The low-frequency Raman and far infrared spectra of 4,4’-difluorobenzophenone and 4,4’-dichlorobenzophenone have been recorded in the high-temperature phase. The assignment of the lattice vibrations based on the semirigid-molecule model is presented. The strong influence of intermolecular forces on a number of molecular normal modes is predicted by the lattice dynamics calculations. Particularly, the torsional molecular modes, involving librations of phenyl rings, are sensitive to the external influences. The calculations also predict that several molecular modes are almost unaffected by the crystal field and an explanation of such behavior is proposed. Comparison with experimental and calculated results for other flexible molecules is discussed.