Theoretical and experimental barriers to internal rotation in 2,6-difluorobenzaldehyde and 2,4,6-trifluorobenzaldehyde. Relatively low barriers

Abstract

Abstract: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-', respectively, as determined from 1 9~{ ' H ) dynamic nuclear magnetic resonance measurements. For the parent compound AG* is 32.2 kJ mol-I in the same solvent. These free energy bamers, 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, MP216-31G*, finds a bamer height of 18.6 W 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 bamers (AE values at absolute zero for the free molecules) and the free energy barriers for benzaldehyde and the four fluoro derivatives; the theoretical bamers utilize 6-31G** and correlation-gradient MP216-31G* 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 bamers 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 AG* and AE, 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 W mol-', the AG* values from 18.8(5) to 34.4 kJ mol-I , and the MP216-31G* (correlation-gradient) barriers span 18.6 to 36.8 kJ mol-' 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.RCsumC : En se basant sur des mesures de rksonance magnitique nuclCaire dynamique du 1 9~{ ' H I , on a Ctabli que les Cnergies libres d'activation, 5 110 K, pour la rotation autour des liaisons exocycliques C-C des 2,6-difluoro-et 2,4,6-trifluorobenzaldChydes, en solutuions dans le mCthoxymCthane, sont respectivement de 1 8 3 ? 0,5 et 20,O ? 0,5 kJ mol-I . Pour le ...

1H nuclear magnetic resonance and molecular orbital studies of 2-formylstyrene. Three significant nonplanar conformers at 300 K

“…than, for example, the free energy of activation of 32.2(6) kJ/mol in solution (19)(20)(21). In contrast, the 6-31G* barrier for styrene is only 12.0 kJ/mol (1 1) while, experimentally, the internal bar-' There is a controversy concerning the internal barrier for the free molecule (22). However, it is agreed that the barrier in solution is not less than 27 Idlmol, so that large out-of-plane deviations of the carbonyl group are not expected at 300 K except for 8 values other than 90" in B.…”