Nuclear Magnetic Resonance Spectroscopy

Nuclear Magnetic Resonance was first experimentally observed in late 1945, nearly simultaneously by the research groups of Felis Bloch, at Stanford University and Edward Purcell, at Harvard University. The first NMR spectra were first published in the same issue of Physical Review in January of 1946. Bloch and Purcell were jointly awarded the Nobel Prize in Physics in 1952 for their discovery of Nuclear Magnetic Resonance. Since then, NMR spectroscopy has become an indispensable tool for the determination of molecular structure, the study of molecular dynamics, and the characterization of materials at the molecular level by chemists, physicists, and molecular biologists.

For the first several decades, researchers relied on one-dimensional NMR spectra of NMR active nuclei. These spectra have one frequency axis, and analysis relies upon the relative frequency shifts between chemically inequivalent nuclei. During the 1970s, though, two-dimensional NMR was discovered and rapidly evolved NMR into the powerful tool that it is today for molecular structural determination. Two-dimensional NMR spectra have two frequency axes, which can correspond to like nuclei (i.e. 1H-1H) or different nuclei (i.e. 1H-13C), and a third dimension of peak intensity. More recently, NMR experiments have been developed that contain information in three, four, and even five dimensions. The power of NMR to clarify molecular structure seems almost limitless.

NMR is one of the few non-destructive methods for analyzing the content, purity, and structure of a molecule, as well as molecular dynamics. Because the instruments involved in spectroscopy are portable and inexpensive, they are often used for teaching and field work.

NMR exploits the behavior of certain atoms when they are placed in a very strong magnetic field. For biochemists these are mainly H, N, C and P. H and P are highly abundant isotopes while N and C are present at only low levels. Studies using these nuclei generally require isotopic enrichment, which means the molecule will be produced from media that has been enriched by these particular isotopes.

The principle behind NMR is that many nuclei have spin and all nuclei are electrically charged. If an external magnetic field is applied, an energy transfer is possible between the base energy to a higher energy level. The energy transfer takes place at a wavelength that corresponds to radio frequencies and when the spin returns to its base level, energy is emitted at the same frequency. The signal that matches this transfer is measured in many ways and processed in order to yield an NMR spectrum for the nucleus concerned.

In simple terms, when a sample is placed in the magnet, the nuclei of the atoms align with the magnetic field in the same way that the needle of a compass aligns itself in the earth's magnetic field. Typically the magnets used in NMR spectroscopy are 10,000-15,000 times stronger than the Earth’s magnetic field. The NMR experiment generally consists of applying short bursts or pulses of energy in the radio frequency (RF) range, typically 40-800 MHz, to the sample. These pulses of RF cause the nuclei to rotate away from their equilibrium position and they start to rotate around the axis of the magnetic field. The exact frequency at which the nuclei rotate is related to both the chemical and physical environment of the atom in the molecule. By using different combinations of RF pulses and delays, it is possible to determine how each atom in the molecule interacts with other atoms in the molecule. Using a large set of these interactions it is possible to calculate the three-dimensional structures of molecules.

The NMR Spectrometer is composed of the following:

Superconducting Magnet, which provides a strong, extremely homogenous magnetic field into which the sample is placed.

NMR Probe, which holds the sample and is placed into the bore of the magnet. The probe also contains the antennae for irradiating the sample with radio frequency energy and for receiving the very weak RF resonance back from the sample.

NMR Console, which is the component that generates and controls short bursts (pulses) of high-power RF energy used to excite the sample in the probe. The NMR console also receives and detects the very weak signals coming back from the probe. The performance of the RF system (stability, speed, and accuracy) is a very important factor.

Computer Workstations and software that are used in the NMR spectrometers, both for the control of the various RF pulses, as well as for the storage and processing of the NMR data.