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

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aluminium nuclei in the metal and reference compound. A consider-
able improvement in the accuracy of Knight shift calculations will
be necessary to enable a precise comparison between theory and
experiment to be made. However the value reported here of
1.640 x 10 3 does show a small but significant departure from the
previously accepted estimate of 1.61 x
10 3.
7.3 RECORDED LINESHAPES AND SECOND MOMENT VALUES
7.3.1 INTRODUCTION
Although theory is unable to predict the exact form of
the absorption spectrum, a comparison between the theoretical and
experimental second moments can be made.
27A1
is the only natur-
ally occurring aluminium isotope. It occupies a FCC lattice, the
lattice constant being 4.0496 X.
moment predicted by Van Vleck's calculation
From this the dipolar second
(16)
is 9.18 kU1z2
(7.44 G2). (It has also been quoted as 9.42 kHz2 (7.63 G2). )
Previous experimental determinations on pure annealed specimens
have consistently yielded values above this in the range 11.8 to
15.4 kHz2 (9.5-12.5 G2)(72,103-110). The discrepancy between theory
and experiment is well known. In the main it has been attribut-
ed to a nuclear quadrupole interaction, but the mechanism for such
an interaction has never been demonstrated conclusively.
7.3.2 MEASUREMENTS
The resonance spectra of the static aluminium powders
were obtained by Fourier transformation of the detected FIDs as
detailed in Section 5.6. In each case about 1 cm3 of the loose