Xenon NMR Studies

One of the earliest properties of xenon studied by NMR was the gas-phase relaxation, which had been investigated in a detail as early as 1961. Xenon was observed to have a T relaxation time that is inversely dependent on the pressure, indicating the presence of a strong spin-rotation interaction. Detailed studies of xenon relaxation in the presence of paramagnetic gases has been studied in detail by the Jamesons. 

Over the years, a number of studies have probed the interaction of xenon in numerous liquid environments. The xenon chemical shift is known to vary widely depending on the particular solvent. For example, the xenon chemical shift in methanol is 148 ppm while in methyl iodide it is 333 ppm, with more typical values around 180-220 ppm. The xenon chemical shift is very sensitive to its liquid environment such that it can detect the difference between protonated and deute-rated solvents, with changes of the order of l ppm seen for various solvents. Most liquid-phase studies involving xenon now probe other solute species, as described below, however fundamental studies of xenon in various solvents are still reported.  

The xenon relaxation mechanism in aqueous solution was examined in detail recently and determined to result essentially from dipolar interactions, as 

Locci et al. describe a method to use xenon to monitor chemical transformations, which they describe as the Spin-Spy methodology. Xenon was added to a solution of a- and p-D-glucose and the change in concentration of these speeies was monitored via the Xe chemical shift as equilibrium concentrations of the interconverting sugars were reached over a period of 300 min. The xenon chemieal shift difference in a 1 M concentration of these two species is approximately 1.3 ppm. Xenon s interaction with liquid crystal environments has also been reported over the past several years. This area was reviewed extensively by Jokisaari in 1994. 

In a recent paper Jokisaari and coworkers investigated the effect of long-range attractive van der Waals (vdW) forces on the chemical shift anisotropy due to the anisotropic medium of the liquid crystal.  

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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. 

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. 

J. Fraissard, T. Ito, 1988 (Xe-129 NMR-study of adsorbed xenon - a new method for studying zeolites and metal-zeolites), Zeolites 8, 350. 

L. M. Schwartz, R. L. Walswofhry 2001, (Tortuosity measurement and the effects of finite pulse widths on xenon gas diffusion NMR studies of porous media), Mag. Reson. Imag. 19, 345. 

With the rare exception of xenon gas NMR of fluidized beds, which we discuss later, granular flow studies by NMR detect signals from the particles and not the surrounding medium. Because it is technically easier to obtain NMR signals from liquids rather than solids, the majority of granular NMR studies so far use solid particles containing liquids. 

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. 

F. Junker, Materialforschung an Porosen Festkorpern Mittles Xenon -Diffusionsmessungen eine 129Xe-PFGE-NMR-studie, Duisburg, 2000. [PhD Thesis]... 

Following the pioneering works of Ito and Fraissard (57) and Ripme-ester (58), 129Xe NMR of xenon adsorbed on zeolite has proven sensitive probe of its local environment due to its chemical inertness and excellent sensitivity (59). In this work, we used 11 and 13C NMR measurements of the adsorbed benzene in conjunction with 129Xe NMR and adsorption isotherm measurements of the co-adsorbed xenon to study the homogeneous adsorption behavior of benzene on faujasite-type zeolites with various Si/Al ratios. Detailed macroscopic and microscopic adsorption states of the benzene in various NaX and NaY zeolites are discussed in terms of NMR linewidths and chemical shifts and are compared with results obtained from other studies. 

Kinetic study of the reactions of anions involving xenon has barely begun. In an nmr study using oxygen-17, exchange of Xe(OH)6 was shown to be fairly fast-complete in three minutes at 23 A study of the oxidation of water by... 

There is no evidence for discrete ionic fluoroxenon. species in WFt solution. In addition to the lack of spectroscopic correspondence with the XeFJ and XeiF), cations, the following pieces of evidence may be cited to support this view (i) Compounds such as XeFJBFr are insoluble in WFj, (ii) Removal of solvent from solutions of XcFe in WFs by pumping at low temperature leaves pure xenon hexafluoride, (iii) A F-NMR study of xenon hexafluoride in WF, shows no exchange between xenon hexafluoride, solvent, and the XeOF4 impurity. 

Since the first 29xe NMR study of xenon adsorbed on a zeolite, this technique has been shown to be of interest for the investigation of the distribution and the size of supported metal particles, the quantitative distribution of phases chemisorbed on these particles, the dimensions of the void spaces of zeolites, the detection of structure defects, the location of cations and the effect of electric fields they create [1,2], We report here some typical applications related to the study of the location and the electronic structure of the cations. 

Table 5 shows the results of diffusion measurements with carbon monoxide, methane, and xenon in a comparative C, H, and Xe PFG NMR study [178,179]. For all adsorbates the values of timra and are found to be on... 

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... 

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. 

A long-term goal of HP xenon research is to develop methods to polarize surface nuclei, and thereby enhance surface NMR studies. Applications in a variety of areas of surface science, catalysis, biophysics, semiconductor physics, and many other areas could be envisioned if such methods become routine. Various groups have explored a variety of methods to transfer polarization from HP xenon to other atoms, including rotating-frame cross-polarization (CP), ° thermal mixing in low magnetic field, and SPINOE, to date, however, successful polarization transfer have been limited to a select subset of materials and experiments. 

Tremendous progress has been made in the past few years in the area of biomedical studies using xenon NMR and MRI. However, more work is needed before these types of analyses can be made at a diagnostic clinical level. 

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