Rotational isomerism

Abstract

A calculation of the energies of the dipolar and quadrupolar electric fields of a 1,2-disubstituted ethane and their dependence on the dielectric constant of the medium gave results in good agreement with experiment for the energy differences between the rotational isomers in the gas and liquid phases. The theory is used to derive the proton-proton and proton-fluorine coupling constants of the rotational isomers of 1-chloro-2-bromo, 1-chloro-2-fluoro and 1-bromo-2-fluoro ethane from the solvent dependence of the proton resonance spectra. Notable deviations from this classical theory are found with aromatic solvents and are due to specific interactions of the solvents with the more polar rotational isomer. The theory also shows that the energy difference between the rotational isomers in the liquid phase is not constant but a function of temperature, and the determination of the coupling constants of the individual isomers from the temperature dependence of the spectrum is shown to be critically dependent on the assumptions made concerning this energy difference. The coupling constants obtained show the effects of the orientation of the substituents in that J _g ^HH in the trans isomer (5·1–5·5 c.p.s.) is almost twice that in the gauche isomer (2·7–2·2 c.p.s.). J _t ^HF equals 53 c.p.s., a much larger value than has hitherto been reported. Consideration of these and other couplings in similar ethane-like fragments leads to the conclusions that J _t is much more dependent on the substituents on the carbon atoms than J _g. The latter is only dependent on the nature of the substituents which are in a trans orientation to the coupling nuclei.