Guest-induced symmetry lowering of an ionic clathrate material for carbon capture.

We report a new lattice structure of the ionic clathrate hydrate of tetra-n-butylammonium bromide induced by guest CO2 molecules, which is found to provide high CO2 storage capacity. The structure was characterized by a set of methods, including single crystal X-ray diffraction, NMR, and MD simulations.

Raman spectroscopic investigations on natural samples from the Integrated Ocean Drilling Program (IODP) Expedition 311: indications for heterogeneous compositions in hydrate crystals.

Natural gas hydrates usually are found in the form of structure I, encasing predominantly methane in the hydrate lattices as guest molecules, sometimes also minor amount of higher hydrocarbons, CO2 or H2S. Raman spectroscopy is an approved tool to determine the composition of the hydrate phase. Thus, in this study Raman spectroscopic analyses have been applied to hydrate samples obtained from Integrated Ocean Drilling Program (IODP) Expedition 311 in two different approaches: studying the samples randomly taken from the hydrate core, and--as a new application--mapping small areas on the surface of clear hydrate crystals. The results obtained imply that the gas composition of hydrate, in terms of relative concentrations of CH4 and H2S, is not homogeneous over a core or even within a crystal. The mapping method yielded results with very high lateral resolution, indicating the coexistence of different phases with the same structure but different compositions within a hydrate crystal.

Nonequilibrium adiabatic molecular dynamics simulations of methane clathrate hydrate decomposition.

Nonequilibrium, constant energy, constant volume (NVE) molecular dynamics simulations are used to study the decomposition of methane clathrate hydrate in contact with water. Under adiabatic conditions, the rate of methane clathrate decomposition is affected by heat and mass transfer arising from the breakup of the clathrate hydrate framework and release of the methane gas at the solid-liquid interface and diffusion of methane through water. We observe that temperature gradients are established between the clathrate and solution phases as a result of the endothermic clathrate decomposition process and this factor must be considered when modeling the decomposition process. Additionally we observe that clathrate decomposition does not occur gradually with breakup of individual cages, but rather in a concerted fashion with rows of structure I cages parallel to the interface decomposing simultaneously. Due to the concerted breakup of layers of the hydrate, large amounts of methane gas are released near the surface which can form bubbles that will greatly affect the rate of mass transfer near the surface of the clathrate phase. The effects of these phenomena on the rate of methane hydrate decomposition are determined and implications on hydrate dissociation in natural methane hydrate reservoirs are discussed.

Synthesis and characterization of a new structure of gas hydrate.

Atoms and molecules <0.9 nm in diameter can be incorporated in the cages formed by hydrogen-bonded water molecules making up the crystalline solid clathrate hydrates. For these materials crystallographic structures generally fall into 3 categories, which are 2 cubic forms and a hexagonal form. A unique clathrate hydrate structure, previously known only hypothetically, has been synthesized at high pressure and recovered at 77 K and ambient pressure in these experiments. These samples contain Xe as a guest atom and the details of this previously unobserved structure are described here, most notably the host-guest ratio is similar to the cubic Xe clathrate starting material. After pressure quench recovery to 1 atmosphere the structure shows considerable metastability with increasing temperature (T <160 K) before reverting back to the cubic form. This evidence of structural complexity in compositionally similar clathrate compounds indicates that the reaction path may be an important determinant of the structure, and impacts upon the structures that might be encountered in nature.

Simulations of structure II H2 and D2 clathrates: potentials incorporating quantum corrections.

Molecular dynamics simulations are used to study the stability of structure II H(2) and D(2) clathrates with different large and small guest occupancies at 160 and 250 K and 2.0 kbars. Simulations are performed with the recently proposed anisotropic site-site potentials of Wang for H2 and D2 [J. Quant. Spectrosc. Radiat. Transf. 76, 23 (2003)] which are parameterized to account for quantum corrections of order variant Planck's over 2pi(2) in the second virial coefficient. Occupancies of 0-2 in the small cages and 2-5 in the large cages are considered. Thermodynamic integration is used to determine the most stable guest occupancy at each temperature. Since lattice free energy and configurational energy differences are small for a number of different combinations of cage occupancies, one must expect that in bulk samples various combinations will indeed be observed. Special attention is given to the differences between H(2) and D(2) guests and implications on the hydrogen storage capacity of the clathrates are discussed.

Molecular dynamics study of the stability of methane structure H clathrate hydrates.

Molecular dynamics simulations are used to study the stability of structure H (sH) methane clathrate hydrates in a 3 x 3 x 3 sH unit cell replica. Simulations are performed at experimental conditions of 300 K and 2 GPa for three methane intermolecular potentials. The five small cages of the sH unit cell are assigned methane guest occupancies of one and large cage guest occupancies of one to five are considered. Radial distribution functions, unit cell volumes, and configurational energies are studied as a function of large cage CH(4) occupancy. Free energy calculations are carried out to determine the stability of clathrates for large cage occupancies. Large cage occupancy of five is the most stable configuration for a Lennard-Jones united-atom potential and the Tse-Klein-McDonald potential parametrized for condensed methane phases and two for the most stable configuation for the Murad and Gubbins potential.

What we have learned from the study of solid p-tert-butylcalix[4]arene compounds.

The study of solid p-tert-butylcalix[4]arene and its compounds with a variety of techniques has provided a good understanding of the versatility of this host molecule, how to induce a number of distinct host-guest motifs, its molecular recognition properties, the complex phase relationships and unique properties such as gas adsorption without having obvious channels.

Stability of rare gas structure H clathrate hydrates.

Molecular dynamics simulations are used to study the stability of structure H (sH) clathrate hydrates with the rare gases Ne, Ar, Kr, and Xe. Simulations on a 3 x 3 x 3 sH unit cell replica are performed at ambient pressure at 40 and 100 K temperatures. The small and medium (s+m) cages of the sH unit cell are assigned rare gas guest occupancies of 1 and for large (l) cages guest occupancies of 1-6 are considered. Radial distribution functions for guest pairs with occupancies in the l-l, l-(s+m), and (s+m)-(s+m) cages are presented. The unit cell volumes and configurational energies are studied as a function of large cage occupancy for the rare gases. Free energy calculations are carried out to determine the stability of clathrates for large cage occupancies at 100 K and 1 bar and 20 kbar pressures. These studies show that the most stable argon clathrate has five guests in the large cages. For krypton and xenon the most stable configurations have three and two guests in the large cages, respectively.

Molecular dynamics simulations of binary structure H hydrogen and methyl-tert-butylether clathrate hydrates.

Binary structure H (sH) hydrogen and methyl-tert-butylether (MTBE) clathrate hydrates are studied with molecular dynamics simulations. Simulations on a 3 x 3 x 3 sH unit cell with up to 4.7 mass % hydrogen gas are run at pressures of 100 bars and 2 kbars at 100 and 273 K. For the small and medium cages of the sH unit cell, H2 guest molecule occupancies of 0, 1 (single occupancy), and 2 (double occupancy) are considered with the MTBE molecule occupying all of the large cages. An increase of the small and medium cage occupancies from 1 to 2 leads to a jump in the unit cell volume and configurational energy. Calculations are also set up with 13, 23, and 89 of the MTBE molecules in the large cages replaced by sets of three to six H2 molecules, and the effects on the configurational energy and volume of the simulation cell are determined. As MTBE molecules are replaced with sets of H2 guests in the large cages, the configurational energy of the unit cell increases. At the lower temperature, the energy and volume of the clathrate are not sensitive to the number of hydrogen guests in the large cages; however, at higher temperatures the repulsions among the H2 guest molecules in the large cages cause an increase in the system energy and volume.

Molecular-dynamics simulations of binary structure II hydrogen and tetrahydrofurane clathrates.

The binary structure II hydrogen and tetrahydrofurane (THF) clathrates are studied with molecular-dynamics simulations. Simulations are done at pressures of 120 and 1.013 bars for temperatures ranging from 100 to 273 K. For the small cages of the structure II unit cell, H2 guest molecule occupancies of 0, 16 (single occupancy), and 32 (double occupancy) are considered. THF occupancies of 0-8 in the large cages are studied. For cases in which THF does not occupy all large cages in a unit cell, the remaining large cages can be occupied with sets of four H2 guest molecules. The unit-cell volumes and configurational energies are compared in the different occupancy cases. Increasing the small cage occupancy leads to an increase in the unit-cell volume and thermal-expansion coefficient. Among simulations with the same small cage occupancy, those with the large cages containing 4H2 guests have the largest volumes. The THF guest molecules have a stabilizing effect on the clathrate and the configurational energy of the unit cell decreases linearly as the THF content increases. For binary THF + H2 clathrates, the substitution of the THF molecules in the large cages with sets of 4H2 molecules increases the configurational energy. For the binary clathrates, various combinations of THF and H2 occupancies have similar configurational energies.

NMR shielding constants for hydrogen guest molecules in structure II clathrates.

Proton NMR shielding constants and chemical shifts for hydrogen guests in small and large cages of structure II clathrates are calculated using density-functional theory and the gauge-invariant atomic-orbital method. Shielding constants are calculated at the B3LYP level with the 6-311++G(d,p) basis set. The calculated chemical shifts are corrected with a linear regression to reproduce the experimental chemical shifts of a set of standard molecules. The calculated chemical shifts of single hydrogen molecules in the small and large structure II cages are 4.94 and 4.84 ppm, respectively, which show that within the error range of the method the H2 guest molecules in the small and large cages cannot be distinguished. Chemical shifts are also calculated for double occupancy of the hydrogen guests in small cages, and double, triple, and quadruple occupancy in large cages. Multiple occupancy changes the chemical shift of the hydrogen guests by approximately 0.2 ppm. The relative effects of other guest molecules and the cage on the chemical shift are studied for the cages with multiple occupancies.

Molecular-dynamics study of structure II hydrogen clathrates.

Molecular-dynamics simulations are used to study the stability of structure II hydrogen clathrates with different H2 guest occupancies. Simulations are done at pressures of 2.5 kbars and 1.013 bars and for temperatures ranging from 100 to 250 K. For a structure II unit cell with 136 water molecules, H2 guest molecule occupancies of 0-64 are studied with uniform occupancies among each type of cage. The simulations show that at 100 K and 2.5 kbars, the most stable configurations have single occupancy in the small cages and quadruple occupancy in the large cages. The optimum occupancy for the large cages decreases as the temperature is raised. Double occupancy in the small cages increases the energy of the structures and causes tetragonal distortion in the unit cell. The spatial distribution of the hydrogen guest molecules in the cages is determined by studying the guest-water and guest-guest radial distribution functions at various temperatures.

Probing the geometry and interconnectivity of pores in organic aerogels using hyperpolarized 129XE NMR spectroscopy.

Hyperpolarized (HP) 129Xe NMR was used for the first time to probe the geometry and interconnectivity of pores in RF aerogels and to correlate them with the [resorcinol]/[catalyst] (R/C) ratio. We have demonstrated that HP 129Xe NMR is an ideal method for accurately measuring the volume-to-surface-area (Vg/S) ratios for soft mesoporous materials without using any geometric models. The Vg/S parameter, which is related both to the geometry and the interconnectivity of the porous 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 5-fold rise in Vg/S. Unlike conventional techniques, HP 129Xe NMR spectroscopy probes the geometry and interconnectivity of the nano- or mesopores in soft materials, providing new insights into the pore structure.

(6,17-Dimethyl-8,15-diphenyldibenzo[b,i][1,4,8,11]-tetra-aza[14]annulenato)nickel(II) in solids: two guest-free polymorphs and inclusion compounds with methylene chloride and fullerene (C(60))-carbon disulfide.

Two guest-free polymorphs and two inclusion compounds of the macrocyclic title complex [NiL] have been isolated and characterized with single-crystal and/or powder XRD, solid-state (13)C NMR, and other methods. The inclusion compound with methylene chloride, [NiL](CH(2)Cl(2)), is stable in air and thermally stable up to approximately 128 degrees C. Its crystal structure is consistent with van der Waals packing of the host [NiL] and guest CH(2)Cl(2) molecules. The host complex has square-planar coordination of the nickel(II) center with four nitrogen atoms of the macrocycle with an average Ni-N distance of 1.86 A. The molecule has a saddle-shaped conformation with the guest molecule located between one phenylene and two phenyl rings of the host molecule. Isostructural compounds with chloroform and 2-chloropropane form only as mixtures along with a guest-free host polymorph. The inclusion compound with C(60) has a composition 3[NiL]*(C(60))*2(CS(2)) and here also the crystal structure is consistent with a van der Waals type of packing. Three crystallographically inequivalent [NiL] molecules have geometries similar to that in the inclusion compound with methylene chloride. The concave surfaces of the complex molecules form a spherical cavity for the C(60) molecule. At -100 degrees C the C(60) molecule is disordered over two orientations centered at the same site. (13)C NMR studies at room temperature show that the C(60) molecule is undergoing rapid pseudo-isotropic rotation. The stability and other properties of the title and related complexes are discussed.

Novel 4-vinylpyridine-extended metal-dibenzoylmethanate host frameworks: structure, polymorphism, and inclusion properties.

In this contribution we show that host materials based on metal dibenzoylmethanates (DBM) can be extended in a versatile way by decreasing the packing efficiency of the simpler metal DBM's reported earlier. Specifically, this can be accomplished by coordinating two 4-vinylpyridines (4-ViPy) to the metal (Ni or Co) DBM units to give [M(4-ViPy)2(DBM)2] host complexes. These display a remarkable polymorphism and an ability to form inclusion compounds with a large variety of organic species. Five non-clathrate phases representing three polymorphic types and twenty-eight inclusion compounds with nineteen guests, representing five structural types were isolated and studied in varying degrees of detail. The inclusion compounds can be prepared by recrystallization or by interaction of the solid host with guest vapor. In the latter case, the process realization, kinetics and final product strongly depend on the host polymorph chosen as starting material. Kinetic studies executed with powder XRD suggest that transient formation of inclusion compounds may occur even during solvent vapor induced transformation of one guest-free polymorph to another. The beta polymorph of the Ni-host reveals the strongest clathratogenic ability as well as a high selectivity towards certain homologues and isomers. Its properties give insight into the concept of "flexible zeolite mimics", or "apohosts", as this empty host form is energetically and structurally predisposed towards inclusion processes. In all eleven (three host and eight clathrate) structures studied by single crystal X-ray diffraction the [M(4-Vi-Py)2(DBM)2] complex molecule is transconfigured. In most, the host molecules show effective packing in one dimension by forming parallel chains. Guest species are located between the chains in cages or channels formed by combining voids in the host molecules belonging to adjacent chains. The corresponding Ni and Co versions of the compounds studied were similar.

The effects of anion variation and ligand derivatization on silver coordination networks based upon weaker interactions.

This article presents a series of silver(I) coordination networks based upon nonchelating bidentate thioether ligands. Frameworks using AgOTs as the silver(I) starting material form two-dimensional frameworks and are quite stable as shown by differential scanning calorimetry/thermogravimetric analysis (DSC/TGA) data. The networks are sufficiently robust as to maintain the same layered motif when the basic skeleton of the ligand is sequentially derivatized with -OEt, OBu, and OHex groups. Crystal structures of the AgOTs complexes of the underivatized and bis(hexoxy) derivatives, compounds 5 and 8, respectively, are presented as well as powder X-ray diffraction (PXRD) data of the other complexes. For 5, C20H20S3O3Ag, crystal data are as follows: monoclinic, space group P2(1)/n, a = 11.8117(5) A, b = 7.8813(5) A, c = 22.3316(10) A, beta = 102.245(5) degrees, V = 2031.6(2) A(3), Z = 4. For 8, C30H44S3O6Ag, crystal data are as follows: triclinic, space group Ponebar a = 8.445(4) A, b = 10.855(5) A, c = 19.308(9) A, alpha = 84.53(1) degrees, beta = 78.76(1) degrees, gamma = 68.43(1) degrees V = 1613.9(13) A(3), Z = 2. Changing the silver(I) starting material to AgPF6 results in a shift to a one-dimensional structure, 9, as shown by X-ray crystallography and in highly compromised stability. For 9, C14H16S2N2PF6Ag, crystal data are as follows: monoclinic, space group P2/n, a = 11.9658(11) A, b = 3.9056(4) A, c = 19.6400(18) A, beta = 92.87(1) degrees, V = 916.70(15) A(3), Z = 4.

In situ switching of sorbent functionality as monitored with hyperpolarized (129)Xe NMR spectroscopy.

In this contribution, we demonstrate that a material (organic zeolite mimetic coordination polymer [CuL(2)], where L = L(-) = CF(3)COCHCOC(OCH(3))(CH(3))(2)) can be endowed with its functionality in situ under molecular-level control. This process involves the isomerization of the ligands followed by phase interconversion from a dense to an open, porous form. The porous (beta) form of the complex reveals zeolite-like behavior but, unlike zeolites and many other hard porous frameworks, porosity may be created or destroyed at will by the application of suitable external stimuli. Contact with methylene chloride vapor was used to switch on the sorbent functionality, whereas switching off was accomplished with a temperature pulse. The transformations between functionally inactive alpha and active beta forms, as well as the amount of vacant pore space, were monitored in situ by observing the NMR spectrum of hyperpolarized (HP) Xe atom probes. For methylene chloride, the chemical shift of the coabsorbed HP Xe correlated directly with the amount of adsorbate in the pore system of the open framework, illustrating the use of HP Xe for following sorption kinetics. The adsorption of propane, as an inert adsorbate, was also monitored directly with (1)H NMR, with HP Xe and by BET measurements, revealing more complex behavior.