Electro-oxidation of dimethyl ether on platinum nanocubes with preferential {1 0 0} surfaces

Platinum nanocubes with preferential {1 0 0} surfaces are synthesized by colloidal method and characterized by TEM image and hydrogen adsorption/desorption process. The mean diameter of Pt nanocubes is ca. 10.5 nm. The hydrogen adsorption/desorption wave shows clearly the <1 0 0> preferential orientation. It is proved that dimethyl ether oxidation on Pt nanocubes exhibits similar features with bulk Pt single crystal electrode. The catalytic activity of Pt nanocubes for DME electro-oxidation is nearly 3× higher than that on commercial available Pt black catalyst. After potential step to 1.2 V, the surfaces of Pt nanocubes are reconstructed and the activity for DME electro-oxidation decreases.     

Comportement de quelques Arylbenzo[b]-furannes sous l'Impact électronique

A fragmentation mechanism for arylbenzo[b]furans is proposed to account for the similarity of the spectra of isomers with substituents in the 2 and 3 positions. This mechanism implies the migration of the aryl group during the decomposition of the 2-substituted species.     

Obtention de phosphore élémentaire et de phosphures de nickel par électrolyse de solutions de pentoxyde de phosphore dans des bains à base de cryolithe

By electrolysis of cryolitic solutions of P₂O₅, elementary pure phosphorus is obtained on carbon cathode and Ni phosphides on Ni cathode. The most probable mechanism of phosphorus formation in these conditions seems to be the final dissociation of P₂O₅ in P⁵⁺ and O^2? and the primary electrodic discharge of these ions.     

Aziridines N-Non substituées. Détermination des Paramètres Thermodynamiques ΔG°, ΔH°, ΔS° Liés à l'Equilibre Conformationnel à partir de Mesures RMN 1H à Haut Champ

Although nitrogen inversion in unsubstituted aziridines has been shown by ¹H NMR, it was only possible to study quantitatively the distribution of both invertomers and determine the thermodynamic parameters by a systematic high field NMR study (CAMECA 250 MHz). Thermodynamic parameters have been determined for 32 compounds. In C-aryl aziridines, the steric hindrance of the aromatic group and its variation with other aziridine ring substituents can be studied. These observations are in agreement with a conjugation phenomenon between the aromatic system and the aziridine ring.     

Spectrométrie de Masse d'Hétérocycles Azotés. X–Contribution à l'Etude du Comportement de ll'Ion Aniline et des Ions Aminopyridines avant Fragmentation par Perte de HCN

The study of the mass analysed ion kinetic energy spectra of deuterated derivatives of aniline, aminopyridines and 2-chloro-5-aminopyridine shows that prior to HCN loss, hydrogen scrambling does not occur for aminopyridines and is limited but noticeable for aniline. In the case of this last compound the extent of scrambling varies markedly for small variations in the energy of the ions studied, these variations being within the energy window corresponding to metastable ions. Furthermore, an examination of the mass analysed ion kinetic energy spectra of monodeuterated derivatives of aminopyridines leads to the rejection of the generally accepted mechanism for HCN loss from the molecular ions of these compounds.     

Vibronic interactions in {6} and {18}hetero(A,B)annulenes

The vibronic (vibrational–electronic) interactions and the Jahn–Teller effects in the monoanions and trianions of {6}hetero(B,N), (C,N), and (B,O)annulenes and {18}hetero(B,N), (C,N), and (B,O)annulenes are discussed. All the heteroannulenes have threefold axis of symmetry and the twofold degenerate lowest unoccupied molecular orbital (LUMOs), and the E^′ or E vibrational modes can cause Jahn–Teller distortions in their monoanions and trianions. State vibronic coupling constants of the monoanions and trianions and orbital vibronic coupling constants concerning the LUMOs are calculated for each Jahn–Teller active vibrational mode at the B3LYP/6-31G* level of theory. Vibrational modes near 1500 cm⁻¹ of the {6}hetero(A,B)annulenes and low-frequency modes (<500 cm⁻¹) of the {18}hetero(A,B)annulenes give large coupling constants, and therefore, these modes are essential in the Jahn–Teller distortions and the vibronic interactions. The coupling constants are qualitatively analyzed by looking at the nuclear motions of the Jahn–Teller active modes and the shapes of the LUMOs on the basis of one-electron approximation.     

Étude par chromatographie liquide à haute pression des alcaloïdes de l''ergot

High-performance liquid chromatography of ergot alkaloid.

A method for the analysis of ergot alkaloids has been developed by high-performance liquid chromatography. A silica column was used for preparation of ergotinine and ergotoxine from ergot. Separation of the individual ergot alkaloids was successfully accomplished on a column of Hitachi Gel No. 3011-O (porous polystyrene modified by hydroxymethyl) with an eluent of n-hexane—ethanol—triethylamine (70:30:0.5, v/v). The method was applied to the analysis of -ergocryptinine, ergocorninine, ergocristinine, -ergocryptine, ergocornine and ergocristine in ergot and dihydroergocryptine ( + β), dihydroergocornine and dihydroergocristine in pharmaceutical tablets.     

Mécanisme de solvolyse des cis - et trans-(p-toluénesulfonates) d'aryl1-2-cyclopentyle II. Etude des étapes ultérieures à l'ionisation lors de la solvolyse des cis-(p-toluénesulfonates) d'aryl-2-cyclopentyle

Solvolysis Mechanism of cis- and trans-2-Arylylopentylp- Toluenesulfonates, Subsequent Step in cis-2-Arylcyclopentyl p-Toluenesulfonate Solvolysis The solvolysis of a series of 1-deuteriated, 2-deuteriated, and undeuteriated cis-2-arylcyclopentyl p-toluenesulfonates in HCOOH, AcOH, and EtOH has been studied. We have shown that classical secondary carbocation arising from substrate ionisation undergoes two concurrent processes, namely k_c (direct products formation) and k_ip (hydrogenium bridged ion formation as tertiary carbocation or products precursor). The intrinsic properties of solvent and the aryl substitutant electronic effects the total solvolysis rate and the respective contributions of k _c and k _ip processes. Comparing the observed and calculated kinetic isotope of D–C(2) allows one to conclude that no step can called ‘rate-determining step’ but that the steps following ionisation have a preponderent effect on the total solovolysis rate.     

Agglomération contrôlée de latex par formation de complexes tensioactifs anioniques : copolymères à blocs à base de poly(oxyde d’éthylène)

For polystyrene–poly(ethylene oxide) (PS–PEO) diblock copolymers, as micellar dispersions in aqueous medium, the formation of complexes with anionic surfactants, such as sodium dodecylsulfate (SDS) could be confirmed. The number of SDS molecules fixed per EO unit is close to the values reported for the SDS–PEO homopolymer interaction. Advantage of this type of complexation was taken to develop a controlled agglomeration process for SDS stabilized PS and PVC latexes by using as agglomerants ‘hairy’ latexes of PS and PVC that have been synthesized in the presence of PS–PEO block copolymers and that carry therefore a fringe of PEO sequences on their surface. The complexation of SDS by these surface-anchored PEO chains leads to the destabilization of the anionic latex, which has a tendency to precipitate onto the surface of the agglomerant latex. The average particle size and the size distribution of the agglomerated particles were studied as a function of the weight and number ratio of the two types of latexes involved in the agglomeration process, as well as in function of the surface coverage by SDS and PEO respectively. By adjusting these parameters, it was possible to obtain, with an efficiency of almost 100%, latex agglomerates with a monomodal distribution in the size range of 1 to 40 μm. An agglomeration mechanism could be outlined taking into account the complexation capacity and the specific surface of the agglomerating ‘hairy’ latex. To cite this article: P. Peter et al., C. R. Chimie 6 (2003).     

Nouvelle voie de synthèse des dérivés de l'ellipticine et analogues alcaloïdes à activité antitumorale

A new snythesis of 6H-pyrido[4,3-b]carbazoles is described starting from substituted cyclohexanones involving the combination of Borsche's carbazole preparation and Cranwell and Saxton's ellipticine synthesis where the starting materials are easily available. The dehydrogenation step is accomplished at an early stage. In some cases (11-desmethyl ellipticines derivatives), 7H-pyrido[3,4-c]carbazoles were also obtained.     

Construction de complexes tétranucléaires de ruthénium à ossature planaire par condensation de deux unités diruthénium à l'aide de ligands pontants: Synthèse et structure moléculaire de [Ru4(CO)8{μ2-P(Cy)2}4] et de [Ru4(CO)8{μ4-P(Cy)}2{μ2-P(Cy)2}2] (Cy = cyclohexyl)

Assembly of Tetranuclear Ruthenium Complexes with Planar Metal Core by Condensation of Two Diruthenium Units Using Bridging Ligands: Synthesis and Molecular Structure of [Ru₄(CO)₈{μ₂-P(Cy)₂}₄] and [Ru₄(CO)₈{μ₄-P(Cy)}₂{μ₂}₂](Cy = Cyclohexyl) The dinuclear complexes [Ru₂(CO)₆{μ-P(Cy)₂}₂] ( 1 ) or [Ru₂(CO)₄{μ-(HCO₂)}₂{P(Cy)₂H}₂] ( 2 ) react in THF solution at 160° to give the tetranuclear complexes [Ru₄(CO)₈{μ₂-P(Cy)₂}₄] ( 3 ) and [Ru₄(CO)₈{μ₄-P(Cy)}₂{μ₂-P(Cy)₂}₂] ( 4 ), as well as the trinuclear complex [Ru₃(CO)₇(μ₂-H){μ₂-P(Cy)₂}₃] ( 5 ). If the reaction is performed at 200°, the bicapped tetranuclear species 4 is obtained in a higher yield, whereas 3 and 5 are formed in trace amounts only. The phenyl derivatives [Ru₂(CO)₆{μ-P(Ph)₂}₂] ( 6 ) or [Ru₂(CO)₄{μ-(EtCO₂)}₂{P(Ph)₂H}₂] ( 7 ) react in a similar manner to give only the complex [Ru₄(CO)₈{μ₄-P(Ph)}₂{μ₂-P(Ph)₂}₂] ( 8 ), analogous to 4 . The molecular structure of 3 consists of a planar framework of four Ru-atoms, each Ru—Ru bond being bridged by a μ₂-dicyclohexylphosphino ligand. The complex 4 represents a planar rectangular Ru core, both faces being capped by μ₄-cyclohexylphosphinidene ligands and two opposite edges being bridged by μ₂-dicyclohexylphosphino ligands.     

Complexes macrocycliques des lanthanides: Stabilité et comportement électrochimique dans le méthanol et le carbonate de propylène

Macrocyclic Complexes of Lanthanides: Stability and Electrochemical Behaviour in Methanol and Propylene Carbonate The stabilities of the 1:1 complexes of the trivalent lanthanides with the diazapolyoxamacrocycles (2.1.) and (2.2.1.) in anhydrous methanol and propylene carbonate have been determined at 25°, by competitive potentiometric methods using H⁺ or Ag⁺ as auxiliary cations, with Et₄NClO₄ as supporting-electrolyte. Additional data are also reported for the crown ethers 15C5 and 18C6 in propylene carbonate. It is shown that the diazapolyoxamacrocycles are much stronger complexing agents for trivalent lanthanides than macrocyclic polyethers, and that the bicyclic (2.2.1.) cryptates are more stable than the monocyclic (2.1.) complexes. With increasing atomic number of the lanthanides, the stability increases with diazapolyoxamacrocycles and decreases with cyclic polyethers. The electrochemical reduction of the trivalent samarium and europium cryptates has been investigated by polarography on a dropping Hg-electrode, in water and methanol. In both solvating solvents, the +2 oxidation states of the cations are stabilized by complexation.     

Contribution à l'étude de l'ionisation des films insolubles de polyélectrolytes stéréoréguliers

The surface potentials of monolayers of poly(methacrylic acid) (PMA) have been studied as a function of the pH of the subsolution. The influence of the macromolecular configuration on the properties of the ionizable groups was more particularly investigated. Orientation of molecules in the monolayer may be imposed by the stereoregularity of PMA (isotactic, atactic) and by the nature of the interface (air–water, cyclohexane–water). The results are analyzed in terms of theories of the ionic double layer (Gouy model, Donnan equilibrium) which permit the determination of the degree of ionization α of the film. The value of α gives indications about the orientation of specific groups of the macromolecular chain, i.e., of the ionized monomer units. Indeed, at the air–water interface, the film of isotactic PMA is more acid than that of the atactic sample, and at the cyclohexane–water interface, the degree of ionization is greater than at the air–water interface. These properties are the consequence of a modification of orientation of the hydrophilic groups with respect to the aqueous phase as a function of the stereoregularity of the sample and the nature of the interface. The variation of α with the pH of the substrate phase may be used to calculate an intrinsic dissociation constant K_s° of the molecules in the film. The value of K_s° is not modified by spreading conditions and remains very similar to that obtained in solution.     

RMN du 13C de Bromo et de Fluoro Cyclanones à Structure Contrainte: Variation des Effets α et γ en Fonction des Facteurs Stériques et de la Nature de l'Halogène

¹³C n.m.r. spectra of various halo-bicyclo[3.2.1]octan-3-ones and 7,7-dimethylbicyclo[3.1.1]heptan-3-ones are described. It is possible to correlate in an empirical way the α and γ effects with the geometric features of these molecules. Thus, the reflex and anti-reflex effects can be studied by ¹³C n.m.r. spectroscopy.     

Etude en RMN de l'Association des Sels de Lanthanides avec des Phénols; Application à la Détermination de Structure de Polyphénols Naturels

Interaction of lanthanide salts with phenols in DMSO solution was investigated as a tool for ¹H NMR structure determination in the field of naturally-occurring polyphenolic compounds. Intermolecular competition experiments evidenced reactivity differences with respect to the nature and position of substituents. Empirical rules of reactivity are suggested from which variations observed in the PMR chemical shifts on addition of lanthanide salts to natural polyphenols (flavones, xanthones) were rationalized.     

Etude des mécanismes de réarrangement dans la fragmentation des benzoyloxy-cyclopentànes par impact électronique

The fragmentation modes of 3-benzoyloxy-cyclopentene and of 4-benzoyloxy-cyclopentene have been studied on deuterium-labelled analogues. A mathematical model is proposed for the study of the rearrangement reactions which occur during the expulsion of H₂O and of \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CO}_2 {\rm H}^. $\end{document} from the molecular ion of 3-benzoyloxy-cyclopentene.     

Préparation d'isoxazoles trisubstitués par réduction chimique et électrochimique d'α-acyl β-nitrostilbènes

Nitration of α-acylstilbenes with dinitrogen tetroxide leads to corresponding Z-α-acyl-β-nitrostilbenes. By chemical or electochemical reduction of these compounds, trisubstituted isoxazoles were prepared in good yields.