CCFOM and CCFDF models to interpret infrared intensities: A comparative study

MP2/aug-cc-pVTZ and MP2/6-311++G(3d,3p) calculations were employed to compare infrared intensity parameters of the AX diatomic molecules with A = H, Li, Na and X = F, Cl, Br, and the triatomics HCN and HNC using the charge–charge flux-overlap modified (CCFOM) and charge–charge flux–dipole flux (CCFDF) models. Our results have shown that the corrected Mulliken charges obtained from the CCFOM model represent the sum of the AIM charges with the derivatives of the atomic dipoles perpendicular to the A–X bond axis. This leads to the element of the atomic polar tensor which is only represented from its rotational contribution since its vibrational part is null.Our results have also revealed that the equilibrium hydrogen charges from CCFOM are smaller than the CCFDF ones, with the largest difference being observed for HF. Indeed, the CCFDF charge for the hydrogen in HF is comparable to the corresponding values obtained for lithium and sodium in LiF and NaF. We have compared CCFOM and CCFDF hydrogen charges with the μ/RAX ratios, where μ is the dipole moment and RAX is the A–X bond length with A = H, Li, Na and X = F, Cl, Br. This analysis has revealed that the CCFOM charges show a better linear relationship with μ/RAX than those derived from the CCFDF model when the atomic dipoles contributions are not included.With respect to dynamic part, i.e., charge fluxes due to the H–X stretching vibration, our results have shown that the CCFOM charge fluxes are very small or nearly zero. This is expected for these molecules as well as organic acids as shown by the ECCF (equilibrium charge–charge flux) model, which uses experimental results obtained from infrared intensities, and thus are more chemically sounded. Except for HF, the charge flux and dipole flux terms from CCFDF are large and with opposite signs.