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 (3), Cl (4)) were studied by gas-phase electron diffraction, low-temperature NMR spectroscopy, and high-level quantum chemical calculations. In the gas phase only three (3) and two (4) stable conformers differing in the axial or equatorial location of the phenyl group and the angle of rotation about the Si-CPh bond (axi and axo denote the Ph group lying in or out of the X-Si-CPh plane) contribute to the equilibrium. In 3 the ratio Pheq:Phaxo:Phaxi is 40(12):55(24):5 and 64:20:16 by experiment and theory, respectively. In 4 the ratio Pheq:Phaxo 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 3-Pheq conformer lies 0.5 kcal/mol higher than the 3-Phaxo 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 3 and 4 could be determined. The ratio of the conformers is 3-Pheq:3-Phax = (75-77):(23-25) and 4-Pheq:4-Phax = 82:18.