Paradigms and paradoxes: O- and N-protonated amides, stabilization energy, and resonance energy

Protonation of amides most often occurs on oxygen, unless the amide bond is twisted out of planarity allowing the more favored nitrogen protonation to occur. Protonation of oxygen is believed to take place over that of nitrogen due to increased resonance stabilization of the O-protonated species. Resonance energies for O- and N-protonated dimethylacetamide are quantitated using quantum chemically calculated proton affinities for the two sites.