Quadrupolar hydrated cations

Information derived from nmr of quadrupolar hydrated cations. 

It is evident that the aluminum released from the framework upon hydro-thermal treatment remains in the sample, either as an intracrystalHne oxidic or cationic aluminum species or as intercrystalline material, i.e., as a separate crystalline or amorphous aluminum oxide phase [77-79]. Typical signals appear in the Al MAS NMR spectra of Y zeolites upon hydrothermal dealumination (Fig. 4). A line at 0 ppm associated with octahedrally coordinated aluminum is indicative of hydrated cationic species [79-82]. Amorphous oxidic aluminum species, which are subject to large second-order quadrupolar... 

Cooper et al. reported that solid-state 23Na NMR is useful for observing the sodium cation in sulfonated polystyrene ionomers (NaSPS).44 Three NMR peaks were detected, corresponding to isolated ion pairs, aggregated ions and hydrated ions (Fig. 14). The distributions of these three types of sodium cations are systematically influenced by hydration treatment, sulfonation level and neutralization level. Fully dried NaSPS at low ion content shows that the isolated and aggregated sodium ions in NaSPS are available for hydration. As the sulfonation level increases, the fraction of sodium ions held in isolated ion pairs decreases while the fraction of ionic species in the ionic aggregates increases. This coincides with a shift in the peak position of the aggregated sodium ions to low frequency, indicative of increased quadrupolar interactions. 

Various types of NMR experiments have previously been performed on the cations of Y zeolites. The work of Basler (9) on hydrated Y zeolites is particularly relevant. Performing static sodium-23 NMR measurements at 16 MHz, he observed a single NMR line which disappeared as he exchanged in increasing amounts of calcium. He concluded that sodium in the supercage was being preferentially removed, and that sodium in the Type I and I sites was unobservable at such a low frequency, due to quadrupolar interactions. In order to use the sodium-23 NMR spectra of Figures 2 and 4 to determine how... 

The effect of hydration and dehydration on the electric field gradient at the Al nucleus in ZSM-5 zeolites was investigated by Al MAS NMR [83K3]. The dehydration causes an increase of the A1 NMR linewidth due to an increased quadrupolar interaction. The dehydration therefore causes a redistribution of the electric charges and/or a conformation change of the AIO4 tetrahedra. The latter possibihty was considered to be most importarrt since the effect is cation dependent [83K3]. 

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