Chemistry Reference
In-Depth Information
OCH
3
H
HO
H
H
CH
2
N
H
H
N
Figure 7.21
The structure of quinine.
which the most important are
1
H,
13
C,
15
N and
31
P, which have spin 1/2. The
nuclei can exist with spins of
1/2 which are normally randomly
oriented in space and have equal energy. However, if these nuclei are placed
in an external magnetic field they may align themselves either
with
the
direction of the external field (the low energy orientation) or
opposed
to the
direction of the external field (a higher energy situation). If the nucleus is in
the lower energy state (analogous to the 'ground state' in spectroscopy), and
is irradiated with energy of the correct frequency, the nucleus can flip to the
higher energy state. The energy required to bring about this effect is in the
radiofrequency part of the electromagnetic spectrum. The nucleus can
return to the lower energy state by losing energy to its surroundings - a
process known as
relaxation.
Critically, the frequency of energy required to
flip a nucleus from one state to another varies slightly depending on the
electronic environment of the atom in question. This frequency is what is
measured in a spectrophotometer and what gives rise to the NMR spec-
trum. It also means that NMR is exquisitely sensitive to tiny changes in the
electronic environment of an atom - invaluable for the elucidation of
unknown chemical structures.
1/2 or
Instrumentation
The apparatus required for obtaining an NMR spectrum comprises a
powerful magnet capable of producing a homogeneous magnetic field, a
radiofrequency oscillator and receiver and electronics capable of integrating
and displaying spectra. Modern instruments are interfaced with a computer
and the hard disc of the computer is used for data storage, a library of past
spectra, and so on. A schematic diagram of an NMR spectrophotometer is
shown in Figure 7.22.
Solvents for NMR
In NMR, the sample is usually analysed in solution (solid-state NMR does
exist, but this is a specialised technique). Since hydrogen is active in NMR,
