Conformations of silicon-containing rings. Part 4. Gas-phase structure of 1,3,5-trisilacyclohexane and comparison with cyclohexane and cyclohexasilane

Abstract

The gas-phase structure of 1,3,5-trisilacyclohexane 1 has been determined by gas electron diffraction (GED). The experimental structure is compared to the result of quantum chemical (QC) calculations (HF, MP2 and B3LYP) using the 6-31G∗ basis set. It is found that all three QC methods give similar results. The Si–C–Si bond angle is found by experiment/B3LYP calculation to be 113.0(4)/113.5°. whereas the C–Si–C bond angle is 110.7(14)/110.1° The large Si–C–Si bond angle makes 1 rather flat with an endocyclic dihedral angle of 53.7(4)/53.8° compared with 55.1(7)/54.7 and 57.9(9)/56.0° for cyclohexane and cyclohexasilane, respectively. The Si–C bond length is found to be 1.872(1)/1.893Å. It is shown that HF, MP2 and B3LYP calculations using the 6-31G∗ basis set commonly predict Si–C and Si–Si bond lengths in small molecules to be about 0.02Å longer than experimental values. The energy difference between the chair and twisted boat conformers of 1 is calculated to be 2.2kcalmol−1 compared to 6.5 and 1.9kcalmol−1 for cyclohexane and cyclohexasilane, respectively.