Conformational properties of 1-fluoro-1-methyl-silacyclohexane and 1-methyl-1-trifluoromethyl-1-silacyclohexane: Gas electron diffraction, low-temperature NMR, temperature-dependent Raman spectroscopy, and quantum chemical calculations

Two new 1,1-disubstitued silacyclohexanes, C₅H ₁₀SiFCH₃ (1) and C₅H₁₀SiCF ₃CH₃ (2) were synthesized. The molecular structure of their axial and equatorial conformers as well as the thermodynamic equilibrium between these species was investigated by means of gas electron diffraction (GED), dynamic nuclear magnetic resonance (DNMR), temperature-dependent Raman spectroscopy, and quantum chemical calculations (CCSD(T), MP2, and DFT methods). 1a, 2a and 1e, 2e are used to denote the conformers having the CH₃ group in axial and equatorial positions, respectively. According to GED, both compounds exist as a mixture of two conformers possessing the chair conformation of the six-membered ring and C_s symmetry and differing in the axial or equatorial position of the two substituents (axial-CH₃:equatorial- CH₃ ratio of45(6)%:55(6)%and 51(5)%:49(5)% was found for 1 and 2, respectively). Hence, G_ax-G_eq = 0.11(13) kcal - mol ^-1 for 1, whereas 2a and 2e have virtually the same free energy. Low-temperature ¹⁹F NMR experiments resulted in G_ax-G _eq = 0.26(2) kcal mol^-1 at 125 K for 1 and G _ax-G_eq = 0.36(2) kcal mol^-1 at 118 K for 2. Temperature-dependent Raman spectroscopy in the temperature range of 210-300 K of the neat liquids and their solutions in THF and hexane indicates that 1e and 2e are favoured over 1a and 2a by 0.50(15) and 0.73(15) kcal mol^-1, respectively (AH values). The Raman results seem not to depend on the polarity of the medium. CCSD(T)/CBS calculations at the NMR temperatures predict G _ax-G_eq = 0.28 and 0.36 kcal mol^-1 for 1 and 2, respectively, and are thus in excellent agreement with the DNMR results. The agreement of CCSD(T)/CBS with the GED results is slightly worse, predicting G_ax-G_eq = 0.31 and 0.22 kcal mol^-1 for 1 and 2, respectively. The CCSD(T)/CBS calculations are also in slight disagreement with the Raman results, predicting AH values of 0.25 and 0.48 kcal mol^-1 for 1 and 2, respectively. The CCSD(T)/CBS calculations of both mono- and disubstituted silacyclohexanes with F, CH₃, and CF₃ substituents revealed a remarkable good additivity of substituent effects, which is not shown by the analogous cyclohexanes.