Resonance in formamide: resolution of contending models

Resonance in formamide has been investigated by carrying out ab initio VBSCF calculations in an extended Gaussian basis set. It is found that the special properties of the planar geometry C–N bond arise from a π space interaction that involves a N→C donation to a highly polarized carbonyl group. The net result is that the double bond structure in planar formamide looks like the classical H2N+C(H)O− distribution. In the C–N rotated geometry the corresponding n–σ* interaction is much less stabilizing. The N→C model resolves some of the differences between the classical amide resonance and Wiberg models for formamide.