Vibrational spectroscopic studies, conformations and ab initio calculations of 2-chloroethyl trifluorosilane

Infrared spectra of 2-chloroethyl trifluorosilane (CETFS) (ClCH2CH2SiF3) we re obtained in the vapour, amorphous and crystalline solid phases in the ra nge 4000-50 cm(-1). Additional variable temperature spectra in liquefied xe non and spectra in argon and nitrogen matrices at 5 K were recorded. Raman spectra of the compound as a Liquid were recorded at various temperatures b etween 298 and 180 K and amorphous and crystalline solids were obtained. The spectra of CETFS revealed the existence of two conformers (anti and gau che) in the vapour and in the liquid. Large variations in the infrared and Raman spectra were observed when the vapour was shock-frozen on a window at 80 K and subsequently annealed. An intermediate phase appeared at ca. 125 K containing both conformers. A crystal was formed at 160 K, and ca, 18 inf rared and/or Raman bands present both in the fluid phases and in the 125 K solid vanished in this crystal. From the intensity variations between 293 and 183 K of four Raman band pair s, DeltaH(0) (anti - gauche) = 0.8 +/- 0.3 kJ mol(-1) was obtained in the l iquid. In Liquid xenon under pressure a DeltaH value of -0.7 +/- 0.1 kJ mol (-1) was calculated from the intensity variations between 218 and 173 K of two band pairs in the infrared spectra. Annealing experiments indicate that the anti conformer has slightly lower energy in argon and nitrogen matrice s. Also! the anti conformer has the lower energy in liquid xenon, whereas t he more polar gauche rotamer is the low energy conformer in the liquid and is also present in the crystal. The spectra of both conformers have been in terpreted in detail. Ab initio calculations were performed using the GAUSSIAN 94 program with th e HF/6-311G* basis set and gave optimized geometries, infrared and Raman in tensities and scaled vibrational frequencies for the anti and gauche confor mers. The conformational energy difference derived was 3.8 kJ mol(-1) with anti being the low energy conformer. (C) 2001 Elsevier Science B.V. All rig hts reserved.