Infrared and Raman spectra, conformational stability, and ab initio calculations of cyclobutyl trifluorosilane

The infrared (3500-40 cm(-1)) spectra of gaseous and solid and the Raman sp ectrum (3500-30 cm(-1)) of liquid and solid cyclobutyl trifluorosilane, c-C 4H7SiF3, have been recorded. From the Raman spectrum of the liquid qualitat ive depolarization values have been obtained. Both the axial and equatorial conformers have been identified in the fluid phases. Variable temperature (-55 to -150 degrees C) studies of the infrared spectra of the sample disso lved in liquid xenon and krypton have been carried out. From these data, th e enthalpy difference has been determined to be 25 +/- 10 cm(-1) (0.30 +/- 0.12 kJ/mol), with the equatorial conformer being the more stable form. A c omplete vibrational assignment is proposed for both conformers based on inf rared band contours, relative intensities, depolarization values and group frequencies and the assignments are supported by normal coordinate calculat ions utilizing ab initio force constants. Complete equilibrium geometries h ave been determined for both rotamers by ab initio calculations employing d ifferent basis sets at the levels of restricted Hartree-Fock (RHF) and/or M oller-Plesset (MP) to second order. The results are discussed and compared to those obtained for some similar molecules. (C) 2000 Elsevier Science B.V . All rights reserved.