Nuclear Magnetic Resonance Spectroscopy: Abnormal Splitting of Ethyl Groups Due to Molecular Asymmetry, III

The protons of a methylene group removed by one or more bonds from a center of molecular asymmetry may be magnetically nonequivalent and display AB-type nuclear magnetic resonance (n.m.r.) spectra [1-4]. It has been suggested that the chemical shift between the two methylene protons arises from unequal populations of the possible rotational conformations. However, even assuming equal populations and rapid interconversion of the three conformers I, II, and III, Ha and Hb are always distinct and identifiable, since no two conformers are identical except for interchange of Ha and Hb [5,6]. This "intrinsic asymmetry" of the methylene group might also be the cause of the observed magnetic nonequivalence of the methylene protons.