The vibrational spectra and dihedral angles of biphenyl and the 4,4'-dihalogenobiphenyls

The vibrational frequencies of biphenyl and its 4,4'-dihalogen derivatives have been computed for various values of the dihedral angle between the two rings. It is shown that certain modes with frequencies below 700 cm⁻¹ are sensitive to this angle. Agreement between experimental and calculated frequencies is good. The spectra of biphenyl and 4,4'-difluorobiphenyl are in complete accord with a planar D_2h structure in the crystal phase. In solution, melt and gas the dihedral angle of these systems is 45 ± 15°. The observed frequency shifts of certain A modes are shown to be due to considerable force constant changes for the central C-C bond and its neighbouring internal angle deformations. No significant frequency shifts with phase changes were observed for 4,4'-dichloro- or 4,4'-dibromobiphenyl. In these cases the structures are D₂.