Vinyl dichlorosilane and vinyl dibromosilane [formula omitted] conformational structure and vibrational properties determined by gas-phase electron diffraction, ab initio molecular orbital calculations, and variable-temperature Raman spectroscopy

Abstract

The molecular structures, conformations, vibrational spectra, and torsional potentials of vinyl dichlorosilane (VDC) H2CCH–SiHCl2, and vinyl dibromosilane (VDB) H2CCH–SiHBr2, have been studied using gas-phase electron diffraction (GED) data at 23–25°C and variable-temperature Raman spectroscopy, together with ab initio molecular orbital calculations. The GED data were handled by a dynamic theoretical model using a cosine Fourier potential function in describing the torsional coordinate. According to the GED refinements, these molecules exist in the gas phase at room temperature as a mixture of two minimum energy conformers, syn (torsional angle φ(CCSiH)=0°) and gauche (torsional angle φ(CCSiH)≈120°). Relevant structural parameters for syn-VDC are as follows: Bond lengths (rg): r(Si–C)=1.847(5)Å,r(Si–Cl)=2.042(2)Å,r(CC)=1.357(7)Å. Bond angles (∠α): ∠CSiCl=110.3(6)°, ∠CCSi=121.8° (calc.). Relevant structural parameters for syn-VDB are as follows: bond lengths (rg): r(Si–C)=1.827(9)Å,r(Si–Br)=2.206(2)Å,r(CC)=1.366(10)Å. Bond angles (∠α): ∠CSiBr=110.1(8)°, ∠CCSi=121.7° (calc.). Uncertainties are given as 2σ (σ includes estimates of uncertainties in voltage/height measurements and correlation in the experimental data). From the variable-temperature Raman investigation in the liquid phase, the energy differences are: VDC, ΔE°S−G=+0.11±0.06kcalmol−1; VDB, ΔE°S−G=+0.23±0.07kcalmol−1. The Raman energies are average values obtained from two separate line doublets for each molecule, and they have been used in the GED least-squares refinements as valuable constraints.