An effective atomic charge model for infrared intensities

Infrared intensities, in particular the dipole derivatives with respect to internal (symmetry) coordinates derived from the intensities, can be explained in terms of an effective atomic charge model which includes both the equilibrium charges and their first-order fluxes. For diatomic molecules it is found that most of the intensity arises from the equilibrium charges in the case of the hydrogen halides, but conversely, in CO, most of the intensity is due to the charge flux. The parameters found can be nicely related to elementary bonding theory.In small symmetrical polyatomic molecules the number of parameters is sufficiently restricted by symmetry and charge conservation that the parameters would be uniquely determinate except for the ambiguity in sign of the experimental dipole derivatives. The examples of AB₂ (D_∞h and C_2v), AB₃ (D_3h and C_3v), and AB₄ (Td) are discussed in detail; a simple generalization for molecular ions is included.For nonpolar molecules, the effective equilibrium charges are determined by the motion (not the equilibrium value) of the electronic centroid.