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

Gas electron diffraction data are applied to determine the geometrical para meters of the 1,1,3,3-tetramethyl-1,3-disilacyclobutane molecule using a dy namic model where the ring puckering was treated as a large amplitude motio n. The structural parameters and parameters of the potential function were refined taking into account the relaxation of the molecular geometry estima ted 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(phi) = A phi(2) with A = (4.9 +/- 1.8) x 10(-4) kcal/mol deg(2), with the minimum at the puckering angle phi = 0 degrees. The planar (D-2h) equil ibrium structure was supported by theoretical calculations. The geometric p arameters at the minimum of V(phi) (r(a) in Angstrom, angle(alpha) in degre es and errors given as three times the standard deviations including a scal e error) are: r(Si-C-m) = 1.853(4), r(Si-C-r) = 1.910(5), r(C-H-av) = 1.084 (5), angle CmSiCm = 109.2(14), angle CrSiCr = 92.2(4), angle SiCHm = 113.8( 13), angle HCrH = 107.9, angle delta(CmSiCm) = 0, angle delta(HCrH) = 0, wh ere the tilts delta and angle HCrH are estimated from ab initio constraints , indexes m and r refer to methyl and ring. The structural parameters are c ompared with those obtained for related compounds. (C) 1999 Elsevier Scienc e B.V. All rights reserved.