Nuclear magnetic resonance (NMR) is a powerful analytical spectroscopy technique that has far-reaching applications in the physical sciences and industry. It uses a large magnet to probe the intrinsic spin properties of atomic nuclei. The radio frequency waves that are used in NMR promote transitions between nuclear energy levels and produce an electromagnetic signal corresponding to the natural frequencies of the spinning nuclei.
NMR spectra typically reflect chemical shifts and/or coupling constants that are important for structure determination of small molecules. They are also useful for investigating molecular interactions and detecting compounds that exhibit toxic properties.
Typical NMR spectra of organic and inorganic substances include: 1H, 13C, 15N, 19F, and many others. These can be determined by placing a sample in a magnetic field and then excitation it with radio waves.
Most chemists use NMR to obtain the chemical shifts and other information necessary for the determination of a molecule’s structure. NMR spectra are based on the concept that all nuclei have spin and can transfer energy from their base level to higher energy states by a process called’spin flip’, when an external magnetic field is applied. This ‘flip’ also transfers energy when the spin comes back to its base state.
The chemical shift is usually defined by the difference between the resonant frequency of the spinning protons and the signal from a reference molecule. It is the most commonly used property in NMR spectroscopy for structure determination.
Other properties such as scalar and one-bond couplings are also reflected in NMR spectra. These are also important for determining a molecule’s structure and can be compared to the spectra of known references to determine their relative symmetry.
NMR has several advantages over spectroscopic techniques such as MS. First, it is a non-invasive technique that can be performed repeatedly over time. This allows measurement of processes occurring within the cell, which are often difficult to monitor with traditional methods. Second, atomic parameters such as relaxation times and homonuclear and heteronuclear spin couplings contain unique information about cellular metabolism that can be used to distinguish physiologically active or pathological cells.
Moreover, NMR spectroscopy can provide a structural profile of an individual molecule in sufficient detail to reveal its interaction with other molecules and the molecular structure of the receptors that sense it. This is particularly helpful for the study of complex lipids and proteins in large-scale biological systems, including cells.
Besides its high resolution, modern NMR spectroscopy provides excellent signal to noise and is fully quantitative if the sample has been prepared in the correct solvent. It can be applied to any soluble material, and if the samples are cryogenically cooled with nitrogen or helium, it is possible to increase the number of signals detected. In addition, it is a robust and versatile tool for the identification of environmental pollutants, waste water, and other mixtures.
