Nuclear magnetic resonance spectroscopy, most
commonly known as NMR spectroscopy, is a research
technique that exploits the magnetic properties
of certain atomic nuclei.
It determines the physical and chemical properties
of atoms or the molecules in which they are
contained.
It relies on the phenomenon of nuclear magnetic
resonance and can provide detailed information
about the structure, dynamics, reaction state,
and chemical environment of molecules.
The intramolecular magnetic field around an
atom in a molecule changes the resonance frequency,
thus giving access to details of the electronic
structure of a molecule.
Most frequently, NMR spectroscopy is used
by chemists and biochemists to investigate
the properties of organic molecules, although
it is applicable to any kind of sample that
contains nuclei possessing spin.
Suitable samples range from small compounds
analyzed with 1-dimensional proton or carbon-13
NMR spectroscopy to large proteins or nucleic
acids using 3 or 4-dimensional techniques.
The impact of NMR spectroscopy on the sciences
has been substantial because of the range
of information and the diversity of samples,
including solutions and solids.
NMR spectra are unique, well-resolved, analytically
tractable and often highly predictable for
small molecules.
Thus, in organic chemistry practice, NMR analysis
is used to confirm the identity of a substance.
Different functional groups are obviously
distinguishable, and identical functional
groups with differing neighboring substituents
still give distinguishable signals.
NMR has largely replaced traditional wet chemistry
tests such as color reagents for identification.
A disadvantage is that a relatively large
amount, 2�50 mg, of a purified substance
is required, although it may be recovered.
Preferably, the sample should be dissolved
in a solvent, because NMR analysis of solids
requires a dedicated MAS machine and may not
give equally well-resolved spectra.
The timescale of NMR is relatively long, and
thus it is not suitable for observing fast
phenomena, producing only an averaged spectrum.
Although large amounts of impurities do show
on an NMR spectrum, better methods exist for
detecting impurities, as NMR is inherently
not very sensitive.
NMR spectrometers are relatively expensive;
universities usually have them, but they are
less common in private companies.
Modern NMR spectrometers have a very strong,
large and expensive liquid helium-cooled superconducting
magnet, because resolution directly depends
on magnetic field strength.
Less expensive machines using permanent magnets
and lower resolution are also available, which
still give sufficient performance for certain
application such as reaction monitoring and
quick checking of samples.
There are even benchtop NMR spectrometers.
Over the past fifty years nuclear magnetic
resonance spectroscopy, commonly referred
to as nmr, has become the preeminent technique
for determining the structure of organic compounds.
Of all the spectroscopic methods, it is the
only one for which a complete analysis and
interpretation of the entire spectrum is normally
expected.
Although larger amounts of sample are needed
than for mass spectroscopy, nmr 
is non-destructive, and 
with modern instruments good data may be obtained
from samples weighing less than a milligram.
To be successful in using nmr as an analytical
tool, it is necessary to understand the physical
principles on which the methods are based.
This important and well-established application
of nuclear magnetic resonance will serve to
illustrate some of the novel aspects of this
method.
To begin with, the nmr spectrometer must be
tuned to a specific nucleus, in this case
the proton.
The actual procedure for obtaining the spectrum
varies, but the simplest is referred to as
the continuous wave (CW) method.
A typical CW-spectrometer is shown in the
following diagram.
A solution of the sample 
in a uniform 5 mm glass tube is oriented between
the poles of a powerful magnet, and is spun
to average any 
magnetic field variations, as well as tube
imperfections.
Radio frequency radiation of appropriate energy
is broadcast into the sample from an antenna
coil (colored red).
A receiver coil surrounds the sample tube,
and emission of absorbed rf energy is monitored
by dedicated electronic devices and a computer.
An nmr spectrum is acquired by varying or
sweeping the magnetic field over a small range
while observing the rf signal from the sample.
An equally effective technique is to vary
the frequency of the rf radiation while holding
the external field constant.
Unlike infrared and uv-visible spectroscopy,
where absorption peaks are uniquely located
by a frequency or wavelength, the location
of different nmr resonance signals is dependent
on both the external 
magnetic field strength and the rf frequency.
Since no two magnets will have exactly the
same field, resonance frequencies will vary
accordingly and an alternative method for
characterizing and specifying the location
of nmr signals is needed.
This problem is illustrated by the eleven
different compounds shown in the following
diagram.
Although 
the eleven resonance signals are distinct
and 
well separated, an unambiguous numerical locator
cannot be directly assigned to each.
