Manifestation of exo-cyclic aromaticity in triangular heterocyclic B2F2X 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 pi-electrons from exo-cyclic substituents. A considerable sigma-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.