Ab Initio Study of the Geometry, Stability, and Aromaticity of the Cyclic S2N3+ Cation Isomers and Their Isoelectronic Analogues

A theoretical study of the geometries, energies, dissociation pathways, and aromaticity of the isomeric sulfur−nitrogen S2N3+ rings reveals that the experimentally known 1,2-isomer is only stable kinetically. A rather high barrier inhibits its dissociation into the slightly lower energy N2 and NSS+ fragments via a stepwise mechanism. A second possible dissociation mode, into NNS and NS+ via a concerted [3 + 2] mechanism, is endothermic. Instead, the reverse cycloaddition reaction has a low barrier and offers an exothermic route for the formation of cyclic 1,2-S2N3+. Despite being thermodynamically more stable, the 1,3-isomer has only fleeting existence: its facile exothermic [3 + 2] cycloreversion into N2 and SNS+ fragments precludes observation. Nucleus independent chemical shifts (NICS) analysis reveals considerable six π electron aromaticity for both cyclic S2N3+ isomers, as well as their five membered ring valence isoelectronic analogues, N5−, SN4, and S3N22+. The decomposition routes and the energeti...