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.
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The help of Professor Joop van Lenthe and Dr. Pinchas Aped with the TURTLE code is gratefully acknowledged. This work was supported by a grant from the Israel Science Foundation (484/95).