Oxiranones: α-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, cyclopropanone, and oxirane. Oxiranone possesses a ring critical point in vacuo, and may be considered as an α-lactone with considerable ionic character in the endocyclic C_α-O_n bond. In water, oxiranone has neither a ring critical point nor a bond critical point for C_α-O_n, and may be considered as a zwitterion, whose carboxylate group has a net charge of -0.63. Geometrically, however, the molecule still possesses an acute-angled three-membered ring with a C_αCO_n angle of only 69°. Electronically, hydroxyoxiranone is acyclic and zwitterionic even in vacuo, but geometrically it still looks like an α-lactone.