Diastereotopism, proton magnetic resonance

Since nuclear magnetic resonance is a scalar probe, enantiotopic nuclei are isochronous (i.e. have the same chemical shift) in achiral media. Such nuclei, however, become diastereotopic in chiral media and thus, in principle (though often not in practice) anisochronous. Among many examples 26,27) are the enantiotopic methyl protons of dimethyl sulfoxide, CH3SOCH3, which are shifted with respect to each other by 0.02 ppm 26c) in solvent QH5CHOHCF3. (Surprisingly the 13C signals of the... 

Figure II. Nuclear spin-.spin-interaction through the bonding electrons in a CHi-group. The low-energy state corresponds to the antiparallel orientation of the magnetic moments of the nuclei (black arrows) and electrons (white arrows). The two possible slates of the magnetic moments of hydrogen nucleus B are shown. As a result the resonance signal of nucleus A shows a splitting into a doublet. The effect is only observable in a methylene group in Ihe case of diastereotopic. i.e.. chemically inequivalent protons.

---