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

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Lorentzian NMR lineshape of Aid 3 solution depends in the first
instance upon the homogeneity of the external magnetic field.
On the other hand the 27A1 resonance line is approximately Gauss-
ian with a width of about 9 kHz. Even if the line centre is
located to an accuracy of 2% of the width the greatest accuracy
to which the Knight shift can be determined in a magnetic field
of around 1 tesla is ±0.02 x
10 3.
Aluminium has a cubic structure at room temperature with only
one naturally occurring isotope. The broadening of the resonance
line from a pure annealed powdered specimen of this metal arises
from the magnetic dipolar interaction between nuclei. The rapid
rotation technique can be used therefore to narrow the resonance
lineshäpe. This enables the line centre and hence the Knight
shift to be determined to a correspondingly greater accuracy.
7.2.2 MEASUREMENTS
Experimental measurements were undertaken following the
procedure described in Section 5.5. Using compressed air, rotors
containing the aluminium powder samples were spun about the magic
angle at spinning frequencies of about'4 kHz. At these speeds the
width of the resonance lineshape is reduced by a factor of approxi-
mately 15. (Typical FIDs and Fourier transformed lineshapes
obtained from static and rotating pure powder specimens are included
in Figures 7.1 and 7.2 respectively. ) The reference material
used was a saturated aqueous solution of 99.999% pure A1C13 as
supplied by Johnson Matthey Chemicals. The solution was enclosed
inside a nylon container, the internal dimensions of which were