MOLECULAR POLARIZABILITIES FROM ELECTRONEGATIVITY EQUALIZATION MODELS

Implications of the electronegativity equalization (EE) principle to molecular polarizabilities are discussed. An expression for molecular polarizabilities that involves simple matrix manipulations is derived based 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 models fail to account for polarizabilities that do not involve interatomic charge transfer, such as the perpendicular polarizability of planar molecules. A modified EE scheme is suggested in which the center of the electronic cloud is allowed to be displaced from its nucleus through a Drude model. This incorporates the notion of atomic dipoles into the theory and enables the EE scheme to be compatible with the existence of perpendicular polarizabilities in planar molecules as well as with binding in homonuclear diatomic molecules. Ab initio calculations for HF and H2O are used to test the formal results. It is found that intramolecular charge transfer between atoms is accurately reproduced by the EE model with a minimal set of parameters. The intraatomic polarization as obtained from the EE-Drude model is found to be anisotropic and oriented mainly along the perpendicular direction to the molecular plane.