Nuclear Magnetic Resonance Spectroscopy

by Gregory Brust
Nuclear Magnetic Resonance Spectroscopy, or NMR as it is usually called, is the same as a medical technique you may have heard of, MRI or Magnetic Resonance Imaging. They changed the name for use in the medical field because the word nuclear might scare some people. No one wants to get nuked! There's no need to fear, NMR and MRI use harmless radio waves to acquire their data, not the gamma rays that "nuke ya." In fact, radio waves are on the opposite end of the electromagnetic spectrum from gamma rays. Take a look.

NMR is also a characterization technique where a sample is immersed in a magnetic field and hit with radio waves. These radio waves encourage the nuclei of the molecule to sing a song for us that can only be picked up on a special radio receiver. But just like opera, the nuclei sing in a language we can't understand, so we need a decoder. This decoder is called the Fourier Transform algorithm. It is a complex equation that translates the language of the nuclei into something we can understand. Here's the algorithm, in case you're curious.

Then the song of the nuclei is analyzed to determine many different things about the molecule and its surroundings, such as the molecule's structure. This may be a little confusing now, but this page should at least give you a basic understanding of the technique.

To fully understand NMR spectroscopy, one must first shrink down to subatomic size and take a look at the nucleus. I'll give you a second to get down to size . . .

Are you tiny yet? Good! Now, if you were not smaller than the wavelength of visible light, you would see that the nucleus is spinning. Scientists can't just call it spinning though. They have to try and sound smart and call it resonance. As the positively charged nucleus spins, this moving charge creates a magnetic moment. You can think of it as a spinning subatomic bar magnet. When no magnetic field is present, these tiny magnets are aligned randomly, but when they are placed in a homogenous magnetic field, the magnetic moments will line up with the magnetic field. Even though the magnetic moments are aligned by the magnetic field, the nuclear spin is not simple and flat like the spin of a merry-go-round. The thermal motion of the molecule creates a torque which makes the magnetic moment "wobble" like a child's top. When the radio waves hit the spinning nuclei, they tilt even more, and even flip over sometimes. When the magnetic moment is tilted away from the applied magnetic field, some of the magnetic moment is detectable perpendicular (90 o) to the applied magnetic field.

Different nuclei resonate at different frequencies. This means that you must hit a carbon atom with a different frequency radio wave than a hydrogen atom to get it to flip. It also means that similar atoms in different environments, such as a hydrogen attached to an oxygen and a hydrogen attached to a carbon, flip at different frequencies. By seeing at which frequencies these different nuclei flip, one can determine how a molecule is put together, as well as many other interesting properties of the molecule.

I understand that two different atoms, like carbon and hydrogen, resonate at different frequencies because they are different, but why do two similar atoms in different environments resonate at different frequencies?

That is a very good question. The answer is shielding. What is shielding you ask? Well I'll tell you. The electrons which surround the spinning nuclei are also charged and spinning, and if you've been paying attention, you know that a spinning charge creates a magnetic field. which is in opposition to the applied magnetic field. This decreases the magnitude of the applied magnetic field which reaches the nuclei. In other words, the electrons "shield" the nuclei from the full magnetic field. Because the resonant frequency of a nuclei depends on the strength of the magnetic field it "feels" . . . I think you get the idea.

Okay, I know what an NMR spectrometer does, but how does it work?

Be patient. I'm getting to that. That big spaceship looking thing in the picture on the right is an NMR spectrometer. The majority of the machine is just a big "cooler" filled with two very cold liquids, liquid helium and liquid nitrogen. How cold? Liquid nitrogen has a temperature of -195oC and liquid helium is -269oC! The liquid helium is in the innermost part of the "cooler" to cool the superconducting coil which creates the magnetic field to -269oC and the liquid nitrogen surrounds it to keep the helium from evaporating too fast.

The hole in the top (where Funda is pointing) is where you put your sample into the spectrometer. When it gets into the machine, an air jet spins the sample tube to give a more uniform sample to scan.

When a sample is made for solution NMR spectroscopy, the solvent or part of the solvent used should be deuterated. This means that there are deuterium atoms in the place of the hydrogens of the solvent molecule. Hydrogen has one proton as its nucleus while deuterium has a proton and a neutron in its nucleus. This is necessary to "lock" the NMR on a specific frequency so the spectrum will not drift around during acquisition.

Now that our sample is in a magnetic field, locked and spinning, we can acquire a spectrum. First, an RF (radio frequency) generator "pulses" the sample with a short burst of radio waves. These waves are absorbed and transmitted through the sample to the receiver which detects the signal from the sample. This information is then transmitted to the computer next to the NMR where it is translated and analyzed. Click here to read learn more.

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