Long-range 13C,1H and 13C,19F coupling constants as indicators of the conformational properties of 4-fluoroanisole, 2,3,5,6-tetrafluoroanisole, and pentafluoroanisole

Abstract

Long-range spin–spin coupling constants over six bonds between 13C nuclei in the methyl group and ring protons or 19F nuclei in the para position are reported for 4-fluoroanisole, 2,3,5,6-tetrafluoroanisole, and pentafluoroanisole in solution. The couplings are σ–π electron mediated, as indicated by INDO MO FPT computations and by measurements on anisole, 2,6-dibromoanisole, 2,6-dichloroanisole, 2,6-dibromo-4-fluoroanisole, and 2,6-dibromo-4-methylanisole. On the basis of the measured coupling magnitudes and a hindered rotor model, it is concluded that the barrier to internal rotation about the [Formula: see text] bond in 4-fluoroanisole lies near 6 kcal/mol, is nearly zero in the tetrafluoroanisole, and is somewhat less than 1 kcal/mol in the pentafluoroanisole. In the latter, the preferred conformation is that in which the methoxy group lies in a plane perpendicular to the pentafluorophenyl plane. Some inconclusive dynamic nmr experiments on anisole, including T1ρ measurements, are briefly discussed.