THE RESONANCE ENERGY OF AMIDES, THE STRUCTURE OF AZIRIDINONE, AND ITS RELATIONSHIP TO OTHER STRAINED LACTAMS

The resonance energy of amides (lactams) is calculated both with and without inclusion of the inversion barrier of model amines. Inclusion of the barrier yields a larger resonance energy for amides than for esters, whereas the reverse is true if the barrier is not included. This is consistent with intuitive views related to electronegativity. The high inversion barrier in aziridine and a lower intrinsic resonance stabilization in aziridinone explain nonplanar geometry at nitrogen in alpha-lactams. A surprisingly good correlation is seen when one plots the difference in carbonyl frequencies of strained lactams (amides) and ketones versus the enthalpy differences between two olefin analogues to the corresponding lactam (amide) resonance contributors. This correlation implies the need to invoke resonance arguments to understand strained lactams. The deviation of aziridinone from the correlation is explicable in terms of its nonplanar structure and its deviation from planarity.