Nuclear magnetic resonance (NMR) is a spectroscopic technique used to study organic molecules. It uses a strong magnetic field to detect signals from proton nuclei within a sample. The field is induced by the nuclei absorbing electromagnetic radiation at a certain frequency; this frequency is called the Larmor frequency, and is usually in the range of a radio wave.
NMR spectroscopy can be used to detect nuclei with spin, and thus is an excellent tool for investigating the structural details of organic molecules. This type of NMR spectroscopy can be applied to many kinds of samples, and it is particularly useful for organic molecules that are abundant in nature.
The process of detecting a nucleus with spin is quite simple and involves placing the sample in a magnetic field. Then, the nucleus absorbs electromagnetic radiation at a certain frequency; this energy is transferred to the protons of the sample, which jump up to a different (higher) energy state when the sample is placed in a magnetic field that is aligned with the magnetic field.
When the protons are in this higher energy state, they absorb more radiowaves than when they are in the lower energy state. This absorption causes them to flip their spin from the aligned state to the anti-aligned state; this is known as a’spin flip’ and is observed in NMR spectra.
Since the difference in energy between the two states is very small, NMR spectroscopy can be used for identifying individual molecules, even in complex mixtures. It is also an important tool for studying proteins.
In medicine, NMR has also been used to image the soft tissues of the body, such as the brain, heart, liver, kidneys, spleen, pancreas and breast, with MRI proving very sensitive to detecting diseased or damaged tissues. This noninvasive and hazard-free technique is now commonly used in the medical field, especially to investigate the organs of the human body, and it has been found effective in detecting cancer cells.
Moreover, NMR is increasingly being used in environmental research and remediation to monitor soils, plants and air-particles in their swollen states. This swollen state is not accessible by most other techniques and this approach allows the detection of molecular conformations and structures in their natural state.
Time-shared NMR experiments are another exciting approach. They combine the sensitivity of high-resolution NMR with computer-assisted structure elucidation to allow structure elucidation from NMR data at natural isotopic abundance, making it possible to determine structures that are not available through other methods.
The basic idea of time-shared NMR is that the NMR spectra of chemically inequivalent environments are Fourier transformed so that each peak corresponds to a specific environment, and that the area underneath each peak represents the number of nuclei in that particular environment. The time-shared sequences can be performed with high-resolution NMR spectroscopy, but can also be based on a solid-state probe.
In order to achieve the high resolution required, NMR spectroscopy requires powerful magnets that have fields ranging from 1 to 20 teslas. While these strong fields make NMR spectroscopy much more expensive than other types of spectroscopy, they are necessary for detecting the very low energy differences between the two spin states. This is because the energy needed to change the atomic spin of an element from one state to the other is very small, less than 0.1 cal/mole. This makes NMR a very useful technique for determining the structure of large and small molecules in the laboratory.
