Nuclear Magnetic Resonance

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Nuclear Magnetic Resonance (NMR) is a physical phenomenon described independently by F. Bloch and E. M. Purcell in 1946 both of whom shared the Nobel Prize in physics in 1952 for their discovery. It involves the interaction of atomic nucleus placed in an external magnetic field with applied electro-magnetic field oscillating at a particular frequency. At that frequency, energy from the electro-magnetic field can be absorbed by the nucleus.

Only nuclei with non zero magnetic moment can undergo NMR. Such nuclei must have an odd number of protons or neutrons (ex. 1H, 2H, 13C, 31P).

NMR is used as a spectroscopy technique to obtain physical and chemical properties of molecules. It is also the underlying principle of Magnetic Resonance Imaging.

1H is frequently used nucleus for NMR spectroscopy and we can use it as an example to see how such spectroscopy works. The hydrogen nucleus can be thought of as a spinning charged body which acts as a tiny magnet. Its spin = 1/2 and so in the presence of an external magnetic field H it can only take up two orientations; with the field or against it. The energy difference between the two different orientations is 2μH where μ is the magnetic moment of the nucleus. If a photon (embodying the oscillating applied field) is supplied with exactly this energy, a nucleus can flip its orientation by absorbing the photon. The useful thing about NMR is, that required photon frequency depends on the local field H that is seen by the nucleus not the applied external field.

The difference is that nuclei are surrounded by orbiting electrons, which are also spinning charged particles [i.e. magnets] and so will partially shield the nuclei. The amount of shielding depends on the exact local environment. For example, a hydrogen bonded to an oxygen will be shielded differently than a hydrogen bonded to a carbon atom. In addition, two hydrogen nuclei can interact via a process known as spin spin couplingif they are on the same molecule, which will split the lines of the spectra in a recognisable way. By studying the peaks of a NMR spectra skilled chemists can determine the structure of many compounds. It can be a very selective technique, distinguishing among many atoms within a molecule or collection of molecules of the same type, but which differ only in terms of their local chemical environment.

A relatively recent example of NMR being used in the determination of a structure is that of Buckminsterfullerene. This now famous form of carbon has 60 carbon atoms forming a football shaped molecule. The carbon atoms are all in identical environments and so should see the same internal H field. Unfortunately Buckminster Fullerene contains no hydrogen and so 13C NMR has to be used [a more difficult form of NMR to do. However in [date here please] the spectra was obtained and sure enough it did contain just the one single spike, confirming the unusual structure of C60.

The development of NMR as a technique of analytical chemistry and biochemistry parallels the development of electromagnetic technology and its introduction into civilian use. Purcell had worked on the development and application of RADAR during World War II at MIT's Radiation Lab. His work during that project on the production and detection of radiofrequency energy, and on the absorption of such energy by matter, preceded his discovery of NMR and probably contributed to his understanding of it and related phenonmena.

Throughout the next several decades, NMR practice utilized a technique known as continuous-wave, or CW, spectroscopy, in which either the magnetic field was kept constant and the oscillating field was swept in frequency to chart the on-resonance portions of the spectrum, or more frequently, the oscillating field was held at a fixed frequency, and the magnetic field was swept through the transitions. This technique is limited in that it probes each frequency individually, in succession, which has unfortunate consequences due to the insensitivity of NMR--that is to say, NMR suffers from poor signal-to-noise ratio.

Fortunately for NMR in general, signal-to-noise ratio (S/N) can be improved by signal averaging. Signal averaging increases S/N by the square-root of the number of signals taken. A technique relying on the Heisenberg Uncertainty Principle can speed the time it takes to acquire a scan by allowing a range of frequencies to be probed at once. This technique, known as Fourier transform NMR spectroscopy (FT-NMR), has been made more practical with the development of computers capable of performing the computationally-intensive mathematical transformation of the data from the time domain to the frequency domain, to produce a spectrum.

This technique, pioneered by Richard R. Ernst, works by irradiating the sample (still held in a static, external magnetic field) with a short pulse of radiofrequency energy (RF). According to the uncertainty principle, the shorter the pulse, the broader the range of frequencies it contains. The pulse perturbs the equilibrium energy states of the nuclei under study (1H for instance). At the end of the pulse, the nuclei relax back to their equilibrium state, emitting the energy absorbed by the system again in the radiofrequency range. Detectors record the time dependent decay of this excitation as a time-dependent pattern, known as the free induction decay (FID). This time-dependent pattern, when processed through the Fourier transform, reveals the frequency-dependent pattern of nuclear resonances, the NMR spectrum.

The use of pulses of various shapes, frequencies, and durations, in specifically-designed patterns, gives the spectroscopist great flexibility in determining what portions of a molecule, or what intra- and intermolecular dynamic processes, to study.

Kurt Wutrich, Ad Bax and many others, developed FT-NMR into a powerful technique for studying biochemistry, in particular for the determination of the structure of biopolymers such as proteins or even small nucleic acids. This technique complements biopolymer X-ray crystallography in that it is most frequently applicable to biomolecules in a liquid or liquid crystal phase, whereas crystallography (as the name implies) is performed on molecules in a solid phase. Though NMR is used to study solids, extensive atomic-level biomolecular structural detail is especially difficult to obtain in the solid state.

Because the intensity of NMR signals, and hence the sensitivity of the technique, depend on the strength of the magnetic field, the technique has also advanced over the decades with the development of more powerful magnets.

The sensitivity of NMR signals is also dependent, as noted above, on the presence of a magnetically-susceptible isotope, and therefore either on the natural abundance of such isotopes, or on the ability of the experimentalist to artificially enrich the molecules under study with such isotopes. The most abundant naturally occuring isotopes of hydrogen and phosphorus, for instance, are both magnetically susceptible and readily useful for NMR spectroscopy. In contrast, carbon and nitrogen have useful nuclei, but which occur only in very low natural abundance.

See also