The orientation and sign-dependence of 4-bond fluorine–fluorine coupling constants

Abstract

An analysis of the reported ⁴J(F–F) couplings in perfluoromethylcyclohexane and some fluorinated propanes enables the magnitudes and signs of all the ⁴J(F–F) couplings in heavily fluorinated C·C·C fragments to be obtained. These are J_tt(trans-trans)–10, J_tg(trans-gauche)ca. 0, J_gg(gauche-gauche)+16; and J_e(eclipsed)+26 Hz.

The noteworthy predictions from these results are that ⁴J(F–F) couplings can be of either sign in the same molecule and that the large F–F couplings observed when two fluorine atoms are in close proximity are probably always positive.