Spectra and structure of small ring compounds. Part LVII —Raman and infrared spectra, vibrational assignment and ab initio calculations of cyclopropyltrifluorosilane

The Raman (3500–20 cm −1 ) and infrared (3500–40 cm −1 ) spectra of gaseous and solid cyclopropyltrifluorosilane have been recorded. Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization ratios have been obtained. From these data a complete assignment of the normal vibrational modes is provided. The fundamental vibrational frequencies have also been calculated from ab initio Hartree‐Fock calculations with the 3–21G* basis set. The force field obtained from these calculations has been used to carry out a normal coordinate analysis with a chosen set of symmetry coordinates. The molecular structure and potential surface governing internal rotation of the SiF 3 moiety have been calculated using both the 3–21G* and 6–31G* basis sets. Additionally, the SiF 3 torsional mode has been observed in the far‐infrared spectrum of the gas as a broad band centered at 46 cm −1 and from this frequency a periodic threefold barrier to internal rotation has been determined to be 420 cm −1 (1.2 kcal mol −1 ). These results are compared with corresponding quantities for some similar molecules.