Ab-initio quantum-mechanical GIAO calculation of the anisotropic effect of C-C and X-C single bonds -: application to the 1H NMR spectrum of cyclohexane

The anisotropic effects of the C-C single bond in ethane and various other X-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 C-C and X-C single bonds on especially H-1 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)-C(3) [C(5)-C( 6)] bonds at the C(1)-H-ax/H-equ magnetic shielding tensors determines both position and chemical shift difference, dominated by hyperconjugation.