A DFT Study of Mono-, Bi- and Tricyclic Examples of Semi-conjugated Heterocyclic Mesomeric Betaines

The properties of heterocyclic mesomeric betaines (HMBs) are determined by the connectivity between the ring 2 π heteroatoms and the odd conjugated fragments that complete the ring. In contrast to conjugated HMBs (1,3-dipoles) and cross-conjugated HMBs (1,4-dipoles), semi-conjugated HMBs are a class of heterocycle that is unexplored. The only known example is 1,4-dimethyl-1,2,4,5-tetrazinium-3,6-diolate. In this study density functional theory (DFT) methodology is employed to further investigate the structures and properties of examples of mono- bi-and tricyclic semi-conjugated HMBs. A common feature associated with the ‘semi - conjugation’ is a small energy gap between the frontier orbitals