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

- 15 -
This contact term manifests itself as a small difference in
the resonance frequencies of a particular nucleus in the metal and
in a diamagnetic compound. This frequency shift, called the Knight
shift, is defined as
v-v
mr
v r
where m and yr are the resonance frequencies of the metal and refer-
ence compound respectively in the same external field.
Because of the existence of chemical shifts any Knight shift
value will depend upon the particular reference compound used.
Similar chemical shifts are also present in metals usually arising
from the shielding of nuclei by the inner shell electrons. Although
this effect may not be negligible, it is usually approximately
equal for the metal and reference compound so it can be ignored.
Reference compounds are normally chosen which have no large para-
magnetic contribution to the chemical shift. Measured Knight
shifts are then usually much greater than the chemical shifts bet-
ween different possible reference samples.
From equation (2.9) the Knight shift can be written
K=
Vm
v
ro
This predicts that the Knight shift is
yr
tv _ $3 XSýF (2.10)
(i) Positive and independent of the applied field.
(ii) Insensitive to temperature.
(iii) Dependent upon atomic number.
These predictions are generally confirmed but other interactions
also contribute to the experimental value(5).