Z. Xu et Jf. Stebbins, OXYGEN SITES IN THE ZEOLITE STILBITE - A COMPARISON OF STATIC, MAS, VAS, DAS AND TRIPLE QUANTUM MAS NMR TECHNIQUES, Solid state nuclear magnetic resonance, 11(3-4), 1998, pp. 243-251
Static, magic angle spinning (MAS), variable angle spinning (VAS), dyn
amic angle spinning (DAS) and triple quantum magic angle spinning (3QM
AS) NMR techniques were applied to separate and quantify oxygen signal
s from Al-O-Si and Si-O-Si sites of O-17-enriched samples of the miner
al stilbite, a natural zeolite. DAS experiments showed that there was
a distribution of quadrupolar coupling constants, asymmetry parameters
and isotropic chemical shifts. Two methods were used to study the qua
ntification problem of DAS and 3QMAS. Our results showed that DAS was
quantitative. In 3QMAS, signal intensity from sites with larger quadru
polar coupling constants was reduced because of less efficient excitat
ion. All techniques have shown a clear difference in rates of exchange
between the two types of sites with interchannel H2O molecules. (C) 1
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