Nuclear
magnetic resonance spectroscopy (NMR) has become the main technique for
determining the structure of organic molecules. NMR uses larger quantities,
roughly 1 milligram, of a substance than mass spectroscopy and is non-destructive.
A
spinning charge generates a magnetic field. Protons, neutrons and electrons
spin on their axis in either an up or down configuration. This spinning
of charge creates a very small magnetic field.
As shown on the right, the
proton in a hydrogen atom spins on its axis.
Where the
nucleus contains more than one nucleon their spins are paired so that they
cancel out.
However, in
a nucleus with an odd number of nucleons the nucleus has a permanent overall
spin. This is why atoms, such as the 1H and the 13C
are used in nmr.
Subatomic
particles such as electrons, protons and neutrons are thought of as spinning
on their axes. In many atoms, such as 12C, these spins are paired
against each other so that they cancel each other out and so the nucleus
of the atom has no overall spin. In some atoms, with an odd number of nucleons,
such as 1H and 13C the nucleus does have an overall
spin. How do we determine if a nucleus will have an overall spin? Follow
the rules below.
1. If the number of neutrons and the number of protons are both even, then
the nucleus has no overall spin.
2. If the atomic mass is an odd number, such as 15N, then the nucleus has
a half-integer spin (i.e. 1/2, 3/2, 5/2)
3. If the atomic mass is an even number and the number of protons is an odd number, as
in 10B, then the nucleus has an integer spin (i.e. 1, 2, 3).
View the video on the right.
NMR is used as a diagnostic tool. It can produce a three dimensional view of the internal body. This can be extremely useful for doctors diagnosing cancer or faulty blood vessels.
