1,2-Dibromoethyl-trichlorosilane (CH2Br-CHBr-SiCl3):: conformational structure and vibrational properties by gas-phase electron diffraction, infrared and Raman spectroscopy, and ab initio molecular orbital and density functional theory calculations

The molecular structure and conformational properties of 1, 2-dibromoethyl-trichlorosilane (CH2Br-CHBr-SiCl3) have been investigated using gas-phase electron diffraction (GED) data recorded at a temperature of 100 degrees C, together with ab initio molecular orbital (MO) and density functional theory (DFT) calculations, infrared (IR) and Raman spectroscopy in the liquid and solid phases, and normal coordinate analysis (NCA). The molecule exists in the gas- and liquid phases as a mixture of three conformers, gauche(-) [G(-)], with a refined torsion angle phi(BrCCBr) = -71(6)degrees, anti [A], with a torsion angle phi(BrCCBr) approximate to -170 degrees, and gauche(+) [G(+)], with a torsion angle phi(BrCCBr) approximate to +70 degrees. The second torsion angle of importance, the rotation about the C-Si bond, has been refined to a value of +175(13)degrees. Torsion angles were only refined for the more abundant G(-) conformer. In the solid phase, only the G(-) conformer was observed. The temperature-dependent Raman spectra have provided an estimate of the relative conformational entropies, AS. The obtained composition from GED refinements was (%)G(-)/A/G(+) = 64(27)/23(13)/13(18) (values with estimated 2 sigma uncertainties), giving a conformational stability order in agreement with both the Raman enthalpy measurements and the ab initio MO and DFT calculations using the 6-311G(d) basis set and scaled zero-point energies. Relevant structural parameter values obtained from the GED refinements (with the ab initio HF values used as constraints) were as follows (G(-) values with estimated 2 sigma uncertainties): bond lengths (r(g)): r(C-C) = 1.501(18) angstrom, r(Si-C) 1.865(15) angstrom, r((C-Br)) = 1.965(8),angstrom (average), r((Si-Cl)) = 2.028(3) angstrom (average). Bond angles (angle alpha): < CCSi = 114.1(33)degrees, angle C1C2Br = 114.0(21)degrees, angle(CSiCl) = 109.6(7)degrees (average). Experimental IR/Raman and obtained vibrational wavenumbers based on both the unsealed, fixed-scaled as well as the scale-refined quantum-mechanical force fields [HF/6-311G(d)] are presented. The results are discussed and compared with some similar molecules from the literature. (c) 2004 Elsevier B.V. All rights reserved.