Molecular‐orbital calculations on some mesoionic compounds

Molecular‐orbital calculations have been performed on sydnone, sydnone imine, ψ‐oxatriazole, and isosydnone rings and their positive ionized forms by the Pariser–Parr–Pople–Brown–Heffernan method. The importance of elaborate open‐shell calculations and the effect of introducing nonneighbor core resonance integrals in this order of approximation are investigated. The available experimental data on closely related compounds are qualitatively in agreement with the calculated values. The computed charges and bond orders demonstrate that certain structural features are common to all five‐membered mesoionic ring systems and in agreement with recent experimental findings.