Infrared and Raman spectra, conformational stability, normal coordinate analysis, ab initio calculations, and vibrational assignment of 1-fluoro-1-methylsilacyclobutane

The infrared (3500-40 cm(-1)) spectra of gaseous and solid 1-fluoro-1-methy lsilacyclobutane, c-C3H6SiF(CH3), have been recorded. Additionally, the Ram an spectrum (3500-30 cm(-1)) of the liquid has been recorded and quantitati ve depolarization values have been obtained. Both the axial and equatorial (with respect to the methyl group) conformers have been identified in the f luid phases. Variable temperature (-55 - -100 degrees C) studies of the inf rared spectra of the sample dissolved in liquid xenon have been carried out . From these data, the enthalpy difference has been determined to be 267 +/ - 10 cm(-1) (3.19 +/- 0.12 kJ mol(-1)), with the axial conformer being the more stable form and the only conformer remaining in the polycrystalline so lid. A complete vibrational assignment is proposed for the axial conformer and many of the fundamentals for the equatorial conformer have also been id entified. The vibrational assignments are supported by normal coordinate ca lculations utilizing ab initio force constants. Complete equilibrium geomet ries have been determined for both rotamers by ab initio calculations emplo ying the 6-31G* and 6-311 + + G** basis sets at the levels of restricted Ha rtree-Fock (RHF) and/or Moller-Plesset (MP) to second order. The results ar e discussed and compared to those obtained for some similar molecules. (C) 1999 Elsevier Science B.V. All rights reserved.