Octabetaines: a DFT Study of Unexplored Eight-Membered 10π Heterocycles

Eight-membered 10π heterocyclic mesomeric betaines (HMBs) are unusual in that they are associated with two 1,3-dipolar fragments in mutual conjugation; they are conjugated HMBs belonging to Class 1A. Apart from the characterization of five derivatives of the 1,5-dithia-2,4,6,8-tetrazocine ring in the 1980s, this large family of "aromatic" heterocycles has received no attention. The density functional theory (DFT) study reported here investigates the structure, aromaticity, and bonding of representative examples. The parent structures and aza derivatives are planar, and bond lengths are in agreement with a reported crystal structure. Calculated HOMA aromaticity index values and aromatic stabilization energy (ASE) are consistent with high classical aromaticity. Their magnetic aromaticity, measured by the NICS(1) index and π electron current density maps, is also high. The introduction of electron-donating ring substituents (Me, OH, and NH₂) results in distortion from planarity. The energy values of the frontier orbitals, vertical ionization potentials (VIPs), and vertical electron affinities (VEAs) are reported. Their significant variation with position of aza substitution is rationalized by a perturbation model based on the frontier orbitals of the cyclooctatetraene dianion.