Electron Coupling of Nuclear Spins. Viii. Temperature Dependence of Jff in 2-Fluorobenzotrifluorides

摘要

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 the coupling constant 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. An approximate analysis of the temperature dependence of the coupling constant indicates that the energy shift is about 1 kcal for X = H, C1, and I; however, the stable orientation of the CF3 group is rotated by 60 degrees in the 6-C1 and 6-I derivatives from that in the 6-H. 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.