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's high magnetic field (800 MHz, 18.8 T ) NMR spectrometer being loaded with a sample.]] NMR Magnet at HWB-NMR, Birmingham, UK being loaded with a sample]] Nuclear magnetic resonance ('''NMR''') is a physical phenomenon based upon the Quantum Mechanical Magnetic properties of an Atom 's Nucleus . NMR also commonly refers to a family of scientific methods that exploit nuclear magnetic resonance to study Molecules . All nuclei that contain odd numbers of Proton s or Neutron s have an intrinsic Magnetic Moment and Angular Momentum . The most commonly measured nuclei are Hydrogen -1 (the most receptive Isotope at natural abundance) and Carbon -13, although nuclei from isotopes of many other elements (e.g. 15 N , 14N 19 F , 31 P , 17 O , 29 Si , 10 B , 11B, 23 Na , 35 Cl , 195 Pt ) can also be observed. NMR resonant frequencies for a particular substance are directly proportional to the strength of the applied magnetic field. NMR studies magnetic nuclei by aligning them with an applied constant Magnetic Field and perturbing this alignment using an alternating Electromagnetic Field , the fields being Orthogonal . The resulting response to the perturbing electromagnetic field is the phenomenon that is exploited in NMR Spectroscopy and Magnetic Resonance Imaging , which use very powerful applied magnetic fields in order to achieve high resolution spectra. NMR phenomena are also utilised in Low Field NMR and Earth's Field NMR spectrometers, and some kinds of Magnetometer . HISTORY Discovery Nuclear magnetic resonance was first described and measured in molecular beams by Isidor Rabi in 1938.Rabi, I. I., Zacharias, J. R., Millman, S. and Kusch, P. (1938). A New Method of Measuring Nuclear Magnetic Moment. Physical Review 53, 318-318 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''. Interestingly, for analytical scientists, 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 and its introduction into civilian use. 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 Orbital s, so do even numbers of protons and neutrons (which are also spin ½ particles and hence Fermion s) 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 is associated with a non-zero magnetic moment, μ, via : where the proportionality constant, γ, is the Gyromagnetic Ratio . It is this magnetic moment that is exploited in NMR. Electron Spin Resonance is a related technique which exploits the spin of electrons instead of nuclei. The basic principles are otherwise similar. Values of spin angular momentum As with any quantum object, the Angular Momentum associated with nuclear spin is Quantized , both in the sense that the magnitude of angular momentum is quantized i.e. ''I'' can only take on a restricted range of values (integer or half-integer), but the 'orientation' of the associated angular momentum is also quantized. The associated quantum number is known as the Magnetic Quantum Number , ''m'', and can take values from +''I'' to –''I'' in integral steps. Hence for any given nucleus, there is a total of 2''I''+1 angular momentum states. The ''z'' component of the angular momentum vector, ''I''''z'', is therefore: : where is Planck's Reduced Constant . The ''z'' component of the magnetic moment is simply : Spin behavior in a magnetic field Consider nuclei which have a spin of one-half, like 1H, 13C or 19F. The nucleus has two possible spin states: ''m'' = ½ or ''m'' = -½ (also referred to as up and down or α and β, respectively). The energies of these states are degenerate—that is to say that they are the same. Hence the ''populations'' of the two states (i.e. number of atoms in the two states) will be exactly equal at Thermal Equilibrium . If a nucleus is placed in a magnetic field, however, the interaction between the nuclear magnetic moment and the external magnetic field mean the two states no longer have the same energy. The Energy of a magnetic moment μ when in a magnetic field ''B''0 (the zero subscript is used to distinguish this Magnetic Field from any other applied field) is given by the negative scalar product of the vectors: : where the magnetic field has been oriented along the ''z'' axis. Hence : As a result the different nuclear spin states have different energies in a non-zero magnetic field. In hand-waving terms, we can talk about the two spin states of a spin ½ as being ''aligned'' either with or against the magnetic field. If γ is positive (true for most isotopes) then ''m'' = ½ is the lower energy state. The energy difference between the two states is : and this difference results in a small population bias toward the lower energy state. Resonance Resonant absorption will occur when Electromagnetic Radiation of the correct frequency to match this energy difference is applied. The energy of a Photon is ''E'' = ''hν'', where ''ν'' is its frequency. Hence absorption will occur when : These frequencies typically correspond to the Radio Frequency range of the Electromagnetic Spectrum . It is this resonant absorption that is detected in NMR. Nuclear shielding It might appear from the above that all nuclei of the same nuclide (and hence the same γ) would resonate at the same frequency. This is not the case. The most important perturbation of the NMR frequency for applications of NMR is the 'shielding' effect of the surrounding electrons. In general, this electronic shielding reduces the magnetic field ''at the nucleus'' (which is what determines the NMR frequency). As a result the energy gap is reduced, and the frequency required to achieve resonance is also reduced. This shift of the NMR frequency due to the chemical environment is called the Chemical Shift , and it explains why NMR is a direct probe of chemical structure. Unless the local Symmetry is particularly high, the shielding effect depends on the orientation of the molecule with respect to the external field. In Solid-state NMR , Magic Angle Spinning is required to average out this orientation dependence. This is unnecessary in conventional NMR of molecules in solution since rapid molecular tumbling averages out the anisotropic component of the chemical shift. Relaxation The process called population relaxation refers to nuclei that return to the thermodynamic state in the magnet. This process is also called ''T''1 relaxation, where ''T''1 refers to the mean time for an individual nucleus to return to its equilibrium state. Once the population is relaxed, it can be probed again, since it is in the initial state. The Precessing nuclei can also fall out of alignment with each other (returning the net magnetization vector to a nonprecessing field) and stop producing a signal. This is called ''T''2 relaxation. It is possible to be in this state and not have the population difference required to give a net magnetization vector at its thermodynamic state. Because of this, ''T''1 is always larger (slower) than ''T''2. This happens because some of the spins were flipped by the pulse and will remain so until they have undergone population relaxation. In practice, the ''T''2 time is the life time of the observed NMR signal, the Free Induction Decay . In the NMR spectrum, meaning the Fourier Transform of the Free Induction Decay , the ''T''2 time defines the width of the NMR signal. Thus, a nucleus having a large ''T''2 time gives rise to a sharp signal, whereas nuclei with shorter ''T''2 times give rise to more broad signals. The length of ''T''1 and ''T''2 is closely related to molecular motion. NMR SPECTROSCOPY NMR Spectroscopy is one of the principal techniques used to obtain physical, chemical, electronic and structural information about Molecule s. It is a powerful technique that can provide detailed information on the topology, dynamics and three-dimensional structure of molecules in solution and the solid state. Also, nuclear magnetic resonance is one of the techniques that has been used to build elementary Quantum Computer s. Continuous wave (CW) spectroscopy In its first few decades, nuclear magnetic resonance spectrometers used a technique known as continuous-wave (CW) spectroscopy. Although NMR spectra could be obtained using a fixed magnetic field and sweeping the frequency of the electromagnetic radiation, this more typically involved using a fixed frequency source and varying the current (and hence magnetic field) in an electromagnet to observe the resonant absorption signals. (This is the origin of the now anachronistic but still common "high" and "low" field terminology for low frequency and high frequency regions respectively of the NMR spectrum.) CW spectroscopy is inefficient in comparison to Fourier techniques (see below) as it probes the NMR response at individual frequencies in succession. As the NMR signal is intrinsically weak, the observed spectra suffer from a poor Signal-to-noise Ratio (S/N). This can be mitigated by signal averaging i.e. adding the spectra from repeated measurements. While the NMR signal is constant between scans and so adds linearly, the noise is random adds so more slowly–as the square-root of the number of spectra (see Random Walk ). Hence the overall ratio of the signal to the noise increases as the square-root of the number of spectra measured. Fourier spectroscopy Most applications of NMR involve full NMR Spectra , that is, the intensity of the NMR signal as a function of frequency. Early attempts to acquire the NMR spectrum more efficiently than simple CW methods involved irradiating simultaneously with more than one frequency. It was soon realised, however, that a simpler solution was to use short pulses of radio-frequency (centred at the middle of the NMR spectrum). In simple terms, a short square pulse of a given "carrier" frequency "contains" a range of frequencies centred about the carrier frequency, with the range of excitation ( Bandwidth ) being inversely proportional to the pulse duration (the Fourier Transform of an approximate Square Wave contains contributions from all the frequencies in the neighborhood of the principal frequency). The restricted range of the NMR frequencies made it relatively easy to use RF pulses to excite the entire NMR spectrum. Applying such a pulse to a set of nuclear spins simultaneously excites all the NMR transitions. In terms of the net magnetisation vector, this corresponds to tilting the magnetisation vector away from its equilibrium position (aligned along the external magnetic field). The out-of-equilibrium magnetisation vector Precesses about the external magnetic field at the NMR frequency of the spins. This oscillating magnetisation Induces a current in a nearby pickup coil, creating an electrical signal oscillating at the NMR frequency. This signal is known as the Free Induction Decay (FID) and contains the sum of the NMR responses from all the excited spins. In order to obtain the frequency-domain NMR Spectrum (intensity vs. frequency) this time-domain signal (intensity vs. time) must be Fourier Transformed . Fortunately the development of FT-NMR coincided with the development of digital computers and Fast Fourier Transform algorithms. Richard R. Ernst was one of the pioneers of pulse (FT) NMR and won a Nobel Prize In Chemistry in 1991 for his work on FT-NMR and his development of multi-dimensional NMR (see below). Multi-dimensional The use of pulses of different shapes, frequencies and durations in specifically-designed patterns or ''pulse sequences'' allows the spectroscopist to extract many different types of information about the molecule. Multi-dimensional nuclear magnetic resonance spectroscopy is a kind of FT-NMR in which there are at least two pulses and, as the experiment is repeated, the pulse sequence is varied. In ''multidimensional nuclear magnetic resonance'' there will be a sequence of pulses and, at least, one variable time period. In three dimensions, two time sequences will be varied. In four dimensions, three will be varied. There are many such experiments. In one, these time intervals allow—among other things—magnetization transfer between nuclei and, therefore, the detection of the kinds of nuclear-nuclear interactions that allowed for the magnetization transfer. Interactions that can be detected are usually classified into two kinds. There are ''through-bond'' interactions and ''through-space'' interactions, the latter usually being a consequence of the Nuclear Overhauser Effect . Experiments of the nuclear-Overhauser variety may establish distances between atoms. Although the fundamental concept of 2D NMR was proposed by the Belgian scientist Jean Jeener , professor at the Université Libre De Bruxelles , this idea was largely developed by Richard Ernst who won the 1991 Nobel Prize In Chemistry for his work in FT and multi-dimensional NMR. Multi-dimensional NMR experiments were further developed into powerful methodologies for studying biomolecules in solution, in particular for the determination of the structure of Biopolymer s such as Protein s or even small Nucleic Acid s. Kurt Wüthrich shared the 2002 Nobel Prize In Chemistry for his work in Protein Nuclear Magnetic Resonance Spectroscopy . Solids This technique complements biopolymer X-ray Crystallography in that it is frequently applicable to Biomolecule s in a Liquid or Liquid Crystal phase, whereas crystallography, as the name implies, is performed on molecules in a Solid phase. Though nuclear magnetic resonance is used to study solids, extensive atomic-level biomolecular structural detail is especially challenging to obtain in the solid state. There is no signal averaging by thermal motion in the solid state, where molecules are held still, each in a slightly different electronic environment, giving a different signal. This variation in electronic environment lowers resolution greatly and makes interpretation more difficult. Raymond Andrew was a pioneer in the development of high-resolution Solid-state Nuclear Magnetic Resonance . He introduced the Magic Angle Spinning (MAS) technique and allowed for an increase in resolution by several orders of magnitude. In MAS, the sample is averaged by spinning it at several kilohertz. Alex Pines together with John S. Waugh revolutionized the area with the introduction of the cross-polarization technique in order to enhance low abundance and sensitivity nuclei. Sensitivity Because the intensity of nuclear magnetic resonance signals and, hence, the sensitivity of the technique depends on the strength of the magnetic field the technique has also advanced over the decades with the development of more powerful magnets. Advances made in audio-visual technology have also improved the signal-generation and processing capabilities of newer machines. The sensitivity of nuclear magnetic resonance signals is also dependent—as noted above—on the presence of a magnetically-susceptible nuclide and, therefore, either on the natural abundance of such nuclides or on the ability of the experimentalist to artificially enrich the molecules, under study, with such nuclides. The most abundant naturally-occurring isotopes of hydrogen and phosphorus—for instance—are both magnetically susceptible and readily useful for nuclear magnetic resonance spectroscopy. In contrast, carbon and nitrogen have useful isotopes but which occur only in very low natural abundance. Other limitations on sensitivity arise from the quantum-mechanical nature of the phenomenon. For quantum states separated by energy equivalent to radio frequencies, thermal energy from the environment causes the populations of the states to be close to equal. Since incoming radiation is equally likely to cause stimulated emission (a transition from the upper to the lower state) as absorption, the NMR effect depends on an excess of nuclei in the lower states. Several factors can reduce sensitivity, including
Isotopes Almost any chemical element can be used for NMR analysis. {Link without Title}
APPLICATIONS Medicine The use of nuclear magnetic resonance best known to the general public is in Magnetic Resonance Imaging for medical diagnosis, however, it is also widely used in chemical studies, notably in NMR Spectroscopy such as Proton NMR and Carbon-13 NMR . These studies are possible because nuclei are surrounded by orbiting electrons, which are also spinning charged particles such as Magnet s 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 Coupling , if they are on the same molecule, which will split the lines of the spectra in a recognisable way. Chemistry By studying the peaks of nuclear magnetic resonance 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.
''T''2 information can give information about dynamics and molecular motion.
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