An ab initio study of the valence tautomerism of type B mesoionic rings

Abstract

The MP2 calculated Gibbs free energies of a series of type B mesoionic rings and their acyclic valence tautomers suggest that in the gas phase the relative stability of the mesoionic ring increases with single bond strength (S-S > RN-NR > O-O). Inclusion of aqueous solvation in the calculations further favours the stability of the mesoionic ring by ~10-15 kcal mol-1. Replacement of CR groups at ring positions by nitrogen atoms results in a significant increase in the relative stability of the mesoionic ring. Calculations of aromatic stabilization energy (ASE), together with the Bird aromaticity index (I5), suggest that aromaticity decreases with aza substitution and the increase in relative stability is attributable to charge stabilisation by the electronegative nitrogen atoms, which more than compensates for any loss of aromaticity.