Raman and infrared spectra, conformational stability, normal coordinate analysis,ab initio calculations and vibrational assignment of 1-chloro-1-methylsilacyclobutane

Abstract

The Raman spectrum (3500–30 cm−1) of liquid 1-chloro-1-methylsilacyclobutane, c-C3H6SiCl(CH3), was recorded and quantitative depolarization values were obtained. Additionally, the infrared (3500–40 cm−1) spectra of the gas and solid were recorded. Both the axial and equatorial (with respect to the methyl group) conformers were identified in the fluid phases. Variable temperature (−55 to −100 °C) studies of the infrared spectra of the sample dissolved in liquid xenon were carried out. From these data, the enthalpy difference was determined as 178 ± 15 cm−1 (2.13 ± 0.18 kJ mol−1), with the axial conformer being the more stable form and the only conformer remaining in the polycrystalline solid. A complete vibrational assignment is proposed for the axial conformer and many of the fundamentals of the equatorial conformer were also identified. The vibrational assignments are supported by normal coordinate calculations utilizing ab initio force constants. Complete equilibrium geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities and depolarization ratios were determined for both rotamers by ab initio calculations employing the 6–31G* and 6–311++G** basis sets at the levels of restricted Hartree–Fock (RHF) and/or Moller–Plesset (MP) to second order. The results are discussed and compared with those obtained for some similar molecules. Copyright © 1999 John Wiley & Sons, Ltd.