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 S(2)N(3)(+) 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 N(2) 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-S(2)N(3)(+). Despite being thermodynamically more stable, the 1,3-isomer has only fleeting existence: its facile exothermic [3 + 2] cycloreversion into N(2) and SNS(+) fragments precludes observation. Nucleus independent chemical shifts (NICS) analysis reveals considerable six pi electron aromaticity for both cyclic S(2)N(3)(+) isomers, as well as their five membered ring valence isoelectronic analogues, N(5)(-), SN(4), and S(3)N(2)(2+). The decomposition routes and the energetics of these analogues also provide comparisons along the series.