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

Abstract

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 ¹H 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.