FREE ESSAY ON NUCLEAR MAGNETIC RESONANCE

NUCLEAR MAGNETIC RESONANCE

The measurement of nuclear and electron magnetic resonance on bulk materials was made
possible by Felix Bloch and Edward Purcell and in 1952 they shared the Nobel Prize in
Physics for their work. Until then, magnetic resonance was a measurable phenomena in
which atoms were shot through a magnet as a beam. This was the work of Rabi. Therefore,
the Nobel Prize quality in Bloch and Purcell's work was not in the theory of magnetic
resonance itself, but in the development of instruments which would measure this
phenomena in bulk material such as liquids and solids. These two laboratories were
uniquely suited for this work. Bloch was a great quantum mechanic and is credited with
much quality work in the quantum mechanics of solids. Purcell is an electricity and
magnetism physicist who worked on projects like radar during the war. His experience in
radiofrequency electronics aided in the development of his NMR instrument. Bloch's firm
grasp of quantum mechanics allowed him to look at the problem of magnetic resonance
classically, while Purcell's classical background forced him to look at NMR with a
quantum mechanic approach and develop a spectrometer much like the optical spectrometers.

There is a quantum mechanical property of electrons and some nuclei called spin. In the
absence of an externally applied magnetic field, this property is not observable.
However, even in optical experiments, this property can be observed when a magnetic field
is applied. It was the appearance of split lines in spectroscopic experiments that caused
the need for quantum mechanics to include this property. Purcell looked at the magnetic
resonance experiment with the eyes of an optical spectroscopist. He built a double beam
spectrometer. The two channels were identical except that in one channel an external
magnetic field was applied. His instrument scanned through the magnetic field and when
transitions between the spin energy levels were made, he would notice that the power in
the channel with the magnet would be less than that without. Thus he could precisely
measure the frequency and the difference in energy between the two energy levels. This
energy level picture is simple and the method of measuring correlates directly to that of
double beam optical methods with which we are all familiar. However, it does not
correspond well with the methods used to measure NMR today. It does correlate with
continuous wave spectrometers such as those used in EPR. However, it was a perfectly good
way to measure the phenomena and one which we would have been able to invent as well with
our knowledge of optical spectroscopy. 
The nuclei of some atoms, but not all, behave as if they were rotating, or spinning,
about an axis passed through them, much as a top spins about its central axis. This is
quite easily visualized in terms of the particle nature of the nucleus; it is less easily
visualized in terms of the wave picture, which by now we know is another face of nature
that is manifested in the atomic/molecular realm. Spin of nuclei is similar to electron
spin. Examples of atoms whose nuclei spin are hydrogen, fluorine, the 13 isotope of
carbon, and the 15 isotope of nitrogen.