| Abstract
| - The molecular structures and conformations ofmethylvinyldifluorosilane (VFS),H2CCHSiF2CH3,andmethylvinyldichlorosilane (VCS),H2CCHSiCl2CH3, have beenstudied by using gas-phase electron diffraction(GED) data at 25 °C. Ab initio molecular orbital (MO)calculations were used to establish constraints in thetheoretical model used to analyze the GED data. These moleculesexist in the gas phase as a mixture of twoconformers, syn (S, torsional angle φ(CCSiC) = 0°) andgauche (G, torsional angle φ(CCSiC) close to120°).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 experimentalgas-phase composition(%) was (syn/gauche) 35/65 (±41)%. An estimated conformationalenergy difference ΔE°G-S =0.0(±1.2)kcal/mol was obtained for VFS. Relevant structural parameters forVCS (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 VCSthe experimental composition was (syn/gauche) 45/55 (±64)%. Anestimated conformational energy differenceΔE°G-S = 0.3(±1.8) kcal/mol wasobtained from this composition. Error estimates are given as 2σ(σ includesestimates of uncertainties in voltage/height measurements andcorrelation in the experimental data).
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