Fluorine magnetic resonance studies. V. Fluorine-fluorine coupling in 1,4-difluorobenzenes

Abstract

A number of substituted 1,4-difluorobenzenes were examined by ¹⁹F n.m.r. spectroscopy and ⁵J_FF was found to vary with the nature of the substituent in much the same way that it does in more heavily substituted difluorobenzenes. Only in 2',5'-difluoroacetophenone is ⁵J_FF larger than it is for 1,4-difluorobenzene itself. The magnitude of ⁵J_FF is increased when a small ring is fused to the benzene ring, i.e. in 4,7-difluorodihydroindenes and 2,5-difluorobicyclo[4,2,0]octa-1,3,5- trienes. Similar effects have been reported for ⁵J_HH in aromatic systems, and the value of ⁵J_HH for tricyclo-[6,2,0,0^3,6]deca-1,3(6),7-triene is found to be 1.61 Hz.