Spectra and structure of small ring compounds. LXVII vibrational spectra, variable temperature FT-IR spectra of krypton solutions, conformational stability and ab initio calculations of 1-bromosilacyclobutane

The infrared (3200-30 cm(-1)) spectra of gaseous and solid 1-bromosilacyclo butane, c-C3H6SiBrH, have been recorded. Additionally, the Raman spectra of the liquid (3200-30 cm(-1)) with quantitative depolarization values and th e solid have been recorded. Both the equatorial and the axial conformers ha ve been identified in the fluid phases. Variable temperature (- 105 to - 15 0 degreesC) studies of the infrared spectra of the sample dissolved in liqu id krypton have been carried out. From these data the enthalpy difference h as been determined to be 182 +/- 18 cm(-1) (2.18 +/- 0.22 kJ/mol) with the equatorial conformer the more stable rotamer and only conformer remaining i n the annealing solid. At ambient temperature there is similar to 22% of th e axial conformer present in the vapor phase. A complete vibrational assign ment is proposed for both conformers based on infrared contours, relative i ntensities, depolarization values and group frequencies. The vibrational as signments are supported by normal coordinate calculations utilizing the for ce constants from ab initio MP2/6-31G(d) calculations. From the frequencies of the Si-H stretch, the Si-H bond distance of 1.483 Angstrom has been det ermined for both the equatorial and the axial conformers. Complete equilibr ium geometries have been determined for both rotamers by ab initio calculat ions employing the 6-31G(d) and 6-311 + G(d,p) basis sets at levels of Hart ree-Fock (RHF) and/or Moller-Plesset with full electron correlation by the perturbation method to the second order (MP2). The results are discussed an d compared to those obtained for some similar molecules. (C) 2000 Elsevier Science B.V. All rights reserved.