Nuclear magnetic resonance
Nuclear magnetic resonance is a physical phenomenon in which nuclei in a strong constant magnetic field are disturbed by a weak oscillating magnetic field and respond by producing an electromagnetic signal with a frequency characteristic of the magnetic field at the nucleus. This process occurs near resonance, when the oscillation frequency matches the intrinsic frequency of the nuclei, which depends on the strength of the static magnetic field, the chemical environment, and the magnetic properties of the isotope involved; in practical applications with static magnetic fields up to ca. 20 tesla, the frequency is similar to VHF and UHF television broadcasts. NMR results from specific magnetic properties of certain atomic nuclei. High-resolution nuclear magnetic resonance spectroscopy is widely used to determine the structure of organic molecules in solution and study molecular physics and crystals as well as non-crystalline materials. NMR is also routinely used in advanced medical imaging techniques, such as in magnetic resonance imaging. The original application of NMR to condensed matter physics is nowadays mostly devoted to strongly correlated electron systems. It reveals large many-body couplings by fast broadband detection and should not be confused with solid state NMR, which aims at removing the effect of the same couplings by Magic Angle Spinning techniques.
The most commonly used nuclei are and, although isotopes of many other elements, such as,, and, can be studied by high-field NMR spectroscopy as well. In order to interact with the magnetic field in the spectrometer, the nucleus must have an intrinsic angular momentum and nuclear magnetic dipole moment. This occurs when an isotope has a nonzero nuclear spin, meaning an odd number of protons and/or neutrons. Nuclides with even numbers of both have a total spin of zero and are therefore not NMR-active.
In its application to molecules the NMR effect can be observed only in the presence of a static magnetic field. However, in the ordered phases of magnetic materials, very large internal fields are produced at the nuclei of magnetic ions, which allow NMR to be performed in zero applied field. Additionally, radio-frequency transitions of nuclear spin I > with large enough electric quadrupolar coupling to the electric field gradient at the nucleus may also be excited in zero applied magnetic field.
In the dominant chemistry application, the use of higher fields improves the sensitivity of the method and the spectral resolution. Commercial NMR spectrometers employing liquid helium cooled superconducting magnets with fields of up to 28 Tesla have been developed and are widely used.
It is a key feature of NMR that the resonance frequency of nuclei in a particular sample substance is usually directly proportional to the strength of the applied magnetic field. It is this feature that is exploited in imaging techniques; if a sample is placed in a non-uniform magnetic field then the resonance frequencies of the sample's nuclei depend on where in the field they are located. This effect serves as the basis of magnetic resonance imaging.
The principle of NMR usually involves three sequential steps:
- The alignment of the magnetic nuclear spins in an applied, constant magnetic field B0.
- The perturbation of this alignment of the nuclear spins by a weak oscillating magnetic field, usually referred to as a radio frequency pulse. The oscillation frequency required for significant perturbation is dependent upon the static magnetic field and the nuclei of observation.
- The detection of the NMR signal during or after the RF pulse, due to the voltage induced in a detection coil by precession of the nuclear spins around B0. After an RF pulse, precession usually occurs with the nuclei's Larmor frequency and, in itself, does not involve transitions between spin states or energy levels.
NMR phenomena are also utilized in low-field NMR, NMR spectroscopy and MRI in the Earth's magnetic field, and in several types of magnetometers.
History
Nuclear magnetic resonance was first described and measured in molecular beams by Isidor Rabi in 1938, by extending the Stern–Gerlach experiment, and in 1944, Rabi was awarded the Nobel Prize in Physics for this work. In 1946, Felix Bloch and Edward Mills Purcell expanded the technique for use on liquids and solids, for which they shared the Nobel Prize in Physics in 1952.Russell H. Varian filed the "Method and means for correlating nuclear properties of atoms and magnetic fields", on October 21, 1948 and was accepted on July 24, 1951. Varian Associates developed the first NMR unit called NMR HR-30 in 1952.
Purcell had worked on the development of radar during World War II at the Massachusetts Institute of Technology's Radiation Laboratory. His work during that project on the production and detection of radio frequency power and on the absorption of such RF power by matter laid the foundation for his discovery of NMR in bulk matter.
Rabi, Bloch, and Purcell observed that magnetic nuclei, like and, could absorb RF energy when placed in a magnetic field and when the RF was of a frequency specific to the identity of the nuclei. When this absorption occurs, the nucleus is described as being in resonance. Different atomic nuclei within a molecule resonate at different frequencies in the same applied static magnetic field, due to various local magnetic fields. The observation of such magnetic resonance frequencies of the nuclei present in a molecule makes it possible to determine essential chemical and structural information about the molecule.
The improvements of the NMR method benefited from the development of electromagnetic technology and advanced electronics and their introduction into civilian use. Originally as a research tool it was limited primarily to dynamic nuclear polarization, by the work of Anatole Abragam and Albert Overhauser, and to condensed matter physics, where it produced one of the first demonstrations of the validity of the BCS theory of superconductivity by the observation by Charles Slichter of the Hebel-Slichter effect. It soon showed its potential in organic chemistry, where NMR has become indispensable, and by the 1990s improvement in the sensitivity and resolution of NMR spectroscopy resulted in its broad use in analytical chemistry, biochemistry and materials science.
In the 2020s zero- to ultralow-field nuclear magnetic resonance, a form of spectroscopy that provides abundant analytical results without the need for large magnetic fields, was developed. It is combined with a special technique that makes it possible to hyperpolarize atomic nuclei.
Theory of nuclear magnetic resonance
Nuclear spins and magnets
All nucleons, that is neutrons and protons, composing any atomic nucleus, have the intrinsic quantum property of spin, an intrinsic angular momentum analogous to the classical angular momentum of a spinning sphere. The overall spin of the nucleus is determined by the spin quantum number S. If the numbers of both the protons and neutrons in a given nuclide are even then, i.e. there is no overall spin. Then, just as electrons pair up in nondegenerate atomic orbitals, so do even numbers of protons or even numbers of neutrons, giving zero overall spin.However, an unpaired proton and unpaired neutron will have a lower energy when their spins are parallel, not anti-parallel. This parallel spin alignment of distinguishable particles does not violate the Pauli exclusion principle. The lowering of energy for parallel spins has to do with the quark structure of these two nucleons. As a result, the spin ground state for the deuteron, which has only a proton and a neutron, corresponds to a spin value of 1, not of zero. On the other hand, because of the Pauli exclusion principle, the tritium isotope of hydrogen must have a pair of anti-parallel spin neutrons, plus a proton of spin. Therefore, the tritium total nuclear spin value is again, just like the simpler, abundant hydrogen isotope, 1H nucleus. The NMR absorption frequency for tritium is also similar to that of 1H. In many other cases of non-radioactive nuclei, the overall spin is also non-zero and may have a contribution from the orbital angular momentum of the unpaired nucleon. For example, the nucleus has an overall spin value.
A non-zero spin is associated with a non-zero magnetic dipole moment,, via the relation where γ is the gyromagnetic ratio. Classically, this corresponds to the proportionality between the angular momentum and the magnetic dipole moment of a spinning charged sphere, both of which are vectors parallel to the rotation axis whose length increases proportional to the spinning frequency. It is the magnetic moment and its interaction with magnetic fields that allows the observation of NMR signal associated with transitions between nuclear spin levels during resonant RF irradiation or caused by Larmor precession of the average magnetic moment after resonant irradiation. Nuclides with even numbers of both protons and neutrons have zero nuclear magnetic dipole moment and hence do not exhibit NMR signal. For instance, is an example of a nuclide that produces no NMR signal, whereas,, and are nuclides that do exhibit NMR spectra. The last two nuclei have spin S > and are therefore quadrupolar nuclei.
Electron spin resonance is a related technique in which transitions between electronic rather than nuclear spin levels are detected. The basic principles are similar but the instrumentation, data analysis, and detailed theory are significantly different. Moreover, there is a much smaller number of molecules and materials with unpaired electron spins that exhibit ESR absorption than those that have NMR absorption spectra. On the other hand, ESR has much higher signal per spin than NMR does.