The proton magnetic resonance spectra of some substituted ethanes

The A2B2 proton resonance spectra given by 1,2-disubstituted ethanes are reported and analysed. A large number of these spectra are examples of the ‘deceptively simple’ type of spectra which do not give explicit values of the coupling constants. In those cases which are not of this type, it is shown that the average coupling constant, i.e. 1/3(J t + 2J g), can be obtained from such spectra, even though the individual vicinal coupling constants are dependent on the percentage of the rotational isomers, and thus change with solvent, etc. This change with solvent is discussed, and the results show that in many 1,2-disubstituted ethanes the gauche isomer is the more stable isomer. Also, the results suggest that, in many of the compounds investigated, the dihedral angle between the substituents in the gauche isomer differs appreciably from 60°. The values of the average coupling constant obtained from these systems are combined with measurements of this coupling in other substituted ethanes to investigate the dependence of this coupling on the substituents. It is shown that the influence of the substituents can be described in terms of the equation J av = 17·97-0·80 ∑E, where ∑E is the sum of the electronegativities of all the atoms directly bonded to the CH-CH fragment. The geminal coupling constants obtained from the A2B2 spectra are also discussed, and shown to be much more sensitive to small changes in the environment than the vicinal couplings.