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

The geometrical parameters for 1,1-dimethylsilacyclobiitane and 1,1,3,3-tetramethyl-13-disilacyclobutune are detemiined by gas phase electron diffraction analysis using a dynamic model, which considers ring inversion as a large-amplitude motion. The structural and potential function parameters were refined with allowance for molecular geometry relaxation estimated by quantum chemical calculations with an HF/6-311G** basis. The potential function of 1,1-dim ethylsilacyclobutane is represented as V(ϕ) = V₀[(ϕ/ϕ_e)² − 1]² with V₀ = 1.3 ± 1.2 kcallmole and ϕ_e = 29.7±4.5°, where ϕ is the ring puckering angle. A more reliable estimate for the height of the barrier (V₀ = 0.56 kcallmole) was obtained by solving the one-dimensional quantum vibrational problem and by fitting the frequency of the 0 →2 transition to the experimental value. For 1,1,3,3-tetram ethyl-1,3-disilacyclobutane, the potential function is adequately represented bx V(ϕ) = Aϕ², where A = (4.9 ± 1.8) · 10⁻⁴ kcall(mole · deg²) with a minimum coiresponding to a planar ring conformation with ϕ = 0°. The calculated structural parameters are compared with the data for related compounds. Translated fromZhumal Struktumoi Khimii. Vol. 41, No. 2, pp. 269–284, March–April, 2000