How Accessible Is Atomic Charge Information from Infrared Intensities? A QTAIM/CCFDF Interpretation

Abstract

Infrared fundamental intensities calculated by the quantum theory of atoms in molecules/charge-charge flux-dipole flux (QTAIM/CCFDF) method have been partitioned into charge, charge flux, and dipole flux contributions as well as their charge-charge flux, charge-dipole flux, and charge flux-dipole flux interaction contributions. The interaction contributions can be positive or negative and do not depend on molecular orientations in coordinate systems or normal coordinate phase definitions, as do CCFDF dipole moment derivative contributions. If interactions are positive, their corresponding dipole moment derivative contributions have the same polarity reinforcing the total intensity estimates whereas negative contributions indicate opposite polarities and lower CCFDF intensities. Intensity partitioning is carried out for the normal coordinates of acetylene, ethylene, ethane, all the chlorofluoromethanes, the X(2)CY (X = F, Cl; Y = O, S) molecules, the difluoro- and dichloroethylenes and BF(3). QTAIM/CCFDF calculated intensities with optimized quantum levels agree within 11.3 km mol(-1) of the experimental values. The CH stretching and in-plane bending vibrations are characterized by significant charge flux, dipole flux, and charge flux-dipole flux interaction contributions with the negative interaction tending to cancel the individual contributions resulting in vary small intensity values. CF stretching and bending vibrations have large charge, charge-charge flux, and charge-dipole flux contributions for which the two interaction contributions tend to cancel one another. The experimental CF stretching intensities can be estimated to within 31.7 km mol(-1) or 16.3% by a sum of these three contributions. However, the charge contribution alone is not successful at quantitatively estimating these CF intensities. Although the CCl stretching vibrations have significant charge-charge flux and charge-dipole flux contributions, like those of the CF stretches, both of these interaction contributions have opposite signs for these two types of vibrations.