The free energies of activation at 110 K for rotation about the exocyclic C—C bonds in 2,6-difluorobenzaldehyde and 2,4,6-trifluorobenzaldehyde, in dimethyl ether solutions, are 18.8 ± 0.5 and 20.0 ± 0.5 kJ mol−1, respectively, as determined from19F{1H} dynamic nuclear magnetic resonance measurements. For the parent compound ΔG≠is 32.2 kJ mol−1in the same solvent. These free energy barriers, the lowest available for benzaldehyde derivatives, are likely a result of steric and electrostatic repulsions between the C+—O−and C+—F−bonds. Computations of the spectroscopic barrier in the 2,6-difluoro compound at various levels of molecular orbital theory imply that the barrier is predominantly twofold, with a fourfold component of opposite sign, whose magnitude is about 10% of the twofold component. A correlation-gradient computation, MP2/6-31G*, finds a barrier height of 18.6 kJ mol−1for this compound, lower by 3.0 kJ mol−1than found with the 6-31G* basis and 2.9 kJ mol−1with 6-31G**. Similar computations are compared for the parent compound and the 4-fluoro, 2,4,6-trifluoro, and 3,5-difluoro derivatives. Linear relationships exist between the computed spectroscopic barriers (ΔEvalues at absolute zero for the free molecules) and the free energy barriers for benzaldehyde and the four fluoro derivatives; the theoretical barriers utilize 6-31G** and correlation-gradient MP2/6-31G* procedures. For the 2,6-difluoro derivative, the computed frequencies of the torsional motions about the exocyclic C—C bond yield spectroscopic twofold barriers. These barriers are much lower than the computed energy differences between the planar and perpendicular conformers, perhaps because the negative fourfold components flatten the potential at its minimum. A rough estimate of the relationship between ΔG≠and ΔE0for the 2,6-difluorobenzaldehyde suggests that the solvent increases the internal barrier by only about 3 kJ mol−1. By way of contrast, the AM1 barriers, scaled by a factor of 1.9 (as previously recommended) range from 17.3 to 22.6 kJ mol−1, the ΔG≠values from 18.8(5) to 34.4 kJ mol−1, and the MP2/6-31G* (correlation-gradient) barriers span 18.6 to 36.8 kJ mol−1for benzaldehyde and the four fluorine derivatives. It seems likely that the internal barrier in benzaldehyde is considerably larger than that modeled on torsional frequencies.Keywords: Free energies of activation, internal rotational barriers in 2,6-difluoro- and 2,4,6-trifluorobenzaldehyde; molecular orbital computations, internal rotational barriers in 2,6-difluoro- and 2,4,6-trifluorobenzaldehyde; correlation gradient computations on internal barriers in benzaldehyde and four of its fluorine derivatives.