United atom models for polyatomic ligands. Part II. Methyl and fluoro derivatives

Abstract

The United Atom model is extended to the fluoro and methyl derivatives of Group V and VI central atoms. An additional second-order effect, the Anisotropic Potential Stabilization (APS) is identified associated with the orbitals of the highly electronegative fluorine substituent. The current spectroscopic assignments for these molecules are compared to those predicted by the correlation diagrams and in some cases alternative assignments are proposed. In all of the methyl derivatives the highest occupied orbital appears to be localized in the methyl groups. The Lewis σ basicity of the highest occupied and the π acidity of the first unoccupied orbitals are described using spectroscopic criteria of Part 1. Both π acidity and its extreme form, synergism are important if the frontier orbitals display molecular d or f quantum members and originate on the central atom.