Electron diffraction study of molecular structure and inversion potential for 1,1-dimethylsilacyclobutane and 1,1,3-tetramethyl-1,3-disilacyclobutane

The geometrical parameters for 1,1-dimethylsilacyclobutane and 1,1,3,3-tetr amethyl-1,3-disilacyclobutane are determined by gas phase electron diffract ion analysis using a dynamic model, which considers ring inversion as a lar ge-amplitude motion. The structural and potential function parameters were refined with allowance for molecular geometry relaxation estimated by quant um chemical calculations with an HF/6-311G** basis. The potential function of 1,1-dimethylsilacyclobutane is represented as V(phi) = V-0[(phi/phi(e))( 2) - 1](2) with V-0 = 1.3 +/- 1.2 kcal/mole and phi(e) = 29.7 +/- 4.5 degre es, where phi is the ring puckering angle. A more reliable estimate for the height of the barrier (V-0 = 0.56 kcal/mole) was obtained by solving the o ne-dimensional quantum vibrational problem and by fitting the frequency of the 0 --> 2 transition to the experimental value. For 1,1,3,3-tetramethyl-1 ,3-disilacyclobutane, the potential function is adequately represented by V (phi) = A phi(2), where A = (4.9 +/- 1.8) . 10(-4) kcal/(mole . deg(2)) wit h a minimum corresponding to a planar ring conformation with phi = 0 degree s. The calculated structural parameters are compared with the data for rela ted compounds.