NMR chemical shifts of129Xe dissolved in liquid ethers, secondary amines, and alkyl sulfides

Xenon-129 NMR spectra have been measured for solutions containing Xe dissolved in a variety of linear aliphatic ethers, secondary amines, and alkyl sulfides. The chemical shifts obtained from these data are found to be linearly related to the composition of the solvent molecules expressed as volume fraction of the constituent methyl groups, methylene groups, and the respective oxygen, nitrogen, and sulfur atoms. These results support a model used previously to interpret chemical shifts measured for¹²⁹Xe dissolved in a variety ofn-alkanes and their terminally substituted derivatives. This model treats dispersion force contributions to the chemical shift as resulting from pairwise additive interactions between each dissolved Xe atom and the individual constituent groups of each solvent molecule.     

NMR chemical shifts of129Xe dissolved in liquid haloalkanes and their mixtures

Xenon-129 NMR spectra have been measured for solutions containing Xe dissolved in a variety of linear 1-haloalkanes, α, ω-dihaloalkanes, and their mixtures. The data are found to be linearly related to solvent composition expressed as volume fraction of the constituent methyl groups, methylene groups, and halogen atoms. This behavior is consistent with a model in which the large dispersion force contributions to the chemical shift of¹²⁹Xe result from pair interactions between dissolved Xe atoms and the methyl, and methylene groups, and the halogen atom, from which the solvent molecules are composed.     

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.     

Reaction field model with free volume for the NMR chemical shift of129Xe dissolved in organic solvents

The temperature dependence of the NMR chemical shift of¹²⁹Xe dissolved in liquid alkanes is examined in the context of the reaction field model. An essential feature of the theory is the inclusion of the temperature dependence of the density of thesolvent. The theory of free volume for liquids is incorporated into the reaction field model to account for this temperature dependence. Comparison of the theory with previously reported measurements indicates the sensitivity of the¹²⁹Xe chemical shift to the free volume of liquids. Incorporation of free volume improves the agreement between measurement and theory for branched alkane solvents, and resolves the origin of the 62 ppm intercept in the plot of reaction field as a function of¹²⁹Xe chemical shift for the n-alkanes.     

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.     

Solvent-induced H/D isotope effect on129Xe chemical shifts

An effect of substitution of deuterium for protium in molecules of 17 solvents on^l29Xe NMR shifts was studied. The variation range of solvent-induced isotope xenon shifts σ _s ⁱ exceeds 5 ppm and lies in the range from ?1.18 ppm (in CH₃OD solution) to 3.92 ppm (in D₂O) at 28°C. Experimental results agree with the concept that interaction of xenon atoms with solvent molecules is determined predominantly by dispersion forces. Correlations of σ _s ⁱ with the isotope effect on polarizability, boiling point, and vapor pressure of solvents were established. Anomalous manifestation of properties of aqueous solution was detected.     

129Xe NMR in study of tissues and plants

The high sensitivity of the¹²⁹Xe nuclear magnetic resonance (NMR) chemical shift to the environment was used for characterization of biological tissues and plants. The xenon gas was dissolved under moderate pressure by means of a special device in small pieces of human and animal tissues (heart, muscle, lung, kidney, liver, spleen, brain, sinew, cartilage and hypodermic fat) or plants (leaves, stems, grains, fruits) and the NMR parameters were measured in vitro. The observed line with the chemical shift ～ 180 ppm was attributed to the xenon located in various cellular structures such as lipid shells, intracellular formations. A xenon spectrum in the lungs obtained in vitro coincides with that in the lungs of a mouse measured in vivo by other investigators. The NMR parameters were found to reveal noticeable distinctions between normal and tumour-affected tissues. The analysis of the¹²⁹Xe NMR spectra of the sinew and the cartilage revealed the dependence of the magnetic parameters on the age of the substance. This fact could be accounted for by the changes of the absorption ability of a biological system due to age transformations. The results obtained in comparison with biochemical data reveal the promissory outlook of¹²⁹Xe NMR for the investigation of the state of biological tissues and for medical diagnostics.     

Nuclear magnetic resonance on oriented 11/2− 129mXe and131mXe in Fe

The magnetic hyperfine splitting frequenciesν _M=¦gμ _NB_HF/h.¦ of the 11/2^? isomeric states^129m Xe (T_1/2=8.9d) and^131mXe (T_1/2=11.8d) in Fe were measured with nuclear magnetic resonance on oriented nuclei at temperatures of 10–15 mK as 188.0(1) MHz and 209.8(1) MHz, respectively, the samples being prepared with the technique of recoil implantation after (α, x n) reactions. The magnetic moments of^129m Xe and^131m Xe are deduced to be (?)0.8914(6)μ _N and (?)0.9943(6)μ _N, respectively. The missing γ-anisotropies for allγ-transitions following the decay of 36.4d ¹²⁷Xe indicateI=1/2 for the ground state spin of¹²⁷Xe.     

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.     

Nuclear orientation measurements of the magnetic dipole moments of the 11/2− Isomeric states in129Xe,131Xe and133Xe

The low-temperature nuclear orientation technique was used to measure the magnetic moments of¹²⁹ mXe,¹³¹ mXe and¹³³ mXe implanted in iron by isotope separator. The magnitudes of the magnetic dipole moments, extracted from the gamma-ray anisotropy measurements areμ=?0.80(10)μ _N for^129m Xe,μ=?0.80(10)μ _N for¹³¹Xe andμ=?0.87(12)% for¹³³Xe. The results for these 11/2^? isomers agree with theoretical calculations by the spin polarization procedure using wave functions from the pairing-plus-quadrupole model.     

Magnetic resonance imaging of dissolved hyperpolarized Xe using a membrane-based continuous flow system

A technique for continuous production of solutions containing hyperpolarized ¹²⁹Xe is explored for MRI applications. The method is based on hollow fiber membranes which inhibit the formation of foams and bubbles. A systematic analysis of various carrier agents for hyperpolarized ¹²⁹Xe has been carried out, which are applicable as contrast agents for in vivo MRI. The image quality of different hyperpolarized Xe solutions is compared and MRI results obtained in a clinical as well as in a nonclinical MRI setting are provided. Moreover, we demonstrate the application of ¹²⁹Xe contrast agents produced with our dissolution method for lung MRI by imaging hyperpolarized ¹²⁹Xe that has been both dissolved in and outgassed from a carrier liquid in a lung phantom, illustrating its potential for the measurement of lung perfusion and ventilation.     

129Xe,29Si and27Al NMR studies of dealuminated Y zeolite

The dealumination of a commercial Y zeolite by contact with SiCl₄ is monitored with NMR spectroscopy of²⁹Si,²⁷Al and¹²⁹Xe. The results are interpreted in terms of the changes in the material as silicon is incorporated in the framework and aluminum is deposited in the pores.     

Experiment and dynamic simulations of radiation damping of laser-polarized liquid<Superscript>129</Superscript>Xe at low magnetic field in a flow system

Radiation damping is generally observed when a sample with high spin concentration and high gyromagnetic ratio is placed in a high magnetic field. However, we firstly observed liquid-state¹²⁹Xe radiation damping with laser-enhanced nuclear polarization at low magnetic field in a flow system in which the polarization enhancement factor for the liquid-state¹²⁹Xe was estimated to be 5000, and, furthermore, theoretically simulated the envelopes of the¹²⁹Xe free induction decay and spectral lineshape in the presence of both relaxation and radiation damping with different pulse flip angles and ratios ofT ₂ ^* /T _rd. The radiation damping time constantT _rd of 5 ms was derived on the basis of the simulations. The reasons of depolarization and the further possible improvements were also discussed.     

Application of129Xe NMR spectroscopy and EXAFS to probe the formation of a bimetallic cluster in zeolite Y

Palladium was supported on CaY zeolite by ion exchange of Pd(NH₃) ₄ ²⁺ into CaY zeolite, calcination in O₂ and subsequent reduction with H₂. Platinum and silver were added to this Pd/CaY zeolite by ion exchange of Pt(NH₃) ₄ ²⁺ and Ag⁺, respectively, and subsequent reduction with Hg. Extended X-ray absorption fine structure of these metals indicates that the successive metal-loading treatments lead first to the formation of a 1-nm Pd cluster inside the supercage of CaY zeolite and to the subsequent formation of bimetallic clusters through incorporation of Pt and Ag atoms into the small Pd cluster. The line width and chemical shift of¹²⁹Xe NMR spectrum of xenon gas adsorbed on the zeolite show dramatic changes during the metal clustering procedure, which indicates that¹²⁹Xe NMR spectroscopy can be used to probe the formation of bimetallic clusters.     

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     

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.     

Laser-polarized Xe NMR and MRI at Ultralow Magnetic Fields

Laser-polarized ¹²⁹Xe and a high-T_csuperconducting quantum interference device (SQUID) are used to obtain magnetic resonance images in porous materials at a magnetic field of 2.3 mT, corresponding to a Larmor frequency of 27 kHz. Image resolution of 1 mm is obtained with gradients of only 1 mT/m. The resolution of xenon chemical shifts in different physicochemical environments at ultralow fields is also demonstrated. Details of the circulating flow optical pumping apparatus and the SQUID spectrometer are presented.     

Configuration and Performance of a Mobile 129Xe Polarizer

A stand-alone, self-contained and transportable system for the polarization of ¹²⁹Xe by spin exchange optical pumping with Rb is described. This mobile polarizer may be operated in batch or continuous flow modes with medium amounts of hyperpolarized ¹²⁹Xe for spectroscopic or small animal applications. A key element is an online nuclear magnetic resonance module which facilitates continuous monitoring of polarization generation in the pumping cell as well as the calculation of the absolute ¹²⁹Xe polarization. The performance of the polarizer with respect to the crucial parameters temperature, xenon and nitrogen partial pressures, and the total gas flow is discussed. In batch mode the highest ¹²⁹Xe polarization of P_Xe = 40 % was achieved using 0.1 mbar xenon partial pressure. For a xenon flow of 6.5 and 26 mln/min, P _Xe = 25 % and P _Xe = 13 % were reached, respectively. The mobile polarizer may be a practical and efficient means to make the applicability of hyperpolarized ¹²⁹Xe more widespread.     

Structure and dynamics of radicals in zeolite matrices: ESR and theoretical studies

Amine radical cations of the type R₃N^·+ and [R₃NCH₂]^·+, R=CH₃, C₃H₇, and nitric oxide, NO, have been used to probe the bonding to the surface and the dynamics of the radicals trapped in the confined space of cages or channels in the zeolite. Regular continuous-wave electron spin resonance (ESR) was employed to study the internal motion of the cation radicals formed by γ-irradiation of amines and related ammonium ions, introduced during the synthesis of the zeolites Al-offretite, SAPO-37, SAPO-42 and AlPO₄-5. The ESR spectra of [(CH₃)₃NCH₂]^·+ radical cation in several studied systems changed reversibly with temperature, indicating dynamical effects. Free rotation about the >N?CH₂ bond of the [(CH₃)₃NCH₂]^·+ species was found to occur in the temperature range of 110 to 300 K, while the rotation about the >N?CH₃ bonds was hindered. The observations confirm the theoretical prediction on the basis of density functional theory calculations, which indicate that the corresponding barriers are of the order of 0.3 and 7 kJ/mol, respectively. The radical cations of the type R₃N^·+ with R=C₂H₅, C₃H₇ were found to undergo a different type of dynamics, involving a two-jump process of the methylene hydrogens next to the nitrogen. A cage or channel size effect on the stability and molecular dynamics was inferred in some cases. Pulsed ESR was employed to study the (NO)₂ triplet-state dimers in Na-A type zeolite, with the purpose to resolve the interaction with surface groups, and to elucidate the role of the zeolite on stabilizing the triplet rather than the usual singlet state. Measurements performed at 5 K gave rise to Fourier transform spectra that were assigned to the dimer species interacting with one or more²³Na nuclei, with approximative parameters A(²³Na)=(4.6, 4.6, 8.2) MHz and Q(²³Na)=(?0.3, ?0.3, 0.6) MHz for the hyperfine and nuclear quadrupole coupling tensors, respectively. The values are of similar magnitude as those determined for the NO?Na⁺ complex. The stability of the triplet-state structure was attributed to unusual geometric structure imposed by the zeolite matrix, with the N?O bonds along a line as in [O?N?Na⁺?N?O], which according to UHF ab initio calculations has a triplet ground     

Consequences of Xe–H Cross Relaxation in Aqueous Solutions

We have investigated the transfer of polarization from ¹²⁹Xe to solute protons in aqueous solutions to determine the feasibility of using hyperpolarized xenon to enhance ¹H sensitivity in aqueous systems at or near room temperatures. Several solutes, each of different molecular weight, were dissolved in deuterium oxide and although large xenon polarizations were created, no significant proton signal enhancement was detected in -tyrosine, α-cyclodextrin, β-cyclodextrin, apomyoglobin, or myoglobin. Solute-induced enhancement of the ¹²⁹Xe spin–lattice relaxation rate was observed and depended on the size and structure of the solute molecule. The significant increase of the apparent spin–lattice relaxation rate of the solution phase ¹²⁹Xe by α-cyclodextrin and apomyoglobin indicates efficient cross relaxation. The slow relaxation of xenon in β-cyclodextrin and -tyrosine indicates weak coupling and inefficient cross relaxation. Despite the apparent cross-relaxation effects, all attempts to detect the proton enhancement directly were unsuccessful. Spin–lattice relaxation rates were also measured for Boltzmann ¹²⁹Xe in myoglobin. The cross-relaxation rates were determined from changes in ¹²⁹Xe relaxation rates in the α-cyclodextrin and myoglobin solutions. These cross-relaxation rates were then used to model ¹H signal gains for a range of ¹²⁹Xe to ¹H spin population ratios. These models suggest that in spite of very large ¹²⁹Xe polarizations, the ¹H gains will be less than 10% and often substantially smaller. In particular, dramatic ¹H signal enhancements in lung tissue signals are unlikely.