Electron Coupling of Nuclear Spins. VIII. Temperature Dependence of JFF in 2-Fluorobenzotrifluorides

Abstract

The F–F coupling constant between the CF3 group and the fluorine atom at the 2 position was measured and found to have an appreciable temperature dependence in several 2-F, 6-X-benzotrifluorides. The magnitude of JoFF (F–CF3) and the direction of its change with temperature is consistent with the existence of two different rotational conformations of the CF3 group, whose relative stability depends upon the balance of repulsive steric interactions between the CF3 group and the 2 and 6 substituents. These results were supported and extended by measurements of the fluorine chemical shifts for the CF3 and 2-F, both of which move downfield with increasing temperature. For a particular compound, the ΔE between the two conformations is the potential barrier to internal rotation of the CF3 group, and the temperature dependences of JoFF (F–CF3) and of the fluorine chemical shifts are attributed to thermally excited torsional oscillations and reorientations of the CF3 group in the potential well. An approximate analysis of the temperature dependence of JoFF (F–CF3) indicates that ΔE is about 1 kcal for X = H, Cl, and I; however, the stable orientation of the CF3 group is rotated by 60° in the 6 Cl and 6 I derivatives from that in the 6 H. Similar conformational isomers should exist for 2,6-asymmetrically disubstituted toluenes, and more complex isomers are possible when the threefold symmetry of the CF3 and CH3 groups is reduced by substitution to CF2X and CH2X or to CFXY and CHXY. Values are reported for the long-range F–F and F–H coupling between the CF3 group and fluorines and protons at the meta and para positions in several benzotrifluorides.