Clathrate ices -- recent results

Abstract The last five years have seen an increasing interest in clathrate ices as a result of the discovery of extensive deposits of natural gas hydrates in permafrost regions. Twenty-six new clathrate hydrates have been identified, mainly by NMR, including a tetragonal hydrate of dimethyl ether. N-butane and neopentane have been found to be enclathrated in natural gas hydrates, the former as a gauche conformer. As a result of their high symmetries, encaged neopentane, CF4, SF6, and SeF6 exhibit a Resing apparent-phase-change effect in the temperature range of NMR line narrowing. There is increasing evidence that reorientational jumps of water molecules are more frequent than translational jumps in clathrate ices. This is certainly so for ethylene oxide-d4 and tetrahydrofuran-d8 hydrates for which two regions of proton line narrowing and two T 1ρ minima have been observed. The reorientational motions of most guest molecules in structure II hydrates only become isotropic on a time scale long enough to permit the cage configurations to be averaged to 4 3m symmetry by reorientation of the water molecules. The orientations of the water molecules remain disordered to the lowest temperatures

Song and the city : a comparison between urban and forest blackbirds

Merels in de stad blijken te verschillen van soortgenoten in naburige bossen ten aanzien van lichaamsbouw, zang, en genetica. Nederlandse stadsmerels zijn zwaarder dan hun soortgenoten in het bos. Daarnaast zingen stadsmerels met een hogere frequentie dan bosmerels wat de hoorbaarheid bij laagtonig verkeerslawaai ten goede komt. Merelmannen merken zelf daadwerkelijk de verschillen op tussen de zang van stads- en bosmerels, zo blijkt uit experimenten waarbij de zang werd afgespeeld in merelterritoria in stad en bos. Stadsmerels reageren namelijk sterker op hogere zang en bosmerels juist meer op lagere. Variatie in zang en reactie op zang kan mogelijk de onderlinge uitwisseling tussen stad- en bospopulaties be_nvloeden. Dit wordt ondersteund door de bevinding dat populaties uit steden en naburige bossen inderdaad genetisch anders zijn. De gevonden verschillen zijn opmerkelijk omdat de studiepopulaties in stad en bos slechts 5 _ 10 km uit elkaar liggen. Het onderzoek toont dus aan dat een stedelijke omgeving invloed kan hebben op belangrijke soortskenmerken waarbij het echter nog maar de vraag is of deze evolutie ooit zal leiden tot het ontstaan van twee gescheiden merelsoorten.LEI Universiteit LeidenGedragsbiologi

Observation of micropores in hard-carbon using Xe-129 NMR porosimetry

The existence of micropores and the change of surface structure in pitch-based hard-carbon in xenon atmosphere were demonstrated using Xe-129 NMR. For high-pressure (4.0 MPa) Xe-129 NMR measurements, the hard-carbon samples in Xe gas showed three peaks at 27, 34 and 210 ppm. The last was attributed to the xenon in micropores (<1 nm) in hard-carbon particles. The NMR spectrum of a sample evacuated at 773 K and exposed to 0.1 MPa Xe gas at 773 K for 24 h showed two peaks at 29 and 128 ppm, which were attributed, respectively, to the xenon atoms adsorbed in the large pores (probably mesopores) and micropores of hard-carbon. With increasing annealing time in Xe gas at 773 K, both peaks shifted and merged into one peak at 50 ppm. The diffusion of adsorbed xenon atoms is very slow, probably because the transfer of molecules or atoms among micropores in hard-carbon does not occur readily. Many micropores are isolated from the outer surface. For that reason, xenon atoms are thought to be adsorbed only by micropores near the surface, which are easily accessible from the surrounding space.</p

Willkommen in Deutschland: wie internationale Studierende den Hochschulstandort Deutschland wahrnehmen

Warum entscheidet sich ein ausländischer Studierender oder Doktorand für eine deutsche Hochschule? Um diese Frage zu beantworten wurden rund 50.000 international mobile Studieninteressierte im In- und Ausland im Auftrag des Konsortiums für internationales Hochschulmarketing (GATE-Germany) befragt. In drei Befragungen wurde ermittelt, welche Faktoren für die Wahl einer deutschen Hochschule entscheidend waren und ob die Erwartungen erfüllt wurden. Darüber hinaus informiert die Publikation über Hindernisse, die einem weiteren Verbleib der Studierenden in Deutschland entgegenstanden. Studieninteressierte im Ausland wurden außerdem befragt, wie sie den Hochschulstandort Deutschland wahrnehmen. Die Ergebnisse dieser Befragungen bilden die Grundlage für eine zielgruppenadäquate Weiterentwicklung der Serviceangebote deutscher Hochschulen.Why does a foreign student or doctoral student decide to study at a German University? In order to answer this question approximately 50,000 internationally mobile prospective students at home and abroad were interviewed on behalf of the Konsortium für internationales Hochschulmarketing (GATE-Germany, Consortium for international University marketing). With the help of three surveys it was determined which factors are important for the choice of a German University and whether the expectations were met. Furthermore, the publication provides information about challenges, which impeded the further stay of students in Germany. Prospective students abroad were also asked about their perceptions relating to the University location Germany. The results of these interviews provide the basis for an adequate target-group orientated development of service provisions at German Universities

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

Species-specific relationships between water transparency and male coloration within and between two closely related Lake Victoria cichlid species

Environmental variation in signalling conditions affects animal communication traits, with possible consequences for sexual selection and reproductive isolation. Using spectrophotometry, we studied how male coloration within and between populations of two closely related Lake Victoria cichlid species (Pundamilia pundamilia and P. nyererei) covaries with water transparency. Focusing on coloration patches implicated in sexual selection, we predicted that in clear waters, with broad-spectrum light, (1) colours should become more saturated and (2) shift in hue away from the dominant ambient wavelengths, compared to more turbid waters. We found support for these predictions for the red and yellow coloration of P. nyererei but not the blue coloration of P. pundamilia. This may be explained by the species difference in depth distribution, which generates a steeper gradient in visual conditions for P. nyererei compared to P. pundamilia. Alternatively, the importance of male coloration in intraspecific sexual selection may differ between the species. We also found that anal fin spots, that is, the orange spots on male haplochromine anal fins that presumably mimic eggs, covaried with water transparency in a similar way for both species. This is in contrast to the other body regions studied and suggests that, while indeed functioning as signals, these spots may not play a role in species differentiation.</p

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