C₆F₆ and sym-C₆F₃H₃: Ab initio and DFT studies of structure, vibrations, and inelastic neutron scattering spectra

The inelastic neutron scattering spectra of crystalline hexafluorobenzene (HFB) and 1,3,5-trifluorobenzene (TFB) are reported and compared to the results of calculations of the full spectral intensity based on density functional calculations (DFT). It is shown that several of the previous assignments of optically inactive fundamentals are incorrect, bringing the experimental picture into much better agreement with the calculated frequencies. For HFB the scattering is predominantly coherent, in contrast to the TFB case where hydrogen atom scattering predominates. There is generally good agreement between the calculated scattering intensities and the observed spectra. For TFB a comparison of the calculated and observed neutron spectra shows some significant differences attributed at least in part to hydrogen bonding in the crystal. The structures of these two molecules are discussed with reference to the reliability of the density functional methods and the effect of hydrogen bonding on the structure of TFB. It is argued that the CC bond length in HFB is shorter than for benzene consistent with similar effects for other π-electron systems.