Rotational barriers of amides and polyisocyanates: A spin-coupled study

The spin-coupled method has been applied to the study of the rotational bar riers of dimethylacetamide and of 1-acetyl-1,3,3-trimethylurea-a model comp ound for polyisocyanates. A significant amount of nitrogen lone-pair stabil ization, achieved by delocalization toward the neighboring carbonyls, is sh own to be present in all conformational minima and transition states of the latter. This explains the observed lowering of the C-N torsional barrier, on passing from amides to polyisocyanates. (C) 1999 John Wiley & Sons, Inc.