Conformational stability, Raman and infrared spectra, vibrational assignment, and ab initio calculations of allyltrifluorosilane

The Raman spectra (3500 to 30 cm(-1)) of allyltrifluorosilane, CH2CHCH2SiF3 , in the liquid with quantitative depolarization ratios and solid states, a nd the infrared spectra (3500 to 30 cm(-1)) of the gas and solid have been recorded. Additionally, the mid-infrared spectra of the sample dissolved in liquified xenon as a function of temperature (-100 degrees to -55 degrees C) have been recorded. All of these data indicate there are two conformers, the more stable gauche rotamer and a very small amount of the cis conforme r in the fluid states, but only the gauche form remains in the polycrystall ine solid. The variable temperature studies of the infrared spectrum of the xenon solution indicate a relatively large enthalpy difference of 354 +/- 30 cm(-1) (4.23 +/- 0.36 kJ/mol) between the conformers. The fundamental fr equencies for the asymmetric (54 cm(-1)) and SiF3 (48 cm(-1)) torsions for the gauche conformer were observed in the far infrared spectrum, and from t he SiF3 torsional frequency the barrier to internal rotation is calculated to have a value of 525 cm(-1) (6.28 kJ/mol). A complete vibrational assignm ent is presented for the gauche conformer that is consistent with the predi cted wavenumbers utilizing the force constants from ab initio MP2/6-31G* ca lculations. The optimized geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios , and vibrational wavenumbers have been obtained from RHF/6-31G* and/or MP2 /6-31G* ab initio calculations. These quantities are compared to the corres ponding experimental quantities when appropriate as well as with correspond ing results for some similar molecules.