The structure of smectic A phases of compounds with cyano end groups

The smectic layer spacings (d) have been determined for a number of cyano derivatives which have an anomalous d/l ratio of ∼ 1.4, where l is the molecular length. It is shown that the quantity ( d — l) is directly related to the length of the alkyl or alkoxy tail of the molecule but is not simply related to the length of the aromatic core. This is consistent with a bimolecular layer structure having overlapping molecular cores at the layer centre. Using a combination of X-ray and neutron diffraction measurements of the intensity of the layer reflections on several isotopically substituted forms we have positively confirmed and quantified this model for one compound (4-cyano-4'-n-octylbiphenyl, 8CB).Les épaisseurs (d) des couches smectiques déterminées jusqu'à maintenant pour un certain nombre de dérivés cyano présentent une valeur anormale du rapport d/l ∼ 1,4 où l est la longueur moléculaire. Nous avons montré que la quantité (d — l) est directement reliée à la longueur de la chaîne alkyl ou alkoxy de la molécule mais que cette quantité n'a pas de rapport simple avec la longueur du coeur aromatique. Ceci est en accord avec une structure en couches bimoléculaires qui présentent en leur centre des recouvrements de coeurs moléculaires. En utilisant un ensemble de mesures de l'intensité de diffusion des couches par rayons X et diffraction de neutrons sur plusieurs formes substituées d'une façon isotopique, nous avons pu confirmer et quantifier ce modèle pour un composé (4-cyano-4'-n-octylbiphényl, 8CB)

The Leeuwin Current and its eddies: An introductory overview

The Leeuwin Current (LC) is an anomalous poleward-flowing eastern boundary current that carries warm, low-salinity water southward along the coast of Western Australia. We present an introduction to a new body of work on the physical and biological dynamics of the LC and its eddies, collected in this Special Issue of Deep-Sea Research II, including (1) several modelling efforts aimed at understanding LC dynamics and eddy generation, (2) papers from regional surveys of primary productivity and nitrogen uptake patterns in the LC, and (3) the first detailed field investigations of the biological oceanography of LC mesoscale eddies. Key results in papers collected here include insight into the source regions of the LC and the Leeuwin Undercurrent (LUC), the energetic interactions of the LC and LUC, and their roles in the generation of warm-core (WC) and cold-core (CC) eddies, respectively. In near-shore waters, the dynamics of upwelling were found to control the spatio-temporal variability of primary production, and important latitudinal differences were found in the fraction of production driven by nitrate (the f-ratio). The ubiquitous deep chlorophyll maximum within LC was found to be a significant contributor to total water column production within the region. WC eddies including a single large eddy studied in 2000 contained relatively elevated chlorophyll a concentrations thought to originate at least in part from the continental shelf/shelf break region and to have been incorporated during eddy formation. During the Eddies 2003 voyage, a more detailed study comparing the WC and CC eddies illuminated more mechanistic details of the unusual dynamics and ecology of the eddies. Food web analysis suggested that the WC eddy had an enhanced "classic" food web, with more concentrated mesozooplankton and larger diatom populations than in the CC eddy. Finally, implications for fisheries management are addressed