Xenon shielding

The averaging of the xenon shielding tensor, and consequently, the derivation of structure information from the Xe shielding data necessitates the... 

Clathrates were the first systems investigated by Xe NMR of natural xenon gas. The xenon atom is of the same size and shape as methane and it also forms a clathrate hydrate with water. The xenon shielding is much more sensitive (by a factor of about 30) than the shielding of methane to the... 

We apply this technique to study the effect of the spin-orbit coupling on an NMR shielding tensor and the shielding polarizability of the xenon atom. The shielding polarizabilities are defined as the second derivatives of nuclear shielding constants with respect to an electric field E... 

Table 5. The NMR shielding constant and shielding polarizabilities of the xenon atom calculated at the Hartree-Fock level using the Drrac-Coulomb Hamiltonian (SR + SO), its spin-free version (SR) as well as the non-relativistic Levy-Leblond Hamiltonian. The shielding constant is given in ppm and shielding polarizabilities in ppm/(au field2) (1 a.u. field = 5.14220642X 10" V...

I would like to dedicate this paper to Jens Oddershede on his 60th birthday. I would like to thank Magdalena Pecul for suggesting an additional s -exponent in the basis set used for the calculation of the shielding polarizability of the xenon atom. 

The theoretical understanding of xenon chemical shifts was placed on a firm footing by the work of Jameson and co-workers, who were able to reproduce the chemical shifts for xenon clusters of different size in NaA and AgA zeolites remarkably well [13-17]. However, the procedure is not trivial, as it requires a knowledge of the structure, the shielding functions for all possible Xe-framework atom pairs, the Xe-framework atom potential functions, and a calculation over the accessible dynamic states of the xenon atoms in the cage. To work back from chemical shifts to pore size clearly is not trivial, so the establishment of some general shift - size correlations still is extremely useful. 

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. 

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