On the structures, stabilities, and potential energy surfaces of planar BnN (n = 1–6) clusters

The structures, stabilities, and potential energy surfaces (PESs) of the low-lying isomers of planar BnN (n = 1–6) clusters are searched at the CCSD(T)/6-311+G(d)//B3LYP/6-311+G(d) level. The lowest-energy structures (1a–6a) of planar BnN (n = 1–6) are located. It is worthy to note a structural transition occurring in the series of BnN (n = 1–6) clusters upon increase of the boron content from n = 3 to n = 4. The evolution of the binding energy per atom, incremental binding energy, and second order difference of total energy with the size of BnN reveals that the lowest-energy isomers 2a (B2N) and 5a (B5N) are highly stable. Results demonstrate that the stability of lowest-energy isomers of BnN (n = 1–6) is attributed to the delocalized π, σ-radial, and σ-tangential molecular orbitals (MOs) interactions. The isomers of BnN (n = 4–6) with planar multicoordinate boron or nitrogen are unfavorable in energy. The double aromaticity of isomer 3d of B3N is discussed in terms of valence molecular orbital, electron localization function (ELF) and adaptive natural density partitioning (AdNDP), and NICS analyses. Interestingly, some isomer pairs can be transferred directly to each other by two or three isomerization channels. Some isomers of BnN with large thermodynamic and kinetic stability are predicted in terms of their total energies and potential energy surfaces, which is important for future experimental studies.