6:15 PM - 7:30 PM
[SMP42-P07] Nature of Si-O bonding via molecular orbital calculation
Keywords:Molecular orbital calculation, Si-O-Si bridging
Molecular orbital calculations were performed using the GAUSSIAN 09 code. We firstly calculate the optimized structure of disiloxane (Almenningen et al., 1963) by Hartree-Fock (HF), second-order Moller-Plesset perturbation theory (MP2) , and two density functional theory (Becke’s density functional (Becke, 1988) with three correlation functionals by Lee, Yang and Parr (B3LYP) (Lee et al., 1988), and generalized gradient approximation by Perdew, Burke and Ernzerhof (PBE) (Perdew et al., 1996)) with 6-311G(d,p) split valence double zeta basis set (Raghavachari et al., 1980). The bending of Si-O-Si bridging is not reproduced by HF method as shown in Tsuneyuki (1996). The bending of Si-O-Si bridging is reproduced by use of MP2 and density functional theory with PBE. The optimized angle of Si-O-Si in disiloxane molecule by MP2 is closer to experimental value than that by PBE. Then we apply the MP2 method with 6-311G(d,p) basis set to the calculation of pyrosilisic acid, H6Si2O7. NBO analysis (Foster and Weinhold, 1980, Reed et al., 1985; 1988) is used to analyze the electronic state of bonding.
We found the equilibrium geometries for bended two pyrosilisic acid molecules (C2V and 60⁰ torsion) using Moller-Plesset perturbation theory and with 6-311G(d,p) split valence double zeta basis set. We calculated the energy surface with varying Si-Obr length and Si-O-Si angle and found the relationship between Si-Obr length and bridging angle. From the energy surface, the stable Si-O bond length decrease with spreading Si-O-Si angle. The bending of Si-O-Si angle in equilibrium geometries can be explained by explained by the balance of Coulombic repulsion between tetrahedra and lone pair electrons of bridging oxygen atom without concerning the contribution of d-p π-bonding. The Si-O bonding strengthen with increasing Si-O-Si angle because of stabilization in energy of Si-O bonding orbital with decreasing the hybridization index λ in spλ orbital of bridging oxygen and increase of coulombic interaction between Si and bridging oxygen atom.