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

Qv ==
,
2
28 kHz
-111-
which is slightly larger than the width of the vanadium resonance
line. That the origin of this large linewidth was mainly quadrup-
olar in origin was made evident by the large quadrupolar echoes
observed following for example a 90-T-35 pulse sequence
(118,119)ý
As expected, rapid sample rotation about the magic angle at speeds
of around 5 kHz failed to have any effect upon the nuclear lineshape.
There are very few compounds of niobium which can be observed
by NMR. The niobium Knight shift has been measured previously
with respect to Nb205 dissolved in hydrofluoric acid. However in
our work the Knight shift was determined against a solution of
NbC15 in pure acetone. The solution is light yellow upon the
NbC15 first being introduced, but later turns brown and produces
a brown precipitate. Although the 93Nb resonance observed during
this time varies considerably both in frequency and in T2, after
several hours it becomes constant. The Knight shift value obtained
by direct substitution of the reference and metal samples into
the Bruker coil was 0.768 ± 0.0087. Values quoted in the literat-
ure with respect to Nb205 in HF are 0.85(120) and 0.875%(127)
view of the unknown nature of the reference compounds, the discrep-
ancy is not meaningful.
The large experimental Knight shifts measured for both vanadium
and niobium have been explained'in terms of a large orbital contrib-
ution to the hyperfine coupling in these two metals(123).
.
In