The aromaticity of mesoionic compounds

Abstract

Magnetic (dissected NICS) and energetic (Indene/Iso-indene type evaluations of aromatic stabilization) criteria confirm that the closed forms of sydnone and münchnone, are indeed aromatic, comparable to benzene, furan, or pyrrole. While CH3, NH2, OH, F, CN, and NO2 substituents on nitrogen have nearly the same effects on the energy differences (ΔE) between closed and open forms of both sydnones and münchnones, all the sydnones prefer cyclic minima, whereas some of the münchnones (including the parent) favor open structures. Aromaticity stabilizes the closed forms of münchnones, while amide resonance stabilizes the open forms to a comparable extent. Sydnones prefer the closed form more than münchnones because the energy of the N=O bond of open sydnones is far less favorable than that of the C=O amide bond in open münchnones. For sydnones and münchnones, the ΔE values plot best against Taft σ inductive (rather than Hammett σ) constants.