Oxiranones: alpha-lactones or zwitterions? Insights from calculated electron density distribution analysis

Electron density distributions for oxiranone and hydroxyoxiranone have been analysed in vacuo [MP2/6-31+G(d,p)] and in water [SCI-PCM/MP2/6-31+G(d,p)/ /HF/6-31+G(d,p)] and compared with those for cyclopropane, cyclo-propanone, and oxirane. Oxiranone possesses a ring critical point in vacuo, and may b e considered as an alpha -lactone with considerable ionic character in the endocyclic C-alpha-O-n bond. In water, oxiranone has neither a ring critica l point nor a bond critical point for C-alpha-O-n, and may be considered as a zwitterion, whose carboxylate group has a net charge of -0.63. Geometric ally, however, the molecule still possesses an acute-angled three-membered ring with a CalphaCOn angle of only 69 degrees. Electronically, hydroxyoxir anone is acyclic and zwitterionic even in vacuo, but geometrically it still looks like an alpha -lactone.