Chemical Shift Of Xenon Research Articles

Exploring the structure of halomethanes with xenon: An NMR and MD investigation

The liquid structure of dihalomethanes (CH2X2, X = Cl, Br, I) loaded with xenon gas was investigated both experimentally, by 129Xe NMR Spectroscopy, and computationally, by molecular dynamics (MD) simulations and relativistic DFT calculations. The chemical shifts of xenon have been measured in a wide range of temperatures; in addition, the spin-lattice relaxation times, T1, and the diffusion coefficients of dissolved xenon were also determined. The MD simulations of the three dihalomethanes with xenon dissolved were used to generate a collection of clusters, representing xenon with its first solvation shell, which was subsequently submitted to the relativistic DFT calculation of the 129Xe NMR shielding constant. This was finally averaged over the trajectory to obtain the corresponding chemical shift at various temperatures. The experimental data were then rationalized in terms of the density of the bulk phase and the average orientation of the halogens with respect to the xenon atom. Diffusion coefficients were also obtained from the MD simulations and compared with the experimental values, showing a good agreement.

Structural Characterization of Micro- and Mesoporous Carbon Materials Using In Situ High Pressure 129Xe NMR Spectroscopy

In situ high pressure 129Xe NMR spectroscopy in combination with volumetric adsorption measurements were used for the textural characterization of different carbon materials with well-defined porosity including microporous carbide-derived carbons, ordered mesoporous carbide-derived carbon, and ordered mesoporous CMK-3. Adsorption/desorption isotherms were measured also by NMR up to relative pressures close to p/p0 = 1 at 237 K. The 129Xe NMR chemical shift of xenon adsorbed in porous carbons is found to be correlated with the pore size in analogy to other materials such as zeolites. In addition, these measurements were performed loading the samples with n-nonane. Nonane molecules preferentially block the micropores. However, 129Xe NMR spectroscopy proves that the nonane also influences the mesopores, thus providing information about the pore system in hierarchically structured materials.

Cage-Like Local Structure of Ionic Liquids Revealed by a (129)Xe Chemical Shift.

The chemical shift of xenon (at natural abundance) dissolved in a variety of 1-butyl-3-methylimidazolium-based ionic liquids (ILs) has been measured with (129)Xe NMR spectroscopy. The large chemical shift differences observed are mainly related to the type of anion; the strongest deshielding effect is observed ILs with I(-), Br(-), and Cl(-) anions, and the strongest shielding is found for the bis(trifluoromethanesulfonyl)imide ([Tf2N](-))-based IL. The measured (129)Xe chemical shift variations correlate well with the IL structure organization imposed by the anions and with the size of the empty voids due to charge alternation patterns. Descriptors taken from literature data on X-ray and neutron scattering, as well as single-crystal structures where available, support this interpretation. The proposed methodology adds a new investigating tool to the elucidation of the short-range order in ILs. The observed chemical shift trend provides information about how these solvents are organized.

Local structure of xenon adsorbed in the nanospaces of zeolites as studied by high-pressure 129Xe NMR

Pressure (0-10 MPa) and local density dependence of 129Xe NMR chemical shift of xenon in various microporous materials was investigated using an in situ high-pressure probe. The density dependence of the chemical shift was analyzed using virial expansion of the chemical shift by xenon density. Results indicate that the second virial coefficient depends on the pore size and shape, and that the void space affects xenon-xenon interaction in both microporous and mesoporous materials. Furthermore, to interpret the magnitude of the virial coefficient in terms of the local structure of the adsorbed xenon, we analyzed the local structure of adsorbed xenon in molecular sieve 5A using Xe(n) clusters, thereby allowing description of the density dependence of the chemical shift. We also demonstrated the cluster model's validity by applying it to molecular sieves 13X and ZSM-5. The latter showed that the adsorbed xenon exists as a xenon monomer up to the filling of about 0.6 in micropores. Larger xenon clusters up to n = 4 have been grown with increasing filling of xenon. According to analyses using the Xe(n) cluster model, the second virial coefficient is related closely with the xenon cluster size, which contributes greatly to the chemical shift in the low loading region.

Application of continuously circulating flow of hyperpolarized (HP)129Xe-NMR on mesoporous materials

The continuously circulating flow of hyperpolarized (HP) 129Xe NMR technique has been used to study the porous structure of ordered purely siliceous and Al-containing MCM-41 and SBA-15 mesoporous materials. In this paper, we report 129Xe chemical shift measurements of xenon adsorbed on mesoporous materials having different porosities and surface composition. The use of hyperpolarized xenon allows us to measure spectra at very low xenon concentration where xenon reflects mainly the interaction between the adsorbed xenon atoms and the surface. The effect of compression on the mesopore structure has also been studied by HP 129Xe NMR. The obtained NMR spectra are interpreted in terms of an exchange between adsorbed xenon in the pores and gazeous Xe atoms in the interparticle spaces. Variable temperature measurments allowed us to obtain information on the heat of adsorption of xenon on the surface and to evaluate the chemical shift of xenon in interaction with the surface.

Chemical shift of hyperpolarized129Xedissolved in liquid nitrogen

We report NMR measurements of hyperpolarized xenon dissolved in liquid nitrogen. The dependence of the ${}^{129}\mathrm{Xe}$ frequency shift on liquid nitrogen temperature was measured along the nitrogen saturated vapor curve from 77 to 93 K. Plotted as a function of the liquid nitrogen density, the chemical shift of xenon is very well described by a simple proportionality relation, with a slope of 0.2135(15) ppm/amagat. The relationship between the chemical shift and the longitudinal spin relaxation is considered in terms of the spin-rotation interaction, and estimates of Xe relaxation time in liquid nitrogen are discussed.

Continuous-flow optical pumping NMR in a closed circuit system.

In a typical continuous-flow optical pumping setup, the chemical shift of xenon in the adsorbed phase depends on the gas flow rate due to warming of the sample surface by the gas stream. Calibration of the system using the (207)Pb resonance of solid lead nitrate is necessary to determine the actual sample temperature. Optimum pulse repetition rates are strongly affected by gas flow and spin-lattice relaxation rates. The interplay of flow and pulse repetition rate alters signal intensity ratios and may lead to the complete suppression of signals.

A129Xe NMR study of highly cadmium-exchanged zeolite Y

The adsorption isotherms of carbon monoxide and xenon as well as the129Xe NMR chemical shifts of xenon in highly (68 and 87%) cadmium-exchanged zeolite NaY were measured. The complete set of experimental data can quantitatively be reproduced with a model that considers localized adsorption of both adsorbate molecules on cadmium and sodium cation sites in the supercages. The concentrations of the supercage cadmium cations as well as their characteristics like adsorption constants for Xe and CO and129Xe NMR chemical shifts were determined.

Grand canonical Monte Carlo simulations of the distribution and chemical shifts of xenon in the cages of zeolite NaA. II. Structure of the adsorbed fluid

The quantitative agreement between the results of a grand canonical Monte Carlo (GCMC) simulation and the various direct experimental measures of the distribution of the Xe atoms between adsorbed phase and gas phase, of intrazeolitic xenon among the alpha cages, and of the distribution of n Xe atoms in a Xen cluster within one alpha cage permit us to consider the structure of the adsorbed fluid in the GCMC simulation as a reasonable description of the actual structure. We provide here the adsorption sites for a single Xe atom in the alpha cage of zeolite NaA, the transition states between these adsorption sites, the one-body distribution functions for the individual clusters Xen inside the alpha cage, the Xe–Xe pair distribution functions for Xe2 through Xe8 at two temperatures, and some of the local minima in the configuration space of the clusters Xe2 through Xe8, i.e., some of the minimum energy configurations of the clusters.

Characterization of CuY zeolites after dehydration, oxidation and reduction with carbon monoxide An adsorption and 129Xe nuclear magnetic resonance spectroscopy study

The adsorption isotherms and 129Xe nuclear magnetic resonance (NMR) chemical shifts of xenon and the adsorption isotherms of carbon monoxide of Cu(II)- and Cu(I)-exchanged zeolites NaY were measured. The former zeolites of 53, 75, and 95% exchange degrees were investigated after various pretreatment steps comprising dehydration, oxidation and reduction with CO at 420°C as well as long-term CO reduction at 470°C. The Cu(I)Y zeolite of 70% exchange degree was prepared via a solid-state exchange procedure with CuCl and subjected to dehydration at 420°C. In all cases, except the dehydrated zeolites, almost linear xenon adsorption isotherms and linear 129Xe NMR chemical shift versus xenon concentration curves running parallel to each other are obtained. In contrast, the chemical shift curves for the dehydrated zeolites are non-linear at low xenon concentrations turning towards negative chemical shift values at very low pressures. The whole body of the experimental xenon data can be explained quantitatively with a unifying approach on the basis of a site adsorption model where the sites are (i) two types of cuprous ions of much different adsorption strength and 129Xe chemical shift, (ii) Na + cations, (iii) Lewis acid sites generated through autoreduction and reduction of Cu 2+ by CO, and (iv) framework sites free of cations. These five types of sites are each characterized by Langmuir adsorption isotherm constants and local 129Xe NMR chemical shifts. The adsorption site concentrations in the various zeolites are evaluated. The supercage Cu(I) concentration values are in nice agreement with the results deduced from the CO adsorption isotherm measurements.

Adsorption and 129Xe n.m.r. of xenon in silver-exchanged Y zeolites: Application to the location of silver cations

The adsorption isotherms and 129Xe n.m.r. chemical shifts of xenon in silver-exchanged yAgNaY zeolites ( y from 5 to 100%) after high-temperature (400°C) dehydration as well as subsequent oxidation were measured at 26°C. A three-site sorption model taking into account adsorbateadsorbate interaction was developed that reproduces simultaneously the adsorption isotherms and the chemical shift vs. concentration curves up to xenon loadings of about 6 sol atoms supercage . The upfield displacements of the chemical shifts in the silver-exchanged zeolites with respect to NaY are attributed to the interaction of xenon with a small portion of the supercage silver cations, probably those unlocalized or residing on crystallographic SIII sites.