Exploration on stability, aromaticity, and potential energy surface of planar BnC2 (n = 3–8)

We perform a comprehensive investigation on the geometry, stability, aromaticity, bonding nature, and potential energy surface of low-lying isomers of planar BnC2 (n = 3–8) at the CCSD(T)/6-311+G(d)//B3LYP/6-311+G(d) level. The geometries of lowest-energy isomers of CnB2 (n = 3–8) clusters are similar to the results of Wang and Zeng, respectively. CnB2 (n = 3–6) clusters are analogous to pure boron clusters in structure. Interestingly, the lowest-energy isomers of BnC2 (n = 3–8) clusters undergo polycyclic to wheel-type structure transition as the number of boron atom increases. Energy analysis reveals that BnC2 clusters with even n have relatively higher stability. The valence molecular orbital (VMO), electron localization function (ELF), and Mayer bond order (MBO) are used to reveal the bonding feature of BnC2 (n = 3–8) isomers. The aromaticity of B6C2 and B8C2 is discussed in terms of VMO, ELF, adaptive natural density partitioning (AdNDP), and nucleus-independent chemical shift (NICS) analyses. Some B3C2 and B5C2 isomers with large thermodynamic and kinetic stability are predicted at the CCSD(T)/6-311+G(d)//B3LYP/6-311+G(d) level and observable in laboratory.