129Xe and131Xe NMR of xenon on silica surfaces at 77 K

Little is known about¹²⁹Xe NMR spectral features and spin-lattice relaxation behavior, and the dynamics of xenon atoms, for xenon adsorbed on solid surfaces at cryogenic temperatures (≤77 K), where exchange with gas-phase atoms is not a significant complication. We report¹²⁹Xe NMR experiments at 9,4 T that provide such information for xenon adsorbed onto the hydroxylated surface of a number of microporous silica samples at 77 K. A convenient design for these cryogenic experiments is described. Dynamics of surface-adsorbed xenon atoms on the time scale of seconds can be observed by¹²⁹Xe NMR hole-burning experiments; much slower dynamics occurring over hours and days are evidenced from changes with time of the¹²⁹Xe NMR chemical shifts. The peak maxima occur in the region ca. 180–316 ppm, considerably downfield of¹²⁹Xe shifts previously reported on surfaces at higher temperatures, and closer to the shift of xenon bulk solid (316.4±1 ppm). The¹²⁹Xe spin-lattice relaxation timesT ₁ range over five orders of magnitude; possible explanations for both nonexponential relaxation behavior and extremely shortT ₁ values (35 ms) are discussed. Preliminary¹³¹Xe and¹H NMR results are presented, as well as a method for greatly increasing the sensitivity of¹²⁹Xe NMR detection at low temperatures by using closely-spaced trains of rf pulses.     

NMR of Xe on CO/Ir(1 1 1) and on multilayer Xe/Ir(1 1 1)

Nuclear magnetic resonance (NMR) is performed on monolayer (ML) amounts of adsorbed ¹²⁹Xe on a single crystal substrate. The inherently low sensitivity of NMR is overcome by using highly nuclear spin polarized ¹²⁹Xe that has been produced by optical pumping. A polarization of 0.8 is regularly achieved which is 10⁵ times the thermal (Boltzmann) polarization. The experiments are performed with a constant flux of xenon atoms impinging on the surface, typically 4 ML/s. The chemical shift (σ) of ¹²⁹Xe is highly sensitive to the Xe local environment. We measured profoundly different shifts for the Xe bulk, for the surface of the Xe bulk, and for Xe on CO/Ir(1 1 1). The growth of the bulk is seen in a phase transition like change of σ as a function of temperature at constant Xe flux. At temperatures where no bulk forms at a flux of 4 ML/s, the xenon exchange rate was measured by a spin inversion/recovery method. The exchange time of Xe is found to be 0.24 s at 63.4 K and 64.4 K and somewhat longer at 61.2 K. An analysis is given involving the desorption out of the second layer and fast mixing of first and second layer atoms at these temperatures.     

Production of nitrogen-free, hyperpolarized 129Xe gas

129 Xe with a nuclear polarization far above the thermal equilibrium value (hyperpolarized) is used in NMR studies to increase sensitivity. Gaseous, adsorbed, or dissolved xenon is utilized in physical, chemical, and medical applications. With the aim in mind to study single-crystal surfaces by NMR of adsorbed hyperpolarized ¹²⁹Xe, three problems have to be solved. The reliable production of ¹²⁹Xe with highest nuclear polarization possible, the separation of the xenon gas from the necessary quench gas nitrogen without polarization loss, and the dosing/delivery of small amounts of polarized xenon gas to a sample surface. Here we describe an optical pumping setup that regularly produces xenon gas with a ¹²⁹Xe nuclear polarization of 0.7(±0.07). We show that a freeze–pump–thaw separation of xenon and nitrogen is feasible without a significant loss in xenon polarization. The nitrogen partial pressure can be suppressed by a factor of 400 in a single separation cycle. Dosing is achieved by using the low vapor pressure of a frozen hyperpolarized xenon sample. Received: 12 June 1998     

Observation of micropores in hard-carbon using Xe NMR porosimetry

The existence of micropores and the change of surface structure in pitch-based hard-carbon in xenon atmosphere were demonstrated using ¹²⁹Xe NMR. For high-pressure (4.0 MPa) ¹²⁹Xe NMR measurements, the hard-carbon samples in Xe gas showed three peaks at 27, 34 and 210 ppm. The last was attributed to the xenon in micropores (<1 nm) in hard-carbon particles. The NMR spectrum of a sample evacuated at 773 K and exposed to 0.1 MPa Xe gas at 773 K for 24 h showed two peaks at 29 and 128 ppm, which were attributed, respectively, to the xenon atoms adsorbed in the large pores (probably mesopores) and micropores of hard-carbon. With increasing annealing time in Xe gas at 773 K, both peaks shifted and merged into one peak at 50 ppm. The diffusion of adsorbed xenon atoms is very slow, probably because the transfer of molecules or atoms among micropores in hard-carbon does not occur readily. Many micropores are isolated from the outer surface. For that reason, xenon atoms are thought to be adsorbed only by micropores near the surface, which are easily accessible from the surrounding space.     

纤维碳孔洞形态和质地的~(129)Xe核磁共振研究(英文)

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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.     

129Xe NMR study of the homogeneity and the size distribution of small sodium metal particles in NaY zeolite

NaY zeolite samples loaded with sodium metal by vapor phase deposition have been investigated using¹²⁹Xe NMR spectroscopy. At low sodium concentration, the¹²⁹Xe NMR spectrum showed three resonance lines which clearly indicate the existence of distinct domains in the zeolite sample. Such an observation suggests that the diffusion of the xenon atoms into each domain only occurs with respect to the NMR time scale (2.9 ms). As the sodium concentration increases, observation of a single broad line indicate a macroscopic homogenization of the system. The shift of this line is explained in part due to a paramagnetic interaction between the xenon atoms and the unpaired electrons of particles containing an odd number of sodium atoms. The linewidth is due to the distribution of the local magnetic fields partially averaged by the rapid motion of the xenon atoms and to the statistical distribution of the sodium particles in the supercage cavities. The paramagnetic interaction vanishes with the oxidation of the sample leading to a narrowing and a shift of the line to higher magnetic fields.     

129Xe NMR studies of the dynamics of xenon in siliceous ZSM-12 zeolite

The adsorption of xenon in siliceous zeolite ZSM-12 has been studied by static, magic angle spinning and 2D-EXSY¹²⁹Xe NMR. Anisotropic lines were observed with parameters dependent on the Xe loading and the temperature of the experiment. The observed dependence of the isotropic chemical shift is at variance with the predictions of the mean-free-path model, which casts further doubt on the applicability of this model to the interpretation of Xe NMR data in porous systems. Based on the continuous changes of anisotropic parameters with the loading, we conclude that there are several adsorption sites for xenon in the pores. A qualitative model for the distribution and rapid exchange of the xenon atoms between several sites is discussed. The observed lines arise from a dynamic average of the chemical shift tensors for the different types of site weighted by their populations. 2D-EXSY spectra show two kinds of slow exchange of Xe: (a) particle to particle and, (b) particle to interparticle gas phase.     

Interdependence of in-cell xenon density and temperature during Rb/129Xe spin-exchange optical pumping using VHG-narrowed laser diode arrays

The (129)Xe nuclear spin polarization (P(Xe)) that can be achieved via spin-exchange optical pumping (SEOP) is typically limited at high in-cell xenon densities ([Xe](cell)), due primarily to corresponding reductions in the alkali metal electron spin polarization (e.g. P(Rb)) caused by increased non-spin-conserving Rb-Xe collisions. While demonstrating the utility of volume holographic grating (VHG)-narrowed lasers for Rb/(129)Xe SEOP, we recently reported [P. Nikolaou et al., JMR 197 (2009) 249] an anomalous dependence of the observed P(Xe) on the in-cell xenon partial pressure (p(Xe)), wherein P(Xe) values were abnormally low at decreased p(Xe), peaked at moderate p(Xe) (~300 torr), and remained surprisingly elevated at relatively high p(Xe) values (>1000 torr). Using in situ low-field (129)Xe NMR, it is shown that the above effects result from an unexpected, inverse relationship between the xenon partial pressure and the optimal cell temperature (T(OPT)) for Rb/(129)Xe SEOP. This interdependence appears to result directly from changes in the efficiency of one or more components of the Rb/(129)Xe SEOP process, and can be exploited to achieve improved P(Xe) with relatively high xenon densities measured at high field (including averaged P(Xe) values of ~52%, ~31%, ~22%, and ~11% at 50, 300, 500, and 2000 torr, respectively).     

NMR Study of Laser polarized ~(129) Xe in Low Pressure Natural Xenon Gas

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NMR Study of Laser-polarized 129Xe in Low Pressure Natural Xenon Gas

The NMR signal from the laser-polarized t29 Xe in low-pressure natural xenon gas has been observed with a Bruker WP-80SY NMR spectrometer. The laser-polarized 129 Xe was produced by the method of laser pumping and spin exchange in a magnetic field of 1.87 Tesla. It is obtained experimentally that the nuclear spin relaxation rate 1/T1 of laser-polarized 129Xe are (4.03±1.97)×10-3/see～(2.21±0.78)×10-3/see in the range of the 3.33×103 Pa～8.29×104 Pa Xe gas pressures, the apparent wall relaxation rate 1/Tw* =(1.98±0.18)×10-3/see, and the relaxation rate coefficient C of 133Cs-129Xe spin exchange is (2.81±0.74)×10-16 em3/sec.     

Temperature response of 129Xe depolarization transfer and its application for ultrasensitive NMR detection

Trapping xenon in functionalized cryptophane cages makes the sensitivity of hyperpolarized (HP) 129Xe available for specific NMR detection of biomolecules. Here, we study the signal transfer onto a reservoir of unbound HP xenon by gating the residence time of the nuclei in the cage through the temperature-dependant exchange rate. Temperature changes larger than approximately 0.6 K are detectable as an altered reservoir signal. The temperature response is adjustable with lower concentrations of caged xenon providing more sensitivity at higher temperatures. Ultrasensitive detection of functionalized cryptophane at 310 K is demonstrated with a concentration of 10 nM, corresponding to a approximately 4000-fold sensitivity enhancement compared to conventional detection. This makes HPNMR capable of detecting such constructs in concentrations far below the detection limit of benchtop uv-visible light absorbance.     

Dynamics of confined Xe monolayers adsorbed on the Pt(997) vicinal surface

The dynamics of a confined discrete xenon monolayer adsorbed on the vicinal Pt(997) surface is analyzed within a semi-empirical potential approach. We have investigated two monolayer geometries, namely the commensurate phase and the quasi-hexagonal incommensurate phase, which seem to be stable on the (111) face of Pt, depending on the terrace size. We show that the number and the behavior of localized and side modes characteristic of the confinement are completely different in the two phases. Inelastic helium scattering experiments with a resolution better than 0.2 meV should be an efficient tool to detect the localized modes of the monolayer. This could be an additional way to elucidate the structure of Xe/Pt(997), which has not yet been well established.     

A scanning tunneling microscopy study of the adsorption of Xe on Pt(111) up to one monolayer

The adsorption of Xe on Pt(111) has been investigated from the arrival of the very first atoms up to completion of the monolayer using a variable-temperature Scanning Tunneling Microscope (STM). Surprisingly, in the initial stages of the adsorption Xe preferentially binds to a low coordination site, theupper edge of the platinum steps. The strong binding to these sites leads to a local repulsive interaction with further Xe atoms. Therefore, the Xe atoms located at the upper edge of the steps do not serve as nuclei for 2D Xe islands, which, instead, form on the terraces and at thelower edges of the platinum steps. Only during completion of the monolayer do these islands make contact with the atoms adsorbed at the beginning in the upper-edge positions. The full monolayer exhibits the Hexagonal Incommensurate Rotated (HIR) phase already known from earlier helium-diffraction experiments.     

Thermodynamics of xenon adsorption on Pd(s)[8(100) × (110)]: From steps to multilayers

The thermodynamic properties of the adsorption of xenon on the stepped Pd(s)[8(100)×(110)] surface have been studied over a wide range of pressure (5×10^?11 to 1×10^?4 Torr) and temperature (40–140 K). We have measured adsorption isobars using AES in order to evaluate the surface coverage. By choosing pressure and temperature we have studied under equilibrium conditions, the successive adsorption of xenon on the steps and on the terraces until the first layer is formed, the condensation of the second layer as well as the formation of xenon multilayers. For a small range of pressure and temperature, adsorption takes place only on the atomic steps. The LEED pattern shows that only every other site along the steps is occupied. The extrapolated initial heat of adsorption for steps is E_ad^S = 10.2 kcal/mol, decreasing monotonically by about 2 kcal/mol as the relative coverage of the step sites increases. The dipole moment of the Xe atoms adsorbed on steps is 1.12 D. During adsorption on the terraces the LEED observations suggest that the xenon adlayer is non-localized up to completion of the hexagonally close packed monolayer. The initial heat of adsorption on the terraces, E_ad^T is 8.2 kcal/mol and decreases continuously to a value of 6.9 kcal/mol for a complete monolayer due to lateral repulsive interactions between the adsorbed xenon atoms. The induced dipole moment of Xe on terraces is reduced to 0.49 D. The 5p_{$\text{1}\text{2}$} binding energy of Xe adsorbed on terrace sites is 0.3 eV smaller than that of Xe occuping step sites. The differential molar entropy of the adsorbed layer on the terraces as a function of coverage compares fairly well with the calculated value for an ideally mobile two-dimensional gas. No indication of the growth of two-dimensional xenon islands has been found under these conditions. The isosteric heat of adsorption for the second layer is E_ad^sec = 5.8 kcal/mol independently of the coverage. The condensation of the second layer is a first order two-dimensional gas ? two-dimensional solid phase transition in opposition to the continuous nature of the adsorption of the first layer (extending over a wide range of temperature for a given pressure). The induced dipole moment is further reduced for the Xe second layer to a value of 0.11 D. Finally, the condensation of multilayers proceeds with a latent heat of transformation of E_cond = 3.8 kcal/mol in excellent agreement with the known bulk value for the heat of sublimation of xenon. The line shape of the NVV low energy Auger transitions of xenon or the UPS binding energies of the Xe 5p_{$\text{3}\text{2}$,$\text{1}\text{2}$} spectra allow a clear distinction between first, second and higher layer Xe atoms. We have also established the temperature/pressure conditions for equilibrium between first, second and bulk xenon layers, i.e. a so-called “roughening point”.     

An apparatus for NMR of laser polarized 129Xe on single crystal surfaces

Due to a lack of at least 2 orders of magnitude in the amount of sample nuclei, single crystal surfaces are out of reach for conventional NMR measurements. Our aim is to prove that highly polarized ¹²⁹Xe provides a technique to overcome this restriction. Therefore an apparatus for polarizing ¹²⁹Xe up to 0.7 by spin transfer from optically pumped Rb has been designed as well as an NMR spectrometer in combination with a UHV chamber with sample cleaning, cooling and characterization abilities and a special manifold of glass stopcocks with a liquid nitrogen cooled trap for dosing nitrogen free polarized Xe into the chamber onto the surface. This revised version was published online in August 2006 with corrections to the Cover Date.     

NMR detection using laser-polarized xenon as a dipolar sensor

Hyperpolarized (129)Xe can be used as a sensor to indirectly detect NMR spectra of heteronuclei that are neither covalently bound nor necessarily in direct contact with the Xe atoms, but coupled through long-range intermolecular dipole-dipole interactions. To reintroduce long-range dipolar couplings the sample symmetry has to be broken. This can be done either by using an asymmetric sample arrangement, or by breaking the symmetry of the spin magnetization with field gradient pulses. Experiments are performed where only a small fraction of the available (129)Xe magnetization is used for each point, so that a single batch of xenon suffices for the point-by-point acquisition of a heteronuclear NMR spectrum. Examples with (1)H as the analyte nucleus show that these methods have the potential to obtain spectra with a resolution that is high enough to determine homonuclear J couplings. The applicability of this technique with remote detection is discussed.     

Hyperpolarized (131)Xe NMR spectroscopy

Hyperpolarized (hp) (131)Xe with up to 2.2% spin polarization (i.e., 5000-fold signal enhancement at 9.4 T) was obtained after separation from the rubidium vapor of the spin-exchange optical pumping (SEOP) process. The SEOP was applied for several minutes in a stopped-flow mode, and the fast, quadrupolar-driven T(1) relaxation of this spin I = 3/2 noble gas isotope required a rapid subsequent rubidium removal and swift transfer into the high magnetic field region for NMR detection. Because of the xenon density dependent (131)Xe quadrupolar relaxation in the gas phase, the SEOP polarization build-up exhibits an even more pronounced dependence on xenon partial pressure than that observed in (129)Xe SEOP. (131)Xe is the only stable noble gas isotope with a positive gyromagnetic ratio and shows therefore a different relative phase between hp signal and thermal signal compared to all other noble gases. The gas phase (131)Xe NMR spectrum displays a surface and magnetic field dependent quadrupolar splitting that was found to have additional gas pressure and gas composition dependence. The splitting was reduced by the presence of water vapor that presumably influences xenon-surface interactions. The hp (131)Xe spectrum shows differential line broadening, suggesting the presence of strong adsorption sites. Beyond hp (131)Xe NMR spectroscopy studies, a general equation for the high temperature, thermal spin polarization, P, for spin I ≥ 1/2 nuclei is presented.     

Magnetic resonance imaging of convection in laser-polarized xenon

We demonstrate nuclear magnetic resonance (NMR) imaging of the flow and diffusion of laser-polarized xenon (129Xe) gas undergoing convection above evaporating laser-polarized liquid xenon. The large xenon NMR signal provided by the laser-polarization technique allows more rapid imaging than one can achieve with thermally polarized gas-liquid systems, permitting shorter time-scale events such as rapid gas flow and gas-liquid dynamics to be observed. Two-dimensional velocity-encoded imaging shows convective gas flow above the evaporating liquid xenon, and also permits the measurement of enhanced gas diffusion near regions of large velocity variation.     

Investigating hyperpolarized Xe and CPMAS for spin polarization transfer to surface nuclei: a model study

Several experimental techniques have been developed to utilize spin-polarized xenon gas for sensitivity and selectivity enhancement in surface studies using solid-state NMR. Although previously reported as a viable spin polarization transfer mechanism, the details of high-field cross-polarization (CP) have not been thoroughly investigated. We recently reported observations of CP from an adsorbed layer of hyperpolarized xenon (HP Xe) to a variety of surface nuclei at temperatures as high as 323 K [J. Am. Chem. Soc. 105 (2001) 1412]. In this paper, we investigate many of the issues associated with HP Xe surface CP studies, including polarization transfer kinetics and the effects of temperature on the dynamics. Protonated and methylated silica samples are used as model systems for comparison. A comparison of the rate analysis data from CP and SPINOE (Spin Polarization-Induced Nuclear Overhauser Effect) experiments provides information on the origin of the difference in polarization transfer efficiencies between the two techniques. Lineshape analysis of ¹H spectra for CP and SPINOE experiments demonstrates the difference in selectivity of methods due to longer SPINOE evolution times that lead to greater spin diffusion. The results of this work help to assess the viability of HP Xe CP as a surface analysis technique.