Computations and measurements of the structures and conformations of 2- and 3-fluorobenzaldehyde in the gas and in solution

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. 69, 1039 (1991). The 'H nuclear magnetic resonance spectra of 2-cyanobenzaldehyde (2CNB) and 3-cyanobenzaldehyde (3CNB) in CS2/C6D12 and acetone-d6 solutions at 300 K yield precise stereospecific long-range proton-proton coupling constants. These are used to establish the conformational population of the o-cis and o-trans conformers of these relatively polar molecules. For example, the fractional o-cis population of 2CNB changes from 0.12(4) in CS2/C6DI2 to 0.46(6) in acetone-d6, whereas that of 3CNB is 0.48(2) in both solvents. Extrapolation to the vapor phase, using a dielectric model, implies a negligible concentration of the o-cis conformer of 2CNB and a roughly 50% abundance of each conformer of 3CNB. Computations at various levels of molecular orbital theory provide estimates of the rotational bamer of the aldehyde moiety and confirm the planar structure of each conformer. The geometries of three conformers are given as obtained from the 6-3 IG MO basis and may be useful to molecular spectroscopists. Theoretical and experimental dipole moments are interpolated to yield estimates of their magnitudes for the four planar conformers. Somewhat less precise 'H nrnr spectral parameters (than for the above solutions) are also obtained for dilute solutions in benzene-d6 at 300 K. ' The conformational distributions based on these parameters are compared with their only other measurement, based on dipolar moments in benzene at 298 K. Good agreement between the results of the two methods is found for 3CNB but not for 2CNB. It is suggested that specific interactions occur between benzene solvent and solute molecules, particularly for 3CNB, for which these interactions stabilize the conformer having a low dipole moment. Remarkable changes in the intraring proton-proton coupling constants occur in going from CS2/C6DI2 to acetone-d6 solution.Key words: 2-and 3-cyanobenzaldehyde (2CNB and 3CNB): 'H NMR, conformations, long-range spin-spin coupling constants, MO computations.TED SCHAEFER, KERRY J. COX et RUDY SEBASTIAN. Can. J. Chem. 69, 1039 (1991). Les spectres de resonance magnttique nuclCaire du 'H du 2-cyanobenzaldChyde (2CNB) et du 3-cyanobenzaldthyde (3CNB), mesurtes B 300 K, dans des solutions dans le CS2/C6DI2 et dans I'acCtone-d6, fournissent des constantes de couplage proton-proton 2 1 longue distance trks prCcise. On les a utiliskes pour dCterminer les populations conformationnelles des conformkres o-cis et o-trans de ces molCcules relativement polaires. Par exemple, la fraction de la population o-cis du 2CNB varie de 0,12(4) dans le CS21C6D12 i 0,46(6) dans l'acttone-d4; par ailleurs, celle du 3CNB est Cgale h 0,48(2) dans les deux solvants. Une extrapolation jusqu'h la phase vapeur, utilisant un modkle ditlectrique, suggkre que la concentration du conformkre o-cis du 2CNB est alors ntgligeable, mais que celle du 3CNB est approximativement Cgale a 50%. Des calculs i divers niveaux de la thCorie des orbitales moltculaires fournissent des tvaluations de la barrikre 1 la rotation de la portion aldkhydique et con...

Section: Spectral Analysesmentioning

confidence: 69%

The conformational distribution of 2-cyanobenzaldehyde and 3-cyanobenzaldehyde in solution and in the vapor

Schaefer¹,

Cox²,

Sebastian³

1991

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“…'Except where otherwise noted, the standard deviations in all parameters were <lo-' Hz. /The marked solvent dependence of "J(H, F) within the phenyl group for a variety of fluorobenzene derivatives is well documented (32)(33)(34)44); more polar solvents increase 'J(H, F) algebraically, that is, a negative 'J(H, F), for example, becomes smaller in magnitude (35,36). Tables 1 and 2 for details) were filtered into 5 mm 0.d.…”

Section: Methodsmentioning

confidence: 99%

Experimental and theoretical assessments of the substituent and medium dependence of the internal rotational potentials in benzyl fluoride. 3,5-Difluorobenzyl fluoride and 4-fluorobenzyl fluoride

Schaefer¹,

Schurko²,

Sebastian³

et al. 1995

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The 'H, 1 9~, and I3c (HI nuclear magnetic resonance spectra at 300 K of 4-fluoro-and 3,5-difluorobenzyl fluoride, dissolved in CS,-C6DI2 and acetone-d,, are fully analyzed. Spin-spin coupling constants over four, five, and six formal bonds are used to derive expectation values of sin28 and sin2+ and the apparent twofold internal rotational potentials; +(8) is the angle by which the a C-F (C-H) bond twists out of the ring plane. The conformation of lowest energy has 4 = 0" for the 3,5-difluoro compound in the polar and nonpolar solutions, whereas it has 4 = 90" for the 4-fluoro derivative. The magnitudes of the potentials lie between 2 and 4 kJImol, that is, comparable to thermal energies. These data are compared with previous results for the parent compound and its 3,5-dichloro derivative. Geometry-optimized molecular orbital computations,

“…The dipole moment of 2,6-difluorobenzaldehyde is not known. However, the experimental dipole moment of 2-fluorobenzaldehyde is reasonably well reproduced by the mean of the values calculated with the STO-3G and 6-31G bases (24). For the 2,6-difluoro derivative, the corresponding means is 4.15 D for the planar and 3.78 D for the perpendicular form.…”

Section: Discussionmentioning

confidence: 65%

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

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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 ...