Kinetics and mechanisms of homogeneous catalytic reactions. Part 5. Regioselective reduction of heteroaromatic nitrogen compounds catalysed by [OsH(CO)(NCMe)2(PPh3)2]BF4

The [OsH(CO)(NCMe)₂(PPh₃)₂]BF₄ complex (1) is an efficient and regioselective precatalyst for the hydrogenation of the nitrogen-containing ring of quinoline (Q), isoquinoline (iQ), 5,6- and 7,8-benzoquinoline (BQ), and acridine (A) under mild reaction conditions (125 °C and 4 atm H₂). Kinetic studies of the hydrogenation of Q and iQ to give tetrahydroquinoline (THQ) and tetrahydroisoquinoline (THiQ), respectively, lead to the rate law r = K ₁ k ₂/(1 + K ₁[H₂])[Os][H₂]², which becomes r = K ₁ k ₂[Os][H₂]², at low hydrogen concentrations (below 1 atm H₂); the catalytically active species is of the type [OsH(CO)(L)( ¹-N)(PPh₃)₂]BF₄ [(2a): L = NCMe, N = Q; (2b): L = N = iQ]. The generic mechanisms involve a rapid and partial hydrogenation of the coordinated substrate (N) of complex (2) to yield the corresponding dihydroderivative (DHN) species [OsH(CO)(L)( ¹-DHN)(PPh₃)₂]BF₄ [(3a): L = NCMe, DHN = DHQ; (3b): L = iQ or THiQ, DHN = DHiQ], followed by the rate-determining second hydrogenation of the DHN ligand, which yield [OsH(CO)(L)( ¹-THN)(PPh₃)₂]BF₄ [(4a): L = NCMe, THN = THQ; (4b): L = iQ or THiQ, THN = THiQ]; substitution of the THN ligand by a new molecule of the respective substrate regenerates the active species and restarts the catalytic cycle. For the hydrogenation of acridine to give 9,10-dihidroacridine (acridane), the rate law was r = k ₁[Os][H₂]; the mechanism involves the hydrogenation of the active species [OsH(CO)(NCMe)( ¹-A)(PPh₃)₂]BF₄ (2c) to yield acridane and the unsaturated species [OsH(CO)(NCMe)(PPh₃)₂]BF₄ as the rate-determining step.     

Design and synthesis of a new class of arginine analogues with an improved anion binding site in the side chain

Replacing the guanidinium group in arginine (1) by a guanidiniocarbonyl pyrrole moiety provides a new class of artificial amino acids (2), that can be used as building blocks in standard solid phase peptide synthesis.     

High-flux nanofiltration membranes prepared by adsorption of multilayer polyelectrolyte membranes on polymeric supports

Layer-by-layer deposition of anionic and cationic polyelectrolytes readily converts polymeric ultrafiltration membranes into materials capable of nanofiltration. ATR-FTIR spectra confirm that layer-by-layer deposition occurs on the ultrafiltration substrates, and adsorption of as few as 2.5 bilayers of poly(styrenesulfonate) (PSS)/protonated poly(allylamine) (PAH) or 3.5 bilayers of PSS/poly(diallyldimethylammonium chloride) (PDADMAC) reduces the molecular weight cutoff of polyethersulfone ultrafiltration supports from 50 kDa to <500 Da. Deposition of multilayer polyelectrolyte films on 300 and 500 kDa membranes also decreases molecular weight cutoffs, but solute rejections are significantly lower when using these supports, suggesting that the polyelectrolyte films do not completely cover large (0.2-0.4 microm in diameter) pores. On the 50 kDa substrates, PSS/PDADMAC films containing 3.5 bilayers exhibit a 95% rejection of SO(4)(2-) and a chloride/sulfate selectivity of 27, whereas 4.5-bilayer PSS/PAH coatings show a glucose/raffinose selectivity of 100. Pure water flux for [PSS/PAH](3)PSS-coated membranes at 4.8 bar is 1.6 m(3)/(m(2)day), which is more than 2-fold higher than that through a commercial 500 Da membrane.     

Estimation des potentiels chimiques de melange et des chaleurs latentes de fusion a partir des diagrammes binaires liquides-solides

The simultaneous estimation of chemical potentials of mixing and the latent heat of fusion of the pure constituents from liquid-solid equilibrium data of binary mixing is rather difficult. Since the activity coefficients change before the considered molar fraction and the temperature. Moreover chemical reaction may occur in the binary mixing therefore the application of the Gibbs-Duhem relation becomes impossible.The activity, the latent heat of fusion in the pure state and the C _p of fusion of NaI from the experimental data of the diagram of fusion of NaI-NaIO₃ were calculated. The application of the results obtained near the eutectic point allowed the determination of the latent heat of fusion of the pure NaIO₃. When the expression of the activity becomes complicated, the experimental data of heat of mixing in addition to the data obtained from the equilibrium of fusion were applied.

---

Zusammenfassung Eine simultane SchÄtzung der chemischen Potentiale von MischvorgÄngen und der SchmelzwÄrme der reinen Komponenten aus Liquid-Solid-Gleichgewichtsdaten von binÄren MischvorgÄngen ist recht schwer, da sich eher die AktivitÄtskoeffizienten Ändern als die betrachteten Molenbrüche und die Temperatur. Au\erdem können sich bei den binÄren MischvorgÄngen chemische Reaktionen abspielen, wodurch die Anwendung der Gibbs-Duhemschen Gleichung unmöglich wird. Es wurde die AktivitÄt, die SchmelzwÄrme im reinen Zustand und aus den experimentellen Daten des Schmelzdiagrammes von NaI-NaIO₃ C _p für das Schmelzen von NaI berechnet. Die Anwendung der erhaltenen Ergebnisse in der NÄhe des Eutektikums ermöglichte die Bestimmung der SchmelzwÄrme von reinem NaIO₃.Wird der Ausdruck für die AktivitÄt kompliziert, werden zusÄtzlich zu den aus dem Schmelzgleichgewicht ermittelten Daten die experimentellen Daten für die MischungswÄrme verwendet.

     

Immersed Boundaries in Large Eddy Simulation of Compressible Flows

Methods to immerse walls in a structured mesh are examined in the context of fully compressible solutions of the Navier–Stokes equations. The ghost cell approach is tested along with compressible conservative immersed boundaries in canonical flow configurations; the reflexion of pressure waves on walls arbitrarily inclined on a cartesian mesh is studied, and mass conservation issues examined in both a channel flow inclined at various angles and flow past a cylinder. Then, results from Large Eddy Simulation of a flow past a rectangular cylinder and a transonic cavity flow are compared against experiments, using either a multi-block mesh conforming to the wall or immersed boundaries. Different strategies to account for unresolved transport by velocity fluctuations in LES are also compared. It is found that immersed boundaries allow for reproducing most of the coupling between flow instabilities and pressure-signal properties observed in the transonic cavity flow. To conclude, the complex geometry of a trapped vortex combustor, including a cavity, is simulated and results compared against experiments.     

Natural and artificial ageing of an alkyd based wood finish

Wood protection in exterior use is generally achieved with a stain. This protective product is often obtained from an alkyd resin. A natural and artificial agening have been studied by measuring the glass transition temperature (T _g) of the finish variations in terms of time. In both ageings, theT _g variations are the same; there is an increase inT _g during the first steps of ageing and then a stabilization. A behaviour equation is proposed and is perfectly suitable to both ageings. Time constants are calculated. This equation shows that the artificial ageing machine chosen is a good means of artificial ageing. It allows the reproduction and the acceleration by 10 times of phenomena which are observed during the natural ageing.     

Recognition of anionic carbohydrates by an artificial receptor in water

A tris-cationic artificial receptor 1 efficiently binds anionic carbohydrates even in aqueous solution as shown by NMR and UV titration experiments. Complex formation involves both ion pair formation and H-bonds to the sugar, explaining the preference for saccharides compared to simple anions and the observed selectivities among different sugars. [structure: see text]     

Impulsive stimulated Raman scattering: comparison between phase-sensitive and spectrally filtered techniques

Impulsive stimulated Raman scattering is used to generate and control coherent phonons and other low-frequency modes. In transparent materials, pump-probe experiments are usually performed by spectrally resolving the probe beam and measuring the spectral shift as a function of pump-probe time delay. By measuring the optical phase of the probe pulse as a function of time delay, we find that the phonon signal can be increased by a factor alpha(omegadelta)(-1), where omega is the phonon frequency and delta is the pulse duration.