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 from 19F{1H)dynamic nuclear magnetic resonance measurements. For the
parent compound dG* is 32.2 kJ mol-1 in 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+--0- 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-I for this compound, lower by 3.0 kJ mol-I than found with the 6-
31G* basis and 2.9 W mol-' with 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 (dE values 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 baniers. 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 dG* and dE, for
the 2,6-difluorobenzaldehyde suggests that the solvent increases the internal barrier by only about 3 kJ mol-I .
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 dG* values from 18.8(5) to 34.4 kJ mol-I , and the MP2/6-31G* (correlation-gradient) barriers
span 18.6 to 36.8 kJ mol-1 for benzaldehyde and the four fluorine derivatives. It seems likely that the internal
banier in benzaldehyde is considerably larger than that modeled on torsional frequencies.
Key words: Free energies of activation, internal rotational baniers 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.
各種検索
サポート
ヘルプ