Knowledge Bank

Infrared and Raman spectra of hexafluorobenzene have been observed. It has been possible to select eleven fundamental bands and lines whose frequencies as well as polarization character indicate that the molecule is planar and belongs to the $D6h$ symmetry, as in the case of benzene. A potential function containing four valency deformation force constants and seven other parameters characterizing certain bond-bond and angle-angle interactions was calculated. The C-C force constant, amounting to $5.741\times 105$ dynes/cm, is weaker than in benzene where it is equal to $6.3\times 105$. The C-F force constant, which is equal to $7.5\times 105$, is greater than in p. $C6H4F2$ where it was found to be equal to $6.5\times 105$. This should mean a small decrease in the CF bond-length when the number of these bonds in the benzene ring increases. The signs obtained for the bond-bond and bond-angle interactions nicely check the theory.