METHYLVINYLDIFLUOROSILANE AND METHYLVINYLDICHLOROSILANE (H2C=CHSIX2CH3, X=F, CL) - STRUCTURE, CONFORMATION, AND TORSIONAL POTENTIAL AS DETERMINED BY GAS-PHASE ELECTRON-DIFFRACTION AND AB-INITIO CALCULATIONS

The molecular structures and conformations of methylvinyldifluorosilan e (VFS), H2C=CHSiF2CH3, and methylvinyldichlorosilane (VCS), H2C=CHSiC l2CH3, have been studied by using gas phase electron diffraction (GED) data at 25 degrees C. Ab initio molecular orbital (MO) calculations w ere used to establish constraints in the theoretical model used to ana lyze the GED data. These molecules exist in the gas phase as a mixture of two conformers, syn (S, torsional angle phi(CCSiC) = 0 degrees) an d gauche (G, torsional angle phi(CCSiC) close to 120 degrees). Relevan t structural parameters for VFS (syn) are as follows: bond lengths (r( g)), r(Si-C-vinyl) 1.846(3) Angstrom, r(Si-CH3) = 1.851(3) Angstrom, r (C=C) 1.344(5) Angstrom, r(Si-F) = 1.592(2) Angstrom; bond angles (ang le(alpha)), angle CSiC = 113.4-(11)degrees, angle CCSi = 123.3(8)degre es; torsional angle, phi(G) = 117(14)degrees. For VFS the experimental gas-phase composition (%) was (syn/gauche) 35/65 (+/-41)%. An estimat ed conformational energy difference Delta E degrees(G-S) = 0.0(+/-1.2) kcal/mol was obtained for VFS. Relevant structural parameters for VCS (syn) are as follows: bond lengths (r(g)), r(Si-C-vinyl) = 1.843(4) A ngstrom, r(Si-CH3) = 1.855(4) Angstrom, r(Si-Cl) = 2.051(2) Angstrom, r(C=C) = 1.341(6) Angstrom; bond angles (angle(alpha)), angle CSiC = 1 11.6(17)degrees, angle CCSi = 123.3(17)degrees; torsional angle, phi(G ) = 121(16)degrees. For VCS the experimental composition was (syn/gauc he) 45/55 (+/-64)%. An estimated conformational energy difference Delt a E degrees(G-S) = 0.3(+/-1.8) kcal/mol was obtained from this composi tion. Error estimates are given as 2 sigma (sigma includes estimates o f uncertainties in voltage/height measurements and correlation in the experimental data).