Aluminium shift correlations

In view of the chemistry of this inert element, the main application of Xe NMR is as a surface probe for studying meso and microporous solids and the free volume in polymers. The relaxation time for Xe adsorbed in solids is typically 10 ms to a few seconds. The use of Xe NMR as a probe for studying microporous solids has been extensively reviewed by Barrie and Klinowski (1992). A more recent example of the use of Xe NMR to study surface interactions is provided by a study of borosilicalites with the ZSM-5 structure (Ngokoli-Kekele et al. 1998). The Xe shift of adsorbed xenon (referred to the shift of the pure gas extrapolated to zero pressure) was found to change regularly with boron content, with a discontinuity at a boron content of about one atom per unit cell ascribed to a change in the distribution of boron atoms in the lattice. A similar correlation between the Xe NMR shift and the aluminium content has been reported for the zeolite ZSM-5, in which the discontinuity occurred at about 2 Al atoms per unit cell (Chen et al. 1992). 

A really improved agreement was obtained using the optimized structures. The shifts of the atomic positions lead to reduced octahedron distortions. The optimized structures provided a reliable assignment of the Al quadrupolar parameters to the aluminium sites in the four studied compounds, which contain two sites with the same multiplicity [15,16,124]. The correlation between experimental and calculated EFG tensor elements (Figure 5.12) allowed the determination of the value of the Al nuclear quadrupole moment with an improved accuracy [124]. Moreover the DFT calculations provided the orientation of the... 

---