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  • Jewish studies  (3)
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  • Jewish studies  (3)
  • 1
    Online Resource
    Online Resource
    Wiley ; 1980
    In:  Israel Journal of Chemistry Vol. 20, No. 1-2 ( 1980-01), p. 12-19
    In: Israel Journal of Chemistry, Wiley, Vol. 20, No. 1-2 ( 1980-01), p. 12-19
    Abstract: Starting from the concept of the mean plane of a general nonplanar N ‐membered ring, the substituent orientation angles α and β are derived. These angles define the position of a ring substituent in an unambiguous way, the angle α indicating the direction of the ring‐substituent bond relative to the mean plane (perpendicular, inclined or parallel) and the angle β revealing whether the bond vector is directed inward (towards the geometrical center of the ring) or outward (away from the geometrical center). The orientation angle α allows a generalization of the terms “axial” and “equatorial” including Barton's original specification of these terms and vivifying their semantics. Unlike earlier descriptions of ring substituent orientations the new definitions completely avoid terms like “quasi‐axial”, “quasi‐equatorial”, etc. Examples are given which show their usefulness in discussions of steric effects.
    Type of Medium: Online Resource
    ISSN: 0021-2148 , 1869-5868
    URL: Issue
    Language: English
    Publisher: Wiley
    Publication Date: 1980
    detail.hit.zdb_id: 2066481-3
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  • 2
    Online Resource
    Online Resource
    Wiley ; 1993
    In:  Israel Journal of Chemistry Vol. 33, No. 4 ( 1993-01), p. 369-385
    In: Israel Journal of Chemistry, Wiley, Vol. 33, No. 4 ( 1993-01), p. 369-385
    Abstract: NMR chemical shift calculations provide the basis for an intensive collaboration between quantum chemists and experimentalists. Calculated shift data can be used to describe the magnetic properties of a molecule, to identify unknown compounds by comparison of experimental and theoretical shift values, to determine equilibrium geometries, to investigate conformational changes, to elucidate the mechanism of molecular rearrangements, to determine solvent effects on NMR data, to identify complexation or coordination of soluted molecules by solvent molecules, to detect electronic structure changes caused by the medium, and to describe chemical bonding. This is demonstrated by three examples, namely the determination of the equilibrium structure of the homotropylium cation, the description of BH 3 NH 3 in solution or condensed phases, and the investigation of stannyl cation complexes in solution. IGLO calculations of 13 C, 11 B, 15 N, and 119 Sn chemical shifts with DZ+P or TZ+P basis sets lead to the following results: (1) The homotropylium cation possesses an equilibrium 1,7 distance of 2 Å that is indicative of strong through‐space interactions and, as a consequence, homoaromatic character. (2) In solution, the charge transfer from NH 3 to BH 3 is increased, which leads to a decrease of the BN bond length, an increase of the dipole moment, and a shielding of both the B and the N nucleus. The experimental δ( 11 B) and δ( 15 N) values can be reproduced when the geometry effect and the direct solvent effect are included in the shift calculations. (3) Stannyl cations form strongly‐bounded coordination complexes with solvent molecules (binding energy: ≥ 50 kcal/mol) that make the cation properties, in particular δ( 119 Sn) values, similar to those of covalently‐bounded stannyl compounds. An experimental detection of stannyl cations in solution by NMR spectroscopy should only be possible by extensive solvent variations.
    Type of Medium: Online Resource
    ISSN: 0021-2148 , 1869-5868
    URL: Issue
    Language: English
    Publisher: Wiley
    Publication Date: 1993
    detail.hit.zdb_id: 2066481-3
    Library Location Call Number Volume/Issue/Year Availability
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  • 3
    Online Resource
    Online Resource
    Wiley ; 1983
    In:  Israel Journal of Chemistry Vol. 23, No. 1 ( 1983-01), p. 72-84
    In: Israel Journal of Chemistry, Wiley, Vol. 23, No. 1 ( 1983-01), p. 72-84
    Abstract: Structural, conformational and energetic properties of cyclopentane (1) and the seven oxolanes monoxolane (2), 1,3‐dioxolane (3), 1,2‐dioxolane (4), 1,2,4‐trioxolane (5), 1,2,3‐trioxolane (6), tetroxolane (7), and pentoxolane (8) are investigated employing the polarized 6‐31G* basis set at the Hartree‐Fock level of theory. Extensive geometry optimization is carried out within the model of the semirigid pseudorotor. The conformational potentials V of compounds 1–8 are evaluated as a function of the puckering amplitude q and the pseudorotation phase angle Φ. Ring molecules 1 and 8 are free pseudorotors, while pseudorotation is hindered by barriers ≤ 3.3 kcal/mol for oxolanes 2–7. Puckering and inversion barriers increase with the number of O‐O bonds but decrease with the number of ether bridges. Puckered C 2 ‐symmetrical twist forms are the most stable conformations for compounds 2–7 but 6, where highest stability is found for the C 2 ‐symmetrical envelope forms. At room temperature a multitude of conformers of 1–8 coexists either because of free pseudorotation (barriers 〈 RT) or large amplitudes of pseudolibration. These results are rationalized in terms of the rotor potentials of appropriate reference compounds (C 2 H 6 , CH 3 OH, H 2 O 2 ). A more elegant approach leads to a simple π electron count and an analysis of bonding and antibonding overlap in the π‐type HOMO's. In this way the effects of substituents can be predicted and conformational preferences of the furanose ring in nucleotides, nucleosides and carbohydrates explained. The relative stability of the oxolanes is analyzed by calculating O‐O bond energies, bond‐bond interactions and ring strain for each compound. The lability of the higher oxolanes is traced back to increased ring strain. A new method of the conformational analysis of ring compounds is outlined.
    Type of Medium: Online Resource
    ISSN: 0021-2148 , 1869-5868
    URL: Issue
    Language: English
    Publisher: Wiley
    Publication Date: 1983
    detail.hit.zdb_id: 2066481-3
    Library Location Call Number Volume/Issue/Year Availability
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