Microwave spectra, r0 structural parameters, and conformational stability from xenon solutions of silylcyclohexane

The FT-microwave spectrum of silylcyclohexane, c-C6H11SiH3 has been investigated from 11,000 to 21,000 MHz and 43 transitions for the 28Si isotopomer have been observed and assigned for the chair-equatorial conformer and likewise for 23 and 18 transitions for the 29Si and 30Si isotopomers, respectively. The SiHa and SiHs distances have been determined from their stretching frequencies with values of 1.486 and 1.487 Å, respectively. With these parameters and by utilizing ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r0 have been obtained. The determined heavy atom structural parameters for the most stable chair-equatorial conformer are: the distances (Å) r0(C1–C10,11) = 1.534(3); r0(C10,11–C16,17) = 1.530(3); r0(C4–C16,17) = 1.544(3); r0(C4–Si6) = 1.880(5) and the angles in degrees: C1C10,11C16,17 = 111.1(5)°; Si6C4C16,17 = 111.6(5)° with the dihedral angle C1C10,11C16,17C4 = 56.0(10)°. Variable temperature (−55 to −100 °C) studies of the infrared spectra (4000 to 400 cm−1) of silylcyclohexane dissolved in liquefied xenon have been carried out as well as the infrared spectra of the gas and solid. By measuring the temperature dependency of three conformer pairs from the spectra of xenon solutions, a standard enthalpy difference of 414 ± 19 cm−1 (1.18 ± 0.05 kcal/mol) was obtained with the chair-equatorial conformer the more stable form. At ambient temperature, the abundance of the axial conformer is 12.7 ± 1%. From MP2 ab initio calculations utilizing various basis sets, the equatorial conformer is predicted to be more stable by 525 ± 10 cm−1 from the four largest basis set calculations, which is consistent with experimental results. However, the average from the corresponding B3LYP density functional theory calculations is 671 ± 5 cm−1 which is certainly too large. A few of the previously reported vibrational assignments have been corrected. The results of these spectroscopic and theoretical studies are discussed and compared to the corresponding results for some similar molecules.