Nuclear magnetic resonance

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"NMR" redirects here. For other uses, see NMR
(disambiguation).

Pacific
Northwest National Laboratory's high magnetic field (800 MHz,
18.8 T) NMR spectrometer being
loaded with a sample.

900MHz, 21.2 T NMR Magnet at HWB-NMR,
Birmingham, UK being loaded with a sample

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 ~ 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).

NMR phenomena are also utilized in low field NMR, Earth's field NMR
spectrometers, and several types of magnetometers.

Contents

1 History

1.1 Discovery

2 Theory of nuclear magnetic
resonance

2.1 Nuclear spin and
magnets

2.2 Values of spin angular
momentum

2.2.1 Spin behavior in a
magnetic field

2.2.2 Resonance

2.2.3 Nuclear
shielding

2.3 Relaxation

3 NMR spectroscopy

3.1 Continuous wave (CW)
spectroscopy

3.2 Fourier
spectroscopy

3.3 Multi-dimensional NMR
Spectroscopy

3.4 Solid-state NMR
spectroscopy

3.5 Sensitivity

3.6 Isotopes

4 Applications

4.1 Medicine

4.2 Chemistry

4.3 Non-destructive
testing

4.4 Data acquisition in the
petroleum industry

4.5 Process
control

4.6 Earth's field
NMR

4.7 Magnetometers

5 Makers of NMR
equipment

6 See also

7 Notes

8 References

9 External links

[edit]
History

[edit]
Discovery

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 and its introduction into civilian
use.

[edit]
Theory of nuclear magnetic resonance

[edit]
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, μ,
via μ = γ I ,

where the proportionality constant, γ, 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]

[edit]
Values of spin angular momentum

The angular momentum
associated with nuclear spin is quantized. This means both
that the magnitude of angular momentum is quantized (i.e. I can only take
on a restricted range of values), and also that the 'orientation' of the
associated angular momentum is 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 2I+1
angular momentum states.

The z- component of the angular momentum vector, I is therefore:
Iz = mh(2π), where h is Planck's
constant.

The z-component of the magnetic moment is simply:

μz = γ Iz = m γ (h/2π)

[edit]
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 approximately 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 B0 (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:

E= - μ B0= -μz
B0,

where the magnetic field has the orientation along the z axis.

Hence: E = - m h(2π)γ B0.

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:

ΔE = h(2π) γ B0,

and this difference results in a small population bias toward the lower
energy state.

[edit]
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 ΔE
= hν = γ B0/(2π). These frequencies typically correspond to the radio frequency range of
the electromagnetic
spectrum for magnetic fields up to ~ 20T.

It is this (magnetic) resonant absorption that is detected in NMR.

[edit]
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 in the NMR
frequency due to the electron (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 moelcular 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 will
be measured).

Unless the local symmetry of such molecular
orbitals is very high (that is, in the `isotropic shift' case), the shielding
effect will depend on the orientation of the molecule with respect to the
external field, H0. In solid-state NMR
spectroscopy, magic angle
spinning is required to average out this orientation dependence in order to
obtain values close to the average chemical shifts. This is obviously
unnecessary in conventional NMR of molecules in solution since rapid molecular
tumbling averages out the chemical shift
anisotropy (CSA) to the `average
chemical shift' (ACS).

[edit]
Relaxation

For more details on this topic, see Relaxation
(NMR).

The process called population relaxation refers to nuclei that return to the
thermodynamic state in the magnet. This process is also called
T1 relaxation, where T1 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
T2 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, T1 is always larger
(slower) than T2. 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 T2 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 T2 time defines the width of the NMR signal.
Thus, a nucleus having a large T2 time gives rise to a sharp
signal, whereas nuclei with shorter T2 times give rise to more
broad signals. The length of T1 and T2 is
closely related to molecular motion.

[edit]
NMR spectroscopy

Main article: NMR
spectroscopy

NMR spectroscopy is one
of the principal techniques used to obtain physical, chemical, electronic and
structural information about molecules due to the chemical shift Zeeman
effect, and/or Knight shift effect on the resonant frequencies of the nuclei
present in the sample. 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. Thus, structural and dynamic
information is obtainable (with or without magic-angle spinning (MASS)) from NMR
studies of quadrupolar nuclei (that is, those nuclei with spin I > ½, such as
1, 3/2, 5/2, and so on) even in the presence of dipolar broadening which is
always much smaller than the quadrupolar interaction strength.

Additional structural and chemical information may be obtained by performing
double-quantum NMR experiments for quadrupolar nuclei such as 2H.

Also, nuclear magnetic resonance is one of the techniques that has been
used to build elementary quantum computers {reference
needed}.

[edit]
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, and have been, 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 random noise adds 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.

[edit]
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 short (millisecond to microsecond) radiofrequency (RF) pulses to excite
the entire NMR spectrum.

Applying such a pulse to a set of nuclear spins simultaneously excites all
the single-quantum 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
magnetization vector precesses about the
external magnetic field vector at the NMR frequency of the spins. This
oscillating magnetization vector 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 vector-sum of the NMR responses from all the
excited spins. In order to obtain the frequency-domain NMR spectrum (NMR absorption
intensity vs. NMR 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).

[edit]
Multi-dimensional NMR Spectroscopy

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-FT NMR was proposed by the Belgian
scientist Jean
Jeener, professor at the Free University
of Brussels, 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 biopolymers such as proteins or even
small nucleic acids.[3]

Kurt Wüthrich shared
(with John B. Fenn) in 2002 the Nobel Prize in
Chemistry for his work in protein
nuclear magnetic resonance spectroscopy.

[edit]
Solid-state NMR spectroscopy

Main article: Solid-state
nuclear magnetic resonance

This technique complements biopolymer X-ray
crystallography in that it is frequently applicable to biomolecules 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 little signal averaging by thermal motion in the solid state, where most
molecules can only undergo restricted vibrations and rotations at room
temperature, each in a slightly different electronic environment, therefore
exhibiting a different NMR absorption peak. Such a variation in the electronic
environment of the resonating nuclei results in a blurring of the observed
spectra--which is often only a broad Gaussian band for non-quadrupolar spins in
a solid- thus making the interpretation of such "dipolar and chemical shift
anisotropy (CSA)" broadened spectra either very difficult or impossible.
Raymond Andrew at Nottingham University in UK pioneered the development of
high-resolution solid-state
nuclear magnetic resonance. He was the first to report the introduction of
the 'magic-angle'
spinning (MAS) 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 MAS, the
sample is spun at several kilohertz around an axis that makes the so-called
'magic angle' (540.44') with the static magnetic field
H0, whereby and the spin interactions are averaged to their isotropic
values.

A concept developed by Sven Hartmann and Erwin Hahn was utilized in
transferring magnetization from protons to less sensitive nuclei (popularly
known as cross-polarization) by Gibby[who?],
Alex Pines and John S.
Waugh. Schaefer[who?]
and coworkers demonstrated the powerful use of cross-polarization under MAS
which is now routinely used to detect low-abundance and low-sensitivity
nuclei.

[edit]
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

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 radio energy. This manifests itself in both
CW and pulsed NMR, the first by using too much continuous power, and the
second by pulsing frequently without allowing time for the nuclei to return to
thermal equilibrium. For nuclei such as 29Si this is a serious
problem as the relaxation time is measured in seconds.

Non-magnetic effects, such as electric-quadrupole coupling of
spin-1 and spin-3/2 nuclei with their local environment, which broaden and
weaken absorption peaks. 14N, a very common spin-1
nucleus, is difficult to study for this reason. High resolution NMR instead
probes molecules using the rarer Nitrogen 15N
isotope, which has spin-½.

[edit]
Isotopes

Many chemical elements can be used for NMR analysis. [1]

1H, the most
commonly used, very useful. Highly abundant, the most sensitive nucleus apart
from tritium
which is not commonly used as it is unstable and radioactive. Narrow chemical
shift with sharp signals. In particular, the 1H signal is used in
magnetic resonance imaging.

2H, commonly
used in the form of deuterated
solvents to avoid interference of solvents in measurement of
1H. Also used in determining the behavior of lipids in lipid
membranes and other solids or liquid crystals as it is a relatively
non-perturbing label which can selectively replace 1H.

3He, very
sensitive. Low percentage in natural helium, has to be enriched. Used mainly
in studies of endohedral
fullerenes.

13C, commonly
used. There is a low percentage in natural carbon, therefore spectrum
acquisition on unlabelled takes a long time. Frequently used for labeling of
compounds in synthetic and metabolic studies. Has low sensitivity and wide
chemical shift, yields sharp signals. Low percentage makes it useful by
preventing spin-spin couplings and makes the spectrum appear less crowded.

15N,
relatively commonly used. Can be used for labeling compounds. Nucleus very
insensitive but yields sharp signals. Low percentage in natural nitrogen
together with low sensitivity requires high concentrations or expensive
isotope enrichment.

14N, medium
sensitivity nucleus with wide chemical shift. 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.

19F,
relatively commonly measured. Sensitive, yields sharp signals, has wide
chemical shift.

31P, 100% of
natural phosphorus. Medium sensitivity, wide chemical shift range, yields
sharp lines. Used in biochemical studies.

17O, low
sensitivity and very low natural abundance.

10B, lower
sensitivity than 11B. Quartz tubes must be used as borosilicate glass
interferes with measurement.

11B, more
sensitive than 10B, yields sharper signals. Quartz tubes must be
used as borosilicate glass
interferes with measurement.

35Cl and
37Cl, broad signal. 35Cl significantly more
sensitive, preferred over 37Cl 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.

43Ca, used in
biochemistry to study calcium binding to DNA, proteins, etc. Moderately
sensitive, very low natural abundance.

195Pt, used in
studies of catalysts and complexes.

Other nuclei, usually used in the studies of their complexes and chemical
binding, or to detect presence of the element: 6Li, 7Li,
9Be, 19F, 21Ne, 23Na,
25Mg, 27Al, 29Si, 31P,
33S, 39K, 40K, 41K,
45Sc, 47Ti, 49Ti, 50V,
51V, 53Cr, 55Mn, 57Fe,
59Co, 61Ni, 63Cu, 65Cu,
67Zn, 69Ga, 71Ga, 73Ge,
77Se, 81Br, 87Rb, 87Sr,
95Mo, 109Ag, 113Cd, 125Te,
127I, 133Cs, 135Ba, 137Ba,
139La, 183W, 199Hg.

[edit]
Applications

[edit]
Medicine

The use of nuclear magnetic resonance best known to the general public is magnetic
resonance imaging for medical diagnosis and MR
Microscopy in research settings, however, it is also widely used in chemical
studies, notably in NMR spectroscopy such
as proton
NMR, carbon-13 NMR, deuterium
NMR and phosphorus-31 NMR. Biochemical information can also be obtained from
living tissue (e.g human brain tumours) with the
technique known as in
vivo magnetic resonance spectroscopy or chemical
shift NMR Microscopy.

These studies are possible because nuclei are surrounded by orbiting
electrons, which are also spinning charged particles such as magnets 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
recognizable way.

[edit]
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. See the articles on carbon-13 NMR and proton NMR
for detailed discussions.

By studying T2* information a chemist can determine the
identity of a compound by comparing the observed nuclear precession frequencies
to known frequencies. Further structural data can be elucidated by observing
spin-spin coupling, a process by which the precession frequency of a
nucleus can be influenced by the magnetization transfer from nearby nuclei.
Spin-spin coupling is most commonly observed in NMR involving common isotopes,
such as Hydrogen-1 (HNMR).

T2 information can give information about dynamics and
molecular motion.

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 buckminsterfullerene.
This now famous form of carbon has 60 carbon atoms forming a sphere. The carbon
atoms are all in identical environments and so should see the same internal H
field. Unfortunately, buckminsterfullerene contains no hydrogen and so
13C nuclear magnetic resonance has to be used. 13C spectra
require longer acquisition times since carbon-13 is not the common isotope of
carbon (unlike hydrogen, where 1H is the common isotope). However, in
1990 the spectrum was obtained by R. Taylor and co-workers at the University of
Sussex and was found to contain a single peak, confirming the unusual
structure of C60.[4]

[edit]
Non-destructive testing

Nuclear magnetic resonance is extremely useful for analyzing samples
non-destructively. Radio waves and static magnetic fields easily penetrate many
types of matter and anything that is not inherently ferromagnetic. For
example, various expensive biological samples, such as nucleic
acids, including RNA and DNA, or proteins, can be studied using
nuclear magnetic resonance for weeks or months before using destructive
biochemical experiments. This also makes nuclear magnetic resonance a good
choice for analyzing dangerous samples.

[edit]
Data acquisition in the petroleum industry

Another use for nuclear magnetic resonance is data acquisition in the
petroleum industry
for petroleum and natural
gas exploration and recovery. A borehole is drilled into rock
and sedimentary strata into which nuclear magnetic resonance logging equipment
is lowered. Nuclear magnetic resonance analysis of these boreholes is used to
measure rock porosity, estimate permeability from pore size distribution and
identify pore fluids (water, oil and gas). These instruments are typically low
field NMR spectrometers.

[edit]
Process control

NMR has now entered the arena of real-time process control and process
optimization in oil refineries and petrochemical plants. Two
different types of NMR analysis are utilized to provide real time analysis of
feeds and products in order to control and optimize unit operations. Time-domain
NMR (TD-NMR) spectrometers operating at low field (2-20 MHz for 1H)
yield free induction
decay data that can be used to determine absolute hydrogen content values, rheological information, and
component composition. These spectrometers are used in mining, polymer production, cosmetics and
food manufacturing as
well as coal
analysis. High resolution FT-NMR spectrometers operating in the 60 MHz range
with shielded permanent magnet systems yield high resolution 1H NMR
spectra of refinery and petrochemical streams. The
variation observed in these spectra with changing physical and chemical
properties is modelled utilizing chemometrics to yield
predictions on unknown samples. The prediction results are provided to control systems via
analogue or digital outputs from the spectrometer.

[edit]
Earth's field NMR

In the Earth's magnetic
field, NMR frequencies are in the audio frequency range. EFNMR is typically stimulated by
applying a relatively strong dc magnetic field pulse to the sample and,
following the pulse, analysing 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[5].
Their inexpensive portable nature makes these instruments valuable for field use
and for teaching the principles of NMR and MRI.

[edit]
Magnetometers

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

[edit]
Makers of NMR equipment

Major NMR instrument makers include Oxford Instruments,
Bruker, SpinLock
SRL, General Electric, JEOL, Kimble
Chase, Philips, Siemens
AG, Varian, Inc. and SpinCore
Technologies, Inc.

[edit]
See also

Carbon-13 NMR

Chemical shift

Dynamic
Nuclear Polarisation (DNP)

Earth's field NMR
(EFNMR)

Electromagnetism

Electron
spin/paramagnetic resonance

Ferromagnetic
resonance

Free induction
decay (FID)

Gyromagnetic ratio

Hyperpolarization

In
vivo magnetic resonance spectroscopy (MRS)

J-coupling

Larmor equation (Not
to be confused with Larmor formula).

Larmor precession

Low field NMR

Magic angle
spinning

Magnetic
resonance imaging (MRI)

Magnetometer

NMR spectra
database

NMR spectroscopy

NMR Microscopy

Nuclear
quadrupole resonance (NQR)

Overhauser
magnetometer

Protein NMR

Proton NMR

Proton
precession magnetometer (PPM)

Proton
magnetometer

Rabi
cycle

Relaxometry

Relaxation (NMR)

Solid-state NMR

Stark effect

Zeeman effect

[edit]
Notes

^
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

^
I.C.
Baianu et al (1980). "Ferromagnetic Resonance and Spin Wave Excitations in
Metallic Glasses". J.
Phys. Chem. Solids 40 (2): 941-950.

^
I.C. Baianu.
"Two-dimensional Fourier transforms". 2D-FT
NMR and MRI. PlanetMath,. http://planetmath.org/encyclopedia/TwoDimensionalFourierTransforms.html. Retrieved on 2009-02-22.

^
Taylor, Roger; Hare, Jonathan P.; Abdul-Sada, Ala'a
K.; Kroto, Harold W (1990), "Isolation, separation and characterization of the
fullerenes C60 and C70: the third form of carbon", Journal of the Chemical
Society, Chemical Communications 20: 1423–5

^
http://en.wikipedia.org/wiki/Magnetic_resonance_imaging#Portable_instruments

[edit]
References

Martin, G.E; Zekter, A.S., ‘’Two-Dimensional NMR Methods for Establishing
Molecular Connectivity’’; VCH Publishers, Inc: New York, 1988 (p.59)

Akitt, J.W.; Mann, B.E., ‘’NMR and Chemistry’’; Stanley Thornes:
Cheltenham, UK, 2000. (p273)

Akitt, J.W.; Mann, B.E., ‘’NMR and Chemistry’’; Stanley Thornes:
Cheltenham, UK, 2000. (p287)

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.

Tyszka, J. M. & Fraser, S. E. & Jacobs, R. E. (2005). Magnetic
resonance microscopy: recent advances and applications. Current
Opinion in Biotechnology, 16(1):93-99.

Edwards, John C. NMR
Overview

[edit]
External links

NMR/MRI
tutorial

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 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").

Acronyms, Abbreviations and Initialisms - Nuclear Magnetic
Resonance

A free
interactive simulation of NMR principles

Bionmr.com - discussion of NMR
applications in biological systems

NMR Wiki Open NMR,EPR,MRI web
project

The International Society of
Magnetic Resonance

AUREMOL (Universitaet
Regensburg)

NMR processing software from ACD/Labs for 1D and 2D NMR spectra. DB interface available.

NMR Prediction software ACD/NMR Predictors

Automated elucidation of chemical structures ACD/Structure Elucidator

NMR simulation software QSim

Free software for simulation of spin coupled multiplets and DNMR spectra
WINDNMR-Pro

NMR processing software NMRPipe

BMRB BioMagResBank - A
repository for experimental data from NMR spectroscopy of proteins, peptides,
nucleic acids and small biomolecules.

RMN - An NMR data processing program for the Macintosh.

Applications of Time-Domain NMR in Process Control

Applications of
High Resolution FT-NMR in Real-Time Process Control

Introduction to Process NMR

NMR and Diesel Manufacturing

HWB-NMR Henry Wellcome Building
for Biomolecular NMR, Birmingham, UK.

CARA - Computer Aided Resonance
Assignment, freeware, developed at the group of Prof. Kurt Wüthrich

CCPN - The Collaborative
Computing Project for NMR, based at the University of Cambridge, UK

Vanderbilt Structural
Biology General site for structural biology at Vanderbilt University. NMR,
x-ray crystallography, and computational biology all included.

Janocchio
Conformation-dependent coupling and NOE prediction for small molecules.

Earth's field NMR (EFNMR)

Animation of NMR spin 1/2 precession

Article on
helium scarcity and potential effects on NMR and MRI communities

CERMAX - CEntro de Ressonancia
Magnetica Antonio Xavier

Retrieved from "http://en.wikipedia.org/wiki/Nuclear_magnetic_resonance"

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