization technique which improves the selectivity as well as the sensitivity. Charged complexes were formed through the addition of central complexing ions such as silver(I), cobalt(II), copper(II), and lithium(I) to the sheath flow. Advantages of CIS-MS detection compared to the ESI-MS mode are discussed. In the CIS-MS mode, increased sensitivity and high selectivity was attained through different possibilities of complexation. The superiority of pressure-supported CEC compared to pHPLC in the hyphenation with CIS-MS is demonstrated.     

Rate-equilibrium relationships in hydride transfer reactions: the role of intrinsic barriers

A literature survey on the kinetics of hydride abstractions from CH-groups by carbocations reveals a general phenomenon: Variation of the hydride acceptor affects the rates of hydride transfer to a considerably greater extent than an equal change of the thermodynamic driving force caused by variation of the hydride donor. The origin of this relationship was investigated by quantum chemical calculations on various levels of ab initio and DFT theory for the transfer of an allylic hydrogen from 1-mono- and 1,1-disubstituted propenes (XYC=CH-CH(3)) to the 3-position of 1-mono- and 1,1-disubstituted allyl cations (XYC=CH-CH(2)(+)). The discussion is based on the results of the MP2/6-31+G(d,p)//RHF/6-31+G(d,p) calculations. Electron-releasing substituents X and Y in the hydride donors increase the exothermicity of the reaction, while electron-releasing substituents in the hydride acceptors decrease exothermicity. In line with Hammond's postulate, increasing exothermicity shifts the transition states on the reaction coordinate toward reactants, as revealed by the geometry parameters and the charge distribution in the activated complexes. Independent of the location of the transition state on the reaction coordinate, a value of 0.72 is found for Hammond-Leffler's alpha = deltaDeltaG/deltaDelta(r)G degrees when the hydride acceptor is varied, while alpha = 0.28 when the hydride donor is varied. The value of alpha thus cannot be related with the position of the transition state. Investigation of the degenerate reactions XYC=CH-CH(3) + XYC=CH-CH(2)(+) indicates that the migrating hydrogen carries a partial positive charge in the transition state and that the intrinsic barriers increase with increasing electron-releasing abilities of X and Y. Substituent variation in the donor thus influences reaction enthalpy and intrinsic barriers in the opposite sense, while substituent variation in the acceptor affects both terms in the same sense, in accord with the experimental findings. Marcus theory is employed to treat these effects quantitatively.     

Cyclisierungen unter beteiligung von fluoridionen. 2. Mitteil. [1], [2]. 4.5-perfluor-1.3-dioxolane

4.5-Perfluoro-1.3-dioxolanes $\text{2}$ are available by reaction of 2(α-chloroalkoxy)perfluoro-carbonyl halides $\text{3}$ or -ketone $\text{9}$ with fluoride ions. A mechanism for the intramolecular ring- closure-reaction is proposed. Hydrogen atoms at C-2 in $\text{2}$ can be exchanged photochemically by chlorine. Starting from the 2-monochloro-derivatives $\text{16}$ the 2-monofluoro-4.5-perfluoro- 1.3-dioxolanes $\text{18}$ are formed by reaction with triethylamine- hydrofluoride.