Manifestation of exo-cyclic aromaticity in triangular heterocyclic \(\hbox {B}_{2}\hbox {F}_{2}\)X systems (X \(=\) O, S, Se, NH)

Aromaticity is an important concept in chemistry which extends over a wide range of molecular systems and imparts unique features to the molecules possessing it. In the present work, novel heteroatomic molecular systems are proposed which demonstrate non-conventional aromaticity where the molecules accomplish the aromatic sextet and hence stabilization through the conjugation of \(\uppi \)-electrons from exo-cyclic substituents. A considerable \(\upsigma \)-aromaticity is also observed which does not involve the exo-cyclic atoms. At first, the stability of these molecular systems is theoretically ascertained through various density functional theory and ab-initio calculations along with the energy decomposition analysis, T1 diagnostic run, estimation of ring strain energy and highest occupied molecular orbital–lowest unoccupied molecular orbital gap which indicate towards the viability of these molecular systems. Then, a detailed study of aromaticity with the aid of different computational probes such as nucleus-independent chemical shift (NICS), dissected canonical molecular orbital-NICS analysis, multi-centre bond index (MCI), adaptive natural density partitioning and theoretical tools such as aromatic stabilization energy based on a fully ab-initio approach are performed which establish unique exo-cyclic aromaticity in these systems.