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  • 1
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: Underivatized short-chain polysaccharides such as two inulins from different plants containing up to 35-40 monosaccharide units with molecular weights of up to 6500 Da and dextran T 1.5 containing up to 16-20 monosaccharide units with molecular weights of up to 3200 Da have been investigated by direct chemical lionization. Under soft ionization conditions such as ammonia chemical ionization and reduced ion-source temperature, it was possible to obtain spectra of the native polysaccharides showing dominant ion series corresponding to ammonia adduct ions of oligosaccharides, and also ion series corresponding to ammonia adduct ions of anhydro-oligosaccharides.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1052-9306
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 11 Ill.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: A number of different low molecule weight and volatile compounds have been tested and compared as matrices in 252Cf plasma-desorption mass spectrometry (PDMS) measurements of oligosaccharides. Some heteroaromatic amines (2-aminothiazole and 3-aminopyridine), hydroxyanthraquinones (alizarin and quinalizarin) and phthaleins (fluorescein) proved to be especially suited to enhancing the quasimolecular ion intensity of oligosaccharides. Discrimination effects could be reduced by matrix addition and thus, quantitative results of PDMS could be improved. Oligosaccharides with broad molecular weight distribution (dextrin 10 and dextran T 1.5) have been successfully analyzed by PDMS. The mass spectra obtained compared quite well with ion chromatograms of the respective oligosaccharides.
    Additional Material: 8 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
    Description / Table of Contents: We have recorded the 13C n.m.r. spectra of thiones and thioethers in the 1,3-diazole and 1,3-diazine series with various alkyl substituents at the nitrogen atoms. Some analogous oxygen containing heterocycles were also examined. We have shown that in the thiocarbonylated compounds the thiol ⇄ thione equilibrium is displaced towards the thione form, but that 13C n.m.r. gives only qualitative results. In the sulphur containing derivatives the isopropyl group is in a fixed conformational position because of the steric hindrance of the sulphur atom. Substitution by a tert-butyl group leads to unexpected γ values. We ascribe this phenomenon to ring deformation or to variations in the valence angles of the substituted nitrogen atoms.
    Notes: Nous avons enregistré les spectres de RMN du carbone-13 de thiones et de thioethers, en série diazolique et diazinique-1,3, diversement substitués sur les atomes d'azote par des groupements alkyles, ainsi que certaines des structures oxygénées homologues. Toutes les raies de résonance ont été attribuées. Dans le cas des dérivés thiocarbonylés, nous montrons que l'équilibre thiol ⇄ thione est déplacé vers la forme thione, mais que la RMN du carbone-13 ne peut conduire qu'à une évaluation qualitative de la position de cet équilibre. L'analyse des effets de substituant montre que, contrairement aux composés oxygénés, le groupement isopropyle adopte dans les dérivés soufrés une conformation privilégiée par suite de l'encombrement stérique dǔ à l'atome de soufre. La substitution par un groupement tertiobutyle conduit à des valeurs des effets γ inattendus. Nous attribuons ce phénomène à des déformations de cycle ou à des variations des angles valentiels de l'azote porteur du substituant, provoquées par des interactions stériques entre le substituant et le cycle correspondant. Nous relions ces modifications à certaines modalités de la réactivité, et à des résultats de spectroscopie photoélectronique.
    Additional Material: 5 Tab.
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  • 5
    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 effect of alkyl groups (Me, Et, i-Pr, t-Bu) on the ortho methyl proton chemical shifts is shown to be linearly correlated with the Taft (Es) scale in model compounds 1. Experimental results can be expressed by Δδ = δR - δMe = λ′Es in which λ′ is solvent and model dependent. The calculations of the chemical shifts according to ApSimon and Buckingham are used to show the predominant influence of steric effects on the observed substituent effect.
    Notes: L'effet des groupements alkyle de la série Me, Et, i-Pr, t-Bu sur le déplacement des hydrogènes d'un groupe méthyle, situé en ortho, est relié linéairement aux paramètres d'effets stériques (Es) de Taft dans des structures du type 1. Une équation Δδ = δR - δMe = λ′Es rend compte des résultats expérimentaux. λ′ dépend du solvant et du modèle utilisé. Les calculs des déplacements chimiques suivant les approches d'ApSimon et de Buckingham confirment la prépondérance des effets stériques sur l'effet de substituant observé.
    Additional Material: 5 Tab.
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  • 6
    ISSN: 0030-4921
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The 13C NMR spectra of some 1,3-diaryltriazenes, 3(1)-aryl-1(3)-hetaryltriazenes and their methylated derovatives have been recorded. From the comparison of the 13C chemical shifts between 3-aryl-1-(3,4-dimethyl-5-isoxazolyl)triazenes 1 and compounds with fixed structure, such as 1-aryl-3,3-dimentyltriazenes and methylated derivatives of 5-amino-3,4-dimethylisxazole, we found that the triazenes 1 have the sturcture (A) in which the azo group is conjugated with the heterocycllic moiety. para-Substituent on the aromatic ring have a week influence on the displacement of this tautomeric equilibrium. 13C NMR gives only qualitatives results. Theoretical calculation (HMO, CNDO-2) are also consistent with the greater stability of tautomeric form A.
    Additional Material: 1 Ill.
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  • 7
    ISSN: 0030-493X
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The isobutane and methane direct chemical ionization (DCI) mass spectra of mono- and dianhydrosugars are substantially influenced by their stereochemistry at temperatures of 150-250°C. The best results are obtained under isobutane CI at 200 or 250°C. l,6:3,5-Dianhydrogulofuranose gives proton-bridge stabilized MH+ ions and fewer fragment ions compared with its ido epimer. The four diastereomeric 1,5-anhydropentofuranoses show increased MH+ and M2H+ abundances for the lyxo and ribo isomers with cis-1,2-diol sites capable of proton bridging. The xylo isomer gives a dominant [M — OH]+ ion. All isomers, but especially xylo and ribo, give abundant [C3H5O2]+ skeleton cleavage ions. They are attributed to an RDA fragmentation of pyranoglycosyl-type MH+ ions. The eight diastereomeric 1,6-anhydrohexofuranoses with endo/exo configured triol structures show very characteristic MH+ abundances (4-41% Σ). They parallel the amount of proton solvation in cis-diol sites of the isomers (gluco ≤ ido ≤ altro 〈 gulo 〈 galacto 〈 talo 〈 allo 〈 manno). The [M — OH]+ ion abundances (15-47% Σ) are also very characteristic (galacto ≤ talo 〈 manno 〈 altro ≪ atto 〈 gluco ≤ ido ≪ gulo). They reflect the possible H2O or C4H,OH loss from the MH+ and (M · C4H9]+ ions, respectively, with formation of protonated 1,6:3,5-and 1,6: 23-dianhydrehexofuranose species; the slightly flattened C(5) site of M might favour the departure of the 5-OH group. In addition, there are especially high abundances of skeleton fragment ions, 15% Σ [MH — HCOOH]+, 10% Σ [M — OH — H3O — CH3O]+ and 42% Σ [M — OH — C2H4O2)+, for the ido, galacto and gluco isomers, respectively.
    Additional Material: 6 Tab.
    Type of Medium: Electronic Resource
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