The conformation of α,α,α,-trifluoroanisoles investigated via the n.m.r. spectra of liquid crystalline solutions

Abstract

The proton and fluorine n.m.r. spectra of samples of 4-fluoro-and 4-iodo-sc,a,x-trifluoromethoxybenzene dissolved in the nematic liquid crystal solvent 135 have been obtained. Analysis of these spectra yields a set of dipolar coupling constants whose values are used to investigate the form of V ( q ) , the potential governing rotation about the phenyl 0-bond. V(cp) is found to be very different from the form found for the corresponding potential in methoxybenzene in that the minimum occurs when the 0-CF, bond is in the plane orthogonal to the phenyl ring (cp = 90"). The data cannot, however, distinguish between a potential of the form V, + V,cos 2q, and one containing a substantial V4 term, that is V ( q ) = V, + v, cos 2q + V4 cos 4q.

Long-range 1H,19F and 13C,19F coupling constants and molecular orbital computations as indicators of internal motion in C6H5OCF3 and its 4-fluoro derivative

“…It also suggests that the simple twofold potential, based on dipolar couplings, is quantitatively incorrect because it strongly favors large 0 values. On the other hand, 4J is roughly consistent with the alternative potential based on the dipolar couplings in a nematic solvent (9).…”

“…By way of contrast, the potentials derived from intramolecular dipolar coupling constants for this compound in a nematic solvent can be written as V(0)/kl mol-I = -13.5 sin20 or V(@)/kJ mol-' = -1.8 sin2@ -14.1 sin220, the latter producing a slighter better fit (9). Both potentials have the conformer as most stable, and yield (sin2@) as 0.885 and 0.656 at 300 K. Comparison with the MO potentials (Tables 1 and 2) would indicate-a large solvent dependence of the internal rotational potential.…”

Structure and Conformation of 4-Fluoro(trifluoromethoxy)benzene: Gas Electron Diffraction and Quantum Chemical Calculations

“…From 1 H and 19 F NMR spectra of this compound in a nematic solvent, a potential function for internal rotation around the C-O bond with a minimum at φ(C-Ο) ) 90°(perpendicular conformer) was derived. 12 The data, however, cannot differentiate between a potential function with a single minimum at φ(C-Ο) ) 90°and one with two minima at φ(C-Ο) ) 90°and 0°, i.e., between a single perpendicular conformer and a mixture of both forms. On the other hand, longrange NMR coupling constants are interpreted in terms of a planar or nearly planar structure, with the barrier to internal rotation lower than that in anisole.…”

“…It is expected that fluorine substitution in para position has only a minor effect on the conformational properties and that GED intensities are more characteristic for the orientation of the CF 3 group due to additional interatomic distances between the p -fluorine atom and the CF 3 group. From 1 H and 19 F NMR spectra of this compound in a nematic solvent, a potential function for internal rotation around the C−O bond with a minimum at φ(C−Ο) = 90° (perpendicular conformer) was derived . The data, however, cannot differentiate between a potential function with a single minimum at φ(C−Ο) = 90° and one with two minima at φ(C−Ο) = 90° and 0°, i.e., between a single perpendicular conformer and a mixture of both forms.…”

1H, 19F, and 13C nuclear magnetic resonance approaches to the barrier to rotation about the Csp2—Csp3 bond in 1,1,1 -trifluoro-2-phenylethane. Proximate coupling constants and molecular orbital computations

“…The magnitude of the barrier to rotation is uncertain, although it appears that the perpendicular conformer is most stable, as indicated by 6-3 1G computations (27) and by alternative internal rotational potentials derived from residual dipolar coupling constants measured for solutions in nematic solvents (28). In the 4-fluoro derivative, 5~(~, CF,) is +0.95Hz (27).…”