Vibrational spectroscopic studies, conformations and ab initio calculations of 1,1,1-trifluoropropyltrifluorosilane

Infrared spectra of 1,1,1‐trifluoropropyltrifluorosilane (CF3CH2CH2SiF3) were obtained in the vapour, amorphous and crystalline solid phases in the range 4000–50 cm−1. Additional spectra in argon and nitrogen matrices at 4.8 K were recorded before and after annealing to temperatures of 20–36 K. Raman spectra of the compound as a liquid were recorded at various temperatures between 298 and 163 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 vapour was shock‐frozen on a cold‐finger at 78 K and subsequently annealed to 120–150 K, 12 mostly weak Raman bands vanished in the crystal. Much smaller variations were observed in the corresponding infrared spectra after annealing. However, the spectra indicated the existence of two different crystals, formed after annealing the amorphous solid to 120 and to 150 K; both crystals contained the same conformer (anti). Raman and IR spectra of 1,1,1‐trifluoropropyltrifluorosilane in cyclohexane, carbon tetrachloride and acetonitrile revealed conformational displacements with solvent polarity. From intensity variations between 298 and 223 K of the Raman pair 639/665 cm−1, ΔH° (gauche–anti) = 3.4 ± 0.3 kJ mol−1 was obtained in the liquid. Annealing experiments indicate that the anti conformer also has a lower energy in argon and nitrogen matrices; moreover, the low‐energy conformer is present in the crystal. The spectra of both conformers have been interpreted in detail. Ab initio calculations at the HF/6–311(d), HF/6–311 G (3df,3pd) and MP2/G–31(d) levels gave optimized geometries, infrared and Raman intensities and scaled vibrational wavenumbers for the anti and gauche conformers. The conformational energy difference derived was 5.3 kJ mol−1 with anti being the low‐energy conformer. Copyright © 2003 John Wiley & Sons, Ltd.