Chemical reaction in chromatographic columns : Dehydrogenation of ethane over cadmium-exchanged zeolite 4A

Pulsed-flow techniques were used to detect considerable differences in the heats of adsorption of ethane and ethylene on various cadmium-exchanged zeolites 4A at temperatures up to 500°C. Higher values (about 10.0 kcal/mole) were observed for ethylene than for ethane (5.0 kcal/mole) at 300–400°C. Experimental verification is provided pertaining to the dehydrogenation of ethane in a gas chromatographic reactor. By appropriate choice of the reaction conditions, conversions up to 80% per pass could easily be obtained at temperatures (400–500°C) at which the thermodynamic equilibrium for a diluted ethane stream (Pc⁺₂ = 0.01-0.1 P_tot.) would not permit more than 25%.     

Isolation and structural characterization of anionic and neutral compounds resulting from the oxidative addition of HI or CH3I to [IrI2(CO)2](-)

The active iridium species in the methanol carbonylation reaction has been crystallized as the [PPN][IrI(2)(CO)(2)] complex and the X-ray structure solved, showing a cis-geometry and a square planar environment. Hydriodic acid reacts very quickly with this compound to provide [PPN][IrHI(3)(CO)(2)], the X-ray crystal structure of which has been determined. The two CO ligands remain in mutual cis-position in a pseudooctahedral environment. The same cis-arrangement has been observed from the X-ray structure for [PPN][IrI(3)(CH(3))(CO)(2)] resulting from the slower oxidative addition of CH(3)I to [PPN][IrI(2)(CO)(2)]. By iodide abstraction with InI(3), the anionic methyl complex gave rise to the dimeric neutral complex [Ir(2)(mu-I)(2)I(2)(CH(3))(2)(CO)(4)]. An X-ray structure showed that the methyl ligands are in the equatorial positions of the two octahedrons sharing an edge, formed by the two bridging iodide ligands. All these four complexes have been fully characterized by mass spectrometry, (1)H and (13)C NMR, and infrared both in solution and in the solid state. When necessary, the (13)CO- or (13)CH(3)-enriched complexes have been prepared and analyzed.     

Zinc dissolution and passivation in buffered phosphate solutions: Part I. A comparative study with sodium hydroxide solutions

Zn dissolution and passivation in buffered phosphate electrolyte in the pH range between 7 and 13.5 has been studied under a wide variety of experimental conditions. The electrode rest potential shows three regions depending on pH. Between pH 7 and 10 a constant value was found while a slope of 60 mV is obtained from a plot of E_r vs. pH. In the pH range 10–11.5 a mixed potential could be established due to the minimum buffer capacity of the system. The necessary charge for passivation was found to be 1.5 mC cm^?2 and this was attributed to a monolayer film. From l.s.v. and RDE results a change on Zn dissolution mechanism from a dissolution precipitation to a solid phase process with pH can be postulated. Time and potential effects on product reduction would show that there are chemical transformations on the electrode surface as a second step on the electrode poassivation. From experimental kinetic parameters an empirical rate equation for Zn dissolution is found asi_a=2k_aFc_OH^?c_{(PO4^3?)t^0.5}exp[(3/2)EF/RT)]Accordingly a reaction mechanism is postulated where PO₄^3? ions act as dissolution promoters, while HPO₄^2?, through the formation of NaZnPO₄·H₂O glass, would act as a dissolution inhibitor.     

Impacts of solar ultraviolet-B radiation on terrestrial ecosystems of Tierra del Fuego (southern Argentina). An overview of recent progress.

The southern part of Tierra del Fuego, in the southernmost tip of South America, is covered by dense Nothofagus spp. forests and Sphagnum-dominated peat bogs, which are subjected to the influence of ozone depletion and to increased levels of solar ultraviolet-B radiation (UV-B). Over the last 5 years we have studied some of the biological impacts of solar UV-B on natural ecosystems of this region. We have addressed two general problems: (i) do the fluctuations in UV-B levels under the influence of the Antarctic ozone 'hole' have any measurable biological impact, and (ii) what are the long-term effects of solar (ambient) UV-B on the Tierra del Fuego ecosystems? In this paper, we provide an overview of the progress made during the first 4 years of the project. We highlight and discuss the following results: (1) ambient UV-B has subtle but significant inhibitory effects on the growth of herbaceous and graminoid species of this region (growth reduction < or = 12%), whereas no consistent inhibitory effects could be detected in woody perennials; (2) in the species investigated in greatest detail, Gunnera magellanica, the inhibitory effect of solar UV-B is accompanied by increased levels of DNA damage in leaf tissue, and the DNA damage density in the early spring is clearly correlated with the dose of weighted UV-B measured at ground level; (3) the herbaceous species investigated thus far show little or no acclimation responses to ambient UV-B such as increased sunscreen levels and DNA repair capacity; and (4) ambient UV-B has significant effects on heterotrophic organisms, included marked inhibitory effects on insect herbivory. The results from the experiments summarized in this review clearly indicate that UV-B influences several potentially important processes and ecological interactions in the terrestrial ecosystems of Tierra del Fuego.     

On representations of a general solution in the theory of micropolar thermoelasticity without energy dissipation

In the present paper, the linear theory of micropolar thermoelasticity without energy dissipation is considered. This work is organized as follows: Section 2 is devoted to basic equations for micropolar thermoelastic materials, supposed to be isotropic and homogeneous, and to assumptions on constitutive constants. In Section 3, some theorems related to representations of a general solution are studied. Published in Ukrains’kyi Matematychnyi Zhurnal, Vol. 59, No. 10, pp. 1391–1398, October, 2007.     

A new spirocyclic proline-based lactam as efficient type II′ β-turn inducing peptidomimetic

A new proline-based spirotricyclic lactam is reported as an efficient type II′ β-turn inducing peptidomimetic. After investigations of the reverse turn properties by computational techniques, the scaffold has been synthesized by a straightforward sequence relying on a key RCM reaction for the construction of the spirocyclic lactams ring. For its conformational properties, the scaffold can be considered a privileged structure to be employed as a mimic of the β-turn motif of the potent antibiotic Gramicidin S.     

Enantioselective alkylidenecyclopropanation of norbornenes with terminal alkynes catalyzed by palladium–phosphinous acid complexes

Pd(II)-coordinated phosphinous acids catalyzed the formal enantioselective [2+1] cycloaddition of norbornene derivatives with terminal alkynes. The absolute configuration of (+)-3aa was assigned using VCD.     

Aspidosperma Alkaloids. A New Didehydrodimerization Mode of β-Anilinoacrylic Alkaloids by Anodic Oxidation

A regio- and stereoselective didehydrodimerization procedure, whose key step involves the anodic oxidation, allows the conversion of β-anilinoacrylic alkaloids belonging to the Aspidosperma class, typified by tabersonine ( 1 ) and its 3-oxo derivative 2 , into the hitherto unknown 16,10′-didehydro dimers 3 and 7 , respectively.     

Enthalpy relaxation of polymers: comparing the predictive power of two configurational entropy models extending the AGV approach

The Tool-Narayanaswamy-Moynihan (TNM) phenomenological model is widely accepted in order to describe the structural relaxation of glasses. However several quantitative discrepancies can be found in the literature that cannot be entirely ascribed to the experimental errors. In this work we compare the predictive power of two recently proposed configurational entropy approaches extending the TNM formalism. Both of them change the treatment of non linearity by adding a free parameter. We use Differential Scanning Calorimetry (DSC) experiments in order to test the models in two different polymers. One of them is a commercial PMMA sample, the other is a side chain liquid crystal azo-benzene polymer properly synthesized for optical nanorecording purposes. Different results were found for the two systems. In the PMMA sample only one of the new models was able to improve the agreement between DSC experiments and theory with respect to the TNM model, whereas in the second polymer both the approaches were able to describe the experiments better than TNM model.Received: 25 February 2004, Published online: 21 October 2004PACS: 64.70.Pf Glass transitions - 61.43.Fs Glasses - 61.41. + e Polymers, elastomers, and plastics