Conformational stabilities, ab initio calculations and structural parameters for 2-chloroethyl silane and 2-chloroethyl trifluorosilane

The infrared spectra of gaseous and solid 2-chloroethyl silane, ClCH2CH2SiH 3 and 2-chloroethyl trifluorosilane, ClCH2CH2SiF3, was recorded. Additional ly, the variable temperature infrared spectra of the sample dissolved in li quid xenon was obtained. Both the trans and gauche conformers are present i n the fluid phases. For 2-chloroethyl silane only the trans conformer is pr esent in the solid form but for the 2-chloroethyl trifluorosilane molecule the gauche rotamer is the more stable form in the solid. The enthalpy diffe rence between the two conformers for 2-chloroethyl silane was determined to be 181 +/- 12 cm(-1) (2.17 +/- 0.14 kJ/mol) and for 2-chloroethyl trifluor osilane 55 +/- 11 cm(-1) (658 +/- 132 J/mol) with the trans conformers more stable for both molecules. Ab initio calculations were carried out at the RHF/6-31G* and MP2/6-31G*levels for both molecules from which structural pa rameters and conformational stabilities were determined. The predicted conf ormational stabilities are consistent with the experimental results. (C) 19 99 Elsevier Science B.V. All rights reserved.