Linear solvation energy relationship. Part 11. An analysis of nitrogen-15 solvent shifts in amides

Abstract

The solvatochromic comparison method is used to unravel and rationalize solvent effects on the ¹⁵N n.m.r. spectra of some N-unsubstituted, N-monoalkyl-, and NN-dialkyl-amides. It is shown that, in addition to solvent polarity–polarizability and type-A hydrogen bonding effects, type-B hydrogen bonding by the second protons of self-associated formamide leads to a significant dependence of the ¹⁵N shifts on solvent β values. In the case of the N-monoalkylamides, however, self association is sufficiently strong that the dependence of the shifts on solvent β values is negligible.