Si6-nCnH6 (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 Si6-nCnH6 (n = 0-6) series. We performed unbiased Coalescence Kick global minimum and low-lying isomers search for the Si6H6, Si5CH6, Si4C2H6, Si3C3H6, Si2C4H6, 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, Si5CH6, 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 (SiC5H6) along with other factors, such as stronger C-C sigma bonds compared to weaker C-Si and Si-Si sigma bonds.