Molecular Structure and Conformational Analysis of 1‑Phenyl-1-X-1-Silacyclohexanes (X = F, Cl) by Electron Diffraction, Low-Temperature NMR, and Quantum Chemical Calculations

The molecular structure and conformational preferences of 1-phenyl-1-X-1-silacyclohexanes C₅H₁₀Si­(Ph,X) (X = F (<b>3</b>), Cl (<b>4</b>)) were studied by gas-phase electron diffraction, low-temperature NMR spectroscopy, and high-level quantum chemical calculations. In the gas phase only three (<b>3</b>) and two (<b>4</b>) stable conformers differing in the axial or equatorial location of the phenyl group and the angle of rotation about the Si–C_Ph bond (ax<i>i</i> and ax<i>o</i> denote the Ph group lying in or out of the X–Si–C_Ph plane) contribute to the equilibrium. In <b>3</b> the ratio Ph_eq:Ph_ax<i>o</i>:Ph_ax<i>i</i> is 40(12):55(24):5 and 64:20:16 by experiment and theory, respectively. In <b>4</b> the ratio Ph_eq:Ph_ax<i>o</i> is 79(15):21(15) and 71:29 by experiment and theory (M06-2X calculations), respectively. The gas-phase electron diffraction parameters are in good agreement with those obtained from theory at the M06-2X/aug-ccPVTZ and MP2/aug-cc-pVTZ levels. Unlike the case for M06-2X, MP2 calculations indicate that <b>3</b>-Ph_eq conformer lies 0.5 kcal/mol higher than the <b>3</b>-Ph_ax<i>o</i> conformer. As follows from QTAIM analysis, the phenyl group is more stable when it is located in the axial position but produces destabilization of the silacyclohexane ring. By low-temperature NMR spectroscopy the six-membered-ring interconversion could be frozen at 103 K and the present conformational equilibria of <b>3</b> and <b>4</b> could be determined. The ratio of the conformers is <b>3</b>-Ph_eq:<b>3</b>-Ph_ax = (75–77):(23–25) and <b>4</b>-Ph_eq:<b>4</b>-Ph_ax = 82:18