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Methylvinyldifluorosilane and Methylvinyldichlorosilane (H 2 CCHSiX 2 CH 3 , X = F, Cl): Structure, Conformation, and Torsional Potential As Determined by Gas-Phase Electron Diffraction and ab Initio Calculations

1997; American Chemical Society; Volume: 101; Issue: 19 Linguagem: Inglês

10.1021/jp9633862

1520-5215

Tore H. Johansen, Kolbjørn Hagen, Reidar Stølevik, Karl Hassler,

Crystallography and molecular interactions

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).