Hidden in plain sight: Aromaticity of hexagonal boron nitride

Hexagonal boron nitride (hBN) is of interest for a wide range of applications in electronics, photonics, catalysis, and quantum information science. Despite this broad interest and being isoelectronic analog of graphene, it is not known if hBN is aromatic. Aromaticity (or, lack thereof) of a planar structure is related to its structural and chemical stability, and hence, its determination is of fundamental importance to different applications. With the help of energetic and magnetic criteria, we determine the aromaticity of hBN flakes of different sizes. We show that while the graphene flakes are strongly aromatic, the hBN flakes are at the best weakly aromatic. Unlike the robust global diatropic ring currents seen in the graphene flakes, the hBN flakes display only a weak global diatropic current flowing thorough the outer π-ring, along with local diatropic currents centered at the nitrogen atoms. Ultimately, the weaker aromaticity of hBN flakes as compared to graphene flakes can be linked to the differences in their symmetry-allowed transitions between the occupied and unoccupied molecular orbitals.