Ab initio molecular orbital calculations optimized at the 6-31G * level have been performed for aziridinone, 2-azetidinone and acetamide and their O- and N-protonated structures. Aziridinone is calculated to have a strongly pyramidal nitrogen and only 12.7 kcal mol - 1 of ''operational'' resonance energy compared with the 20.5 kcal mol - 1 of ''operational'' resonance energy in acetamide. The very low N-inversion barrier for aziridinone is associated with the 27.9 kcal mol - 1 of ''pure'' resonance energy calculated in planar aziridinone, the N-inversion transition state. This is further supported by the exceedingly short C-N bond (1.302 a) which is reminiscent of cyclopropene. For comparison, the ''operational'' and ''pure'' resonance energies of 2-azetidinone are 19.2 and 24.9 kcal mol - 1 respectively and the ''pure'' resonance energy of acetamide is 26.5 kcal mol - 1 . 2-Azetidinone is O-protonated (anti to N) but aziridinone protonates at nitrogen to generate a ring-opened acylium ion. It appears that ring-closed and -opened N-protonated aziridinones exist as isomers and this is consistent with mass spectral determinations of the corresponding phenyl derivatives. Structural and energy data and core eigenvalues (N, O and carbonyl C) are given for aziridinone, 2-azetidinone, acetamide, two earlier-studied bridgehead bicyclic lactams (1-azabicyclo[2.2.2]octan-2-one (''2-quinuclidone'') and 1-azabicyclo[3.3.1]nonan-2-one) and their protonated derivatives.