Compared to the well-known conjugated (1,3-dipolar) and
cross-conjugated
(1,4-dipolar) heterocyclic mesomeric betaines (HMBs), semi-conjugated
HMBs are unexplored and almost unknown. The three discrete classes
of HMB are defined by the connectivity between their ring 2π
heteroatoms and the odd-conjugated fragments that complete the ring.
A single example of a stable, fully-characterized semi-conjugate HMB
has been reported. This study employs the density functional theory
(DFT) methodology to investigate the properties of a series of six-membered
semi-conjugated HMBs. The electronic character of ring substituents
is found to significantly influence the structure and electronic properties
of the ring. The aromaticity measured by HOMA and NICS(1)zzindices is increased by π-electron-donating substituents whereas
π-electron-withdrawing substituents decrease the calculated
aromatic character and ultimately lead to non-planar boat or chair
structures. A notable property of all derivatives is the small energy
gap between their frontier orbitals.