129Xe chemical shift

Figure 5. HP 129Xe Chemical Shift Imaging (left and centre) of a phantom consisting of a 7 mm porous Vycor tube filled with NaY zeolite and placed inside an open 9 mm ID glass tube (right). Images from Xe in the three different chemical shift environments can be clearly separated. The NMR spectrum is shown bottom left.

Xenon is a very useful molecular probe for adsorption studies. 129Xe is a spin nucleus of 26.44% natural abundance and a very wide range of chemical shifts (334). The shielding of the xenon atom with respect to the bare nucleus has been estimated to be 5642 ppm (335), and the 129Xe chemical shift is extremely sensitive to physical environment as shown by its strong dependence on density in the pure phases the liquid at 224 K resonates 161 ppm downfield from the gas at zero density, whereas the solid at 161 K has its resonance at — 274 ppm. The atomic diameter of xenon is 4.6 A, i.e., comparable to the size of zeolitic channels. 

Fraissard and co-workers were the first to take advantage of these properties of l29Xe for the study of xenon adsorbed in zeolites (336-344). They have demonstrated that the 129Xe chemical shift is then a sum of several... 

In studies of competitive adsorption, the usually measured quantity is the overall composition of the adsorbed phase for a given composition of the bulk phase in equilibrium with it. It has been found that chemical shifts can provide a more detailed description. In a mixture of Xe and Kr in N 4 zeolite it was possible to observe the individual signals from XenKr mixed clusters as well as the Xen clusters under magic angle spinning (28). The absolute 129Xe chemical shifts of the XenKr mixed clusters and the increments between XenKr and the Xen+1 in various Xe-Kr mixtures in Na4 zeolite, are shown in Table I. 

Table I. 129Xe chemical shifts of mixed clusters XenKr in the alpha cages of zeolite Na/f (ppm) ...

Figure 1. 129Xe chemical shifts of Xen with CH4 under fast exchange in zeolite NaA, obtained from GCMC simulations, compared with experimental values. The shifts are in ppm relative to the isolated Xe atom. 

Figure 4. 129Xe chemical shift versus the number of adsorbed xenon atoms per gram of anhydrous zeolite N. 

Because of the collisions between the Xe atoms in the gas phase, the chemical shift of 129Xe in the gas phase is temperature and pressure dependent. Quantitatively, a relationship between the 129Xe chemical shift in ppm and the density of the gas has been found to exist [3] ... 

Figure 12.9 The 129Xe chemical shift as a function of the composition of the EP copolymer. The two values for pure iPP result from two different iPP samples...

This must be due to a high mobility of Xe on the carbon black surface. For the three carbon blacks shown here the 129Xe chemical shift is proportional to the average particle diameter [14]. 

Figure 12.18 shows the temperature dependence of the 129Xe chemical shifts of Xe in EPDM/carbon black N110, in the bound rubber fraction of EPDM/N110 and in the carbon black N110 itself. It shows that the temperature dependence of the chemical shift of the broad resonance in the bound rubber sample shows a similar temperature dependence as the chemical shift of the pure EPDM and that of Xe adsorbed on the carbon black. 

Figure 2. The variations of 129Xe chemical shift with the number of xenon atoms co-adsorbed per supercage for various benzene/zeolite systems (a) NaX, Si/Al = 1.23. (b) NaY, Si/Al = 2.49, (c) NaY, Si/Al = 2.70.

Figure 3. Dependence of (a) 129Xe chemical shift at zero xenon loading (<5SJ, and (b) slopes (

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