Fluorine magnetic resonance studies. VI. Fluorine-fluorine coupling over four bonds in α-Fluoro- and α,α-Difluoro-toluenes

Abstract

The 4J fluorine-fluorine coupling over four bonds involving an sp3 carbon and two sp2 carbons has been studied by using constrained structures to calibrate the dependence of the size of coupling on the dihedral angle (8). Both CF and CF2 groups follow the pattern established for CF3, with the largest couplings (38, 62 and 86 Hz respectively) arising from closest approaches of two fluorines (θ = 0�) and the zero couplings resulting from θ = 180�.