CARBONYL CARBON AND NITROGEN CHEMICAL-SHIFT TENSORS OF THE AMIDE FRAGMENT OF ACETANILIDE AND N-METHYLACETANILIDE

Both carbon-13 and nitrogen-15 solid-state NMR spectroscopy have been employed to characterize the carbonyl carbon and nitrogen chemical shift (CS) tensors of the amide fragment of (Z)-acetanilide (I) and (E)-N-methylacetanilide (II). These two;elated compounds exhibit very different structural features in the solid state, as shown by previous X-ray diffraction studies. The orientation of the principal axis system (PAS) of both the carbon and nitrogen CS tensors has been determined using dipolar-chemical shift NMR spectroscopy in conjunction with IGLO chemical shielding calculations. For I and II, the carbon CS tensors are found to be very similar. In each, the most shielded direction is perpendicular to the amide plane while the intermediate component lies approximately along the carbonyl bond. Unlike the carbon results, the three principal components of the nitrogen CS tenser reveal striking variations which can be attributed predominantly to differences in the orientation of the N-phenyl substituent with respect to the amide plane. The orientation of the PAS of the nitrogen CS tenser has-been unambiguously determined for I and II from the results of two separate dipolar-chemical shift NMR experiments for each compound. The intermediate component of the nitrogen CS tensor lies perpendicular to the amide plane in both compounds while the most shielded direction lies close to the C-N bond, toward C-alpha. Although the shielding calculations adequately reproduce the experimental shielding trends; they are less successful in determining the absolute magnitudes of the principal components of the experimental shielding tensors, which, in the case of I, can be partly attributed to the neglect of intermolecular N-H- -O=C hydrogen bonding in the-calculations.