Adaptive σ Aromaticity and Triplet Ground State in Tetraatomic Boron Species

In comparison with
the widely recognized π aromaticity, σ
aromaticity is a less developed concept in chemistry, especially for
unsaturated systems. Moreover, most studies on σ aromaticity
have been mainly limited to the ground state of saturated systems;
unsaturated species with σ aromaticity in the excited state
have never been reported. Here we demonstrate that a recently synthesized
tetraatomic boron species is σ aromatic not only in the lowest
singlet state but also in the lowest triplet state according to density
functional theory (DFT) calculations, supported by numerous aromaticity
indices, such as nucleus-independent chemical shift (NICS), anisotropy
of the induced current density (ACID), and electron density of delocalized
bonds (EDDB). In addition, several isomers are predicted to possess
a triplet ground-state character. Our findings not only highlight
the importance of developing the concept of σ aromaticity in
unsaturated systems, which are traditionally reserved as the domain
of π aromaticity, but also extend the concept of adaptive aromaticity
(being aromatic in both the lowest singlet and triplet states) into
the σ system.