Xenon NMR Research Articles

Xenon NMR Spectroscopy

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1H and 129Xe NMR absorption line shapes in the presence of highly polarized and concentrated xenon solutions in high magnetic field

The presence of highly concentrated dissolved laser-polarized xenon (∼1 mol/L, polarization up to 0.2) induces numerous effects on proton and xenon NMR spectra. We show that the proton signal enhancements due to 129Xe– 1H cross-relaxation (SPINOE) and overall shifts of the proton resonances due to the average dipolar shift created by the intense xenon magnetization are correlated. Protons behave as very useful sensors of the xenon magnetization. Indeed the xenon resonances exhibit many features such as superimposition of narrow lines on the main resonance due to clustering effects, or such as a polarization-dependent line broadening that is tentatively assigned to the effects of temperature fluctuations that decorrelate some distant dipolar field effects from local interactions, transforming xenon spins from “like” to “unlike” spins. These spectral features make difficult the determination of the average dipolar field by means of the xenon resonance but have interesting consequences on the heteronuclear polarization transfer experiment in Hartmann–Hahn conditions (SPIDER).

Local Structure and Xenon Adsorption Behavior of Metal−Organic Framework System [M2(O2CPh)4(pyz)]n (M = Rh and Cu) As Studied with Use of Single-Crystal X-ray Diffraction, Adsorption Isotherm, and Xenon-129 NMR

The local structure and xenon adsorption behavior of a metal−organic framework system [M(II)2(bza)4(pyz)]n (bza and pyz = benzoate and pyrazine, M = Rh (1a) and Cu (1b)) were investigated by using single-crystal X-ray diffraction, xenon adsorption isotherm, and 129Xe NMR measurements. Single-crystal X-ray diffraction analysis revealed that rare gas atoms were accommodated in a one-dimensional (1D) nanochannel with the dimer structure. Xenon adsorption reached saturation at the xenon uptake of 1.93 Xe per molecular unit in 1a and of 1.85 Xe in 1b, suggesting accommodation of two Xe atoms per host formula unit. Analysis of the xenon adsorption isotherm based on the Fowler−Guggenheim equation gave the xenon−xenon interaction and the isosteric heat of adsorption. The 129Xe NMR spectrum suggested that the environment of the adsorption site for xenon in 1a is very tight and anisotropic. Furthermore, the temperature dependence of the 129Xe chemical shift was explained by using xenon loading, supporting the xenon dimer as the local structure of xenon in 1a. These aspects revealed that cooperative adsorption of xenon to 1a and 1b occurs according to the xenon−xenon interaction. Xenon is stabilized in the nanochannel through formation of a dimer structure.

A single-crystal imprints macroscopic orientation on xenon atoms

A porous single-crystal collects xenon atoms from the gas phase and orients them macroscopically, as highlighted by hyperpolarized xenon NMR.

Characterization of violet emission from Rb optical pumping cells used in laser-polarized xenon NMR experiments

Visible emission from Rb optical pumping cells was characterized under a range of conditions relevant to the production of laser-polarized xenon (including temperature, partial pressures, and D 1-resonant 795 nm laser power). Bright 421 nm (6P → 5S) emission was consistent with energy-pooling processes of the type: Rb ∗(5P 1/2) + Rb ∗(5P 1/2,3/2) → Rb ∗(6P 1/2,3/2) + Rb(5S 1/2), with processes transiting through 5D states likely contributing at higher temperatures/lower N 2 partial pressures. Under such conditions a number of Rb lines may be observed, indicating population of Rb states to ⩾9D (∼31 822 cm −1). Such energies exceed those required for efficient production of laser-induced plasma.

Study of the Hydrophobic Cavity of β‐Cryptogein through Laser‐Polarized Xenon NMR Spectroscopy

The interaction of xenon with beta-cryptogein, a basic 10 kDa protein belonging to the elicitin family, has been studied by using dissolved thermal and laser-polarized gas in liquid-state NMR. 13C and 1H chemical-shift-mapping experiments were unfruitful, the proton lines only experienced a slight narrowing but no significant frequency variation when the xenon concentration was increased. Nevertheless magnetization transfer from hyperpolarized xenon to protons of the protein demonstrates an undoubted interaction and enables localization of the noble-gas-binding site. Due to the proton-proton cross-relaxation efficiency, however, this experiment is subjected to important spin-diffusion. An automatic procedure that takes spin-diffusion into account when assigning the protons that interact with xenon is then used. The binding site, as defined by 30 Xe--H interactions, is situated in the inner core of the protein. The protons that interact with xenon border the channel by which sterols are known to enter into the cavity. These results support the idea that xenon is a good probe for hydrophobic protein regions.

Nanoporosity of an organo-clay shown by hyperpolarized xenon and 2D NMR spectroscopy

Interlayer nanoporosity of hectorite pillared by tetraethylammonium ions is explored by hyperpolarized xenon NMR and relevant gases such as carbon dioxide revealing the adsorption capacity of the open galleries.

Xenon NMR as a Probe for Microporous and Mesoporous Solids, Polymers, Liquid Crystals, Solutions, Flames, Proteins, Imaging

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Determination of Pore Sizes and Volumes of Porous Materials by 129Xe NMR of Xenon Gas Dissolved in a Medium

In our previous paper (J. Phys. Chem. B 2005, 109, 757) it was illustrated that the 129Xe NMR spectra of xenon dissolved in acetonitrile confined into mesoporous materials give detailed information on the system, especially about the pore sizes. A resonance signal originating from xenon atoms sited in very small cavities built up inside the pores during the freezing transition (referred to as signal D) turned out to be highly sensitive to the pore size. The emergence of this signal reveals the phase transition temperature of acetonitrile inside the pores, which can also be used to determine the size of the pores. In addition, the difference in the chemical shifts of two other signals arising from xenon dissolved in bulk and confined acetonitrile (B and C) provides another method for determining the pore sizes. In the present work, the observed correlations have been investigated using an extensive set of measurements with a variety of porous materials (silica gels and controlled pore glasses) with the mean pore diameters ranging from 43 to 2917 A. The usefulness of the correlations has been demonstrated by calculating the pore size distributions from the spectral data. The distributions are in agreement with those reported by the manufacturers, when the mean pore diameter is smaller than approximately 500 A. In addition, it has been shown that the porosity of the materials can be determined by comparing the intensities of the signals arising from the bulk and confined liquid. When acetonitrile is replaced by cyclohexane in the sample, the dependence of the chemical shift difference between the B and C signals on the pore size becomes more sensitive, but no D signal appears below the freezing point. In addition, the influence of xenon gas on the melting points of bulk and confined acetonitrile has been studied by 1H NMR cryoporometry. The measurements show that the temperature of the latter transition lowers slightly more, and consequently affects the pore sizes calculated by means of the difference in the phase transition temperatures. Hysteresis in the phase transitions in a cooling-warming cycle has also been studied as a function of the temperature stabilization time by 129Xe NMR of xenon dissolved in acetonitrile.

Analysis of porosity in porous silicon using hyperpolarized 129Xe two-dimensional exchange experiments

The porosity in porous silicon was characterized using hyperpolarized (HP) xenon as a probe. HP xenon under conditions of continuous flow allows for the rapid acquisition of xenon NMR spectra that can be used to characterize a variety of materials. Two-dimensional exchange spectroscopy (EXSY) 129Xe NMR experiments using HP xenon were performed to obtain exchange pathways and rates of xenon mobility between pores of different dimensions within the structure of porous silicon and to the gas phase above the sample. Pore sizes are estimated from chemical shift information and a model for pore geometry is presented.

Xenon NMR measurements of permeability and tortuosity in reservoir rocks

In this work we present measurements of permeability, effective porosity and tortuosity on a variety of rock samples using NMR/MRI of thermal and laser-polarized gas. Permeability and effective porosity are measured simultaneously using MRI to monitor the inflow of laser-polarized xenon into the rock core. Tortuosity is determined from measurements of the time-dependent diffusion coefficient using thermal xenon in sealed samples. The initial results from a limited number of rocks indicate inverse correlations between tortuosity and both effective porosity and permeability. Further studies to widen the number of types of rocks studied may eventually aid in explaining the poorly understood connection between permeability and tortuosity of rock cores.

Development of a Functionalized Xenon Biosensor

NMR-based biosensors that utilize laser-polarized xenon offer potential advantages beyond current sensing technologies. These advantages include the capacity to simultaneously detect multiple analytes, the applicability to in vivo spectroscopy and imaging, and the possibility of "remote" amplified detection. Here, we present a detailed NMR characterization of the binding of a biotin-derivatized caged-xenon sensor to avidin. Binding of "functionalized" xenon to avidin leads to a change in the chemical shift of the encapsulated xenon in addition to a broadening of the resonance, both of which serve as NMR markers of ligand-target interaction. A control experiment in which the biotin-binding site of avidin was blocked with native biotin showed no such spectral changes, confirming that only specific binding, rather than nonspecific contact, between avidin and functionalized xenon leads to the effects on the xenon NMR spectrum. The exchange rate of xenon (between solution and cage) and the xenon spin-lattice relaxation rate were not changed significantly upon binding. We describe two methods for enhancing the signal from functionalized xenon by exploiting the laser-polarized xenon magnetization reservoir. We also show that the xenon chemical shifts are distinct for xenon encapsulated in different diastereomeric cage molecules. This demonstrates the potential for tuning the encapsulated xenon chemical shift, which is a key requirement for being able to multiplex the biosensor.

Synthesis and dissolved hyperpolarized xenon NMR studies of sucrose octaoleate-F 104

Sucrose octaoleate-F 104 has been prepared from sucrose and the acid chloride of R F-oleic acid-F 13 in 86% yield using dry pyridine as solvent. The product has been characterized by MALDI-TOF mass spectrometry and 1 H and 13 C NMR methods. Some preliminary 129 Xe NMR results are included using hyperpolarized xenon gas dissolved in an aqueous emulsion containing egg yolk phospholipid and the octaester. The in vitro spin lattice relaxation time of dissolved xenon as well as the 129 Xe chemical shift suggest that highly fluorinated sucrose polyesters (HFSPE’s) may be promising molecules for use as part of in vivo delivery systems for hyperpolarized xenon.

Mapping hydrophobic molecular regions using dissolved laser-polarized xenon NMR

Molecular hydrophobic cavities can be mapped thanks to the detection of magnetization transfer from laser polarized xenon to nearby protons. This so called SPINOE approach is described. The study of the spin dynamics during this experiment and its consequences on the practical implementation are detailed. We show that thanks to the knowledge of the physical properties of the system, it becomes possible to choose the best experimental conditions in order to be able to assign magnetization transfer through two dimensional NMR methods. As an illustration, the first 2D SPIROE-TOCSY experiment is reported. To cite this article: L. Dubois et al., C. R. Physique 5 (2004).

In situ NMR spectroscopy of combustion.

The first successful in situ studies of free combustion processes by one- and two-dimensional NMR spectroscopy are reported, and the feasibility of this concept is demonstrated. In this proof-of-principle work, methane combustion over a nanoporous material is investigated using hyperpolarized (hp)-xenon-129 NMR spectroscopy. Different inhomogeneous regions within the combustion cell are identified by the xenon chemical shift, and the gas exchange between these regions during combustion is revealed by two-dimensional exchange spectra (EXSY). The development of NMR spectroscopy as an analytical tool for combustion processes is of potential importance for catalyzed reactions within opaque media that are difficult to investigate by other techniques.

Solvation, interaction and dynamics of xenon atoms in HPLC column materials studied by variable-temperature dependent 129Xe, 1H– 129Xe cross-polarization, and two-dimensional exchange NMR experiments

Xenon NMR is a useful method for probing structure and dynamics of micro-porous materials due to the sensitivity of xenon’s chemical shifts to its local interactions, and the diffusion property of xenon atoms. Here, we report a study of solvation, interaction and diffusion of xenon atoms inside the HPLC column materials, Zorbax SB-C18 and XDB-C18 which were made of siloxane surface coatings of porous silica, by variable-temperature dependent (VT) 129Xe, 1H– 129Xe cross-polarization (CP), and two-dimensional exchange (2D EXSY) NMR experiments. The VT NMR experiment showed the solvation and dynamics of xenon atoms in the column materials. The CP experiment at low temperature provided evidence for probing the direct interaction of xenon atoms with the hydrocarbon chains of the stationary phase, and helped for assigning the 129Xe peaks in the VT NMR spectra. The 2D EXSY NMR experimental result showed the diffusion of xenon atoms within the accessible spaces in the column materials. Combined with our previous study, a full picture of xenon’s behavior inside the column materials has been described. This study provides a basic understanding of xenon NMR of the column materials, which enables us to conduct further investigation of retention mechanisms of column materials in terms of molecular interaction and diffusion by xenon NMR method.

Amplification of xenon NMR and MRI by remote detection.

A technique is proposed in which an NMR spectrum or MRI is encoded and stored as spin polarization and is then moved to a different physical location to be detected. Remote detection allows the separate optimization of the encoding and detection steps, permitting the independent choice of experimental conditions and excitation and detection methodologies. In the initial experimental demonstration of this technique, we show that taking dilute 129Xe from a porous sample placed inside a large encoding coil and concentrating it into a smaller detection coil can amplify NMR signal. In general, the study of NMR active molecules at low concentration that have low physical filling factor is facilitated by remote detection. In the second experimental demonstration, MRI information encoded in a very low-field magnet (4-7 mT) is transferred to a high-field magnet (4.2 T) to be detected under optimized conditions. Furthermore, remote detection allows the utilization of ultrasensitive optical or superconducting quantum interference device detection techniques, which broadens the horizon of NMR experimentation.

Ordering and Clathrate Hydrate Formation in Co‐deposits of Xenon and Water at Low Temperatures

AbstractLocal ordering in co‐deposits of water and xenon atoms produced at low temperatures can be followed uniquely by 129Xe NMR spectroscopy. In water‐rich samples deposited at 10 K and observed at 77 K, xenon NMR results show that there is a wide distribution of arrangements of water molecules around xenon atoms. This starts to order into the definite coordination for the structure I, large and small cages, when samples are annealed at ∼140 K, although the process is not complete until a temperature of 180 K is reached, as shown by powder Xray diffraction. There is evidence that Xe⋅20 H2O clusters are prominent in the early stages of crystallization. In xenon‐rich deposits at 77 K there is evidence of xenon atoms trapped in Xe⋅20 H2O clusters, which are similar to the small hydration shells or cages observed in hydrate structures, but not in the larger water clusters consisting of 24 or 28 water molecules. These observations are in agreement with results obtained on the formation of Xe hydrate on the surface of ice surfaces by using hyperpolarized Xe NMR spectroscopy. The results indicate that for the various different modes of hydrate formation, both from Xe reacting with amorphous water and with crystalline ice surfaces, versions of the small cage are important structures in the early stages of crystallization.

The potential of the xenon "spin-spy" methodology for the study of configurational equilibria in solution.

Xenon, a gas of rare ability: The mutarotation of D-glucose (see structure) was monitored using 129Xe NMR spectroscopy. This is the first example of the use of xenon NMR spectroscopy to probe a configurational equilibrium in solution of a system which does not complex xenon.

Low-Frequency NMR of Laser-Polarized Xenon on a Liquid Surface

We have optically polarized the nuclear spin of xenon atoms using a frequency-narrowed high-power laser-diode. The free-induction signals of polarized xenon in gas and liquid phases were measured in a magnetic field of less than 3 mT. The polarized xenon plays the role of a probe atom sensitive to the local environment. For example, we could detect the atoms, selectively, on or below the surface of liquid ethanol utilizing chemical shift imaging. The free-induction signal is so large that we can track the polarized xenon atoms dissolving into liquid ethanol through its surface. We also observed the spin-polarization of protons of ethanol transferred from polarized xenon.