Environmental Engineering Reference
In-Depth Information
the radiofrequency region of the electromagnetic
spectrum. NMR absorbs photon energy equal to the
difference between these levels, causing a transition
from a lower to a higher energy state. The resonance
frequency depends only upon the applied magnetic
fi eld and the nature of the nucleus. NMR allows the
identifi cation of different elements in a sample
because the resonance frequency differs for different
nuclei (Abraham & Loftus 1985; Wilson 1987). The
main application of NMR is as a technique for chem-
ical analysis and structure determination known as
NMR spectroscopy.
In an NMR experiment, nuclei are excited by a
radio-frequency pulse and the excited nuclei undergo
a relaxation process, whereupon they return to their
ground state. While the excited nuclei relax back to
equilibrium, the emitted energy is recorded as a peak
after Fourier transformation. This excitation-relax-
ation cycle is repeated until a clear spectrum is
obtained. There are two principal types of relaxation
processes: spin-lattice relaxation and spin-spin
relaxation (Pavia
et al.
2001; McDowell
et al.
2006).
Spin-lattice, or longitudinal, relaxation processes
occur in the direction of the fi eld. The spins transfer
their energy to their surroundings - the lattice - as
thermal energy. The rate of this process is related to
the spin-lattice relaxation time,
T
1
. Intramolecular
and intermolecular processes contribute to spin-lat-
tice relaxation, but the principal contributor is
dipole-dipole interaction
, where the excited nuclei
relax by exchanging energy with other magnetic
nuclei that are in the same molecule or in nearby
molecules. For carbon nuclei, this process is espe-
cially successful if there are hydrogen atoms nearby.
The relaxation of the excited carbon nuclei is fastest
if hydrogen atoms are directly bonded, as in CH,
CH
2
, and CH
3
groups. Spin-spin, or transverse,
relaxation processes occur only between nuclei of the
same type, in a plane perpendicular to the direction
of the fi eld. The rate of this process is related to the
spin-spin relaxation time,
T
2
. Spin-spin relaxation is
often described as an entropy process; it does not
change the energy of the spin system (Pavia
et al.
2001).
In solutions, spin-lattice processes dominate,
whereas spin-spin relaxation is negligible (Pavia
et al.
2001; McDowell
et al.
2006).
3.4.2 The chemical shift
The nucleus is sensitive to the effects of small mag-
netic fi elds in its local molecular environment. The
magnetic fi eld generated by circulating neighbouring
electrons affects the nucleus and may either oppose
or enhance the much larger external fi eld
B
o
. When
this local molecular magnetic fi eld opposes
B
o
, reduc-
ing its magnitude, the nucleus is shielded from the
full effect of
B
o
. A shielded nucleus feels a lower
effective fi eld strength and resonates at a lower fre-
quency (Fig. 3.8). The opposite phenomenon, called
deshielding, occurs, for example in the benzene mol-
ecule, where the moving electrons in the
orbitals
give rise to a magnetic fi eld in the hydrogen nuclei
which reinforces the
B
o
fi eld (McGregor 1997; Skoog
et al.
1998).
In a molecule, the electron density around each
nucleus changes for different types of nuclei and
bonds. The opposing fi eld and therefore the effective
π
Spin = -½ ,
β
I
= ½
h
g
B
o
/2
π
=
Δ
E
Fig. 3.8
Spin energy level diagram for a
nucleus with spin = ½ which is brought
into a magnetic fi eld
B
o
, showing the
shielded (dashed line) and deshielded
(dotted line) cases.
Spin = +½,
α
