The predicted value of potential barrier to internal rotation of benzaldehyde: Is there a conflict between the theory and the experiment or not?

The ratio of potential barrier heights for the internal rotation of benzaldehyde molecule calculated by quantum mechanical methods and determined experimentally from UV spectra has been discussed. Based on the joint analysis of the results of ab initio MP2/6-311G** calculations of normal vibrations of benzaldehyde in the ground state and the results of interpretation of the observed UV spectra of the compound, possible approximations for a correct description of the hindered internal rotation of the aldehyde group have been considered. The two-dimensional model including the kinematic interaction between torsion and out-of-plane deformation of aldehyde group has been established as more efficient for the correct description of the hindered rotation in benzaldehyde.