The molecular structure and the puckering potential function of 1,1,3,3-tetramethyl-1,3-disilacyclobutane determined by gas electron diffraction and relaxation constraints from ab initio calculations

Abstract

Gas electron diffraction data are applied to determine the geometrical parameters of the 1,1,3,3-tetramethyl-1,3-disilacyclobutane molecule using a dynamic model where the ring puckering was treated as a large amplitude motion. The structural parameters and parameters of the potential function were refined taking into account the relaxation of the molecular geometry estimated from ab initio calculations at the Hartree–Fock level of theory using a 6-311G** basis set. The potential function was found to be well described as V(ϕ)=Aϕ2 with A=(4.9±1.8)×10−4kcal/moldeg2, with the minimum at the puckering angle ϕ=0°. The planar (D2h) equilibrium structure was supported by theoretical calculations.The geometric parameters at the minimum of V(ϕ) (ra in Å, ∠α in degrees and errors given as three times the standard deviations including a scale error) are: r(Si–Cm)=1.853(4), r(Si–Cr)=1.910(5), r(C–Hav)=1.084(5), ∠CmSiCm=109.2(14), ∠CrSiCr=92.2(4), ∠SiCHm=113.8(13), ∠HCrH=107.9, ∠δ(CmSiCm)=0, ∠δ(HCrH)=0, where the tilts δ and ∠HCrH are estimated from ab initio constraints, indexes m and r refer to methyl and ring. The structural parameters are compared with those obtained for related compounds.