THE MOLECULAR-STRUCTURE AND THE PUCKERING POTENTIAL FUNCTION OF 1,1-DICHLORSILACYCLOBUTANE DETERMINED BY GAS ELECTRON-DIFFRACTION AND RELAXATION CONSTRAINTS FROM AB-INITIO CALCULATIONS

Gas electron diffraction data are applied to determine the geometrical parameters of the 1,1-dichlorosilacyclobutane molecule using a dynami c model where the ring puckering was treated as a large amplitude moti on. The structural parameters and parameters of the potential function were refined taking into account the relaxation of the molecular geom etry estimated from ab initio calculations at the Hartree-Fock level o f theory using a 6-311+G* basis set. The potential function has been described as V(phi)=V-0[(phi/phi(e))(2)-1](2) with the following param eters V-0=0.57 +/- 0.32 kcal/mol and phi(e)=25.9 +/- 2.6 degrees, wher e phi is the puckering angle of the ring. The classic distribution fun ction used for averaging the local molecular configurations was found to underestimate the value V-0 by 8% as compared with the exact quantu m mechanical distribution function. The geometric parameters at the mi nimum V(phi) (r(a) in Angstrom, angle(alpha) in degrees and errors giv en 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), angle ClSiCl=105.2(8), angle CSiC=8 1.1(10), angle SiCHeq=118.9(54), angle SiCHax=109.7(54), angle CC5Heq= 105.3(63), angle CC5Hax=100.9(63), angle HC3H=108.0, angle delta(ClSiC l)=4.1, angle delta(HC3H)=3.0, where the tilt angle delta, and angle H C3H are estimated from ab initio constraints. The structural parameter s 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. (C) 1998 Elsevier Science B.V.