Bis(guanidinium) cyananilate

The asymmetric unit of the title compound, 2CH6N3 +·C8N2O4 2−, contains one half of a centrosymmetric 2,5-di­cyano-3,6-dioxocyclo­hexa-1,4-diene-1,4-diolate (cyananil­ate) anion and one guanidinium cation, which are connected by N—H⋯O and N—H⋯N hydrogen bonds into a three-dimensional network

Selective occupancy of methane by cage symmetry in TBAB ionic clathrate hydrate.

Methane trapped in the two distinct dodecahedral cages of the ionic clathrate hydrate of TBAB was studied by single crystal XRD and MD simulation

Towards a Green Hydrate Inhibitor: Imaging Antifreeze Proteins on Clathrates

The formation of hydrate plugs in oil and gas pipelines is a serious industrial problem and recently there has been an increased interest in the use of alternative hydrate inhibitors as substitutes for thermodynamic inhibitors like methanol. We show here that antifreeze proteins (AFPs) possess the ability to modify structure II (sII) tetrahydrofuran (THF) hydrate crystal morphologies by adhering to the hydrate surface and inhibiting growth in a similar fashion to the kinetic inhibitor poly-N-vinylpyrrolidone (PVP). The effects of AFPs on the formation and growth rate of high-pressure sII gas mix hydrate demonstrated that AFPs are superior hydrate inhibitors compared to PVP. These results indicate that AFPs may be suitable for the study of new inhibitor systems and represent an important step towards the development of biologically-based hydrate inhibitors

C-Methyl­calix[4]resorcinarene–1,4-bis­(pyridin-3-yl)-2,3-diaza-1,3-butadiene (1/2)

In the title compound, 2C12H10N4·C32H32O8, the calixarene adopts a rctt conformation with dihedral angles of 138.40 (1) and 9.10 (1)° between the opposite rings. The dihedral angles between the rings of the pyridine derivative are 8.80 (1) and 9.20 (1)°. In the crystal, adjacent C-methylcalix[4]resorcinarene molecules are connected into columns parallel to [010] by O—H⋯O hydrogen bonds. O—H⋯N hydrogen bonds between the axial phenoxyl groups and bipyridine molecules link the columns into sheets parallel to (011), which are connected by O—H⋯N hydrogen bonds. Further O—H⋯N hydrogen bonds link the bipyridine and C-methylcalix[4]resorcinarene molecules, giving rise to a three-dimensional network

Bis(guanidinium) chloranilate

The asymmetric unit of the title co-crystal, 2CH6N3 +·C6Cl2O4 2−, contains one half of a chloranilate anion and one guanidinium cation, which are connected by strong N—H⋯O hydrogen bonds into a two-dimensional network

Efficient Recovery of CO2 from Flue Gas by Clathrate Hydrate Formation in Porous Silica Gels

Thermodynamic measurements and NMR spectroscopic analysis were used to show that it is possible to recover CO2 from flue gas by forming a mixed hydrate that removes CO2 preferentially from CO2/N2 gas mixtures using water dispersed in the pores of silica gel. Kinetic studies with 1H NMR microimaging showed that the dispersed water in the silica gel pore system reacts readily with the gas, thus obviating the need for a stirred reactor and excess water. Hydrate phase equilibria for the ternary CO2-N2-water system in silica gel pores were measured, which show that the three-phase hydrate-water-rich liquid-vapor equilibrium curves were shifted to higher pressures at a specific temperature when the concentration of CO2 in the vapor phase decreased. 13C cross-polarization NMR spectral analysis and direct measurement of the CO2 content in the hydrate phase suggested that the mixed hydrate is structure I at gas compositions of more than 10 mol % CO2, and that the CO2 molecules occupy mainly the more abundant 51262 cages. This makes it possible to achieve concentrations of more than 96 mol % CO2 gas in the product after three cycles of hydrate formation and dissociation. 1H NMR microimaging showed that hydrate yields of better than 85%, based on the amount of water, could be obtained in 1 h when a steady state was reached, although ~90% of this yield was achieved after ~20 min of reaction time.NRC publication: Ye

Structural Characterization of Hydrate Formation in Black Oil and Condensate Systems

NRC publication: Ye

Nuclear magnetic resonance study of molecular motion in some solids

A number of solid substances ware examined by nuclear magnetic resonance methods with a view to investigating possible molecular motion. The possibility of using the adiabatic rapid passage technique as a method for investigating the molecular motion in the solid state was studied. Two systems, namely benzene and furan were studied. It was found that spin-lattice relaxation times and in some cases, second moments could be obtained using the adiabatic rapid passage technique; the results obtained were in good agreement with values obtained using standard techniques. Also it was found that the presence of a certain type of molecular motion severely affects the shape and amplitude of the adiabatic rapid passage signal. A number of charge-transfer complexes were investigated using standard broadline (1)H and (19)F nuclear magnetic resonance techniques. Some strong charge-transfer complexes studied in this manner include: a number of amine complexes of BF(3) and some halogen complexes of trimethylamine. Second moment and linewidth changes with temperature were interpreted in terms of reorientations of molecular groups within the complexes. Also studied were a number of weak complexes of benzene. Linewidth, second moment, and also spin-lattice relaxation time measurements showed that the benzene rings were reorientating about their hexad axes at frequencies greater than about 10(5) sec.(-1) at temperatures above 120°K. The activation energies for this motion depended strongly on the environment of the benzene ring in the complex. A study of some arene-chromium-tricarbonyl compounds indicated that the motional properties of the arene rings in the complexes resembled very much the motional properties of the free ring compounds; this suggests that specific bonding effects are relatively inimportant. Finally, linewidth, second moment and spin-lattice relaxation time measurements are reported for some soap systems, the alkali metal stearates and oleates. Transition temperatures observed could in some cases be correlated with values obtained by different methods. Some motional models were suggested in order to explain decreases in linewidths and second moments with increasing temperature. It was shown that methyl group rotation provides a relaxation mechanism in the low temperature phase for all the soaps studied. Activation energies found for this process ranged from 1.8-2.5 Kcal/mole. The effect of thermal history on phase transitions in the alkali metal stearates was also investigated. Thermograms were obtained using samples with different thermal histories using a differential scanning calorimeter. It was found that the thermal history of a sample may affect transition temperatures, and also the absence of presence of some transitions.Science, Faculty ofChemistry, Department ofGraduat

Clathrate Hydrates: Occurrence, Uses, and Problems

Clathrate hydrates are supramolecular framework materials in which guest molecules are physically trapped inside cages made of hydrogen-bonded water molecules. Naturally occurring hydrocarbon gas hydrates constitute a vast untapped energy resource, generally little known by the layman, though receiving increasing attention in the media. Ice-like in appearance, methane hydrate can generate about 160 times its own volume of gas at standard temperature and pressure (STP). Gas hydrates already have a huge impact on industry, as the cause of numerous problems for the oil and gas industry, but they also have a few specialized beneficial applications, and they have potential for use in a number of other areas. This review will be rather eclectic, because the study of hydrates cuts across many sciences, from the basic physics and chemistry of hydrates to their involvement in biological systems, in geological processes, in astronomy, and in climatology. They even have a place as a source of entertainment. The sources for much of the current information, especially regarding natural gas hydrates, are a number of books, reviews, and conference proceedings, to which the interested reader may refer.1-151, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 First, we will present a little history and basic science. Although gas hydrates have been known to scientists since the early nineteenth century, with the work of Davy (in 1811) and Faraday (in 1823) on chlorine hydrate, their true nature as clathrates was not demonstrated until the advent of x-ray crystallographic studies in the 1950s. Gas hydrates began to gain more attention when their potential for causing blockages in natural gas pipelines was first noted in 1934. Their existence as natural deposits in permafrost regions of the Earth was recognized in Siberia in 1965 and in Canada in 1974. Off-shore deposits were found with the advent of the Deep Sea Drilling Project, with the first indications of hydrate recorded around 1972 and the first samples recovered around 1983. The detailed physical science of hydrates can be found in a number of reviews.2, 4-94, 5, 6, 7, 8, 9 Three principal crystal structures are known: Structures I and II, which are cubic, and Structure H, which is hexagonal. All have small cages together with cages of increasing size (in the order I, II, H) that can accommodate larger guest molecules. They are nonstoichiometric, and their stabilities depend on the particular guest molecules and the pressure (P) and temperature (T) conditions. Stability models are based on the statistical thermodynamic description formulated by van der Waals and Platteeuw.16 Many hydrates can exist above the melting point of ice, some up to 28\ub0C under pressure. Because the guest does not have any chemical bonds to the host, it has considerable translational and rotational freedom within its cage. Resonant coupling between these guest motions and the low-frequency lattice vibrational modes results in a thermal conductivity for hydrates that is considerably lower than that in ice. Many different techniques have been applied to study hydrate compositions and physical properties. The most reliable methods for determining structure type are x-ray diffraction, solid-state nuclear magnetic resonance (NMR), and (to a lesser extent) Raman spectroscopy.NRC publication: Ye