Dynamic atomic contributions to infrared intensities of fundamental bands

Abstract

Dynamic atomic intensity contributions to fundamental infrared intensities are defined as the scalar products of dipole moment derivative vectors for atomic displacements and the total dipole derivative vector of the normal mode. Intensities of functional group vibrations of the fluorochloromethanes can be estimated within 6.5 km mol⁻¹ by displacing only the functional group atoms rather than all the atoms in the molecules. The asymmetric CF₂ stretching intensity, calculated to be 126.5 km mol⁻¹ higher than the symmetric one, is accounted for by an 81.7 km mol⁻¹ difference owing to the carbon atom displacement and 40.6 km mol⁻¹ for both fluorine displacements. Within the Quantum Theory of Atoms in Molecules (QTAIM) model differences in atomic polarizations are found to be the most important for explaining the difference in these carbon dynamic intensity contributions. Carbon atom displacements almost completely account for the differences in the symmetric and asymmetric CCl₂ stretching intensities of dichloromethane, 103.9 of the total calculated value of 105.2 km mol⁻¹. Contrary to that found for the CF₂ vibrations intramolecular charge transfer provoked by the carbon atom displacement almost exclusively explains this difference. The very similar intensity values of the symmetric and asymmetric CH₂ stretching intensities in CH₂F₂ arise from nearly equal carbon and hydrogen atom contributions for these vibrations. All atomic contributions to the intensities for these vibrations in CH₂Cl₂ are very small. Sums of dynamic contributions of the individual intensities for all vibrational modes of the molecule are shown to be equal to mass weighted atomic effective charges that can be determined from atomic polar tensors evaluated from experimental infrared intensities and frequencies. Dynamic contributions for individual intensities can also be determined solely from experimental data.