Exploration of structures and stability of planar CnB3 (n = 1–8)

The structures and relative stabilities of C_nB₃ (n = 1–8) clusters are explored and analyzed at the CCSD(T)/6-311+G(d)//B3LYP/6-311+G(d) level. For low-lying isomers of C_nB₃ (n = 1–8), cyclic structures are more favorable in energy than branch as well as linear structures, and rings with exocyclic chains. For most isomers of C_nB₃ (n = 1–8), the structures with doublet states are lower in energy than the ones with quartet states. The lowest-energy isomers of C_nB₃ are all cyclic structures. The lowest-energy structures of C_nB₃ (n = 1–3) appear to be similar to those of pure boron clusters which have polycylic structures, while the lowest-energy structures of C_nB₃ (n = 4–8) tend to be analogous to the corresponding geometries of pure carbon, C_nB, and C_nB₂ clusters which possess single-ring structures. The most stable structures of planar C_nB₃ (n = 3–8) clusters can be derived from combining a B–C–B–C–C–B building unit and a carbon chain. Some physical properties, such as the binding energy per atom, incremental binding energy, and second order energy difference suggest that for C_nB₃ clusters, odd-n clusters are more stable than even-n clusters. Results from NBO and molecular orbital analysis illustrate that the formation of the delocalized π MOs, the σ-radial and σ-tangential MOs are favorable to the stabilization of structures for lowest-energy isomers. It is interesting to find from the valence molecular orbital and NBO analyses that for the most stable isomers of C_nB₃ (n = 1–8), the numbers of π electrons for C_nB₃ (n = 1–8) is (n + 1) with an exception of C₄B₃ (6 π electrons), suggesting that CB₃, C₄B₃ and C₅B₃ may have aromaticity, which is consistent with the results of the nucleus independent chemical shifts (NICS).