A study of the structure, bonding and aromatic character of 1,3-dihydro-1,3-diazocines: Class 1A heterocyclic mesomeric betaines and amidinium inner salts

Abstract 1,3-Dihydro-1,3-diazocines and their aza derivatives are eight-membered 10π heterocycles that are Class 1A heterocyclic mesomeric betaines (HMBs); members of this class are formally associated with two discrete 1,3-dipolar fragments. DFT calculations are consistent with these structures being planar with a high degree of classical aromaticity measured by the HOMA aromaticity index and aromatic stabilisation energy (ASE). Calculated NICS(1) zz values and electron current density maps are consistent with a high degree of cyclic conjugation and high magnetic aromaticity. DFT Calculated NPA atomic charges suggest that these molecules can be considered as amidinium inner salts but no derivatives of these fundamental heteroaromatic ring systems have been reported. There is good linear correlation between calculated HOMO energy and VIP in accord with Koopmans’ theorem. A lack of correlation between LUMO and VEA is attributed to the LUMOs being σ orbitals located on only a fragment of the whole molecule.