Kinetics and mechanisms of homogeneous catalytic reactions. Part 2. Hydrogenation of cyclohexane catalysed by [RuH(CO)-(NCMe)2(PPh3)2]BF4

Summary Kinetic and mechanistic studies of the homogeneous hydrogenation of cyclohexane were carried out using the cationic complex [RuH(CO)(NCMe)₂(PPh₃)₂]BF₄ as the catalyst precursor, which was very efficient under mild reaction conditions in xylene solution. The experimental rate law was found to ber = {K ₁ K ₂ k ₃/([MeCN]+K _1–(1+K ₂)[C₆H₁₀]}[Ru][C₆H₁₀][H₂], which became {K ₂ k ₃/(1 +K ₂)}[Ru][H₂] at high cyclohexane concentrations. The activation parameters were calculated. The kinetic data are consistent with a mechanism involving the oxidative addition of hydrogen as the rate-determining step of the catalytic cycle.     

Optic phonon anomalies and f-d hybridization in SmS and SmB6

The optic phonons of semiconducting SmS and of (homogeneously) mixed-valent SmS and SmB₆ have been investigated by means of Raman scattering. The dominant electron - phonon interaction upon resonantly exciting the 4f shell is revealed as a coupling of the strongly localized 4f hole at the Sm site to full symmetric (A_{j g}) displacements of the nearest neighbour anions i.e. mainly to LO(L) phonons. Semiconducting and mixed-valent, metallic SmS show a “softening” of the LO(L) phonon frequencies with respect to the other divalent and trivalent rare earth sulfides, respectively, going in parallel with a “softening” of the bulk modulus. The f-d hybridization is considered as the common origin of these phonon frequency renormalizations.     

A DFT/TDDFT study of the structural and spectroscopic properties of Al(III) complexes with 4-nitrocatechol in acidic aqueous solution

The complexation of 4-nitrocatechol in aqueous solution at pH 5 has been studied by molecular spectroscopy combined with quantum chemical calculations. In these physico-chemical conditions, the formation of the two complexes [4ncatAl(H₂O)₄]⁺ and [(4ncat)₂Al(H₂O)₂]⁻ has been highlighted. The electronic absorption spectra of the 1:1 and 1:2 complexes of Al(III) with 4-nitrocatechol have been computed using the time-dependent density functional theory and the polarizable continuum model. It turns out that the 6-311+G(d,p) basis set provides a good agreement between experimental and theoretical absorption spectra. This good agreement has allowed the determination of the preferential conformation of the 1:2 complex in aqueous solution. A complete assignment of the UV–Vis absorption and Raman spectra of the complexes has been proposed.     

Fast high performance liquid chromatography analysis in lipidomics: Separation of radiolabelled fatty acids and phosphatidylcholine molecular species using a monolithic C18 silica column

HPLC procedures using conventional C₁₈ columns are usually used to separate simple and complex lipid mixtures but these methods of separation remain often laborious and very slow. Here, monolithic columns were successfully applied to separate lipids - radiolabelled fatty acid mixtures and individual phosphatidylcholine (PC) molecular species. For that, isocratic elution was performed using two Chromolith™ Performance RP-18e columns connected in series. Detection was achieved by online measurement of radioactivity for radiolabelled fatty acids and by UV absorbance at 205 nm for PC molecular species. The performances of such silica rods were compared to conventional reverse-phase silica columns. Monolithic stationary phase separated radiolabelled fatty acids and PC molecular species two times and four times faster, respectively. In each analysis, monolithic columns allowed better separation efficiency per unit of time, with lower inlet pressure. The main advantages of this method for lipid separation are that, under isocratic conditions, it is simpler and much faster, while remaining accurate and selective when compared to conventional methods. Therefore, monolithic columns may represent a powerful tool for the near future in the field of lipidomics.     

Gas barrier and adhesion of interpenetrating polymer networks based on poly(diurethane bismethacrylate) and different epoxy-amine networks

Interpenetrating polymer networks (IPNs) composed of poly(methacrylate) and epoxy-amine networks made in situ between two oriented polypropylene sheets were examined in terms of their oxygen barrier and adhesion to the substrate properties. A particular attention was devoted to a system presenting an obvious phase separation. Kinetics of network formation and phase behavior were investigated by infrared and UV-visible spectroscopy, respectively. Since the poly(methacrylate) network could be instantaneously generated by photoirradiation, IPNs differing in network sequence formation were prepared. The influence of the moment at which irradiation was induced, on gas barrier properties of different films was examined. It was shown that similar oxygen transmission rates are obtained whatever the moment of irradiation.     

Kinetics and mechanisms of homogeneous catalytic reactions. Part 3. Regioselective,catalysed [RuH(CO)(NCMe)2(PPh3)2]BF4 reduction of quinoline

Summary A kinetic study of the regioselective homogeneous hydrogenation of quinoline (Q) to 1,2,3,4-tetrahydroquinoline (THQ) was carried out using the cationic complex [RuH(CO)(NCMe)₂(PPh₃)₂]BF₄ (1) as the precatalyst. The experimentally determined rate law wasr = {k ₂ K ₁/(1+K ₁[H₂])}[Ru₀][H₂]², which becomesr = {k ₂ K ₁[Ru₀]–[H₂]² at low hydrogen concentrations (k ₂ K ₁ = 28.5M ^–2 s^–1 at 398 K). The corresponding activation parameters were found to be H = 42 + 6 kJ mol^–1, S = – 115 ± 2JK^–1mol^–1 and G = 92 ± 8 kJ mol^–1. Complex(1) was found to react with Q in CHCl₃ under reflux to yield [RuH(CO)(NCMe)(N-Q)(PPh₃)₂]BF₄ (2) which was also isolated from the hydrogenation runs. These experimental findings, together with the results ofab initio self-consistent-field molecular orbital calculations on the free organic molecules involved, are consistent with a mechanism involving a rapid and reversible partial hydrogenation of(2) to yield the corresponding dihydroquinoline (DHQ) species [RuH(CO)(NCMe)(DHQ)(PPh₃)₂]BF₄ (4), followed by a rate-determining second hydrogenation of DHQ to yield [RuH(CO)(NCMe)(THQ)(PPh₃)₂]BF₄ (3).     

Electropolymerization of <Emphasis Type="Italic">trans</Emphasis>-[RuCl<Subscript>2</Subscript>(vpy)<Subscript>4</Subscript>] complex—EQCM and Raman studies

The electropolymerization of trans-[RuCl₂(vpy)₄] (vpy=4-vinylpyridine) on Au or Pt electrodes was studied by cyclic voltammetry, electrochemical quartz crystal microbalance (EQCM) technique, and Raman spectroscopy. Cyclic voltammetry of the monomer at a microelectrode shows the typical Ru(III/II) and Ru(IV/III) waves, together with the vinyl reduction waves at −1.5 and −2.45 V and adsorption wave at −0.8 V. Electrodeposition on EQCM technique performed under potential cycling between −0.9 and −2.0 V revealed that the polymerization proceeded well in advance of the vinyl reduction waves. At potentials more positive than −0.9 V, soluble oligomers were deposited irreversibly on the electrode during the oxidative sweep. The film also showed reversible mass changes due to the oxidation and accompanying ingress of charge-balancing anions and solvent into the film. In contrast, potentiostatic growth of the polymer at −1.6 V was slower because the oligomeric material was lost completely from the electrode. Unreacted vinyl groups were detected by in situ Raman spectroscopy for films grown at −0.7, −0.9, and −1.6 V but were absent when the polymerization was carried out at −2.9 V vs Ag/Ag⁺.