QTAIM charge–charge flux–dipole flux models for the fundamental infrared intensities of BF3 and BCl3

Abstract

Quantum Theory of Atoms in Molecules Charge–Charge Flux–Dipole Flux (QTAIM/CCFDF) models have been determined for the BF3 and BCl3 molecules. Model parameters were obtained from MP2/6–31G(2d,2p) wave functions owing to their accurate estimations of the BF3 intensities and were found to be insensitive to changes in basis sets with polarization functions and to the level of electron correlation treatment, MP2, QCISD and DFT. The BF3 stretching intensity has a very dominant equilibrium charge contribution with small charge and dipole fluxes occurring in the opposite direction to the charge movement. Large equilibrium charge and small dynamic contributions are also characteristic of stretching vibrations in the ionic diatomic molecules, NaF, NaCl, LiF and LiCl. Furthermore the Laplacians of the electron density at the bond critical points of BF3 and these diatomics are all positive indicating electron depletion in their bonding regions relative to large electronic densities concentrated around their nuclei that is characteristic of ionic bonds. The MP2/6–31G(2d,2p) BCl3 stretching intensity can be accurately estimated by equilibrium charge movement since the charge and dipole fluxes almost exactly cancel one another. Both in-plane and out-of-plane BF3 and BCl3 bending deformations are described by equilibrium charge movements that are partially canceled by opposing dipole fluxes that measure the effect on the dipole moment change from electron densities polarized in the opposite direction. Charge fluxes are calculated to be small for the in-plane deformations and are zero by symmetry for the out-of-plane ones.