Analytical Techniques - Deuterium use in ¹H NMR (A-Level Chemistry)

Deuterium use in ¹H NMR

Preparing a Sample for ¹H NMR


Samples are investigated in dilute solution to separate molecules from each other and prevent them from interacting.

The solvents used have to fulfill certain criteria:

  • They are good solvents. They are very good solvents for organic molecules.
  • They do not contain ¹H atoms. They will, therefore, not produce a peak on the ¹H NMR spectrum.

The solvents of choice normally are tetrachloromethane (CCl₄) or deuterated trichloromethane (CDCl₃).

Deuterated Solvents

Tetrachloromethane has no hydrogen atoms in its structure and hence can be used as a solvent in proton NMR.

However, not all organic molecules can dissolve well in it so that deuterated solvents such as deuterated trichloromethane tend to be more widely used.

In deuterated trichloromethane, H atoms have been replaced for deuterium atoms.

Deuterium (²H or D) is an isotope of hydrogen whose nucleus contains one proton and one neutron.

Deuterium use in ¹H NMR
Deuterium use in ¹H NMR

As its atomic mass is an even number, deuterium atoms do not have nuclear spin and hence do not interact with the external magnetic field used in NMR spectroscopy. Absorptions from deuterium atoms will not be detected and hence will not produce a peak.

Identifying -OH and -NH Signals

Labile Protons

The signals produced by the proton in -OH and -NH- groups in high resolution proton NMR spectroscopy are singlets.

This is because the proton in these groups are very rapidly exchanged for protons in any traces of water or acid in the sample. This reversible exchange is so quick that only peak gets produced.

This rapidly exchanged proton in -OH and -NH- is referred to as a labile proton.

Deuterium use in ¹H NMR
Deuterium use in ¹H NMR

Using Deuterium to identify Labile Protons

-OH and -NH- groups can be difficult to identify in proton NMR spectra because their chemical shifts are:

  • Too broad – The chemical shift ranges for -OH and -NH- are very broad and overlap with that for other proton types.
  • Outside the given ranges – Under some conditions, for example when using specific solvents or sample concentrations, the peak corresponding to the -OH or -NH- group may lie outside of the normal quoted ranges

In order to make sure that a peak on a proton NMR spectrum corresponds to one of these groups, a small amount of deuterium oxide (D₂O) can be added to the sample.

The deuterium atoms in the deuterium oxide molecules will be rapidly exchanged for the labile protons on the -OH and -NH- groups as shown below:

Deuterium use in 1H NMR
Deuterium use in ¹H NMR

As deuterium atoms do not show up in proton NMR spectra, the resonance for the -OH or -NH- groups will disappear, confirming its identity

→What is NMR spectroscopy, and how is it used in chemistry?

NMR (nuclear magnetic resonance) spectroscopy is an analytical technique used to determine the structure and properties of molecules by analyzing the interaction of atomic nuclei with an external magnetic field. It is widely used in chemistry to identify and quantify organic compounds.

→What is deuterium, and how is it used in ¹H NMR spectroscopy?

Deuterium is an isotope of hydrogen that has a neutron in addition to its proton. In ¹H NMR spectroscopy, deuterium can be used as a solvent or added to a sample as a “lock” to improve the accuracy of the measurement.

→How does deuterium as a solvent affect ¹H NMR spectra?

Deuterium is often used as a solvent in ¹H NMR spectroscopy because it has a very different chemical shift from proton (¹H) spectra. This means that the signal from the deuterium solvent does not interfere with the signals from the sample being analyzed.

→What is a deuterated solvent, and how is it prepared?

A deuterated solvent is a solvent that has had most or all of its hydrogen atoms replaced with deuterium. Deuterated solvents are prepared by reacting the solvent with deuterium oxide (D2O) or by distilling the solvent over deuterium gas.

→What is the purpose of using deuterium as a “lock” in ¹H NMR spectroscopy?

In ¹H NMR spectroscopy, deuterium can be added to a sample as a “lock” to improve the accuracy of the measurement. This is because the deuterium signal is very sharp and does not change with temperature or other conditions, so it can be used to accurately calibrate the spectrometer.

→How does deuterium labeling affect ¹H NMR spectra?

Deuterium labeling involves replacing one or more hydrogen atoms in a molecule with deuterium atoms. This can affect the ¹H NMR spectra by changing the chemical shift of the affected protons and altering the intensity of the signals. Deuterium labeling can be used to study the metabolism and kinetics of biological systems.

→What are some common applications of ¹H NMR spectroscopy in chemistry?

¹H NMR spectroscopy is widely used in chemistry for the identification, quantification, and structural analysis of organic compounds. It is also used in the study of chemical reaction mechanisms, protein-ligand interactions, and the characterization of synthetic and natural products.

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