Purpose of D20 in NMR samples?

Samples need to be in solution for NMR analysis, so D2O is used as a solvent. Deuterated water is used (or any other deuterated solvent) since the protons on the solvent will also show a signal on the NMR. Deuterium has an integer spin number (1) and consequently does not show up in an NMR spectrum. This allows only the spectrum of the analyte, not the carrier solvent, to be displayed, making identification of the product easier.
Heavy water is used in certain types of nuclear reactors where it acts as a neutron moderator to slow down neutrons so that they can react with the uranium in the reactor. Light water also acts as a moderator but because light water absorbs more neutrons than heavy water, reactors using light water must use enriched uranium rather than natural uranium, otherwise critically is impossible. The use of heavy water essentially increases the efficiency of the nuclear reaction.
In simple NMR, the deuterium isotope of hydrogen has a different magnetic moment and spin, hence it is invisible in a spectrometer tuned to protons. The presence of deuterium in the solvent will allow the spectrophotometer to "lock" on the frequency of the D in the solvent allowing the instrument to scan the appropriate frequency range in question.

D2O is also used to detect acidic hydrogens in the sample examined...alcohol/phenol OH, and carboxylic acid COOH hydrogens are exchanged by deuterium when D2O is added and the signal will diminish allowing you to "prove" that you have these types of acidic hydrogens in your sample. A typical example of this is looking at, say, a phenolic compound in CDCl3...a drop of D2O in the deuterated chloroform will result in the disappearance of the phenolic OH in the spectrum.

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