For NMR, the sample is dissolved in a solvent. The solvent is present in far greater amount than the sample, so any signal it gives would swamp the peaks from the sample.In 1H NMR, an ordinary solvent containing hydrogen atoms would give a huge peak. Instead, a deuterated solvent is used, in which every hydrogen atom has been replaced by deuterium, 2H or D.The most common is CDCl3, deuterated trichloromethane (deuterochloroform), made from CHCl3. Deuterium nuclei do not absorb in the same frequency range as 1H, so the solvent gives no peak on a 1H NMR spectrum.A good NMR solvent also:• dissolves the sample
• is inert, so it does not react with the sample
• is volatile, so the sample can be recovered13C NMR: CDCl3 still contains a carbon atom, so it does give a 13C signal (a small peak at about 77 ppm). This peak is known and is ignored when interpreting the spectrum. Solvents with no carbon at all avoid this completely.Worked example: why would CHCl3 be a poor solvent for 1H NMR? It contains one H atom per molecule, and as the solvent it is present in large excess, so it would give a very large peak that could hide the sample's peaks. CDCl3 gives no 1H peak.Deuterium is an isotope of hydrogen, so CDCl3 has the same chemical properties as CHCl3.
• is inert, so it does not react with the sample
• is volatile, so the sample can be recovered13C NMR: CDCl3 still contains a carbon atom, so it does give a 13C signal (a small peak at about 77 ppm). This peak is known and is ignored when interpreting the spectrum. Solvents with no carbon at all avoid this completely.Worked example: why would CHCl3 be a poor solvent for 1H NMR? It contains one H atom per molecule, and as the solvent it is present in large excess, so it would give a very large peak that could hide the sample's peaks. CDCl3 gives no 1H peak.Deuterium is an isotope of hydrogen, so CDCl3 has the same chemical properties as CHCl3.