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Continuous Channels (continuous + channel)

Distribution by Scientific Domains

Chemistry	50%

Selected Abstracts

Spatial and temporal variations in bank erosion on sand-bed streams in the seasonally wet tropics of northern Australia

EARTH SURFACE PROCESSES AND LANDFORMS, Issue 9 2006
M. J. Saynor
Abstract Bank erosion rates and processes across a range of spatial scales are poorly understood in most environments, especially in the seasonally wet tropics of northern Australia where sediment yields are among global minima. A total of 177 erosion pins was installed at 45 sites on four sand-bed streams (Tributaries North and Central, East Tributary and Ngarradj) in the Ngarradj catchment in the Alligator Rivers Region. Bank erosion was measured for up to 3·5 years (start of 1998/99 wet season to end of 2001/02 wet season) at three spatial scales, namely a discontinuous gully (0·6 km2) that was initiated by erosion of a grass swale between 1975 and 1981, a small continuous channel (2·5 km2) on an alluvial fan that was formed by incision of a formerly discontinuous channel between 1964 and 1978, and three medium-sized, continuous channels (8·5,43·6 km2) with riparian vegetation. The bank erosion measurements during a period of average to above-average rainfall established that substantial bank erosion occurred during the wet season on the two smaller channels by rapid lateral migration (Tributary Central) and by erosion of gully sidewalls due to a combination of within-gully flows and overland flow plunging over the sidewalls (Tributary North). Minor bank erosion also occurred during the dry season by faunal activity, by desiccation and loss of cohesion of the sandy bank sediments and by dry flow processes. The larger channels with riparian vegetation (East Tributary and Ngarradj) did not generate significant amounts of sediment by bank erosion. Deposition (i.e. negative pin values) was locally significant at all scales. Bank profile form and channel planform exert a strong control on erosion rates during the wet season but not during the dry season. Copyright © 2006 Commonwealth Government of Australia. [source]

Polymorphs and pseudopolymorphs of N,N,-dithiobisphthalimide

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 2 2002
Dorcas M. M. Farrell
N,N,-Dithiobisphthalimide, C16H8N2O4S2 (I), forms a wide range of polymorphs and solvates (pseudopolymorphs). When (I) is crystallized from methanol it yields a solvent-free polymorph (4), in Pna21 with Z, = 1, in which the molecules are linked into chains by a single C,H,O hydrogen bond: crystallization from either acetonitrile or dimethylformamide produces a monoclinic polymorph (5), in P21/c with Z, = 2, also solvent-free, in which the molecules are linked into molecular ladders. Nitromethane forms a monosolvate, C16H8N2O4S2·CH3NO2 (6), in P21/c with Z, = 1, in which the solvent molecules are linked to the molecules of (I) not only via a conventional C,H,O hydrogen bond but also via a polarized multicentre interaction involving all three C,H bonds of the solvent molecule. Chlorobenzene forms a precise hemisolvate, C16H8N2O4S2·0.5C6H5Cl (7), in P with Z, = 1, while ethylbenzene forms an approximate hemisolvate 2C16H8N2O4S2·0.913C6H5C2H5·0.087H2O (8), in P21/c with eight molecules of (I) per unit cell. In both solvates the molecules of (I) are linked, in (7) by ,,, stacking interactions augmented by weak C,H,O hydrogen bonds and in (8) by stronger C,H,O hydrogen bonds: the solvent molecules lie in isolated cavities, disordered across inversion centres in (7) and fully ordered in general positions in (8). Crystallization of (I) either from tetrahydrofuran or from wet tert -butanol yields isomorphous solvates (9) and (10), respectively, in C2/c with Z, = 0.5, in which molecules of (I) lie across twofold rotation axes and are linked by ,,, stacking interactions and very weak C,H,O hydrogen bonds, forming a framework enclosing continuous channels: highly disordered solvent molecules lie within these channels. p -Xylene and toluene form isomorphous hemisolvates (11) and (12) with unit cells metrically very similar to those of (9) and (10), but in P21/n with Z, = 1: in these two solvates the molecules of (I) are linked into a framework by very short C,H,O hydrogen bonds; the solvent molecules lie within continuous channels, but they are localized across inversion centres so that the toluene is disordered across an inversion centre. [source]

(E)-3-(4-Methylphenyl)-2-(2-thienyl)acrylonitrile has Z, = 0.75 in the space group C2/m: fourfold disordered molecules lie in channels enclosed by fully ordered molecules

ACTA CRYSTALLOGRAPHICA SECTION C, Issue 9 2009
Debora Cobo
The title compound, C14H11NS, crystallizes with Z, = 0.75 in the space group C2/m. Two independent molecules are present, one of which lies with all the non-H atoms on a mirror plane, while the other is fourfold disordered across a site of 2/m symmetry. The ordered molecules are stacked such that they enclose continuous channels running along twofold rotation axes, and the disordered molecules are positioned within these channels. [source]