N
uclear magnetic resonance (NMR) is a form of
spectroscopy that provides detailed
information about a molecule's structure.
It is based on the observation that some atomic
nuclei have nuclear spin. Hydrogen nuclei (protons),
written as 1H, are often used
although several other nuclei including 13C, 14N,
19F, 31P and 35Cl are also used.
NMR spectroscopy involves measuring the absorption
of electromagnetic radiation
during transitions between different spin states of
atoms and molecules. Because nuclei
have "spin," they also have an angular momentum
associated with them. Since nuclei
possess "spin" and charge, each has a magnetic moment
called a spin magnetic moment.
When placed in a strong external magnetic field, some
nuclei act as magnets, aligning
either with (parallel) or against (antiparallel) the
field. They spin at a rate characteristic
of the type of nucleus and proportional to the magnetic
field strength. The magnetic
moment will precess about the z axis at a certain
rate or frequency. If the frequency of
the electromagnetic radiation from the incident
photon has energy comparable to the
frequency of the precessing proton, the energy can
be absorbed and cause the magnetic
moment to "flip" with respect to the magnetic field.
Because this resonant frequency is in
the radio-wave region, it can be measured by applying
a radio frequency signal to a
sample and changing the frequency until absorbance is
noticed.
The low abundance 13C isotope is an important tool in
NMR since all organic compounds
contain this element. Compared to 1H NMR, 13C NMR
spectra are more difficult to
measure. This is due not only to the low natural
abundance of the nuclei, but also to the
weak magnetic resonance they produce. The behavior
of the 13C nucleus in the presence
of an external magnetic field is similar to that of 1H.
Click here for an overview of
MRI, a clinical use of NMR spectroscopy.
The following links contain more information on NMR spectroscopy:
http://www.nmss.org/cmsi/cmsi47.html
- gives an overview of current research and applications of NMR
spectroscopy
http://www-wilson.ucsd.edu/education/spectroscopy/nmr.html
- provides information on the fundamentals of NMR spectroscopy.
The technique is explained thoroughly through the use of diagrams and
movies as well as text.
Group 13 - November 25, 1997