Free Xe Research Articles

NMR chemical shift of confined 129Xe: coordination number, paramagnetic channels and molecular dynamics in a cryptophane-A biosensor.

Advances in hyperpolarisation and indirect detection have enabled the development of xenon nuclear magnetic resonance (NMR) biosensors (XBSs) for molecule-selective sensing in down to picomolar concentration. Cryptophanes (Crs) are popular cages for hosting the Xe "spy". Understanding the microscopic host-guest chemistry has remained a challenge in the XBS field. While early NMR computations of XBSs did not consider the important effects of host dynamics and explicit solvent, here we model the motionally averaged, relativistic NMR chemical shift (CS) of free Xe, Xe in a prototypic CrA cage and Xe in a water-soluble CrA derivative, each in an explicit H2O solvent, over system configurations generated at three different levels of molecular dynamics (MD) simulations. We confirm the "contact-type" character of the Xe CS, arising from the increased availability of paramagnetic channels, magnetic couplings between occupied and virtual orbitals through the short-ranged orbital hyperfine operator, when neighbouring atoms are in contact with Xe. Remarkably, the Xe CS in the present, highly dynamic and conformationally flexible situations is found to depend linearly on the coordination number of the Xe atom. We interpret the high- and low-CS situations in terms of the magnetic absorption spectrum and choose our preference among the used MD methods based on comparison with the experimental CS. We check the role of spin-orbit coupling by comparing with fully relativistic CS calculations. The study outlines the computational workflow required to realistically model the CS of Xe confined in dynamic cavity structures under experimental conditions, and contributes to microscopic understanding of XBSs.

Ultrafast Dissociation Dynamics Induced in [Fe(CO)5] n Xe m Mixed Clusters by Resonant Femtosecond Infrared Laser Radiation.

Real-time dissociation dynamics induced in [Fe(CO)5] n Xe m mixed molecular clusters by femtosecond IR radiation in the 5 μm region was studied for the first time by means of time-resolved methods based on resonant excitation of C≡O vibrations in the molecular core of the cluster and photoionization probing (λ = 400 nm) of its decay products. It was found that IR-excited clusters in the initially cold particle beam are heated and dissociated as a result of relaxation processes, giving rise to free neutral Xe aggregates and Fe(CO)5 molecules. Thus, the formed particles are the origin of signal variations from Xe+ and Fe(CO)5 + ions, which grow on a picosecond time scale. It is concluded that the initial laser excitation of C≡O vibrations in clusterized molecules is followed by the process of cluster dissociation accompanied with the formation of free neutral particles according to the hierarchy of binding energies: weakly bound shells of Xe atoms are evaporated first and much faster than the Fe(CO)5 molecules from the cluster core. The characteristic times of relaxation processes as well as the cluster temperature were estimated.

An improved xenon equation of state for nanobubbles in UO2

To understand the thermodynamic behavior of xenon (Xe) bubbles in uranium dioxide (UO2), the pressure of Xe nanobubbles as a function of temperature was systematically investigated using molecular dynamics (MD) simulations at temperatures ranging from 300 to 2100 K. The results show that the maximum pressure decreases with increasing temperature and bubble size. Interestingly, for a critical Xe/Schottky Defect (SD) ratio of 1 to 1.25, within a temperature range from 300 to 1500 K, the increase of pressure attributed to the change of kinetic energy offsets the decrease of pressure due to the change of potential energy with respect to volume, resulting in a roughly constant bubble pressure. The critical Xe/SD ratio slightly decreases with increasing bubble size. As Xe/SD ratios are lower or higher than the critical ratio, the bubble pressure increases or decreases with increasing temperature, respectively. Based on MD simulations of the pressure of unconfined (free) Xe at temperatures of 300, 900, 1500 and 2100 K, a virial equation of state (EOS), which was expanded to the fifth term, is reported for gas, fluid and solid, unconfined Xe at temperatures higher than 300 K. The EOS for free Xe can also be used to assess the pressure of large bubbles in a matrix, since the confinement effect can be negligible. The confinement effect of the UO2 matrix causes an overestimation of Xe density in highly pressurized nanobubbles. Considering the interaction between Xe and the UO2 lattice, the free Xe EOS is modified to calculate the pressure of small intragranular Xe nanobubbles in UO2.

129Xe NMR analysis reveals efficient gas transport between inborn micro-, meso- and macropores in geopolymers

Geopolymers are alumino-silicates with inborn mesoporous structures. For construction applications, the knowledge on the porous structure and fluid dynamics is important. In this work, the interconnected pore structures of metakaolin-based geopolymers and dynamics of guest molecules were studied by 129Xe nuclear magnetic resonance (NMR), by exploiting the high sensitivity of the 129Xe chemical shift to its local environment. The 129Xe NMR spectra revealed three different pore structures experienced by xenon gas in the geopolymers: the first associated with microporous zeolite-analogous cages (≤2 nm), the second with the mesopores (2–50 nm) and the third with free Xe in larger voids. The sizes of mesopores, heat of adsorption as well as pore connectivity were determined as a function of water-to-solid (w/s) ratio. The results show that the pore structures can be tailored by changing the w/s ratio, providing tunability both for potential construction, catalysis, and adsorbent applications.

Xenon Nanobubbles for the Image-Guided Preemptive Treatment of Acute Ischemic Stroke via Neuroprotection and Microcirculatory Restoration.

Early lesion site diagnosis and neuroprotection are crucial to the theranostics of acute ischemic stroke. Xenon (Xe), as a nontoxic gaseous neuroprotectant, holds great promise for ischemic stroke therapy. In this study, Xe-encapsulated lipid nanobubbles (Xe-NBs) have been prepared for the real-time ultrasound image-guided preemptive treatment of the early stroke. The lipids are self-assembled at the interface of free Xe bubbles, and the mean diameter of Xe-NBs is 225 ± 11 nm with a Xe content of 73 ± 2 μL/mL. The in vitro results show that Xe-NBs can protect oxygen/glucose-deprived PC12 cells against apoptosis and oxidative stress. Based on the ischemic stroke mice model, the biodistribution, timely ultrasound imaging, and the therapeutic effects of Xe-NBs for stroke lesions were investigated in vivo. The accumulation of Xe-NBs to the ischemic lesion endows ultrasound contrast imaging with the lesion area. The cerebral blood flow measurement indicates that the administration of Xe-NBs can improve microcirculatory restoration, resulting in reduced acute microvascular injury in the lesion area. Furthermore, local delivery of therapeutic Xe can significantly reduce the volume of cerebral infarction and restore the neurological function with reduced neuron injury against apoptosis. Therefore, Xe-NBs provide a novel nanosystem for the safe and rapid theranostics of acute ischemic stroke, which is promising to translate into the clinical management of stroke.

Direct inner-shell photoionization of Xe atoms embedded in helium nanodroplets

We present the first measurements of photoelectron spectra of atomic clusters embedded in superfluid helium (He) nanodroplets. Owing to the large absorption cross section of xenon (Xe) around 100 eV photon energy (4d inner-shell ionization), direct dopant photoionization exceeds charge transfer ionization via the ionized He droplets. Despite the predominant creation of Xe2+ and Xe3+ by subsequent Auger decay of free Xe atoms, for Xe embedded in He droplets only singly charged , k = 1, 2, 3 fragments are observed. Broad Xe+ ion kinetic-energy distributions indicate Coulomb explosion of the ions due to electron transfer to the primary Auger ions from surrounding neutral atoms. The electron spectra correlated with Xe ions emitted from the He nanodroplets contain a low-energy feature and nearly unshifted Xe photolines. These results pave the way to extreme ultraviolet and x-ray photoelectron spectroscopy of clusters and molecular complexes embedded in He nanodroplets.

Photoionization of Xe and Xe@C60 from the 4d shell in RABBITT fields

We consider photoemission from the $4d$ shell of the free Xe and encapsulated Xe@${\mathrm{C}}_{60}$ atoms by ionizing XUV and probing IR fields typical for a RABBITT (reconstruction of attosecond beating by interference of two-photon transitions) measurement. Our theoretical model is based on the numerical solution of the time-dependent Schr\odinger equation in the single-active-electron approximation. The fullerene ${\mathrm{C}}_{60}$ cage is represented by a finite-width well potential. We test our model against an analogous set of nonrelativistic [Phys. Rev. A 89, 053424 (2014)] and relativistic [Phys. Rev. A 96, 053407 (2017)] calculations for the $4d$ shell of Xe and Xe@${\mathrm{C}}_{60}$ driven by a continuous XUV field. Based on this verification, we make predictions for the total ionization probability, angular anisotropy $\ensuremath{\beta}$ parameter, and the angular-dependent atomic time delay ${\ensuremath{\tau}}_{a}$ from the threshold to several hundred eV of excess energy.

Continuous-wave saturation considerations for efficient xenon depolarization.

The combination of hyperpolarized Xe with chemical exchange saturation transfer (Hyper-CEST) is a powerful NMR technique to detect highly dilute concentrations of Xe binding sites using RF saturation pulses. Crucially, that combination of saturation pulse strength and duration that generates the maximal Hyper-CEST effect is a priori unknown. In contrast to CEST in proton MRI, where the system reaches a steady-state for long saturation times, Hyper-CEST has an optimal saturation time, i.e. saturating for shorter or longer reduces the Hyper-CEST effect. Here, we derive expressions for this optimal saturation pulse length. We also found that a pulse strength, B1, corresponding to five times the Xe exchange rate, k(BA) (i.e. B1 = 5 k(BA)/γ with the gyromagnetic ratio of (129)Xe, γ), generates directly and without further optimization 96% of the maximal Hyper-CEST contrast while preserving spectral selectivity. As a measure that optimizes the amplitude and the width of the Hyper-CEST response simultaneously, we found an optimal saturation pulse strength corresponding to √2 times the Xe exchange rate, i.e. B1=√2k(BA)/γ. When extremely low host concentration is detected, then the expression for the optimum saturation time simplifies as it approaches the longitudinal relaxation time of free Xe.

Encapsulation of xenon by a self-assembled Fe4L6 metallosupramolecular cage.

We report (129)Xe NMR experiments showing that a Fe4L6 metallosupramolecular cage can encapsulate xenon in water with a binding constant of 16 M(-1). The observations pave the way for exploiting metallosupramolecular cages as economical means to extract rare gases as well as (129)Xe NMR-based bio-, pH, and temperature sensors. Xe in the Fe4L6 cage has an unusual chemical shift downfield from free Xe in water. The exchange rate between the encapsulated and free Xe was determined to be about 10 Hz, potentially allowing signal amplification via chemical exchange saturation transfer. Computational treatment showed that dynamical effects of Xe motion as well as relativistic effects have significant contributions to the chemical shift of Xe in the cage and enabled the replication of the observed linear temperature dependence of the shift.

Quantitative chemical exchange saturation transfer with hyperpolarized nuclei (qHyper-CEST): sensing xenon-host exchange dynamics and binding affinities by NMR.

The reversible binding of xenon to host molecules has found numerous applications in nuclear magnetic resonance studies. Quantitative characterization of the Xe exchange dynamics is important to understand and optimize the physico-chemical behavior of such Xe hosts, but is often challenging to achieve at low host concentrations. We have investigated a sensitive quantification technique based on chemical exchange saturation transfer with hyperpolarized nuclei, qHyper-CEST. Using simulated signals we demonstrated that qHyper-CEST yielded accurate and precise results and was robust in the presence of large amounts of noise (10%). This is of particular importance for samples with completely unknown exchange rates. Using these findings we experimentally determined the following exchange parameters for the Xe host cryptophane-A monoacid in dimethyl sulfoxide in one type of experiment: the ratio of bound and free Xe, the Xe exchange rate, the resonance frequencies of free and bound Xe, the Xe host occupancy, and the Xe binding constant. Taken together, qHyper-CEST facilitates sensitive quantification of the Xe exchange dynamics and binding to hydrophobic cavities and has the potential to analyze many different host systems or binding sites. This makes qHyper-CEST an indispensable tool for the efficient design of highly specific biosensors.

Off-center effect on the photoabsorption spectra of encapsulated Xe atoms

The photoabsorption spectra of the Xe atoms encapsulated at different locations of the C${}_{60}$ have been evaluated using the time-dependent density-functional theory. The calculations are performed in the energy region of the Xe $4d$ giant resonance. The results demonstrate that the main confinement resonances can result only when the Xe atom is located within a very small sphere of radius 0.3 \AA{} around the center of the fullerene. The small resonance peak around 100 eV has now been identified as a shape resonance of the free Xe $4d$-$\ensuremath{\epsilon}f$ transition; it is not a confinement resonance.

Probing confinement resonances by photoionizing Xe inside a C60+molecular cage

Double photoionization accompanied by loss of $n$ C atoms ($n=0$, 2, 4, 6) was investigated by merging beams of Xe@C${}_{60}^{+}$ ions and synchrotron radiation and measuring the yields of product ions. The giant $4d$ dipole resonance of the caged Xe atom has a prominent signature in the cross section for these product channels, which together account for 6.2 $\ifmmode\pm\else\textpm\fi{}$ 1.4 of the total Xe $4d$ oscillator strength of 10. Compared to that for a free Xe atom, the oscillator strength is redistributed in photon energy due to multipath interference of outgoing Xe $4d$ photoelectron waves that may be transmitted or reflected by the spherical C${}_{60}^{+}$ molecular cage, yielding so-called confinement resonances. The data are compared with an earlier measurement and with theoretical predictions for this single-molecule photoelectron interferometer system. Relativistic $R$-matrix calculations for the Xe atom in a spherical potential shell representing the fullerene cage show the sensitivity of the interference pattern to the molecular geometry.

Photoabsorption spectrum of the Xe@C60 endohedral fullerene

Photoabsorption spectrum of the Xe@C60 endohedral fullerene has been studied using the time-dependent-density-functional-theory (TDDFT), which represents the dynamical polarizability of an interacting electron system by an off-diagonal matrix element of the resolvent of the Liouvillian superoperator and solves the problem with the Lanczos algorithm. The method has been tested with the photoabsorption spectra for the free Xe atom and C60 fullerene. The result of the Xe atom encapsulated inside C60 confirms the three main peaks observed in the recent measurement in the energy region of the Xe 4d giant resonance and indicates the possibility that the Auger decay of the Xe+ has been greatly suppressed if the ion is encapsulated inside C60. It is suggested to use the current theoretical result around 22 eV to check this possibility.

Enhancement of Double Auger Decay Probability in Xenon Clusters Irradiated with a Soft-X-Ray Laser Pulse

The interaction of large Xe clusters with a soft x-ray laser pulse having a wavelength of 13.9 nm and an intensity of up to 2x10(10) W/cm2 was investigated using a time-of-flight ion mass spectrometer. The corresponding laser photon energy was sufficiently high to photoionize Xe 4d innershell electrons. It was found that Xe3+ ions (which result from double Auger decay of 4d vacancies) became the dominant final ionic product with increasing cluster size and x-ray intensity. This is in contrast to the results of synchrotron radiation experiments involving free Xe atoms, in which Xe2+ is the dominant resultant ion species. Possible mechanisms responsible for the enhancement of the double Auger transition probability in x-ray laser and cluster interaction are discussed.

Size effects in angle-resolved photoelectron spectroscopy of free rare-gas clusters

The photoionization of free Xe clusters is investigated by angle-resolved time-of-flight photoelectron spectroscopy. The measurements probe the evolution of the photoelectron angular distribution parameter as a function of photon energy and cluster size. While the overall photon-energy-dependent behavior of the photoelectrons from the clusters is very similar to that of the free atoms, distinct differences in the angular distribution point at cluster-size-dependent effects. Multiple scattering calculations trace their origin to elastic photoelectron scattering.

Characterization of an organometallic xenon complex using NMR and IR spectroscopy

Photolysis of Re(iPrCp)(CO)2(PF3) in liquid or supercritical Xe yields two new compounds [Re(iPrCp)(CO)2Xe and Re(iPrCp)(CO)(PF3)Xe]. Re(iPrCp)(CO)(PF3)Xe has been characterized by NMR and IR spectroscopies. The compound is an organometallic Xe complex that has been characterized by using NMR spectroscopy and is shown to be longer-lived than other organometallic Xe complexes by IR spectroscopy. 19F, 31P, and 129Xe chemical shifts have been determined. The 129Xe chemical shift of Re(iPrCp)(CO)(PF3)Xe, delta -6,179, is a Xe shift that is significantly shielded, on the order of 1,000 ppm, with respect to free Xe. The coupling constants between coordinated 129Xe and both the 19F and 31P nuclei present have been extracted, confirming the identity of the compound. Observed line widths give a lower limit to the lifetime of the coordinated Xe of 27 ms at 163 K.

Observation of charged excimer complexes radiating in the VUV range in Xe–Ne cryoalloys

Comparative measurements are performed of the cathodoluminescence of Xe–Ne solid alloys and free Xe clusters. A nonelementary band is observed on the low-energy side of the well-known transition in the neutral excimer complex Xe2* (7.1 eV). The structure and intensity of this band depend on Xe concentration. It is concluded on the basis of an analysis of the experimental data for cryocrystals and ionized clusters that this new band is a superposition of the luminescence of homo- and heteronuclear charged excimer complexes. It is shown that the neon matrix can serve as an effective reservoir for accumulation of hole centers and localized electrons.

Molecular dynamics averaging of Xe chemical shifts in liquids

The Xe nuclear magnetic resonance chemical shift differences that afford the discrimination between various biological environments are of current interest for biosensor applications and medical diagnostic purposes. In many such environments the Xe signal appears close to that in water. We calculate average Xe chemical shifts (relative to the free Xe atom) in solution in eleven liquids: water, isobutane, perfluoro-isobutane, n-butane, n-pentane, neopentane, perfluoroneopentane, n-hexane, n-octane, n-perfluorooctane, and perfluorooctyl bromide. The latter is a liquid used for intravenous Xe delivery. We calculate quantum mechanically the Xe shielding response in Xe-molecule van der Waals complexes, from which calculations we develop Xe (atomic site) interpolating functions that reproduce the ab initio Xe shielding response in the complex. By assuming additivity, these Xe-site shielding functions can be used to calculate the shielding for any configuration of such molecules around Xe. The averaging over configurations is done via molecular dynamics (MD). The simulations were carried out using a MD technique that one of us had developed previously for the simulation of Henry's constants of gases dissolved in liquids. It is based on separating a gaseous compartment in the MD system from the solvent using a semipermeable membrane that is permeable only to the gas molecules. We reproduce the experimental trends in the Xe chemical shifts in n-alkanes with increasing number of carbons and the large chemical shift difference between Xe in water and in perfluorooctyl bromide. We also reproduce the trend for a given solvent of decreasing Xe chemical shift with increasing temperature. We predict chemical shift differences between Xe in alkanes vs their perfluoro counterparts.

From localised to delocalised electronic states in free Ar, Kr and Xe clusters

We present new results for the inner valence levels of clusters of the three inert gases Ar, Kr and Xe based on photoelectron spectroscopy studies. The inner valence levels are compared to the localised core levels and to the delocalised outer valence levels. This comparison shows a gradual change from a relatively localised behaviour for Ar inner valence 3s, over the intermediate case of Kr inner valence 4s, to a more delocalised behaviour for Xe inner valence 5s. This change correlates well with the ratio between the orbital sizes and the interatomic distances. The Kr4s intermediate case is found to exhibit characteristics of both localised and delocalised behaviour.

The nuclear magnetic resonance line shapes of Xe in the cages of clathrate hydrates.

We report, for the first time, a prediction of the line shapes that would be observed in the (129)Xe nuclear magnetic resonance (NMR) spectrum of xenon in the cages of clathrate hydrates. We use the dimer tensor model to represent pairwise contributions to the intermolecular magnetic shielding tensor for Xe at a specific location in a clathrate cage. The individual tensor components from quantum mechanical calculations in clathrate hydrate structure I are represented by contributions from parallel and perpendicular tensor components of Xe-O and Xe-H dimers. Subsequently these dimer tensor components are used to reconstruct the full magnetic shielding tensor for Xe at an arbitrary location in a clathrate cage. The reconstructed tensors are employed in canonical Monte Carlo simulations to find the Xe shielding tensor component along a particular magnetic field direction. The shielding tensor component weighted according to the probability of finding a crystal fragment oriented along this direction in a polycrystalline sample leads to a predicted line shape. Using the same set of Xe-O and Xe-H shielding functions and the same Xe-O and Xe-H potential functions we calculate the Xe NMR spectra of Xe atom in 12 distinct cage types in clathrate hydrates structures I, II, H, and bromine hydrate. Agreement with experimental spectra in terms of the number of unique tensor components and their relative magnitudes is excellent. Agreement with absolute magnitudes of chemical shifts relative to free Xe atom is very good. We predict the Xe line shapes in two cages in which Xe has not yet been observed.