Why does NMR use deuterated solvents?

Question

Nolan Allen · Accepted Answer

There are three reasons why deuterated solvents are used in NMR spectroscopy.

First, to prevent the solvent signal from swamping. An NMR tube typically contains far more solvent than sample. 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. Second: To maintain a steady magnetic field strength. Superconducting magnets have a tendency for their field strength to slowly drift. In order to maintain a constant resonance frequency (field strength), modern NMR spectrometers measure the solvent's deuterium absorption and modify the field strength accordingly. Third: To precisely define 0 ppm. It is commonly known that the deuterium frequency differs from 0 ppm (TMS). Nowadays, spectrometers are able to "lock" onto the deuterium signal, negating the need for an external reference such as TMS.

Isaac Aronson · Answer

undefined Deuterated solvents are used in NMR to avoid interference from proton signals in the solvent, which could overshadow the sample's signals. Deuterium has a different resonance frequency than hydrogen, reducing background noise and providing clearer spectra.