MNDO study of the site of protonation in methylsydnone

The electronic and geometric structures of the parent methylsydnone and its conjugate acids were studied by the semiempirical MNDO method. Protonation at the exocyclic carbonyl oxygen yields the most stable acid in agreement with some findings obtained for FSO3H–SbF5 solution. Since other experimental evidence indicates that protonation in aqueous acid occurs at the N(5) position, it appears that the influence of solvent cannot always be neglected. It is found that protonation at the endocyclic oxygen site leads to formation of an acyclic chain possessing a keten-like fragment.