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Nuclear magnetic = resonance

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"NMR" redirects here. For other uses, see NMR=20 (disambiguation).

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Pacific=20 Northwest National Laboratory's high magnetic field (800 MHz,=20 18.8 T) NMR = spectrometer being=20 loaded with a sample.

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900MHz, 21.2 T NMR Magnet at = HWB-NMR,=20 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=20 properties of an atom's nucleus in the = presence=20 of an external magnetic field applied to a molecular system and = crystalline or=20 non-crystalline materials.

NMR also commonly refers to a family of scientific methods and = techniques=20 that exploit the nuclear magnetic resonance resonance phenomenon to = study molecules, crystals = and=20 non-crystalline materials ("NMR spectroscopy" is perhaps the most = important, as=20 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=20 other words a spin > 0. = The most=20 commonly measured nuclei are ¹H (the = most=20 NMR-sensitive isotope after the = radioactive ³H = isotope, and=20 also after the stable ¹³C = nucleus,=20 although nuclei from isotopes of many other elements (e.g. ²H,=20 ¹⁰B,=20 ¹¹B, ¹⁴N, ¹⁵N,=20 ¹⁷O,=20 ¹⁹F, ²³Na, ²⁹Si, ³¹P,=20 ³⁵Cl, ¹¹³Cd,=20 ¹⁹⁵Pt) are readily = measured by=20 high-field NMR spectroscopy as well.

NMR resonant frequencies for a particular substance are directly = proportional=20 to the strength of the applied magnetic field, in accordance with the = equation=20 for the Larmor = precession=20 frequency. The scientific literature as of February 2009 includes NMR = spectra at=20 magnetic=20 fields in a wide range: from ~ 5 nT up to ~ 24 T. Very high magnetic = fields=20 are often preferred since 1D-NMR detection sensitivity increases = proportionally=20 with the magnetic field strength (the "Golden Rule of NMR"). Other = methods to=20 increase either the NMR signal strentgth or the detection sensitivity = include hyperpo= larization=20 and two-dimensional (2D) FT NMR techniques.

The principle of NMR usually involves two sequential steps: (1) the = alignment=20 or polarization of the magnetic nuclear spins being studied in an = applied,=20 constant magnetic field=20 H₀, and (2) the perturbation of this alignment of the = nuclear=20 spins (in the constant external magnetic field) by employing a second,=20 alternating magnetic field (rf) H_1rf, with the two = fields=20 being usually orthogonal for = maximum=20 detected NMR signal intensity. The resulting response by the total=20 magnetization, M =3D $\vec{M}$, of the nuclear spins to the = perturbing=20 magnetic field is the phenomenon that is exploited in NMR = spectroscopy and magnetic= =20 resonance imaging, which both use intense applied magnetic fields=20 H₀, in order to achieve high spectral = resolution,=20 the details of which are described by the chemical shift, = the Zeeman=20 effect, and Knight shifts (in = metals).

NMR phenomena are also utilized in low field NMR, = Earth's field = NMR=20 spectrometers, and several types of magnetometers.

=
1 History=20
- 1.1 Discovery=20
2 Theory of = nuclear magnetic=20 resonance=20
- 2.1 Nuclear = spin and=20 magnets=20
- 2.2 Values of = spin angular=20 momentum=20
  
  2.2.1 Spin = behavior in a=20 magnetic field=20
  2.2.2 Resonance=20
  2.2.3 Nuclear = shielding
- 2.3 Relaxation=20
3 NMR = spectroscopy=20
- 3.1 Continuous = wave (CW)=20 spectroscopy=20
- 3.2 Fourier=20 spectroscopy=20
- 3.3 Multi-dimensional NMR=20 Spectroscopy=20
- 3.4 Solid-state = NMR=20 spectroscopy=20
- 3.5 Sensitivity=20
- 3.6 Isotopes=20
4 Applications=20
- 4.1 Medicine=20
- 4.2 Chemistry=20
- 4.3 Non-destructive=20 testing=20
- 4.4 Data = acquisition in the=20 petroleum industry=20
- 4.5 Process=20 control=20
- 4.6 Earth's = field=20 NMR=20
- 4.7 Magnetometers
5 Makers of NMR=20 equipment=20
6 See = also=20
7 Notes=20
8 References=20
9 External = links=20

[edit]=20 History

[edit]=20 Discovery

Nuclear magnetic resonance was first described and measured in = molecular=20 beams by Isidor Rabi in = 1938.[1]=20 Eight years later, in 1946, Felix Bloch and Edward Mills=20 Purcell refined the technique for use on liquids and solids, for = which they=20 shared the Nobel Prize = in=20 physics in 1952.

Purcell had worked on the development and application of RADAR during World War=20 II at Massachusetts=20 Institute of Technology's Radiation=20 Laboratory. His work during that project on the production and = detection of=20 radiofrequency energy, and on the absorption of such energy by matter, = preceded=20 his discovery of NMR.

They noticed that magnetic nuclei, like ¹H and = ³¹P,=20 could absorb RF energy when = placed in=20 a magnetic field of a strength specific to the identity of the nuclei. = When this=20 absorption occurs, the nucleus is described as being in = resonance.=20 Different atoms within a molecule resonate at different = frequencies at a=20 given field strength. The observation of the resonance frequencies of a = molecule=20 allows a user to discover structural information about the molecule.

The development of nuclear magnetic resonance as a technique of analytical=20 chemistry and biochemistry = parallels the=20 development of electromagnetic technology and its introduction into = civilian=20 use.

[edit]=20 Theory of nuclear magnetic = resonance

[edit]=20 Nuclear spin and magnets

The elementary particles, neutrons and protons, composing an = atomic nucleus<= /A>, have=20 the intrinsic quantum mechanical property of spin. The = overall spin of=20 the nucleus is determined by the spin quantum = number=20 I. If the number of both the protons and neutrons in a given isotope are even then = I=20 =3D 0, i.e. there is no overall spin; just as electrons pair up in atomic=20 orbitals, so do even numbers of protons and neutrons (which are also = spin =C2=BD=20 particles and hence fermions) pair up = giving zero=20 overall spin. In other cases, however, the overall spin is non-zero. For = example=20 ²⁷Al has an overall spin I =3D 5/2.

A non-zero spin, I, is associated with a non-zero magnetic = moment, =CE=BC,=20 via =CE=BC =3D =CE=B3 I ,

where the proportionality constant, =CE=B3, is the gyromagnetic=20 ratio.

It is this magnetic moment that allows the observation of NMR = absorption=20 spectra caused by transitions between nuclear spin levels. Most = radioactive=20 nuclei (with some rare exceptions, such as tritium) that have both even = numbers=20 of protons and even numbers of neutrons, also have zero nuclear magnetic = moments-and also have zero magnetic dipole and quadrupole = moments;therefore,=20 such radioactive isotopes do not exhibit any NMR absorption spectra. = Thus,=20 ¹²C, ³²P and ³⁶Cl are examples of = radioactive=20 nuclear isotopes that have no NMR absorption, whereas ¹³C,=20 ³¹P, ³⁵Cl and ³⁷Cl are stable nuclear = isotopes=20 that do exhibit NMR absorption spectra.

Electron = spin=20 resonance is a related technique which detects transitions between = electron=20 spin levels instead of nuclear ones. The basic principles are similar; = however,=20 the instrumentation, data analysis and detailed theory are significantly = different. Moreover, there is a much smaller number of molecules and = materials=20 with unpaired electron spins that exhibit ESR (or EPR) absorption than = those=20 that have NMR absorption spectra. Significantly also, is the much = greater=20 sensitivity of ESR in comparison with NMR. Furthermore, ferromagnetic = materials=20 and thin films may exhibit highly resolved ferromagnetic resonance (FMR) = spectra, or spin wave excitations (SWR) beyond the single-quantum = transitions=20 common to most routine NMR and EPR studies. ^[2]

[edit]=20 Values of spin angular momentum

The angular = momentum=20 associated with nuclear spin is quantized. This = means both=20 that the magnitude of angular momentum is quantized (i.e. I can = only take=20 on a restricted range of values), and also that the 'orientation' of the = associated angular momentum is quantized. The associated quantum number = is known=20 as the magnetic = quantum=20 number, m, and can take values from +I to = =E2=80=93I in=20 integral steps. Hence for any given nucleus, there is a total of = 2I+1=20 angular momentum states.

The z- component of the angular momentum vector, I is = therefore:=20 I_z =3D mh(2=CF=80), where h is Planck's=20 constant.

The z-component of the magnetic moment is simply:

=CE=BC_z =3D =CE=B3 I_z =3D m =CE=B3 = (h/2=CF=80)

[edit]=20 Spin behavior in a magnetic field

Consider nuclei which have a spin of one-half, like ¹H,=20 ¹³C or ¹⁹F. The nucleus has two possible spin = states:=20 m =3D =C2=BD or m =3D -=C2=BD (also referred to as up and = down or =CE=B1 and =CE=B2,=20 respectively). The energies of these states are degenerate=E2=80=94that = is to say that=20 they are the same. Hence the populations of the two states (i.e. = number=20 of atoms in the two states) will be approximately equal at thermal=20 equilibrium.

If a nucleus is placed in a magnetic field, however, the interaction = between=20 the nuclear magnetic moment and the external magnetic field mean the two = states=20 no longer have the same energy. The energy of a magnetic = moment=20 =CE=BC when in a magnetic field B₀ (the zero = subscript is=20 used to distinguish this magnetic field = from any=20 other applied field) is given by the negative scalar product of the = vectors:

E=3D - =CE=BC B₀=3D = -=CE=BC_z=20 B₀,

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

Hence: E =3D - m h(2=CF=80)=CE=B3 = B₀.

As a result the different nuclear spin states have different = energies in=20 a non-zero magnetic field. In hand-waving terms, = we can=20 talk about the two spin states of a spin =C2=BD as being aligned = either with=20 or against the magnetic field. If =CE=B3 is positive (true for most = isotopes) then=20 m =3D =C2=BD is the lower energy state.

The energy difference between the two states is:

=CE=94E =3D h(2=CF=80) =CE=B3 B₀,

and this difference results in a small population bias toward the = lower=20 energy state.

[edit]=20 Resonance

Resonant absorption will occur when electroma= gnetic=20 radiation of the correct frequency to match this energy difference = is=20 applied. The energy of a photon is = E =3D=20 h=CE=BD, where =CE=BD is its frequency. Hence absorption = will occur when =CE=94E=20 =3D h=CE=BD =3D =CE=B3 B₀/(2=CF=80). These frequencies = typically correspond to the radio = frequency range of=20 the electromag= netic=20 spectrum for magnetic fields up to ~ 20T.

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

[edit]=20 Nuclear shielding

It might appear from the above that all nuclei of the same nuclide = (and hence=20 the same =CE=B3) would resonate at the same frequency. This is not the = case. The most=20 important perturbation of the NMR frequency for applications of NMR is = the=20 'shielding' effect of the surrounding electrons. In general, this = electronic=20 shielding reduces the magnetic field at the nucleus (which is = what=20 determines the NMR frequency). As a result the energy gap is reduced, = and the=20 frequency required to achieve resonance is also reduced. This shift in = the NMR=20 frequency due to the electron (molecular) orbital coupling to the = external=20 magnetic field is called chemical shift, = and it=20 explains why NMR is able to probe the chemical structure of molecules = which=20 depends on the electron density distribution in the corresponding = molecular=20 orbitals. If a nucleus in a specific chemical group is shielded to a = higher=20 degree by a higher electron density of its surrounding moelcular = orbital, then=20 its NMR frequency will be shifted upfield (that is, a lower chemical = shift),=20 whereas if it is less shielded by such surrounding electron density, = then its=20 NMR frequency will be shifted downfield (that is, a higher chemical = shift will=20 be measured).

Unless the local symmetry of such = molecular=20 orbitals is very high (that is, in the `isotropic shift' case), the = shielding=20 effect will depend on the orientation of the molecule with respect to = the=20 external field, H₀. In solid-state = NMR=20 spectroscopy, magic angle=20 spinning is required to average out this orientation dependence in = order to=20 obtain values close to the average chemical shifts. This is obviously=20 unnecessary in conventional NMR of molecules in solution since rapid = molecular=20 tumbling averages out the chemical = shift=20 anisotropy (CSA) to the `average=20 chemical shift' (ACS).

[edit]=20 Relaxation

For more details on this topic, = see Relaxation=20 (NMR).

The process called population relaxation refers to nuclei that return = to the=20 thermodynamic state in the magnet. This process is also called=20 T₁ relaxation, where T₁ refers to = the mean=20 time for an individual nucleus to return to its equilibrium state. Once = the=20 population is relaxed, it can be probed again, since it is in the = initial=20 state.

The precessing = nuclei can=20 also fall out of alignment with each other (returning the net = magnetization=20 vector to a nonprecessing field) and stop producing a signal. This is = called=20 T₂ relaxation. It is possible to be in this state and = not have=20 the population difference required to give a net magnetization vector at = its=20 thermodynamic state. Because of this, T₁ is always = larger=20 (slower) than T₂. This happens because some of the = spins were=20 flipped by the pulse and will remain so until they have undergone = population=20 relaxation. In practice, the T₂ time is the life time = of the=20 observed NMR signal, the free = induction=20 decay. In the NMR spectrum, meaning the Fourier = transform of=20 the free = induction=20 decay, the T₂ time defines the width of the NMR = signal.=20 Thus, a nucleus having a large T₂ time gives rise to a = sharp=20 signal, whereas nuclei with shorter T₂ times give rise = to more=20 broad signals. The length of T₁ and = T₂ is=20 closely related to molecular motion.

[edit]=20 NMR spectroscopy

Main article: NMR=20 spectroscopy

NMR = spectroscopy is one=20 of the principal techniques used to obtain physical, chemical, = electronic and=20 structural information about molecules due to the = chemical shift = Zeeman=20 effect, and/or Knight shift effect on the resonant frequencies of = the nuclei=20 present in the sample. It is a powerful technique that can provide = detailed=20 information on the topology, dynamics and three-dimensional structure of = molecules in solution and the solid state. Thus, structural and dynamic=20 information is obtainable (with or without magic-angle spinning (MASS)) = from NMR=20 studies of quadrupolar nuclei (that is, those nuclei with spin I > = =C2=BD, such as=20 1, 3/2, 5/2, and so on) even in the presence of dipolar broadening which = is=20 always much smaller than the quadrupolar interaction strength.

Additional structural and chemical information may be obtained by = performing=20 double-quantum NMR experiments for quadrupolar nuclei such as = ²H.

Also, nuclear magnetic resonance is one of the techniques that = has been=20 used to build elementary quantum = computers {reference=20 needed}.

[edit]=20 Continuous wave (CW) spectroscopy

In its first few decades, nuclear magnetic resonance spectrometers = used a=20 technique known as continuous-wave (CW) spectroscopy. Although NMR = spectra could=20 be, and have been, obtained using a fixed magnetic field and sweeping = the=20 frequency of the electromagnetic radiation, this more typically involved = using a=20 fixed frequency source and varying the current (and hence magnetic = field) in an=20 electromagnet to observe the resonant absorption signals. (This is the = origin of=20 the now anachronistic but still common "high" and "low" field = terminology for=20 low frequency and high frequency regions respectively of the NMR = spectrum.)

CW spectroscopy is inefficient in comparison to Fourier techniques = (see=20 below) as it probes the NMR response at individual frequencies in = succession. As=20 the NMR signal is intrinsically weak, the observed spectra suffer from a = poor signal-to-noi= se=20 ratio (S/N). This can be mitigated by signal averaging i.e. adding = the=20 spectra from repeated measurements. While the NMR signal is constant = between=20 scans and so adds linearly, the random noise adds more slowly =E2=80=94 = as the=20 square-root of the number of spectra (see Random walk). = Hence the=20 overall ratio of the signal to the noise increases as the square-root of = the=20 number of spectra measured.

[edit]=20 Fourier spectroscopy

Most applications of NMR involve full NMR spectra, that is, the = intensity=20 of the NMR signal as a function of frequency. Early attempts to acquire = the NMR=20 spectrum more efficiently than simple CW methods involved irradiating=20 simultaneously with more than one frequency. It was soon realised, = however, that=20 a simpler solution was to use short pulses of radio-frequency (centred = at the=20 middle of the NMR spectrum). In simple terms, a short square pulse of a = given=20 "carrier" frequency "contains" a range of frequencies centred about the = carrier=20 frequency, with the range of excitation (bandwidth) = being=20 inversely proportional to the pulse duration (the Fourier = transform of=20 an approximate square wave = contains=20 contributions from all the frequencies in the neighborhood of the = principal=20 frequency). The restricted range of the NMR frequencies made it = relatively easy=20 to use short (millisecond to microsecond) radiofrequency (RF) pulses to = excite=20 the entire NMR spectrum.

Applying such a pulse to a set of nuclear spins simultaneously = excites all=20 the single-quantum NMR transitions. In terms of the net magnetisation = vector,=20 this corresponds to tilting the magnetisation vector away from its = equilibrium=20 position (aligned along the external magnetic field). The = out-of-equilibrium=20 magnetization vector precesses = about the=20 external magnetic field vector at the NMR frequency of the spins. This=20 oscillating magnetization vector induces a=20 current in a nearby pickup coil, creating an electrical signal = oscillating at=20 the NMR frequency. This signal is known as the free = induction=20 decay (FID) and contains the vector-sum of the NMR responses from = all the=20 excited spins. In order to obtain the frequency-domain NMR spectrum (NMR = absorption=20 intensity vs. NMR frequency) this time-domain signal (intensity vs. = time) must=20 be Fourier = transformed.=20 Fortunately the development of FT-NMR coincided with the development of = digital=20 computers and Fast = Fourier=20 Transform algorithms.

Richard R. = Ernst was=20 one of the pioneers of pulse (FT) NMR and won a Nobel = Prize in=20 chemistry in 1991 for his work on FT-NMR and his development of=20 multi-dimensional NMR (see below).

[edit]=20 Multi-dimensional NMR Spectroscopy

The use of pulses of different shapes, frequencies and durations in=20 specifically-designed patterns or pulse sequences allows the=20 spectroscopist to extract many different types of information about the=20 molecule.

Multi-dimensional nuclear magnetic resonance spectroscopy is a kind = of FT-NMR=20 in which there are at least two pulses and, as the experiment is = repeated, the=20 pulse sequence is varied. In multidimensional nuclear magnetic = resonance=20 there will be a sequence of pulses and, at least, one variable time = period. In=20 three dimensions, two time sequences will be varied. In four dimensions, = three=20 will be varied.

There are many such experiments. In one, these time intervals = allow=E2=80=94among=20 other things=E2=80=94magnetization transfer between nuclei and, = therefore, the detection=20 of the kinds of nuclear-nuclear interactions that allowed for the = magnetization=20 transfer. Interactions that can be detected are usually classified into = two=20 kinds. There are through-bond interactions and = through-space=20 interactions, the latter usually being a consequence of the nuclear = Overhauser=20 effect. Experiments of the nuclear-Overhauser variety may establish=20 distances between atoms.

Although the fundamental concept of 2D-FT NMR was proposed by the = Belgian=20 scientist Jean=20 Jeener, professor at the Free = University=20 of Brussels, this idea was largely developed by Richard Ernst = who won=20 the 1991 Nobel = prize in=20 Chemistry for his work in FT and multi-dimensional NMR. = Multi-dimensional=20 NMR experiments were further developed into powerful methodologies for = studying=20 biomolecules in solution, in particular for the determination of the = structure=20 of biopolymers such as = proteins or even=20 small nucleic = acids.[3]

Kurt = W=C3=BCthrich shared=20 (with John B. Fenn) in 2002 the Nobel = Prize in=20 Chemistry for his work in protein=20 nuclear magnetic resonance spectroscopy.

[edit]=20 Solid-state NMR spectroscopy

Main article: Solid-state=20 nuclear magnetic resonance

This technique complements biopolymer X-ray=20 crystallography in that it is frequently applicable to biomolecules in a = liquid = or liquid=20 crystal phase, whereas crystallography, as the name implies, is = performed on=20 molecules in a solid phase. Though = nuclear=20 magnetic resonance is used to study solids, extensive atomic-level = biomolecular=20 structural detail is especially challenging to obtain in the solid = state. There=20 is little signal averaging by thermal motion in the solid state, where = most=20 molecules can only undergo restricted vibrations and rotations at room=20 temperature, each in a slightly different electronic environment, = therefore=20 exhibiting a different NMR absorption peak. Such a variation in the = electronic=20 environment of the resonating nuclei results in a blurring of the = observed=20 spectra--which is often only a broad Gaussian band for non-quadrupolar = spins in=20 a solid- thus making the interpretation of such "dipolar and chemical = shift=20 anisotropy (CSA)" broadened spectra either very difficult or = impossible.=20 Raymond Andrew at Nottingham University in UK pioneered the development = of=20 high-resolution solid-state=20 nuclear magnetic resonance. He was the first to report the = introduction of=20 the 'magic-angle'=20 spinning (MAS) technique that allowed him to achieve spectral = resolution in=20 solids sufficient to distinguish between chemical groups with either = different=20 chemical shifts = or=20 distinct Knight shifts. In = MAS, the=20 sample is spun at several kilohertz around an axis that makes the = so-called=20 'magic angle' (54⁰.44^') with the static magnetic = field=20 H₀, whereby and the spin interactions are averaged to their = isotropic=20 values.

A concept developed by Sven Hartmann and Erwin Hahn was = utilized in=20 transferring magnetization from protons to less sensitive nuclei = (popularly=20 known as cross-polarization) by Gibby^[who?],=20 Alex Pines and = John S.=20 Waugh. Schaefer^[who?]=20 and coworkers demonstrated the powerful use of cross-polarization under = MAS=20 which is now routinely used to detect low-abundance and low-sensitivity=20 nuclei.

[edit]=20 Sensitivity

Because the intensity of nuclear magnetic resonance signals and, = hence, the=20 sensitivity of the technique depends on the strength of the magnetic = field the=20 technique has also advanced over the decades with the development of = more=20 powerful magnets. Advances made in audio-visual technology have also = improved=20 the signal-generation and processing capabilities of newer machines.

The sensitivity of nuclear magnetic resonance signals is also = dependent=E2=80=94as=20 noted above=E2=80=94on the presence of a magnetically-susceptible = nuclide and,=20 therefore, either on the natural abundance of such nuclides or on the = ability of=20 the experimentalist to artificially enrich the molecules, under study, = with such=20 nuclides. The most abundant naturally-occurring isotopes of hydrogen and = phosphorus=E2=80=94for instance=E2=80=94are both magnetically = susceptible and readily useful for=20 nuclear magnetic resonance spectroscopy. In contrast, carbon and = nitrogen have=20 useful isotopes but which occur only in very low natural abundance.

Other limitations on sensitivity arise from the quantum-mechanical = nature of=20 the phenomenon. For quantum states separated by energy equivalent to = radio=20 frequencies, thermal energy from the environment causes the populations = of the=20 states to be close to equal. Since incoming radiation is equally likely = to cause=20 stimulated emission (a transition from the upper to the lower state) as=20 absorption, the NMR effect depends on an excess of nuclei in the lower = states.=20 Several factors can reduce sensitivity, including

Increasing temperature, which evens out the population of states.=20 Conversely, low temperature NMR can sometimes yield better results = than=20 room-temperature NMR, providing the sample remains liquid.=20
Saturation of the sample with radio energy. This manifests itself = in both=20 CW and pulsed NMR, the first by using too much continuous power, and = the=20 second by pulsing frequently without allowing time for the nuclei to = return to=20 thermal equilibrium. For nuclei such as ²⁹Si this is a = serious=20 problem as the relaxation time is measured in seconds.=20
Non-magnetic effects, such as electric-quadrupole = coupling of=20 spin-1 and spin-3/2 nuclei with their local environment, which broaden = and=20 weaken absorption peaks. ¹⁴N, a very common = spin-1=20 nucleus, is difficult to study for this reason. High resolution NMR = instead=20 probes molecules using the rarer Nitrogen = ¹⁵N=20 isotope, which has spin-=C2=BD.

[edit]=20 Isotopes

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

¹H, = the most=20 commonly used, very useful. Highly abundant, the most sensitive = nucleus apart=20 from tritium=20 which is not commonly used as it is unstable and radioactive. Narrow = chemical=20 shift with sharp signals. In particular, the ¹H signal is = used in=20 magnetic resonance imaging.=20
²H, = commonly=20 used in the form of deutera= ted=20 solvents to avoid interference of solvents in measurement of=20 ¹H. Also used in determining the behavior of lipids in = lipid=20 membranes and other solids or liquid crystals as it is a relatively=20 non-perturbing label which can selectively replace ¹H.=20
³He, = very=20 sensitive. Low percentage in natural helium, has to be enriched. Used = mainly=20 in studies of endohedral = fullerenes.=20
¹³C, = commonly=20 used. There is a low percentage in natural carbon, therefore spectrum=20 acquisition on unlabelled takes a long time. Frequently used for = labeling of=20 compounds in synthetic and metabolic studies. Has low sensitivity and = wide=20 chemical shift, yields sharp signals. Low percentage makes it useful = by=20 preventing spin-spin couplings and makes the spectrum appear less = crowded.=20
¹⁵N, = relatively commonly used. Can be used for labeling compounds. Nucleus = very=20 insensitive but yields sharp signals. Low percentage in natural = nitrogen=20 together with low sensitivity requires high concentrations or = expensive=20 isotope enrichment.=20
¹⁴N, = medium=20 sensitivity nucleus with wide chemical shift. Its large quadrupole moment = interferes in acquisition of high resolution spectra, limiting = usefulness to=20 smaller molecules and functional groups with a high degree of symmetry = such as=20 the headgroups of lipids.=20
¹⁹F, = relatively commonly measured. Sensitive, yields sharp signals, has = wide=20 chemical shift.=20
³¹P, = 100% of=20 natural phosphorus. Medium sensitivity, wide chemical shift range, = yields=20 sharp lines. Used in biochemical studies.=20
¹⁷O, = low=20 sensitivity and very low natural abundance.=20
¹⁰B, = lower=20 sensitivity than ¹¹B. Quartz tubes must be used as borosilicate = glass=20 interferes with measurement.=20
¹¹B, = more=20 sensitive than ¹⁰B, yields sharper signals. Quartz tubes = must be=20 used as borosilicate = glass=20 interferes with measurement.=20
³⁵Cl = and=20 ³⁷Cl, broad signal. ³⁵Cl significantly = more=20 sensitive, preferred over ³⁷Cl despite its slightly broader = signal.=20 Organic chlorides yield very broad signals, its use is limited to = inorganic=20 and ionic chlorides and very small organic molecules.=20
⁴³Ca, = used in=20 biochemistry to study calcium binding to DNA, proteins, etc. = Moderately=20 sensitive, very low natural abundance.=20
¹⁹⁵Pt, = used in=20 studies of catalysts and = complexes.=20
Other nuclei, usually used in the studies of their complexes and = chemical=20 binding, or to detect presence of the element: ⁶Li, = ⁷Li,=20 ⁹Be, ¹⁹F, ²¹Ne, ²³Na,=20 ²⁵Mg, ²⁷Al, ²⁹Si, ³¹P,=20 ³³S, ³⁹K, ⁴⁰K, ⁴¹K,=20 ⁴⁵Sc, ⁴⁷Ti, ⁴⁹Ti, ⁵⁰V,=20 ⁵¹V, ⁵³Cr, ⁵⁵Mn, ⁵⁷Fe,=20 ⁵⁹Co, ⁶¹Ni, ⁶³Cu, ⁶⁵Cu,=20 ⁶⁷Zn, ⁶⁹Ga, ⁷¹Ga, ⁷³Ge,=20 ⁷⁷Se, ⁸¹Br, ⁸⁷Rb, ⁸⁷Sr,=20 ⁹⁵Mo, ¹⁰⁹Ag, ¹¹³Cd, ¹²⁵Te, = ¹²⁷I, ¹³³Cs, ¹³⁵Ba, ¹³⁷Ba, = ¹³⁹La, ¹⁸³W, ¹⁹⁹Hg.

[edit]=20 Applications

[edit]=20 Medicine

The use of nuclear magnetic resonance best known to the general = public is magnetic= =20 resonance imaging for medical diagnosis and MR=20 Microscopy in research settings, however, it is also widely used in = chemical=20 studies, notably in NMR = spectroscopy such=20 as proton=20 NMR, carbon-13 NMR, = deuterium=20 NMR and phosphorus-31 NMR. Biochemical information can also be obtained = from=20 living tissue (e.g human brain tumours) = with the=20 technique known as in=20 vivo magnetic resonance spectroscopy or chemical=20 shift NMR Microscopy.

These studies are possible because nuclei are surrounded by orbiting=20 electrons, which are also spinning charged particles such as magnets and, so, will = partially=20 shield the nuclei. The amount of shielding depends on the exact local=20 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=20 nuclei can interact via a process known as spin-spin = coupling,=20 if they are on the same molecule, which will split the lines of the = spectra in a=20 recognizable way.

[edit]=20 Chemistry

By studying the peaks of nuclear magnetic resonance spectra, skilled = chemists=20 can determine the structure of many compounds. It can be a very = selective=20 technique, distinguishing among many atoms within a molecule or = collection of=20 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=20 for detailed discussions.

By studying T₂* information a chemist can determine = the=20 identity of a compound by comparing the observed nuclear precession = frequencies=20 to known frequencies. Further structural data can be elucidated by = observing=20 spin-spin coupling, a process by which the precession frequency = of a=20 nucleus can be influenced by the magnetization transfer from nearby = nuclei.=20 Spin-spin coupling is most commonly observed in NMR involving common = isotopes,=20 such as Hydrogen-1 (HNMR).

T₂ information can give information about dynamics = and=20 molecular motion.

Because the nuclear magnetic resonance timescale is rather = slow,=20 compared to other spectroscopic methods, changing the temperature of a=20 T₂* experiment can also give information about fast = reactions,=20 such as the Cope = rearrangement or=20 about structural dynamics, such as ring-flipping in cyclohexane. At = low enough=20 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=20 structure is that of buckminsterful= lerene.=20 This now famous form of carbon has 60 carbon atoms forming a sphere. The = carbon=20 atoms are all in identical environments and so should see the same = internal H=20 field. Unfortunately, buckminsterfullerene contains no hydrogen and so=20 ¹³C nuclear magnetic resonance has to be used. ¹³C = spectra=20 require longer acquisition times since carbon-13 is not the common = isotope of=20 carbon (unlike hydrogen, where ¹H is the common isotope). = However, in=20 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=20 structure of C₆₀.^[4]

[edit]=20 Non-destructive testing

Nuclear magnetic resonance is extremely useful for analyzing samples=20 non-destructively. Radio waves and static magnetic fields easily = penetrate many=20 types of matter and anything that is not inherently ferromagnetic. = For=20 example, various expensive biological samples, such as nucleic=20 acids, including RNA and DNA, or proteins, can be = studied using=20 nuclear magnetic resonance for weeks or months before using destructive=20 biochemical experiments. This also makes nuclear magnetic resonance a = good=20 choice for analyzing dangerous samples.

[edit]=20 Data acquisition in the petroleum = industry

Another use for nuclear magnetic resonance is data = acquisition in the=20 petroleum = industry=20 for petroleum and natural=20 gas exploration and recovery. A borehole is drilled = into rock=20 and sedimentary strata into which nuclear magnetic resonance logging = equipment=20 is lowered. Nuclear magnetic resonance analysis of these boreholes is = used to=20 measure rock porosity, estimate permeability from pore size distribution = and=20 identify pore fluids (water, oil and gas). These instruments are = typically low=20 field NMR spectrometers.

[edit]=20 Process control

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

[edit]=20 Earth's field NMR

In the Earth's = magnetic=20 field, NMR frequencies are in the audio frequency range. EFNMR is typically = stimulated by=20 applying a relatively strong dc magnetic field pulse to the sample and,=20 following the pulse, analysing the resulting low frequency alternating = magnetic=20 field that occurs in the earth's magnetic field due to free = induction=20 decay (FID). These effects are exploited in some types of magnetometers, = EFNMR=20 spectrometers, and MRI imagers^[5].=20 Their inexpensive portable nature makes these instruments valuable for = field use=20 and for teaching the principles of NMR and MRI.

[edit]=20 Magnetometers

Various magnetometers use NMR effects to measure magnetic fields, = including=20 proton=20 precession magnetometers (PPM) (also known as proton=20 magnetometers), and Overhauser=20 magnetometers. See also Earth's field=20 NMR.

[edit]=20 Makers of NMR equipment

Major NMR instrument makers include Oxford = Instruments,=20 Bruker, SpinLock=20 SRL, General = Electric, JEOL, Kimble=20 Chase, Philips, Siemens=20 AG, Varian, Inc. and = SpinCore=20 Technologies, Inc.

[edit]=20 See also

Carbon-13 NMR=20
Chemical = shift=20
Dynami= c=20 Nuclear Polarisation (DNP)=20
Earth's = field NMR=20 (EFNMR)=20
Electromagnetism=20
Ele= ctron=20 spin/paramagnetic resonance=20
Ferromagnet= ic=20 resonance=20
Free = induction=20 decay (FID)=20
Gyromagnetic = ratio=20
Hyperpo= larization=20
In=20 vivo magnetic resonance spectroscopy (MRS)=20
J-coupling=20
Larmor = equation (Not=20 to be confused with Larmor = formula).=20
Larmor = precession=20
Low field NMR=20
Magic angle = spinning=20
Magnetic= =20 resonance imaging (MRI)=20
Magnetometer=20
NMR spectra = database=20
NMR = spectroscopy=20
NMR = Microscopy=20
Nuclea= r=20 quadrupole resonance (NQR)=20
Overhauser=20 magnetometer=20
Protein NMR=20
Proton NMR=20
Proton=20 precession magnetometer (PPM)=20
Proton=20 magnetometer=20
Rabi=20 cycle=20
Relaxometry=20
Relaxation = (NMR)=20
Solid-state = NMR=20
Stark effect=20
Zeeman effect=20

[edit]=20 Notes

^=20 Rabi, I. I., Zacharias, J. R., Millman, S. and Kusch, P. (1938). A New = Method=20 of Measuring Nuclear Magnetic Moment. Physical Review 53, 318-318=20
^=20 I.C.=20 Baianu et al (1980). "Ferromagnetic Resonance and Spin Wave = Excitations in=20 Metallic Glasses". J.=20 Phys. Chem. Solids 40 (2): 941-950. =20
^=20 I.C. Baianu.=20 "Two-dimensional Fourier transforms". 2D-FT=20 NMR and MRI. PlanetMath,. http://planetmath.org/encyclopedia/TwoDimensionalFourierTr= ansforms.html. Retrieved on = 2009-02-22. =20
^=20 Taylor, Roger; Hare, Jonathan P.; = Abdul-Sada, Ala'a=20 K.; Kroto, Harold W (1990), "Isolation, separation and = characterization of the=20 fullerenes C60 and C70: the third form of carbon", Journal of the = Chemical=20 Society, Chemical Communications 20: = 1423=E2=80=935 =20
^=20 http://en.wikipedia.org/wiki/Magnetic_resonance_imaging#Po= rtable_instruments=20

[edit]=20 References

Martin, G.E; Zekter, A.S., =E2=80=98=E2=80=99Two-Dimensional NMR = Methods for Establishing=20 Molecular Connectivity=E2=80=99=E2=80=99; VCH Publishers, Inc: New = York, 1988 (p.59)=20
Akitt, J.W.; Mann, B.E., =E2=80=98=E2=80=99NMR and = Chemistry=E2=80=99=E2=80=99; Stanley Thornes:=20 Cheltenham, UK, 2000. (p273)=20
Akitt, J.W.; Mann, B.E., =E2=80=98=E2=80=99NMR and = Chemistry=E2=80=99=E2=80=99; Stanley Thornes:=20 Cheltenham, UK, 2000. (p287)=20
Hornak, Joseph P. The = Basics of=20 NMR=20
J. Keeler, Understanding NMR Spectroscopy=20
Wuthrich, Kurt NMR of Proteins and Nucleic Acids=20 Wiley-Interscience, New York, NY USA 1986.=20
Tyszka, J. M. & Fraser, S. E. & Jacobs, R. E. (2005). Magnetic=20 resonance microscopy: recent advances and applications. Cu= rrent=20 Opinion in Biotechnology, 16(1):93-99.=20
Edwards, John C. NMR=20 Overview

[edit]=20 External links

NMR/MRI=20 tutorial=20
Richard Ernst,=20 NL - Developer of Multdimensional NMR techniques Freeview video = provided=20 by the Vega Science Trust.=20
2D-FT NMR , MR--Imaging and related Nobel awards = Illustrated=20 applications of 2D-FT NMR and MR-Imaging.=20
'An = Interview=20 with Kurt Wuthrich' Freeview video by the Vega Science Trust = (W=C3=BCthrich was=20 awarded a Nobel Prize in Chemistry in 2002 "for his development of = nuclear=20 magnetic resonance spectroscopy for determining the three-dimensional=20 structure of biological macromolecules in solution").=20
Acronyms, Abbreviations and Initialisms - Nuclear = Magnetic=20 Resonance=20
A = free=20 interactive simulation of NMR principles=20
Bionmr.com - discussion = of NMR=20 applications in biological systems=20
NMR Wiki Open = NMR,EPR,MRI web=20 project=20
The International = Society of=20 Magnetic Resonance=20
AUREMOL (Universitaet=20 Regensburg)=20
NMR processing software from ACD/Labs for 1D and 2D NMR spectra. DB interface available.=20
NMR Prediction software ACD/NMR Predictors=20
Automated elucidation of chemical structures ACD/Structure Elucidator=20
NMR simulation software QSim=20
Free software for simulation of spin coupled multiplets and DNMR = spectra=20 WINDNMR-Pro=20
NMR processing software NMRPipe=20
BMRB = BioMagResBank - A=20 repository for experimental data from NMR spectroscopy of proteins, = peptides,=20 nucleic acids and small biomolecules.=20
RMN - An NMR data processing program for the = Macintosh.=20
Applications of Time-Domain NMR in Process Control=20
Applications of=20 High Resolution FT-NMR in Real-Time Process Control=20
Introduction to Process NMR=20
NMR and Diesel Manufacturing=20
HWB-NMR Henry = Wellcome Building=20 for Biomolecular NMR, Birmingham, UK.=20
CARA - Computer Aided = Resonance=20 Assignment, freeware, developed at the group of Prof. Kurt = W=C3=BCthrich=20
CCPN - The = Collaborative=20 Computing Project for NMR, based at the University of Cambridge, UK=20
Vanderbilt = Structural=20 Biology General site for structural biology at Vanderbilt = University. NMR,=20 x-ray crystallography, and computational biology all included.=20
Janocchio=20 Conformation-dependent coupling and NOE prediction for small = molecules.=20
Earth's field NMR (EFNMR)=20
Animation of NMR spin 1/2 precession=20
Article on=20 helium scarcity and potential effects on NMR and MRI communities=20
CERMAX - CEntro de = Ressonancia=20 Magnetica Antonio Xavier

Retrieved from "http://e= n.wikipedia.org/wiki/Nuclear_magnetic_resonance"

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