Interpretation of infrared intensities. I. Mass-weighted squared effective charges.

Abstract

Relationships between atomic polar tensors (APTs) and experimental infrared intensities are reviewed. The invariant known as the square of the effective charge on the αth atom and its relationship to the intensity sum allow the definition of the atomic contribution to the sum of all vibrational and rotational infrared intensities for the molecule, and the contribution to the experimental dipole moment derivative (∂μ x/∂Q s) from each atom in the molecule. It is seen that this atomic contribution to the intensity sum (≡ the mass-weighted squared effective change, 3χ 2 α/m α) has very interesting properties. For one thing, values calculated from an ab initio molecular orbital calculation for a number of atoms appear to be relatively transferable for atoms in the same chemical bond in different molecules. Furthermore, the sum of intensities (or sum of 3χ 2 α/m α values) are found to be constant for different tautomeric forms of a molecule. These interesting relationships are discussed briefly.