On the problem of theoretical evaluation of the rotational barrier in aromatics with adjacent conjugated group: benzaldehyde and N-methylbenzamide

The rotational barrier in benzaldehyde was evaluated using DFT methods and CCSD. Compared to experimental values, the calculated values are typically a factor 2 too high. Evidence is provided this also applies to N-methylbenzamide. The results once more suggest that molecules comprising a benzene ring with a π-conjugated substituent form a special problem with respect to calculation of the rotational barrier around the C(sp2)–C(aryl) bond. It must be concluded that current DFT functionals cannot properly handle such barriers. Conversely, successfully describing these rotational barriers may be an appropriate and stringent test for new functionals.