A method for the determination of fluorine—fluorine internuclear distances using “through-space” coupling contributions. A NMR and u.v. spectroscopic study of 1-pentafluorophenyl-1-trifluoromethylethylene

Abstract The hydrogen and fluorine NMR spectra of the title compound have been analysed completely and the u.v. absorption spectrum has been obtained and decomposed into individual bands with the aid of an analogue computer. A relationship has been established between internuclear distance and the “through-space” contribution to the coupling constant between an ortho-fluorine nucleus and the trifluoromethyl group. This has enabled the dihedral angle between the aromatic ring and the olefinic group to be evaluated as being 69 ± 10°: absorption band shift data produced a value for the angle of 70 ± 8°. The good agreement between the totally unconnected methods suggests that certain through-space NMR coupling contributions may be used to evaluate internuclear distances.