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Nuclear Magnetic Resonance Spectroscopy

Background

Nuclear SpinNuclear Magnetic Resonance (NMR) is not easy to describe in a few paragraphs, let alone understand. Therefore, this description of NMR takes a simplistic point of view. There is much more to NMR than is contained here.

The nuclei of certain atoms (hydrogen, for example) behave as if they are spinning like tiny, positively-charged tops. A spinning nucleus (because it is charged) generates its own magnetic field. If we place a sample containing these nuclei in an external magnetic field (between the poles of a powerful magnet), the magnetic field of the spinning nuclei can either be aligned with the external magnetic field, or opposed to it. It is more favorable for the nuclear magnetic field to be aligned with the external magnetic field.

The nucleus can change its orientation to the opposed state if it absorbs the right energy of radiofrequency radiation (energy from radio waves). The size of the energy difference between the two orientations is dependent on the strength of the magnetic field that the nucleus feels. Because molecular structures can shield or expose the atom from the external magnetic field, the energy of radiation absorbed provides information about the location of that nucleus within the molecule. In addition, adjacent nuclei interact to give us information about which nuclei are near one another.

Older NMR instruments worked by bombarding a sample with 60 MHz radiation (just below the end of your FM dial), and increasing the magnetic field to differentiate between hydrogen atoms in a molecule. In the example below, the magnetic field strength increases from left to right. When a hydrogen nucleus feels the right magnetic field to absorb the 60 MHz radiation, we see a peak. H's in different parts of the molecule require different field strengths to absorb that specific energy.

Ethylbenzene Molecule and Spectrum

KU NMR Instrumentation

Varian EM360A NMR Spectrometer with Anasazi Eft Upgrade

Varian EM360AThe Varian EM360A NMR was purchased in the early to mid 1980s. This instrument was state of the art in the early 80s, but the advances in NMR technology outpaced our equipment. However, we kept it functioning with the original chart recorder and some "custom-engineered" modifications for the felt-tipped pen. In 2002 we purchased an Anasazi Instruments Eft upgrade for this instrument. The Eft upgrade converted our continuous wave instrument (that swept through a series of magnetic field strengths one at a time) to a pulsed Fourier transform NMR controlled by a Windows 2000 PC. Our instrument is now capable of performing both one- and two-dimensional NMR techniques.

COSY

 

 

The figure at left is a COSY(H-H Correlation Spectroscopy), a two-dimensional NMR spectrum that gives information about what hydrogens within the molecule are close to one another. The peaks labeled A-B, for example, indicate that hydrogens A and B in the structure are on adjacent carbons.

C-13 APT

 

We also have the capability to detect not only hydrogen-1, but carbon-13, fluorine-19, silicon-29, and phosphorous-31. The adjacent spectrum is an example of a carbon-13 NMR. The reason it has both positive and negative peaks is that it is the result of an Attached Proton Test. This spectrum not only gives information about the electronic environment surrounding the carbons in our sample molecule, it also indicates how many hydrogens are attached to each carbon. The peaks pointing down correspond to carbons with 0 or 2 hydrogens attached. The peaks pointing up correspond to carbons with 1 or 3 hydrogens attached.

Applications of Nuclear Magnetic Resonance Spectroscopy

NMR spectroscopy is one of the most powerful tools in determining the structure of organic compounds. Using the techniques described above (and a number of others), an NMR spectroscopist can ascertain the structure of a compound. All that is needed is approximately 50 milligrams of a pure sample. As a result, NMR spectroscopy is used extensively in a number of applications.

Suppose you decided to come up with a new, cheaper synthesis of aspirin. How would you know if you had actually made aspirin. NMR spectroscopy could help you confirm that your product is aspirin.

Suppose you isolated a new cure for malaria from some natural source (like skunk fur, for instance). Since there are inherent disadvantages to isolating compounds from natural sources it is often advantageous to identify the active ingredient and then make it synthetically in the lab. NMR spectroscopy would be the tool to help determine the structure of the active ingredient.

NMR spectroscopy can even be used to determine the three-dimensional shape of a molecule. Techniques have been designed to identify atoms that are close to one another in space while not necessarily being attached to adjacent atoms. These techniques are important in biochemistry, where the overall shape of a molecule often dictates its behavior.

Links to NMR Resources

SDBS Spectral Database System for Organic Compounds