The molecular structure and the puckering potential function of octamethylcyclotetrasilane, Si4Me8, 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 octamethylcyclotetrasilane molecule using a dynamic model in which the ring puckering is treated as a large amplitude motion. The struct ural parameters and parameters of the potential function were refined, taki ng into account the relaxation of the molecular geometry estimated from ab initio calculations at the Hartree-Fock level of theory using a 6-311G** ba sis set. The potential function has been described as V(phi) = V-o[(phi/phi (e))(2) - 1](2) with V-o = 1.0 +/- 0.5 kcal/mol and phi(e) = 28.3 +/- 1.9 d egrees, where co is the puckering angle of the ring. The geometric paramete rs at the minimum of V(phi) (r(a) in Angstrom, angle(alpha) in degrees and errors given as three times the standard deviations including a scale error ) are as follows: r(Si-C)(av) = 1.894(3), r(Si-Si)= 2.363(3), r(C-H)= 1.104 (3), angle CSiC = 109.5(6), angle SiSiSi = 88.2(2), angle SiCH = 111.7(6), angle delta C = 4.1, where the tilt delta C was estimated from nb initio co nstraints. The structural parameters are compared with those obtained for r elated compounds.