Chapter Three - Quantification and Visualization of the Anisotropy Effect in NMR Spectroscopy by Through-Space NMR Shieldings

Abstract

Abstract The anisotropy effect of functional groups (respectively the ring-current effect of aryl moieties) in 1 H NMR spectra has been computed as spatial NICS (through-space NMR chemical shieldings) and visualized by iso-chemical-shielding surfaces of various size and low(high)field direction. Hereby, the anisotropy/ring-current effect, which proves to be the molecular response property of spatial NICS, can be quantified and can be readily employed for assignment purposes in proton NMR spectroscopy—characteristic examples of stereochemistry and position assignments (the latter in supramolecular structures) will be given. In addition, anisotropy/ring-current effects in 1 H NMR spectra can be quantitatively separated from the second dominant structural effect in proton NMR spectra, the steric compression effect, pointing into the reverse direction, and the ring-current effect, by far the strongest anisotropy effect, can be impressively employed to visualize and quantify (anti)aromaticity and to clear up standing physical-organic phenomena as are pseudo-, spherical, captodative, homo- and chelatoaromaticity, to characterize the π-electronic structure of, for example, fulvenes, fulvalenes, annulenes or fullerenes and to differentiate aromatic and quinonoid structures.