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Theory of NMR Spectroscopy: Procedure & Types

Instructor: Scott Larkin
The preparation of NMR samples covers how to make the sample for an NMR instrument. In this lesson, you will learn what to do when you have trouble preparing a sample. You will also find what basic types of information you can expect from an NMR.

Prepare the Sample

Choose the Solvent

You are more than likely going to encounter a modern NMR which requires you use a deuterated solvent. This means that instead of having a small molecule with hydrogens, the hydrogens have been substituted with deuterium. If the NMR you are using has an automatic acquisition program, the machine uses the deuterated solvent as its reference.

These are used so you need not run a standard every time you obtain NMR spectra. The most common deuterated solvent is D2 O. If you are running your spectra as part of an organic chemistry lab, you may also use CDCl3, d-chloroform.

Amount to Use

The amount of solid or liquid is a concentration based consideration. This means the larger the molecular weight of the molecule wish to use to obtain an NMR spectrum, the greater the mass in milligrams is necessary to dissolve. Whatever the amount of material you dissolve, you should use no more than 1 mL of the deuterated solvent.

Often times you may be concerned about the material you must use to obtain the sample. If that is the case, it is perfectly feasible to retain the sample you dissolved, and then recover it by evaporation of the solid or recrystallization in the NMR tube.

In the Tube

Make certain all of the sample is cleanly dissolved and there is no solid suspended in the solution or precipitate at the bottom of the tube. If there is a solid material, there are two common methods to obtain a transparent homogeneous solution: filtration and centrifugation.

Filtration

Prepare a Pasteur pipette with a cotton plug and 0.5 mm of either silica or alumina gel as the stationary phase. Load the sample in the deuterated solvent onto the dry column. The pipette tip should be held so it drains directly into an NMR tube. It is often advantageous to use a pipette bulb to force the solution through the column.

Pasteur pipette mini-column for NMR sample preparation
mini-column for sample preparation

Sometimes you may encounter a situation where suspended solids are too finely divided to easily filter with a miniature column. In that case, do not force the mixture through the column with a pipette bulb. Let the column's mobile phase carry your sample using gravity.

Centrifugation

If a great deal of precipitate is present, cap the NMR tube. Seal the NMR tube and tube cap with parafilm to prevent the cap from leaking. Prepare a counterbalance by filling a separate NMR tube with the same solvent and the same volume. (The solvent, in this case, does not have to be a deuterated solvent). The counterbalance should also be sealed with parafilm. Shake up the NMR tube with your sample and quickly invert the tube.

You should now have most of the solid gathering towards the cap end. Both tubes go in the centrifuge on opposite tube holders with the caps facing inwards inside the holders. After one to two minutes, your sample will now be clear of any precipitates. For the best results, remove the parafilm from your sample tube.

The two most common NMR spectra are taken on 1 H and 13 C. Although it is possible you may encounter other nuclei, if you are obtaining spectra for an undergraduate teaching lab, these are almost certainly the only type you will be exposed to.

Analyse the Spectra

1 H NMR

Proton NMR is the default nuclei for NMR instruments. These spectra vary mostly between δ12 ppm and δ0 ppm. A 1 H NMR spectrum immediately provides four kinds of information:

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