NMR xenon

Several reviews on xenon NMR spectroscopy have appeared [1, 2], therefore in this introduction only the most important aspects will be discussed. The xenon atom has two isotopes which are suited for NMR studies, the 129Xe and 131Xe isotopes. For most studies 129Xe is more convenient than 131Xe, while the former nucleus with spin I=V2 does not have an electrical quadrupole moment. In some cases, however, the quadrupolar interaction can provide additional (spectral and relaxation) information. Flere we will only consider the 129Xe isotope. 

Xenon NMR spectroscopy was used to characterize xenon in Ca, Mg Ni, Ag, Cu, Zn, and Cd - exchanged Y and X zeolites. We report here some examples concerning the location of these cations and the effect of their charge and electronic structure. 

Brunner, Pines and coworkers reported on the enhancement of NMR signals in solid Cgo and C70 using a laser-polarized xenon. NMR signals emanating from surface nuclei of solids may be enhanced by the transfer of spin polarization from laser-polarized noble gases via SPINOE (spin polarization induced nuclear Overhauser effect). The paper describes experiments in which the spin polarization is transferred under MAS from laser-polarized - Xe to a nuclear spin with a low gyromagnetic ratio in the fullerenes Ceo and C70, which are polycrystalline materials with a low surface area. In C70, a different degree of enhancement of the NMR spectrum is observed for the different atomic sites in the molecule. 

Davis and coworkers [104] studied " Xe NMR of xenon adsorbed in several SAPOs, ALPOs, and Y zeolites. From a comparison of the xenon chemical shift extrapolated to zero pressure, these authors concluded that Xe atoms feel significantly smaller electrostatic fields and field gradients in the aluminophosphates compared to aluminosilicates. The extrapolated chemical shift decreased from 97 ppm in erionite to 60 ppm in Y zeolite and to 27 ppm in AIPO4-5, with the values for SAPOs being intermediate to Y zeolites and AlPOs as would be expected from the acidity trends. They concluded as well that SAPO-37 does not contain separate aluminophosphate and aluminosilicate islands. Dumont et al. [105] also carried out xenon NMR experiments in SAPO-37. From xenon sorption capacity and the decrease in the chemical shift, their conclusion was that the framework of calcined SAPO-37 is unstable when exposed to moist air. 

The oral presentation in the form of a ke>uote speech will also mention the study of diflusion by xenon NMR published in this Proceedings... 

Raftery D (2006) Xenon NMR spectroscopy. In Webb GA (ed) Annual reports on NMR spectroscopy. Vol 57, Acadamic Press, Salt Lake City, USA, pp. 205-270... 

Xenon NMR spectroscopy has evolved in status over the past two decades from exotic to routine such that it is now used to provide detailed information for an enormous variety of applications. Thus, xenon NMR has become a useful and complementary experiment for the investigation of a large range of materials as will be seen below. Also evident is the growing emphasis on the development and use of hyperpolarized (HP) xenon experiments that can provide much enhanced sensitivity. These experiments have therefore greatly enhanced the utility of xenon NMR studies, and exciting new developments promise even more advances to come. 

This chapter follows a number of excellent reviews, some comprehensive and others more focused on the subject of xenon NMR spectroscopy. I have chosen to focus this review primarily on the most recent and exciting developments (in my own opinion) during the period 1998 to the present as the previous xenon work has been reviewed comprehensively by Ratcliffe in volume 36 of the Annual Reports on NMR Spectroscopy. Thus, results from approximately 100 papers are described from the over 500 that have been published on Xe NMR during the past 7 years. References are made to many other papers, although not all that have appeared since 1998 due to time and space considerations. I apologize to those authors whose work was not featured in this review. 

Xenon continues to be useful to study micro- and mesoporous materials, and this is by far the largest area of application for xenon NMR studies. Variable pressure. 

Much of the current xenon NMR work in zeolites dates back to the pioneering work by Fraissard and coworkers, who showed the utility of xenon as a sensitive probe of its chemical environment. They have mapped out the dependence of the xenon chemical shift on a number of different variables, which are summarized by the following expression ... 

Xenon is very useful to eharaeterize the ehannel and pore structure and numerous papers have been published on this topie in reeent years.Many of the applications of xenon NMR to zeolites have been reviewed previously, including experiments to probe issues related to porosity, crystallinity, the influence of cations, diffusivity of gases as well as eoking, and other factors causing the blocking of pores. Therefore, our treatment will be abbreviated. 

Pietrass et al also used xenon NMR to examine the porosity of three me-soporous silica materials that were synthesized by sol-gel processing. Xenon did not penetrate the pores of a sample of largely disordered silica. The other two samples showed more typical xenon adsorption and chemical shift behavior. Results from variable temperature and Ti experiments indicated that xenon had a stronger interaction with the surface of smaller pores and that it was in fast exchange with the gas phase. 

Xenon has also been applied to the study of humic substances. Previously, there had only been one study on the investigation of surface of soil materials using xenon, namely a study of xenon sorption on to the montmorillonite clay surface. Magusin et al. explored the use of xenon NMR to probe the average volume to area ratio in the pore networks of humidified sand as well as carbon black and kaolin model systems. High-pressure xenon NMR experiments were carried out and showed a broad resonance around 45-55 ppm for three different samples of carbon black with different particle sizes. By comparison, the kaolin and humidified sand showed broad resonances near 8 ppm and zero, respectively. The humidified sand spectrum showed xenon spectral intensity at negative ppm values. The authors derived an expression for the pore size given the measured xenon chemical shift and a constant related to xenon adsorption on the surface. 

The success of ab initio methods have made the calculations of xenon NMR parameters quite popular, with a number of groups now investigating a number of microporous materials, fluids, and even xenon compounds. Many of these studies compare ab initio results with experimental data. 

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