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Tentative Mechanism (tentative + mechanism)
Selected AbstractsRapid crystal growth without inherent supersaturation induced by nanoscale fluid flows?CRYSTAL RESEARCH AND TECHNOLOGY, Issue 1 2006M. J. Jones Abstract Crystal growth is a process that only takes place under non-equilibrium conditions and a necessary prerequisite is that the crystal is exposed to a phase that is supersaturated in the material the crystal is composed of, be it a solution, a vapour or a supercooled melt. In industrial mass crystallization the growth rate for a population of crystals (in suspension growth processes [1]) rarely exceeds mean linear velocities of 10 -7 ms -1. Here we present a mass crystallization process which is accompanied by rapid crystal growth several orders of magnitude faster and into a region of solution that is without inherent supersaturation. The material investigated is a solid hydrate that exhibits a solution mediated phase transition to its anhydrous form in the presence of methanol [2]. The phase transition is initiated simply by placing an amount of hydrate crystals into the solvent and is characterized by the rapid emergence of needle-shaped crystals. The needles emanate from the crystal faces of the hydrate crystals and grow into the solution, which is nominally free of the substance to be crystallized. The high growth rate of the crystals, which of the order of up to 10 -4 ms -1 is surprising. Although rapid needle growth has been observed before [3-9], to date a satisfactory explanation for needles growing under the abovementioned conditions is still outstanding. Based upon the topology of the crystals we propose a tentative mechanism for this phenomenon capable of explaining the unusually rapid growth and highlight those questions that need addressing in order to verify this mechanism. X-ray powder diffraction is used to characterize the crystal phase of the needles; confocal fluorescence microscopy reveals that the needles are hollow. The width of these needles is between 0.5 and 5 ,m, their length appears to be limited only by the amount of hydrate available for their formation. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] From Single-Molecule Precursors to Coupled Ag2S/TiO2 NanocompositesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 28 2008Márcia C. Neves Abstract A single-source approach using mild temperatures was applied to prepare morphological well-defined and coupled TiO2/metal,sulfide nanocomposites. Metal N -alkyldithiocarbamates were used as the precursors to the metal,sulfide nanophases and, in particular, Ag2S nanostructures were investigated in more detail. These were observed as nano-islands at the surface of TiO2 (anatase) particles, which were used as substrates. To explain the formation of these nanocomposite particulates, a tentative mechanism has been proposed which involves the controlled release of sulfide ions from an intermediate coordination compound. Because the growth of the metal sulfide can be controlled at the surface of a photoactive substrate, we anticipate the potential of this synthetic method to chemical design reasonable amounts of semiconductor-sensitized TiO2, such as Ag2S/TiO2 nanocomposites. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Photocatalytic reduction of carbonates and formation of some energy rich systems in the presence of Toluidine BlueINTERNATIONAL JOURNAL OF ENERGY RESEARCH, Issue 2 2001Sarita Jain Abstract Aqueous sodium and potassium carbonates have been photoreduced in the presence of Toluidine Blue solution (which is also the photocatalyst). The photocatalytic formation of formic acid and formaldehyde was measured spectrophotometrically using Nash reagent. The effect of variation of various parameters like pH, amount of photocatalyst (Toluidine Blue concentration), concentration of Na2CO3 and K2CO3, light intensity, etc., on the yield of photoproducts was also investigated. A tentative mechanism for this reduction has been proposed. Copyright © 2001 John Wiley & Sons, Ltd. [source] Ruthenium-Catalyzed Oxidative Homo-Coupling of 2-ArylpyridinesADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 13 2009Xiangyu Guo Abstract A ruthenium-catalyzed oxidative homo-coupling reaction of 2-arylpyridines via CH activation was developed. The reaction could tolerate various functional groups on both the aryl and the pyridyl rings to afford a series of dimerized products with iron(III) chloride (FeCl3) as a stoichiometric oxidant. A tentative mechanism was proposed for this oxidative CH/CH homo-coupling. [source] Morphology and photophysical properties of a thermally responsive fluorescent material based on a rod-coil tri-block copolymerJOURNAL OF APPLIED POLYMER SCIENCE, Issue 1 2008Zhun Ma Abstract A thermally responsive rod-coil poly[poly (N -isopropylacrylamide)- b -polyfluorene- b -poly(N -isopropylacrylamide)] triblock copolymer has been successfully synthesized by atom transfer radical polymerization from an end-functionalized macroinitiator. The thermochromic behavior and relevant morphology of this polymer were investigated by UV-vis spectra, DLS, and AFM, respectively, at various temperatures. A thermally responsive fluorescent material was achieved facilely by combining the optically active polyfluorene with temperature-responsive poly(N -isopropylacrylamide). All the measurements demonstrated that in the region of 25,45°C, the polymer underwent a phase transition and the corresponding change in optical properties in its water solution. However, the polymer did not show completely reversible behavior upon heating and cooling. On the basis of the comparison with two other thermally responsive conjugated polymers in literatures, a tentative mechanism has been proposed that ,,, interaction induced rigid segments to remain chain conformation and packing styles as in collapsed state. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 [source] Graft copolymerization of methyl acrylate onto cellulose initiated by potassium ditelluratoargentate(III)POLYMER INTERNATIONAL, Issue 10 2004Yinghai Liu Abstract A novel redox system, potassium ditelluratoargentate(III) (DTA),cellulose, was employed to initiate the graft copolymerization of methyl acrylate onto cellulose in alkali aqueous solution. Grafting parameters, such as total conversion, grafting efficiency and grafting yield, were evaluated comparatively. The dependence of these parameters on temperature, reaction time, initiator concentration and ratio of monomer to cellulose was also investigated. Graft copolymers with high grafting parameters were obtained, which indicated that the DTA,cellulose redox pair is an efficient initiator for cellulose grafting. The proof of grafting was obtained from gravimetric analysis and infrared spectra. A tentative mechanism involving a two-step single-electron-transfer process of DTA is proposed to explain the generation of radicals and initiation. Thermogravimetry, X-ray diffraction and scanning electron microscopy were also carried out to study the thermal stability, crystallinity and morphology of the grafted copolymers. Copyright © 2004 Society of Chemical Industry [source] Spectroelectrochemical study of N -ethyl-carbazole in the presence of acrylamidePOLYMER INTERNATIONAL, Issue 3 2001Ö Yavuz Abstract Polymerization of N -ethylcarbazole (NECz) in the presence of acrylamide (AAm) has been investigated by in situ and ex situ UV,visible spectrophotometric measurements to obtain information about the reaction pathway, because NECz gives soluble oligomeric species allowing such measurements. A tentative mechanism is proposed in the light of these results. The redox properties of the new polymers have been studied for possible sensor application. © 2001 Society of Chemical Industry [source] | |||||||||||||