Conformational stability, vibrational spectra and ab initio calculations for chloromethyl methyl silane and chloromethyl methyl difluorosilane

The infrared spectra of gaseous and solid chloromethyl methyl silane, ClCH2 SiH2CH3, and chloromethyl methyl difluorosilane, ClCH2SiF2CH3, were recorde d. Additionally, the variable temperature infrared spectra of the sample di ssolved in liquid krypton and/or xenon were obtained. Both the trans and ga uche conformers are present in the fluid phases but only the trans conforme r is present in the solid for both molecules. The enthalpy difference betwe en the two conformers for chloromethyl methyl silane was determined to be 1 80 +/- 15 cm(-1) (2.16 +/- 2.16 +/- 0.18 kJ/mol) and for chloromethyl methy l difluorosilane 236 +/- 15 cm(-1) (2.82 +/- 0.17 kJ/mol) in xenon with the gauche conformers more stable for both molecules. The enthalpy value for t he silane in krypton is 181 +/- 18 cm(-1) (2.16 +/- 0.09 kJ/mol). Ab initio calculations were carried out at the RHF/6-31G* and MP2/6-31G* levels for both molecules from which structural parameters and conformational stabilit ies were determined. (C) 1999 Elsevier Science B.V. All rights reserved.