MOLECULAR-STRUCTURE OF 3,3-DIMETHYL-3-SILATETRAHYDROFURAN FROM GAS-PHASE ELECTRON-DIFFRACTION ANALYSIS AND AB-INITIO CALCULATIONS

The molecular structure of 3,3-dimethyl-3-silatetrahydrofuran has been studied by means of gas-phase electron diffraction (GED) and ab initi o calculations at the HF/6-31G level. Six different conformers of the title compound have been refined in the course of the GED analysis, a llowing the vibrational amplitudes to vary over a wide range. Several bond length differences were taken from the results of the ab initio g eometry refinement. Almost the same value of the R-factor, about 3%, h as been obtained for all six models, whereas the ab initio calculation resulted in one single conformer (''O-envelope''). Based upon the com bined information from the ab initio calculations and the GED work, th e ''O-envelope'' conformer is chosen as the final structure. Geometric al parameters obtained (GED) are as follows (bond lengths in Angstroms , angles in degrees; three times standard deviation estimates in paren theses): r(Si-C-cyc) = 1.891(3) and 1.904; r(Si-C)(Me)) = 1.877 and 1. 879; r(C-C) = 1.556(7); r(C-O)= 1.431(3) and 1.445; r(C-cyc-H) = 1.11( 2); r(C-Me-H) = 1.10(2); phi(O-C-C-Si) = 33.0(14); angle CcycSiCcyc, = 92.2(5); angle CMeSiCMe = 107.2(8); angle SiCC = 102.6(6); angle CCO = 109.9(11); angle HCcycH = 110.5(47); angle SiCMeH = 111.6(14).