16.4 : ¹H NMR of Labile Protons: Deuterium (²H) Substitution

This lesson illustrates the role of deuterium substitution in simplifying the NMR spectrum of compounds comprising labile protons. One method employed is the use of deuterium. Amongst the three isotopes of hydrogen, deuterium (²H) has a nucleus composed of one proton and one neutron. When the D₂O solvent is added to a pure dry ethanol solution, its labile proton is substituted with deuterium.

Figure 1. The comparison of the proton NMR spectra of pure dry ethanol and the same in D₂O solvent.

The signals from deuterium are not observed in a proton NMR spectrum, as deuterium–proton coupling is negligible, and no spin-spin splitting occurs between them. As shown in Figure 1, the NMR spectra of pure ethanol and ethanol in deuterium, the disappearance of the triplet in the latter, confirms that it corresponds to the labile –OH proton. Furthermore, the methylene protons that initially showed up as a multiplet transform to a quartet, proving that the multiplet is due to the coupling with the –OH proton and the methyl protons.