Nuclear magnetic resonance (NMR) is an essential tool for researchers across a wide range of disciplines, including drug development, polymer and plastics chemistry, biochemistry, nanotechnology, and materials engineering. NMR employs the same basic principles as magnetic resonance imaging (MRI), a critical tool for medical imaging of the human body. NMR is used by scientists to confirm they have made the correct material, determine the purity of a sample, determine the structure of a molecule of interest, and to study how molecules move.
Superconducting Magnets for NMR
The critical component of the NMR instrument is a very strong superconducting magnet. In contrast to so-called “permanent magnets” such as those you might have on your refrigerator, superconducting magnets have an electric current running through them. This allows us to have a much stronger magnet. For example, the lowest field (i.e. weakest) magnets we have at the University of Delaware are 9.4 Tesla (400 MHz), while a typical refrigerator magnet is ~0.001 Tesla.
An alloy of niobium (Nb) and tin (Sn) is one of the most commonly used low temperature superconductors for NMR magnets. Nb3Sn is a so-called intermetallic compound, which are formed from electropositive and electronegative metals, which chemically bond to form compounds with a specific composition and crystalline structure. The Nb3Sn is formed into wires, which are wrapped precisely and tightly into a coil.
Superconducting magnets must be kept very cold to maintain their superconductivity. To achieve this, the superconducting magnet coil sits in a vessel of liquid helium (4K, -420°F, -269°C). This inner vessel of liquid helium is surrounded by an outer vessel of liquid nitrogen (77K, -320°F, -196°C) to help slow the evaporation of liquid helium. The bore of the magnet, which contains the sample during an NMR experiment, is at room temperature. At the University of Delaware, our magnets are connected to a helium recovery system to recycle the helium as it slowly evaporates off. To read more about how we recover helium and why it is so important, visit our Helium Recovery page.
From Atoms to an NMR Spectrum
Atoms are comprised of a nucleus, containing protons and neutrons, and electrons surrounding the nucleus. The nucleus of many isotopes have a property known as nuclear spin (typically represented by an arrow), which is related to the number of protons and neutrons in the nucleus. Isotopes with a spin > 0 are NMR active and we can perform NMR experiments on them. Hydrogen atoms are the most routinely studied isotope via NMR.
An NMR experiment starts with a sample, either dissolved in solution or a solid sample. Outside the NMR magnet, nuclear spins of the sample are oriented in all different directions. When the sample is placed into the NMR magnet, the spins orient along the z-axis of the magnetic field.
We then apply a burst or pulse of radio frequency (RF) radiation to the sample. This causes the spins to flip to be perpendicular to the magnetic field.
The nuclear spins then return to their original orientation along the Z-axis of the magnetic field (“relax”). As they relax, the atoms give off signal, that we detect in the form of an NMR spectrum. The frequency of the signal given off depends on the environment around an atom. Shown below is the structure of aspirin (a common pain reliever), as well as its proton (1H) NMR spectrum. Chemists take several pieces of information from the NMR spectrum to determine the structure of the molecule they are studying. (1) The position in the spectrum (known as the ‘chemical shift’) where a peak shows up tells us the type/function group that the atom is in. For example in the spectrum of aspirin, the protons of the CH3 group show up as 1 tall peak at 1.9 ppm (parts per million, the chemical shift scale). The aromatic protons show up in the 7-8 ppm range.
The integration or area under each peak in the NMR spectrum tells us the number of protons that peak corresponds to. The CH3 proton peak has an integral of 2.85 or ~3, telling us there are 3 protons in this peak. Two of the aromatic peaks have integrals of 1.04 and 1.05, telling us each of these peaks corresponds to 1 proton, while the peak at 6.9 ppm has an integral of 2 meaning this peak corresponds to 2 protons. Chemists also get information on the molecule from the fine details of the peak pattern, known as “splitting.” Looking more closely at the aromatic region of aspirin, we can see the peaks have unique splitting patterns.
Looking at the peak at 7.8 ppm, this peak is split twice, telling us that this proton has 2 neighboring protons. The other peaks have more complex splitting patterns due to the larger number of neighboring protons. We are not limited to studying the protons in our samples. Chemists will typically get NMR spectra of several different elements in their sample. Common additional elements that we routinely get NMR spectra for are carbon, fluorine, phosphorous, silicon, boron, and aluminum. Taking all this information together, chemists can piece together what molecule they have made.