The Molecular Structure and the Puckering Potential Function of Octamethylcyclotetrasilane, Si4Me8, Determined by Gas Electron Diffraction and Relaxation Constraints from Ab Initio Calculations1
Gas electron diffraction data are applied to determine the geometrical parameters of the octamethylcyclotetrasilane molecule using a dynamic model in which the ring puckering is 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 has been described as V(ϕ) _ V0[(ϕ/ϕe)2 − 1]2 with V0 = 1.0 ± 0.5 kcal/mol and ϕe _ 28.3 ± 1.9−, where ϕ is the puckering angle of the ring. 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 as follows: r(Si―C)av _ 1.894(3), r(Si―Si) _ 2.363(3), r(C―H) _ 1.104(3), ∠CSiC _ 109.5(6), ∠SiSiSi _ 88.2(2), ∠SiCH _ 111.7(6), ∠δC _ 4.1, where the tilt δC was estimated from ab initio constraints. The structural parameters are compared with those obtained for related compounds.