NMR Introduction
What is Nuclear Magnetic Resonance Spectroscopy?
NMR is a spectroscopic technique which uses electromagnetic radiation
and magnetic fields to determine the structure of organic compounds.
Radio-frequency radiation is used to stimulate nuclei present within
the molecule and from the information we obtain from doing this we can
very accurately determine where the carbon atoms are located and where
hydrogen atoms are located.
The effect was first noticed in 1902 by P. Zeeman, a physicist, who
won a Nobel Prize for noticing that nuclei of certain atoms behave strangely
in a magnetic field. Fifty years later F. Bloch and E. Purcell, both physicists
put this idea to good use by constructing the first NMR spectrometer. They
too received a Nobel Prize for this work.
The principle of NMR is based upon the spin of atomic
nuclei in an external magnetic field. Not all nuclei possess this ability.
The main two which are used are the proton (1H) and an isotope
of carbon (13C). Both these nuclei precess normally, essentially
spinning in random directions (see diagram).
When these nuclei
are placed in a strong magnetic field they line up, precessing in the direction
of the applied field, very much like a compass needle lines up when in
a magnetic field. There are two possible orientations which the nuclei
can adopt; either lined up with the field (parallel), or against the field
(antiparallel). The two orientations are not exactly the same energy. The
parallel alignment is slightly lower in energy than the antiparallel one.
When the nuclei are irradiated with RF radiation the lower energy nuclei
spin-flip to the higher state. When this occurs the nuclei are said
to be in resonance, hence the name NMR.