Electron delocalization in isocyanates, formamides, and ureas: Importance of orbital interactions

The electron delocalization in isocyanates, amides, and ureas has been studied using ab initio MO, and density functional methods. The observed trends in the C-N rotational barriers and N-inversion barriers in these systems have been explained in terms of orbital interactions. NBO analysis indicates that there is n(N) --> pi*([C-X]) electron delocalization in amides, which increases with a decrease in the energy difference (DeltaE) between the two interacting orbitals. This phenomenon, rather than electronegativity, is responsible for the observed increase in the C-N rotational barrier in amides X=C(R)-NH2 (R = H, NH2) in the order X = O < S < Se.