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Is the Conventional Interpretation of the Anisotropic Effects of C[DOUBLE BOND]C Double Bonds and Aromatic Rings in NMR Spectra in Terms of the π-Electron Shielding/Deshielding Contributions Correct?

Authors

  • Prof. Dr. Marija Baranac-Stojanović,

    Corresponding author
    1. Faculty of Chemistry, University of Belgrade, Studentski trg 16, P.O.Box 158 and Center for Chemistry ICTM, P.O.Box 473, 11000 Belgrade (Serbia), Fax: (+381) 11 2636061
    2. Chemisches Institut der Universität Potsdam, Karl-Liebknecht Strasse24-25, 14476 Potsdam/Golm (Germany)
    • Faculty of Chemistry, University of Belgrade, Studentski trg 16, P.O.Box 158 and Center for Chemistry ICTM, P.O.Box 473, 11000 Belgrade (Serbia), Fax: (+381) 11 2636061
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  • Dr. Andreas Koch,

    1. Chemisches Institut der Universität Potsdam, Karl-Liebknecht Strasse24-25, 14476 Potsdam/Golm (Germany)
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  • Prof. Dr. Erich Kleinpeter

    1. Chemisches Institut der Universität Potsdam, Karl-Liebknecht Strasse24-25, 14476 Potsdam/Golm (Germany)
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Abstract

Based on the nucleus-independent chemical shift (NICS) concept, isotropic magnetic shielding values have been computed along the three Cartesian axes for ethene, cyclobutadiene, benzene, naphthalene, and benzocyclobutadiene, starting from the molecular/ring center up to 10 Å away. These through-space NMR spectroscopic shielding (TSNMRS) values, which reflect the anisotropic effects, have been broken down into contributions from localized- and canonical molecular orbitals (LMOs and CMOs); these contributions revealed that the proton NMR spectroscopic chemical shifts of nuclei that are spatially close to the C[DOUBLE BOND]C double bond or the aromatic ring should not be explained in terms of the conventionally accepted π-electron shielding/deshielding effects. In fact, these effects followed the predictions only for the antiaromatic cyclobutadiene ring.

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