4.1: Obtaining and Interpreting NMR Spectra

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Sample Preparation

For a ¹H NMR sample, it is necessary to use a deuterated solvent in order to avoid large signals from the solvent peak. While the ¹H signals from the solvent do not interfere with other nuclei, a deuterated solvent is frequently used so that the sample can be locked and shimmed. The sample should be a homogeneous solution and ideally be free from paramagnetic material. It may be necessary to filter a sample to remove any undissolved material. The height of the sample in the 5 mm NMR tube should be approximately 2”.

Inserting the Sample, Spinning, Locking and Shimming

After the sample is prepared, it is inserted into a spinner and a depth gauge will be used to determine the proper positioning of the sample in the tube. It is very important that the sample be at the proper height in the spinner. This is a good time to check to make sure that you have the proper sample height. Ideally, there should be the same amount of sample above and below the rectangle labelled as the 5 mm sample coil. The rectangle is the location of the receiver coils, and this area must have sample. If you have a very limited amount of sample, you should center your sample so that it is centered in the receiver coil box.

The sample is now inserted into the magnet. A cushion of air is used to remove samples from the magnet. On a Bruker instrument, ej will turn on the air to eject the current sample. Once the new sample is floating on the air cushion, ij will turn off the air and allow the sample to drop slowly into the center of the magnetic field. The sample is then spun. Spinning generally serves to reduce the effects of slight differences in the magnetic field in different parts of the sample.

The next step is then to obtain a lock signal. The lock signal is a spectrum of the ²H from the NMR solvent. Once this signal is obtained (called locking) the instrument can be shimmed. In addition to the large magnet, there are many individual electromagnetic shim coils. During the process of shimming, these coils are adjusted to obtain the most homogeneous field possible around the magnet. Inhomogenities in the magnetic field lead to broader signals. In order to optimize the magnetic field, the intensity of the ²H NMR signal is measured. A more intense signal indicates a narrower peak and a more homogenous magnetic field. Diamagnetic materials, including NMR solvents are repelled from magnetic fields, although this effect is weaker than the attraction of paramagnetic materials to a magnetic field. The shimming process compensates for the changes in the magnetic field from the solvent, and is the reason that shimming is more facile when the sample height is correct. In addition to shimming, the lock signal is used to maintain the field homogeneity during the course of the experiment.

Tuning and Receiver Gain

In the NMR experiment, the magnetic field causes the nuclear spin states to possess different energies. Radiowaves are used to promote an atom from one spin state to a higher spin state. The actual frequency is dependent on both the nuclei and the magnetic field strength. In a 9.3947 Tesla magnetic, the frequency for the ¹H nuclei is 400.0 Mhz. Before acquiring a spectrum, your spectrometer needs to be tuned for the correct frequency, in the same way that you will tune a radio to your preferred station. After tuning the radio, you will make sure the volume is set appropriately. In the NMR experiment, the volume is the receiver gain. If the signal is weak, the volume needs to be turned up in order to hear the signal.

Processing and Examining the Spectra

After data collection is complete, the data is Fourier transformed and a phase correction is usually applied. At this point you can make sure that your spectrum is properly referenced to the residual protons in the deuterated solvent and begin integrating and peak picking your spectra. As you look at your spectra, be sure to examine the spectral quality. If there is not enough signal to noise more scans can be obtained. However, the signal to noise ratio increases with the square root of the number of scans, so you will need to increase the scans by a factor of four in order to double the signal to noise. It is also possible that the signal to noise is poor because of broadened peaks. Broad peaks can represent inhomogeneities in the magnetic field which may have been caused by poor shimming, paramagnetic materials in the sample or particulate matter. Alternatively, peaks can broaden due to exchange processes on the NMR time scale. However, this will usually cause broadening of only a few of the product peaks and have not effect on the line width of the deuterated solvent.

It is always preferable to make sure that your NMR tubes are clean and dry and that you have removed all solvent from your product before obtaining an NMR sample. Water from the air will settle into opened bottles of CDCl₃, so unless you remove the water from the solvent, there will be a water peak. The following link is extremely helpful in identifying 1H NMR resonances for residual solvents that may accidently appear in your NMR samples.

³¹P NMR

Questions

What is the purpose of shimming? How can you tell that the machine is poorly shimmed?
What is the purpose of locking to a solvent signal? Do you lock or shim first?
Why is it important to filter samples to remove undissolved material? How would the solid material affect the observed NMR?
If you run a ³¹P NMR after obtaining a ¹H NMR, which of the following do we need to do again? (Lock, Shim, Tune, Receiver Gain)
Our solvent is diamagnetic. Is this attracted to, repelled from or has no interaction from the magnetic field?
In our ¹H spectra with phosphine ligands, we see splitting from the P atoms, but not from the carbon atoms. Why?
³¹P NMR spectrum were singlets. Why did we not see the H-P splitting in our spectra?
Which parameter is changed to improve the signal-to-noise ratio? Does doubling this parameter increase the signal to noise by a factor of 2?