MOLECULAR-STRUCTURE OF 2,2-DIMETHYL-1,3-DIOXA-2-SILACYCLOHEXANE FROM GAS-PHASE ELECTRON-DIFFRACTION

The gas-phase molecular structure of 2,2-dimethyl-1,3-dioxa-2-silacycl ohexane, (CH3)2Si(OCH2)2(CH2), has been determined by electron diffrac tion at 290 K nozzle temperature. The electron diffraction data are co nsistent with a chair conformation of the six-membered ring having C(s ) symmetry, while the presence of smaller amounts of other conformers cannot be excluded. The following bond lengths (r(g)) and bond angles were obtained: Si-O = 1.651 +/- 0.003 angstrom, Si-C = 1.863 +/- 0.004 angstrom, C-C = 1.529 +/- 0.003 angstrom, O-C = 1.425 +/- 0.003 angst rom, (C-H)mean = 1.125 +/- 0.003 angstrom, O-Si-O = 108.9 +/- 0.4-degr ees, O-Si-C = 108.5 +/- 0.2-degrees, C-Si-C = 113.9 +/- 0.6-degrees, S i-O-C = 118.5 +/- 0.3-degrees, C-C-C = 112.2 +/- 0.4-degrees, (Si-C-H) mean = 109.1 +/- 0.7-degrees, (H-C-H)mean = 107.2 +/- 0.7-degrees. The flap angle characterizing the orientation of the O-Si-O plane with re spect to the four coplanar atoms of the ring (O, C, C, O) is considera bly smaller than the corresponding angle in cyclohexane (27-degrees vs . 50-degrees) indicating flattening of the ring in the vicinity of the Si heteroatom. The flap of the opposite end of the ring is similar to that of cyclohexane.