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

- 93 -
approximately 0.4 ppm. We remark in passing that a separate calc-
ulation(99) has predicted that the bulk susceptibility effect will
in fact be averaged out in a sample undergoing sufficiently rapid
macroscopic rotation at the magic angle. The bulk susceptibility
of the saturated Aid 3 reference solutions is unknown but can prob-
ably be safely neglected in comparison to-the metal. It follows
that the possible contribution of the sample bulk susceptibilities
to the experimental Knight shift is less than the experimental error.
(d) Other Effects. For small grain aluminium powders the Knight
shift has been reported to be dependent upon particle size(100)9
but this effect has only been detected at temperatures below 4K
and for particle sizes under 100 Field dependent oscillations
in the Knight shift due to the de Haas Van Alphen effect have been
measured at very low temperatures(101). The magnitude of these
oscillations is small and the average Knight shift value was found
to be unaffected. Both of these effects should be negligible at
room temperatures.
In conclusion it can be stated that neglecting the possibility
of unsuspected systematic errors, the experimental value of the
isotropic Knight shift was 1640 ppm with a maximum possible error
in measurement of ±1 ppm. If a possible contribution due to the
bulk susceptibility of the aluminium samples is included the error
limits should probably be extended to +1 and -2 ppm. From a detailed
theoretical calculation . Shyu, Das and Gaspari(102) obtained
'a value
for the aluminium Knight shift of 1624 ppm. Although their analysis
included a core polarization correction it did not take into account
any possible differences in the chemical shift interaction
of the