Some unique properties of in 2-fluorotoluene derivatives. Conformational dependence

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A precise analysis of the 'H nmr spectrum of toluene as a dilute solution in carbon disulfide yields a revised set of spectral parameters. The chemical shift of the para proton lies 12.6 ppb to low frequency of that of the ortho protons at 300 K. The ring proton chemical shifts are discussed and compared with 'H and 3H shifts observed in carbon tetrachloride. The long-range couplings between methyl and ring protons can be said to be quantitatively understood in terms of a and u-a electron transmitted mechanisms. The changes observed in these three couplings in phenylacetaldehyde can be quantitatively reproduced in terms of these mechanisms and also illustrate how these changes are direct measures of the conformational preferences in this molecule.TED SCHAEFER, RUDY SEBASTIAN et GLENN H. PENNER. Can. J. Chem. 63, 2597(1985.Une analyse prCcise des spectres rmn du 'H du tolukne, en solution diluCe dans le disulfure de carbone, a permis d'obtenir un nouvel ensemble de paramktres spectraux. A 300 K, le dCplacement chimique du proton para se situe ?I une frkquence de 12,6 ppb plus basse que celle des protons ortho. On discute des dCplacements chimiques des protons du cycle et on les compare avec les dCplacements des 'H et 3~ qui ont Ct C determinks en solution dans le tetrachlorure de carbone. On peut dire que I'on peut expliquer quantitativement les couplages ti longue distance entre les protons du cycle et ceux du groupement mCthyle en faisant appel ?I des mkcanismes impliquant que les Clectrons sont transmis par les liaisons u et u-a. On peut reproduire quantitativement les changements observks dans ces trois couplages du phCnylacCtaldChyde en faisant appel ?I ces mkcanismes; de plus, on peut illustrer comment ces changements sont des mesures directes des prCfCrences conformationnelles dans cette moltcule.[Traduit par le journal] !

Section: The Chemical Shifismentioning

confidence: 91%

¹H nuclear magnetic resonance spectral parameters of toluene. Implications for conformational analysis and isotope shifts

Schaefer¹,

Sebastian²,

Penner³

1985

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“…The assignment of H-3 and H-5 follows umambiguously from the magnitudes of their couplings to the methyl protons, always being larger than 6~,y'CH"n toluene derivatives (36)(37)(38). Furthermore, 3~3 4 is larger than 'Jq5, as expected from known substituent effects on vicinal coupling constants in benzene derivatives (39,40).…”

Section: Methodsmentioning

confidence: 79%

Concerning in 2-methylphenol and its derivatives. A proximate coupling

Schaefer¹,

Wildman²,

Sebastian³

1985

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. Can. J. Chem. 63, 773 (1985). In 2-chloro-6-methylphenol in CCI, solution, ' J :~.~~' , the spin-spin coupling between the hydroxyl and methyl protons, is +68(3) mHz. In this compound, the hydroxyl group lies trans to the methyl group. If the coupling becomes -120 ? 60 5 O H . C H I . mHz in a cis orientation of these two groups, then the apparent vanishing of . J , ,In some derivatives of 2-methylphenol is understandable, as well as its presence in other derivatives. Computations of this coupling imply its proximate mechanism and can rationalize its value of -220 ? 20 mHz in 2-tert-butyl-6-methylphenol. Analogies are noted with ' J , S ) H '~~' in 2-trifluoromethyl phenol and with a coupling in styrene, which also appears to vanish in certain orientations of the vinyl group. TED SCHAEFER, TIMOTHY A. WILDMAN et RUDY SEBASTIAN. Can. J. Chem. 63, 773 (1985). Dans le chioro-2-methyl-o phenol en solution dans le CCI,, la constante de couplage spin-spin, ' J :~.~~' , entre les protons de I'hydroxyle et du groupement methyle est Cgale i +68(3) mHz. Dans ce composC, le groupement hydroxyle occupe une position trans par rapport au groupement methyle. Si la valeur de ce couplage devient egale a -120 2 60 mHz pour une orientation cis de ces deux groupes, on peut alors facilement comprendre I'apparente disparition du 5~: H ' C H ' dans quelques derives du methyl-2 phenol ainsi que sa presence dans d'autres derives. Des calculs de ce couplage suggbrent son m6canisme par proximite et peuvent rationaliser sa valeur de -220 2 20 dans le tert-butyl-2 methyl-6 phenol. On note un certain nombre d'analogies avec les valeurs de ' J :~'~" dans le trifluoromethyl-2 phenol ainsi qu'avec un couplage dans le styrene qui semble aussi disparaitre avec certaines orientations du groupement vinyle.[Traduit par le journal]

“…Thus, it has been shown (27,28) that some long-range A values for 13C and 19F nuclei can be interpreted as arising from a change in electron density at the carbon and fluorine atoms, caused by a change in the bond length of the remote C-H bond. This is an interesting suggestion, although it should be noted that, while 6~$.CH in toluene is transmitted via the IT electrons, it is insensitive to the presence of ring substituents (19,29), whereas the electron densities at the various sites in the ring are markedly dependent on the nature of the substituents.…”

Section: Caveatmentioning

confidence: 93%

Solvent and α-substituent perturbations of the ²H/¹H isotope shifts in the ¹³C nuclear magnetic resonance of toluene-α-d₃

Schaefer¹,

Peeling²,

Sebastian³

1987

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The 2H/1H isotope effect on l3C nmr chemical shifts, "A, n being the number of intervening bonds between 'H and I3C nuclei, in toluene-a-d3 is solvent dependent. For example, ' A ranges from 817ppb in CD30H to 869ppb in acetone-d6 solutions, a positive number indicating increased shielding in the deuterated species. 'A is linearly dependent on a function of the refractive index, n~, of the solvent, allowing extrapolation to n~ = 1. The hyperconjugative model, in which the C-D bond is a poorer electron donor to the aromatic system than is a C-H bond, is tested for the substituents CHID, CHD2, CD3, CHDCH3, CD2CH3, CD(CH3)2, C6H5CHD, and (C6H5)2CD. For these substituents, the negative 5A is linearly related to the expectation value of sin2 0; 0 is the angle by which the C-D bond twists out of the benzene plane. The model fails quantitatively for C6H5CD2X (X = C1, COOH, CN, OH). For X = OH, very large negative 5A and 3A values are observed. "A is also reported for 4-ethyltoluene-a-d3 and benzaldehyde-a-dl. For the latter, all "A values are positive other than 5A, which vanishes in acetone-d6 solution.