Rotational isomerism. Part XI. The nuclear magnetic resonance spectra and rotational isomerism of 1,2-difluoro- and 1,1,2-trifluoroethane

Abstract

The 1H and 19F spectra of 1,2-difluoroethane (I) and 1,1,2-trifluoroethane (II) are given and analysed.The solvent dependence of the coupling constants enables the rotamer energies and coupling constants to be obtained. In this process, the intrinsic solvent dependence of the vicinal HF and FF couplings (given by a control study of 1,1,1,2-tetrafluoroethane) was explicitly considered.The rotamer energies ΔE(Eg–Et) are for (I), ΔEv– 0·6, ΔE1– 2·6 and for (II)ΔEv 1·4, ΔE1 0·0 kcal/mol.The couplings in the rotamers of (I), (II), and 1,1,2,2-tetrafluoroethane are considered in detail. Increasing fluorine substitution gives a progressive but very nonlinear decrease in JtHH and JtHF which is much larger, for the latter, if the fluorine is introduced on the CH end of the CH·CF fragment.In (I), JgHH varies from 0 to 6·2 Hz depending on the orientation of the F nuclei, but increasing fluorine substitution evens out these effects to give JgHH 1·5—2·0 Hz. J t FF Is very dependent on the fluorine substitution, varying from –30 to 0 Hz but in contrast JgFF shows no dependence at all and is relatively constant at –13 to –5 Hz.