Environmental Engineering Reference
In-Depth Information
(a)
(b)
(c)
(d)
Fig. 3.6
Transmission electron micrographs of clay in soil samples, prepared with 70-nm thickness are shown at different
magnifi cations: (a)
×
3300, (b)
×
10500; (c)
×
32000; (e, d)
×
110,000. From Bortoluzzi (2003), unpublished data.
nucleus itself which are important to chemistry, such
as the magnetic properties of the nucleus, which are
the basis for nuclear magnetic resonance (NMR)
spectroscopy.
All atomic nuclei have charge, but not all of them
are magnetically active and thus accessible to NMR
spectroscopy. The magnetic activity of the nucleus is
due to the charge of the nucleus fl owing about a loop
around a rotation axis, creating a magnetic dipole.
Consequently, the nucleus behaves like a small
magnet. The fundamental property of the atomic
nucleus involved in NMR spectroscopy is nuclear
spin. According to quantum mechanics, the angular
momentum of the moving nuclear charge can be
described in terms of a quantized spin number (I),
which can have values of 0, 1/2, 1, 3/2, etc., depend-
ing on the nucleus under consideration.
In those nuclei that have no angular momentum
(
I
= 0) it is not possible to induce an NMR signal;
this is the case for
12
C,
16
O, and
32
S. Although these
nuclei do not have spin (i.e. no associated magnetic
moment) they are free to rotate in the classical sense,
forming a current loop. However, the quantum
mechanical concept of spin is different for classical
rotation of charged nuclei. The particles that make
up the nucleus (neutrons and protons) are called
nucleons; as with electrons in atoms, nucleons
possess an intrinsic spin. Nucleons of opposite spin
can pair, in a similar manner as electrons do.
However, only nucleons of the same type can be
