Nuclear Overhauser effect
Encyclopedia
The Nuclear Overhauser Effect (NOE) is the transfer of nuclear spin polarization
from one nuclear spin population to another via cross-relaxation
. It is a common phenomenon observed by nuclear magnetic resonance (NMR) spectroscopy
. The theoretical basis for the NOE was described and experimentally verified by Anderson and Freeman in 1962. The NOE is an extension of the seminal work of American
physicist
Albert Overhauser
who in 1953 proposed that nuclear spin polarization could be enhanced by the microwave irradiation of the conduction electrons in certain metals. The general Overhauser effect was first demonstrated experimentally by T. R. Carver and C. P. Slichter, also in 1953. Another early explication and experimental observation of the NOE was by Kaiser in 1963 in an NMR experiment where the spin polarization was transferred from one population of nuclear spins to another, rather than from electrons spins to nuclear spins. However, the theoretical basis and the applicable Solomon equations had already been published by Ionel Solomon in 1955.
Subsequent to its discovery, the NOE was shown to be highly useful in NMR spectroscopy for characterizing and refining organic chemical structures. In this application, the NOE differs from the application of spin-spin coupling in that the NOE occurs through space, not through chemical bonds. Thus, atoms that are in close proximity to each other can give a NOE, whereas spin coupling is observed only when the atoms are connected by 2-3 chemical bonds. The inter-atomic distances derived from the observed NOE can often help to confirm a precise molecular conformation, i.e. the three-dimensional structure of a molecule. In 2002, Kurt Wüthrich was awarded the Nobel Prize in Chemistry for demonstrating that the NOE could be exploited using two-dimensional NMR spectroscopy to determine the three-dimensional structures of biological macromolecules in solution (see: ).
Some examples of two-dimensional NMR experimental techniques exploiting the NOE include:
Spin polarization
Spin polarization is the degree to which the spin, i.e., the intrinsic angular momentum of elementary particles, is aligned with a given direction. This property may pertain to the spin, hence to the magnetic moment, of conduction electrons in ferromagnetic metals, such as iron, giving rise to...
from one nuclear spin population to another via cross-relaxation
Relaxation (NMR)
In nuclear magnetic resonance spectroscopy and magnetic resonance imaging the term relaxation describes several processes by which nuclear magnetization prepared in a non-equilibrium state return to the equilibrium distribution. In other words, relaxation describes how fast spins "forget" the...
. It is a common phenomenon observed by nuclear magnetic resonance (NMR) spectroscopy
NMR spectroscopy
Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is a research technique that exploits the magnetic properties of certain atomic nuclei to determine physical and chemical properties of atoms or the molecules in which they are contained...
. The theoretical basis for the NOE was described and experimentally verified by Anderson and Freeman in 1962. The NOE is an extension of the seminal work of American
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
physicist
Physicist
A physicist is a scientist who studies or practices physics. Physicists study a wide range of physical phenomena in many branches of physics spanning all length scales: from sub-atomic particles of which all ordinary matter is made to the behavior of the material Universe as a whole...
Albert Overhauser
Albert Overhauser
Albert W. Overhauser is an American physicist and member of the National Academy of Sciences. He is best known for his theory of dynamic nuclear polarization, also known as the Overhauser Effect....
who in 1953 proposed that nuclear spin polarization could be enhanced by the microwave irradiation of the conduction electrons in certain metals. The general Overhauser effect was first demonstrated experimentally by T. R. Carver and C. P. Slichter, also in 1953. Another early explication and experimental observation of the NOE was by Kaiser in 1963 in an NMR experiment where the spin polarization was transferred from one population of nuclear spins to another, rather than from electrons spins to nuclear spins. However, the theoretical basis and the applicable Solomon equations had already been published by Ionel Solomon in 1955.
Subsequent to its discovery, the NOE was shown to be highly useful in NMR spectroscopy for characterizing and refining organic chemical structures. In this application, the NOE differs from the application of spin-spin coupling in that the NOE occurs through space, not through chemical bonds. Thus, atoms that are in close proximity to each other can give a NOE, whereas spin coupling is observed only when the atoms are connected by 2-3 chemical bonds. The inter-atomic distances derived from the observed NOE can often help to confirm a precise molecular conformation, i.e. the three-dimensional structure of a molecule. In 2002, Kurt Wüthrich was awarded the Nobel Prize in Chemistry for demonstrating that the NOE could be exploited using two-dimensional NMR spectroscopy to determine the three-dimensional structures of biological macromolecules in solution (see: ).
Some examples of two-dimensional NMR experimental techniques exploiting the NOE include:
- NOESY, 'Nuclear Overhauser Effect Spectroscopy
- HOESY, Heteronuclear Overhauser Effect Spectroscopy
- ROESY, Rotational Frame Nuclear Overhauser Effect Spectroscopy
- TRNOE, Transferred Nuclear Overhauser Effect
- DPFGSE-NOE, Double Pulsed Field Gradient Spin Echo NOE experiment