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

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

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

Halogen and hydrogen host-guest bonding in cathrate hydrates

Clathrate hydrates with low melting points (often below \u201320\ub0C) are difficult subjects for single crystal data collection. A high level of guest molecule disorder inside the high symmetry cages causes difficulty for structure determination of such crystals as well. Recent advances in single crystal X-ray diffraction have allowed this technique to be used as a valuable tool for the analysis of hydrate structure and composition. With detailed analysis of guest and water molecules disorder, not only the guest positions are clearly defined, but also it becomes possible to find interactions between guest and water molecules. For the first time, single crystal x-ray crystallography is used to detect the presence of guest host hydrogen and halogen bonding in structure I, II and structure H clathrate hydrates. Clathrates studied are the tert- butylamine (tBA) sII clathrate with H2S and Xe help gases, the pinacolone + H2S binary sH clathrate, 1,3-Dioxolane hydrate, chlorine, bromine, and mixed chlorine with bromine hydrates. X-ray structural analysis shows that the tBA nitrogen atom has a distance of 2.64 \uc5 from the closest large cage oxygen atom. This water molecule is pulled inwards toward the tBA guest (cage center) and the structure of the large cage is substantially distorted in comparison to the ideal cage structure. The pinacolone oxygen atom is determined to have a distance of 2.96 \uc5 from the closest large cage oxygen atom.Peer reviewed: YesNRC publication: Ye

Water-fluorine chains in (n-Bu)\u2084NF\ub75.5H\u2082O hydrate

The structure and composition of (n-Bu)4NF?5.5H2O hydrate were determined from single crystal X-ray diffraction data at 173 K. It is monoclinic C2/c with cell parameters a=16.590(3), b=17.040(3), c=16.510(3) \uc5, \u3b2=103.18(3)\ub0. Water molecules and fluorine ions connected by hydrogen bonds form cages consisting of two square faces and four pentagonal faces. These cavities are connected to each other, forming an infinite chain parallel to the c axis of the crystal.NRC publication: Ye

Electrostatic and short-range interactions compete in directing the structure of p-tert-butylcalix[4]arene inclusion compounds of fluorinated benzenes

p-tert-Butylcalix[4]arene guest-host compounds with fluorinated benzenes show several structural motifs, thus indicating that the guest-host structure can be tuned to produce either a form with included guests or a form where the host self-includes with the guests outside the cavity.NRC publication: Ye

Effect of guest\u2013host hydrogen bonding on the structures and properties of clathrate hydrates

To provide improved understanding of guest\u2013host interactions in clathrate hydrates, we present somecorrelations between guest chemical structures and observations on the corresponding hydrate properties. From these correlations it is clear that directional interactions such as hydrogen bonding between guest and host are likely, although these have been ignored to greater or lesser degrees because there has been no direct structural evidence for such interactions. For the first time, single-crystal X-ray crystallography has been used to detect guest\u2013host hydrogen bonding in structure II (sII) and structure H (sH) clathrate hydrates. The clathrates studied are the tert-butylamine (tBA) sII clathrate with H2S/Xe help gases and the pinacolone + H2S binary sH clathrate. X-ray structural analysis shows that the tBA nitrogen atom lies at a distance of 2.64 \uc5 from the closest clathrate hydrate water oxygen atom, whereas the pinacolone oxygen atom is determined to lie at a distance of 2.96 \uc5 from the closest water oxygen atom. These distances are compatible with guest\u2013water hydrogen bonding. Results of molecular dynamics simulations on these systems are consistent with the X-ray crystallographic observations. The tBA guest shows long-lived guest\u2013host hydrogen bonding with the nitrogen atom tethered to a water HO group that rotates towards the cage center to face the guest nitrogen atom. Pinacolone forms thermally activated guest\u2013hosthydrogen bonds with the lattice water molecules; these have been studied for temperatures in the range of 100\u2013250 K. Guest\u2013host hydrogen bonding leads to the formation of Bjerrum Ldefects in the clathrate water lattice between two adjacent water molecules, and these are implicated in the stabilities of the hydrate lattices, the water dynamics, and the dielectric properties. The reported stable hydrogen-bonded guest\u2013host structures also tend to blur the longstanding distinction between true clathrates and semiclathrates.Peer reviewed: YesNRC publication: Ye

AB INITIO STRUCTURE DETERMINATION OF GAS HYDRATES AND REFINEMENT OF GUEST MOLECULE POSITIONS BY POWDER X-RAY DIFFRACTION

Structure determination of powdered crystals is still not a trivial task. For gas hydrates, the difficulty lies in how to determine the rotational disorder and cage occupancies of the guest molecules without other supporting information or constraints because the complexity of the problem for the powder diffraction technique generally depends on the number of atoms to be located in the asymmetric unit. Here, the crystal structures of gas hydrates of CO2, C2H6, C3H8, and Methylcyclohexane/CH4, as determined by the direct-space and Rietveld techniques are reported. The resultant structures and cage occupancies were consistent with results found from conventional experimental methods using single crystal x-ray diffraction or solid-state 13C-NMR. It was shown that the procedures reported in this study make it possible to determine guest disorder and absolute cage occupancy of gas hydrates even from powder crystal.Non UBCUnreviewe