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  • 1995-1999  (4)
  • 1999  (4)
  • 1
    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: An appreciable degree of pyramidalization of the amine N atom is observed in the title compound. The existence of polar chains, induced by N—H...O synthons, is confirmed. C—H...O interactions, not noted in a previous X-ray study, were found to stabilize further the known head-to-tail assembling of the chains. The structure can be described as non-polar (101) layers, embodying chains interlinked by centrosymmetric dimers, connected by C(aryl)—H...π interactions. The latter are not present in m-nitroaniline, 2-methyl-4-nitroaniline and other related compounds with chains built from similar N—H...O synthons and assembled head-to-head. This finding implies that an obvious relationship between molecular recognition patterns and crystal structures should not be assumed.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 55 (1999), S. 867-881 
    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: X-ray diffraction data sets collected on both Nonius and Siemens (Bruker) goniometers equipped with charge-coupled device (CCD) area detectors have been tested for the electron-density determination of the aluminosilicate mineral compound α-spodumene LiAl(SiO3)2, aluminium lithium silicon oxide. Data collection strategies, reflection intensity peak integration methods and experimental error estimates are different for the two instruments. Therefore, the consistency and quality of the two types of CCD measurements have been carefully compared to each other and to high-resolution data collected on a conventional CAD-4 point-detector diffractometer. Multipole density model refinements were carried out against the CCD data and the statistical factors analysed in terms of experimental weighting schemes based on the standard uncertainties of the diffraction intensities derived by the Nonius and Siemens software programs. Consistent experimental electron-density features in the Si–O–Si and Si–O–Al bridges were found from both CCD data sets. The net atomic charges obtained from the kappa refinements against each CCD data set are also in good agreement and quite comparable with the results of the conventional CAD-4 experiment.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1434-193X
    Keywords: Amino acids ; Spiro compounds ; Michael addition ; Copper reagents ; Curtius degradation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Racemic spiropentylglycine (8) has been synthesized by sodium borohydride reduction of benzyl (E/Z)-2-chloro-2-spiropentylideneacetate (5-Bn), nucleophilic substitution of the chlorine in the product 6 with azide and hydrogenolytic deprotection of the resulting 7 (overall yield 15%). An alternative approach to 8 consisted of the coupling of the higher-order cuprate 10, generated by halogen-metal exchange from bromospiropentane (9), with the electrophilic glycine equivalent 11 followed by deprotection (overall yield 47%). Enantiomerically pure (1′-aminospiropentyl)acetic acid [(R)-16] (overall yield 16% from 5-Me) and 1-aminospiropentanecarboxylic acid [(R)-23] (29% from 5-Me) were obtained from the Michael adduct 14-Me of (4R,5S)-4,5-diphenyloxazolidin-2-one (13) and methyl (E/Z)-2-chloro-2-spiropentylideneacetate (5-Me). Racemic 1-aminospiropentanecarboxylic acid (R/S-23) was prepared by rhodium-catalyzed addition of dimethyl diazomalonate to methylenecyclopropane and subsequent Curtius degradation of the halfester 28 via the azide 29 (overall yield 14%). Upon standing in aqueous solution, 23 underwent complete rearrangement to the new 1-amino-2-methylenecyclobutanecarboxylic acid (24). The interesting derivative of azabicyclo[3.1.0]hexane-1-carboxylate 34 with an annelated spiropentane moiety and a β-amino acid fragment was incidentally obtained in a one-step intermolecular domino reaction starting with the addition of lithium benzylamide to methyl 2-chloro-2-cyclopropylideneacetate (32, 41% yield).
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1434-1948
    Keywords: Niobium ; Tin ; Hydrides ; Group 5 ; Group 14 ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of niobocene trihydride [Cp2NbH3] with one equivalent of an alkyl- or aryltin halide XnSnR4-n (n = 1, 2) leads to products of the displacement of hydride by tin, i.e. [Cp2NbH2(SnX3-nRn)] [R = Me, n = 3 (1); R = Me, × = Cl, n = 2 (3); R = Ph, n = 3 (4); R = Ph, × = Cl, n = 2 (5); R = Et, × = Br, n = 2 (6)]. Use of an excess of the tin reagent results in a halodealkylation exchange reaction at the tin atom bound to the metal center. Reaction of the mono(silyl) complexes [Cp2NbH2(SiXMe2)] (X = Cl, Ph) with organotin halides XnSnR4-n gives products of the displacement of silicon by tin, whereas the bis(silyl) complex [Cp2NbH(SiPhMe2)2] does not undergo this reaction. The bis(tin)-substituted niobocene [Cp2NbH(SnMe3)2] has been prepared by the reaction of [Cp2NbH2(SnMe3)] with the tin amide Me3SnNMe2 under thermal conditions, as well as by the literature method of thermolysis of [Cp2NbH(η2-C2H3Ph)] in the presence of Me3SnH. Reaction of [Cp2NbH(SnMe3)2] with two equivalents of Cl2SnMe2 affords the bis(tin)-substituted complex [Cp2NbH(SnClMe2)2] (9). The possible presence of non-classical hypervalent H···Sn-Cl interactions in this compound has been assessed by means of X-ray structure analysis. The structural data for 9 are in accord with an essentially classical description as a tin-substituted dihydride complex without significant Sn···H interactions.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
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