Conformational control by halogen substitution and by crystallisation: a study of the molecular structures of CH(SiMe2H)3 and CH(SiMe2Br)3 by gas-phase electron diffraction, and ab initio molecular orbital calculations

Abstract

The molecular structures of CH(SiMe₂H)₃ and CH(SiMe₂Br)₃ have been determined by gas-phase electron diffraction, ab initio calculations, and, for CH(SiMe₂Br)₃, X-ray diffraction. In each case, 11 distinct conformations, with energies lying within a range of 8.5 kJ mol⁻¹ for CH(SiMe₂H)₃ and 26 kJ mol⁻¹ for CH(SiMe₂Br)₃, were calculated and thus many conformers for both compounds are likely to exist in the gas phase. The structures are compared with those of related alkylsilanes to assess the changes in molecular geometry resulting from crowding at the carbon centre.