Conformational properties of 1-fluoro-1-silacyclohexane, C5H10SiHF:: Gas electron diffraction, low-temperature NMR, temperature-dependent Raman spectroscopy, and quantum chemical calculations

The molecular structures of axial and equatorial conformers of 1-fluorosilacyclohexane, C5H10SiHF, as well as the thermodynamic equilibrium between these species were investigated by means of gas electron diffraction (GED), dynamic nuclear magnetic resonance, temperature-dependent Raman spectroscopy, and quantum chemical calculations (MP2, DFT, and composite methods). According to GED, the compound exists in the gas phase 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 Si-F bond (axial = 63(8) mol %/equatorial = 37(8) mol %) at T = 293 K, corresponding to an A value of -0.31(20) kcal mol(-1). Density functional theory (DFT) calculations were employed to obtain the minimal energy path of the conformational inversion. The MP2, G3B3, and CBS-QB3 methods were also employed to calculate the equilibrium geometries and energies of the local minima in the gas phase and in solution. The gas-phase results are in good agreement with the experiment, whereas a combined PCM/IPCM(B3LYP/6-311G(d)) approach overestimates the stabilization of the axial conformer by 0.3-0.4 kcal mol(-1) in solution at 112 K. Temperature-dependent Raman spectroscopy in the temperature ranges of 210-300 K (neat liquid), 120-300 K (pentane solution), and 200-293 K (dichloromethane solution) also indicates that the axial conformer is favored over the equatorial one by 0.25(5), 0.22(5), and 0.28(5) kcal mol(-1) (Delta H values), respectively.