Rotational spectra of the Xe-(H2O)2 van der Waals trimer: xenon as a probe of electronic structure and dynamics

Rotational spectra of three isotopomers of the Xe-(H2O)2 van der Waals trimer were recorded using a pulsed-nozzle, Fourier transform microwave spectrometer. Nine [nine, four] a-type and twelve [eleven, seven] b-type transitions were measured for the 132Xe-(H2O)2 [129Xe-(H2O)2, 131Xe-(H2O)2] isotopomer. The determined rotational and centrifugal distortion constants were used to extract information about the structure and vibrational motions of the complex. The nuclear quadrupole hyperfine structures due to the 131Xe (nuclear spin quantum number I=3/2) nucleus were also detected. The large value of the off-diagonal nuclear quadrupole coupling constant chiab in particular provides detailed insight into the electronic environment of the xenon atom and the orientations of the water molecules within the complex. An effective structure that best reproduces the experimental 131Xe nuclear quadrupole coupling constants is rationalized by ab initio calculations. An overall goal of this line of work is to determine how the successive solvation of a xenon atom with water molecules affects the xenon electron distribution and its intermolecular interactions. The results may provide molecular level interpretations of 129Xe NMR data from, for example, imaging experiments.     

Solid-state 129Xe and 131Xe NMR study of the perxenate anion XeO(6)4-

Results of the first solid-state 131Xe NMR study of xenon-containing compounds are presented. The two NMR-active isotopes of xenon, 129Xe (I=1/2) and 131Xe (I=3/2), are exploited to characterize the xenon magnetic shielding and quadrupolar interactions for two sodium perxenate salts, Na4XeO6.xH2O (x=0, 2), at an applied magnetic field strength of 11.75 T. Solid-state 129/131Xe NMR line shapes indicate that the local xenon environment in anhydrous Na4XeO6 adopts octahedral symmetry, but upon hydration, the XeO6(4-) anion becomes noticeably distorted from octahedral symmetry. For stationary, anhydrous samples of Na4XeO6, the heteronuclear 129/131Xe-23Na dipolar interaction is the principal contributor to the breadth of the 129/131Xe NMR lines. For stationary and slow magic-angle-spinning samples of Na4XeO(6).2H2O, the anisotropic xenon shielding interaction dominates the 129Xe NMR line shape, whereas the 131Xe NMR line shape is completely dominated by the nuclear quadrupolar interaction. The xenon shielding tensor is approximately axially symmetric, with a skew of -0.7+/-0.3, an isotropic xenon chemical shift of -725.6+/-1.0 ppm, and a span of 95+/-5 ppm. The 131Xe quadrupolar coupling constant, 10.8+/-0.5 MHz, is large for a nucleus at a site of approximate Oh symmetry, and the quadrupolar asymmetry parameter indicates a lack of axial symmetry. This study demonstrates the extreme sensitivity of the 131Xe nuclear quadrupolar interaction to changes in the local xenon environment.     

Theory of stochastic dipolar recoupling in solid-state nuclear magnetic resonance

Dipolar recoupling techniques in solid-state nuclear magnetic resonance (NMR) consist of radio frequency (rf) pulse sequences applied in synchrony with magic-angle spinning (MAS) that create nonzero average magnetic dipole-dipole couplings under MAS. Stochastic dipolar recoupling (SDR) is a variant in which randomly chosen rf carrier frequency offsets are introduced to cause random phase modulations of individual pairwise couplings in the dipolar spin Hamiltonian. Several aspects of SDR are investigated through analytical theory and numerical simulations: (1) An analytical expression for the evolution of nuclear spin polarization under SDR in a two-spin system is derived and verified through simulations, which show a continuous evolution from coherent, oscillatory polarization exchange to incoherent, exponential approach to equilibrium as the range of random carrier offsets (controlled by a parameter f(max)) increases; (2) in a many-spin system, polarization transfers under SDR are shown to be described accurately by a rate matrix in the limit of large f(max), with pairwise transfer rates that are proportional to the inverse sixth power of pairwise internuclear distances; (3) quantum mechanical interferences among noncommuting pairwise dipole-dipole couplings, which are a complicating factor in solid-state NMR studies of molecular structures by traditional dipolar recoupling methods, are shown to be absent from SDR data in the limit of large f(max), provided that coupled nuclei have distinct NMR chemical shifts.     

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.     

Frequency-selective homonuclear dipolar recoupling in solid state NMR

We introduce a new approach to frequency-selective homonuclear dipolar recoupling in solid state nuclear magnetic resonance (NMR) with magic-angle spinning (MAS). This approach, to which we give the acronym SEASHORE, employs alternating periods of double-quantum recoupling and chemical shift evolution to produce phase modulations of the recoupled dipole-dipole interactions that average out undesired couplings, leaving only dipole-dipole couplings between nuclear spins with a selected pair of NMR frequencies. In principle, SEASHORE is applicable to systems with arbitrary coupling strengths and arbitrary sets of NMR frequencies. Arbitrary MAS frequencies are also possible, subject only to restrictions imposed by the pulse sequence chosen for double-quantum recoupling. We demonstrate the efficacy of SEASHORE in experimental (13)C NMR measurements of frequency-selective polarization transfer in uniformly (15)N, (13)C-labeled L-valine powder and frequency-selective intermolecular polarization transfer in amyloid fibrils formed by a synthetic decapeptide containing uniformly (15)N, (13)C-labeled residues.     

Distribution and modification of sorption sites in amphiphilic calixarene-based solid lipid nanoparticles from hyperpolarized 129Xe NMR spectroscopy

The site distribution and accessibility in amphiphilic calixarenes-based solid lipid nanoparticles were determined as a function of lipid chain length using in situ 129Xe NMR spectroscopy with flowing hyperpolarized Xe gas. The study illustrates that host cavities in as-prepared materials are increasingly occluded by the lipid chain for compounds with chain lengths from C6 to C12 and are almost completely occluded for C14 and C16 chain lengths. Host cavities present at the surface of the particles are still accessible to small atoms (xenon) and organic molecules (methylene chloride, etc). The Xe spectra show that the accessible void space can be increased remarkably by exposure of the particle surface to suitably sized guest molecules that appear to displace the occluding hydrocarbon chains from the host cavities by competitive adsorption. This postsynthesis treatment thus modifies the state of self-assembly and improves sorption capability. The HP Xe NMR approach presented is suitable for small samples (a few milligrams) of SLNs, likely also for other biomaterials such as vesicles, model membranes, etc.     

Determination of long-range distances and dynamic order parameters by dipolar recoupling in high-resolution magic-angle spinning NMR spectroscopy

By introducing dipolar recoupling methods to high-resolution magic-angle spinning (HRMAS) NMR spectroscopy, a class of experiments has been delevoped that allows the measurement of residual dipole-dipole couplings of approximately 1 Hz in weakly immobilized molecules. Using homonuclear 1H-1H recoupling, distances of up to approximately 8 A can be selectively determined, while heteronuclear 1H-13C recoupling provides access to dynamic order parameters of individual molecular segments on the order of approximately 10-3. The experiments are demonstrated on functionalized oligopeptides that are attached to polymer resins.     

129Xe NMR study of the localization of PdCl2 supported on carbon nanotubes

Localization of PdCl₂ clusters supported on multi-wall carbon nanotubes (MWCNT) has been investigated using ¹²⁹Xe NMR of adsorbed xenon. As-made MWCNTs with channels initially inaccessible for adsorption and ball-milled MWCNTs with the totally accessible internal surface were used as supports. The observed ¹²⁹Xe NMR spectra were determined by the dynamics of xenon exchange between the aggregate pores and nanotube channels. No considerable changes of the ¹²⁹Xe NMR spectrum with the concentration of supported PdCl₂ were observed for the as-made MWCNT, while an additional resonance appeared for the ball-milled nanotubes. The ¹²⁹Xe NMR experiments evidenced the supported species to be localized on the internal surface of the ball-milled MWCNT.     

Understanding a Host–Guest Model System through 129Xe NMR Spectroscopic Experiments and Theoretical Studies

Gaining an understanding of the nature of host–guest interactions in supramolecular complexes involving heavy atoms is a difficult task. Described herein is a robust simulation method applied to complexes between xenon and members of a cryptophane family. The calculated chemical shift of xenon caged in a H₂O₂ probe, as modeled by quantum chemistry with complementary‐orbital, topological, and energy‐decomposition analyses, is in excellent agreement with that observed in hyperpolarized ¹²⁹Xe NMR spectra. This approach can be extended to other van der Waals complexes involving heavy atoms.     

Simulations of 129Xe NMR chemical shift of atomic xenon dissolved in liquid benzene

The isotropic ¹²⁹Xe NMR chemical shift of atomic Xe dissolved in liquid benzene was simulated by combining classical molecular dynamics and quantum chemical calculations of ¹²⁹Xe nuclear magnetic shielding. Snapshots from the molecular dynamics trajectory of xenon atom in a periodic box of benzene molecules were used for the quantum chemical calculations of isotropic ¹²⁹Xe chemical shift using nonrelativistic density functional theory as well as relativistic Breit?CPauli perturbation corrections. Thus, the correlation and relativistic effects as well as the temperature and dynamics effects could be included in the calculations. Theoretical results are in a very good agreement with the experimental data. The most of the experimentally observed isotropic ¹²⁹Xe shift was recovered in the nonrelativistic dynamical region, while the relativistic effects explain of about 8% of the total ¹²⁹Xe chemical shift.     

Probing polymer colloids by Xe NMR

Model aqueous dispersions of polystyrene, poly(methyl methacrylate), poly(n-butyl acrylate) and a statistical copolymer poly(n-butyl acrylate-co-methyl methacrylate) were studied using xenon NMR spectroscopy. The ¹²⁹Xe NMR spectra of these various latexes reveal qualitative and quantitative differences in the number of peaks and in their line widths and chemical shifts. Above the glass transition temperature, exchange between xenon sorbed in the particle core and free xenon outside the particles is fast on the ¹²⁹Xe spectral time-scale and a single ¹²⁹Xe signal is observed. At temperatures below the glass transition temperature, the exchange between sorbed and free xenon is slow on the ¹²⁹Xe spectral time-scale and two ¹²⁹Xe NMR signals can be observed. If the signal of sorbed ¹²⁹Xe is observed, its chemical shift, line width and integral relative to the integral of free ¹²⁹Xe can be used for the characterization of the particle core. The line width of free ¹²⁹Xe provides the residence time of xenon outside the particles and can be used to determine the rate constant characterizing the kinetics of penetration of xenon in the particles. This rate constant emerges as promising parameter for the characterization of the polymer particle surface.     

Spectroscopic study of the Ne-Xe-NH3 van der Waals trimer

Rotational spectra of the Ne-Xe-NH3 van der Waals trimer were recorded using a pulsed-nozzle, Fourier transform microwave spectrometer. Both a- and b-type transitions of eight isotopologues, namely 20Ne-132Xe-14NH3, 20Ne-129Xe-14NH3, 20Ne-132Xe-15NH3, 20Ne-129Xe-15NH3, 20Ne-131Xe-15NH3, 22Ne-132Xe-15NH3, 22Ne-129Xe-15NH3, and 22Ne-131Xe-15NH3 were measured and assigned. Nuclear quadrupole hyperfine structures arising from the 14N (nuclear spin quantum number I = 1) and 131Xe (I = 3/2) nuclei were detected and analyzed. The determined rotational constants were used to fit structural parameters. A harmonic force field analysis was performed based on centrifugal distortion constants to extract information about vibrational motions of the complex. A comparison of van der Waals bond lengths and stretching force constants between the Ne-Xe-NH3 trimer and the corresponding dimers indicates that non-additive three-body effects are present in the trimer system. Analyses of the 14N and 131Xe nuclear quadrupole coupling constants suggest that the NH3 unit undergoes nearly free internal rotation within the complex and that the presence of Ne has little effect on the orientation of NH3 with respect to the Xe atom.     

Broadband rotational resonance in solid state NMR spectroscopy

A new technique for restoring nuclear magnetic dipole-dipole couplings under magic-angle spinning (MAS) in solid state nuclear magnetic resonance (NMR) spectroscopy is described and demonstrated. In this technique, called broadband rotational resonance (BroBaRR), the coupling between a pair of nuclear spins with NMR frequency difference close (but not necessarily equal) to the MAS frequency is restored by the application of a train of weak radio-frequency pulses at a carrier frequency close to the average of the two NMR frequencies. Phase or amplitude modulation of the pulse train at half the MAS frequency splits the carrier into sidebands close to the two NMR frequencies. The pulse train then removes offsets from the exact rotational resonance condition, leading to dipolar recoupling over a bandwidth controlled by the amplitude of the pulse train. (13)C NMR experiments on uniformly (15)N,(13)C-labeled L-valineHClH(2)O powder validate the theoretical analysis. BroBaRR will be useful in studies of molecular structures by solid state NMR, for example in the detection of long-range couplings between carbons in uniformly labeled organic and biological materials.     

129Xe and 131Xe nuclear magnetic dipole moments from gas phase NMR spectra

³He, ¹²⁹Xe and ¹³¹Xe NMR measurements of resonance frequencies in the magnetic field B₀ = 11.7586 T in different gas phase mixtures have been reported. Precise radiofrequency values were extrapolated to the zero gas pressure limit. These results combined with new quantum chemical values of helium and xenon nuclear magnetic shielding constants were used to determine new accurate nuclear magnetic moments of ¹²⁹Xe and ¹³¹Xe in terms of that of the ³He nucleus. They are as follows: μ(¹²⁹Xe) = ?0.7779607(158)μ_N and μ(¹³¹Xe) = +0.6918451(70)μ_N. By this means, the new ‘helium method’ for estimations of nuclear dipole moments was successfully tested. Gas phase NMR spectra demonstrate the weak intermolecular interactions observed on the ³He and ¹²⁹Xe and ¹³¹Xe shielding in the gaseous mixtures with Xe, CO₂ and SF₆. Copyright © 2015 John Wiley & Sons, Ltd.     

2D 129Xe EXSY of xenon atoms in a thermotropic liquid crystal confined to a controlled-pore glass

Two-dimensional (129)Xe exchange spectroscopy (EXSY) NMR measurements are presented for xenon atoms dissolved in a thermotropic nematic Liquid Crystal (LC), Merck Phase 4, confined to a mesoporous Controlled-Pore Glass (CPG) material with an average pore diameter of 81 A. Experiments were carried out as a function of mixing time at two different temperatures in which Phase 4 appears in nematic and isotropic phases. The exchange rate constants of xenon atoms between two different sites were determined utilizing the intensities of diagonal and off-diagonal signals measured in the EXSY spectra. In the studied system, the sites are: (a) xenon dissolved in the bulk LC between the CPG particles; and (b) xenon in the LC confined inside the pores. The diffusion rate of xenon atoms between the various sites was observed to be very slow.     

Grand canonical Monte Carlo simulations of the 129Xe NMR line shapes of xenon adsorbed in (+/-)-[Co(en)3]Cl3

The 129Xe NMR line shapes of xenon adsorbed in the nanochannels of the (+/-)-[Co(en)3]Cl3 ionic crystal have been calculated by grand canonical Monte Carlo (GCMC) simulations. The results of our GCMC simulations illustrate their utility in predicting 129Xe NMR chemical shifts in systems containing a transition metal. In particular, the nanochannels of (+/-)-[Co(en)3]Cl3 provide a simple, yet interesting, model system that serves as a building block toward understanding xenon chemical shifts in more complex porous materials containing transition metals. Using only the Xe-C and Xe-H potentials and shielding response functions derived from the Xe@CH4 van der Waals complex to model the interior of the channel, the GCMC simulations correctly predict the 129Xe NMR line shapes observed experimentally (Ueda, T.; Eguchi, T.; Nakamura, N.; Wasylishen, R. E. J. Phys. Chem. B 2003, 107, 180-185). At low xenon loading, the simulated 129Xe NMR line shape is axially symmetric with chemical-shift tensor components delta(parallel) = 379 ppm and delta(perpendicular) = 274 ppm. Although the simulated isotropic chemical shift, delta(iso) = 309 ppm, is overestimated, the anisotropy of the chemical-shift tensor is correctly predicted. The simulations provide an explanation for the observed trend in the 129Xe NMR line shapes as a function of the overhead xenon pressure: delta(perpendicular) increased from 274 to 292 ppm, while delta(parallel) changed by only 3 ppm over the entire xenon loading range. The overestimation of the isotropic chemical shifts is explained based upon the results of quantum mechanical 129Xe shielding calculations of xenon interacting with an isolated (+/-)-[Co(en)3]Cl3 molecule. The xenon chemical shift is shown to be reduced by about 12% going from the Xe@[Co(en)3]Cl3 van der Waals complex to the Xe@C2H6 fragment.     

Characterization of the metal-organic framework compound Cu3(benzene 1,3,5-tricarboxylate)2 by means of 129Xe nuclear magnetic and electron paramagnetic resonance spectroscopy

129Xe NMR measurements of adsorbed xenon are shown for the first time to be a suitable tool to characterize the porosity and the properties of the metal-organic framework Cu3(BTC)2(H2O)3 (BTC = benzene 1,3,5-tricarboxylate). The NMR experiments are performed at room temperature and over a wide range of xenon pressure and on two different synthesized Cu3(BTC)2 samples. 129Xe NMR results reveal that in dependence on the kind of the synthesis pathway either one or two signals are observed which can be attributed to two kinds of fast exchange of xenon atoms in two pores with different pore sizes. Coadsorption experiments of xenon and ethylene demonstrate that the xenon atoms prefer to fill the greater pores of the material because the smaller pores are occupied with residual molecules from the synthesis procedure and additionally adsorbed ethylene. Besides the NMR experiments a series of electron paramagnetic resonance (EPR) measurements are performed to estimate the state of copper having a strong influence on the chemical shift of the adsorbed xenon. The EPR experiments demonstrate that spin exchange between the interconnected copper dimers is taking place across the BTC linker molecules in the Cu3(BTC)2 framework.     

A study of homonuclear dipolar recoupling pulse sequences in solid-state nuclear magnetic resonance

Dipolar recoupling pulse sequences are of great importance in magic angle spinning solid-state NMR. Recoupling sequences are used for excitation of double-quantum coherence, which, in turn, is employed in experiments to estimate internuclear distances and molecular torsion angles. Much effort is spent on the design of recoupling sequences that are able to produce double-quantum coherence with high efficiency in demanding spin systems, i.e., spin systems with small dipole-dipole couplings and large chemical-shift anisotropies (CSAs). The sequence should perform robustly under a variety of experimental conditions. This paper presents experiments and computer calculations that extend the theory of double-quantum coherence preparation from the strong coupling/small CSA limit to the weak coupling limit. The performance of several popular dipole-dipole recoupling sequences-DRAWS, POST-C7, SPC-5, R1, and R2-are compared. It is found that the optimum performance for several of these sequences, in the weak coupling/large CSA limit, varies dramatically, with respect to the sample spinning speed, the magnitude and orientation of the CSAs, and the magnitude of dipole-dipole couplings. It is found that the efficiency of double-quantum coherence preparation by gamma-encoded sequences departs from the predictions of first-order theory. The discussion is supported by density-matrix calculations.     

DFT study of the NMR properties of xenon in covalent compounds and van der waals complexes-implications for the use of 129Xe as a molecular probe

The NMR properties (chemical shift and spin-spin coupling constants) of (129)Xe in covalent compounds and weakly bound complexes have been investigated by DFT methods including relativistic effects. For covalent species, a good agreement between experimental and calculated results is achieved without scalar relativistic effects, but their inclusion (with a triple-zeta, double-polarization basis set) leads to some improvement in the quality of the correlation. The spin-orbit coupling term has a significant effect on the shielding constant, but makes a small contribution to the chemical shift. Coupling constants contain substantial contributions from the Fermi contact and paramagnetic spin-orbit terms; unlike light nuclei the spin-dipole term is also large, whereas the diamagnetic spin-orbit term is negligible. For van der Waals dimers, the dependence of the xenon chemical shift and anisotropy is calculated as a function of the distance. Small (<1 Hz) but non-negligible through-space coupling constants between (129)Xe and (13)C or (1)H are predicted. Much larger couplings, of the order of few Hz, are calculated between xenon and (17)O in a model silicate residue.     

129Xe and 131Xe NMR of gas adsorption on single- and multi-walled carbon nanotubes

129Xe and 131Xe nuclear magnetic resonance (NMR) spectroscopy was used to study the adsorption of xenon gas on as-produced single-walled and multi-walled carbon nanotubes. Overall, the adsorption was weak, with slightly stronger interaction between xenon and multi-walled nanotubes. Temperature-dependent spectra, relaxation times, line widths, and signal intensities provide evidence that xenon forms a multilayer bulk phase rather than a homogeneous surface coating. The estimated adsorption energy of 1.6 kJ/mol is significantly lower than 23 kJ/mol predicted for monolayer adsorption but is in keeping with the Xe-Xe attractive potential. Xenon preferentially adsorbs on metallic particles in single-walled tubes, while defects are the nucleation sites for the stronger adsorption on multi-walled tubes.