Infrared and Raman spectra, conformational analysis, ab initio calculations and vibrational assignment of 2-chloroethylsilyl chloride

Abstract

The infrared (3100–40 cm −1) spectra of gaseous and solid and Raman (3200–20 cm −1) spectra of liquid 2-chloroethylsilyl chloride, ClCH 2CH 2SiH 2Cl, have been recorded. There are five possible stable conformers, Gg, Tg, Gt, Tt and Gg′ for this molecule where the capital letter G ( gauche) or T ( trans) refer to rotation around the C–C bond and the lower case letters to rotation around the Si–C bond. Most ab initio calculations at the MP2(full) level predicted the order of the stability as Tg>Gg>Gt>Tt>Gg′ whereas all density function theory calculations with the B3LYP method predicted the stability as Tg>Tt>Gg>Gt>Gg′. The four more stable conformers have been identified in the fluid phases with the Tg rotamer the only form remaining in the solid. Variable temperature (−105 to −150 °C) studies of the infrared spectra of the samples dissolved in liquid krypton have been recorded and the enthalpy differences determined to be: 50±20 (0.59±0.24 kJ/mol), 172±17 (2.06±0.20 kJ/mol) and 290±40 cm −1 (3.45±0.48 kJ/mol) for the Tg/Gg, Tg/Gt and Tg/Tt conformer pairs with the Tg conformer the most stable form. It is estimated that there is 42±2% of the Tg form, 33±1% of the Gg form, 20±2% of the Gt form, and 5±1% of the Tt form present at ambient temperature. A relatively complete vibrational assignment is proposed for the Tg conformer and many of the fundamentals have been identified for the other three (Gg, Gt and Tt) conformers based on the ab initio MP2(full)/6-31G(d) predicted frequencies, the relative infrared and Raman spectral intensities, and infrared band contours which are supported by normal coordinate calculations. Since the predicted energies between Tg and Gg′ conformers from all calculations are very large, it is not surprising that no evidence in either the infrared or Raman spectra could be found for the Gg′ conformer. The geometrical parameters, harmonic force constants, vibrational frequencies, infrared intensities, Raman activities, depolarization ratios, and energies have been obtained for all five conformers from ab initio MP2/6-31G(d) calculations. Structural parameters and energies have also been obtained utilizing several larger basis sets up to 6-311+G(3df,2pd). The results from these vibrational and theoretical studies are discussed and compared to those obtained for some similar molecules.