Infrared and Raman spectra, conformational stability, ab initio calculations, and vibrational assignments for cyclopropyldichlorosilane

Abstract

The infrared (3200–30 cm −1) spectra of gaseous and solid cyclopropyldichlorosilane, c-C 3H 5SiCl 2H, have been recorded. Additionally, the Raman spectra (3200–30 cm −1) of the liquid with quantitative depolarization values and the solid have been recorded. Both the cis and gauche conformers have been identified in the fluid phases but only the gauche conformer remains in the solid. Variable temperature (−55 to −100°C) studies of the infrared spectra of the sample dissolved in liquid xenon have been carried out. From these data the enthalpy difference has been determined to be 99±7 cm −1 (283±20 cal mol −1), with the gauche conformer being the more stable rotamer, which is in agreement with the predictions from ab initio calculations with electron correlation. A complete vibrational assignment is proposed for both conformers based on infrared band contours, relative intensities, depolarization values and group frequencies. The vibrational assignments are supported by normal coordinate calculations utilizing the force constants from ab initio MP2/6-31G * calculations. Utilizing the frequencies of the Si–H stretch, the Si–H bond distances of 1.474 and 1.477 Å have been obtained for the gauche and cis conformers, respectively. Complete equilibrium geometries have been determined for both rotamers by ab initio calculations employing the 6-31G * and 6-311+G ** basis sets at levels of restricted Hartree–Fock (RHF) and/or Møller–Plesset (MP) to second order. The potential energy terms for the conformer interconversion have been obtained from the MP2/6-31G * calculation. The results are discussed and compared to those obtained for some similar molecules.