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 = 0º) and the zero couplings resulting from θ = 180º.