Nuclear Magnetic Resonance Studies of Multi-site Chemical Exchange. III. Hindered Rotation in Dimethylacetamide, Dimethyl Trifluoro-acetamide, and Dimethyl Benzamide

Abstract

The experimental application of a multi-site exchange theory developed recently has been made to three restricted rotation barriers in amido-type systems. Inclusion of 8 spin-sites in the analyses of the line shapes for spectra of N,N-dimethyl trifluoroacetamide and dimethyl acetamide leads to improved values for the activation parameters. Variation of chemical shifts with temperature is included in the iterative fitting procedure. The determination of the relative signs of long range couplings and the inclusion and measurement of unresolved couplings has been accomplished. The barrier for hindered rotation in N,N-dimethyl benzamide has been re-investigated including the effect of relatively large chemical shift changes with temperature. The internal consistency of the measurements from all aspects indicates that systematic errors common in the determination of energy barriers by steady-state nuclear magnetic resonance (n.m.r.) have been largely eliminated and hence the activation parameters obtained form a reliable addition to a compilation of experimental data pertaining to the bonding in a related series of simple C-substituted amides.