Characterization of the deposition of organic molecules at the surface of gold by the electrochemical reduction of aryldiazonium cations

The deposition of 4-X phenyl groups (X = NO2, COOH, N-(C2H5)2) on polycrystalline gold electrode was achieved by the electrochemical reduction of the corresponding 4-substituted phenyldiazonium tetrafluoroborate salts in anhydrous acetonitrile media. The electrochemical quartz crystal microbalance measurements evidenced a two-step deposition process: the first one is the deposition of close to a monolayer and the second one is the relatively slower growth of multilayers. In this second region, the deposition is less efficient than for the first one. The electrochemical behavior of the resulting modified gold electrode was investigated in the presence of an electroactive redox probe and these results, together with the electrochemical quartz crystal microbalance data, demonstrated significant differences in reactivity and in deposition efficiency between the diazonium salts. The characterization of the modified electrodes by cyclic voltammetry and electrochemical impedance spectroscopy, as well as X-ray photoelectron spectroscopy measurements, showed that the formation of multilayers is possible and that a significant fraction of the deposited material remained at the electrode surface, even following ultrasonic treatment. The X-ray photoelectron spectroscopy data indicate that the existence of Au-C and Au-N=N-C linkages (where C represents a carbon atom of the phenyl group) is uncertain. Nonetheless, the deposition of the aryl groups by electrochemical reduction of diazonium cations yielded a film that adheres well to the gold surface and the deposited organic film hindered gold oxides formation in acidic medium.     

Total synthesis of (+)-hyacinthacine A2 based on SmI2-induced nitrone umpolung

[reaction: see text] A concise total synthesis of (+)-hyacinthacine A(2), a polyhydroxylated pyrrolizidine alkaloid, is described using our recently discovered inversion of the C-N bond polarity in nitrones. In the key step, the diastereoselective reductive coupling of a L-xylose-derived cyclic nitrone with ethyl acrylate allowed the assembly of the bicyclic core of the target molecule, by way of a tandem formation of the C-C and C-N bonds. The method opens a novel, short, and general route for the synthesis of other pyrrolizidine alkaloids.     

Effects of pulsed low-frequency electromagnetic fields on water characterized by light scattering techniques: role of bubbles

Well-characterized purified water was exposed for 6 h to pulsed low-frequency weak electromagnetic fields. After various time periods, nondegassed and degassed water samples were analyzed by static light scattering. Just after electromagnetic exposure (day 0), a reduction of over 20% in the maximum light scattering intensity at 488 nm wavelength in both nondegassed and degassed samples was observed. By contrast, on day 12 the difference was observed only in nondegassed water samples. The latter effect was attributed to the different geometries of the containers combined with the basic origin of the whole phenomenon due to gas bubbles present in water. By the use of dynamic light scattering, the bubble mean diameter was estimated to be around 300 nm. Our results suggest that the electromagnetic exposure acts on gas nanobubbles present in water and emphasizes the role of the gas/liquid interface. The possibility that exposure to electromagnetic fields disturbs the ionic double layer that contributes to bubble stabilization in water is discussed.     

Adsorption and heterocoagulation of nonionic surfactants and latex particles on cement hydrates

The adsorption of nonionic surfactants of the alkyl-phenol-poly(ethylene oxide) family and of acrylic latex particles on several anhydrous (but hydrating) or fully hydrated mineral phases of Portland cement was studied. No or negligible adsorption of the surfactant was observed. This was assigned to the ionized character of the surface silanol groups in calcium-silicate-hydrates and to the strongly ionic character of the OH groups in calcium hydroxide and in the calcium-sulfoaluminate-hydrates, which prevents the formation of surface-ethoxy hydrogen bonds. In contrast, provided they are properly stabilized by the surfactant, the latex particles form a loose monolayer on the surface of hydrating tricalcium silicate particles. The attractive interaction between the positive mineral surface and the negative latex surface appears to be the driving force for adsorption. In line with this, adsorption is reduced by sulfate anions, which adsorb specifically onto the silicate surface. Compared to tricalcium silicate, portlandite and gypsum interact only marginally with the latex particles. Our results show that the stability of the nonionic surfactant/latex/cement systems is essentially controlled by the latex colloidal stability and the latex-cement interactions, the surfactant having little direct interaction, if any, with the mineral surfaces.     

Properties of Co-Mo coatings obtained by electrodeposition at pH 6.6

Cobalt-molybdenum coatings were prepared by electrodeposition in a sulfate-citrate bath and their morphology, structure and magnetic properties were analysed. Concentrations of 0.1 mol dm^–3 CoSO₄ and 0.005 mol dm^–3 Na₂MoO₄ at pH 6.6 led to Co-Mo deposits of 20–23% Mo that can be grown to several microns over graphite or copper substrates. At low deposition potentials or current densities, the deposits presented a close-packed hexagonal structure (hcp) that evolved to a (100)+(110) preferred orientation and acicular morphology as the deposit thickness increased. When the deposition potential or the current density was made more negative, a mixed crystalline+amorphous structure was obtained. The degree of crystallinity depended on the thickness: thin films were more amorphous than the thicker ones. Co-Mo deposits showed lower saturation magnetization (M_s) and coercivity (H_c) than the pure cobalt deposits. The crystalline+amorphous films showed the lowest H_c values (around 40 Oe).     

Diffuse reflectance FT-IR spectroscopic study of interactions of alpha-Al2O3/molten alkali nitrate coexisting systems

The diffuse reflectance spectra of alpha-Al(2)O(3)/molten MNO(3) (M=K(+), Li(+)) coexisting systems have been measured for the investigation of the physicochemical properties of molten nitrate at the interface of aluminum oxide. In the region of overtone and combination modes from 3000 to 1700 cm(-1), the absorption band nu(3)+nu(4) decreases with increasing specific surface area of alpha-Al(2)O(3) powder for the potassium mixtures. Gaussian resolution of nu(3)+nu(4) also exhibits changes in band positions of components with the variation in the alumina specific surface area and the nitrate melt content. The second-order derivative of the spectra in the region from 1700 to 950 cm(-1) shows a set of peaks at 1645, 1554-1516, and 1452 cm(-1) characteristic of a coordinated nitrate ion. The peak at 1337 cm(-1) suggests a possible formation of nitrite species. The maxima of nu(3)+nu(4), nu(1)+nu(2), and nu(1)+nu(4) shift toward lower frequencies with increasing temperature above the nitrate melting point. This information shows that the intramolecular bonds of NO(3)(-) are affected. These perturbations are attributed to the effect of the solid phase interacting with the liquid phase. The interaction between solid and liquid phases affects the geometry of the ionic species and, therefore, influences the electrical properties of nitrate ions.     

Energy and electron transfer in bifunctional non-conjugated dendrimers

Nonconjugated dendrimers, which are capable of funneling energy from the periphery to the core followed by a charge-transfer process from the core to the periphery, have been synthesized. The energy and electron donors involve a diarylaminopyrene unit and are incorporated at the periphery of these dendrimers. The energy and electron acceptor is at the core of the dendrimer, which involves a chromophore based on a benzthiadiazole moiety. The backbone of the dendrimers is benzyl ether based. A direct electron-transfer quenching of the excited state of the periphery or a sequential energy transfer-electron-transfer pathway are the two limiting mechanisms of the observed photophysical properties. We find that the latter mechanism is prevalent in these dendrimers. The energy transfer occurs on a picosecond time scale, while the charge-transfer process occurs on a nanosecond time scale. The lifetime of the charge separated species was found to be in the range of microseconds. Energy transfer efficiencies ranging from 80% to 90% were determined using both steady-state and time-resolved measurements, while charge-transfer efficiencies ranging from 70% to 80% were deduced from fluorescence quenching of the core chromophore. The dependence of the energy and charge-transfer processes on dendrimer generation is analyzed in terms of the backfolding of the flexible benzyl ether backbone, which leads to a weaker dependence of the energy and charge-transfer efficiencies on dendrimer size than would be expected for a rigid system.