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Ear:.y studies of rotational isomerism e:mployed dielectric constant measurements and vibrational spectra. Most of such results have been summarized by Mizushimal. Problems studied include the energy differences and potential barriers between the various molecular configurations and the dependence of these quantities upon factors such as substituents, the solvent and the state ofthe sample. The basic question, which remains in large part, is the nature of the forces restricting intramolecular motions. Other mean:-~ of making such studies have been provided by the advent of high resolution nuclea.r magnetic resonance2 • 3. The present work deals with the application o::N.M.R. techniques to the rotational isomerism of substituted, liquid ethanes2-8. The factors governing the appearance of the N.M.R. spectra of these compounds include the relative energies of the three rotational isomers, the potential barriers to internal rotation about the C-C bond, and the chemical shifts and coupling constants characteristic of the magnetic nuclei in each rotamer2 • 3, These quantities can be obtained most completely and directly for a compound if the potential barriers are high enough that 1:he spectrum at lower temperatures is a superposition of spectra for the 1:hree rotamers. In the most general case of a substituted ethane, there will be three stable ::otational forms, as shown in Figure 1. Each ofthese forms has a mirror image,

N.M.R. studies of conformational equilibria in substituted ethanes