MOLECULAR POLARIZABILITIES FROM ELECTRONEGATIVITY EQUALIZATION MODELS

Implications of the electronegativity equalization (EE) principle to m olecular polarizabilities are discussed. An expression for molecular p olarizabilities that involves simple matrix manipulations is derived b ased on the standard EE scheme. The polarizability within this scheme is shown to be completely determined by the atomic hardness parameters and repulsion integrals. However, it is pointed out that current EE m odels fail to account for polarizabilities that do not involve interat omic charge transfer, such as the perpendicular polarizability of plan ar molecules. A modified EE scheme is suggested in which the center of the electronic cloud is allowed to be displaced from its nucleus thro ugh a Drude model. This incorporates the notion of atomic dipoles into the theory and enables the EE scheme to be compatible with the existe nce of perpendicular polarizabilities in planar molecules as well as w ith binding in homonuclear diatomic molecules. Ab initio calculations for HF and H2O are used to test the formal results. It is found that i ntramolecular charge transfer between atoms is accurately reproduced b y the EE model with a minimal set of parameters. The intraatomic polar ization as obtained from the EE-Drude model is found to be anisotropic and oriented mainly along the perpendicular direction to the molecula r plane.