An experimental and theoretical investigation of orientational dependence of H–F couplings in fluoro-olefins

Abstract

The ¹H and ¹⁹F n.m.r. spectra of several monofluoro-olefins are reported. Coupling of vinylic fluorine to hydrogen bound to sp³-hybridised carbon has been observed over three, four, and five bonds. The four-bond ‘allylic’ F–H coupling has been examined in detail, with FPT-INDO calculations for several cases. The trans-CH·C:CF coupling changes sign as the HC·C:C torsion angle varies, as predicted, but the cis-CH·C:CF coupling is positive for all H·C·C:C angles, contrary to predictions. Alkyl substituents have an appreciable effect on the allylic couplings; this effect is primarily due to changes in geometry on substitution. The ⁵J_HF couplings are also sensitive to orientation; in particular, the observation of a cis-⁵J_HF coupling depends on the proximity of the fluorine and the coupled hydrogen.