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  • 1
    ISSN: 0749-1581
    Keywords: NMR ; LIS ; Conformations ; 2-Substituted 1,3-dioxanes ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A LIS analysis of 2-isopropyl-1,3-dioxane (1) and 2-methyl-2-phenyl-1,3-dioxane (2) is reported. The monodentate complexing model gives good agreement with the observed shifts in both compounds, whereas the bidentate model (lanthanum binding to both oxygens) does not give an acceptable solution. In 1 the lanthanide complexes almost exclusively (90%) with the axial lone pair of the oxygen atom. The four-site binding model gives excellent agreement with the observed shifts (Rx 3.5%). In 2 a similar lanthanide binding is found, and the analysis of the LIS allows both the deduction of the molecular conformation (C-2-methyl equatorial, C-2-phenyl axial) and also the orientation of the phenyl group (perpendicular).
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1057-9257
    Keywords: MOCVD ; MAS-NMR ; CARS ; Quantum effects ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Coherent anti-Stokes Raman scattering (CARS) has been utilised to monitor the decomposition of trimethylgallium (TMGa) on the reactive internal surface of the protonated form of zeolite Y under metal organic chemical vapour deposition (MOCVD) conditions designed to produce GaP deposits that exhibit quantum-size effects. The CARS data clearly reveal the facile room temperature decomposition of TMGa in the zeolite, as evidenced by the detection of increasing levels of methane with increasing reaction time. In addition, the zeolite samples were analysed by solid state nuclear magnetic resonance with magic angle spinning (MAS NMR) before reaction, after reaction of TMGa and after subsequent reaction with phosphine at 300°C. These data reveal that TMGa reacts with acid sites in the zeolite 12-rings, resulting in species containing methyl groups. Subsequent exposure to phosphine removes these methyl group protons, leaving various, as yet not fully characterised, phosphorus species. Exposure to phosphine does not appear to regenerate the acid sites on the zeolite.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 0021-8995
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: The effects of thermal and mechanical fatigue on the flexural strength of G40-600/PMR-15 cross-ply laminates with ply orientations of (02, 902)2s and (902, 02)2s are examined. The relative improtance of shear and tensile stresses is examined by varying the span-to-depth ratios of flexural test specimens from 8 to 45 Acoustic emission singals are measured during the flexural tests in order to monitor the initiation and growth of damage. Optical microscopy is used to examine speciments for resin cracking, delamination, and fiber breaks after testing. Transverse matrix cracks and delaminations occur in all specimens, for regardless of ply orientation, span-to-depth ratio, or previous exposure of specimens to thermal and mechanical fatigue. A small amount of fiber tensile fracture occurs in the outer 0° ply of specimens with high span-to-depth ratios. Because of the complex failure modes, the flexural test results represent the “apparent” strengths rather than the true flexural or shear strenghts for these cross-ply laminates. Thermal cycling of specimens prior to flexural testing does not reduce the apparent flexural strength or change the mode of failure. However, fewer acoustic events are recorded at all strins during flexural testing of specimens exposed to prior thermal cycling. High temperature thermal cycling (32-260°C, 100 cycles) causes a greater reduction in acoustic events than low temperature thermal cycling (-85 to 85°C, 500 cycles). Mechanical cycling (0-50% of the flexural strength, 100 cycles) has a similar effect, except that acoustic events are reduced only at strains less than the maximum strain applied during flexural fatigue. © 1994 John Wiley & Sons, Inc.This article is a US Government work and, as such, is in the public domain in the United States of America.
    Additional Material: 13 Ill.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    New York : Wiley-Blackwell
    Biopolymers 32 (1992), S. 497-515 
    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A method was developed for computing the free energy (ΔFi) of aggregates of type I collagen. The method was based on a treatment of Matheson and Flory describing phase equilibria of rigid rod polymers. It included a polymer-solvent interaction term that depended on near neighbor transfer energies. Extrahelical portions of the molecule were assigned local interaction energies differing from that assigned to the helix. Free energies of reaction for successive steps along assembly pathways (ΔFi-i+1) were computed. When allowance was made for specific pairing between extrahelical and helical domains, the so-called D-staggered (D = 670 Å) alignment of molecules was preferred, as opposed to a nonstaggered, or nematic, alignment. Based on ΔFi-i+1 alone, it appeared that 1D-staggered oligomers arise first in assembly, followed later by addition of molecules in 4D alignment.Neither 4D dimers nor 4D-8D trimers were predicted to be major intermediates in assembly. This result is contrary to previous hypotheses. When energies of activation were included in the analysis, the prediction was less certain, and specific circumstances were identified in which 4D dimers and 4D-8D trimers were the earliest aggregated species in assembly.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 0021-8995
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Injectable collagen is a concentrated dispersion of collagen fibers in aqueous solution that is used to correct dermal contour defects through intradermal injection. The effect of hydrophobic forces on the rheology of concentrated dispersions of collagen fibers was studied by dynamic rheological measurements over temperatures ranging from 283 to 308 K. The results are interpreted in terms of the associated relaxation time spectra and the theory of Kamphuis et al. for concentrated dispersions. Increases in fiber rigidity are seen from a progressive decrease in the slope of the linear log G′ (or G″) vs. log ω data recorded for these dispersions as temperature is increased. A molecular interpretation of this result was obtained by treating collagen fibers as a liquid crystalline phase of rigid-rod collagen molecules that have phase-separated from aqueous solution. Hydrophobic forces affect the volume fraction of water that is present in the phase-separated fibers, which, in turn, affects the rigidity of the fibers. Distinct yielding behavior (in the nonlinear viscoelastic region) occurs at temperatures above 293 K and reflects a gel transition. Thermal gelation of collagen dispersions is proposed to proceed through hydrophobically driven mechanisms of increased fiber rigidity and enhanced interfiber attractive forces. © 1993 John Wiley & Sons, Inc.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Electroanalysis 4 (1992), S. 97-105 
    ISSN: 1040-0397
    Keywords: Stripping voltammetry ; mercury film electrode ; polymer coating ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A chemically modified mercury thin-film electrode (MTFE) is presented that employs an electrochemically generated nonconducting polymer (poly-N-ethyltyramine), which has been derivatized with surface dithiocarbamate groups. Mercury is deposited chemically onto the electrode by complexation and, following electrochemical reduction of the mercury, is used as a substrate for anodic stripping voltammetry (ASV) of cadmium, lead, and copper. The properties of a conventional MTFE are compared with the modified electrode.
    Additional Material: 11 Ill.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Electroanalysis 4 (1992), S. 139-142 
    ISSN: 1040-0397
    Keywords: Photoelectrochemistry ; speciation ; thallium ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A method that allows speciation and electrochemical detection of thallium in flowing solutions has been developed. Thallous ions are determined using oxidative detection at a platinum electrode in a sulphuric acid-formic acid solution. In the presence of thallic ions, iiradiation of the solution with UV light induces the reduction of Tl(III) to Tl(I) which is subsequently detected as described above. Under these conditions total thallium is determined. Irradiation of the flowing solution is carried out within the detector flow cell.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Electroanalysis 4 (1992), S. 323-326 
    ISSN: 1040-0397
    Keywords: Oxygen removal ; flow analysis ; stripping voltammetry ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A method for the on-line removal of dissolved oxygen has been developed and applied to the determination of metals by anodic stripping voltammetry in flowing solutions. The removal of oxygen results from a photochemical reaction between the supporting electrolyte and oxygen, induced by ultraviolet light directed into the detector flow cell. Organic acids such as citric, formic, and oxalic acids have been used as supporting electrolytes. Citric acid was found to be the most efficient oxygen scavenger. The removal of oxygen with this acid was also more efficient than conventional removal with nitrogen gas. The determination of Cu, Cd, and Pb was used to demonstrate the method.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Electroanalysis 4 (1992), S. 439-445 
    ISSN: 1040-0397
    Keywords: Alcohols ; photoelectrochemistry ; amperometric detection ; food ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A method for photoelectrochemical detection of alcohols in liquid chromatography and flow injection analysis based on the reaction of the analyte with a photosensitized quinone (benzoquinone) has been developed. Irradiation of the flowing solution and electrochemical detection of the generated photoproduct is carried out within the detector flow cell. Pulsed amperometric detection produced higher sensitivities than conventional potentiostatic amperometry. Samples of vodka and beer were successfully analyzed using the proposed method.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
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  • 10
    ISSN: 1040-0397
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis and utilization of electrodes modified with conducting polymers is reviewed. Specific issues evaluated include the various approaches available to produce the required functionality at the electrode surface and the practical considerations to be addressed during synthesis, both in terms of hardware requirements and the chemistry of electropolymerization. The review concludes with a survey of the many applications of polymer-modified electrodes and includes 112 references.
    Additional Material: 2 Ill.
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