THE MAGNETIC ANISOTROPY OF THE CARBONYL BOND

The magnetic susceptibility components of the carbonyl bond have been determined from chemical shift differences in the n.m.r. spectra of amides. Correction of the experimental chemical shift differences for the effect of internal electric fields is found to overestimate the shielding of groups close to the carbonyl bond and it is shown that the sets of susceptibility components determined without this correction give a satisfactory correlation of structure and chemical shifts in amides, α,β-unsaturated ketones, and exo-methylene cycloketones.Four sets of susceptibility components are derived from formamide or acetamide spectra and depending on whether the effect of the carbonyl bond is assumed to be shielding or deshielding. These sets are, in units of 10−6 cm3 mole−1:[Formula: see text]Spectra of 1-ethylpyrrolidinone-2 are interpreted as showing that the effect of the carbonyl bond is to shield distant protons in the plane of the carbonyl group and its substituents.