Si_6-nC_nH₆ (n = 0-6) series: When do silabenzenes become planar and global minima?

In the current work we studied a structural transition from nonplanar three-dimensional structures to planar benzene-like structures in the Si _6-nC_nH₆ (n = 0-6) series. We performed unbiased Coalescence-Kick global minimum and low-lying isomers search for the Si ₆H₆, Si₅CH₆, Si₄C ₂H₆, Si₃C₃H₆, Si ₂C₄H₆, and SiC₅H₆ stoichiometries at the B3LYP/6-31G** level of theory. The lowest isomers were recalculated at the CCSD(T)/CBS//B3LYP/6-311++G** level of theory. It was shown that the pseudo-Jahn-Teller effect, which is responsible for the deformation of planar Si₆H₆, Si ₅CH₆, and Si₄C₂H₆ structures, is suppressed at n = 3 (the planar structure of 1,3,5- trisilabenzene). We also showed that the 3D-2D transition, which occurs only at n = 5, is due to the aromaticity of monosilabenzene (SiC₅H ₆) along with other factors, such as stronger C-C σ bonds compared to weaker C-Si and Si-Si σ bonds.