A DFT and ab initio study of conjugated and semi-conjugated mesoionic rings and their covalent isomers

The properties of type A and type B mesoionic diazolium olates, their fully covalent isomers and their aza derivatives have been investigated using the B3LYP/6-311++G(d,p) and MP2/aug-cc-pVDZ methods. The results, together with those for six-membered heterocyclic mesomeric betaines (HMBs), are in agreement with a revised classification of HMBs based on connectivity-matrix analysis. The calculated properties of the type B mesoionic rings are shown to be consistent with their classification as semi-conjugated HMBs; they have properties comparable to six-membered semi-conjugated HMBs including high classical aromaticity. The type A mesoionic rings are conjugated HMBs and have quite different properties including lower classical aromaticity. Magnetic aromaticity of both type A and type B mesoionic rings, measured by NICS indices, is low compared to imidazole and pyrazole. The results support the view that semi-conjugated HMBs should be recognized as a discrete class of dipolar heterocycle.

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