Nuclear magnetic resonance

Nuclear magnetic resonance (NMR) is a

The most commonly used nuclei are

A key feature of NMR is that the resonant frequency of 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 (polarization) 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 (RF) pulse. The oscillation frequency required for significant perturbation is dependent upon the static magnetic field (B₀) 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 intrinsic Larmor frequency and, in itself, does not involve transitions between spin states or energy levels.^[1]

The two magnetic fields are usually chosen to be perpendicular to each other as this maximizes the NMR signal strength. The frequencies of the time-signal response by the total magnetization (M) of the nuclear spins are analyzed in NMR spectroscopy and magnetic resonance imaging. Both use applied magnetic fields (B₀) of great strength, often produced by large currents in superconducting coils, in order to achieve dispersion of response frequencies and of very high homogeneity and stability in order to deliver spectral resolution, the details of which are described by chemical shifts, the Zeeman effect, and Knight shifts (in metals). The information provided by NMR can also be increased using hyperpolarization, and/or using two-dimensional, three-dimensional and higher-dimensional techniques.

NMR phenomena are also utilized in

Nuclear magnetic resonance was first described and measured in molecular beams by

The improvements of the NMR method benefited from the development of electromagnetic technology and advanced electronics and their introduction into civilian use.^{[citation needed]} 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, and by the 1990s improvement in the sensitivity and resolution of NMR spectroscopy resulted in its broader use in analytical chemistry, biochemistry and materials science.^{[citation needed]}

In the 2020s zero- to ultralow-field nuclear magnetic resonance (ZULF NMR), 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.^[9]

Theory of nuclear magnetic resonance

Nuclear spin and magnets

All nucleons, that is

]

However, a proton and neutron spin vector that aligns itself opposite to the external magnetic field vector will have a lower energy when their spins are parallel, not anti-parallel. This parallel spin alignment of distinguishable particles does not violate the

²⁷
Al

nucleus has an overall spin value S = 5/2.

A non-zero spin ${\displaystyle {\vec {S}}}$ is always associated with a non-zero magnetic dipole moment, ${\displaystyle {\vec {\mu }}}$, via the relation

$${\displaystyle {\vec {\mu }}=\gamma {\vec {S}}}$$

where γ is the

Nuclear spin is an intrinsic angular momentum that is quantized. This means that the magnitude of this angular momentum is quantized (i.e. S can only take on a restricted range of values), and also that the x, y, and z-components of the angular momentum are quantized, being restricted to integer or half-integer multiples of ħ. The integer or half-integer quantum number associated with the spin component along the z-axis or the applied magnetic field is known as the magnetic quantum number, m, and can take values from +S to −S, in integer steps. Hence for any given nucleus, there are a total of 2S + 1 angular momentum states.^{[citation needed]}

The z-component of the angular momentum vector (${\displaystyle {\vec {S}}}$) is therefore S_z = mħ, where ħ is the reduced Planck constant. The z-component of the magnetic moment is simply:

$${\displaystyle \mu _{z}=\gamma S_{z}=\gamma m\hbar .}$$

Spin energy in a magnetic field

If a nucleus is placed in a magnetic field, however, the two states no longer have the same energy as a result of the interaction between the nuclear magnetic dipole moment and the external magnetic field. The energy of a magnetic dipole moment ${\displaystyle {\vec {\mu }}}$ in a magnetic field B₀ is given by:

$${\displaystyle E=-{\vec {\mu }}\cdot \mathbf {B} _{0}=-\mu _{x}B_{0x}-\mu _{y}B_{0y}-\mu _{z}B_{0z}.}$$

Usually the z-axis is chosen to be along B₀, and the above expression reduces to:

$${\displaystyle E=-\mu _{\mathrm {z} }B_{0}\,,}$$

$${\displaystyle E=-\gamma m\hbar B_{0}\,.}$$

As a result, the different nuclear spin states have different energies in a non-zero magnetic field. In less formal language, we can talk about the two spin states of a spin 1/2 as being aligned either with or against the magnetic field. If γ is positive (true for most isotopes used in NMR) then m = 1/2 is the lower energy state.

The energy difference between the two states is:

$${\displaystyle \Delta {E}=\gamma \hbar B_{0}\,,}$$

Precession of the spin magnetization

A central concept in NMR is the precession of the spin magnetization around the magnetic field at the nucleus, with the angular frequency

$${\displaystyle \omega =-\gamma B}$$

${\displaystyle \omega =2\pi \nu }$

${\displaystyle \nu }$

expectation value

of the spin vector in quantum mechanics), moves on a cone around the B field. This is analogous to the precessional motion of the axis of a tilted spinning top around the gravitational field. In quantum mechanics, ${\displaystyle \omega }$ is the Bohr frequency^[10] ${\displaystyle \Delta {E}/\hbar }$ of the ${\displaystyle S_{x}}$ and ${\displaystyle S_{y}}$ expectation values. Precession of non-equilibrium magnetization in the applied magnetic field B₀ occurs with the Larmor frequency

$${\displaystyle \omega _{L}=2\pi \nu _{L}=-\gamma B_{0},}$$

without change in the populations of the energy levels because energy is constant (time-independent Hamiltonian).^[11]

Magnetic resonance and radio-frequency pulses

A perturbation of nuclear spin orientations from equilibrium will occur only when an oscillating magnetic field is applied whose frequency ν_rf sufficiently closely matches the Larmor precession frequency ν_L of the nuclear magnetization. The populations of the spin-up and -down energy levels then undergo Rabi oscillations,^[10] which are analyzed most easily in terms of precession of the spin magnetization around the effective magnetic field in a reference frame rotating with the frequency ν_rf.^[12] The stronger the oscillating field, the faster the Rabi oscillations or the precession around the effective field in the rotating frame. After a certain time on the order of 2–1000 microseconds, a resonant RF pulse flips the spin magnetization to the transverse plane, i.e. it makes an angle of 90° with the constant magnetic field B₀ ("90° pulse"), while after a twice longer time, the initial magnetization has been inverted ("180° pulse"). It is the transverse magnetization generated by a resonant oscillating field which is usually detected in NMR, during application of the relatively weak RF field in old-fashioned continuous-wave NMR, or after the relatively strong RF pulse in modern pulsed NMR.^{[citation needed]}

Chemical shielding

It might appear from the above that all nuclei of the same nuclide (and hence the same γ) would resonate at exactly 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 shells of electrons.^[13] Electrons, similar to the nucleus, are also charged and rotate with a spin to produce a magnetic field opposite to the applied magnetic field. In general, this electronic shielding reduces the magnetic field at the nucleus (which is what determines the NMR frequency). As a result, the frequency required to achieve resonance is also reduced. This shift in the NMR frequency due to the electronic molecular orbital coupling to the external magnetic field is called chemical shift, and it explains why NMR is able to probe the chemical structure of molecules, which depends on the electron density distribution in the corresponding molecular orbitals. If a nucleus in a specific chemical group is shielded to a higher degree by a higher electron density of its surrounding molecular orbital, then its NMR frequency will be shifted "upfield" (that is, a lower chemical shift), whereas if it is less shielded by such surrounding electron density, then its NMR frequency will be shifted "downfield" (that is, a higher chemical shift).

Unless the local

chemical shift anisotropy

(CSA). In this case, the "average" chemical shift (ACS) or isotropic chemical shift is often simply referred to as the chemical shift.

Relaxation

Further information: Relaxation (NMR)

The process of population relaxation refers to nuclear spins that return to thermodynamic equilibrium in the magnet. This process is also called

spin-lattice" or "longitudinal magnetic" relaxation, where T₁ refers to the mean time for an individual nucleus to return to its thermal equilibrium state of the spins. After the nuclear spin population has relaxed, it can be probed again, since it is in the initial, equilibrium (mixed) state.^{[citation needed}

]

T₂ or transverse relaxation. Because of the difference in the actual relaxation mechanisms involved (for example, intermolecular versus intramolecular magnetic dipole-dipole interactions ), T₁ is usually (except in rare cases) longer than T₂ (that is, slower spin-lattice relaxation, for example because of smaller dipole-dipole interaction effects). In practice, the value of T₂* which is the actually observed decay time of the observed NMR signal, or free induction decay (to 1/e of the initial amplitude immediately after the resonant RF pulse), also depends on the static magnetic field inhomogeneity, which is quite significant. (There is also a smaller but significant contribution to the observed FID shortening from the RF inhomogeneity of the resonant pulse).^{[citation needed]} In the corresponding FT-NMR spectrum—meaning the Fourier transform of the free induction decay—the T₂* time is inversely related to the width of the NMR signal in frequency units. Thus, a nucleus with a long T₂ relaxation time gives rise to a very sharp NMR peak in the FT-NMR spectrum for a very homogeneous ("well-shimmed") static magnetic field, whereas nuclei with shorter T₂ values give rise to broad FT-NMR peaks even when the magnet is shimmed well. Both T₁ and T₂ depend on the rate of molecular motions as well as the gyromagnetic ratios of both the resonating and their strongly interacting, next-neighbor nuclei that are not at resonance.^{[citation needed]}

A Hahn echo decay experiment can be used to measure the dephasing time, as shown in the animation. The size of the echo is recorded for different spacings of the two pulses. This reveals the decoherence that is not refocused by the 180° pulse. In simple cases, an exponential decay is measured which is described by the T₂ time.

NMR spectroscopy

NMR spectroscopy is one of the principal techniques used to obtain physical, chemical, electronic and structural information about

Structure and molecular dynamics can be studied (with or without "magic angle" spinning (MAS)) by NMR of quadrupolar nuclei (that is, with spin S > 1/2) even in the presence of magnetic "dipole-dipole" interaction broadening (or simply, dipolar broadening), which is always much smaller than the quadrupolar interaction strength because it is a magnetic vs. an electric interaction effect.^{[citation needed]}

Additional structural and chemical information may be obtained by performing double-quantum NMR experiments for pairs of spins or quadrupolar nuclei such as

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 illuminating the target simultaneously with more than one frequency. A revolution in NMR occurred when short radio-frequency pulses began to be used, with a frequency centered at the middle of the NMR spectrum. In simple terms, a short pulse of a given "carrier" frequency "contains" a range of frequencies centered about the

The use of pulses of different durations, frequencies, or shapes in specifically designed patterns or pulse sequences allows production of a spectrum that contains many different types of information about the molecules in the sample. In multi-dimensional nuclear magnetic resonance spectroscopy, there are at least two pulses: one leads to the directly detected signal and the others affect the starting magnetization and spin state prior to it. The full analysis involves repeating the sequence with the pulse timings systematically varied in order to probe the oscillations of the spin system are point by point in the time domain. Multidimensional Fourier transformation of the multidimensional time signal yields the multidimensional spectrum. In two-dimensional nuclear magnetic resonance spectroscopy (2D-NMR), there will be one systematically varied time period in the sequence of pulses, which will modulate the intensity or phase of the detected signals. In 3D-NMR, two time periods will be varied independently, and in 4D-NMR, three will be varied.

There are many such experiments. In some, fixed 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 and through-space interactions. Through-bond interactions relate to structural connectivity of the atoms and provide information about which ones are directly connected to each other, connected by way of a single other intermediate atom, etc. Through-space interactions relate to actual geometric distances and angles, including effects of dipolar coupling and the nuclear Overhauser effect.

Although the fundamental concept of 2D-FT NMR was proposed by

This technique complements

Professor Raymond Andrew at the University of Nottingham in the UK pioneered the development of high-resolution solid-state nuclear magnetic resonance. He was the first to report the introduction of the MAS (magic angle sample spinning; MASS) technique that allowed him to achieve spectral resolution in solids sufficient to distinguish between chemical groups with either different chemical shifts or distinct Knight shifts. In MASS, the sample is spun at several kilohertz around an axis that makes the so-called magic angle θ_m (which is ~54.74°, where 3cos²θ_m-1 = 0) with respect to the direction of the static magnetic field B₀; as a result of such magic angle sample spinning, the broad chemical shift anisotropy bands are averaged to their corresponding average (isotropic) chemical shift values. Correct alignment of the sample rotation axis as close as possible to θ_m is essential for cancelling out the chemical-shift anisotropy broadening. There are different angles for the sample spinning relative to the applied field for the averaging of electric quadrupole interactions and paramagnetic interactions, correspondingly ~30.6° and ~70.1°. In amorphous materials, residual line broadening remains since each segment is in a slightly different environment, therefore exhibiting a slightly different NMR frequency.

Dipolar and J-couplings to nearby ¹H nuclei are usually removed by radio-frequency pulses applied at the ¹H frequency during signal detection. The concept of cross polarization developed by Sven Hartmann and Erwin Hahn was utilized in transferring magnetization from protons to less sensitive nuclei by M.G. Gibby, Alex Pines and John S. Waugh. Then, Jake Schaefer and Ed Stejskal demonstrated the powerful use of cross polarization under MAS conditions (CP-MAS) and proton decoupling, which is now routinely employed to measure high resolution spectra of low-abundance and low-sensitivity nuclei, such as carbon-13, silicon-29, or nitrogen-15, in solids. Significant further signal enhancement can be achieved by dynamic nuclear polarization from unpaired electrons to the nuclei, usually at temperatures near 110 K.

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 instruments.

As noted above, the sensitivity of nuclear magnetic resonance signals is also dependent 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 example) 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:

• Increasing temperature, which evens out the population of states. Conversely, low temperature NMR can sometimes yield better results than room-temperature NMR, providing the sample remains liquid.
• Saturation of the sample with energy applied at the resonant radiofrequency. This manifests in both CW and pulsed NMR; in the first case (CW) this happens by using too much continuous power that keeps the upper spin levels completely populated; in the second case (pulsed), each pulse (that is at least a 90° pulse) leaves the sample saturated, and four to five times the (longitudinal) relaxation time (5T₁) must pass before the next pulse or pulse sequence can be applied. For single pulse experiments, shorter RF pulses that tip the magnetization by less than 90° can be used, which loses some intensity of the signal, but allows for shorter recycle delays. The optimum there is called an Ernst angle, after the Nobel laureate. Especially in solid state NMR, or in samples containing very few nuclei with spin (diamond with the natural 1% of carbon-13 is especially troublesome here) the longitudinal relaxation times can be on the range of hours, while for proton-NMR they are more in the range of one second.
• Non-magnetic effects, such as electric-
  ¹⁵
  N
  
  isotope, which has spin-1/2.

Isotopes

Many isotopes of chemical elements can be used for NMR analysis.^[23]

Commonly used nuclei:

• ¹
  H
  signal has been the sole diagnostic nucleus used for clinical magnetic resonance imaging
  (MRI).
• NMR spectroscopy
  to monitor drifts in the magnetic field strength (lock) and to monitor the homogeneity of the external magnetic field.
• endohedral fullerenes
  , where its chemical inertness is beneficial to ascertaining the structure of the entrapping fullerene.
• borosilicate
  glass interferes with measurement.
• ¹³
  C
  spin-1/2, is widely used, despite its relative paucity in naturally occurring carbon (approximately 1.1%). It is stable to nuclear decay. Since there is a low percentage in natural carbon, spectrum acquisition on samples which have not been experimentally enriched in ¹³
  C
  takes a long time. Frequently used for labeling of compounds in synthetic and metabolic studies. Has low sensitivity and moderately wide chemical shift range, yields sharp signals. Low percentage makes it useful by preventing spin-spin couplings and makes the spectrum appear less crowded. Slow relaxation means that spectra are not integrable unless long acquisition times are used.
• ¹⁴
  N
  , spin-1, medium sensitivity nucleus with wide chemical shift range. Its large quadrupole
  moment interferes in acquisition of high resolution spectra, limiting usefulness to smaller molecules and functional groups with a high degree of symmetry such as the headgroups of lipids.
• ¹⁵
  N
  
  , spin-1/2, relatively commonly used. Can be used for isotopically labeling compounds. Very insensitive but yields sharp signals. Low percentage in natural nitrogen together with low sensitivity requires high concentrations or expensive isotope enrichment.
• ¹⁷
  O
  , spin-5/2, low sensitivity and very low natural abundance (0.037%), wide chemical shift range (up to 2000 ppm). Quadrupole moment causing line broadening. Used in metabolic and biochemical studies in studies of chemical equilibria.
• ¹⁹
  F
  
  , spin-1/2, relatively commonly measured. Sensitive, yields sharp signals, has a wide chemical shift range.
• ³¹
  P
  
  , spin-1/2, 100% of natural phosphorus. Medium sensitivity, wide chemical shift range, yields sharp lines. Spectra tend to have a moderate amount of noise. Used in biochemical studies and in coordination chemistry with phosphorus-containing ligands.
• ³⁵
  Cl
  is significantly more sensitive, preferred over ³⁷
  Cl
  
  despite its slightly broader signal. Organic chlorides yield very broad signals. Its use is limited to inorganic and ionic chlorides and very small organic molecules.
• ⁴³
  Ca
  
  , spin-7/2, relatively small quadrupole moment, moderately sensitive, very low natural abundance. Used in biochemistry to study calcium binding to DNA, proteins, etc.
• catalysts
  and complexes.

NMR is extensively used in medicine in the form of magnetic resonance imaging. NMR is used in organic chemistry and industrially mainly for analysis of chemicals. The technique is also used to measure the ratio between water and fat in foods, monitor the flow of corrosive fluids in pipes, or to study molecular structures such as catalysts.^[24]

Medicine

The application of nuclear magnetic resonance best known to the general public is

As one of the two major spectroscopic techniques used in metabolomics, NMR is used to generate metabolic fingerprints from biological fluids to obtain information about disease states or toxic insults.

Chemistry

By studying the peaks of nuclear magnetic resonance spectra, 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. NMR spectroscopy is used to unambiguously identify known and novel compounds, and as such, is usually required by scientific journals for identity confirmation of synthesized new compounds. See the articles on

A chemist can determine the identity of a compound by comparing the observed nuclear precession frequencies to known frequencies. Further structural data can be

Because the nuclear magnetic resonance timescale is rather slow, compared to other spectroscopic methods, changing the temperature of a T₂* experiment can also give information about fast reactions, such as the Cope rearrangement or about structural dynamics, such as ring-flipping in cyclohexane. At low enough temperatures, a distinction can be made between the axial and equatorial hydrogens in cyclohexane.

An example of nuclear magnetic resonance being used in the determination of a structure is that of

• w_std: weight of internal standard
• w_spl: weight of sample
• n[H]_std: the integrated area of the peak selected for comparison in the standard, corrected for the number of protons in that functional group
• n[H]_spl: the integrated area of the peak selected for comparison in the sample, corrected for the number of protons in that functional group
• MW_std:
  molecular weight
  of standard
• MW_spl:
  molecular weight
  of sample
• P: purity of internal standard

Non-destructive testing

Nuclear magnetic resonance is extremely useful for analyzing samples non-destructively. Radio-frequency magnetic fields easily penetrate many types of matter and anything that is not highly conductive or inherently

In addition to providing static information on molecules by determining their 3D structures, one of the remarkable advantages of NMR over X-ray crystallography is that it can be used to obtain important dynamic information. This is due to the orientation dependence of the chemical-shift, dipole-coupling, or electric-quadrupole-coupling contributions to the instantaneous NMR frequency in an anisotropic molecular environment.^[26] When the molecule or segment containing the NMR-observed nucleus changes its orientation relative to the external field, the NMR frequency changes, which can result in changes in one- or two-dimensional spectra or in the relaxation times, depending on the correlation time and amplitude of the motion.

Data acquisition in the petroleum industry

Another use for nuclear magnetic resonance is

NMR logging, a subcategory of electromagnetic logging, measures the induced magnet moment of hydrogen nuclei (protons) contained within the fluid-filled pore space of porous media (reservoir rocks). Unlike conventional logging measurements (e.g., acoustic, density, neutron, and resistivity), which respond to both the rock matrix and fluid properties and are strongly dependent on mineralogy, NMR-logging measurements respond to the presence of hydrogen. Because hydrogen atoms primarily occur in pore fluids, NMR effectively responds to the volume, composition, viscosity, and distribution of these fluids, for example oil, gas or water. NMR logs provide information about the quantities of fluids present, the properties of these fluids, and the sizes of the pores containing these fluids. From this information, it is possible to infer or estimate:

• The volume (porosity) and distribution (permeability) of the rock pore space
• Rock composition
• Type and quantity of fluid hydrocarbons
• Hydrocarbon producibility

The basic core and log measurement is the T₂ decay, presented as a distribution of T₂ amplitudes versus time at each sample depth, typically from 0.3 ms to 3 s. The T₂ decay is further processed to give the total pore volume (the total porosity) and pore volumes within different ranges of T₂. The most common volumes are the bound fluid and free fluid. A permeability estimate is made using a transform such as the Timur-Coates or SDR permeability transforms. By running the log with different acquisition parameters, direct hydrocarbon typing and enhanced diffusion are possible.

Flow probes for NMR spectroscopy

Recently, real-time applications of NMR in liquid media have been developed using specifically designed flow probes (flow cell assemblies) which can replace standard tube probes. This has enabled techniques that can incorporate the use of

In the Earth's magnetic field, NMR frequencies are in the audio frequency range, or the very low frequency and ultra low frequency bands of the radio frequency spectrum. Earth's field NMR (EFNMR) is typically stimulated by applying a relatively strong dc magnetic field pulse to the sample and, after the end of the pulse, analyzing the resulting low frequency alternating magnetic field that occurs in the Earth's magnetic field due to free induction decay (FID). These effects are exploited in some types of magnetometers, EFNMR spectrometers, and MRI imagers. Their inexpensive portable nature makes these instruments valuable for field use and for teaching the principles of NMR and MRI.

An important feature of EFNMR spectrometry compared with high-field NMR is that some aspects of molecular structure can be observed more clearly at low fields and low frequencies, whereas other aspects observable at high fields are not observable at low fields. This is because:

• Electron-mediated heteronuclear
  spin-spin couplings) are field independent, producing clusters of two or more frequencies separated by several Hz, which are more easily observed in a fundamental resonance of about 2 kHz."Indeed it appears that enhanced resolution is possible due to the long spin relaxation times and high field homogeneity which prevail in EFNMR."^[31]
• Chemical shifts of several
  ppm
  are clearly separated in high field NMR spectra, but have separations of only a few millihertz at proton EFNMR frequencies, so are usually not resolved.

In zero field NMR all magnetic fields are shielded such that magnetic fields below 1 nT (nanotesla) are achieved and the nuclear precession frequencies of all nuclei are close to zero and indistinguishable. Under those circumstances the observed spectra are no-longer dictated by chemical shifts but primarily by J-coupling interactions which are independent of the external magnetic field. Since inductive detection schemes are not sensitive at very low frequencies, on the order of the J-couplings (typically between 0 and 1000 Hz), alternative detection schemes are used. Specifically, sensitive magnetometers turn out to be good detectors for zero field NMR. A zero magnetic field environment does not provide any polarization hence it is the combination of zero field NMR with hyperpolarization schemes that makes zero field NMR desirable.

Quantum computing

NMR

Magnetometers

Various magnetometers use NMR effects to measure magnetic fields, including proton precession magnetometers (PPM) (also known as proton magnetometers), and Overhauser magnetometers.

SNMR

Surface magnetic resonance (or magnetic resonance sounding) is based on the principle of nuclear magnetic resonance (NMR) and measurements can be used to indirectly estimate the water content of saturated and unsaturated zones in the earth's subsurface.^[32] SNMR is used to estimate aquifer properties, including quantity of water contained in the aquifer, porosity, and hydraulic conductivity.

Makers of NMR equipment

Major NMR instrument makers include

References

Further reading

• ISBN 9780198520146
  .
• J.W. Akitt; B.E. Mann (2000). NMR and Chemistry. Cheltenham, UK: Stanley Thornes. pp. 273, 287.
  ISBN 978-0-7487-4344-5
  .
• K.V.R. Chary,
  ISBN 978-1-4020-6680-1
  .
• PMID 2047852
  .
• ISBN 9781483184081
  .
• The Feynman Lectures on Physics Vol. II Ch. 35: Paramagnetism and Magnetic Resonance
• David M. Grant; Robin Kingsley Harris (2002). "Advances in NMR". Encyclopedia of Nuclear Magnetic Resonance. John Wiley.
  ISBN 9780471490821
  .
• R.L. Haner; P.A. Keifer (2009). "Flow Probes for NMR Spectroscopy". Encyclopedia of Magnetic Resonance. John Wiley.
  ISBN 978-0470034590
  .
• J.P. Hornak. "The Basics of NMR". Retrieved 23 February 2009.
• J. Keeler (2005). Understanding NMR Spectroscopy. John Wiley & Sons.
  ISBN 978-0-470-01786-9
  .
• ISBN 978-0-471-18707-3
  .
• J.A.Pople; W.G.Schneider; H.J.Bernstein (1959). High-resolution Nuclear Magnetic Resonance. McGraw-Hill Book Company.
• ISBN 9781258811662
  .
• ISBN 9783540084761
  .
• J.M. Tyszka; S.E. Fraser; R.E. Jacobs (2005). "Magnetic resonance microscopy: recent advances and applications". Current Opinion in Biotechnology. 16 (1): 93–99.
  PMID 15722021
  .
• ISBN 978-0-471-11917-3
  .

External links

Wikimedia Commons has media related to Nuclear magnetic resonance.

Tutorial

• NMR/MRI tutorial
• NMR Library NMR Concepts
• NMR Course Notes
• Downloadable NMR exercises as PowerPoint (english/german) and PDF (german only) files

Animations and simulations

• A free interactive simulation of NMR principles
• Interactive simulation on the Bloch sphere

Video

• introduction to NMR and MRI
• Richard Ernst, NL – Developer of multidimensional NMR techniques Freeview video provided by the Vega Science Trust.
• 'An Interview with Kurt Wuthrich' Freeview video by the Vega Science Trust (Wüthrich 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").

Other

• Qian, C.; Pines, A.; Martin, R. W. (September 2007). "Off Magic Angle Spinning". Journal of Magnetic Resonance. 188 (1): 183–189.
  PMID 17638585
  .
• Spotlight on nuclear magnetic resonance: a timeless technique