Zeolite asymmetry parameters

Al Quadrupole Coupling Constant, C-rjcc. ond Asymmetry Parameter, tj, of Framework Al Atoms in Dehydrated Zeolites Y (faujasite), MOR (mordenite), and ZSM-5... 

Figure 24 reports 13C MAS spectra of the ferf-butyl cation (43) and the methylcyclopentyl cation 17 (45) on the solid metal halides A1C13 and AlBr3 the asymmetry parameters, CSAs, and isotropic shifts (Table III) are unambiguous for the species indicated. Repeated attempts in various laboratories to observe the ferf-butyl cation as a persistent species in a zeolite have thus far been unsuccessful. Detailed theoretical work will be required to determine whether or not the ferf-butyl cations are local minima (i.e., true intermediates) on typical reaction pathways in zeolites. The ease with which these cations form in true superacids (liquid or solid) should be contrasted with the history of negative observations in zeolites. 

Conventional single-crystal studies of aluminosilicates date back to the early 1950 s, and principal components of the quadrupole-coupling tensors together with asymmetry parameters have been determined for a number of minerals (see Table XI). Wide-line-NMR measurements of the dependence of the 27A1 and 23Na quadrupole-coupling constants in analcime and zeolites Na-X and Na-A on the water content were carried out by Gabuda et al. (137). 

The spectrum for unsubstituted zeolite samples gave a sharp line with chemical shift 5 = 58.9 ppm and a line width of 300 Hz. with a very feeble these side bands and the width of the isotropic signal showed a systematic increase in their values. The Fe substitution did not affect the chemical shift values From the intensity analysis of the spinning sidebands, the values of the components of the chemical shift tensor aji, the asymmetry parameter ti and the anisotropy parameter 5 have been calculated by following the graphical method developed by Herzfeld (10) and the values are listed in table 1. 

Table 3. Quadrupole coupling constants, Cq, asymmetry parameters, 7]q, and isotropic chemical shifts, 5, (referenced to solid NaCl) of sodium cations in dehydrated zeolites NaY and NaX calculated by a point charge model (see text) and experimentally derived by solid-state Na MAS NMR spectroscopy [194,201,203] ...

---