A number of cyclo-and bicyclosilanes have been prepared and structurally characterized by X-ray crystallography and Raman spectroscopy. 1,1,4,4-and 1,1,3,3-tetrakis(trimethylsilyl)octamethylcyclohexasilanes were found to exhibit unusual twist-and twisted boat-conformations. The UV absorption properties of all compounds were studied and found to show absorption maxima red shifted compared to the parent compound dodecamethylcyclohexasilane.

The molecular structures of axial and equatorial conformers of 1-fluorosilacyclohexane, C 5 H 10 SiHF, 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 (∆H values), respectively.

From variable temperature vibrational Raman spectra, the axial/equatorial enthalpy differences for the substituted silacyclohexanes C5H10SiHMe, C5H10SiH(CF3) and C5H10SiCl(SiCl3) were determined. The pure liquids and solutions in various solvents were investigated. Preferred conformations are equatorial for methylsilacyclohexane and axial for trifluoromethylsilacyclohexane, consistent with earlier results from nuclear magnetic resonance experiments and ab initio calculations. For C5H10SiCl(SiCl3) an enthalpy difference close to zero was found, which is supported by high‐level which is supported by high‐level quantum chemical calculations at the second‐order Møller‐Plesset (MP2) and coupled cluster with single, double, and perturbative triple excitations (CCSD(T)) levels, which employed various basis sets. A novel synthesis for C5H10SiCl(SiCl3) was developed using ClMg(CH2)5MgCl instead of BrMg(CH2)5MgBr as a starting material. The procedure avoids the formation of partially brominated products, facilitating the purification of the compound. 1H, 13C and 29Si nuclear magnetic resonance data are reported. Copyright © 2012 John Wiley & Sons, Ltd.

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