AbstractThe structures of 7‐oxo‐1‐azabicyclo[3.2.0] heptane and its 4‐oxa, 3‐ethylene‐4‐oxa, and 3‐ethylene‐6‐methyl‐4‐oxa derivatives, and of 8‐oxo‐1‐azabicyclo[4.2.0]octane and its 5‐oxa derivative, were studied by ab initio methods. Conformations were refined without constraints using the 4‐21G and the 4‐21G* basis sets, and energies and charge distributions were improved by single‐point 6‐31G*/4‐21G* calculations. The results are interpreted in terms of structural trends re