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 resu lts of calculations of the full spectral intensity based on density functio nal calculations (DFT). It is shown that several of the previous assignment s of optically inactive fundamentals are incorrect, bringing the experiment al picture into much better agreement with the calculated frequencies. For HFB the scattering is predominantly coherent, in contrast to the TFB case w here hydrogen atom scattering predominates. There is generally good agreeme nt between the calculated scattering intensities and the observed spectra, For TFB a comparison of the calculated and observed neutron spectra shows s ome significant differences attributed at least in part to hydrogen bonding in the crystal, The structures of these two molecules are discussed with r eference to the reliability of the density functional methods and the effec t of hydrogen bonding on the structure of TFB. It is argued that the CC bon d length in HFB is shorter than for benzene consistent with similar effects for other pi-electron systems.