Nuclear Magnetic Resonance Fluorine—Fluorine Coupling Constants

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Abstract
The pattern of fluorine—fluorine coupling constants is explained on the basis of two mechanisms for nuclear spin—spin coupling: the ``through‐bond'' and the ``through‐space'' mechanisms. In the former, the interaction proceeds via the electronic structure in the intervening bonds. It is highly dependent on the electron withdrawing power of the substituents on the carbon skeleton. This contribution to the coupling constant goes to zero when the sum of the electronegativities of the substituents becomes sufficiently high. The latter mechanism is operative when two fluorine atoms are sufficiently close in space for there to be appreciable overlap of their electronic clouds. This ``through‐space'' interaction proceeds via an electronic structure where there is no bond per se. Experimental evidence for the existence of both mechanisms is given.