NMR spectroscopy measures the magnetic characteristics
of a specific atom, usually hydrogen, in a molecule. Thus, hydrogens
in different local environments of the sample molecule will give
different peaks according to the electronic character of the environment.
If the local environment of the hyrogen atom nucleus has a high
electron density then the peak appears "upfield," or towards the
right of the spectrum. If the local environment of a hydogen has
a low electron density (eg the hydrogen atom is attached to an electronegative
element) , the peak appears farther "downfield," or towards the
left of the spectrum.
The spectrum measures the ease with which the nuclei
of the hyrogen atoms can be changes from one nuclear spin state
to another (hydtrogen can have two 'spin' states) One of the spin
states may be thought of as being aligned with the magnetic field
and the other (higher spin) against the magnetic field.
When electromagnetic radiation is appied to the nuclei
when they are in the magnetic field they can be changed to the higher
spin state by absorbing radiation. This radiation can be measured
When the local electrons are high in density they 'sheild'
the nucleus from the effect of the magnetic field.
NMR peaks are also split, as shown in Figure d. The
splitting pattern is a function of the number of hydogens adjacent
to the peak hydrogens. For example, in ethanol (Figure a), the three
hydrogens of the CH3 group are adjacent to two hydrogens of the
CH2 group. The number of peaks when split is equal to the number
of adjacent peaks plus one. For the CH3 group of ethanol there are
two adjacent hydrogens, so we expect the CH3 peak to be split into
3 peaks (2 adjacent hydrogens + 1). This is exactly what is observed
in Figure d. In NMR nomenclature, splitting of a peak into one is
a singlet, two is a doublet, three is a triplet, four is a quartet,
and so on.
The first NMR spectrum of a fluid sample was taken of
ethanol at Stanford University in 1951. Even from this early spectrum,
the three hydrogen peaks can be resolved.
The splitting pattern described above is clearly shown
in this figure. The OH peak, not shown, would be a singlet, not
a triplet. This happens because the sample is contamintated by water.
The OH hydrogen exchanges with hydrogens from water, preventing
a peak from being recorded. As a result, the OH hydrogen does not
split the CH2 peak.
The modern day spectrum
This type of modern, full-scale NMR spectrum is easily
recognizable by any modern chemist. The line that rises above the
peaks of the spectrum is the integration, or the area under the
peaks. The integration of each peak is proportional to the number
of hydrogens that peak represents.
The second, smaller spectrum at top is a 13C spectrum.
The 13C spectrum uses a slightly different form of analysis to probe
the electronic environments of the carbons in a sample. Because
there are two carbons in ethanol, two main peaks are seen. The rest
of the peaks are the solvent (CDCl3) or impurities.
Carbon-13 also can have a more complicated spectrum
as its nucleus has more than two possible spin states.