Vinyl dichlorosilane and vinyl dibromosilane (H2C=CH-SiHX2, X = Cl, Br): conformational structure and vibrational properties determined by gas-phase electron diffraction, ab initio molecular orbital calculations, and variable-temperature Raman spectroscopy

The molecular structures, conformations, vibrational spectra, and torsional potentials of vinyl dichlorosilane (VDC) H2C=CH-SiHCl2, and vinyl dibromos ilane (VDB) H2C=CH-SiHBr2, have been studied using gas-phase electron diffr action (GED) data at 23-25 degrees C and variable-temperature Raman spectro scopy, together with ab initio molecular orbital calculations. The GED data were handled by a dynamic theoretical model using a cosine Fourier potenti al function in describing the torsional coordinate. According to the GED re finements, these molecules exist in the gas phase at room temperature as a mixture of two minimum energy conformers, syn (torsional angle phi(CCSiH) = 0 degrees) and gauche (torsional angle phi(CCSiH) approximate to 120 degre es). Relevant structural parameters for syn-VDC are as follows: Bond length s (r(g)): r(Si-C) = 1.847(5) Angstrom, r(Si-Cl) = 2.042(2) Angstrom, r(C=C) = 1.357(7) Angstrom. Bond angles (angle(alpha)): angle CSiCl = 110.3(6)deg rees, angle CCSi = 121.8 degrees (calc.), Relevant structural parameters fo r syn-VDB are as follows: bond lengths (r(g)): r(Si-C) = 1.827(9) Angstrom, r(Si-Br) = 2.206(2) Angstrom, r(C=C) = 1.366(10) Angstrom. Bond angles (an gle alpha): angle CSiBr = 110.1(8)degrees, angle CCSi = 121.7 degrees (calc .). Uncertainties are given as 2 sigma (sigma includes estimates of uncerta inties in voltage/height measurements and correlation in the experimental d ata). From the variable-temperature Raman investigation in the liquid phase , the energy differences are: VDC, Delta E degrees(S-G) = +0.11 +/- 0.06 kc al mol(-1); VDB, Delta E degrees(S-G) = +0/23 +/- 0.07 kcal mol(-1). The Ra man energies are average values obtained from two separate line doublets fo r each molecule, and they have bean used in the GED least-squares refinemen ts as valuable constraints. (C) 2000 Elsevier Science B.V, All rights reser ved.