Surface analysis of plasma‐treated polydimethylsiloxane by x‐ray photoelectron spectroscopy and surface voltage decay

Surface states of polydimethylsiloxane (PDMS) treated by plasma were investigated by x‐ray photoelectron spectroscopy and surface voltage decay. X‐ray photoelectron spectroscopy confirmed the formation of a silica‐like (SiO_x, x = 3–4) oxidative surface layer. This layer increased in thickness with increasing exposure duration of plasma. Plasma exposure lowers the surface resistivity from 1.78 × 10¹⁴ to 1.09 × 10¹³ Ω □^?1 with increasing plasma treatment time. By measuring the decay time constant of surface voltage, the calculated surface resistivity was compared with the value measured directly by a voltage–current method; good agreement between the two methods was obtained. It was observed that plasma treatment led to a decrease in the thermal activation energy of the surface conduction from 31.0 kJ mol^?1 for an untreated specimen to 21.8 kJ mol^?1 for a plasma‐treated specimen for 1 h. Our results allow the examination of effects of plasma on the electrical properties of PDMS. Copyright © 2003 John Wiley & Sons, Ltd.     

Co-adsorption NO and organic molecules on Cu sites in zeolites: Quantum chemical DFT calculations

The co-adsorption of organic molecules: acetone, formaldehyde, ethene and acetylene together with NO on the same Cu⁺ site in zeolite CuZSM-5 was investigated by DFT calculations. The aim of this study was to follow the effect of NO on activation of multiple bonds in organic molecules and the effect of organic molecules on the activation of NO bond. The extent of activation of CO, CC, CC as well as of NO bonds was characterized by the result of calculation as the elongation of the multiple bonds, decrease of bond order as the decrease of stretching frequency, while population analysis gave information on the mechanism of activation. It has been found that the presence of NO co-adsorbed on the same Cu⁺ site as organic molecule resulted in more effective activation of CO bond in acetone and formaldehyde, but resulted in a less effective activation of CC and CC bond in ethane and acetylene. On the other hand, the presence of organic molecule resulted in more effective activation of NO bond (more important bond weakening) in NO molecule. The most significant NO bond weakening took place if NO was co-adsorbed with acetone or formaldehyde. Both acetone and formaldehyde transmit the most negative charge to the Cu⁺-zeolite system if adsorbed “solo” in Cu-zeolite. This negative charge may be next transmitted to antibonding NO orbitals resulting in so important NO bond weakening.     

Structure and catalytic mechanism of methane monooxygenase and approaches to its modelling

The mechanisms of activation of O₂ and CH₄ by methane monooxygenase (MMO) are discussed. A new concept for the catalytic cycle of MMO is suggested, and approaches to its chemical modelling are considered.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1011–1020, June, 1995.The author is grateful to Academician A. E. Shilov for interest in this work and helpful discussions. The work was financially supported by the Russian Foundation for Basic Research (Project No. 94-03-08529), International Scientific Foundation (Grant REU000), and INTAS (Project No. 93-315).     

Activation thermodynamic parameters of binary mixtures of propionic acid ando-substituted anilines

Variations of densities and viscosities with temperature and composition are reported for binary liquid mixtures containing propionic acid+aniline (I),+o-toluidine (II),+o-anisidine (III), and+o-chloroaniline (IV). Entropies S _m and enthalpies H _m of activation as functions of the composition of the mixtures, as well as free energies of activation G _m at 10, 20, 30, 40, and 50°C and different compositions were calculated by means of Eyring's equation. The formation of activated complexes between the components of these binary mixtures is postulated and claimed to result from acid-base and hydrogen bonding exchange interactions.     

Complexation and C-H bond activation of 1,5-cyclooctadiene on triosmium carbonyl clusters

Reaction of Os₃(CO)₁₀(NCMe)₂ and 1,5-cyclooctadiene (C₈H₁₂) affords the diene complex Os₃(CO)₁₀(η⁴-C₈H₁₂) (1) with the two alkene moieties coordinated to an equatorial and an axial positions of one osmium atom. Thermolysis of 1 in refluxing n-hexane results in a vinylic C-H bond activation to form (μ-H)Os₃(CO)₉(μ,η⁴-C₈H₁₁) (2) in good isolated yield. The crystal structures of 1 and 2 have been established by an X-ray diffraction study.     

Rhodium(I) catalyzed four-component reaction of Fischer alkenyl carbene complexes and 1,1-diphenylallene

Alkyl substituted chromium Fischer carbene complexes react with 1,1-diphenylallene in the presence of rhodium(I) catalysts (10 mol%) to yield highly substituted dienyl indenone derivatives. In this process a catalytic chromium(0)-rhodium(I) exchange occurs, four new C-C bonds are created, and four-components (two allenes, the carbene ligand and one CO ligand) are joined in a chemo- and regioselective manner.     

Assembly maps,K-theory, and hyperbolic groups

Following Connes and Moscovici, we show that the Baum-Connes assembly map forK _*(C*v) is rationally injective when is word-hyperbolic, implying the Equivariant Novikov conjecture for such groups. Using this result in topologicalK-theory and Borel-Karoubi regulators, we also show that the corresponding generalized assembly map in algebraicK-theory is rationally injective.     

A review of Mg metal-promoted CF bond activation; a reliable synthetic approach to difluorinated organic compounds

The carbon-fluorine bond of a trifluoromethyl group attached to a π system can be activated by metallic magnesium in the system TMSCl-THF or DMF providing a variety of difluorinated compounds via a selective CF bond cleavage. Thus, trifluoromethyl ketones, α,β-unsaturated ketones, esters, imines, and aromatics can be transformed into the corresponding difluoroenol silyl ethers, enamines, and difluoro(trimethylsilyl)methyl aromatics in good to excellent yields, respectively. This protocol enabled us to synthesize β,β-difluoroamino acids, difluoroheterocycles, and functionalized difluoroacetates.     

NMR Studies of Mixed-Ligand Lanthanide Complexes in Solution: Pseudorotation and Ring Inversion of 18-Crown-6 Molecule in Cerium Subgroup Chelates

¹H and ¹³C NMR measurements are reported for the CDCl₃ and CD₂Cl₂ solutions of [La(NO₃)₃(18-crown-6)] (I), [Pr(NO₃)₃(18-crown-6)] (II) and [Ce(NO₃)₃(18-crown-6)] (III) complexes. Temperature dependencies of the ¹H NMR spectra of II have been analyzed using the dynamic NMR methods for multi-site exchange. Two types of conformational dynamic processes in II were identified (the first one with activation enthalpy ΔH ^‡=26 ± 4 kJ/mol is conditioned by interconversion of complex enantiomeric form and pseudorotation of macrocycle molecule upon the C ₂ symmetry axis, the second one with ΔH ^‡=46 ± 5 kJ/mol is conditioned by macrocycle molecule inversion). Studies of the values of the lanthanide-induced shifts revealed that the structure of complexes in solution is similar to that reported for the complex I in the crystal state.This revised version was published online in July 2005 with a corrected issue number.