NNR IN RAPIDLY ROTATED METALS By - Nottingham eTheses ...

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In powdered samples a weak interaction of this form usually
results in an increase in the linewidth and an enhancement of the
wings of the absorption lineshape(4). In cases where the quad-
rupole interaction is large it can no longer be considered as a
perturbation of.; r , and the central ;- transition is also
z
shifted.
2.2.3 FREQUENCY SHIFT MECHANISMS
(a) Chemical Shifts
The chemical shift interaction arises via the magnetic coupling
between the external magnetic field H0 and the motion of electronic
charges. For a fixed value of Ho the nuclear resonance frequency
of any nuclear species depends in principle upon the particular
chemical sample observed. The precession of electrons about H0
produces a diamagnetic contribution to the magnetic field at the
nucleus. Polarization of the electronic orbitals gives rise to
a second order paramagnetic contribution to this field. Although
in most samples the orbital angular momentum is quenched, the
applied field produces an admixing of electronic states which results
in states having non-zero expectation values of angular momentum.
The total additional magnetic field at the nucleus is proportional
to Ho, so the effective field is given by
-eff ' H0(I - Q)
where I is the unit tensor and a is the chemical shift tensor. The
chemical shift interaction may be written