Limits to barriers to internal rotation in benzal fluoride, chloride, and bromide solution by the J method

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A complete analysis of the 1H and 19F nmr spectra of 2,5- and 3,4-difluorobenzotrifluoride, together with multiple resonance experiments, yields the signs and magnitudes of the long-range 19F,19F and 1H,19F spin–spin coupling constants. The coupling mechanisms are discussed. In particular, the coupling over six bonds, [Formula: see text], whose sign is interpretable in terms of a σ–π mechanism, is too large in magnitude when compared to [Formula: see text], and [Formula: see text] in the analogous compounds. These latter three couplings are consistent in sign and magnitude with what is known about hyperfine interaction constants. The magnitudes of [Formula: see text] are reported for 4-fluorobenzotrifluoride, 3-amino-4-fluorobenzotrifluoride, 3-nitro-4-fluorobenzotrifluoride, as are 6JpF,F values for p-fluorobenzal fluoride and p-fluorobenzyl fluoride. In contrast to 6JpH,CH and 6JpF,CH it seems unlikely that, unless its coupling mechanism becomes more precisely understood, 6JpF,CF will be a reliable indicator of conformational preferences.

Section: Long-range 19fc19~ Couplingssupporting

confidence: 62%

Section: Introductionmentioning

confidence: 99%

Spin–spin coupling constants between side-chain and ring fluorine nuclei in some benzotrifluoride, benzal fluoride, and benzyl fluoride derivatives: coupling mechanisms

Schaefer¹,

Niemczura²,

Wong³

et al. 1979

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“…Nevertheless, assume a 6~0 of -0.02 Hz for benzal chloride. Then (sin2 0) becomes 0.15 (1) for the CDC13 solution and the apparent V2 rises to 10.2(6) kT/mol only. Finally, although not supported by the MO calculations, consider the possible presence of a fourfold component in the internal rotational potential.…”

Section: Solvent Dependence Of the Barriermentioning

confidence: 95%

“…The magnitude of 6~9 0 depends on the electronegativity of the substituents on the side chain. The large uncertainty in 6~9 0 at that time ( I ) led to the range of barriers quoted above, although a value of 9.2 t 1.3 kJ/mol was assessed with a lesser degree of confidence (1). The measured value of 6J suggested that 1 was the conformation of lowest energy in CS2 solution.…”

Section: Introductionmentioning

confidence: 95%

Concerning the internal rotational potential in benzal chloride. ³⁷Cl/³⁵Cl isotope effect on the methine ¹H NMR

Schaefer¹,

Sebastian²

1989

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. Can. J. Chem. 67, 2053Chem. 67, (1989. 'H NMR spectral parameters are reported for benzal chloride in CS2/CJIl2, CDC13, and acetone-d6 solutions. The values of 6J (H,CH), the long-range spin-spin coupling constant between the side-chain and para protons, are consistent with twofold barriers to rotation about the exocyclic C-C bond of 8.6(4), 8.9(4), and 10.2(6) kJ/mol, in the order of the solvents above. These numbers are somewhat smaller than the twofold barrier of 11.6 kJ/mol available from STO-3G MO computations for the free molecule. Alternatively, an internal rotational potential consisting of a twofold term of 17.0 W/mol and a fourfold term of -5.5 kJ/mol is also compatible with the 6 J (~,~H ) measured for the CDC13 solution and with the 13c spin-lattice relaxation data reported for such a solution. The 6~ (H,CH) values demonstrate that the conformer with the a C-H bond situated in the benzene plane is the most stable. A local minimum in the potential for a conformer in which this bond lies in a plane perpendicular to the phenyl group, leading to a population of about 8% for this conformer at 300 K and compatible with the magnitude of 6J (H,CH), is represented by twofold and fourfold terms of 6.0 and 4.5 kJ/mol, respectively. The barrier height is then only 8.0 kJ/mol, however. 37C1/35CI isotope effects on the methine proton amount to -0.17, -0.19, and -0.22 parts per billion in CS2, CDC13, and acetone-d6 solutions, respectively (increased screening).

“…Substitution in the methyl group to give a CH2X or a CHX, sidechain changes the sixfold barrier for a CH, group to one of a predominantly twofold nature. These twofold barriers have been investigated in solution for X = F, C1, Br, I, CH, (3)(4)(5) and a variety of other X substituents (6,7). The investigations are mainly based on the sin2 8 dependence of 6~p H .…”

mentioning

confidence: 99%

Conformational preferences of the fluoromethyl group in p-methylbenzyl fluoride and some derivatives

Schaefer¹,

Niemczura²,

Sebastian³

et al. 1980

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. Can. J . Chem. 58, 1178Chem. 58, (1980.The coupling over seven bonds between IyF nuclei and protons in p-methylbenzyl fluoride and a series of ring-substituted derivatives is used to estimate the value for a conformation in which the C-F bond lies in a plane perpendicularto the benzene ring. The J method is then used to show that in CS, solution, the conformational preference of the fluoromethyl group is very weak in 33-dichloro-4-methylbenzyl fluoride and in 33-dibromo-4-methylbenzyl fluoride. The barrier to rotation about the exocyclic carbon-carbon bond is perhaps as large as 0.2 kcallmol. Inp-methylbenzyl fluoride, the C-F bond prefers a plane perpendicular to the benzene plane by ca. 0.9 kcallmol. Good agreement is found with the conformational preferences based oncouplings over six or seven bonds between the methylene protons and the pnrcr ring or methyl protons. The deductions are compared with calculations at various levels of molecular orbital theory.TED SCHAEFER, WALTER P. NIEMCZURA, RUDY SEBASTIAN, LEONARD J. KRUCZYNSKI et WERNER DANCHURA. Can. J. Chem. 58, 1178Chem. 58, (1980.On a utilise les couplages a travers sept liaisons, entre le I9F et les protons du fluorure dep-methylbenzy le et d'une seriede derives cycliques substitues dans le but de determiner cette valeur pour une conformation de la liaison C-F qui se situe dans le plan perpendiculaire au noyau benzenique. On a alors utilise la methode J pour montrer qu'en solution dans le CS,, la preference conformationnelle dugroupe fluoromethyle est trks faible dans le cas du fluorure desdichloro-3,5, et dibromo-3,5 methyl-4 benzyles. La barrittre a la rotation autour de la liaison exocyclique carbone-carbone est peut 6tre de 0.2 kcallmol. Dans le cas du fluorure de p-mkthylbenzyle, la liaison C-F se situe de preference dans le plan perpendiculaire a celui du benzene avec une barrittre a la rotation de 0.9 kcallmol. On a trouve que ces resultats s'accordent parfaitement avec les preferences conformationnelles evaluees sur la base des couplages a travers 6 ou 7 liaisons entre les protons mkthyleniques et les protons situes en positionpnr.~ sur le cycle ou les protons du groupe methyle. On a compare les deductions ainsi faites avec des calculs d'orbitales moleculaires effectues a divers niveaux.[Traduit par le journal]