NMR Spectrum Acquisition

Sample Preparation

To prepare a sample for NMR spectroscopy, dissolve 10-50 mg of the sample in about a mL of a deuterated solvent, usually CDCl₃. FT-NMR instruments require that samples be run in a solvent containing deuterium because the instrument locks on the resonance of deuterium to achieve field-frequency stabilization. Deuterated solvents must be extremely pure and are expensive, with CDCl₃ being the least expensive (because it only has one deuterium atom) as well as the most versatile.

The dissolved sample is then transferred to an NMR tube. NMR tubes are special-purpose glass tubes that are manufactured to certain specifications of high-quality glass. In the instrument, the tubes are spun rapidly, thus the tubes must be balanced so that they both spin evenly and do not break.

All solid material must be removed from the solution before it is placed in the NMR tube. Suspended or solid particles cause broadening of the absorption peaks in the spectrum. After you prepare your sample, measure the depth of the liquid in the NMR tube; it should be about 6 cm: adjust the volume of your sample tube as necessary.

A sample run in CDCl₃ will always show a peak at 7.26 ppm because deuterochloroform is never 100% pure and a tiny amount of residual ordinary chloroform, CHCl₃, is in the sample. This peak is used to calibrate the spectrum during the workup process.

Acquiring an NMR Spectrum

The NMR facility in the Chemistry Department at CU Boulder is under the direction of Dr. Rich Shoemaker. To operate one of the NMR instruments in this facility, you must be trained by Dr. Shoemaker. The Lab Coordinator and all of the TAs have been trained, but students are only trained if they join a research lab in the Chemistry Department. Therefore, if you want to have an NMR sample run, you will need to ask your TA or the Coordinator to run it for you (you can probably go to the facility with them to watch the process). The NMR website has instruction manuals for the various instruments in the NMR facility.

Work-up of the Spectrum

After acquiring the raw NMR data, the instrument operator will save the spectrum on the NMR lab computer as a FID file. The FID can then be worked up (changed into the familiar NMR spectrum format) at one of several computers/workstations in the NMR lab or elsewhere. The NMR website has instruction manuals for NMR data processing in the NMR facility.

Alternatively, you can ask the coordinator to transfer the FID file to your own computer (or a computer in the teaching labs area) and you can work up the spectrum using a program called NUTS. Acorn NMR, Inc., offers this NMR data processing software for PC and Mac operating systems. You can download free, functioning demo software and manuals on how to use this software on the Acorn NUTS web site.

Common Problems

Solubility: If your sample is not soluble in chloroform, other deuterated solvents are available, such as deuterium oxide (D₂O) or deuterobenzene (C₆D₆).

Contaminants and impurities: Often a sample contains a trace amount of a solvent used in the preparation of the compound. Solvents are organic compounds too and will show up in the NMR spectrum. Usually, solvents are only present in trace amounts, showing small peaks. For your reference, a table of the ¹H-NMR peaks of common solvents is presented below. The data in this table is both from an article in the Journal of Organic Chemistry ("NMR Chemical Shifts of Common Laboratory Solvents as Trace Impurities", J. Org. Chem. 1997, 62, 7512-7515) and from data collected by running the various solvents on the Varian 300 NMR instrument in the CU Boulder NMR facility.

Also remember from above that the most common NMR solvent, CDCl₃, will itself show up as a peak due to residual ordinary chloroform at 7.24 ppm.

¹H-NMR peaks of common solvents when run in CDCl₃

Peaks of the most common solvents are listed below. For a more complete list, consult the paper.