High resolution fluorine-coupled 13C nuclear magnetic resonance spectra of substituted pentafluorobenzenes. Theoretical and empirical correlations of JCF

Abstract

The high resolution ¹⁹F and ¹⁹F-coupled ¹³C n.m.r. spectra of several substituted pentafluorobenzenes, C₆F₅X (X = H, F, NH₂, NO₂, Cl, Br, CHCH₂, OCH₃, CH₃, I, CN, OH, and CHO), have been completely analysed, and all the signs and magnitudes of the ¹³C–¹⁹F coupling constants determined. INDO MO calculations of ⁿJ_FF and ⁿJ_CF have been performed, and comparisons are made with the experimental values. The calculations include the Fermi contact, orbital, and spin–dipolar contributions to the coupling and are shown to reproduce the substituent effects upon ⁿJ_FF. The calculations of ⁿJ_CF values are less successful. ¹J_C(4)F(4) correlates well with σ_p whereas the correlation of the other ¹J_CF values is less successful. No improvement in the correlations occur with a dual parameter equation. All the couplings, ⁿJ_CF, that show a sufficient variation with the substituent change are successfully correlated with a three parameter equation involving F, R, and Q. In particular ²J_C(1)F(2) and ³J_C(2)F(6), in close proximity to the substituent, are successfully correlated by this equation.