Investigation of The Basis of The Valence Shell Electron Pair Repulsion Model By Ab Initio Calculation of Geometry Variations In A Series of Tetrahedral and Related Molecules

Energy-optimized geometries were calculated ab initio for NH₃, NF₃. OH₂, OF₂, PH₃, PF₃, SH₂. SF₂, SO₂, SOF₂, SOH₂, SO₂H_2> HSF, SH3⁺, NH₂^-, NF₂^-, and NH₄⁺using consistent basis sets and optimization criteria. An understanding of the predictions of the valence shell electron pair repulsion (VSEPR) model was sought by a comparison of the calculated geometries and various properties of the localized bonding and lone-pair orbitals. The calculated relative sizes of bonds and lone pairs agreed very well with the VSEPR assumptions. Some apparent failures of the VSEPR model can be explained by examining the total angular space requirements of the bond and lone-pair orbitals, rather than restricting attention only to the angles formed between bonds. An extensive investigation was made of the effect of polarization functions in the basis set both on calculated geometries and on the properties of the resulting localized orbitals.