Why does NMR use deuterated solvents?

Reason 1: To avoid swamping by the solvent signal.

There is usually much more solvent than sample in an NMR tube.

An ordinary proton-containing solvent would give a huge solvent absorption that would dominate the #""^1"H"#-NMR spectrum.

Most #""^1"H"#- NMR spectra are therefore recorded in a deuterated solvent, because deuterium atoms absorb at a completely different frequency.

But deuteration is never complete, so in #"CDCl"_3#, for example, there is always some residual #"CHCl"_3#.

You always get a solvent signal from #"CHCl"_3# at 7.26 ppm.

Reason 2: To stabilize the magnetic field strength.

The field strength of superconducting magnets tends to drift slowly.

Modern NMR spectrometers measure the deuterium absorption of the solvent and adjust the field strength to keep the resonance frequency (field strength) constant.

Reason 3: To accurately define 0 ppm.

The difference between the deuterium frequency and 0 ppm (TMS) is well known.

Modern spectrometers can "lock" onto the deuterium signal, so the addition of an internal reference like TMS is not usually required.