Infrared band intensities and polar tensors: properties of effective charge _ξα and its relation to the sign of dipole moment derivatives

Abstract

Polar tensors for a variety of molecules, XY₄, XY₃, CH₃X, and C₂F₆, are evaluated from infrared band intensities. The effective charges ξ_α for all atoms are reported. It is found that the difference in the ξ_α² value of the central atom and the sum of the ξ_α² values the terminal atoms is maximal for the preferred sign combination of dipole moment derivatives δμ_g/δQ_i in all the molecules considered. Making use of this property, we find it possible to determine unequivocally the sign of dipole moment derivatives with respect to normal coordinates δμ_g/δQ_i. It is noted that the ξ_α and ξ_αβ values follow the trend in electronegativity of atoms and ionic character of bonds, respectively, in a series of molecules.