19F nuclear magnetic resonance studies of aromatic compounds. Part V. Conformational preferences of side chains in ortho-substituted para-fluoro-acetophenones,-α-methylstyrenes, and -αα-dimethylbenzyl alcohols

Abstract

The shielding of para-fluorine nuclei in the series of acetophenones and α-methylstyrenes named in the title is used to estimate the interplanar angle between the side chain and the ring in the preferred conformations. In the series of dimethylbenzyl alcohols, an analogous torsion angle (between the C–OH bond and the aromatic ring) is defined and estimated. No information is obtainable from the ¹⁹F n.m.r. studies about the cisoid or transoid relationship between the side chain and ortho-substituents, since the estimated angles may correspond to either situation. The method of analysis utilises CNDO/2 MO calculations of the way in which charge-density distribution varies with the side chain conformation in a representative member of each series.

The effect of temperature variation, in the range –40 to 140°, upon the ¹⁹F spectra of the series of 2-substituted 4-fluoroacetophenones is studied. The averaging about the zero-point energy of the barrier appears to be temperature-dependent; consequently no information can be obtained about the height of the barrier or the energy differences between conformations.