Si(6-n)C(n)H6 (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-n)C(n)H(6) (n = 0-6) series. We performed unbiased Coalescence-Kick global minimum and low-lying isomers search for the Si(6)H(6), Si(5)CH(6), Si(4)C(2)H(6), Si(3)C(3)H(6), Si(2)C(4)H(6), and SiC(5)H(6) 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 Si(6)H(6), Si(5)CH(6), and Si(4)C(2)H(6) 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 (SiC(5)H(6)) along with other factors, such as stronger C-C σ bonds compared to weaker C-Si and Si-Si σ bonds.