Vibrational spectra of 4, 4′‐difluorobiphenyl‐ d 8 and the structure of biphenyls in solution

4, 4′‐Difluorobiphenyl‐ d 8 was synthesized by two routes and its infrared and Raman spectra were recorded. These were analysed with the assistance of calculated frequencies using a force field based on fluorobenzenes and using phase effects. A perturbation treatment of the frequencies of biphenyl, of its 4, 4′‐difluoro derivative and of their fully deuteriated isotopomers yielded a refined force field essentially in accord with previous aromatic fields. By studying the changes in the frequencies of certain skeletal modes on going from crystal to solution, the dihedral angle of the difluoro compound was shown to be very similar to that of biphenyl itself in solution (35–40°). There is, however, a possibility that the present model underestimates the in‐plane B 1g skeletal deformation mode. In this case the biphenyl dihedral angle will be slightly overestimated and that of 4, 4′‐difluorobiphenyl slightly underestimated (by up to 5°).