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  • 1
    ISSN: 0030-4921
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: The 13chemical shifts of simple polymethines (cyanines and merocyanines) show the very pronounced charge alternation in these compounds. The 13C shifts of polar merocyanines are also susceptible to solvent polarity, which in this case has a strong influence upon the electron structure.
    Notes: Die 13C-NMR-chemischen Verschiebungen von einfachsten Polymethinen bestätigen die alternierende Elektronendichteverteilung bei diesen Verbindungen. Die Verschiebungen polarer Merocyanine sind von der Polarität des Lösungsmittels abhängig und beweisen damit eine lösungsmittelinduzierte Veränderung der Elektronenstruktur.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 0030-4921
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: The chemical shifts of aromatic nitriles of the general structure para-Y—C6H4—X—CN with X = O, S, Se and N(CH3) have been investigated by the 13C NMR technique. For cyanates (X = O) the 14N shifts and for Y = F the 19F shifts were likewise measured. The chemical shifts and the corresponding 13C shift increments Δn have been found to correlate with the appropriate substituent constants σR0, σp0 and σI, as well as with the π-electron densities calculated in the PPP approximation.
    Notes: An aromatischen Nitrilverbindungen der allgemeinen Struktur para-Y—C6H4—X—CN mit X = O, S, Se und N(CH3) wurden die 13C-NMR- sowie für X = O die 14N-NMR-und für Y = F die 19F-NMR-chemischen Verschiebungen bestimmt. Die Meßergebnisse wurden in Zusammenhang mit der elektronischen Struktur der Moleküle (Substituentenkonstanten, π-Elektronendichten in PPP-Näherung) diskutiert.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 5 (1973), S. 561-566 
    ISSN: 0030-4921
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: The 29Si-NMR chemical shifts δ(29Si) of (CH3)4-nSiXn compounds and some 13C-NMR chemical shifts δ(13C) of analogous carbon compounds are discussed by means of relative paramagnetic screening constants σ*, calculated by a simplified model. In this model only the Si(3P)- and C(2P)-orbitals are considered; for the calculations, the electronegativities of Si, C and the X-substituents and a single empirical parameter are necessary. The calculated values of σ* are in good agreement with the change of the chemical shifts which are observed for the (CH3)4-nMXn compounds with different X and n. These results clearly show that δ(29Si) and δ(13C) depend primarily on the σ-charge of the Si- and C-atom, and that (P—d)π-interactions on the Si-atom are of minor importance.
    Notes: Zusammenfassung-die 29Si-NMR-chemischen Verschiebungen δ(29Si) einer größeren Zahl von Verbindungen des Typs (CH3)4-nSiXn sowie δ(13C) einiger analoger Kohlenstoffverbindungen werden mit Hilfe von relativen paramagnetischen Abschirmungskonstanten σ* diskutiert, die nach einem vereinfachten Berechnungsverfahren bestimmt werden. Für die Berechnung von σ* werden nur p-Orbitale berücksichtigt, als Ausgangsgrößen sind lediglilch die Elektronegativitäten von Si, C und X sowie ein einziger empirischer Korrekturfaktor erforderlich. Die berechneten Änderungen von σ* befinden sich in guter Übereinstimmung mit dem Gang der experimentellen Verschiebungen, die in den (CH3)4-nMXn-Verbindungen in Abhängigkeit von Zahl und Art der Substituenten X beobachtet werden. Die erhaltenen Ergebnisse zeigen, daß δ(29Si) und δ(13C) im wesentlichen durch die σ-Ladung am Si-bzw. C-Atom bestimmt werden und daß (p - d)π Wechselwirkungen am Si-Atom nur eine untergeordnete Bedeutung besitzen.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 0030-4921
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An alternative explanation for δ-syn-axial substituent effects of 13C chemical shifts is given using the concept that the δ-syn-axial interaction in the substituted compound replaces the γ-gauche interaction in the parent molecule. The application of this principle to substituted norbornanes and steroids leads to the conclusion that the ‘net steric’ δ-syn-axial shift is upfield rather than downfield.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 9 (1977), S. 239-240 
    ISSN: 0030-4921
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: It is shown that a γ-methyl group causes approximately a 3 ppm upfield shift of the silicon resonance in (CH3)3Si—O—C—CH3 fragments. It is estimated that polar effects contribute about 0.8 ppm to this shift leaving a net steric γ upfield shift of 2 ppm. Variations around these average values are interpreted by different conformer populations, the importance of which are illustrated by the shifts in trimethylsiloxy derivatives of adamantane.
    Additional Material: 1 Tab.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 18 (1982), S. 71-73 
    ISSN: 0030-4921
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 29Si NMR spectra of various methylphenyl-substituted cyclotri- and -tetrasilazanes, composed of different combinations of the units d = Me2SiNH, dph = MePhSiNH and dph2 = Ph2SiNH, have been investigated. For most of the compounds having more than one asymmetric centre, the spectra of both the individual isomers and their mixtures have been studied, and the signals of the individual isomers have been assigned. The effect of a different arrangement of phenyl groups in the rings, and the influence of the stereochemistry of the isomers on the 29Si chemical shifts in cyclotri- and -tetrasilazanes have been studied. The 29Si chemical shifts in cyclotrisilazanes are additive and can be represented as the sum of increments corresponding to the number and positions of the phenyl groups in the rings. In these compounds, the spatial structure of the isomers does not exert any noticeable effect on the spectra. On the contrary, chemical shifts are not simply additive in cyclotetrasilazanes: a pronounced stereochemical dependence is apparent.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 9 (1977), S. 237-238 
    ISSN: 0030-4921
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 29Si and 13C NMR spectra of trimethylphenoxysilane and three bis(trimethylsiloxy)benzenes have been investigated. Crowding in the ortho derivative leads to a small but observable deshielding of both silicon and carbon nuclei of the trimethylsiloxy group. In comparison to analogous effects observed in trimethylsilyl-substituted benzenes the oxygen link appears to increase the susceptibility of silicon to this steric effect but to decrease that of the methyl carbons. It is suggested that the operative steric interaction might not be that between the terminal methyl groups but involves the oxygen atom.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
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