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.

Local Electronic Structure in AlN Studied by Single-Crystal 27Al and 14N NMR and DFT Calculations.

Both the chemical shift and quadrupole coupling tensors for 14 N and 27 Al in the wurtzite structure of aluminum nitride have been determined to high precision by single-crystal NMR spectroscopy. A homoepitaxially grown AlN single crystal with known morphology was used, which allowed for optical alignment of the crystal on the goniometer axis. From the analysis of the rotation patterns of 14 N ( I = 1 ) and 27 Al ( I = 5 / 2 ), the quadrupolar coupling constants were determined to χ ( 14 N ) = ( 8 . 19 ± 0 . 02 ) kHz, and χ ( 27 Al ) = ( 1 . 914 ± 0 . 001 ) MHz. The chemical shift parameters obtained from the data fit were δ i s o = - ( 292 . 6 ± 0 . 6 ) ppm and δ Δ = - ( 1 . 9 ± 1 . 1 ) ppm for 14 N, and (after correcting for the second-order quadrupolar shift) δ i s o = ( 113 . 6 ± 0 . 3 ) ppm and δ Δ = ( 12 . 7 ± 0 . 6 ) ppm for 27 Al. DFT calculations of the NMR parameters for non-optimized crystal geometries of AlN generally did not match the experimental values, whereas optimized geometries came close for 27 Al with χ ¯ calc = ( 1 . 791 ± 0 . 003 ) MHz, but not for 14 N with χ ¯ calc = - ( 19 . 5 ± 3 . 3 ) kHz.

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.

Local Electronic Structure in γ-LiAlO2 Studied by Single-Crystal 27Al NMR and DFT Calculations.

From single-crystal 27Al NMR experiments, the full tensors for both the electrical field gradient (EFG) and the chemical shift (CS) for the aluminum atoms in γ-LiAlO2 have been determined. A simultaneous fit of the quadrupolar splittings observed for the four 27Al in the unit cell gave the EFG tensor in the crystal frame, from which a quadrupolar coupling constant of χ = CQ = 3.330 ± 0.005 MHz and an asymmetry parameter of ηQ = 0.656 ± 0.002 were derived. The experimentally determined quadrupolar splittings were sufficiently sensitive to quantify small deviations of both rotation axis direction and starting direction by the data fitting routine. For determination of the CS tensor, the evolution of the outer satellite centers over the crystal rotation was tracked, and the contribution of the quadrupolar shift was subtracted according to the previously determined EFG tensor. The resulting CS tensor of 27Al yields an isotropic chemical shift of δiso = 81.8 ± 0.25 ppm and an asymmetry parameter of ηCS = 0.532 ± 0.004, in good agreement with the fit of a MAS NMR spectrum acquired at B0 = 21.1 T. From both experiments and DFT calculations using the Castep code, we find the eigenvectors of the EFG and CS tensors to be practically colinear.

Disorder of Hydrofluorocarbon Molecules Entrapped in the Water Cages of Structure†I Clathrate Hydrate.

Water versus fluorine: Clathrate hydrates encaging hydrofluorocarbons as guests show both isotropic and anisotropic distributions within host water cages, depending on the number of fluorine atoms in the guest molecule; this is caused by changes in intermolecular interactions to host water molecules in the hydrates.

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.

Kinetics of methane hydrate replacement with carbon dioxide and nitrogen gas mixture using in situ NMR spectroscopy.

In this study, the kinetics of methane replacement with carbon dioxide and nitrogen gas in methane gas hydrate prepared in porous silica gel matrices has been studied by in situ (1)H and (13)C NMR spectroscopy. The replacement process was monitored by in situ (1)H NMR spectra, where about 42 mol % of the methane in the hydrate cages was replaced in 65 h. Large amounts of free water were not observed during the replacement process, indicating a spontaneous replacement reaction upon exposing methane hydrate to carbon dioxide and nitrogen gas mixture. From in situ (13)C NMR spectra, we confirmed that the replacement ratio was slightly higher in small cages, but due to the composition of structure I hydrate, the amount of methane evolved from the large cages was larger than that of the small cages. Compositional analysis of vapor and hydrate phases was also carried out after the replacement reaction ceased. Notably, the composition changes in hydrate phases after the replacement reaction would be affected by the difference in the chemical potential between the vapor phase and hydrate surface rather than a pore size effect. These results suggest that the replacement technique provides methane recovery as well as stabilization of the resulting carbon dioxide hydrate phase without melting.

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.

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.

Assembling photoluminescent silicon nanocrystals into periodic mesoporous organosilica.

A contemporary question in the intensely active field of periodic mesoporous organosilica (PMO) materials is how large a silsesquioxane precursor can be self-assembled under template direction into the pore walls of an ordered mesostructure. An answer to this question is beginning to emerge with the ability to synthesize dendrimer, buckyball, and polyhedral oligomeric silsesquioxane PMOs. In this paper, we further expand the library of large-scale silsesquioxane precursors by demonstrating that photoluminescent nanocrystalline silicon that has been surface-capped with oligo(triethoxysilylethylene), denoted as ncSi:(CH(2)CH(2)Si(OEt)(3))(n)H, can be self-assembled into a photoluminescent nanocrystalline silicon periodic mesoporous organosilica (ncSi-PMO). A comprehensive multianalytical characterization of the structural and optical properties of ncSi-PMO demonstrates that the material gainfully combines the photoluminescent properties of nanocrystalline silicon with the porous structure of the PMO. This integration of two functional components makes ncSi-PMO a promising multifunctional material for optoelectronic and biomedical applications.

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.

Molecularly imprinted mesoporous organosilica.

We have prepared molecularly imprinted mesoporous organosilica (MIMO) using a semicovalent imprinting technique. A thermally reversible covalent bond was used to link a bisphenol A (BPA) imprint molecule to a functional alkoxysilane monomer at two points to generate a covalently bound imprint precursor. This precursor was incorporated into a cross-linked periodic mesoporous silica matrix via a typical acid-catalyzed, triblock copolymer-templated, sol-gel synthesis. Evidence of imprint sites buried in the pore walls was found through careful characterization of the imprinted material and its comparison to similarly prepared non-imprinted mesoporous organosilica (NIMO) and pure periodic mesoporous silica (PMS). After thermal treatment, the imprinted material (MIMO-ir) removed more than 90% of appropriately sized bisphenol species from water, yet showed significantly lower binding for both smaller and larger molecules containing phenol moieties. Identically treated NIMO-ir showed much poorer retention behavior than MIMO-ir for the same bisphenol species and behaved only slightly better than PMS-ir.

Thermodynamic and molecular-scale analysis of new systems of water-soluble hydrate formers + CH4.

Among a variety of cyclic ether, cyclic ester, and cyclic ketone compounds, six new formers were found to form binary sII or sH hydrates with CH(4) gas. Hydrate-phase equilibria for all the hydrate formers were measured. The results obtained showed distinct relationships between the hydrate-phase equilibrium curve and the molecular size of the guests. In addition, 2-methyltetrahydrofuran and 3-methyltetrahydrofuran, or 4-methyl-1,3-dioxane and 4-methyl-1,3-dioxolane, showed different hydrate structures even though they have similar chemical structures. Such structural differences can provide useful information on the critical guest size, which determines hydrate crystal structures according to the size of the captured guest.

Structure and dynamics of ammonium borohydride.

Synchrotron powder X-ray diffraction, ab initio molecular dynamics calculations and solid state (1)H and (2)H NMR are used to refine the structure of crystalline NH(4)BH(4) including H atoms. Rapid reorientations of both ions mean that on average half-hydrogens occupy the corners of a cube around B or N.

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.

Testing the sensitivity limits of ³³S NMR: an ultra-wideline study of elemental sulfur.

Preliminary DFT investigations into the feasibility of using (33)S solid-state NMR to study organic and biological molecules suggest that very large (33)S quadrupolar coupling constants (>40MHz) are not uncommon. We have therefore investigated the possibility of using recently developed ultra-wideline techniques to record such (33)S powder patterns at a high magnetic field (21.1T). A WURST-echo sequence was used to record the spectrum from a>99.9% enriched sample of elemental sulfur, resulting in the largest (33)S quadrupolar coupling constant yet measured by solid-state NMR (C(Q)=43.3 MHz). Implications of this experiment are briefly discussed.

Comparing quantum-chemical calculation methods for structural investigation of zeolite crystal structures by solid-state NMR spectroscopy.

Combining quantum-chemical calculations and ultrahigh-field NMR measurements of (29)Si chemical shielding (CS) tensors has provided a powerful approach for probing the fine details of zeolite crystal structures. In previous work, the quantum-chemical calculations have been performed on 'molecular fragments' extracted from the zeolite crystal structure using Hartree-Fock methods (as implemented in Gaussian). Using recently acquired ultrahigh-field (29) Si NMR data for the pure silica zeolite ITQ-4, we report the results of calculations using recently developed quantum-chemical calculation methods for periodic crystalline solids (as implemented in CAmbridge Serial Total Energy Package (CASTEP) and compare these calculations to those calculated with Gaussian. Furthermore, in the context of NMR crystallography of zeolites, we report the completion of the NMR crystallography of the zeolite ITQ-4, which was previously solved from NMR data. We compare three options for the 'refinement' of zeolite crystal structures from 'NMR-solved' structures: (i) a simple target-distance based geometry optimization, (ii) refinement of atomic coordinates in which the differences between experimental and calculated (29)Si CS tensors are minimized, and (iii) refinement of atomic coordinates to minimize the total energy of the lattice using CASTEP quantum-chemical calculations. All three refinement approaches give structures that are in remarkably good agreement with the single-crystal X-ray diffraction structure of ITQ-4.

Natural abundance high field (43)Ca solid state NMR in cement science.

This work is a systematic attempt to determine the possibilities and the limitations of the (43)Ca high field solid state NMR in the study of cement-based materials. The low natural abundance (0.135%) and small gyromagnetic ratio of (43)Ca present a serious challenge even in a high magnetic field. The NMR spectra of a number of cement compounds of known structure and composition are examined. The spectra of several phases important in cement science, e.g., anhydrous beta di-calcium silicate (beta-C(2)S) and tri-calcium (C(3)S) silicate were obtained for the first time and the relation of spectroscopic and structural parameters is discussed. The method was also applied to the hydrated C(3)S and synthetic calcium silicate hydrates (C-S-H) of different composition in order to understand the state of calcium and transformations in the structure during hydrolysis. The spectra of hydrated C(3)S reveals a calcium environment similar to that of the C-S-H samples and 11 A Tobermorite. These observations support the validity of using layered crystalline C-S-H systems as structural models for the C-S-H that forms in the hydration of Portland cement.

Direct space methods for powder X-ray diffraction for guest-host materials: applications to cage occupancies and guest distributions in clathrate hydrates.

Structural determination of crystalline powders, especially those of complex materials, is not a trivial task. For non-stoichiometric guest-host materials, the difficulty lies in how to determine dynamical disorder and partial cage occupancies of the guest molecules without other supporting information or constraints. Here, we show how direct space methods combined with Rietveld analysis can be applied to a class of host-guest materials, in this case the clathrate hydrates. We report crystal structures in the three important hydrate crystal classes, sI, sII, and sH, for the guests CO(2), C(2)H(6), C(3)H(8), and methylcyclohexane + CH(4). The results obtained for powder samples are found to be in good agreement with the experimental data from single crystal X-ray diffraction and (13)C solid-state NMR spectroscopy. This method is also used to determine the guest disorder and cage occupancies of neohexane and tert-butyl methyl ether binary hydrates with CH(4) in the structure H clathrate hydrates. The results are found to be in good agreement with the results from the (13)C solid-state NMR and molecular dynamics simulations. It is demonstrated that the ab initio crystal structure determination methodology reported here is able to determine absolute cage occupancies and the dynamical disorder of guest molecules in clathrate hydrates from powdered crystalline samples.