Conformational and structural studies of ethyl fluorosilane from temperature dependent FT-IR spectra of krypton solutions and ab initio calculations

The infrared spectra (3200–30 cm^?1) of gaseous and solid ethyl fluorosilane, CH₃CH₂SiH₂F, have been recorded. Additionally, the Raman spectra (3200–30 cm^?1) of the liquid and solid have been recorded and quantitative depolarization values obtained. Both the gauche and trans conformers have been identified in the fluid phases but only the gauche conformer remains in the solid. Variable temperature (?105 to ?150°C) studies of the infrared spectra of the sample dissolved in liquid krypton have been carried out. From these data, the enthalpy difference has been determined to be 54±16 cm^?1 (646±191 J/mol) with the gauche conformer the more stable form. This is consistent with the predictions from ab initio, MP2/6-311+G(2d,2p), calculation as well as those with smaller basis sets with full electron correlations. A complete vibrational assignment is proposed for both the trans and gauche conformers based on infrared band contours, relative intensities, depolarization values, and group frequencies, which are supported by normal-coordinate calculations utilizing the force constants from MP2/6-31G(d) ab initio calculations. Complete equilibrium geometries have been determined for both rotamers by ab initio calculations employing a variety of basis sets up to 6-311+G(2d,2p) at levels of restricted Hartree–Fock (RHF) and/or Moller Plesset to the second order (MP2) with full electron correlation. The adjusted r₀ parameters have been obtained for both conformers from a combination of the previously reported rotational constants with ab initio predicted values. All results are compared to similar quantities of some corresponding molecules.