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  • 1
    Electronic Resource
    Electronic Resource
    Effect of electron lone-pairs on nuclear spin-spin coupling constants (1989)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 27 (1989), S. 409-430 
    Details
    ISSN: 0749-1581
    Keywords: NMR ; Multinuclear spin coupling ; Lone-pair effects ; Substituent effects ; Stereochemistry ; MO ; Sum-over-states models ; Electron delocalization ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The nature and types of lone-pair effects on nuclear spin coupling constants are reviewed in the context of a localized bond description of molecular electronic structure. Emphasis is placed on the importance of residual delocalization involving the otherwise lone-pair orbital, in terms of which the effect of an X lone-pair, when compared with an isoelectronic Y—H or X—H+ group or with an X—R group, and its orientational dependence can be interpreted. One-, two- and three-bond coupling constants are considered and the importance of lone-pair effects for configurational and conformational information is stressed and illustrated. They also serve for a better understanding of substituent inductive effects on coupling constants.A large collection of illustrative examples are presented, with particular attention paid to couplings involving H, C, N, F and P nuclei, organized in a systematic manner into nine categories. The signs of the lone-pair effects on the reduced coupling constants are found to be independent of the actual nuclei under study in the same category. This is taken as an indication that the electron lone-pairs mainly affect the Fermi contact contribution to the coupling and, accordingly, an interpretation is given in terms of simple sum-over-states models. In addition, symmetry-based relationships are established involving the sign of lone-pair effects in coupling constants between nuclei which are a different number of bonds apart.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/mrc.1260270502
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  • 2
    Electronic Resource
    Electronic Resource
    Interconversion of diastereomeric complexes involved in Rh(I)-catalyzed asymmetric hydrogenation: A (31P, 31P) EXSY NMR study (1993)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 31 (1993), S. 293-298 
    Details
    ISSN: 0749-1581
    Keywords: Asymmetric hydrogenation ; 31P NMR ; 2D EXSY NMR ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Rhodium(I) complexes containing chiral chelating diphosphines, e.g. R,R-dipamp [(R,R)-1,2-bis(o-methoxy-phenylphenylphosphino)ethane] or S,S-chiraphos [(2S,3S)-bis(diphenylphosphino)butane] and a prochiral olefin, e.g. methyl (Z)-α-acetamidocinnamate (mac), form diastereomeric complexes which differ only in which enantiotopic face of the olefin is bound to rhodium and can be observed by 31P NMR. The interconversion of the two diastereomers has been observed with (31P,31P)-{1H} 2D EXSY techniques for various mixing times, Tm, and in three different solvents (CD2Cl2, CD3OD and CD3COCD3). The results clearly demonstrate that both inter- and intramolecular ligand exchange processes are occurring, and that diastereomeric interconversion cannot proceed exclusively by complete mac dissociation followed by readdition to the opposite enantiotopic olefin face. The new results are consistent with previous DANTE NMR experiments. The differing results in CD2Cl2, CD3OD and CD3COCD3 solutions suggest the possibility of solvent involvement in the rate-limiting step of ligand exchange. The 2D EXSY method appears to be the method of choice for elucidating the qualitative exchange pathway of this complex system.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/mrc.1260310316
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    Paper (German National Licenses)
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