Nuclear Magnetic Resonance
FAQs

Q1. What SEA paperwork do I need to finish before NMR training?

A1. You will need to Join SEA to become a user. After your account is activated, please contact Meri Dix (meri.c.dix@rice.edu) or Rachel Berins (rb108@rice.edu) to request NMR training for the instrument you are interested in.

Q2. How to select the right NMR tubes for my NMR study?

A2. Please use medium- to high-quality sample tubes. We strongly discourage using "disposable" tubes on any NMR instrument. Cheap tubes don’t process perfect cylindrical symmetry, which would limit the attainable field homogeneity. We recommend the precision grade NMR tubes for 600 MHz NMR, such as the Wilmad (535-PP-7), Norell (509-UP-7/S-5-600-7), or NewEra (NE-UP5-7). NMR tubes must be free from scratches on the glass as this distorts the cylinder in which the sample is confined.

Q3. How do I prepare solution NMR samples?

A3.

  • The optimum sample volume for a 5mm tube is 600 uL (i.e., 40mm sample depth).
  • Use deuterated solvent for your sample.
  • The sample solution should be homogeneous. Cloudy solutions will be harder to shim which reduces the spectral resolution. Consider filtration/centrifugation to remove the insoluble material if your sample is unclear.
  • Please note that paramagnetic species in the NMR samples cause the broad line width and unusual chemical shifts. This might also affect the signal frequency of your deuterium solvent. You will have problems with the auto lock and auto shimming. If applicable, please remove paramagnetic species from your NMR sample.

Q4. Why do I need to include the deuterated solvent in my NMR sample?

A4. Deuterated solvents are used in solution NMR for two reasons:

  • Provide a deuterium lock signal. The magnetic field tends to drift slowly. Modern NMR instruments measure the deuterium absorption of the solvent and adjust the field strength to keep the magnetic field constant.
  • Avoid very large solvent signals in the 1H NMR spectrum.

Q5. How do I choose a suitable deuterated solvent for my compound?

A5. Factors to be considered when choosing a suitable solvent are:

  • Good solubility.
  • Make sure the NMR signal from the solvent doesn't interfere with the solute. In 1H NMR, the observed solvent resonance arises from the residual protonated species since deuteration is not 100% complete.
  • Boiling point and melting point: You must not get any closer than 10 oC to the boiling or melting point. When performing high-temperature NMR experiments, it is good practice not to use a sealed NMR tube.
  • H2O content: All solvents are hygroscopic to some degree and should be exposed to the atmosphere as little as possible to prevent them from becoming wet. The H2O peak can be strong and broad, and its chemical shift can vary according to solution conditions. The presence of an H2O peak will degrade the quality of NMR spectra. To minimize the H2O content, you can consider using the deuterated solvents in single-use ampules.

Please read "NMR Solvent Chart" for more information.

Q6. How to clean NMR tubes properly?

A6.

  • For routine cleaning, rinsing with a suitable organic solvent or distilled H2O a few times is usually sufficient.
  • More stubborn soiling is best tackled by chemical rather than mechanical means. Soaking in detergent or strong mineral acids is recommended. Chromic acid is not good for cleaning since the paramagnetic Chromium VI adversely affects your NMR results.
  • Tubes should not be dried by subjecting them to high temperatures for extended period of time as this tends to distort them.
  • Avoid contamination of the outside of the NMR tube because this leads to the transfer of the contaminants into the probe head.

An NMR tube cleaner allows easy cleaning and rinsing.

Q7. My sample quantity is low. How can I improve the signal-to-noise ratio (S/N) of NMR signals?

Q8. What are the sample requirements for a solid-state NMR study?

A8.

  • ~85mg solid sample is required for a 4mm ZrO2 MAS rotor.
  • Solid samples must be finely powdered. Samples can be finely grounded using mortar or a ball mill.
  • All samples must be completely dry.
  • Samples must not be conductive or ferromagnetic.