Rotational barrier of benzaldehyde as determined by c-13 nmr spectroscopy

Lunazzi, Lodovico; Macciantelli, Dante; Boicelli, Andrea

doi:10.1016/s0040-4039(00)72095-8

Cited by 34 publications

(17 citation statements)

References 7 publications

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“…27, assuming a v, of 85 cm-' and that the transition state can be reached in two ways from the planar ground state, yields AG:', = AH* -TAS* = 19.6 -0.6 or 19.0 kJ mol-'. The AS* of 5.5 J mol-I K-' is rather smaller than the 15 J mol-' K-' obtained for benzaldehyde in dimethyl ether solution by DNMR; the range of temperature in this experiment was only 21°, however, so that a rather large error must be expected (18). Of course, had the MP2 result of 18.6 kJ mol-' for AE, been used in this path towards AG*, assuming the same magnitudes of corrections, the estimated value of AG:,, would be 16.1 kJ mol-' for the free molecule.…”

Section: The Free Energy Barrier In 26-difluorobenzaldehydecontrasting

confidence: 65%

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

Schaefer¹,

Takeuchi²,

Bernard³

et al. 1995

Can. J. Chem.

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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 ...

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Section: The Free Energy Barrier In 26-difluorobenzaldehydecontrasting

confidence: 65%

“…The AG* of about 19 kJ mol-' for 2,6-difluorobenzaldehyde at 110 K is roughly 13 kJ mol-' lower than that of 32 kJ mol-' at 160 K, also in dimethyl ether, for benzaldehyde (18). It is likely that dipole-dipole repulsion between Cf=O-and orrho…”

Section: The Free Energy Barrier In 26-difluorobenzaldehydementioning

confidence: 88%

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

Schaefer¹,

Takeuchi²,

Bernard³

et al. 1995

Can. J. Chem.

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“…In other words, there was no significant coupling, this number being a consequence of the least-squares analysis procedure. Because the internal barrier to rotation in benzaldehyde in solution is greater than 30 kJ/mol (27)(28)(29)(30)(31) (32,34). Using the factor of 0.52 above, 'J(H,C) in acetophenone follows as 0.22 Hz, as observed (Table 1).…”

Section: Comparisons With "J(hcho) In Benzaldehydementioning

confidence: 63%

The carbonyl group as a reluctant transmitter of hyperconjugative or σ–π spin–spin coupling interactions in derivatives of benzaldehyde, acetophenone, and benzophenone

Schaefer¹,

Peeling²,

Penner³

et al. 1986

Can. J. Chem.

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Unlike their counterparts in anisole or toluene derivatives, the six-bond spin-spin coupling constants between para ring protons or 19F nuclei and protons or 13c nuclei in the sidechain of derivatives of benzaldehyde, acetophenone, and benzophenone can apparently contain components of opposite sign, at least for the fluorine derivatives. The u-7i components are much smaller in magnitude than in toluene derivatives, leading to very small or unobservable coupling constants. Consequently they are of limited use in conformational analysis. INDO MO FPT computations and their modifications are examined as to the reasons for the small o-TI magnitudes. Although the spin polarizability of the 2pz orbital on oxygen appears to play an important role in the transmission of nuclear spin state information, the computations do not account for a "F coupling mechanism that appears to be significant for planar conformations. On the other hand, spin-spin coupling constants over five formal bonds to meta protons are sizeable and stereospecific.TED SCHAEFER, JAMES PEELING, GLENN H. PENNER et ALBERTA LEMIRE. Can. J. Chem. 64, 1859 (1986).Contrairement leurs contreparties dans les dCrivCs de l'anisole ou du toluene, les constantes de couplage spin-spin a travers six liaisons, entre les protons du cycle en position para ou les noyaux de I9F et les protons ou les noyaux de I3C de la chaine laterale des dCrivCs du benzaldehyde, de I'acCtophCnone et de la benzophCnone peuvent apparernment contenir des composants de signes opposes, au moins dans les derivCs fluores. Les constituants a-7i ont une intensit6 infkrieure a celle des dCriv6s du toluene conduisant ainsi a des constantes de couplage tr&s faibles qu'on ne peut observer. En consequence, ces constantes sont trbs peu utiles dans l'analyse conformationnelle. On a utiIisC des calculs INDO OM FPT ainsi que leurs modifications pour dCterminer les causes des faibles intensitks des constituants cr-TI. Bien que la polarisabilitk du spin de l'orbitale 2pz de I'oxygkne semble jouer un r6le important dans la transmission de I'information sur 1'Ctat du spin nuclkaire, les calculs ne tiennent pas compte du mCcanisme de couplage du 19F qui semble Ctre important dans les conformations planes. Par ailleurs, les constantes de couplage spin-spin a travers cinq liaisons formelles par rapport aux protons en position meta sont quantifiables et stCrCospCcifiques.[Traduit par la revue]

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“…For the high-temperature range, [D 6 ]DMSO (b.p. If the temperature has to be decreased well below -100°C, the samples have to be prepared in a more sophisticated way, with use of liquefied gases such as Me 2 O, [15] vinyl chloride, [16] propane, [17] propene, [18] Freons ® or mixtures of these (see Table 1). CHF 2 Cl/CHFCl 2 , 3:1 -182 [22] CHF 2 Cl/CHFCl 2 , 5:1 -190 [23] CHF 2 Cl/CHFCl 2 /CF 2 Cl 2 , 3:1:1 -170 [24] CHF 2 Cl/CHFCl 2 /CHF 3 , 5:1:1 -188 [25] CHF 2 Cl/CHFCl 2 /CHF 3 , 3:1:1 -182 [26] CCl 2 F 2 /CBrF 3 , 4:1 -157 [27] CF 2 Cl 2 /CBrF 3 , 2:1 -166 [28] CHFCl 2 /CCl 2 F 2 , 1:1 -165 [29] CHF 2 Cl/CHFCl 2 , 1:1 -174 [30] CHCl 2 F/CH 2 =CHCl, 6:1 -160 [31] Me 2 O/THF, 3:1 -140 [32] THF/Et 2 O, 3:2 -135 [32] CHF 2 Cl/CDFCl 2 , 3:1 -171 [33] CH 2 =CHCl/CS 2 , 4:1 -131 [34] CH 2 =CHCl/CS 2 , 3:2 -132 [34] CH 2 =CHCl/CHFCl 2 , 5:2 -152 [35] CD 2 Cl 2 /[D 8 ]toluene, 1:1 -120 [36] The use of hydrogen-containing solvents is often impractical because of their very strong signals, which can overlap with the signals of the dilute solute.…”

Section: Nmr Samplesmentioning

confidence: 99%

Recent Advances in Stereodynamics and Conformational Analysis by Dynamic NMR and Theoretical Calculations

2010

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Rotational barrier of benzaldehyde as determined by c-13 nmr spectroscopy

Cited by 34 publications

References 7 publications

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

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

The carbonyl group as a reluctant transmitter of hyperconjugative or σ–π spin–spin coupling interactions in derivatives of benzaldehyde, acetophenone, and benzophenone

Recent Advances in Stereodynamics and Conformational Analysis by Dynamic NMR and Theoretical Calculations

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