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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 1852-1864 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Slow, large-amplitude chain motions play an important role in determining the macroscopic mechanical properties of polymers. Although such motions have been studied quantitatively by two-dimensional (2D) nuclear magnetic resonance (NMR) exchange experiments, overlapping anisotropic patterns hamper spectral analysis, and limit applications. Variable angle correlation spectroscopy (VACSY) has proven useful in resolving such problems for rapidly spinning samples by separating anisotropic spectral patterns according to isotropic chemical shifts. In a previous study [J. Am. Chem. Soc. 115, 4825 (1993)], we described a three-dimensional (3D) NMR experiment that incorporates the VACSY method and a hop of the rotor axis to correlate the isotropic chemical shifts to 2D anisotropic exchange patterns. The hop of the rotor axis, however, presents experimental difficulties and limits the range of motional rates that may be studied. We present in this paper a new 3D VACSY exchange experiment that obtains the same correlations without the need for the rotor axis hop. A series of 2D exchange spectra are recorded with the sample spinning at different rotation axis angles. Then using the scaling of the anisotropic frequency at the different angles, we construct the data onto a 3D matrix so that a Fourier transformation directly yields the desired correlations. The technique is applied to 13C exchange NMR to study the slow molecular motion of ordered isotactic polypropylene.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 111-117 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We describe the application of a recently developed two-dimensional nuclear magnetic resonance (2D NMR) technique, variable-angle correlation spectroscopy, to the analysis of molecular motions in complex unlabeled solids. This technique separates the broad anisotropic chemical shift line shapes of nuclei in a sample according to the isotropic shift of each site. It can therefore be used to characterize molecular reorientations by monitoring the changes that these processes introduce in the resolved powder patterns as a function of temperature. Using the 13C NMR anisotropies of dimethylsulfone as a test case, we explored the potential applications of following such an approach. It was found that in contrast to what happens in nonexchanging systems, the anisotropic line shapes resolved by the variable-angle technique on an exchanging solid are different from line shapes that at similar temperatures can be recorded from a nonrotating sample. An explanation for these differences is presented, and the complete theory required to extract kinetic and geometric information from the experimental 2D line shapes is introduced and illustrated with computer simulations. The capability of this approach to analyze motions in complex systems is further demonstrated with a natural-abundance 13C variable-temperature NMR analysis of L-tyrosine ethyl ester; a reorienting compound possessing up to 11 inequivalent carbon sites in the solid phase.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Journal of the American Chemical Society 115 (1993), S. 4825-4829 
    ISSN: 1520-5126
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    Journal of materials science 27 (1992), S. 2908-2913 
    ISSN: 1573-4803
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract The variation of optical and mechanical properties in ion-exchanged glasses was investigated. Ion exchange was carried out in KNO3, NaNO3 melts and their mixed melts with AgNO3. The glasses used were soda-lime-silicate glasses produced by the float process. In order to analyse the effects of ion exchange, colour variation for optical properties, bending strength, surface microhardness and softening point for mechanical properties, and resistivity for electrical properties, were measured. In the optical properties, silver ions play an important role in colouring, ion exchange in the mixed melt of KNO3 and AgNO3, and additional heat treatment in air made it possible to obtain a range of colours from yellow to reddish-brown. Bending strength was increased by five times over the values of the parent glasses, surface microhardness was increased about 50 kg mm−2, and the softening point was increased linearly with the amount of ion exchange resulting from ion exchange in the KNO3 melt, because the potassium ion plays a great role in strengthening in the mechanical properties. On the other hand, in the mixed melt of KNO3 and AgNO3, bending strength and surface microhardness were increased slightly in contrast with the KNO3 melt, and the softening point decreased on increasing the amount of ion exchange.
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1612-1112
    Keywords: Reversed phase liquid chromatography ; Water-soluble calixarenes ; Mobile phase additive ; Inclusion complexation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary The utility of water-soluble calix[6]arene-ρ-sulfonate as a mobile phase additive has been investigated for the reversed phase liquid chromatographic separation of some monosubstituted phenol isomers. Retention factors and separation factors for regioisomers of methoxyphenol, aminophenol, and nitrophenol were measured using methanol — water and acetonitrile —water mobile phases of varying composition containing calix[6]arene-ρ-sulfonate, and the values were compared with those obtained using mobile phases containing no additive. It was observed that addition of calix[6]arene-ρ-sulfonate to both acetonitrile — water and methanol —water caused a reduction in the retention of the phenol isomers but generally increased the separation between them, thereby improving the overall separation efficiency.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Springer
    Chromatographia 33 (1992), S. 154-158 
    ISSN: 1612-1112
    Keywords: Inverse gas chromatography ; Solid surface polarity ; Polar interactions ; Linear Free Energy Relationships ; Multiple regression analysis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary Sensitivities of a solid surface to dispersive and nondispersive (polar) interactions can be readily estimated by a multiple regression analysis of inverse gas chromatographic retention data of a set of probe solutes. This analysis is based on linear free energy relationships (LFERs). The sensitivity to the latter type of interactions can be used as a measure of the surface polarity of the solid. This has been shown in the case of a graphitized carbon fiber and the method is also applicable to other solids.
    Type of Medium: Electronic Resource
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