NMR Spectroscopy
Introduction to NMR Spectroscopy
NMR spectroscopy is a powerful technique to elucidate structural information from molecules. There are many different NMR experiments which can be recorded, each providing different information about a molecule. You will learn about NMR spectroscopy from many lecture courses during the degree programme and you will get to grips with interpreting your own spectra through the lab courses.
Preparing an NMR Sample
There are a number of different ways that samples can be analysed using NMR spectroscopy, however for teaching purposes we routinely only use solution phase NMR spectroscopy. This means that any samples need to be prepared as a solution in a non-protic (or deuterated) solvent (usually chloroform-d (CDCl3) or water-d2 although other solvents may occasionally be used). A deuterated solvent is a solvent where all the 1H hydrogen atoms in the solvent molecules have been replaced with 2H hydrogen (known as deuterium, D). These solvents are much more expensive than their protic counterparts and should be used sparingly. The deuteration reduces the signals that would be caused by the solvent in the NMR as deuterium is invisible in proton NMR. The deuteration of solvents is never 100%, so a small peak will be seen in proton NMR for the solvent.
In order to prepare an NMR sample:
Ensure that your compound is soluble in the solvent selected. If you are unsure then test the solubility of your compound using protic solvent first.
Prepare a pipette filter with cotton wool and clamp this above an NMR tube so that the filtrate will run straight into the NMR tube.
Add an amount of compound to a sample vial. The amount required varies depending on the information you wish to obtain. Proton NMR requires very little sample (as 1H accounts for 99.9% of hydrogen atoms) whereas 13C NMR requires much greater amounts (13C is around 1.1% abundant). The molecular mass of your sample also makes a difference as it is the concentration of the solution that is important, but for most low molecular mass compounds this makes little difference. The same sample can be used to record multiple NMR experiments. Typically 20 mg of compound is sufficient to record proton and carbon NMR spectra.
Add approximately 1 mL of detuterated solvent using a Pasteur pipette and use the pipette to dissolve the sample.
Transfer the solution into the pipette filter and allow the solution to run through into the NMR tube.
Ensure there is 4 cm depth of solvent in the NMR tube. If there is insufficient solvent, top the tube up using a clean Pasteur pipette. Avoid overfilling NMR tubes as the solvents are expensive.
Fit a NMR tube cap and label the sample.
Preparing an NMR Sample
Points to note
Deuterated solvents are very expensive. Only use brand new clean Pasteur pipettes to take NMR solvents from the bottle. Even tiny amounts of contamination affect NMR results.
It is essential all samples are filtered to ensure samples are homogeneous. Any solid particles in a sample will result in poor NMR spectra.
Samples with insufficient depth result in poor NMR spectra being recorded. Excess depth is a waste of expensive deuterated solvent.
