The spin-coupled method has been applied to the study of the rotational barriers of dimethylacetamide and of 1-acetyl-1,3,3-trimethylurea—a model compound for polyisocyanates. A significant amount of nitrogen lone-pair stabilization, achieved by delocalization toward the neighboring carbonyls, is shown to be present in all conformational minima and transition states of the latter. This explains the observed lowering of the CN torsional barrier, on passing from amides to polyisocyanates.
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Raos et al. (1999) studied this question.