Heterocyclic mesomeric betaines: the recognition of five classes and nine sub-classes based on connectivity-matrix analysis

The relationship between heterocyclic mesomeric betaines (HMBs) isoconjugate with hydrocarbon dianions and pairs of alternant hydrocarbon fragments is recognised. An analysis of the interactions of these alternant hydrocarbon fragments with the pairs of associated 2π heteroatoms (NR, O, S) leads to the definition of seven connectivity matrices, which in turn identify five discrete classes of HMB (Classes 1–5). Two of these classes, which we describe as semi-conjugated and pseudo-semi-conjugated HMBs, have not previously been recognised. Further examination of the connectivity matrices leads to the recognition of three sub-classes of conjugated (Class 1A–C), cross-conjugated (Class 2A–C) and pseudo-cross-conjugated (Class 4A–C) HMBs. Using this approach similarities and differences in a vast and diverse area of heterocyclic chemistry can be identified and new areas of potential interest identified.