covering a wide frequency range (500–1500 Hz). Following each pulse, excited nuclei return to the
ground state producing a decaying emission signal which is monitored by the receiver coil of a double
coil spectrometer and computer-processed to produce a conventional NMR spectrum (p. 414). The time
interval between pulses is of the order of a second so that data can be accumulated much more rapidly
than with a CW instrument and sensitivity enhancements of one to three orders of magnitude can be
achieved quickly by signal averaging.
The NMR Process
The principle of NMR can be explained in quantum mechanical terms. The angular momentum of a
spinning nucleus, quantized both in magnitude and direction, is given by the equation
where h is Planck's constant and h/2π is defined as an angular momentum unit. In the presence of an
applied magnetic field, the momentum vector can assume only those orientations in space which result
in its component in the direction of the field being an integral or half-integral number of angular
momentum units, i.e.
where Iz, represents the magnitude of the component of the angular momentum in the direction of the
field, usually designated the z direction, and mI = 0, ± 1/2, ± 1, ± 3/2... The total number of values for
mI, the magnetic quantum number, is 2I + 1, as clearly the value of Iz cannot exceed the value of I given
in equation (9.19).
For a given nucleus, the ratio of the magnetic moment to angular momentum is a constant, i.e.
where γ is known as the magnetogyric (gyromagnetic) ratio. If μz is the component of the dipole in the z
direction
and from equation (9.20)
The energy of interaction, between a nucleus and the magnetic field it experiences B is given by