Studies in nuclear magnetic resonance—IX. Rotational barriers in substituted <i>N</i>,<i>N</i>‐dimethylbenzamides

Abstract

Abstract The barriers to rotation about the CN bond in eighteen substituted N , N ‐dimethylbenzamides have been determined by complete line shape analysis of the NMR spectra of the N , N ‐dimethyl protons. The barriers have been correlated with the substituent constants σ and σ + . It has been shown that polar solvents increase the barrier in N , N ‐dimethylbenzamide. Acid catalysis of rotation about the amide CN bond in N ‐( p ‐ N , N ‐dimethylcarboxamidobenzyl)‐pyridinium bromide has been investigated. 18 O exchange studies show that catalysis is due to N ‐protonation rather than the formation of a tetrahedral intermediate. The rate of rotation is a function of the Hammett acidity function, H 0 , and the water activity, and it is shown that proton exchange between the N ‐ and O ‐protonated species involves the intermediacy of a water molecule. The differences in chemical shifts for the non‐equivalent N , N ‐dimethyl groups of the benzamides are also a function of the substituents. Possible explanations of this phenomenon are discussed.