How to Run a Nuclear Magnetic Resonance Spectrum

Опубликовал Admin
13-10-2018, 16:00
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Nuclear magnetic resonance (NMR) is one of the most widely used methods to identify chemical compounds, especially organic ones. It relies on the principle of nuclear spin; when an atom's nucleus is placed in a strong magnetic field, its magnetic field aligns with the direction of the field. When energy, in the form of radio waves, is applied, some of the nuclei spin against the field. The spectrometer measures the energy released when the nuclei go back to spinning with the field. From this information a wealth of structural data can be obtained.

Preparing the Sample

  1. Dry the sample. If your sample contains solvent, the solvent peaks will show up very strongly in the NMR spectrum and could interfere with the peaks you want to see. How easy it is to dry the compound depends, but leaving it under vacuum overnight will usually remove most common organic solvents.
  2. Choose an appropriate solvent. Take about 10-20 mg of the compound and dissolve it in a non-deuterated version of the solvent you want to use. Most NMR spectra are run in chloroform, so start with that. If you have a very polar compound, you may wish to use DMSO, acetone, acetonitrile, or water.
  3. Prepare your sample. Weigh around 10-20mg of compound into a small test tube, and add around 0.75 mL of the deuterated version of the suitable solvent. #*Everything should go into solution. If there are undissolved solids, poor quality spectra will result.
  4. Insert the sample into an NMR tube. An NMR tube is a tube, around 25 cm long and 0.5 cm wide, made out of borosilicate glass. Take the solution of your sample, collect it using a Pasteur pipet, and insert into the NMR tube. Seal the tube using a plastic cap.
    • The height of liquid in the NMR tube should be three fingers tall. Hold your fourth, third, and second fingers against the bottom of the tube. If you can't see the top of the liquid above them, add a little extra solvent.

Running the Sample

  1. Know the safety precautions. While running an NMR isn't very dangerous, it involves being around strong magnetic fields.
    • If you wear a pacemaker or magnetic implants, never enter the red or yellow circles on the floor surrounding the NMR spectrometer.
    • Remove any magnetic items from your pockets, such as credit cards or watches, as these can be permanently damaged, before inserting your sample.
  2. Insert your tube into a spinner. This is a plastic collar that supports your NMR tube inside the magnet. Pick up a spinner using the end of the tube and press it in until the bottom of the spinner is 10cm above the bottom of the tube.
  3. Clean the outside of the tube. Use isopropanol, acetone, or another volatile organic solvent, and wet a tissue/Kimwipe. Wipe down the outside of the tube, below the spinner, to remove any contamination.
  4. Turn on the lift air on the spectrometer. When the air starts rushing upwards, insert your sample above the magnet.
  5. Insert the NMR tube. Turn down the flow of lift air so the sample gently descends into the magnet.
  6. Lock the spectrometer. Choose the appropriate deuterated solvent, and hit the "lock" button. This tells the spectrometer which solvent you're using.
  7. Shim the spectrometer. This compensates for any stray magnetic fields. Wait for the machine to finish.
  8. Select the type, name, and title of the experiment you want to perform.
    • The name of your sample doesn't really matter, but just know what it is so you can find your data easily.
    • The type of experiment depends on which nuclei you want to scan. Most organic molecules contain protons, so a 1D proton NMR is the most common. If there's phosphorus or fluorine, do NMR's on those too. Carbon-13 NMR's take longer than the other three, and are usually only needed if you are told to do so by your instructor, or if you're characterizing a compound not known to the literature.
    • You may wish to run a 2D NMR if your compound is highly complex, or if 1D NMR does not help you identify it adequately.
  9. Set the number of scans. 16 is plenty for proton, fluorine, and phosphorus NMR. Carbon NMR may require 500 or more for an adequate signal to noise ratio.
  10. Set the relaxation delay. The default set by the spectrometer will depend on what nucleus you chose, but you may wish to increase it if you're trying to observe carbonyls in 13C NMR to around 1.5 to 2.0 seconds.
  11. Set the sweep width and middle of the spectrum if needed. These can generally be left alone unless you're working with paramagnetic molecules featuring unusual chemical shifts.
  12. Tune the probe. This is like tuning an old-fashioned radio to listen to the station you want. Wait for the machine to finish.
  13. Stat the experiment. Watch the peaks appear in your spectrum. Once you see a satisfactory signal to noise ratio, you can halt the experiment, or wait for the requested number of scans to finish.
  14. Repeat the scanning process for each additional nucleus you want to scan. There's no need to lock or shim the spectrometer as long as your sample doesn't change.
  15. Eject the sample. Turn on the lift air, and wait for the sample to emerge from the spectrometer. Pick it up and turn off the lift air.
  16. Remove the spinner. Take a Kimwipe and pull it off. Place the spinner back where you found it.

Tips

  • If you don't change solvent, you don't need to lock the spectrometer between samples. You should still shim however.

Warnings

  • Deuterated solvents, chloroform especially, can be toxic. Work in a fume hood when preparing samples.
  • Be careful when inserting spinners. Press straight down, otherwise, the tube may break and cut your hand.
  • In most cases, don't try running a spectrum in non-deuterated solvent. Your solvent peak will be so tall that it'll be very difficult to see any peaks from your compounds.
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