This work is devoted to investigation of the known problem of the discrepancy between the experimental and theoretical values for the barrier to internal rotation of the molecules comprised of a benzene ring and a p-conjugated substituent, such as benzaldehyde and benzoyl fluoride. The possible reasons of such discrepancy are considered. As a result the conclusion was drawn that the origin of the problem is incorrectness of the assignments of the torsional levels higher than first, both for benzaldehyde and benzoyl fluoride. In addition the significant kinematic interaction between torsional vibration and out-of-plane CHO deformation was found for benzaldehyde.