Methylvinyldifluorosilane and Methylvinyldichlorosilane (H2CCHSiX2CH3, X = F, Cl): Structure, Conformation, and Torsional Potential As Determined by Gas-Phase Electron Diffraction and ab Initio Calculations

Abstract

The molecular structures and conformations of methylvinyldifluorosilane (VFS), H2CCHSiF2CH3, and methylvinyldichlorosilane (VCS), H2CCHSiCl2CH3, have been studied by using gas-phase electron diffraction (GED) data at 25 °C. Ab initio molecular orbital (MO) calculations were used to establish constraints in the theoretical model used to analyze the GED data. These molecules exist in the gas phase as a mixture of two conformers, syn (S, torsional angle φ(CCSiC) = 0°) and gauche (G, torsional angle φ(CCSiC) close to 120°). Relevant structural parameters for VFS (syn) are as follows: bond lengths (rg), r(SiCvinyl) = 1.846(3) Å, r(SiCH3) = 1.851(3) Å, r(CC) = 1.344(5) Å, r(SiF) = 1.592(2) Å; bond angles (∠α), ∠CSiC = 113.4(11)°, ∠CCSi = 123.3(8)°; torsional angle, φ(G) = 117(14)°. For VFS the experimental gas-phase composition (%) was (syn/gauche) 35/65 (±41)%. An estimated conformational energy difference ΔE°G-S = 0.0(±1.2) kcal/mol was obtained for VFS. Relevant structural parameters for VCS (syn) are as follows: bond lengths (rg), r(SiCvinyl) = 1.843(4) Å, r(SiCH3) = 1.855(4) Å, r(SiCl) = 2.051(2) Å, r(CC) = 1.341(6) Å; bond angles (∠α), ∠CSiC = 111.6(17)°, ∠CCSi = 123.3(17)°; torsional angle, φ(G) = 121(16)°. For VCS the experimental composition was (syn/gauche) 45/55 (±64)%. An estimated conformational energy difference ΔE°G-S = 0.3(±1.8) kcal/mol was obtained from this composition. Error estimates are given as 2σ (σ includes estimates of uncertainties in voltage/height measurements and correlation in the experimental data).