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SOPHIA OF WISDOM III - NUCLEAR MAGNETIC RESONANCE
LIBRARY OF SOPHIA OF WISDOM III
THE SOPHIA OF ALL SOPHIA OF WISDOMS
AKA
CAROLINE
E. KENNEDY________________________
NOVEMBER 17, 2006
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Nuclear magnetic resonance
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Pacific Northwest
National Laboratory's high magnetic field (800 MHz, 18.8 T) NMR spectrometer being loaded with a sample.
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Pacific
Northwest National Laboratory's high magnetic field (800 MHz, 18.8 T) NMR spectrometer being loaded with a sample.
Nuclear
magnetic resonance (NMR) is a physical phenomenon based upon the magnetic properties of an atom's nucleus. All nuclei that
contain odd numbers of nucleons and some that contain even numbers of nucleons have an intrinsic magnetic moment. The most
commonly used nuclei are hydrogen-1 and carbon-13, although certain isotopes of many other elements nuclei can also be observed.
NMR studies a magnetic nucleus, like that of a hydrogen atom (protium being the most receptive isotope at natural abundance)
by aligning it with a very powerful external magnetic field and perturbing this alignment using an electromagnetic field.
The response to the field by perturbing is what is exploited in nuclear magnetic resonance spectroscopy and magnetic resonance
imaging.
NMR spectroscopy is one of the principal techniques used to obtain physical, chemical, electronic and structural
information about a molecule. It is the only technique that can provide detailed information on the exact three-dimensional
structure of biological molecules in solution. Also, nuclear magnetic resonance is one of the techniques that has been used
to build elementary quantum computers.
Contents
[hide]
* 1 History
o 1.1 Discovery
o 1.2 Continuous
wave (CW) spectroscopy
o 1.3 Fourier spectroscopy
o 1.4 Multi-dimensional
o 1.5 Solids
o 1.6 Sensitivity
*
2 Uses of nuclear magnetic resonance
* 3 Theory of nuclear magnetic resonance
o 3.1 Nuclear spin and magnets
+
3.1.1 Values of spin angular momentum
+ 3.1.2 Spin behaviour in a magnetic field
+ 3.1.3 Resonance
+ 3.1.4 Nuclear
shielding
o 3.2 Relaxation
* 4 See also
* 5 References
* 6 External links
History
Discovery
Nuclear magnetic resonance was first described independently by Felix Bloch and Edward Mills Purcell, in 1946, both
of whom shared the Nobel Prize in physics in 1952 for their discovery.
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 and probably contributed to his understanding of it and related phenomena.
They noticed that magnetic nuclei,
like 1H and 31P, could absorb RF energy when placed in a magnetic field of a specific strength. When this absorption occurs
the nucleus is described as being in resonance. Interestingly, for analytical scientists, different atoms within a molecule
resonate at different frequencies at a given field strength. The observation of the resonance frequencies of a molecule allows
a user to discover structural information about the molecule.
The development of nuclear magnetic resonance as a technique
of analytical chemistry and biochemistry parallels the development of electromagnetic technology and its introduction into
civilian use. Major NMR makers include Bruker, Varian and JEOL.
Continuous wave (CW) spectroscopy
Throughout
its first few decades, nuclear magnetic resonance practice utilized a technique known as continuous-wave (CW) spectroscopy
in which either the magnetic field was kept constant and the oscillating field was swept in frequency to chart the on-resonance
portions of the spectrum or, more frequently, the oscillating field was held at a fixed frequency and the magnetic field was
swept through the transitions.
CW spectroscopy technique is limited in that it probes each frequency individually,
in succession, which has unfortunate consequences due to the insensitivity of nuclear magnetic resonance�that is to say,
nuclear magnetic resonance suffers from poor signal-to-noise ratio. Fortunately for nuclear magnetic resonance in general,
signal-to-noise ratio (S/N) can be improved by signal averaging. Signal averaging is an algorithm where the signals from many
successive runs of an experiment are added together. The noise, which is random in character, tends to cancel itself out while
the actual signal is constant and additive. Signal averaging increases S/N by the square-root of the number of signals taken.
This is a general principle and not unique to NMR.
Fourier spectroscopy
The technique known as Fourier transform
nuclear magnetic resonance spectroscopy (FT-NMR) decreases the time required for a scan by allowing a range of frequencies
to be probed at once. This technique has been made more practical with two technologies: the knowledge of how to create an
array of frequencies at once and computers capable of performing the computationally-intensive mathematical transformation
of the data from the time domain to the frequency domain to produce a spectrum.
Pioneered by Richard R. Ernst, who
won a Nobel Prize in chemistry in 1991, FT-NMR works by irradiating the sample, held in a static external magnetic field,
with a short square pulse of radiofrequency energy containing all the frequencies in the range of interest because the fourier
decomposition of an approximate square wave contains contributions from all the frequencies in the neighborhood of the principle
frequency.
The polarized nuclear magnets of the nuclei begin to spin together, creating a radio frequency signal that
is observable. However, they ultimately lose alignment and simultaniously decay to the equilibrium state in the magnet of
having a net polarization vector that aligns with the field. This decay is known as the free induction decay (FID). This time-dependent
pattern can be converted into a frequency-dependent pattern of nuclear resonances using a mathematical function known as a
Fourier transformation, revealing the nuclear magnetic resonance spectrum.
Multi-dimensional
The use of pulses
of different shapes, frequencies and durations in specifically-designed patterns or pulse sequences allows the spectroscopist
to extract many different types of information about the molecule.
Multi-dimensional nuclear magnetic resonance spectroscopy
is a kind of FT-NMR in which there are at least two pulses and, as the experiment is repeated, the pulse sequence is varied.
In multidimensional nuclear magnetic resonance there will be a sequence of pulses and, at least, one variable time period.
In three dimensions, two time sequences will be varied. In four dimensions, three will be varied.
There are many such
experiments. In one, these time intervals allow�among other things�magnetization transfer between nuclei and, therefore,
the detection of the kinds of nuclear-nuclear interactions that allowed for the magnetization transfer. Interactions that
can be detected are usually classified into two kinds. There are through-bond interactions and through-space interactions.
The latter usually being a consequence of the nuclear Overhauser effect. Experiments of the nuclear-Overhauser variety may
establish distances between atoms.
Richard Ernst and Kurt W�thrich�in addition to many others�developed 2-dimensional
and multidimensional FT-NMR into a powerful technique for studying biochemistry, in particular for the determination of the
structure of biopolymers such as proteins or even small nucleic acids.
This is used in protein nuclear magnetic resonance
spectroscopy. W�thrich shared the 2002 Nobel Prize in Chemistry for this work.
Solids
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 no signal averaging by thermal motion in the solid state, where molecules are held still, each in a
slightly different electronic environment, giving a different signal. This variation in electronic environment lowers resolution
greatly and makes interpretation more difficult. Raymond Andrew was a pioneer in the development of high-resolution solid-state
nuclear magnetic resonance. He introduced the magic angle spinning (MAS) technique and allowed for an increase in resolution
by several orders of magnitude. In MAS, the sample is averaged by spinning it at several kilohertz.
Alex Pines together
with John Waugh revolutionized the area with the introduction of the cross-polarization technique in order to enhance low
abundance and sensitivity nuclei.
Sensitivity
Because the intensity of nuclear magnetic resonance signals
and, hence, the sensitivity of the technique depends on the strength of the magnetic field the technique has also advanced
over the decades with the development of more powerful magnets. Advances made in audio-visual technology have also improved
the signal-generation and processing capabilities of newer machines.
The sensitivity of nuclear magnetic resonance
signals is also dependent�as noted above�on the presence of a magnetically-susceptible nuclide and, therefore, either
on the natural abundance of such nuclides or on the ability of the experimentalist to artificially enrich the molecules, under
study, with such nuclides. The most abundant naturally-occurring isotopes of hydrogen and phosphorus�for instance�are
both magnetically susceptible and readily useful for nuclear magnetic resonance spectroscopy. In contrast, carbon and nitrogen
have useful isotopes but which occur only in very low natural abundance.
Uses of nuclear magnetic resonance
The most obvious use of nuclear magnetic resonance
is in magnetic resonance imaging for medical diagnosis, however, it is also widely used in chemical studies, notably in NMR
spectroscopy such as proton NMR and carbon-13 NMR.
These studies are possible because nuclei are surrounded by orbiting
electrons, which are also spinning charged particles such as 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 recognisable way.
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.
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.
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.
A relatively recent 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, and
is a more difficult form of nuclear magnetic resonance to do. However in 1985 the spectrum was obtained by R. Curl and R.
Smalley of Rice University and sure enough it did contain just the one single spike, confirming the unusual structure of C60.
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.
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).
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.
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