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  • Analytical Chemistry and Spectroscopy  (5)
  • Atomic, Molecular and Optical Physics  (1)
  • Chemical shift anisotropy  (1)
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  • 1
    Digitale Medien
    Digitale Medien
    Low temperature 13C NMR magnetic resonance in solids 4. Cyclopropane, bicyclo[1.1.0]butane and [1.1.1] propellane (1985)
    Springer
    Theoretical chemistry accounts 68 (1985), S. 421-430 
    Details
    ISSN: 1432-2234
    Schlagwort(e): Chemical shift anisotropy ; Highly strained compounds ; Quantum chemical calculation of shielding tensors
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie
    Notizen: Abstract The solid state 13C NMR spectra of bicyclo[1.1.0]butane and [1.1.1]propellane have been measured at low temperature. The orientation of the principal axes of the chemical shielding tensor have been determined with ab initio calculations based on the IGLO (Individual Gauge for Localized Orbitals) method when they are not determined by symmetry. Excellent agreement is obtained between the calculated and experimental principal values of the shielding tensor when basis sets containing polarization functions are used. In most cases the agreement is such that the calculated values are within the experimental error.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00527667
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  • 2
    Digitale Medien
    Digitale Medien
    Analysis of the IGLO bond contributions to the 13C shielding tensors in the local bond frame (1987)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 31 (1987), S. 45-55 
    Details
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A new method is presented to analyze the IGLO (individual gauge for localized orbitals) bond contributions in 13C chemical shielding. The IGLO bond contributions calculated in the molecular frame are rotated to a local bond frame, in which one component is selected along the bond. This procedure removes the explicit angular dependence of the IGLO bond contributions and allows a comparison of the bond contributions in different molecules.The results provide a new method to study the electronic basis of shielding interactions. The problems associated with the multiple gauge origins used in the IGLO method are discussed in their relationship to the bond contribution analysis.
    Zusätzliches Material: 1 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/qua.560310106
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  • 3
    Digitale Medien
    Digitale Medien
    The use of high-field carbon-13 NMR spectroscopy to characterize chiral centers in isopranes (1986)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 24 (1986), S. 191-198 
    Details
    ISSN: 0749-1581
    Schlagwort(e): Chemistry ; Analytical Chemistry and Spectroscopy
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The 13C nuclear magnetic resonance spectra of four diastereomeric, acyclic isoprenoids [farnesane (C15), pristane (C19), phytane (C20), and squalane (C30)] have been obtained as a means for interpreting hydrocarbon mixtures common in some fossil fuels and biosynthesized organic compounds. Multiple resonance environments were detected for carbons influenced by two or three chiral centers. Samples of farnesane and squalane were found to exhibit random stereochemistry, whereas the phytane sample was demonstrated to be a mixture of two isomers, probably the 6(R), 10(S), 14(R,S) compound. The sample of pristane consisted of a single diastereomer and was probably obtained from a natural product. It is shown that general chemical shift assignments can be made in this class of molecules by considering the number of carbons located at positions one, two and three bonds distant, while assignment of closely spaced lines may be made on the basis of diastereomerically induced chemical shifts. Using these principles, the structural features of two fossil fuels were analysed.
    Zusätzliches Material: 7 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/mrc.1260240303
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  • 4
    Digitale Medien
    Digitale Medien
    Solid-state 13C NMR study of drugs: Aspirin (1986)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 24 (1986), S. 768-771 
    Details
    ISSN: 0749-1581
    Schlagwort(e): Solid-state ; 13C NMR ; Aspirin ; Drugs ; Polymorphism ; Chemistry ; Analytical Chemistry and Spectroscopy
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Natural abundance 13C NMR spectra of different crystal habits of pure aspirin and several aspirin tablets in the solid state have been measured by using the combined techniques of high-power decoupling, cross-polarization and magic angle spinning. Solid-state NMR of aspirin had excellent sensitivity compared with liquid-state NMR, and the carbon signals were assigned using a modified pulse sequence to detect non-protonated carbons. The solid-state 13C NMR spectra of two crystalline forms of aspirin were identical, suggesting that the origin of their difference is not polymorphic.The solid-state 13C NMR spectra of several commercial aspirin tablets, some of them containing buffer components, indicated that there were no interactions among the aspirin and the buffer components; only after dissolving the tablets and lyophilization did the 13C NMR spectra of the dry lyophilized powders show strong interactions between aspirin and the buffer components.
    Zusätzliches Material: 3 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/mrc.1260240907
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  • 5
    Digitale Medien
    Digitale Medien
    Structure determination of a new saponin from the plant Alphitonia zizyphoides by NMR spectroscopy (1993)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 31 (1993), S. 472-480 
    Details
    ISSN: 0749-1581
    Schlagwort(e): Alphitonia zizyphoides ; Saponin ; 2D NMR ; INADEQUATE ; Molecular structure ; Computerized spectral analysis ; Chemistry ; Analytical Chemistry and Spectroscopy
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: High-field NMR experiments, including the 2D INADEQUATE technique coupled with a computerized spectral analysis, were used to determine the full structure of a new saponin extracted from a plant (Alphitonia zizyphoides) found in the Samoan rain forests. The saponin, which shows significant pharmacological activity, consists of an aglycone framework of six rings (three saturated six-membered rings, two oxygen-containing five-membered rings and a five-membered carbon ring) to which are attached four sugar units and a six-carbon side-chain. The INADEQUATE experiment, which was carried out on 60 mg of sample, provided absolute confirmation of the assignments for 35 of the possible 53 C—C bonds from the 13C-13C connectivities.
    Zusätzliches Material: 10 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/mrc.1260310511
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  • 6
    Digitale Medien
    Digitale Medien
    Computerized analysis of 2D INADEQUATE spectra to assign chemical shifts in aromatic compounds (1995)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 33 (1995), S. 803-811 
    Details
    ISSN: 0749-1581
    Schlagwort(e): NMR ; 13C NMR ; 2D INADEQUATE ; polycyclic benzenoid aromatics ; Chemistry ; Analytical Chemistry and Spectroscopy
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A computerized analysis of a single 2D INADEQUATE spectra assigns all of the 13C chemical shifts in fairly complex aromatic systems. Without the aid of the computer this information is often difficult, if not impossible, to retrieve owing to overlap of the signals in the double quantum dimension. In addition, the use of the computerized analysis allows for the interpretation of INADEQUATE spectra with much lower signal-to-noise ratios than are practical if the analysis were to be done visually. Finally, narrowing the spectral window to a subset of the entire spectrum and the companion advantages are demonstrated. The assignments of the aromatic chemical shifts in 2-methoxydibenzofuran, 6-methylchrysene, 1-decylpyrene, and 7-methylbenzo[a]pyrene illustrate the power of automated pattern recognition methods while supplying previously unavailable chemical shift assignments on these complex fused polycyclic aromatic systems.
    Zusätzliches Material: 5 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/mrc.1260331007
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  • 7
    Digitale Medien
    Digitale Medien
    Conformational analysis of DL-, L- and D-methionine by solid-state 13C NMR spectroscopy (1986)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 24 (1986), S. 167-170 
    Details
    ISSN: 0749-1581
    Schlagwort(e): Chemistry ; Analytical Chemistry and Spectroscopy
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The conformational analysis of DL-, L- and D-methionine was studied by solid-state 13C NMR with the help of cross-polarization and magic-angle spinning techniques. The spectrum of commercial crystalline DL-methionine showed two chemical shifts (18.7 and 15.8 ppm) for the methyl carbon. Crystallization from water or water-ethanol (95 : 5) allowed the separation of two crystalline forms (α and β), of which one had δ (CH3) 18.7 ppm and the other δ (CH3) 15.8 ppm. Therefore, each crystalline form of DL-methionine consists of a single conformer, a result in agreement with x-ray diffraction data. This conformational difference of the two crystalline forms was also reflected in the carbon resonances of C-β and C-γ. When L- and D-methionine were crystallized from water-ethanol (3 : 2), the solid-state 13C NMR spectra showed that each isomer displayed two methyl carbon peaks at 18.1 and 16.2 ppm of equal intensities, while the α-carbon resonances showed a broad peak due to two unresolved doublets. These results suggest that the L- or D-methionine crystal contains two different conformers in one unit cell, a conclusion that is also supported by the x-ray data.
    Zusätzliches Material: 5 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/mrc.1260240213
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