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

Abstract

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 π-electrons from exo-cyclic substituents. A considerable σ-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, T 1 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.

A New Approach to Design High Spin Organic Systems: Aromatic Stabilization with Exo‐cyclic π Electrons Plays Crucial Role

“…High spin organic systems can be designed using Baird's rule of 4 n electron triplet aromatics [35] . Recently it is shown that delocalized (4 n +2) π electron system can also be achieved through the back donation of electrons from the group which is exo‐cyclic to the ring [36,37] . Here, the concept of this exo‐cyclic aromaticity is categorically used to stabilize high spin systems.…”

“…The electron dot structure of the delocalizable π electrons are given in Figure S1 (Supporting Information) and the optimized bond lengths are given in Figure 1. This type of delocalization of the electron on the p z orbital of exo‐cyclic carbon atom can help the ring to gain (4 n +2) π electrons and impose aromaticity in the systems [36,37] …”