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41.
Robin Zuluaga Jean-Luc Putaux Adriana Restrepo Iñaki Mondragon Piedad Gañán 《Cellulose (London, England)》2007,14(6):585-592
Cellulose microfibrils have been prepared from banana rachis using a combination of chemical and mechanical treatments. The
morphology and structure of the samples were characterized using transmission electron microscopy, atomic force microscopy,
and X-ray diffraction. Fourier-transformed infrared spectroscopy (FTIR) was used to characterize the chemical modifications
of the samples after each treatment. Suspensions of bundled or individualized 5-nm-wide microfibrils were obtained after homogenization
(PH) whereas an organosolv (PO) treatment resulted in shorter aggregates of parallel cellulose microcrystallites. The sharper
rings in the X-ray diffraction pattern of the PO-treated sample suggest a higher crystallinity due to a more efficient removal
of hemicelluloses and dissolution of amorphous zones by the acid treatment. Both microfibrils and microcrystals prepared by
both methods can be used as reinforcing filler in nanocomposite materials. 相似文献
42.
Chromatography has been an important tool in marine laboratories. Since the 1960s, marine laboratories have been involved in the analysis of polychlorinated biphenyls (PCBs), organochlorine pesticides (OCPs), polycyclic aromatic hydrocarbons (PAHs), and brominated flame retardants (BFRs). Column chromatography and liquid chromatography (LC) techniques have been used, mainly in the clean-up phase, while gas chromatography (GC) has been used extensively in the final determination of these contaminants. Developments have been observed from the use of packed GC columns, via capillary columns to the use of heart-cut multi-dimensional GC and comprehensive multi-dimensional GC. The progress made in interlaboratory studies and the availability of certified reference materials are discussed. 相似文献
43.
We show that the two-photon resonances in the third-order susceptibility can be exploited to yield two-photon spectra of molecular gases at moderately high spectral resolution. This form of spectroscopy does not depend on the occurrence of processes (such as fluorescence of photoionization) leading to indirect methods of two-photon absorption. The method is direct and leads in principle to values for a two-photon cross section. Comparisons of two-photon and coherent anti-Stokes Raman resonances leads to ratios of Raman and two-proton cross sections independent of the laser powers and spatial characteristics. The technique is documented with rotationally resolved spectra of SO2 and NO. A value for |αxxαyy| of 1.5 × 10?52 cm6 was measured for the O12 (6) component of the (A)2Σ ← (X)2 Π transition of nitric oxide. 相似文献
44.
Amaresh RR Lakshmikantham MV Baldwin JW Cava MP Metzger RM Rogers RD 《The Journal of organic chemistry》2002,67(8):2453-2458
3,4-Cyanomethyl substituted thiophenes reacted with thionyl chloride in the presence of base to give dicyano substituted thieno(3,4-c)thiophenes. The use of selenium oxychloride furnished the corresponding cyano substituted seleno(3,4-c)thiophene. 1,2-Phenylenediacetonitriles gave the corresponding cyano substituted benzo(c)thiophenes and benzo(c)selenenophenes, respectively, upon reaction with thionyl chloride and selenium oxychloride in the presence of base. 相似文献
45.
The dipole strengths of the 4A2 → 4T1(F), 4T1(P), d-electron transitions of the cobalt(II) tetrahalides are found to decrease with an increase in temperature, in agreement with a dynamic ligand-polarisation model for the absorption mechanism. The reduction in dipole strength is accounted for by a decrease in the coulombic coupling between the quadrupole moment of the d-electron transition of the metal ion and the transient electric dipole moment induced in the ligands, due to the anharmonic increase in the mean bond length and the progressively larger mean-square amplitudes of the bending modes of the complex ion in its ground electronic state as the temperature is raised. 相似文献
46.
Bedford RB 《Chemical communications (Cambridge, England)》2003,(15):1787-1796
Palladacycles are amongst the most active catalysts in Heck-type carbon-carbon bond formation and related carbon-heteroatom bond forming reactions. For instance they have recently emerged as showing the highest activity in the Suzuki coupling of electronically challenging aryl chloride substrates. In addition to the high activity they display, their ease of synthesis, facile modification and comparative stability all act to enhance their appeal. 相似文献
47.
Wang Q Zakeeruddin SM Cremer J Bäuerle P Humphry-Baker R Grätzel M 《Journal of the American Chemical Society》2005,127(15):5706-5713
We report on cross surface ambipolar charge percolation within a monolayer of a molecular triad adsorbed on semiconducting or insulating mesoscopic metal oxide films. The triad consists of a triphenlyamine (TPA) donor and a perylenemonoimide (PMI) acceptor connected by a bithiophene (T2) bridge. The self-assembled PMI-T2-TPA monolayer exhibits p-type or n-type conduction depending on the potential that is applied to the conducting glass (FTO) electrode supporting the oxide films. Cross surface electron transfer is turned on at around -1.24 V (vs Fc+/Fc) where the PMI moiety is electroactive. The color of the film changes from red to blue during the reduction of the PMI. By contrast, lateral hole transfer is turned on at around 0.8 V (vs Fc+/Fc) where the TPA moiety becomes electroactive. The stepwise oxidation of the T2-TPA units at 0.79 and 1.28 V (vs Fc+/Fc) is associated with a color change of the film from red to black. Cyclic voltammetric as well as chronocoulometric and spectroelectrochemical measurements were applied to determine the percolation threshold for cross surface charge transfer and the diffusion coefficients for the electron and hole hopping process. The effect of oxide surface states on the lateral charge motion was also investigated. 相似文献
48.
Bowmaker GA Harris RK Assadollahzadeh B Apperley DC Hodgkinson P Amornsakchai P 《Magnetic resonance in chemistry : MRC》2004,42(9):819-826
Solid-state cross-polarization magic-angle spinning (CP/MAS) NMR spectra were recorded for the compounds [Ag(NH3)2]2SO4, [Ag(NH3)2]2SeO4 and [Ag(NH3))]NO3, all of which contain the linear or nearly linear two-coordinate [Ag(NH3)2]+ ion. The 109Ag CP/MAS NMR spectra show centrebands and associated spinning sideband manifolds typical for systems with moderately large shielding anisotropy, and splittings due to indirect 1J(109Ag,14N) spin-spin coupling. Spinning sideband analysis was used to determine the 109Ag shielding anisotropy and asymmetry parameters Deltasigma and eta from these spectra, yielding anisotropies in the range 1500-1600 ppm and asymmetry parameters in the range 0-0.3. Spectra were also recorded for 15N and (for the selenate) 77Se. In all cases the number of resonances observed is as expected for the crystallographic asymmetric units. The crystal structure of the selenate is reported for the first time. One-bond (107, 109Ag,15N) coupling constants are found to have magnitudes in the range 60-65 Hz. Density functional calculations of the Ag shielding tensor for model systems yield results that are in good agreement with the experimentally determined shielding parameters, and suggest that in the solid compounds Deltasigma and eta are reduced and increased, respectively, from the values calculated for the free [Ag(NH3)2]+ ion (1920 ppm and 0, respectively), primarily as a result of cation-cation interactions, for which there is evidence from the presence of metal-over-metal stacks of [Ag(NH3)2]+ ions in the solid-state structures of these compounds. 相似文献
49.
Simple, inexpensive tricyclohexylphosphine adducts of palladium show the highest activity yet observed in the Stille coupling of non-activated and deactivated aryl chlorides. 相似文献
50.
A simple synthesis has been devised for the tripodal 3,3,4-tetraamine ligand N{(CH2)3NH2}2{(CH2)4NH2} (L). This ligand forms
a copper(II) complex, [Cu (LH)Cl2]ClO4 (7), the structure of which has been determined by X-ray diffraction. The cation contains
a five-coordinate copper atom, bonded to two chloride ions, the two propylamine groups and the tertiary nitrogen atom of the
ligand; the arrangement is a distorted trigonal bipyramid, in which the two primary amine groups occupy the axial positions.
The butylamine group of the ligand does not coordinate to copper but is protonated. It is involved in hydrogen bonding to
the perchlorate ion. The e.p.r. spectrum of [Cu(dpt)Cl2] is very similar to that of (7), suggesting that it also has a trigonal
bipyramidal structure.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献