Properties of atoms in molecules: Atomic polarizabilities

Abstract

The theory of atoms in molecules is applied to the determination of the atomic and group contributions to the molecular polarizability in diatomic and polyatomic systems, covering a wide range of atomic interactions. The calculations are performed through the use of coupled-perturbed Hartree–Fock theory. An applied field polarizes the atomic charge distributions and induces a transfer of charge between them. The resulting changes in the dipole moment per unit applied field determine the corresponding contributions to the polarizability. In general, the component of the polarizability along the internuclear axis in a linear molecule or in the direction of a chain of bonds in a nonlinear polyatomic molecule, exceeds the perpendicular component because of the transfer of electronic charge between the two ends of the molecule, between the methyl groups in ethane, and between the methylene groups in ethene, for example. Atoms shielded from an applied field, carbon in methane for example, polarize in a direction counter to that of the applied field, responding instead to the opposing field created by the transfer of charge between the exterior atoms. The same behavior is found in vibrationally induced polarizations of a molecular charge distribution. The extent of atomic polarization and of interatomic charge transfer caused by an external field are readily related to known properties of the atomic charge distributions and the corresponding contributions to the polarizability may be understood and predicted. The mean molecular polarizability and its group contributions are found to be linearly proportional to the corresponding sum of atomic volumes, making possible the prediction of molecular polarizabilities from tabulated atomic volumes as determined by the theory of atoms in molecules.