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Antifreezes are widely used in preventing hydrate formation in oil and gas flowlines. Recent studies have revealed that methanol and ammonia can be incorporated into clathrate hydrate phases along with a more hydrophobic guest such as THF or propane and that these antifreezes act as catalysts for methane hydrate formation from ice. In this work, we demonstrated that these antifreezes can enhance the methane storage content of binary clathrate hydrates, namely those of THF and TBAB. THF + methane and TBAB + methane binary hydrates with/without methanol or ammonia were synthesized and analyzed with 13C NMR spectroscopy and X-ray diffraction (XRD) methods. In the THF hydrate system, 84% and 81% of the 512 small cages were occupied by methane in the presence of methanol and ammonia respectively, while only 44% of the small cages were occupied in the absence of these antifreezes. In the TBAB-H2O system, the powder XRD (PXRD) patterns of 1TBAB:38H2O samples without antifreeze both before and after methane introduction showed mostly tetragonal structures. On the other hand, it was confirmed that methanol can easily induce TBAB hydrates to form the orthorhombic structure which is more suitable for methane storage than the tetragonal structures of TBAB hydrates. The single crystal XRD analysis of a crystal grown from the 1TBAB:1CH3OH:38H2O solution at 277 K showed that methanol was present in the 512 cage of the orthorhombic TBAB hydrate phase. The 13C NMR spectra of TBAB + methane hydrates also showed an enhanced methane content in the presence of methanol. The present findings on the enhancement of methane storage induced by antifreezes suggest that methanol can be a key material for hydrate-based methane storage systems.

In light of the intense recent interest in the methylammonium lead halides, CH3NH3PbX3 (X = Cl, Br, and I) as sensitizers for photovoltaic cells, the dynamics of the methylammonium (MA) cation in these perovskite salts has been reinvestigated as a function of temperature via 2H, 14N, and 207Pb NMR spectroscopy. In the cubic phase of all three salts, the MA cation undergoes pseudoisotropic tumbling (picosecond time scale). For example, the correlation time, τ2, for the C–N axis of the iodide salt is 0.85 ± 0.30 ps at 330 K. The dynamics of the MA cation are essentially continuous across the cubic ↔ tetragonal phase transition; however, 2H and 14N NMR line shapes indicate that subtle ordering of the MA cation occurs in the tetragonal phase. The temperature dependence of the cation ordering is rationalized using a six-site model, with two equivalent sites along the c-axis and four equivalent sites either perpendicular or approximately perpendicular to this axis. As the cubic ↔ tetragonal phase transition temperature is approached, the six sites are nearly equally populated. Below the tetragonal ↔ orthorhombic phase transition, 2H NMR line shapes indicate that the C–N axis is essentially frozen.

Methanol is one of the most common inhibitors for clathrate hydrate formation. Crystalline clathrate hydrates containing methanol were synthesized and analyzed by powder X‐ray diffraction and 13C NMR spectroscopy. The data obtained demonstrate that methanol can be a helper guest for forming structure I, structure II, and structure H clathrate hydrates, as long as the lattice framework contains NH4F. The latter acts as a lattice stabilizer by providing sites for strong hydrogen bonding of the normally disruptive methanol hydroxy group. NH4F and methanol can be considered key materials for crystal engineering of clathrate hydrates, as the modified lattices allow preparation of hydrates of non‐traditional water‐soluble guests such as alcohols and diols. Methanol takes on the role of an unconventional helper guest. This extends clathrate chemistry to a realm where neither hydrophobic guests nor high pressures are required. This also suggests that more stable lattices can be engineered for applications such as gas storage.

Highly emissive alloyed CdSeS quantum dots (QDs) with a gradient structure exhibiting photoluminescence (PL) peaking at 490 nm and an absolute quantum yield (QY) of 79% (in toluene with excitation wavelength of 430 nm) were designed and synthesized. The cyan-blue emitters were synthesized at 180 °C in 1-octadecene (ODE) with cadmium oleate (Cd(OA)2), tri-n-octylphosphine selenide (TOPSe), and tri-n-octylphosphine sulfide (TOPS) as the Cd, Se, and S precursors, respectively; importantly, a commercial secondary phosphine, diphenyl phosphine (DPP or HPPh2), was used as a beneficial additive. Also, our high Cd/(Se + S) feed molar ratio aids in shifting the equilibrium of the chalcogenide exchange, TOPE + HPPh2 ⇔ TOP + E═PPh2H, to the right. Density functional theory (DFT) calculations suggest that the formation of Se═PPh2H proceeds faster than that of S═PPh2H, which supports our high S/Se feed molar ratio used to synthesize the bright gradient-alloyed CdSeS QDs. Compositional and structural characterization w...

Broad developments in experimental NMR techniques have opened new and exciting opportunities for application of solid state nuclear magnetic resonance (SS NMR) in studies of gas hydrates and inclusion compounds in general. Perhaps the most important advance of the last 10 years was the extension into very high magnetic fields beyond 20 T. This progress is especially significant in studies concerned with low-γ, low natural abundance, and quadrupolar nuclei. This work reports our recent exploration of clathrate hydrates and other inclusion compounds (β-quinol, tert-Bu-Calix[4], and dodecasil-3C) with SS NMR of nuclei that were not so long ago completely out of reach for NMR, namely 131Xe, 83Kr, and 33S. Although 129Xe is a widely used NMR probe, applications of the low-γ isotope 131Xe were very scarce. Being a quadrupolar spin 3/2 nucleus, 131Xe provides an additional probe for sampling the electric field gradients in inclusion compounds. Another nucleus that has been seriously under-explored is 83Kr, with its very low γ being the main obstacle, and along with quadrupolar coupling we report the first detection of the chemical shift anisotropy in krypton. The relative values of the Sternheimer antishielding factors for 131Xe and 83Kr, obtained by comparison of the spectra of the two in identical cage environments, are also discussed. Though 33S NMR of solids is notoriously difficult due to its low γ, low natural abundance, and relatively large quadrupolar moment, working at the field of 21.1 T it was possible to acquire, in a reasonable time, natural abundance 33S SS NMR spectra of various H2S and SO2 gas hydrates and inclusion compounds. In most cases the spectra are dominated by the quadrupolar interactions, providing information on the symmetry of the cages encapsulating the guest molecules, and also show the effects of very rapid reorientation of the encaged H2S and SO2. The impact of the introduction of new NMR nuclei on hydrate research is discussed.

Ionic hydrates are known to form numerous clathrate structures in which either the cations or anions sit in cages and the counterions are incorporated into the water framework. Due to the inclusion of the ionic species, such ionic clathrate hydrates not only show many peculiar features such as metal ion encagement and superoxide ion generation, but also exhibit notable physicochemical properties such as outstanding ionic conductivity and thermal stability. Thus, the ionic clathrate hydrates are considered for their potential applicability in various fields, including those that involve solid electrolytes, gas sensors, and energy storage. In this study, we report the design, synthesis, and characterization of the first ionic clathrate hydrate of the hexagonal structure-H (Str.H) crystal type. Diethyl-dimethyl-ammonium hydroxide hydrate was synthesized with CH4 and Xe as help gases, and the crystal structure was identified by powder X-ray diffraction analysis. Further confirmation of the formation of Str.H was obtained from Raman spectroscopy and 13C, 129Xe, and 2H solid-state NMR spectroscopy. From 13C NMR and ab initio calculations, it was shown that the quaternary ion occupies the large cage of Str.H with a conformation different from that in solution, due to constraints imposed by the dimensions of the cage. The H deficiency introduced by substitution of OH– for a water molecule appears, from 129Xe NMR, to be disordered over the framework, and, from 2H NMR, to substantially increase the rate of reorientational mobility of the D atoms in the framework, over that observed for a Str.I hydrate and for ice. The Str.H hydrates are commonly more stable than other structures, thus the present findings on the ionic Str.H clathrate hydrate may offer a new approach for improving the stability of ionic clathrate hydrates for their practical application.

Systematic analyses have been carried out on two gas hydrate-bearing sediment core samples, HYPV4, which was preserved by CH 4 gas pressurization, and HYLN7, which was preserved in liquid-nitrogen, recovered from the BPXA-DOE-USGS Mount Elbert Stratigraphic Test Well. Gas hydrate in the studied core samples was found by observation to have developed in sediment pores, and the distribution of hydrate saturation in the cores imply that gas hydrate had experienced stepwise dissociation before it was stabilized by either liquid nitrogen or pressurizing gas. The gas hydrates were determined to be structure Type I hydrate with hydration numbers of approximately 6.1 by instrumentation methods such as powder X-ray diffraction, Raman spectroscopy and solid state 13 C NMR. The hydrate gas composition was predominantly methane, and isotopic analysis showed that the methane was of thermogenic origin (mean δ 13 C = −48.6‰ and δ D = −248‰ for sample HYLN7). Isotopic analysis of methane from sample HYPV4 revealed secondary hydrate formation from the pressurizing methane gas during storage.

A combination of density functional and optimal control theory has been used to generate amplitude- and phase-modulated excitation pulses tailored specifically for the (33)S nuclei in taurine, based on one of several reported crystal structures. The pulses resulted in significant signal enhancement (stemming from population transfer from the satellite transitions) without the need for any experimental optimization. This allowed an accurate determination of the (33)S NMR interaction parameters at natural abundance and at a moderate magnetic field strength (11.7 T). The (33)S NMR parameters, along with those measured from (14)N using frequency-swept pulses, were then used to assess the accuracy of various proposed crystal structures.

Atoms and molecules

Homogeneously alloyed ternary CdTeSe magic-sized nanocrystals (MSNs) with bandgap emission were synthesized in 1-octadecene (ODE) via a noninjection one-pot approach featuring synthetic reproducibility and large-scale capability. The noninjection approach used cadmium acetate dihydrate (Cd(OAc)2·2H2O), elemental selenium, and elemental tellurium as Cd, Se, and Te sources, respectively. The growth of the CdTeSe nanocrystals was carried out at temperatures from 120 to 200 °C for several hours in a reaction flask containing the reactants together with a long-chain fatty acid as capping ligands and ODE as the reaction medium. During synthesis, the CdTeSe nanocrystals exhibited persistent bandgap and did not grow in size anymore after their formation, either with longer growth periods or higher reaction temperature. Also, they exhibit an absorption doublet with 288 meV energy difference between the two peaks; the first excitonic transition peak is at 520 nm and bandgap photoemission peak at 524 nm with full width at half-maximum (fwhm) of ca. 20 nm. Both the growth pattern and the optical properties suggest that they are magic-sized and thus termed as single-sized. Nuclear magnetic resonance (NMR), with sensitivity to local environment, provided valuable information regarding the structure and composition of the nanocrystals. Solid-state 13C cross polarization/magic angle spinning (CP/MAS) NMR spectra showed that the carboxylate capping ligand is firmly attached to the crystal, and 1H decoupled 113Cd MAS spectra, with and without CP, distinguished between the surface Cd species and the Cd inside a nanocrystal. The 113Cd NMR results also confirmed, unambiguously, that the nanocrystals are homogeneously alloyed ternary CdTeSe, with 113Cd resonances located between those of CdTe and CdSe nanocrystals indicating a stoichiometry of approximately 1Se:1Te ratio throughout the whole nanocrystal. XRD supported that they are ternary-alloyed CdTeSe rather than binary CdTe or CdSe, with a wurtzite crystal structure. In addition, both energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) supported an approximate stoichiometric ratio of 1Se:1Te of the CdTeSe nanocrystals.

High-quality colloidal photoluminescent (PL) ZnCdS quantum dots (QDs) with gradient distribution of components, consisting of Cd-rich inner cores and Zn-rich outer shells, were synthesized via a noninjection one-pot approach. This newly developed synthetic approach uses zinc stearate (Zn(St)2), cadmium acetate dihydrate (Cd(OAc)2·2H2O), and elemental sulfur as Zn, Cd, and S source compounds, respectively. The growth of the cubic-structured QDs was carried out at 240 °C in a reaction flask consisting of the source compounds, together with stearic acid (SA), 2,2‘-dithiobisbenzothiazole (MBTS), and 1-octadecene (ODE); all of these chemicals were loaded at room temperature. The temporal evolution of the optical properties of the growing QDs, including absorption and photoemission, was monitored in detail; the evolution monitored indicates that the growth kinetics, the composition, and the distribution of the Zn and Cd in the resulting ternary ZnxCd1-xS QDs are sensitive to feed Zn−Cd−S molar ratios. Detailed ...

In a quest for thermoelectric polymeric materials novel polycarbazole and polyindolocarbazole derivatives were synthesized. Alkyl side chains on the carbazole cycle and different side chains (alkyl or benzoyl) on the nitrogen atom of the backbone unit were introduced. Optical, electrochemical, electrical, and thermoelectric properties were investigated on these polymers and on two poly(diindolocarbazole)s. Band structure calculations were used to predict which polymers might be promising as thermoelectric materials. The best combination of Seebeck coefficient and conductivity (power factor) was around 10-7 W m-1 K-2 with copolymers comprising thiophene units alternating with carbazole or indolocarbazole. This family of polymers possesses good Seebeck coefficients, but there is still a need to improve the electrical conductivity, to produce useful thermoelectric materials.

A new highly soluble derivative of poly(2,7-carbazolenevinylene) was synthesized, bearing alkyl side chains in the 3,6 positions. The polymer synthesized was characterized by optical, electrochemical and electrical measurements. The stability towards chemical doping at room temperature is investigated. The Seebeck coefficients at room temperature are measured for different doping levels and compared to poly(3-decylthiophene). At high doping level, the Seebeck coefficient is around 200–250 μV/K, which is one order of magnitude higher than poly(3-decylthiophene). At low doping level, the Seebeck coefficient reaches 600 μV/K. The carbazolenevinylene polymer is not very conductive, the highest conductivity obtained being 5 × 10−3 S/cm. The electronic band structure and the Seebeck coefficients at room temperature are calculated and compared to experimental results.

In this article, we report a detailed study of resorcinol-formaldehyde (RF) aerogels prepared under different processing conditions, [resorcinol]/[catalyst] (R/C) ratios in the starting sol-gel solutions, using continuous flow hyperpolarized (129)Xe NMR in combination with solid-state (13)C and two-dimensional wide-line separation (2D-WISE) NMR techniques. The degree of polymerization and the mobility of the cross-linking functional groups in RF aerogels are examined and correlated with the R/C ratios. The origin of different adsorption regions is evaluated using both coadsorption of chloroform and 2D EXSY (129)Xe NMR. A hierarchical set of Xe exchange processes in RF aerogels is found using 2D EXSY (129)Xe NMR. The exchange of Xe gas follows the sequence (from fastest to slowest): mesopores with free gas, gas in meso- and micropores, free gas with micropores, and, finally, among micropore sites. The volume-to-surface-area (V(g)/S) ratios for aerogels are measured for the first time without the use of geometric models. The V(g)/S parameter, which is related both to the geometry and the interconnectivity of the pore space, has been found to correlate strongly with the R/C ratio and exhibits an unusually large span: an increase in the R/C ratio from 50 to 500 results in about a 5-fold rise in V(g)/S.

The storage of large quantities of hydrogen at safe pressures1 is a key factor in establishing a hydrogen-based economy. Previous strategies—where hydrogen has been bound chemically2, adsorbed in materials with permanent void space3 or stored in hybrid materials that combine these elements3—have problems arising from either technical considerations or materials cost2, 3, 4, 5. A recently reported6, 7, 8 clathrate hydrate of hydrogen exhibiting two different-sized cages does seem to meet the necessary storage requirements; however, the extreme pressures (approx 2 kbar) required to produce the material make it impractical. The synthesis pressure can be decreased by filling the larger cavity with tetrahydrofuran (THF) to stabilize the material9, but the potential storage capacity of the material is compromised with this approach. Here we report that hydrogen storage capacities in THF-containing binary-clathrate hydrates can be increased to approx4 wt% at modest pressures by tuning their composition to allow the hydrogen guests to enter both the larger and the smaller cages, while retaining low-pressure stability. The tuning mechanism is quite general and convenient, using water-soluble hydrate promoters and various small gaseous guests.

A broad based approach, using nitrogen adsorption, X-ray diffraction, DSC, solid-state NMR and X-ray photoelectron spectroscopy, has been used to characterize a series of Ga/MCM-41 composite materials. Rather than filling the mesochannels of the MCM-41 it is found, through this combination of techniques, that most of the Ga is present in the spaces between the particles of MCM-41. High-angle XRD and 71Ga NMR indicate that at room temperature most of the Ga is in the liquid metal state. 29Si and 1H MAS NMR, and XPS reveal that at the Ga/MCM-41 interface the Ga reacts with silanol groups to form new Ga+ states like SiOGa, or bridging Si(OH)Ga hydroxyls.

1H and 2H NMR spin-lattice relaxation times (T1) have been studied as a function of temperature to obtain activation energies for the threefold reorientations of CH3 groups (Ea = 22.83, 18.95, and 18.39 kJ mol–1) and NH3 groups (Ea = 47.64, 45.31, and 28.74 kJ mol–1) in the β-phases of ethylammonium chloride, -bromide, and -iodide, respectively, for comparison with barriers determined from Raman torsional frequencies. T1, 2H NMR lineshapes, and DSC studies have been used to clarify the existence of a β → α phase transition above room temperature in all three salts, and the absence of a transition at low temperature. 2H NMR lineshape analysis indicates a rapid n-fold (probably fourfold) reorientation of the whole molecule in the α-phase. This motion occurs about an axis in the molecular CCN mirror plane in the bromide and iodide. In the chloride, the axis is in the plane perpendicular to the bisector of the DCD bond angle.Key words: ethylammonium halides, solid state 1H and 2H NMR, molecular dynamics, rotational barriers, phase behaviour.

Spin−lattice relaxation time measurements, obtained between 2 and ∼150 K, are presented for 19F in two structure II clathrate hydrates, SF6·17D2O and SeF6·17D2O, as well as for 1H in a double structure II deuteriohydrate of methane and tetrahydrofuran-d8. The 19F results were analyzed in terms of three relaxation mechanisms related to the guest motions, including fluctuations in nuclear dipolar couplings and the anisotropic chemical shift, plus a contribution from spin rotation interactions above ∼50 K. For quantitative agreement, the incorporation of a distribution function into the relaxation equations is required to account for the proton disorder in the hydrate cages, in this case a normal distribution in activation energies. The dynamic parameters derived from relaxation measurements also account for the motional narrowing of the 19F spectra. This is the first instance where a single motional model has accounted for both spectral narrowing and relaxation times for guest dynamics in clathrate hydrates...

Local ordering in co-deposits of water and xenon atoms produced at low temperatures can be followed uniquely by 1 2 9 Xe NMR spectroscopy. In water-rich samples deposited at 10 K and observed at 77 K, xenon NMR results show that there is a wide distribution of arrangements of water molecules around xenon atoms. This starts to order into the definite coordination for the structure I, large and small cages, when samples are annealed at ∼140 K, although the process is not complete until a temperature of 180 K is reached, as shown by powder X-ray diffraction. There is evidence that Xe . 20H 2 O clusters are prominent in the early stages of crystallization. In xenon-rich deposits at 77 K there is evidence of xenon atoms trapped in Xe . 20H 2 O clusters, which are similar to the small hydration shells or cages observed in hydrate structures, but not in the larger water clusters consisting of 24 or 28 water molecules. These observations are in agreement with results obtained on the formation of Xe hydrate on the surface of ice surfaces by using hyperpolarized Xe NMR spectroscopy. The results indicate that for the various different modes of hydrate formation, both from Xe reacting with amorphous water and with crystalline ice surfaces, versions of the small cage are important structures in the early stages of crystallization.

The low-temperature crystal structure of the p-tert-butylcalix[4]arene–toluene inclusion compound, C44H56O4·C7H8, exhibits significant guest-induced distortion of the host calixarene molecule. This distortion persists long enough to establish a correlation in the orientation of guests in adjacent host molecules. Remarkably, the space group and unit cell that best describe the structure appear to depend on the wavelength of the incident radiation. This behaviour appears to arise from spatial averaging over the different scattering volumes required to establish the diffraction peaks.

Structure and dynamics in the Hofmann and Hoffmann-en type pyrrole clathrates, Cd(NH3)2Ni(CN)4·2C4H4NH and Cd(en)Ni(CN)4·2C4H4NH, have been studied by single crystal X-ray crystallography, solid state 2H NMR, PXRD, TGA and DSC. Both show in-plane reorientation of the pyrrole in an inequivalent six-site potential, with two low energy minima where the N of the pyrrole is oriented up or down with respect to the crystal c axis, followed by out-of-plane 4-fold motion about an axis parallel to the crystal c axis. There is also evidence for an intermediate dynamic state where there is 2-fold rather than 4-fold motion. The NH3 or en groups of the host frameworks rotate about the crystal c direction. The Hofmann-en pyrrole clathrate shows two phase transitions at 196 and 259 K, and the dynamics of the host en and pyrrole guest appear to be intimately linked to the latter structural transition. The results for these pyrrole clathrates have been compared with those for the two equivalent benzene clathrates, obtained previously and augmented with new 2H NMR lineshape data for Cd(en)Ni(CN)4·2C6D6 above 300 K.

The nucleation and growth of hydrate on the surface of ice was followed by NMR spectroscopy using hyperpolarized Xe. From the ice surface area and Xe pressure drop, the thickness of the hydrate film formed was ∼ 500−1000 Å before the reaction became extremely slow. The cage occupancy ratio θL/θS was measured as a function of time from the Xe spectrum and was used to monitor the nature of the material formed. The ratio changed from values close to 1 during the early part of the reaction (induction time) to its equilibrium value of ∼3−4 after nucleation processes were finished and rapid growth commenced. The low value can be seen as evidence of a precursor phase that is quite different from the equilibrium hydrate. The induction time was found to be a reproducible function of temperature and pressure for the conditions studied. The kinetics of gas uptake was analyzed according to the Avrami−Erofeyev equation used to describe solid−gas reactions. A surface memory effect was noted on successive cycles of xenon adsorption−desorption−readsorption, as the induction time was absent on the readsorption cycle. The new results are discussed in terms of models for nucleation and growth of hydrate and the various experiments that have been carried out in the past. It is essential to differentiate between later-stage diffusion-limited hydrate formation processes and the initial steps of hydrate formation at a surface, as they have opposite temperature coefficients.

In this study, new complexes of metal(II) (Ni, Zn, Cd) dibenzoylmethanates (DBMs) coordinated with 2-methylpyridine (2-MePy) were prepared and studied for host properties as a function of their structure and molecular isomerism. The novel [Ni(2-MePy)2(DBM)2] complex host exists as the trans isomer in five structurally distinct phases comprising two guest-free polymorphs as well as inclusion compounds with benzene, carbon tetrachloride, chlorobenzene, and toluene. All architectures arise from van der Waals packing, the host materials having guest species located in channels or cages. The stability of inclusions correlates with the shape of the cavity space. For zinc dibenzoylmethanate only one 2-MePy unit coordinates to give the [Zn(2-MePy)(DBM)2] complex that does not exhibit inclusion properties. With cadmium dibenzoylmethanate, two polymorphic forms of the composition [Cd(2-MePy)2(DBM)2] were isolated. A metastable form contains complex molecules in the trans isomeric state, while a stable form contains cis isomers. As earlier, the ability to form clathrates was attributed only to trans-configured molecules of this type; the presence of a more stable cis form, with a trans-to-cis phase conversion enthalpy gain of as much as 17.2(6) kJ/mol, explains the failure to isolate any inclusion compounds of the Cd complex. The precedent of isolating both cis and trans isomers for the same complex of metal dibenzoylmethanate host is discussed in the context of designing novel materials where the potential for inclusion may be switched through molecular-level control of the isomeric state.

The structure and composition of CO2 hydrate were determined from single-crystal X-ray diffraction data at 173 K for a crystal grown over heavy water and liquid CO2. Superior diffraction data allowed the derivation of a structural model of unprecedented quality for the hydrate, giving the location of the disordered CO2 molecules in the two cages. In the large cage, the guests are shown to be off-center, with a bimodal distribution of out-of-plane orientations for the long axis of the molecule (173 K). Also, the absolute cage occupancies were determined from the structural model, thus allowing a reliable and direct evaluation of the hydrate composition for this crystal, CO2·6.20(15) D2O. The temperature dependence of the lattice parameters for the single crystal was measured between 123 and 223 K and found to be in good agreement with recent neutron powder diffraction results, and data from all sources were fit to a single polynomial function. The hydrate composition and density are discussed in terms of the information needed for the deep-sea sequestration of CO2. The guest disorder and dynamics are discussed in terms of the model derived from earlier NMR data.

The 23Na and 29Si NMR spectra of the sodium-silicon clathrate materials Na8Si46 and NaxSi136 (1 < x < 24) and the parent Zintl phase NaSi have been studied in detail with a view to ironing out a number of ambiguities in the published literature and to determining properties of these potential thermoelectric materials. Sharp spectra are obtained only when the clathrate cages are close to fully occupied by Na, so that crystallographic symmetry is achieved. Signals from Na in the small and large cages of both structures have been unequivocally assigned. The pseudospherical cages give isotropic 23Na lines, whereas the other cages produce first-order quadrupolar line shapes. Electric field gradients derived from these spectra, and ab initio calculations are in remarkable agreement. The large Knight shifts of both types of nuclei have rather unusual temperature dependences, which reflect the changing distributions of unpaired electron density in the conduction bands. As the Na content of NaxSi136 is reduced there is drastic broadening of the 23Na and 29Si spectra due to a random distribution of vacant cages and hence of environments, and the Si spectra shift to lower frequencies, indicating a reduction in conduction s-electron density on the Si. Intensity data suggest a preferential loss of Na from the large cages. XPS results show a protective coating of silica on the surface of the clathrates. NMR, XPS, and XANES results all indicate transfer of electrons from Na to the Si framework, as predicted by calculations. XANES shows a lowering of the absorption edge, and hence of the conduction band, of both clathrates relative to crystalline Si.

Four forms of nickel(II) and two of zinc(II) dibenzoylmethanates have been isolated and characterized with powder and single-crystal X-ray diffraction analyses, differential scanning calorimetry, magnetic susceptibility measurements, and solid-state 13C cross-polarization/magic angle spinning NMR. Nickel dibenzoylmethanate, Ni(DBM)2 (DBM = PhCOCHCOPh-), forms three polymorphic forms (light-green, brown, and green) and a fourth clathrate form with guest benzene included. The light-green polymorph is metastable. Substituted benzenes induce recrystallization of the polymorph into a stable brown form (C30H22NiO4; a = 26.502(3) A, b = 5.774(1) A, c = 16.456(2) A, beta = 116.03(1) degrees; monoclinic, C2/c; Z = 4). Unlike the other forms, the brown form is diamagnetic and is comprised of monomers of the low-spin [Ni(DBM)2] complex. The Ni(II) is chelated by two DBM ligands in a square planar environment by four donor oxygen atoms. When heated, the brown form transforms to a green form which is stable above 202 degrees C (C90H66Ni3O12; a = 13.819(2) A, b = 16.252(2) A, c = 17.358(2) A, beta = 108.28(1) degrees; monoclinic, P2(1)/n; Z = 2). This polymorph is formed by van der Waals packing of trimers [Ni3(DBM)6] containing linear Ni3 clusters with an Ni-Ni distance of 2.81 A. The cluster is surrounded by six DBM ligands, providing a distorted octahedral environment about each Ni by six oxygen atoms. Benzene stabilizes the trimeric structure at room temperature, forming a [Ni3(DBM)6].2(benzene) inclusion compound (Ni-Ni distance of 2.83 A) with guest benzene molecules located in channels (C90H66Ni3O12 + 2(C6H6); a = 17.670(2) A, b = 20.945(3) A, c=11.209(2) A, beta = 102.57(1) degrees; monoclinic, P2(1)/c; Z = 2). Zinc dibenzoylmethanate has been prepared in two polymorphic forms. The monomeric form contains [Zn(DBM)2] molecules with the zinc center in a distorted tetrahedral environment of four oxygens from the two chelated DBMs (C30H22O4Zn; a = 10.288(2) A, b = 10.716(2) A, c = 12.243(2) A, alpha = 89.19(1) degrees, beta = 75.39(1) degrees, gamma = 64.18(1) degrees; triclinic, P1; Z = 2). Another, dimeric form contains [Zn2(DBM)4] species, with two zinc atoms separated by a distance of 3.14 A and each zinc coordinated by five oxygen atoms (C60H44O8Zn2; a = 25.792(3) A, b = 7.274(1) A, c = 24.307(2) A, beta = 90.58(1) degrees; monoclinic, C2/c; Z = 4). The polymorphic variety of the title complexes and the peculiarities of the Ni(II) and Zn(II) coordination environments are discussed in the context of using the complexes as precursors for new metal complex hosts.

In this contribution gas-phase 129Xe spin-lattice relaxation time measurements are extended to conditions (pressure, temperature, magnetic-field strength, isotope composition) not previously used. It is shown that wall effects become apparent at densities below ~20 amagat, and that these become dominant below ~3 amagat. A significant new discovery from field-dependent studies is that, in addition to the previously identified field-independent spin�rotation relaxation operative in the bulk gas, there is a contribution from the modulation of the chemical shift that depends on the square of the applied magnetic-field strength. The weak temperature dependence of the relaxation times can be understood in terms of the opposite temperature coefficients of the field-independent and field-dependent contributions to the relaxation. The spin�rotation contribution and its temperature dependence are calculated and found to be in good agreement with the experimental data. The low field, low density limit for the relaxation time is 56.3�2.6 hr�amagat at 295 K in the absence of wall effects.

Applications of hyperpolarized xenon to the measurement of diffusion in porous materials were tested on porous Vycor glass. Ingression of hyperpolarized xenon into Vycor was followed both by measur...

In this contribution we report new approaches to the MRI of materials using continuously produced laser-polarized (129)Xe gas. This leads to vastly improved sensitivity and makes new kinds of information available. The hyperpolarized xenon is produced in a continuous flow system that conveniently delivers the xenon at low partial pressure to probes for NMR and MRI experiments. We illustrate applications to the study of micropore and other kinds of void space and show for the first time that with flowing hyperpolarized xenon it is possible to obtain chemical-shift-resolved images in a relatively short time.

For a number of years now, siloxanes have been the materials of choice for coating vessels used in the production and short-term storage of hyperpolarized xenon. The methods used to apply this material, however, often vary from one research group to another and it is commonly reported that it is difficult to obtain cells with consistently long spin-lattice relaxation times (T1) and high-polarization levels. In a series of controlled experiments individual production variables were altered and optimized, leading to improved protocols for the reliable production of high-quality siloxane-coated cells. During these studies we discovered that the surface-induced relaxation rates in bare and coated Pyrex cells differ profoundly. This information on Xe relaxation helps to define the limits on the way pumping cells can be improved and suggests the need for further fundamental work on relaxation mechanisms.

When Rb+ is incorporated into different cubic halide lattices at very low concentrations, the 87Rb NMR chemical shift becomes more shielded by as much as 282 ppm when the cation is changed from Na+ to Cs+. Similarly, in mixed crystals of KCl and RbCl, the average resonance frequency shifts with the degree of incorporation. The 87Rb chemical shift data for each halide show a near-linear correlation with the average alkali metal−halide interionic distance in the different crystals studied, in good agreement with ab initio calculations which show that indeed the chemical shielding has a strong dependence on the nearest neighbor shell of halide anions and their distance from the Rb+. When the Rb concentration is increased as in KCl/RbCl solid solutions, the 87Rb NMR spectra are influenced by a large distribution of K/Rb configurations in the next-nearest-neighbor shell of 12 cations. While the total shift is still largely dependent on the Rb−Cl distance, shells containing both K and Rb atoms give rise to seco...

Single-crystal X-ray diffraction and 13C solid state NMR spectroscopy were used to characterize the structure and dynamics of the p-xylene clathrate of Dianin's compound. In contrast to conclusions obtained from single-crystal 2H NMR and modelling, the diffraction results suggest there is a single (symmetry disordered) guest site without any significant distortion of the host framework. A single xylene guest statistically disordered over six overlapping equivalent positions can account for the 13C NMR spectrum at room temperature. The high crystal-lographic symmetry arises from space averaging. At high temperatures the 13C spectrum is consistent with the onset of dynamic processes that result in higher effective symmetry.

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We propose two new double resonance experiments that are sensitive to internuclear dipolar couplings, namely Quadrupolar Echo DOuble Resonance (QEDOR) and SOLid Echo DOuble Resonance (SOLEDOR). The experiments are explained by analysis of the spin evolution during the pulse sequences, and are demonstrated for deuterium–proton double resonance experiments with partially deuterated polycrystalline glycine. In addition to molecular dynamics information normally obtained from the standard quadrupolar echo experiment, the QEDOR experiment is able to provide information about the local molecular surroundings by probing heteronuclear dipolar interactions. We have illustrated this with experiments on single-phase and phase-separated samples of tetrahydrofuran clathrate hydrate.

The interaction and reaction of PCl3 and PCl3-H2S mixtures with the microporous materials Silicalite, ALPO-5, NaY, NaX, and NaA have been investigated, with the intention of producing phosphorus sulfide clusters in the pores. A rich chemistry was observed and monitored by solid state 31P NMR. The presence of P4S3, α-P4S5, β-P4S6, and P4S7 inside the NaY α-cages was demonstrated, as well as a new species that is possibly a third geometric isomer of P4S4 with C3v symmetry. 129Xe NMR showed the exclusion of Xe from the micropores by the phosphorus sulfides. Sulfides lower than P4S7 are small enough that they undergo rapid pseudo-isotropic reorientation inside the NaY α-cage. Key words: phosphorus sulfides, intrazeolitic cluster synthesis.

Luminescent silicon clusters have been synthesized by the chemical vapor deposition of Si2H6 into the α-cages of H32Na24Y zeolite. The synthetic process was monitored by FTIR, TGA-MS, and 29Si and 1H solid-state NMR spectroscopies. In the initial step at 100 °C, Si2H6 reacts with the Bronsted acid sites to produce anchored ZO−Si2H5. Si2H6 is also chemisorbed at Na+ cation sites to give Si2H6/NaHY and is possibly physically trapped within the α-cage by the anchored disilyl groups. Multiple quantum 1H NMR spin counting shows that each α-cage contains 38 H atoms. This is equivalent to 14 Si atoms present as a combination of disilyl and disilane. Subsequent thermal treatment of the entrapped disilane precursors leads, via H2 and SiH4 elimination, to the formation of Si clusters. The formation of Si clusters is complete at 550 °C. These clusters are capped by up to 5 H atoms (determined by 1H NMR spin counting) and attached to the zeolite framework through SiOx linkages (determined by Si K-edge XANES). The ave...

The adsorption of xenon in AgA zeolite has been studied by 129Xe NMR spectroscopy, yielding information on the silver distribution, Xe cluster size and distribution, and xenon exchange dynamics. The exchange of xenon is slow between the α-cages of AgA treated in a vacuum at 380 or 410 K (yellow AgA), and separate lines appear in the 129Xe NMR spectra for cages containing different xenon populations. Up to 10 different Xen clusters with n between 1 and 8 can be distinguished; the Xe7 and Xe8 clusters appear to be present in two different states Xe7‘ + Xe7‘‘ and Xe8‘ + Xe8‘‘, reflecting cage differences apparent only at high xenon loading. The population distribution, studied over a broad range of xenon loadings, cannot be described in terms of the simple hypergeometric distribution except at loading levels below n = 4. The Xen cluster lines for “yellow” AgA zeolite are all shifted uniformly to low field relative to the clusters in NaA zeolite, a feature which currently has no firm explanation. Because larg...

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.

Prospective industrial applications of clathrate hydrates as materials for gas separation require further knowledge of cavity distortion, cavity selectivity, and defects induction by guest-host interactions. The results presented in this contribution show that under certain temperature conditions the guest combination of CH3F and a large polar molecule induces defects on the clathrate hydrate framework that allow intercage guest dynamics. 13C NMR chemical shifts of a CH3F/CH4/TBME sH hydrate and a temperature analysis of the 2H NMR powder lineshapes of a CD3F/THF sII and CD3F/TBME sH hydrate, displayed evidence that the populations of CH4 and CH3F in the D and D′ cages were in a state of rapid exchange. A hydrogen bonding analysis using molecular dynamics simulations on the TBME/CH3F and TBME/CH4 sH hydrates showed that the presence of CH3F enhances the hydrogen bonding probability of the TBME molecule with the water molecules of the cavity. Similar results were obtained for THF/CH3F and THF/CH4 sII hydrates. The enhanced hydrogen bond formation leads to the formation of defects in the water hydrogen bonding lattice and this can enhance the migration of CH3F molecules between adjacent small cages. © 2014 AIP Publishing LLC.

There have been very few attempts to apply crystal engineering approaches to modify the clathrate lattice with the aim of modifying structure and properties. To this end, solutions of ammonium fluoride in water were used to prepare clathrate hydrates with ammonium fluoride replacing water molecules in the hydrate lattice. Both modified structure I Xe and structure II tetrahydrofuran/Xe hydrates were prepared, with the hydrate lattices consisting of NH4F-water solid solutions containing up to ~19 and 25 mole% NH4F, respectively. The lattice constants for both hydrates decreased with increase of the amount of NH4F incorporated, and guest positions and cage occupancies were determined from the PXRD patterns with direct space methods and Rietveld analysis. The 129Xe NMR spectra for Xe in the small cavities of each hydrate showed NH4F concentration-dependent fine structure, not evident in the pure water clathrates, and characteristic of the presence of cage configurations with different distributions of ions. Analysis of the spectra along with density functional theory calculations of the chemical shifts allowed reasonable assignments to be made of the ion distribution. As a test of the altered function of the clathrate upon modification, a clathrate of methanol was prepared, something which has not been possible to do with a pure water clathrate. Structural analysis of the PXRD pattern by direct space methods showed that the methanol OH was hydrogen-bonded to one or both of the NH 4 + and F- ions in the lattice, a conclusion corroborated by the absence of molecular motion (except for methyl group rotation) of the methanol guest in the clathrate cages, as determined by static 2H NMR and molecular dynamics simulations. The stability of the modified methanol clathrate can be attributed to the strong methanol CH 3OH⋯F- or CH3OH⋯NH 4 + hydrogen bonding which leaves the water-water hydrogen bonding network intact, as opposed to the situation in a pure water clathrate where the methanol-water hydrogen bonding disrupts the lattice and a stable clathrate cannot be made.