Quantum theory atoms in molecules charge-charge flux-dipole flux models for the infrared intensities of benzene and hexafluorobenzene.

The infrared fundamental intensities of benzene and hexafluorobenzene have been calculated at the MP2/6-311++G(3d,3p) level. The theoretical values are in excellent agreement with the averaged experimental C(6)H(6) results having a rms error of 15.3 km mol(-1). However, the theory badly underestimates the CF stretching and ring deformation intensities of C(6)F(6) having an overall rms error of 141 km mol(-1). The theoretical results confirm the dipole moment derivative signs previously attributed on the basis of the comparison of C(6)H(6) and C(6)D(6) derivatives and semiempirical molecular orbital results. A quantum theory atoms in molecules charge-charge flux-dipole flux interpretation of the theoretical results shows that electronic density changes for out-of-plane vibrations can be explained using a simple bond moment-rehybridization moment model proposed many years ago. However, these changes were found to be much more complicated for the in-plane vibrations involving important charge flux and dipole flux contributions for both molecules as well as contributions from the displacement of equilibrium atomic charges for hexafluorobenzene.