129Xe Chemical Shift Research Articles

Characterization of adsorbed xenon in Zeolite-A, X, and Y by high-pressure 129Xe NMR spectroscopy

The local structure of confined xenon in A-, X- and Y-type zeolites (molecular sieve 5A, 13X and zeolite NaY) was investigated by in situ high-pressure 129Xe NMR spectroscopy. Xenon confined in dehydrated samples gives a resonance peak in the chemical shift range from 50 to 100 ppm at 0.1 MPa, and the chemical shift values increase as pressure increases. The pressure dependence of 129Xe chemical shift on xenon confined in the micropores can be approximately described by the Langmuir adsorption model, resulting in the parameters being able to describe pore diameter, adsorption ability of the pore, and the local structure of the confined xenon atoms. Using these parameters, the pore structure, the mobility and the cluster size of xenon in the micropore are discussed for A- and X-type zeolites. Furthermore, the effect of pre-adsorbed water and the binder for pellet preparation on the pore structure is also examined. In the Y-zeolite, a chemical exchange of xenon between the outside and inside of the micropores was observed above 0.75 MPa. Spectral simulation assuming a simple two-site exchange model revealed the exchange rate constant and the population ratio of xenon in the exchange between the outside and the inside the micropores. The exchange of xenon is optimum around the supercritical point of the bulk xenon (∼6 MPa), and decreases above 6 MPa, and this originates from the cooperative phenomenon in the supercritical fluid.

Theoretical calculations of the Xe chemical shifts in cryptophane cages

Toward an understanding of the factors that affect the chemical shift in the Xe nuclear magnetic resonance spectrum of Xe atoms trapped in cages which may have applications as biosensors, we carry out calculations of Xe nuclear magnetic shielding using Hartree–Fock and density functional methods. The resulting values for various Xe positions within the cage can be described by an analytical function of Xe and cage atom coordinates. This shielding function is used in Monte Carlo canonical averaging of a Xe atom within cryptophane cages to investigate the dependence of the Xe chemical shifts on cage size (cryptophane-A versus cryptophane-E), isotopic substitution, and temperature. We compare our theoretical average Xe chemical shifts with the experimental values in four types of cryptophane cages, and with the temperature and isotopic dependence of Xe chemical shifts in cryptophane-A, and achieve a quantitative understanding of the factors that influence the Xe chemical shifts in these cages. The predicted effects on the Xe chemical shifts of mechanical distortion of the cryptophane-A cage provide some insight into the applications of Xe in cages as biosensors.

Intermolecular Interactions of Xe Atoms Confined in One-dimensional Nanochannels of Tris(o-phenylenedioxy)cyclotriphosphazene as Studied by High-pressure 129Xe NMR

The pressure dependence of the 129Xe chemical shift tensor confined in the Tris(o-phenylenedioxy) cyclotriphosphazene (TPP) nanochannel was investigated by high-pressure 129Xe NMR spectroscopy. The observed 129Xe spectrum in the one-dimensional TPP nanochannel (0.45 nm in diameter) exhibits a powder pattern broadened by an axially symmetric chemical shift tensor. As the pressure increases from 0.02 to 7.0 MPa, a deshielding of 90 ppm is observed for the perpendicular component of the chemical shift tensor δ⊥, whereas a deshielding of about 30 ppm is observed for the parallel one, δ‖. This suggests that the components of the chemical shift tensor, δ‖ and δ⊥, are mainly dominated by the Xe-wall and Xe-Xe interaction, respectively. Furthermore, the effect of helium, which is present along with xenon gas, on the 129Xe chemical shift is examined in detail. The average distance between the Xe atoms in the nanochannel is estimated to be 0.54 nm. This was found by using δ⊥ at the saturated pressure of xenon, and comparing the increment of the chemical shift value in δ⊥ to that of a β -phenol/Xe compound.

Morphological and Structural Features of Activated Iron Silicalites: A 129Xe-NMR and EPR Investigation

129Xe-NMR has been employed in the characterization of the activation of two iron silicalites, with different iron loadings. The 129Xe chemical shift recorded as a function of the gas pressure is e...

129Xe-NMR study of free volume in amorphous perfluorinated polymers: comparsion with other methods

The 129Xe-NMR method was used for evaluation of the sizes of free volume elements in amorphous glassy materials—random copolymers of tetrafluoroethylene and perfluorodioxoles of different structures. 129Xe chemical shifts were measured at different pressures up to 1060 Torr. The zero pressure chemical shifts obtained by extrapolation were used for calculation of the diameters of microcavities having spherical and cylindrical shapes. The sizes of microcavities are consistent with those found by means of positron annihilation lifetimes and inverse gas chromatography methods.

Xe@cryptophane Complexes with C2 Symmetry: Synthesis and Investigations by 129Xe NMR of the Consequences of the Size of the Host Cavity for Xenon Encapsulation

AbstractNew cryptophanes‐223 (1), ‐233 (2), and ‐224 (3) with C2 symmetry, bearing different linkers connecting the two cyclotriveratrylene units were synthesized following a multi‐step procedure. The formation of the xenon@cryptophane complexes was investigated by 129Xe NMR spectroscopy. Cryptophanes‐223 (1) and ‐233 (2) complex xenon efficiently in 1,1,2,2‐[D2]tetrachloroethane solution with somewhat lower binding constants (K = 2810 M−1 and 810 M−1 respectively at 278 K) than that observed with cryptophane‐A (K = 3900 M−1). The free and bound guests were observed at room temperature under slow‐exchange conditions on the 129Xe NMR timescale. A linear relationship was observed between the binding constants and the internal volume of the hosts. Under the same conditions, the Xe@cryptophane‐224 [Xe3] complex underwent a fast‐exchange process on the NMR timescale. The decomplexation activation energies Ea, and the associated parameters ΔH‡ and ΔS‡, were determined for the Xe1 and Xe2 complexes from variable temperature 1D‐EXSY experiments. These investigations emphasized the importance of the size of the internal cavity of the cryptophanes for guest encapsulation, and the extreme sensitivity of the 129Xe chemical shift toward slight structural modifications of the atom’s environment. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

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.

Nanoclusters in Nanocages: Platinum Clusters and Platinum Complexes in Zeolite LTL Probed by 129Xe NMR Spectroscopy

Platinum nanoclusters and mononuclear platinum complexes in zeolite KLTL were characterized by 129Xe NMR spectroscopy at temperatures from 100 to 296 K; the chemical shift increased with decreasing temperature, consistent with xenon's increasingly strong interactions with the platinum. The room-temperature chemical shift of xenon characterizing the zeolite containing the platinum clusters was 148.7 ppm, but that of the zeolite containing the platinum complexes was only 98.3 ppm, slightly greater than the value representing the bare zeolite, 95.4 ppm. These data, combined with literature data characterizing similar samples, indicate that the 129Xe chemical shift increases with the size of the metal species in the zeolite, up to the point at which entry of the Xe atom into the space containing the metal is constrained by the geometry of the metal species and the zeolite cage; in the limiting case, the chemical shift is substantially reduced because a Xe atom interacts with an encaged nanocluster only through a cage window.

Detection and Characterization of Xenon-binding Sites in Proteins by 129Xe NMR Spectroscopy

Xenon-binding sites in proteins have led to a number of applications of xenon in biochemical and structural studies. Here we further develop the utility of 129Xe NMR in characterizing specific xenon–protein interactions. The sensitivity of the 129Xe chemical shift to its local environment and the intense signals attainable by optical pumping make xenon a useful NMR reporter of its own interactions with proteins. A method for detecting specific xenon-binding interactions by analysis of 129Xe chemical shift data is illustrated using the maltose binding protein (MBP) from Escherichia coli as an example. The crystal structure of MBP in the presence of 8 atm of xenon confirms the binding site determined from NMR data. Changes in the structure of the xenon-binding cavity upon the binding of maltose by the protein can account for the sensitivity of the 129Xe chemical shift to MBP conformation. 129Xe NMR data for xenon in solution with a number of cavity containing phage T4 lysozyme mutants show that xenon can report on cavity structure. In particular, a correlation exists between cavity size and the binding-induced 129Xe chemical shift. Further applications of 129Xe NMR to biochemical assays, including the screening of proteins for xenon binding for crystallography are considered.

A 129Xe NMR Study of Functionalized Ordered Mesoporous Silica

In this publication we report the first application of continuous flow hyperpolarized (HP) 129Xe techniques to studies of pore structure in ordered mesoporous materials. In particular, we report extensive 129Xe NMR chemical shift measurements of xenon adsorbed in mesoporous materials of different porosities and surface chemical composition. The use of HP Xe has allowed us to work at very low concentrations of xenon where the contribution of the Xe−Xe interactions is negligible and the observed 129Xe chemical shift reflects mainly interactions between the xenon atoms and the surface. Variable temperature measurements gave access to information on the adsorption parameters of xenon, and allowed us to follow the changes in these properties due to modification of the mesopore voids. Our 129Xe NMR data reveal a nonuniform porosity and irregular pore structure in contrast to N2 adsorption and TEM measurements that indicate regular nanoporous channels.

Calculations of Xe line shapes in model nanochannels: Grand canonical Monte Carlo averaging of the Xe129 nuclear magnetic resonance chemical shift tensor

The nuclear shielding of the Xe atom is a tensor molecular electronic property that is a very sensitive indicator of the local environment. Xe atoms in nanochannels of a crystal exhibit anisotropic NMR line shapes that are characteristic of the average shielding tensor; the line shape is a manifestation of the systematic variation of the observed component of the tensor with the orientation of the nanochannel axis in the static uniform external magnetic field. In this paper, a method of calculating the Xe line shapes in nanochannels is presented. The averaging of the shielding tensor is carried out with a grand canonical ensemble at constant (μ, V, T). The line shapes are obtained by assuming a random distribution of orientations of the crystallites within a sample. The equivalent procedure is carried out by finding the component of the Xe shielding tensor along the magnetic field directions selected uniformly on the surface of a sphere. The approach developed here is used to predict the general behavior of Xe line shapes for Xe in elliptical channels of nanoscale dimensions. The channel architecture of crystalline aluminum phosphate ALPO-11 with dimensions 6.7×4.4 Å is used here as a model channel architecture. ALPO-11 is known to impose on Xe atoms an intermolecular NMR shielding response that is highly deshielded compared to a free Xe atom and with a line shape systematically changing with Xe occupancy [J. A. Ripmeester and C. I. Ratcliffe, J. Phys. Chem. 99, 619 (1995)]. In the present work, model channels are constructed with Ne or Ar atoms in the ALPO-11 architecture, and grand canonical Monte Carlo simulations of Xe in these model channels are carried out. The difficulty lies in the construction of the Xe chemical shift tensor for each Xe in the channel at each configuration. We propose a new approach to calculations of the Xe chemical shift tensor in a nanochannel: the additive dimer tensor model. For a model nanochannel constituted entirely of rare gas atoms (Ne, for example) that are located at the crystallographic positions of the atoms constituting the channel walls, the Xe shielding tensor is determined as follows: For a given configuration of Xe atoms within the channel, the Xe shielding tensor of the Jth Xe atom at position (xJ,yJ,zJ) is calculated by a summation over all i of the contribution of XeJ–Nei dimer, the Ne atom located at the ith position, using the ab initio Xe–Ne rare gas dimer shielding tensor. To this is added the Xe–Xe contributions that are calculated by a summation over all L of the contribution of the XeJ–XeL dimer, using the ab initio Xe–Xe dimer shielding tensor. The systematic variations with Xe occupancy of the line shapes obtained from GCMC simulations using the additive dimer tensor model in the model Ne and Ar channels are used to provide general insight into the average Xe shielding tensor in nanochannels. The invariant qualitative aspects of the behavior of Xe line shapes in the model channels provide general predictions independent of the atoms constituting the channel. The chemical shift response of the Xe to the specific atoms constituting the channel walls provides the quantitative details. The specific application to Xe in ALPO-11 crystals compares favorably with experiment.

129Xe NMR of Zeolite NaY in the Inorganic Chemistry Laboratory

129Xe NMR spectroscopy of xenon physisorbed in commercially available zeolite NaY is introduced as an experiment suitable for the inorganic chemistry laboratory in the senior year. The integration of concepts from inorganic and physical chemistry (ideal gas law, gas adsorption, solid state structure, porous materials, NMR spectroscopy) make this experiment also suitable for an integrated laboratory approach. The chemical shift of 129Xe at different loadings in the α-cages of zeolite NaY is used to derive the intrinsic chemical shift by extrapolation to zero loading.

Osmium Carbonyls in Zeolite NaX: Characterization by 129Xe NMR and Extended X-ray Absorption Fine Structure Spectroscopies

Osmium carbonyls synthesized at low loadings in the supercages of zeolite NaX, including mononuclear osmium carbonyls and the following clusters, [HOs 3 (CO) 1 1 ] - , [H 3 Os 4 (CO) 1 2 ] - , and [Os 5 C(CO) 1 4 ] 2 - , were identified by extended X-ray absorption fine structure and infrared spectroscopies. The samples were also characterized by 1 2 9 Xe NMR spectroscopy over the temperature range 100-310 K. The 1 2 9 Xe chemical shifts were greater for samples containing osmium carbonyls than for the bare zeolite over the entire temperature range. At the lowest temperatures, the chemical shifts representing xenon sorbed in the zeolites containing the osmium carbonyl clusters were essentially the same, but at temperatures close to room temperature, the chemical shift increased with decreasing size of the osmium carbonyl. The relatively large chemical shift observed for the smallest osmium carbonyl is consistent with the presence of Xe atoms in the cages with these mononuclear metal complexes, whereas Xe atoms barely fit into the cages with the larger [HOs 3 (CO) 1 1 ] - , [H 3 Os 4 (CO) 1 2 ] - , or [Os 5 C(CO) 1 4 ] 2 - .

Characterization of Microvoids in Glassy Polymers by Means of 129Xe NMR Spectroscopy

129Xe NMR chemical shift of 129Xe sorbed in glassy polymers has been studied in connection with Xe sorption property. Poly(2,6-dimethyl-1,4-phenylene oxide), polystyrene, Bisphenol A polycarbonate, and tetramethyl-Bisphenol A polycarbonate have been used as glassy polymers. Xe sorption isotherms of glassy polymers have been successfully interpreted by the dual-mode sorption model. 129Xe NMR chemical shifts of 129Xe in glassy polymers have shown non-linear low-field shift against sorption amount of Xe. From the Xe density dependence of the 129Xe NMR chemical shift, it has been able to evaluate mean size of microvoids in glassy polymer. These findings suggest that the 129Xe NMR spectroscopy is a good tool for characterizing microvoids in glassy polymers.

Characterization of the effects of nonspecific xenon-protein interactions on (129)Xe chemical shifts in aqueous solution: further development of xenon as a biomolecular probe.

The sensitivity of (129)Xe chemical shifts to weak nonspecific xenon-protein interactions has suggested the use of xenon to probe biomolecular structure and interactions. The realization of this potential necessitates a further understanding of how different macromolecular properties influence the (129)Xe chemical shift in aqueous solution. Toward this goal, we have acquired (129)Xe NMR spectra of xenon dissolved in amino acid, peptide, and protein solutions under both native and denaturing conditions. In general, these cosolutes induce (129)Xe chemical shifts that are downfield relative to the shift in water, as they deshield the xenon nucleus through weak, diffusion-mediated interactions. Correlations between the extent of deshielding and molecular properties including chemical identity, structure, and charge are reported. Xenon deshielding was found to depend linearly on protein size under denaturing solution conditions; the denaturant itself has a characteristic effect on the (129)Xe chemical shift that likely results from a change in the xenon solvation shell structure. In native protein solutions, contributions to the overall (129)Xe chemical shift arise from the presence of weak xenon binding either in cavities or at the protein surface. Potential applications of xenon as a probe of biological systems including the detection of conformational changes and the possible quantification of buried surface area at protein-protein interfaces are discussed.

Spectroscopic Mesopore Size Characterization and Diffusion Measurement in Closed Porosity by Xenon NMR

Nuclear magnetic resonance measurements of 129Xe chemical shifts in a large variety of nonionic calibrated microporous and mesoporous silica glasses with open porosity provide a calibration of spectroscopic mesopore size measurements. The pore size dependence of the xenon chemical shift is quantitatively interpreted by considering both fast exchange and van der Waals interactions at the surface, which cause chemical shifts. The temperature dependence of the chemical shift supports the proposed model and characterizes the enthalpy of adsorption at the pore surface. The xenon NMR measurement is also useful for very low or even quasi-closed porous systems and may provide assessment of the macroscopic xenon diffusion coefficient in xerogels.

Evidence of nonspecific surface interactions between laser-polarized xenon and myoglobin in solution.

The high sensitivity of the magnetic resonance properties of xenon to its local chemical environment and the large (129)Xe NMR signals attainable through optical pumping have motivated the use of xenon as a probe of macromolecular structure and dynamics. In the present work, we report evidence for nonspecific interactions between xenon and the exterior of myoglobin in aqueous solution, in addition to a previously reported internal binding interaction. (129)Xe chemical shift measurements in denatured myoglobin solutions and under native conditions with varying xenon concentrations confirm the presence of nonspecific interactions. Titration data are modeled quantitatively with treatment of the nonspecific interactions as weak binding sites. Using laser-polarized xenon to measure (129)Xe spin-lattice relaxation times (T(1)), we observed a shorter T(1) in the presence of 1 mM denatured apomyoglobin in 6 M deuterated urea (T(1) = 59 +/- 1 s) compared with that in 6 M deuterated urea alone (T(1) = 291 +/- 2 s), suggesting that nonspecific xenon-protein interactions can enhance (129)Xe relaxation. An even shorter T(1) was measured in 1 mM apomyoglobin in D(2)O (T(1) = 15 +/- 0.3 s), compared with that in D(2)O alone (T(1) = 506 +/- 5 s). This difference in relaxation efficiency likely results from couplings between laser-polarized xenon and protons in the binding cavity of apomyoglobin that may permit the transfer of polarization between these nuclei via the nuclear Overhauser effect.

Adsorption of xenon and CH4 mixtures in zeolite NaA. 129Xe NMR and grand canonical Monte Carlo simulations

Investigation of competitive adsorption is carried out using the Xe–CH4 mixture in zeolite NaA as a model system. The Xen clusters are trapped in the alpha cages of this zeolite for times sufficiently long that it is possible to observe individual peaks in the NMR spectrum for each cluster while the CH4 molecules are in fast exchange between the cages and also with the gas outside. The 129Xe nuclear magnetic resonance spectra of nine samples of varying Xe and CH4 loadings have been observed and analyzed to obtain the 129Xe chemical shifts and the intensities of the peaks which are dependent on the average methane and xenon occupancies. The distributions Pn, the fraction of cages containing n Xe atoms, regardless of the number of CH4 molecules are obtained directly from the relative intensities of the Xen peaks. From the observed 129Xe chemical shift of each Xen peak can be obtained the average number of CH4 molecules in the same cavity as n Xe atoms. Grand canonical Monte Carlo (GCMC) simulations of mixtures of Xe and CH4 in a rigid zeolite NaA lattice provide the detailed distributions and the average cluster shifts, as well as the distributions Pn. The agreement with experiment is reasonably good for all nine samples. The calculated absolute chemical shifts for the Xen peaks in all samples at 300 K range from 80 to 230 ppm and are in good agreement with experiment. We also consider a very simple strictly statistical model of a binary mixture, derived from the hypergeometric distribution, in which the component molecules are distinguishable but equivalent in competition for eight lattice sites per cage under mutual exclusion. The latter simple model provides a limiting case for the distributions, with which both the GCMC simulations and the properties of the actual Xe–CH4 system are compared. The ideal adsorbed solution theory gives a first approximation to the selectivity of the adsorption of the Xe and CH4 from a mixture of gases, but starts to fail at high total pressures, especially at low CH4 mole fraction in the bulk.

On the roughness of the internal surface of MCM-41 materials studied by 129Xe NMR

Different MCM-41 samples have been synthesized and characterized by N 2 adsorption–desorption at 77 K, X-ray diffraction and 129Xe-NMR spectroscopy. The unexpectedly high chemical shift of adsorbed xenon corresponds to a stronger xenon–surface interaction in MCM-41 than in zeolites. Coadsorption of water and xenon leads to a decrease in the 129Xe chemical shift, which is interpreted in terms of a screening of the roughness of the surface. Coadsorption of benzene or cyclohexane shows that there are channel restrictions and, more generally, pore structure defects.

Exploring Surfaces and Cavities in Lipoxygenase and Other Proteins by Hyperpolarized Xenon-129 NMR

This paper presents an exploratory study of the binding interactions of xenon with the surface of several different proteins in the solution and solid states using both conventional and hyperpolarized (129)Xe NMR. The generation of hyperpolarized (129)Xe by spin exchange optical pumping affords an enhancement by 3-4 orders of magnitude of its NMR signal. As a result, it is possible to observe Xe directly bound to the surface of micromolar quantities of lyophilized protein. The highly sensitive nature of the (129)Xe line shape and chemical shift are used as indicators for the conditions most likely to yield maximal dipolar contact between (129)Xe nuclei and nuclear spins situated on the protein. This is an intermediate step toward achieving the ultimate goal of NMR enhancement of the binding-site nuclei by polarization transfer from hyperpolarized (129)Xe. The hyperpolarized (129)Xe spectra resulting from exposure of four different proteins in the lyophilized, powdered form have been examined for evidence of binding. Each of the proteins, namely, metmyoglobin, methemoglobin, hen egg white lysozyme, and soybean lipoxygenase, yielded a distinctly different NMR line shape. With the exception of lysozyme, the proteins all possess a paramagnetic iron center which can be expected to rapidly relax the (129)Xe and produce a net shift in its resonance position if the noble gas atom occupies specific binding sites near the iron. At temperatures from 223 to 183 K, NMR signals were observed in the 0-40 ppm chemical shift range, relative to Xe in the gas phase. The signals broadened and shifted downfield as the temperature was reduced, indicating that Xe is exchanging between the gas phase and internal or external binding sites of the proteins. Additionally, conventional (129)Xe NMR studies of metmyoglobin and lipoxygenase in the solution state are presented. The temperature dependence of the chemical shift and line shape indicate exchange of Xe between adsorption sites on lipoxygenase and Xe in the solvent on the slow to intermediate exchange time scale. The NMR results are compared with N(2), Xe, and CH(4) gas adsorption isotherms. It is found that lipoxygenase is unique among the proteins studied in possessing a relatively high affinity for gas molecules, and in addition, demonstrating the most clearly resolved adsorbed (129)Xe NMR peak in the lyophilized state.