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

The infrared (3500-40 cm(-1)) spectra of gaseous and solid 1-methylsilacycl obutane, c-C3H6SiH(CH3), have been recorded. In addition, the Raman spectru m (3500-30 cm(-1)) of the liquid has also been recorded and quantitative de polarization values obtained. Both the axial and equatorial conformers, wit h respect to methyl group, have been identified in the fluid phases. Variab le temperature studies (-55 to -100 degrees C) of the infrared spectra of t he sample dissolved in liquid xenon have been carried out. From these data, the enthalpy difference has been determined to be 122 +/- 26 cm(-1) (1.46 +/- 0.32 kJ/mol), with the equatorial conformer being the more stable struc ture. However, with repeated annealing of the amorphous solid, it was not p ossible to obtain a polycrystalline solid with a single conformer. A comple te vibrational assignment is proposed for the equatorial conformer based on infrared band contours, relative intensities, depolarization values and gr oup frequencies. Most of the fundamentals for the axial conformer have also been identified. Utilizing the frequency of the silicon-hydrogen (Si-H) st retching mode, the Si-H distance has been determined to be 1.490 Angstrom f or both conformers. The vibrational assignments are supported by normal coo rdinate calculations utilizing ab initio force constants. Complete equilibr ium geometries have been determined for both rotamers by ab initio calculat ions, employing the 3-21G* and 6-31G*: basis sets at the levels of restrict ed Hartree-Fock (RHF) and/or Moller-Plesset (MP) to second order. The resul ts are discussed and compared with those obtained for some similar molecule s. (C) 1999 Elsevier Science B.V. All rights reserved.