AZIRIDINONE AND 2-AZETIDINONE AND THEIR PROTONATED STRUCTURES - AN AB-INITIO MOLECULAR-ORBITAL STUDY MAKING COMPARISONS WITH BRIDGEHEAD BICYCLIC LACTAMS AND ACETAMIDE

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 hav e a strongly pyramidal nitrogen and only 12.7 kcal mol(-1) of ''operat ional'' resonance energy compared with the 20.5 kcal mol(-1) of ''oper ational'' resonance energy in acetamide. The very low N-inversion barr ier 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 Angstrom) which is reminiscent of cyclopropene. For com parison, the ''operational'' and ''pure'' resonance energies of 2-azet idinone are 19.2 and 24.9 kcal mol(-1) respectively and the ''pure'' r esonance energy of acetamide is 26.5 kcal mol(-1). 2-Azetidinone is O- protonated (anti to N) but aziridinone protonates at nitrogen to gener ate a ring-opened acylium ion. It appears that ring-closed and -opened N-protonated aziridinones exist as isomers and this is consistent wit h 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-stu died bridgehead bicyclic lactams (1-azabicyclo[2.2.2]octan-2-one (''2- quinuclidone'') and 1-azabicyclo[3.3.1]nonan-2-one) and their protonat ed derivatives.