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Keywords:

  • vibrational spectra;
  • ab initio calculations;
  • 3,3,3-trifluoropropylsilane;
  • conformational analysis

Abstract

Infrared spectra of 3,3,3-trifluoropropylsilane (CF3CH2CH2SiH3) and 3,3,3-trifluoropropylsilane-d3 (CF3 CH2CH2SiD3) were obtained in the vapour, liquid, and crystalline solid phases in the range 4000–50 cm−1. Additional spectra in argon matrices at 5 K were recorded before and after annealing to temperatures 20–34 K. Raman spectra of the compounds as liquids were recorded at various temperatures between 296 and 183 K and spectra of the amorphous and crystalline solids were obtained.

The spectra revealed the existence of two conformers (anti and gauche) in the fluid phases and in the matrices. When the two vapours were shock-frozen on a cold finger at 78 K, they turned partly crystalline immediately. After subsequent annealing to 140–150 K, ca 7–9 Raman bands of both molecules present in the liquids vanished in the crystal. Similar variations in intensity were observed in the corresponding infrared spectra before and after annealing. The spectra revealed the existence of one conformer (anti) in the crystal.

From Raman intensity variations of three independent pairs of anti and gauche bands between 298 and 173 K for the parent compound, and 298 and 183 K for the deuterated analogue, the values ΔconfHo(gaucheanti) = 4.1 ± 0.3 kJ mol−1 for the parent compound and the same value for the deuterated species were obtained in the liquid state. Annealing experiments in the matrices show that the gauche bands vanish after annealing, demonstrating that the anti conformer also has the lower energy here and that the barrier to gauche [RIGHTWARDS ARROW] anti inter-conversion is around 5–6 kJ mol−1. The spectra of both conformers have been interpreted in detail.

Ab initio and DFT calculations at the HF/6–311G**, B3LYP/6–311 G** and MP2/6–311 G** levels gave optimized geometries, infrared and Raman intensities and vibrational wavenumbers for the anti and gauche conformers. The conformational enthalpy difference derived from the calculations was between 6.0 and 4.1 kJ mol−1 with anti being the low energy conformer. Copyright © 2006 John Wiley & Sons, Ltd.