Nitrogen-15 Magnetic Resonance Spectroscopy. Natural-Abundance Spectra of Secondary Amides

The ^(15)N chemical shifts of 17 N-alkyl- and N-arylethanamides have been determined at the natural-abundance level of ^(15)N using Fourier-transform methods. The shift effects produced by several substituent groups are compared with corresponding substituent effects in amino acids, amino acid derivatives, and dipeptides. The results show that N-alkylethanamides are not especially good models for predicting ^(15)N shifts in dipeptides. The sensitivity of the ^(15)N shift of N-arylethanamides to electronic effects, as reflected in the Hammett p constant, is less than that of the ^(15)N shift in para-substituted benzenamines.