Unusual species of methane hydrate detected in nanoporous media using solid state 13C NMR.

Methane is considered to be a cubic structure I (CS-I) clathrate hydrate former, although in a number of instances, small amounts of structure II (CS-II) clathrate hydrate have been transiently observed as well. In this work, solid-state magic angle spinning 13C NMR spectra of methane hydrate formed at low temperatures inside silica-based nanoporous materials with pores in the range of 3.8-20.0 nm (CPG-20, Vycor, and MCM-41) show methane in several different environments. In addition to methane encapsulated in the dodecahedral 512 (D) and tetrakaidecahedral 51262 (T) cages typical of the CS-I clathrate hydrate phase, methane guests in pentakaidecahedral 51263 (P) and hexakaidecahedral 51264 (H) cages are also identified, and these appear to be stabilized for extended periods of time. The ratio of methane guests among the D and T cages determined from the line intensities is significantly different from that of bulk CS-I samples and indicates that both CS-I and CS-II are present as the dominant species. This is the first observation of methane in P cages, and the possible structures in which they could be present are discussed. Broad and relatively strong methane peaks, which are also observed in the spectra, can be related to methane dissolved in an amorphous component of water adjacent to the pore walls. Nanoconfinement and interaction with the pore walls clearly have a strong influence on the hydrate formed and may reflect species present in the early stages of hydrate growth.

Methylammonium Cation Dynamics in Methylammonium Lead Halide Perovskites: A Solid-State NMR Perspective.

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.

Managing Hydrogen Bonding in Clathrate Hydrates by Crystal Engineering.

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 13 C 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 NH4 F. The latter acts as a lattice stabilizer by providing sites for strong hydrogen bonding of the normally disruptive methanol hydroxy group. NH4 F 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.

Methanol incorporation in clathrate hydrates and the implications for oil and gas pipeline flow assurance and icy planetary bodies.

One of the best-known uses of methanol is as antifreeze. Methanol is used in large quantities in industrial applications to prevent methane clathrate hydrate blockages from forming in oil and gas pipelines. Methanol is also assigned a major role as antifreeze in giving icy planetary bodies (e.g., Titan) a liquid subsurface ocean and/or an atmosphere containing significant quantities of methane. In this work, we reveal a previously unverified role for methanol as a guest in clathrate hydrate cages. X-ray diffraction (XRD) and NMR experiments showed that at temperatures near 273 K, methanol is incorporated in the hydrate lattice along with other guest molecules. The amount of included methanol depends on the preparative method used. For instance, single-crystal XRD shows that at low temperatures, the methanol molecules are hydrogen-bonded in 4.4% of the small cages of tetrahydrofuran cubic structure II hydrate. At higher temperatures, NMR spectroscopy reveals a number of methanol species incorporated in hydrocarbon hydrate lattices. At temperatures characteristic of icy planetary bodies, vapor deposits of methanol, water, and methane or xenon show that the presence of methanol accelerates hydrate formation on annealing and that there is unusually complex phase behavior as revealed by powder XRD and NMR spectroscopy. The presence of cubic structure I hydrate was confirmed and a unique hydrate phase was postulated to account for the data. Molecular dynamics calculations confirmed the possibility of methanol incorporation into the hydrate lattice and show that methanol can favorably replace a number of methane guests.

Facilitating guest transport in clathrate hydrates by tuning guest-host interactions.

The understanding and eventual control of guest molecule transport in gas hydrates is of central importance for the efficient synthesis and processing of these materials for applications in the storage, separation, and sequestration of gases and natural gas production. Previously, some links have been established between dynamics of the host water molecules and guest-host hydrogen bonding interactions, but direct observation of transport in the form of cage-to-cage guest diffusion is still lacking. Recent calculations have suggested that pairs of different guest molecules in neighboring cages can affect guest-host hydrogen bonding and, therefore, defect injection and water lattice motions. We have chosen two sets of hydrate guest pairs, tetrahydrofuran (THF)-CO2 and isobutane-CO2, that are predicted to enhance or to diminish guest-host hydrogen bonding interactions as compared to those in pure CO2 hydrate and we have studied guest dynamics in each using (13)C nuclear magnetic resonance (NMR) methods. In addition, we have obtained the crystal structure of the THF-CO2 sII hydrate using the combined single crystal X-ray diffraction and (13)C NMR powder pattern data and have performed molecular dynamics-simulation of the CO2 dynamics. The NMR powder line shape studies confirm the enhanced and delayed dynamics for the THF and isobutane containing hydrates, respectively, as compared to those in the CO2 hydrate. In addition, from line shape studies and 2D exchange spectroscopy NMR, we observe cage-to-cage exchange of CO2 molecules in the THF-CO2 hydrate, but not in the other hydrates studied. We conclude that the relatively rapid intercage guest dynamics are the result of synergistic guest A-host water-guest B interactions, thus allowing tuning of the guest transport properties in the hydrates by choice of the appropriate guest molecules. Our experimental value for inter-cage hopping is slower by a factor of 10(6) than a published calculated value.

Inter-cage dynamics in structure I, II, and H fluoromethane hydrates as studied by NMR and molecular dynamics simulations.

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. (13)C NMR chemical shifts of a CH3F/CH4/TBME sH hydrate and a temperature analysis of the (2)H 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.

Multiple H2 occupancy of cages of clathrate hydrate under mild conditions.

Experiments were carried out by reacting H(2) gas with N(2) hydrate at a temperature of 243 K and a pressure of 15 MPa. The characterizations of the reaction products indicated that multiple H(2) molecules can be loaded into both large and small cages of structure II clathrate hydrates. The realization of multiple H(2) occupancy of hydrate cages under moderate conditions not only brings new insights into hydrogen clathrates but also refreshes the perspective of clathrate hydrates as hydrogen storage media.

Structural phase transitions induced by pressure in ammonium borohydride.

A combined experimental and theoretical study of hydrogen-rich ammonium borohydride (NH4BH4) subjected to pressures up to 10 GPa indicates two phase transitions, detected by synchrotron radiation powder X-ray diffraction, Raman spectroscopy and Car-Parrinello molecular dynamics calculations, at 1.5 and 3.4 GPa. The ambient pressure, face-centred cubic phase of NH4BH4 transforms into a highly disordered intermediate structure which then evolves upon increasing pressure into an orthorhombic, distorted CsCl structure. The structure of the latter phase was solved using ab initio computational techniques and from a Rietveld full pattern refinement of the powder X-ray diffraction data.

Multinuclear solid-state nuclear magnetic resonance and density functional theory characterization of interaction tensors in taurine.

A variety of experimental solid-state nuclear magnetic resonance (NMR) techniques has been used to characterize each of the elements in 2-aminoethane sulfonic acid (taurine). A combination of (15)N cross-polarization magic angle spinning (CPMAS), (14)N ultrawideline, and (14)N overtone experiments enabled a determination of the relative orientation of the nitrogen electric field gradient and chemical shift tensors. (17)O spectra recorded from an isotopically enriched taurine sample at multiple magnetic fields allowed the three nonequivalent oxygen sites to be distinguished, and NMR parameters calculated from a neutron diffraction structure using density functional theory allowed the assignment of the (17)O parameters to the correct crystallographic sites. This is the first time that a complete set of (17)O NMR tensors are reported for a sulfonate group. In combination with (1)H and (13)C MAS spectra, as well as a previously reported (33)S NMR study, this provides a very broad set of NMR data for this relatively simple organic molecule, making it a potentially useful structure on which to test DFT calculation methods (particularly for the quadrupolar nuclei (14)N, (17)O, and (33)S) or NMR crystallography approaches.

Interaction tensors and local dynamics in common structural motifs of nitrogen: a solid-state 14N NMR and DFT study.

(14)N solid-state NMR powder patterns have been obtained at high field (21.1 T) using broadband, frequency-swept pulses and a piecewise acquisition method. This approach allowed the electric field gradient (EFG) tensor parameters to be obtained from model organic and inorganic systems featuring spherically asymmetric nitrogen environments (C(Q) values of up to ca. 4 MHz). The advantages and limitations of this experimental approach are discussed, and the observation of (14)N T(2) relaxation anisotropy in certain systems is also reported, which can shed light on dynamic processes, allowing motional geometries and jump rates to be probed. In particular, we show that observable effects of dynamics on (14)N spectra can be mediated by modulation of either the EFG tensor or heteronuclear dipolar couplings. It is demonstrated that the QCPMG protocol can be used to selectively enhance certain types of nitrogen environments on the basis of differences in T(2). We also present the results of extensive density functional theory calculations on these systems, which show remarkably good correlation with the experimental results and allow the prediction of tensor orientations, assignment of parameters to crystallographic sites, and a rationalization of the origin of the EFG tensors in terms of contributions from individual molecular orbitals. This work demonstrates that ultra-wideline (14)N solid-state NMR can, under favorable circumstances, be a straightforward, useful, and informative probe of molecular structure and dynamics.

Crystal structure based design of signal enhancement schemes for solid-state NMR of insensitive half-integer quadrupolar nuclei.

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.

13C NMR studies of hydrocarbon guests in synthetic structure H gas hydrates: experiment and computation.

(13)C NMR chemical shifts were measured for pure (neat) liquids and synthetic binary hydrate samples (with methane help gas) for 2-methylbutane, 2,2-dimethylbutane, 2,3-dimethylbutane, 2-methylpentane, 3-methylpentane, methylcyclopentane, and methylcyclohexane and ternary structure H (sH) clathrate hydrates of n-pentane and n-hexane with methane and 2,2-dimethylbutane, all of which form sH hydrates. The (13)C chemical shifts of the guest atoms in the hydrate are different from those in the free form, with some carbon atoms shifting specifically upfield. Such changes can be attributed to conformational changes upon fitting the large guest molecules in hydrate cages and/or interactions between the guests and the water molecules of the hydrate cages. In addition, powder X-ray diffraction measurements revealed that for the hexagonal unit cell, the lattice parameter along the a-axis changes with guest hydrate former molecule size and shape (in the range of 0.1 Å) but a much smaller change in the c-axis (in the range of 0.01 Å) is observed. The (13)C NMR chemical shifts for the pure hydrocarbons and all conformers were calculated using the gauge invariant atomic orbital method at the MP2/6-311+G(2d,p) level of theory to quantify the variation of the chemical shifts with the dihedral angles of the guest molecules. Calculated and measured chemical shifts are compared to determine the relative contribution of changes in the conformation and guest-water interactions to the change in chemical shift of the guest upon clathrate hydrate formation. Understanding factors that affect experimental chemical shifts for the enclathrated hydrocarbons will help in assigning spectra for complex hydrates recovered from natural sites.

Tuning clathrate hydrates for hydrogen storage.

The storage of large quantities of hydrogen at safe pressures is a key factor in establishing a hydrogen-based economy. Previous strategies--where hydrogen has been bound chemically, adsorbed in materials with permanent void space or stored in hybrid materials that combine these elements--have problems arising from either technical considerations or materials cost. A recently reported clathrate hydrate of hydrogen exhibiting two different-sized cages does seem to meet the necessary storage requirements; however, the extreme pressures (approximately 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 material, 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 approximately 4 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.

Facile proton conduction via ordered water molecules in a phosphonate metal-organic framework.

A new phosphonate metal-organic framework (MOF) with a layered motif but not that of the classical hybrid inorganic-organic solid is presented. Zn(3)(L)(H(2)O)(2)·2H(2)O (L = [1,3,5-benzenetriphosphonate](6-)), henceforth denoted as PCMOF-3, contains a polar interlayer lined with Zn-ligated water molecules and phosphonate oxygen atoms. These groups serve to anchor free water molecules into ordered chains, as observed by X-ray crystallography. The potential for proton conduction via the well-defined interlayer was studied by (2)H solid-state NMR spectroscopy and AC impedance spectroscopy. The proton conductivity in H(2) was measured as 3.5 × 10(-5) S cm(-1) at 25 °C and 98% relative humidity. More interestingly, an Arrhenius plot gave a low activation energy of 0.17 eV for proton transfer, corroborating the solid-state NMR data that showed exchange between all deuterium sites in the D(2)O analogue of PCMOF-3, even at -20 °C.

Probing cucurbituril assemblies in water with TEMPO-like nitroxides: a trinitroxide supraradical with spin-spin interactions.

Nitroxide free radicals are demonstrated to be useful guests for probing cucurbit[n]uril (CB[n]) assemblies in water by EPR spectroscopy. CB[7] and CB[8] significantly aggregate either in solution or in the solid state (as evidenced by a single-crystal X-ray structure in which CB[8] forms supramolecular triangles with 4-methoxy-TEMPO). The included nitroxides show a striking resistance to biologically relevant reduction, benefiting from the carcerand effect of CB[7] and CB[8] solutions.

Single-sized colloidal CdTe nanocrystals with strong bandgap photoluminescence.

One brand-new colloidal ensemble of single-sized CdTe nanocrystals, exhibiting a sharp first excitonic absorption peak at 427 nm and bright bandgap photoluminescence at 427 nm with narrow full width at half maximum (FWHM) of only ca. 10 nm, was synthesized in 1-octadecene (ODE) via a non-injection one-pot approach which features high synthetic reproducibility with potential for industrial scale production.

Inclusion complexes of coumarin in cucurbiturils.

Coumarin was found to form stable inclusion complexes with cucurbiturils. In the presence of cucurbit[7]uril (CB[7]), 1 : 1 inclusion complexes were observed in aqueous solution, as monitored by (1)H NMR and UV-visible absorption spectroscopies, and further supported by ab initio calculations, whereas with cucurbit[8]uril (CB[8]) a solid phase 1 : 2 host : guest complex was found in a single crystal X-ray diffraction structure determination.

Single-crystal to single-crystal phase transition of cucurbit[5]uril hydrochloride hydrates: large water-filled channels transforming to layers of unusual stability.

Cucurbit[5]uril hydrochloride hydrate crystals with large water-filled channels transform to a highly stable layer structure via a single-crystal to single-crystal mechanism; (129)Xe NMR showed that porosity in CB[5] samples depends critically on the method of preparation.

Structure, dynamics and ordering in structure i ether clathrate hydrates from single-crystal X-ray diffraction and 2H NMR spectroscopy.

The structure and dynamics of trimethylene oxide (TMO) and ethylene oxide (EO) structure I (sI) hydrates are reported from single-crystal X-ray diffraction and 2H NMR spectroscopic measurements. The guest molecule positions in the large cage were determined with considerable improvement over previous diffraction work so that a dynamic model that was consistent with these orientations could be developed to explain the 2H NMR data. Reorientations are shown to take place among both symmetry-related and symmetry-independent sites, 16 positions in all. Because of the prochiral nature of the molecules, both guests show 2H NMR line shapes with large asymmetry parameters, rather unusual for guest molecules in the sI hydrate large cage. The results also show that the dipolar axis of the TMO molecule lies close to the 4 bar axis of the cage on average, whereas for EO, this is not the case. For TMO, progressive alignment of the polar axis with decrease of temperature then allows the dipoles to interact more strongly until dipole reversal is quenched at the ordering transition. The lack of ordering of EO is consistent with the much weaker alignment of the molecular dipoles along the 4 bar axis. With the new complementary information on the structure and dynamics from crystallography and NMR, it is possible to understand why the large cage guests order in the large cage of sI hydrate for TMO hydrate but not for EO hydrate.