F–F Spin—Spin Coupling in Perfluoro-o-Xylene

The F–F coupling constants between the ortho-ring fluorines and the fluorines of the CF3 group were measured at different temperatures in perfluoro-o-xylene and some related compounds. The rotationally averaged coupling constants calculated by using the approximate dependence of the JFF upon the dihedral angle between the C–F bond of the CFX2 group and that of the 2-fluorine atom are in good agreement with the observed values. Qualitative conclusions about the conformational behavior of two adjacent CF3 groups were supported by calculations of repulsion energies between the fluorine atoms in these groups using a simple van der Waals repulsive potential. It is proposed that the small temperature dependence of the JFF in the compounds studied is due to intermolecular interactions.