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(E_g–E_t) are for (I), ΔE^v– 0·6, ΔE¹– 2·6 and for (II)ΔE^v 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 J_t^HH and J_t^HF which is much larger, for the latter, if the fluorine is introduced on the CH end of the CH·CF fragment.

In (I), J_g^HH 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 J_g^HH 1·5—2·0 Hz.

J _t ^FF Is very dependent on the fluorine substitution, varying from –30 to 0 Hz but in contrast J_g^FF shows no dependence at all and is relatively constant at –13 to –5 Hz.