Deuterium is one of the isotopes of hydrogen, whose nucleus is comprised of one proton and one neutron. Hence, it is represented as ²H.

The deuterium signals are not detected in a proton NMR spectrum.

Adding D₂O solvent to pure ethanol results in substituting its labile –OH proton with deuterium.

As observed in the proton NMR spectra, the triplet of pure ethanol disappears after the deuterium substitution, indicating that the triplet corresponds to the –OH proton.

The methylene protons that show up as a multiplet transform to a quartet, further confirming the exchange of labile –OH proton with deuterium.