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

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Abstract
Polar tensors for a variety of molecules, XY4, XY3, CH3X, and C2F6, are evaluated from infrared band intensities. The effective charges ξα for all atoms are reported. It is found that the difference in the ξα2 value of the central atom and the sum of the ξα2 values the terminal atoms is maximal for the preferred sign combination of dipole moment derivatives δμg/δQi 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 δμg/δQi. It is noted that the ξα and ξαβ values follow the trend in electronegativity of atoms and ionic character of bonds, respectively, in a series of molecules.