Ab initio calculation of the anisotropy effect of multiple bonds and the ring current effect of arenes—application in conformational and configurational analysis

Abstract

Substituents containing magnetically anisotropic chemical bonds, e.g. double bonds, triple bonds or the aromatic phenyl ring, influence the shielding of any nucleus in the molecule by their anisotropy effect dependent on its geometrical position. This effect of the magnetic anisotropy of neighbouring groups on the chemical shift of nuclei is usually specified qualitatively by the anisotropy cone. In this paper, the magnetic anisotropy effect of unsaturated chemical bonds and the ring current effect in arenes have been quantitatively calculated as nuclear independent chemical shieldings (NICSs) in a three dimensional grid of lattice points around the molecule using the GIAO method integrated into the GAUSSIAN 94 calculation program. Plotting the shielding/deshielding data thus obtained as iso-chemical-shielding surfaces (ICSS) around the magnetically anisotropic moieties allows us to quantify both direction and scale of the anisotropy effect.

The calculation of the anisotropy effect of double and triple bonds, and the ring current effect of the phenyl ring, has been applied to a number of stereochemical problems; especially in conformational analysis this method proved very successful in quantitatively assigning ¹H chemical shifts and hereby the stereochemistry of the molecules studied. In addition, contributions to ¹H chemical shifts based on the anisotropy effect of neighbouring groups and based on other substituent effects could be differentiated quantitatively. Considerable deviations from the qualitative sketches of the anisotropy effects of double and triple bonds published in text books were found.