No abstract

The 1H and 19F nmr spectra of benzyl fluoride, although very tightly coupled at 300 MHz, are analyzed for dilute CS2 and acetone solutions. 13C, 19F spin–spin coupling constants for benzyl fluoride in a series of solvents are also measured. Geometry-optimized STO 3G MO computations on benzyl fluoride indicate a small twofold barrier to rotation about the exocyclic C—C bond in the gas phase. All the long-range couplings between 19F and ring protons or carbon-13 nuclei and between methylene and ring protons are consistent with the conclusion that the barrier to internal rotation in benzyl fluoride is small and that the conformation in which the C—F bond lies in a plane perpendicular to the benzene plane is stabilized by 2 kJ/mol in going from CS2 to DMSO or acetone solutions. The solvent dependence of the internal barrier may account for the diversity of conformational conclusions in the literature. Furthermore, it is clear that the internal barrier will depend on the presence of substituents at any site in the ring.

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] !

The 1H nmr spectra of benzyl chloride in dilute CS2 and acetone-d6 solutions are analyzed. The long-range coupling constants are consistent only with a low-energy conformation in which the C—Cl bond lies in a plane perpendicular to the benzene plane. Geometry optimized computations at the STO 3G level of molecular orbital theory agree with this conclusion and yield a nearly pure twofold barrier to internal rotation of 8.6 kJ/mol. In CS2 solution the long-range couplings yield 8.8 kJ/mol, rising to 11.2 kJ/mol in acetone solution. This increase in the internal barrier in a polar solvent is similar to that found for benzyl fluoride, but in the latter the barrier itself is very much smaller than in benzyl chloride.

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,