129Xe NMR Chemical Shift Research Articles

The dependence on temperature and concentration of 129Xe NMR chemical shift values of organoxenon(II) salts [RXe][Y] and of the parent molecule XeF2

The dependence of the 129Xe NMR chemical shift value of XeF2 on temperature and concentration was determined in a variety of prototypic media: in acidic (anhydrous HF, aHF), nonprotic but polar (dichloromethane), and basic (CD3CN-EtCN, 1:3 v/v) solvents. The 129Xe NMR spectra of a representative series of organoxenon(II) salts [RXe][Y] (R = C6F5, heptafluoro-1,4-cyclohexadien-1-yl (cyclo-1,4-C6F7), pentafluoro-1,4-cyclohexadien-3-on-1-yl (cyclo-3-O-1,4-C6F5), CF2=C(CF3), (CF3)2CFC[triple bond]C, C4H9C[triple bond]C; Y = BF4, AsF6) in aHF showed, compared with XeF2-aHF, a quantitatively less distinct but qualitatively related dependence of delta(129Xe) vs temperature. The dependence of their delta(129Xe) values on concentration in aHF is negligible. An explanation for the different behavior of [RXe][Y] and XeF2 is offered.

Local structure of xenon adsorbed in the nanospaces of zeolites as studied by high-pressure 129Xe NMR

Pressure (0-10 MPa) and local density dependence of 129Xe NMR chemical shift of xenon in various microporous materials was investigated using an in situ high-pressure probe. The density dependence of the chemical shift was analyzed using virial expansion of the chemical shift by xenon density. Results indicate that the second virial coefficient depends on the pore size and shape, and that the void space affects xenon-xenon interaction in both microporous and mesoporous materials. Furthermore, to interpret the magnitude of the virial coefficient in terms of the local structure of the adsorbed xenon, we analyzed the local structure of adsorbed xenon in molecular sieve 5A using Xe(n) clusters, thereby allowing description of the density dependence of the chemical shift. We also demonstrated the cluster model's validity by applying it to molecular sieves 13X and ZSM-5. The latter showed that the adsorbed xenon exists as a xenon monomer up to the filling of about 0.6 in micropores. Larger xenon clusters up to n = 4 have been grown with increasing filling of xenon. According to analyses using the Xe(n) cluster model, the second virial coefficient is related closely with the xenon cluster size, which contributes greatly to the chemical shift in the low loading region.

Xe chemical shift tensor in silicalite and SSZ-24.

We report, for the first time, a theoretical prediction of the (129)Xe nuclear magnetic resonance chemical shift tensor of xenon atom in a single crystal of silicalite at near-zero occupancy and the temperature dependence of the Xe NMR chemical shift tensor for the polycrystalline silicalite at maximum occupancy. The former is a measure of the sensitivity of the Xe tensor components to the local structure of the channels without Xe-Xe contributions. The latter is a measure of the sensitivity of the Xe-Xe tensor components to the Xe-Xe distributions, as determined by the Xe-Xe potential function in competition with the Xe-silicalite potential function. Both theoretical predictions can be compared against Xe NMR experiments: the first against the Xe spectra collected as a function of rotation of the single crystal about the three crystalline axes in a magnetic field, and the second against variable temperature Xe NMR studies (below room temperature) of polycrystalline silicalite at maximum Xe occupancy. With the same parameter set (Xe-O potential and shielding functions), we predict the line shapes of Xe in SSZ-24 zeolite under various conditions of occupancy and temperature.

Structural investigation of pig metmyoglobin by 129Xe NMR spectroscopy.

The potentiality of xenon's sensitivity to its local magnetic environment is thoroughly investigated to probe internal structural differences between pig and horse metmyoglobin (MMb). These MMb's differ by 14 amino acids. One of these, Ile142 in horse MMb, is located in the proximal cavity, which is the xenon-binding site in horse MMb, and is replaced by Met142 in pig MMb. Specific and non-specific xenon-protein interactions are investigated here by 129Xe NMR chemical shifts and relaxation rate in aqueous solutions of pig MMb as a function of the xenon and protein concentrations. The results are complemented with 129Xe NMR data of the isostructural carbonmonoxy myoglobin (COMb), with computational calculations in order to highlight the structural differences between the cavities, and 1H NMR spectra to test the dependence of the 1H chemical shift on the addition of xenon. The 129Xe chemical shift NMR parameters are analysed quantitatively in terms of a two-site model. Xenon forms a 1:1 complex with the protein, characterized by an equilibrium binding constant K=[Xe]in/([Xe]out[MMb]), and exchanges rapidly between a cavity within the protein (X(ein)) and all other environments (Xe(out)). A comparison of equilibrium constant, K (74 M(-1)) in pig and K (146 M(-1)) in horse, reveals differences in affinity of xenon to the interior of pig MMb. Changes in xenon binding in both pig and horse MMb are also pointed out by other experimental results, e.g. the difference in the estimated delta(in), which is shifted downfield in pig MMb and upfield in horse MMb, with respect to 129Xe in buffer solution; the xenon-iron distance, 7.4 A, which is longer in the pig than was found in the horse, 5.3 A.

Analysis of gas transport properties of PPO/PS blends by 129Xe NMR spectroscopy

Relationships among variations of microvoids and gas transport properties for miscible poly(2,6-dimethyl-1,4-phenylene oxide) (PPO)/polystyrene (PS) blends in the glassy state have been investigated by Xe sorption, Xe permeation, and 129Xe NMR measurements. Xe sorption isotherms of the blends can be interpreted successfully on the basis of the dual-mode sorption model. Decrease in the permeability of Xe is attributed to the decrease in the diffusivity of that in the Langmuir site. 129Xe NMR spectra of 129Xe in the blends show non-linear low-field shift with increasing sorption amount of Xe because of a fast exchange of Xe atoms between Henry and Langmuir sites. From the analysis of 129Xe NMR chemical shifts, it is found that the mean volume of individual microvoids varies with a negative deviation against volume fraction of PPO in the blend. For PPO/PS blends, it has been clarified that the contraction of individual microvoids occurs by blending and highly affects gas transport properties.

Cover Picture: Eur. J. Org. Chem. 6/2003

AbstractThe cover picture shows the different NMR spectra of xenon atoms encapsulated in cryptophane‐A and C2‐symmetrical analogs. The high sensitivity of the xenon chemical shift to its environment allows the detection of small structural perturbations of the hosts. A slight modification of the internal volume of the cage compounds leads to an appreciable change of the 129Xe NMR chemical shifts. There exists a direct correlation between the stability constant Ka and the van der Waals volume of the cavity. Details of the synthesis of the new cryptophane hosts and complexation dynamics are presented in the article by T. Brotin and J.‐P. Dutasta on p. 973 ff.

Investigation of Enhanced Free Volume in Nanosilica-Filled Poly(1-trimethylsilyl-1-propyne) by129Xe NMR Spectroscopy

The gas permeability of poly[1-(trimethylsilyl)-1-propyne] (PTMSP) containing nanoparticulate fumed silica increases with increasing filler content. This unusual phenomenon is explored using 129Xe NMR spectroscopy to examine the effect of filler on the free volume of the PTMSP host matrix. The 129Xe NMR chemical shift decreases regularly with increasing fumed silica concentration, consistent with an increase in the average size of free volume elements or cavities through which molecular transport can occur. A relationship between the chemical shift and gas permeability in the filled polymer is reported.

Characterization of the Cavity in Poly(4-methyl-1-pentene) Crystal by Gas Permeation and 129Xe NMR Measurements

Characteristic cavity in a crystal part of poly(4-methyl-1-pentene) (PMP) has been investigated by gas permeation and 129Xe NMR measurements for PMP membranes with various crystallinities. For the cavity, permeability coefficients of He, N2, O2, CH4, and CO2 have shown finite values, whereas those of C3H8 and tert-C4H10 have shown zero. From the analysis of 129Xe NMR chemical shifts of 129Xe sorbed in PMP membranes, it has been able to evaluate the size of the cavity in PMP crystal as about 4.5 Å. These findings have indicated that gas molecules less than about 4.5 Å can permeate the cavity in PMP crystal.

Characterization of microvoids in PPO/PS polymer blend by means of 129Xe NMR spectroscopy

Relationships among 129Xe NMR chemical shift, Xe sorption properties, and density have been investigated for poly(2,6-dimethyl-1,4-phenylene oxide) / polystyrene miscible blend system in order to evaluate the variation of microvoids by blending. Xe sorption isotherms of all samples have been explained successfully on the basis of the dual-mode sorption model. Xe sorption properties and densities of blended samples have indicated the reduction of microvoids by blending. 129Xe NMR chemical shifts of 129Xe sorbed in each sample have shown a non-linear low-field shift with increasing total amount of Xe sorption. Estimation of the microvoids by 129Xe NMR spectroscopy has been consistent with the results of density and Xe sorption measurements.

A General Correlation for the 129Xe NMR Chemical Shift−Pore Size Relationship in Porous Silica-Based Materials

A general correlation for the 129Xe NMR chemical shift−pore size relationship (δ versus D) in porous silica-based materials over the range 0.5−40 nm has been demonstrated: δ = δs/(1 + D/b), with δs = 116 ± 3 ppm and b = 117 ± 8 A for the 34 materials studied. The correlation may be used in the characterization of silica samples with unknown pore structure. Even within this general correlation, subsets of materials of similar origin display finer correlations that indicate an acute sensitivity to details of the pore surfaces.

A 129Xe NMR Study of Functionalized Ordered Mesoporous Silica

In this publication we report the first application of continuous flow hyperpolarized (HP) 129Xe techniques to studies of pore structure in ordered mesoporous materials. In particular, we report extensive 129Xe NMR chemical shift measurements of xenon adsorbed in mesoporous materials of different porosities and surface chemical composition. The use of HP Xe has allowed us to work at very low concentrations of xenon where the contribution of the Xe−Xe interactions is negligible and the observed 129Xe chemical shift reflects mainly interactions between the xenon atoms and the surface. Variable temperature measurements gave access to information on the adsorption parameters of xenon, and allowed us to follow the changes in these properties due to modification of the mesopore voids. Our 129Xe NMR data reveal a nonuniform porosity and irregular pore structure in contrast to N2 adsorption and TEM measurements that indicate regular nanoporous channels.

Characterization of Microvoids in Glassy Polymers by Means of 129Xe NMR Spectroscopy

129Xe NMR chemical shift of 129Xe sorbed in glassy polymers has been studied in connection with Xe sorption property. Poly(2,6-dimethyl-1,4-phenylene oxide), polystyrene, Bisphenol A polycarbonate, and tetramethyl-Bisphenol A polycarbonate have been used as glassy polymers. Xe sorption isotherms of glassy polymers have been successfully interpreted by the dual-mode sorption model. 129Xe NMR chemical shifts of 129Xe in glassy polymers have shown non-linear low-field shift against sorption amount of Xe. From the Xe density dependence of the 129Xe NMR chemical shift, it has been able to evaluate mean size of microvoids in glassy polymer. These findings suggest that the 129Xe NMR spectroscopy is a good tool for characterizing microvoids in glassy polymers.

High-pressure [formula omitted] study of supercritical xenon interacting with polymers

High-pressure 129 Xe NMR experiments have been performed by using a newly designed probe. 129 Xe NMR spectrum for the mixture of Xe and a polymer, either bisphenol-A polycarbonate or polytetrafluoroethylene, consists of two lines in a pressure range of 0.3–10 MPa. 129 Xe NMR chemical shift ( δ) for the free Xe is mainly governed by the Xe–Xe interactions. δ for Xe confined in the micropore of polymers is strongly affected by the Xe-wall interactions. It is found that δ of the confined Xe is mainly determined by a sort of `virtual pressure' and is not influenced by a drastic change of the free Xe density in the critical region.

NMR study of the reversible complexation of xenon by cucurbituril

The interaction of xenon with cucurbituril, a synthetic receptor, in aqueous solutions is investigated by 129Xe and 1H NMR spectroscopy. Xenon is reversibly trapped into the cavity to form a 1 ∶ 1 host–guest complex. The exchange between the free and the complexed xenon is slow on the 129Xe NMR chemical shift time scale but fast on the 1H NMR chemical shift time scale. The apparent association constant of xenon for cucurbituril can be obtained from analysis of the 1H spectra and is estimated to be around 200 M−1 at 298 K. Interestingly, even though no H-bonding or electrostatic interactions play a role in the stabilisation of the complex, its stability is comparable to the stability of cucurbituril complexes with certain aryl- and alkyl-ammonium ions.

Positron annihilation and 129Xe NMR studies of free volume in polymers

The existence and the average size of free volume in bisphenol-A polycarbonate (PC), low-density polyethylene (LDPE), poly (2,6-dimethyl-phenyleneoxide)(PPO), and polytetrafluoroethylene (PTFE) were studied by positron annihilation and 129Xe NMR measurements. The 129Xe NMR chemical shifts for xenon adsorbed in the polymers indicated that the average pore size of the free volume increased in the following order: PC, LDPE, PPO, and PTFE. This order of the pore size of the free volume agrees well with that estimated from the longest lifetime ( τ 3) of ortho-positronium formed in the polymers. The unique correlation that δ −1∝ r is established between the 129Xe NMR chemical shift ( δ) and the pore size ( r), which is deduced from the positron annihilation measurements.

Characterization of Phase Transitions and Dynamics in Reentrant Nematic Liquid Crystals by Xenon-129 Nuclear Magnetic Resonance

In this paper, the ability to detect phase transitions and characterize the dynamics in binary and tertiary reentrant nematic liquid crystal mixtures via 129Xe NMR chemical shifts and spin−spin (T2...

The Diverse Nature of Dodecahedral Cages in Clathrate Hydrates As Revealed by 129Xe and 13C NMR Spectroscopy: CO2 as a Small-Cage Guest

In this contribution it is shown that 129Xe NMR chemical shift parameters accurately reflect the relative size and geometry of the small dodecahedral cages in different hydrate structures. The relative sizes and geometric relationships were explored with CO2 as a probe molecule. Whereas CO2 is a relatively poor guest for the D cage in structure I hydrate, it appears to be a very efficient guest for the somewhat larger and asymmetric D cage in structure II hydrate. On the basis of an approximate CO2 distribution over the cages in structure I hydrate a lower limit on the hydration number of CO2 hydrate is determined to be 7.0. On the basis of Xe chemical shift parameters for structure H hydrate, it was predicted that CO2 should also be a reasonable small-cage guest for this structure. Experimental results are presented that support this prediction and show that CO2 indeed can serve as a helpgas for structure H hydrate.

The role of polarization of Xe by di- and monovalent cations in 129Xe NMR studies in zeolite A

We consider the role of polarization in the adsorption of Xe in zeolites of type A by direct comparative analysis of the adsorption isotherms, distributions of occupancies, and 129Xe NMR chemical shifts of Xe n in cages containing Ca x Na 12 −2 x ions per alpha cage ( x = 0, 1, 2, 3, 5). We find that the qualitative trends in the adsorption isotherms, and in the progressions of Xe n chemical shifts (for n = 0–8 in cages with x = 0, 1 Ca 2+ ions and for n = 0–5 in cages with x = 2, 3 Ca 2+ ions) upon increasing the level of Ca 2+ ion for Na + ion substitution could only be accounted for by including polarization of the Xe atom by the zeolite framework and its ions. This system, which permits observation of individual Xe n peaks and of directly comparable adsorption isotherms in several cage types, provides a good model system for the interpretation of the more general case in which only the overall average 129Xe NMR chemical shift is observed in open network zeolites, arising from free exchange of Xe among cavities of variable occupancy and variable cation distribution.

Distribution and Xe129 NMR chemical shifts of Xen clusters in the alpha cages of zeolite AgA

The distributions and 129Xe NMR chemical shifts of xenon in zeolite AgA have been measured in a series of experiments by Moudrakovski, Ratcliffe, and Ripmeester [Proc. Internat. Zeolite Conference, Quebec, 1995; unpublished]. We carry out grand canonical Monte Carlo (GCMC) simulations of xenon in a rigid zeolite AgA lattice to provide the average Xen cluster shifts, and the distributions Pn for comparison with their experiments. The GCMC results for the distributions, the fraction Pn of the alpha cages containing n Xe atoms, are compared with the experimental distributions in 12 samples and the agreement is excellent. The distributions in NaA and in AgA are very similar, as can be established from the comparison of the dispersion of the distributions, {〈n2〉−〈n〉2}, and both are different from the idealized hypergeometric distribution, in which the component atoms occupy eight lattice sites per cage under mutual exclusion. The calculated chemical shift increments [σ(Xen)−σ(Xen−1)]AgA are in good agreement with experiment. The differences between these and the increments in zeolite NaA,{[σ(Xen)−σ(Xen−1)]AgA−[σ(Xen)−σ(Xen−1)]NaA}, are fairly small and are in good agreement with experiment. The absolute 129Xe chemical shifts of Xen in the alpha cages of AgA are nearly uniformly shifted by about 40 ppm compared to the Xen clusters in NaA. This is attributed to the Fermi contact shifts arising from the Ag0 metal atoms that form the linear Ag32+complexes that are found within the beta cages of AgA.

129Xe NMR Chemical Shifts in Zeolites: Effect of Loading Studied by Monte Carlo Simulations

The NMR chemical shift of 129Xe adsorbed in the cavities and channels of zeolites depends on the crystal type (structure and chemistry) and on the loading of Xe. The dependence of the 129Xe shift on Xe loading has frequently been interpreted qualitatively in terms of increasing Xe−Xe interactions, but this dependence can be quantitatively and predictively modeled using computer simulations. We show that Monte Carlo simulations of Xe inside Omega, mordenite, AlPO4-5, L, Y, ZK-4, silicalite, and VPI-5 give Xe−Xe pair correlation functions that are entirely consistent with the shift behavior reported in the literature. The sensitivity of the shift dependence on loading correlates well with the calculated accessible volume; this correlation distinguishes zeolites even with similar unidirectional channels and indicates that an estimate of the available porosity of a zeolite can be made from such data. There is reasonable (but not in all cases quantitative) agreement between the predicted and experimental zeolite shift contribution.