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32 pages, February 23, 2009

Abstract:

Nuclear magnetic resonance (NMR) is the name given to a physical resonance phenomenon involving the observation of specific quantum mechanical magnetic properties of an atom's nucleus in the presence of an external magnetic field applied to a molecular system and crystalline or non--crystalline materials. NMR also commonly refers to a family of scientific methods and techniques that exploit the nuclear magnetic resonance resonance phenomenon to study molecules, crystals and non-crystalline materials ("NMR spectroscopy" is perhaps the most important, as well as routine, group of techniques in this family). All nuclei that contain odd numbers of protons and/or neutrons have an intrinsic magnetic moment and angular momentum, in other words a spin > 0. The most commonly measured nuclei are 1H (the most NMR-sensitive isotope after the radioactive 3H isotope, and also after the stable 13C nucleus, although nuclei from isotopes of many other elements (e.g. 2H, 10B, 11B, 14N, 15N, 17O, 19F, 23Na, 29Si, 31P, 35Cl, 113Cd, 195Pt) are readily measured by high-field NMR spectroscopy as well. NMR resonant frequencies for a particular substance are directly proportional to the strength of the applied magnetic field, in accordance with the equation for the Larmor precession frequency. The scientific literature as of February 2009 includes NMR spectra at magnetic fields in a wide range: from about 5 nT up to 24 T. Very high magnetic fields are often preferred since 1D--NMR detection sensitivity increases proportionally with the magnetic field strength (the "Golden Rule of NMR"). Other methods to increase either the NMR signal strentgth or the detection sensitivity include hyperpolarization and two-dimensional (2D) FT NMR techniques. The principle of NMR usually involves two sequential steps: (1) the alignment or polarization of the magnetic nuclear spins being studied in an applied, constant magnetic field H0, and (2) the perturbation of this alignment of the nuclear spins (in the constant external magnetic field) by employing a second, alternating magnetic field (rf) H1rf, with the two fields being usually orthogonal for maximum detected NMR signal intensity. The resulting response by the total magnetization, M = \vec{M}, of the nuclear spins to the perturbing magnetic field is the phenomenon that is exploited in NMR spectroscopy and magnetic resonance imaging, which both use intense applied magnetic fields H0, in order to achieve high spectral resolution, the details of which are described by the chemical shift, the Zeeman effect, and Knight shifts (in metals). Nuclear magnetic resonance was first described and measured in molecular beams by Isidor Rabi in 1938.[1] Eight years later, in 1946, Felix Bloch and Edward Mills Purcell refined the technique for use on liquids and solids, for which they shared the Nobel Prize in physics in 1952. Purcell had worked on the development and application of RADAR during World War II at Massachusetts Institute of Technology's Radiation Laboratory. 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. They noticed that magnetic nuclei, like 1H and 31P, could absorb RF energy when placed in a magnetic field of a strength specific to the identity of the nuclei. When this absorption occurs, the nucleus is described as being in resonance. Different atoms within a molecule resonate at different frequencies at a given field strength. The observation of the resonance frequencies of a molecule allows a user to discover structural information about the molecule. The development of nuclear magnetic resonance as a technique of analytical chemistry and biochemistry parallels the development of electromagnetic technology. NMR phenomena are also utilized in low field NMR, Earth's field NMR spectrometers, and several types of magnetometers. Theory of nuclear magnetic resonance: Nuclear spin and magnets The elementary particles, neutrons and protons, composing an atomic nucleus, have the intrinsic quantum mechanical property of spin. The overall spin of the nucleus is determined by the spin quantum number I. If the number of both the protons and neutrons in a given isotope are even then I = 0, i.e. there is no overall spin; just as electrons pair up in atomic orbitals, so do even numbers of protons and neutrons (which are also spin--½ particles and hence fermions) pair up giving zero overall spin. In other cases, however, the overall spin is non-zero. For example 27Al has an overall spin I = 5/2. A non-zero spin, I, is associated with a non--zero magnetic moment \mu, via \mu = I . gamma, where the proportionality constant, gamma, is the gyromagnetic ratio. It is this magnetic moment that allows the observation of NMR absorption spectra caused by transitions between nuclear spin levels. Most radioactive nuclei (with some rare exceptions, such as tritium) that have both even numbers of protons and even numbers of neutrons, also have zero nuclear magnetic moments-and also have zero magnetic dipole and quadrupole moments;therefore, such radioactive isotopes do not exhibit any NMR absorption spectra. Thus, 12C, 32P and 36Cl are examples of radioactive nuclear isotopes that have no NMR absorption, whereas 13C, 31P, 35Cl and 37Cl are stable nuclear isotopes that do exhibit NMR absorption spectra. Electron spin resonance is a related technique which detects transitions between electron spin levels instead of nuclear ones. The basic principles are similar; however, 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 (or EPR) absorption than those that have NMR absorption spectra. Significantly also, is the much greater sensitivity of ESR in comparison with NMR. Furthermore, ferromagnetic materials and thin films may exhibit highly resolved ferromagnetic resonance (FMR) spectra, or spin wave excitations (SWR) beyond the single-quantum transitions common to most routine NMR and EPR studies. [2] References: Martin, G.E; Zekter, A.S., "Two-Dimensional NMR Methods for Establishing Molecular Connectivity.", VCH Publishers, Inc: New York, 1988; Akitt, J.W.; Mann, B.E., "NMR and Chemistry"; Stanley Thornes: Cheltenham, UK, 2000. ; Akitt, J.W.; Mann, B.E., "NMR and Chemistry"; Stanley Thornes: Cheltenham, UK, 2000.; Hornak, Joseph P. The Basics of NMR ; J. Keeler, Understanding NMR Spectroscopy; Wuthrich, Kurt. "NMR of Proteins and Nucleic Acids". Wiley--Interscience, New York, NY USA (1986). Richard Ernst, NL--Developer of Multdimensional NMR techniques Freeview video provided by the Vega Science Trust. 2D--FT NMR , MR--Imaging and related Nobel awards Illustrated applications of 2D--FT NMR and MR--Imaging. 'An Interview with Kurt W\"uthrich' Freeview video by the Vega Science Trust (W\"uthrich was awarded a Nobel Prize in Chemistry in 2002 "for his development of nuclear magnetic resonance spectroscopy for determining the three-dimensional structure of biological macromolecules in solution").

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