 |
Chapter 13: Spectroscopy |
 |
Nuclear Magnetic Resonance (NMR) Spectroscopy
Basics:
Nuclei with an odd mass or odd atomic number have "nuclear spin" (in
a similar fashion to the spin of electrons). This includes 1H
and 13C (but not 12C). The spins of nuclei
are sufficiently different that NMR experiments can be sensitive for only
one particular isotope of one particular element. The NMR behaviour
of 1H and 13C nuclei has been exploited by organic
chemist since they provide valuable information that can be used to deduce
the structure of organic compounds. These will be the focus of our attention.
Since a nucleus is a charged particle in motion, it will develop a magnetic
field. 1H and 13C have nuclear spins
of 1/2 and so they behave in a similar fashion to a simple, tiny bar magnet.
In the absence of a magnetic field, these are randomly oriented but when
a field is applied they line up parallel to the applied field, either spin
aligned or spin opposed. The more highly populated state is
the lower energy spin state spin aligned situation. Two schematic
representations of these arrangements are shown below:
 |
In NMR, EM radiation is used to "flip" the alignment of nuclear spins
from the low energy spin aligned state to the higher energy spin opposed
state. The energy required for this transition depends on the strength
of the applied magnetic field (see below) but in is small and corresponds
to the radio frequency range of the EM spectrum. |
| As this diagram shows, the energy required for the spin-flip depends
on the magnetic field strength at the nucleus. With no applied field, there
is no energy difference between the spin states, but as the field increases
so does the separation of energies of the spin states and therefore so
does the frequency required to cause the spin-flip, referred to as resonance. |
 |
 |
The basic arrangement of an NMR spectrometer is shown to the left.
The sample is positioned in the magnetic field and excited via pulsations
in the radio frequency input circuit. The realigned magnetic fields induce
a radio signal in the output circuit which is used to generate the output
signal. Fourier analysis of the complex output produces the actual
spectrum.The pulse is repeated as many times as necessary to allow the
signals to be identified from the background noise. |
