The molecular structure and the puckering potential function of 1,1-dichlorsilacyclobutane 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-dichlorosilacyclobutane 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 6–311 + G∗∗ basis set. The potential function has been described as V(ϕ) = V 0[( ϕ ϕ e ) 2 − 1] 2 with the following parameters V 0 = 0.57 ± 0.32 kcal mol and ϕ e = 25.9 ± 2.6°, where ϕ is the puckering angle of the ring. The classic distribution function used for averaging the local molecular configurations was found to underestimate the value V 0 by 8% as compared with the exact quantum mechanical distribution function. The geometric parameters at the minimum V( ϕ) ( r a in A ̊ , ∠ α in degrees and errors given as three times the standard deviations including a scale error) are: r(Si−Cl ax) = 2.043(2), r(Si−Cl eq) = 2.038(2), r(Si−C) = 1.860(3), r(C−C) = 1.557(4), r(C−H) = 1.091(8), ∠ClSiCl = 105.2(8), ∠CSiC = 81.1(10), ∠SiCH eq = 118.9(54), ∠SiCH ax = 109.7(54), ∠CC 5H eq = 105.3(63), ∠CC 5H ax = 100.9(63), HC 3H = 108.0, ∠ δ(ClSiCl) = 4.1, ∠ δ(HC 3H) = 3.0, where the tilt angle δ, and ∠HC 3H are estimated from ab initio constraints. The structural parameters are compared with those obtained for related compounds. Distortions of the valence angles at the Si atom in silacyclobutanes are shown to be well explained using the VSEPR model complemented by the concept of bent bonds.