Ab-initio quantum-mechanical GIAO calculation of the anisotropic effect of C–C and X–C single bonds—application to the1H NMR spectrum of cyclohexane
Klod, Sabrina; Koch, Andreas; Kleinpeter, Erich; Klod Sabrina; Universität Potsdam, Chemisches Institut; Koch Andreas; Universität Potsdam, Chemisches Institut; Kleinpeter Erich; Universität Potsdam, Chemisches Institut
Журнал:
Journal of the Chemical Society, Perkin Transactions 2
Дата:
2002
Аннотация:
The anisotropic effects of the Câ C single bond in ethane and various other Xâ C single bonds (X = OH, SH, NH2) have been quantitatively calculated as nuclear independent chemical shieldings (NICSs) in a three-dimensional grid of lattice atoms around the single bonds using the GIAO method integrated into the GAUSSIAN 98 calculation program. The shielding/deshielding areas due to the anisotropic effect have been plotted as iso-chemical-shift-surfaces (ICSSs); hereby, both direction and scale of the anisotropic effect were quantified and visualized. The results obtained are not in agreement with McConnell's point dipole model; the influence of the anisotropic effect of Câ C and Xâ C single bonds on especially <sup>1</sup>H chemical shifts must be reevaluated. The various magnetic contributions to the theoretical NMR shielding tensors of the axial/equatorial protons in cyclohexane were calculated by a detailed NCS-NBO analysis. The partition of the C(2)â C(3) [C(5)â C(6)] bonds at the C(1)â Hax/Hequ magnetic shielding tensors determines both position and chemical shift difference, dominated by hyperconjugation.
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