Electron Coupling of Nuclear Spins. VII. JFF for 2-Fluorobenzotrifluorides

The NMR spectra of several 2-F, 6-X-benzotrifluorides yield values for JFF between the 2-fluorine and the fluorines in the CF3 group of 13 to 34 cps, depending upon the 6-substituent. Furthermore, the magnitude of this JFF is roughly proportional to the chemical shift of the CF3 group and to the size of the 6-substituent. Two related factors appear to be involved: (a) the rotational configuration of the CF3 group with respect to the plane of the benzene ring, and (b) electrostatic deformation of the electron distribution about the fluorines in the CF3 group. Our results for the benzotrifluorides and values reported earlier for the cis-F–CF3 coupling in CF3CX=CFY compounds are consistent with little or no hindrance to CF3-group rotation in the latter.