Engineering Chemistry S Datta

528 ENGINEERING CHEMISTRY
The transition of a proton from one possible orientation to another is effected by the
absorption or emission of a discrete amount of energy such that,
E = hν = 2 µH o
where, ν = frequency of electromagnetic radion absorbed or emitted
h = Planck’s constant
µ = nuclear magnetic moment
H o
= applied magnetic field.
For protons if a magnetic field 14000 Gauss is applied the frequency of such energy
remains in the radio frequency region (60 megacycles per second).
Unless the axis of the nuclear magnet is oriented exactly parallel or antiparallel with
the applied magnetic field (Fig. 24.11), there will be a certain force by external magnetic field
to so orient the spinning nucleus. But because the nucleus is spinning, the effect is that its
rotational axis draws a circle perpendicular to the applied field. Such an effect is shown in
Fig. 24.12. This motion of the nucleus is called Precession.
Precessional orbit
Axis of rotation
Spinning nucleus
H o
Applied magnetic field
Fig. 24.12 Precession.
Theory of nuclear resonance
A proton in a static external magnetic field may assume only two possible orientations
corresponding to the energies of ± µH o
. The low energy orientation corresponds to that state in
which the nuclear magnetic moment is aligned parallel to the external magnetic field, and the
high energy orientation corresponds to that state in which the nuclear magnetic moment is
aligned antiparallel (opposed) to the external magnetic field. It is possible to the induce
transitions between these two orientations. The frequency ν of electromagnetic radiation
necessary for such a transition is given by
ν = 2µH o
/h
where H o
is the strength of the external or applied magnetic field and h is Planck’s constant.
The precessional frequency of the spinning nucleus i.e., nuclear magnet is exactly equal
to the frequency of electromagnetic radiation necessary to induce a transition from one nuclear
spin state to another. The nuclear transition corresponds to a change in the angle that the axis
of the nuclear magnet makes with the applied magnetic field. This change can be brought
about through the application of electromagnetic radiation whose magnetic vector component
is rotating in a plane perpendicular to the main magnetic field. When the frequency of the