Quantitative evaluation of the chloride template effect in the formation of dicationic [1(4)]imidazoliophanes

The formation of macrocycles containing two imidazolium rings such as 1.2X and 2.2X is anion-directed through hydrogen bonding. The template effect exerted by the chloride anion in the ring-closure reaction of the monocationic model intermediate 9+ to yield the [1(4)]imidazoliophane 2(2+) has been evaluated. This effect was quantified following the kinetics of the macrocyclization by using a UV-vis technique. The rate of the ring closure of monocation 9+ is increased up to 10 times, in the presence of Bu4NCl 0.04 M. This finding confirms that the template effect is operative in the macrocyclization leading to dicationic [1(4)]imidazoliophanes.     

Solubility and Hydrolysis of La, Pr, Eu, Er, and Lu in 1 M NaCl Ionic Strength at 303 K

The pLn-pC _H diagrams obtained by a radiometric technique were used for the determination of the solubility and the first hydrolysis constants for La, Pr, Eu, Er, and Lu in 1 M NaCl ionic strength at 303 K. The saturation and unsaturation zones, and the borderline of precipitation were determined from these diagrams. The first hydrolysis constants were determined from pH titrations where no precipitation was found; these data were treated with the SUPERQUAD program. Fitting methods involving pLn versus pC _H and the average ligand number vs. pC _H were used to calculate both the first hydrolysis constants and the solubility products. The log K _sp values obtained for La, Pr, Eu, Er, and Lu were –19.53, –20.92, –22.24, –22.62, and –23.05 and the log* ₁ average values obtained were –8.87 ± 0.05, –8.54 ± 0.04, –8.34 ± 0.03, –8.16 ± 0.04, and –8.11 ± 0.03, respectively, under CO₂ free conditions. Finally, the results were compared with those found in the literature.     

Spontaneous resolution, whence and whither: from enantiomorphic solids to chiral liquid crystals, monolayers and macro- and supra-molecular polymers and assemblies

One of the great challenges in stereochemistry is the explanation of why some molecules resolve spontaneously while others do not. In this critical review the recent advances in the creation of chiral systems from achiral and racemic compounds in three-, two- and one-dimensional systems are discussed. There are some groups of molecules in some systems that do tend to display conglomerates, which may suggest that there are enantiophobic and enantiophilic molecules whose assembly is guided by the structural and thermodynamic properties of the systems in question.     

Nondegenerate pi-donor/pi-acceptor [2]catenanes containing proton-ionizable 1H-1,2,4-triazole subunits: synthesis and spontaneous resolution

Chirality can hold the key to inducing directionality of motion in components of molecular devices. With this idea in mind, we describe here 1) the template-directed synthesis of two [2]catenanes wherein cyclobis(paraquat-p-phenylene) is interlocked with polyether macrocycles containing, in addition to one 3,5-bis(oxymethylene)-1H-1,2,4-triazole unit, either one 1,4-dioxybenzene or one 1,5-dioxynaphthalene ring system. We also report 2) the full characterization of both [2]catenanes by fast atom bombardment mass spectrometry (FABMS), X-ray crystallography, and dynamic (1)H NMR spectroscopy. We reveal 3) the fact that the [2]catenanes not only exist, both in the solution-state and in the solid-state, as strictly one of the two possible translational isomers, but that they also exhibit spontaneous resolution on crystallization leading to formation of homochiral crystals, as indicated by X-ray crystallography and circular dichroism (CD) experiments. Finally, we comment 4) on the chances of switching these catenanes chemically.     

Quantum capacities of bosonic channels

We investigate the capacity of bosonic quantum channels for the transmission of quantum information. We calculate the quantum capacity for a class of Gaussian channels, including channels describing optical fibers with photon losses, by proving that Gaussian encodings are optimal. For arbitrary channels we show that achievable rates can be determined from few measurable parameters by proving that every channel can asymptotically simulate a Gaussian channel which is characterized by second moments of the initial channel. Along the way we provide a complete characterization of degradable Gaussian channels and those arising from teleportation protocols.     

Tuning of texture and surface chemistry of carbon xerogels

The influence of different activation processes on the textural and surface chemical properties of carbon xerogels was studied. Carbon xerogels were prepared by the conventional sol-gel approach using resorcinol and formaldehyde; two different pHs of sol-gel processing led to carbon materials with distinct pore size distributions. The materials were subjected to controlled activation by three different methods: activation by oxygen plasma, activation by HNO(3), and activation by diluted air. Treatments with HNO(3) and diluted air created oxygen groups on the external surface as well as inside the pore channels, whereas plasma is more suitable for introducing oxygen groups selectively on the external surface. Nevertheless, it was shown that samples with wider pores can be oxidized to some extent on the pore interiors by plasma. Significant changes in total surface area by air activation were observed.     

Tube-side heat transfer enhancement in flat-plate liquid solar collectors with wire coil inserts

This work presents an experimental study of the thermal-hydraulic characteristics in a flat-plate solar collector with two different geometries of helical coil inserts. Isothermal pressure drop tests are conducted in a horizontal tube using water as working fluid to obtain the fully developed Fanning friction factor for a range of Reynolds number Re = 80–8,000. Heat transfer tests are performed in the solar collector loop using a mixture of propylene-glycol and water at 30 wt%, covering the ranges Pr = 16–28 and Re = 140–600. A significant decrease in the temperature of the absorber is observed when the appropriate wire coil is selected for the given flow conditions, proving the beneficial effect of this type of insert device for the augmentation of the thermal efficiency in flat-plate solar collectors.     

Gametophyte morphology of Platycerium andinum Baker and Platycerium wandae Racif

This paper describes the morphology of the sexual phase and spores of Platycerium andinum and Platycerium wandae. Spores were sown in Thompson's media and the cultures were kept at 24–25 °C, with 12 h light/darkness photoperiod. Developmental phases were fixed in FAA and processed for observation with the scanning electron microscope. Spores of both species are monolete; Vittaria-type germination and Aspidium-type prothallial development were observed. In the phase of development, the gametophytes develop unicellular secretory and as they mature, develop bifurcated or branched pluricellular trichomes, both in the cushion and near the meristematic zone. Adult gametophytes in culture are cordiform-spatulate to cordiform-reniform, most are unisexual and a few are bisexual. Gametangia belong to the leptosporangiate fern type. Archegonial morphology is uniform, with an elongate, thin neck curved toward the base of the gametophyte. Antheridia have a basal cell, an annular cell and an undivided opercular cell. Three hundred days after the spores were sown, sporophytes still had not developed. In both species, some spores germinate inside the sporangial capsule (intra-sporangial germination). We provide new information on morphogenesis in the genus Platycerium.