2. Proton NMR Spectroscopy
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.
As an example, consider a sample of water in a 2.3487 T external magnetic
field, irradiated by 100 MHz radiation. If the magnetic field is smoothly
increased to 2.3488 T, the hydrogen nuclei of the water molecules will at some
point absorb rf energy and a resonance signal will appear. An animation showing
this may be activated by clicking the Show Field Sweep button. The field
sweep will be repeated three times, and the resulting resonance trace is colored
red. For visibility, the water proton signal displayed in the animation is much
broader than it would be in an actual experiment.
Since protons all have the same magnetic moment, we might expect all hydrogen
atoms to give resonance signals at the same field / frequency values.
Fortunately for chemistry applications, this is not true. By clicking the
Show Different Protons button under the diagram, a number of representative
proton signals will be displayed over the same magnetic field range. It is not
possible, of course, to examine isolated protons in the spectrometer described
above; but from independent measurement and calculation it has been determined
that a naked proton would resonate at a lower field strength than the nuclei of
covalently bonded hydrogens. With the exception of water, chloroform and
sulfuric acid, which are examined as liquids, all the other compounds are
measured as gases.
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