The Ultimate Guide to Understanding NMR Spectroscopy

Nuclear Magnetic Resonance (NMR) spectroscopy is an analytical technique that has become an essential tool for chemists to identify and analyze organic molecules. NMR spectroscopy allows us to determine the structure, purity, and concentration of compounds accurately. In this article, we will provide you with a comprehensive guide to NMR spectroscopy, explaining the principles behind the technique, the equipment used, and how to interpret NMR spectra.

Section 1: The Principles of NMR Spectroscopy

NMR spectroscopy is based on the principles of nuclear spin and magnetic fields. In a magnetic field, atomic nuclei that have an odd number of protons and neutrons will possess nuclear spin. When placed in an external magnetic field, the nuclear spins of these nuclei will align either with or against the direction of the magnetic field.

In NMR spectroscopy, a sample containing the molecule of interest is placed in a strong magnetic field, causing the nuclear spins of the atoms in the molecule to align. The sample is then irradiated with radio waves, which causes the nuclei to absorb energy and undergo a change in spin state. As the nuclei return to their original spin state, they release energy, which is detected and recorded as an NMR spectrum.

Section 2: The Equipment Used in NMR Spectroscopy

NMR spectrometers consist of several key components, including the magnet, the radiofrequency (RF) source, and the detector. The magnet is the most crucial component of the NMR spectrometer, providing the strong magnetic field required for the experiment.

RF pulses are used to excite the nuclei in the sample, and the resulting NMR signal is detected by the detector. The signal is then processed by the computer, and a spectrum is produced, which shows the chemical shifts of the atoms in the molecule.

Section 3: Interpreting NMR Spectra

Interpreting an NMR spectrum can be a challenging task, requiring a good understanding of the principles behind the technique and the factors that can influence the spectrum. The most critical feature of an NMR spectrum is the chemical shift, which provides information about the chemical environment of the atoms in the molecule.

Other factors that can affect the NMR spectrum include spin-spin coupling, which occurs when two or more atoms in a molecule are coupled together and influence each other's magnetic fields, and relaxation times, which determine the rate at which the nuclei return to their original spin state.

Conclusion:

In conclusion, NMR spectroscopy is a powerful analytical tool that provides valuable information about the structure, purity, and concentration of organic molecules. Understanding the principles behind the technique, the equipment used, and how to interpret NMR spectra is essential for anyone working in the field of organic chemistry. We hope this comprehensive guide has provided you with the knowledge you need to succeed in your research.

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.

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.

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