Vibrational spectroscopic studies, conformations and ab initio calculations of 3,3,3-trifluoropropyltrichlorosilane

Abstract

Infrared spectra of 3,3,3-trifluoropropyltrichlorosilane (CF 3CH 2CH 2SiCl 3) were obtained in the vapour, amorphous and crystalline solid phases in the range 4000–50 cm −1. Additional spectra in argon matrices at 5.0 K were recorded before and after annealing to 20–36 K. Raman spectra of the compound as a liquid were recorded at various temperatures between 298 and 210 K and spectra of the amorphous and crystalline solids were obtained. The spectra suggested the existence of two conformers ( anti and gauche) in the fluid phases and in the matrix. When the vapour was shock-frozen on a cold finger at 80 K and subsequently annealed to 120–150 K, six weak or very weak Raman bands vanished in the crystal. Similar variations were observed in the corresponding infrared spectra after annealing and four very weak IR bands disappeared after crystallization. From intensity variations between 298 and 210 K of three Raman band pairs an average value Δ conf H° ( gauche–anti) = 6.1 ± 0.5 kJ mol −1 was obtained in the liquid. Annealing experiments indicate that the anti conformer also has a lower energy in the argon matrices. The conformational equilibrium is highly shifted towards anti in the liquid, and the low energy conformer also forms the crystal. The spectra of the abundant anti conformer and the few bands ascribed to the gauche conformer have been interpreted. Ab initio calculations at the HF/6-311G ** and B3LYP/6-311G ** gave optimized geometries, infrared and Raman intensities and vibrational frequencies for the anti and gauche conformers. The conformational energy differences derived were 11.8 and 9.2 kJ mol −1 from the HF and the B3LYP calculations, respectively.