Metal-insulator transition and magnetic properties of La(1-x)Eu(x)NiO3 (0 ≤ x ≤ 1)
Sanchez, Rodolfo Daniel; Causa, Maria Teresa Beatriz; Seoane, A.; Rivas, J.; Rivadulla, F.; López Quintela, M. A.; Pérez Cacho, J. J.; Blasco, J.; García, J.
This paper reports X-ray diffraction patterns, Rietveld fit profiles, electrical resistivity dc magnetic susceptibility, and electron paramagnetic resonance (EPR) measurements of La(1-x)EU(x)NiO3 (0 ≤ x ≤ 1). A shift to high temperature of the metal-insulator transition temperature with x and hysteresis in the electrical resistivity and magnetization versus temperature at intermediate concentration of the compound are observed. The antiferromagnetic order of the Ni sublattice can be observed after subtracting the magnetic contribution of the rare earth ions. A change in space group from R-3c to Pbnm for x ≥ 0.4 was detected, as was the appearance of magnetic order. Samples with 1% Gd3+ probe were studied by EPR. Drastic changes were observed in the shape of the lines with x, though the crystal field symmetry of the Gd sites remained constant in all the samples studied. We observed that when Eu substitution increases, the number of Gd ions that contribute to the crystal-field spectrum diminishes. A possible cause of this effect is a previously reported increase in the Ni-O covalence in the neighborhood of the Eu sites.
Synthesis of acetal (1,1-diethoxyethane) from ethanol and acetaldehyde over acidic catalysts
Capeletti, Maria Rosa; Abovsky, Leandro; de la Puente, Gabriela; Laborde, Miguel Ángel; Sedran, Ulises Anselmo
Various acidic catalysts (zeolitic and amorphous FCC catalysts, mordenite, montmorillonite, and sulfonic ion exchange resin) were tested for the synthesis of acetal from ethanol and acetaldehyde, at 4 and 20C and atmospheric pressure in batch stirred reactors. All the catalysts were active, but the exchange resin showed amuch better performance than the other catalysts, since it quickly reached equilibrium ethanol conversion values. The resin was also tested under different pressures and catalyst to reactants ratios. Clear relationships between the catalyst activity, the amount of acidity and the physical properties of the catalysts were not apparent. A possible reaction mechanism suggests that protonic acid sites are necessary. Water, a reaction product, seems to have an inhibitory effect on the reaction rate.
Increased sulfur tolerance of Pt/KL catalysts prepared by vapor-phase impregnation and containing a Tm promoter
Jacobs, Gary; Ghadiali, Firoz; Pisanu, Adriana; Padro, Cristina; Borgna, Armando; Alvarez, Walter E.; Resasco, Daniel E.
Pt/KL catalysts are highly active and selective for the aromatization of hexane, but they display a very high sensitivity to even small concentrations of sulfur in the feed. Tm-containing Pt/KL catalysts were prepared by incipient wetness impregnation (IWI), ion exchange (IE), and vapor-phase impregnation (VPI) methods. The platinum morphology resulting from the addition of thulium and platinum sequentially, using the VPI method, yielded the greatest enhancement to the aromatization performance of the Pt/KL catalysts. The presence of thulium in the sequential VPI Pt/Tm/KL catalyst resulted in a catalyst with higher platinum dispersion than that in an unpromoted VPI catalyst. VPI catalysts provided more finely dispersed Pt clusters than either than either conventional IWI or IE methods. From temperature programmed oxidation of poisoned catalysts, thulium acted as a getter for sulfur, thus, delaying the poisoning of platinum under sulfur-containing feeds. The initial activity of the Tm-promoted VPI catalysts was higher than that of the unpromoted Pt/KL VPI catalysts, suggesting that thulium may directly modify platinum or even participate in accelerating the aromatization reaction.
Receptor expression for IgG constant fraction in human umbilical vein endothelial cells.
Alberto, Maria Fabiana; Bermejo, Emilse; Lazzari, María Ángela
Endothelial cells (EC) lining the vasculatureform a natural barrier between the circu- immune complex levels, whereby patients with thelating blood and the underlying tissue. One highest disease scores tend to have the highest levelof the major functions of these cells is to prevent of immunocomplexes (IC) [4]. Moreover, patientsplatelet deposition and activation of the coagula- with active SLE contain IC that bind to EC oftion cascade [1].
Single agents and the set of many-to-one stable matching
Martinez, Ruth; Massó, Jordi; Neme, Alejandro José; Oviedo, Jorge Armando
Some properties of the set of many-to-one stable matchings for firms that have responsive preferences and quotas are not necessarily true when firms' preferences are substitutable. In particular, we provide examples in which firms have substitutable preferences but firms and workers may be "single" in one stable matching and matched in another one. We identify a set of axioms on firms' preferences guaranteeing that the set of unmatched agents is the same under every stable matching. We also propose a weaker condition than responsiveness, called separability with quotas or q-separability, that together with substitutability implies this set of axioms.
Sequence stratigraphy of a tidally dominated carbonate-siliciclastic ramp; the Tithonian-Early Berriasian of the southern Neuquen Basin, Argentina
Spalletti, Luis Antonio; Franzese, Juan Rafael; Matheos, Sergio Daniel; Schwarz, Ernesto
The Tithonian-Berriasian Vaca Muerta, Carrin Cura and Picun Leufu formations in the southern Neuquen Basin were deposited on a tidally dominated, mixed carbonate-siliciclastic ramp. Basinal, outer, middle, shallow and back ramp facies associations are recognized and a sequence stratigraphic analysis reveals that the ramp record consists of three shallowing-upwards sequences (Ti1, Ti2 and Ti3) set within a lower-order progradational cycle. A higher order of cyclicity is superimposed on to the middle (Ti2) sequence. The majority of the ramp facies belong to the transgressive and highstand systems tracts; however, at the base of Ti2, a lowstand systems tract is identified, characterized by a basal unconformity and an abrupt basinward shift of the shallow marine lithofacies. Transgressive systems tracts were characterized by slow sedimentation rates and rapid sea-level rises that affected carbonate productivity. Highstand systems tracts show the greatest carbonate productivity and an increased progradation rate on account of a reduction in accommodation space generation. Palaeogeography played a major role in the development of the depositional systems. Partial isolation from the Pacific Ocean reflecting the growth of the Andean magmatic arc and geographic restriction due to tectonic inversion in the central part of the basin resulted in a meso-macrotidal regime that produced a tidally dominated sedimentary record in the shallow and back ramp environments. Coeval anoxic conditions in the central part of the Neuquen Basin favoured distal ramp and basinal black shale deposition during episodes of relative sea-level rise.
Ejemplares tipo de Asteraceae (=Compositae) de A.L. Cabrera.
Freire, Susana Edith; Iharlegui, Laura
El presente trabajo representa un catálogo de los ejemplares tipo de 426 especies y 82 variedades de Asteraceae descriptas por A. L. Cabrera entre los años 1931 y 1999. Los taxones se presentan en orden alfabético. Para cada taxón se indica: diagnosis original, tipo nomenclatural, categoría del tipo, herbario, etiqueta original y nombre actualizado.
Synthesis and properties of [Ru(tpy)(4,4'-X2bpy)H]+ (tpy = 2,2':6',2-terpyridine, bpy = 2,2'-bipyridine, X = H and MeO), and their reactions with CO2
Konno, Hideo; Kobayashi, Atsuo; Sakamoto, Kazuhiko; Fagalde, Florencia; Katz, Néstor Eduardo; Saitoh, Hideki; Ishitani, Osamu
A novel type of hydrido complex [Ru(tpy)(4,4'-X2bpy)H]+ (X = H and MeO) was synthesized. The stronger hydridic character of the complexes compared with [Ru(bpy)2(L)H]+ type complexes (L = CO, PPh3 and AsPh3) was demonstrated by the relatively high chemical shifts of Ru-H in the 1H NMR spectra and by higher reactivities with CO2. The reactions of [Ru(tpy)(4,4'-X2bpy)H]+ with CO2 occurred at second-order rate constants varying from (4.69 ± 0.02) to (5.51 ± 0.04) x 10-3 M-1 s-1 depending on both solvent and X, giving the formato complexes [Ru(tpy)(4,4'-X2bpy)(OCHO)]+ quantitatively. The rate constant was increased with the increase of solvent acceptor number, and the reaction of [Ru(tpy){4,4'-(MeO)2bpy}H]+ with CO2 was found to be 3.6 times faster than that of [Ru(tpy)(bpy)H]+. These results suggest that nucleophilic attack of the hydride ligand to the carbon atom of CO2 is the rate determining step for the formation of the formato complex. The structure of the formato complex [Ru(tpy)(bpy)(OCHO)](PF6) was determined by X-ray crystallographic analysis.
Structural requirements and reaction pathways in condensation reactions of alcohols on MgyAlOx catalysts
Di Cosimo, Juana Isabel; Apesteguia, Carlos Rodolfo; Ginés, M. J. L.; Iglesia, E.
The effect of composition and of surface properties on alcohol-coupling reactions was studied on MgyAlOx catalysts using C2H5OH or 13CH3OH/1–12C3H7OH mixtures as reactants. Samples with Mg/Al ratios of 0.5–9.0 were obtained by thermal decomposition of precipitated hydrotalcite precursors. The nature, density, and strength of surface basic sites were obtained by temperature-programmed desorption (TPD) of CO2 and by 13CO2/12CO isotopic switch methods, whereas the acid site densities were measured by TPD of NH3. The catalyst ability for activating H–H bonds was investigated by performing H2–D2 steady-state equilibration reactions. Isotopic tracer studies were carried out in order to probe chain growth pathways in the synthesis of isobutanol. The rates and product selectivity for C2H5OH or CH3OH/C3H7OH reactions strongly depended on the chemical composition of MgyAlOx samples. In turn, the chemical composition affected the acid–base properties of MgyAlOx samples by modifying surface acid and base site densities and the distribution of strength for such sites. The rate of alcohol dehydration to ethers and olefins increased with increasing Al content. Al-rich MgyAlOx samples contained a high density of Al3+–O2− site pairs and of moderate strength basic sites, the combination of which promoted the formation of ethylene or propylene from primary alcohols via E2 elimination pathways. The competitive dehydration to form ethers involved the adsorption of two alcohol molecules on neighboring active sites offering different acid–base properties. On MgyAlOx samples, the active acid sites for ether formation were probably the Al3+ cations, whereas the basic sites were the neighboring O2− ions. The abundance of surface Al3+–O2− pairs accounted for the high ether formation rates observed on Al2O3 and Al-rich MgyAlOx samples. The dehydrogenation of alcohols to aldehydes (C2H4O or C3H6O) involved the initial alkoxy intermediate formation on weak Lewis acid–strong Brønsted base site pairs. The synthesis of C2H4O or C3H6O was favored on Mg-rich MgyAlOx samples because these samples contained a much larger number of properly positioned Al3+ Lewis acid sites and Mg2+–O2− basic pairs, which are required for hydrogen abstraction steps leading to alkoxy intermediates. Pure MgO showed lower dehydrogenation rates than Mg-rich MgyAlOx samples because the predominant presence of isolated O2− hindered formation of alkoxy intermediates by alcohol dissociative adsorption. Aldol condensation reactions on MgyAlOx samples involved also the formation of a carbanion intermediate on Lewis acid–strong Brønsted base pair sites and yielded products containing a new C–C bond such as n-C4H8O (or n-C4H9OH) and iso-C4H8O (or iso-C4H9OH). Reactions leading to condensation products were also favored on Mg-rich samples, but they took place at much slower rates than those of the corresponding dehydrogenation reactions to aldehydes. This reflected the bimolecular and consecutive character of condensation reactions, which are affected not only by the catalyst acid–base properties but also by the chemical nature of the alcohols and steric factors.
Implementation of the IPPP–CLOPPA–INDO/S method for the study of indirect nuclear spin coupling constants and its application to molecules containing tin nuclei
Botek, Edith L.; Aucar, Gustavo Adolfo; Cory, Marshall G.; Zerner, Michael C.
The inner projection of the polarization propagator, using contributions from localized orbitals, IPPP–CLOPPA, and using the intermediate neglect of the differential overlap model parameterized for spectroscopy, INDO/S, was implemented and used to calculate indirect nuclear spin coupling constants. The resulting model was tested on a group of small- and medium-size model compounds by comparing its performance with that of other semi-empirical methods and experiments where available. It is shown that in general the INDO/S approximation with the use of S N 2 (0) and r N −3 atomic parameters taken from the INDO and AM1 approaches is the most suitable scheme to describe coupling constants. The introduction of atomic parameters for S N 2 (0) and r N −3 in the case of heavy nuclei like Sn, is a critical step. The correction of the bonding beta parameter for this nucleus was also necessary within the INDO/S approximation to improve the accuracy and to better account for indirect relativistic effects. The application of this parameterization was accomplished in a series of tetrastannacyclohexanes and different pathways for coupling transmission were analyzed.
Peptide models XXV. Side-chain conformational potential energy surface, E = E(χ1, χ2) of N-formyl-L-aspartic acidamide and its conjugate base N- formyl-L-aspartatamide in their γ(L) backbone conformations
Salpietro, Salvatore J.; Perczel, Andràs; Farkas, Od̈ön; Enriz, Ricardo Daniel; Csizmadia, Imre Gyula
Ab initio molecular computations were carried out on the γ(L) backbone conformation of N-formyl-L-aspartic acidamide and its conjugate base N- formyl-L-aspartatamide at the HF/3-21G level of theory. All side-chain conformations were explored for the parent amino acid diamide and its conjugate base. Propionate ion, propionic acid, 3,3-difluoropropionate ion and 3,3-difluoropropionic acid were used to model the side-chain in the anionic and neutral compounds.
Evaluation of hydrogen transfer in FCC catalysts. A new approach for cyclohexene as a test reactant
de la Puente, Gabriela; Sedran, Ulises Anselmo
The conversion of cyclohexene over different commercial FCC catalysts in a batch fluidized-bed reactor under very short contact times at 300°C, was described by means of a simple lumped kinetic model that accounted for the observed yield patterns. The main branch of reaction starting from the adsorption of cyclohexene was used to assess an index based on kinetic parameters that defines the relative significance of hydrogen transfer reactions in a set that also includes proton transfer and cracking reactions. Complementary experiments with pulses of reactant on a fixed-bed reactor allowed to establish the best temperature to obtain maximum sensitivity to hydrogen transfer. The same evolutions were observed for the relationships between the index of hydrogen transfer and the paired acid site density with the Y zeolite unit cell size, confirming that two regions can be defined separated by unit cell sizes of about 24.28–24.30 Å, the correlations being stronger at low values of this parameter.
Unitary quasifinite representations of W_{\infty}
Kac, Victor; Liberati, Jose Ignacio
We classify the unitary quasi-finite highest-weight modules over the Lie algebra W and realize them in terms of unitary highest-weight representations of the Lie algebra of infinite matrices with finitely many nonzero diagonals.
Mineral composition of perennial vegetation of shrub patches in northeastern Patagonia
del Valle, Hector Francisco; Rosell, Ramon Antonio
This study was conducted to (1) quantify the mineral composition of litter, new leaves, and stems of two plant species (Atriplex lampa and Prosopis alpataco) in shrub patches, occurring with common abundance in the northeastern Patagonian region, and (2) determine the spatial and temporal variabilities of the mineral elements. There were 240 vegetal samples collected from six different sites, all within about 15 km. The essential nature of 15 mineral elements for litter and plant parts was analyzed. Composition varied significantly for P, Ca, Mg, Na, K, Cu, Zn, Mo, and B between species; N between seasons; and S, Al, Fe, Mn, and Cd among sites. Also Na, K, and B showed secondary variations derived from a seasonal effect, as did P, Ca, Mg, Cu, Zn, and Mo, but associated with secondary differences among sites.
Copper-zinc superoxide dismutase activity in red blood cells and serum in demented patients and in aging
Sacerdote de Lustig, Eugenia; Serra, Jorge Alberto; Kohan, Silvia; Canziani, Gabriela Alicia; Famulari, Arturo L.; Dominguez, Raul Omar
The activity of the enzyme copper-zinc superoxide dismutase (Cu-Zn SOD) has been investigated in serum and red blood cells (RBC) homogenate obtained from demented patients with associated vascular lesions (VD), demented patients with probable Alzheimer's disease (DAT) and healthy controls (CG) of the same age. The increase in SOD activity was statistically significant ( P < 0.01) in RBCs homogenate of DAT and VD patients, when compared to controls, but no differences appear between the two diseased groups. Additionally, a statistically significant increase in SOD activity ( P < 0.01) in DAT patients above 70 years as compared to those 50–70 years old, and a relation between SOD and age were found. No changes in SOD activity with age in healthy controls nor in vascular dementia group were detected. A statistically significant increase in Circulating SOD activity ( P < 0.01) was observed in vascular patients compared to controls. The observed increase in DAT Circulating SOD activity (against CG) was not significant. The increased levels of Cu-Zn SOD, probably represent a general alteration of the oxidative processes characteristic of these dementias and suggest that the enzyme might be used as a marker.
Acción de L. Casei y L. Acidophilus sobre LB productores de IgA en mucosa intestinal: Relación con la capacidad de protección frente a enteropatógenos
Medici, Marta Graciela; Valverde, Marta Hercilia; Perdigon, Gabriela del Valle
Demostramos anteriormente que la alimentación previa a ratón con L. casei fue efectiva en la protección contra enteropatógenos, con incremento de niveles de IgA-s con dosis de 2,4 x 109 cel y que L. acidophillus no protege frente a infecciones y no aumenta los niveles de IgA-s.
El doble mensaje de los mastocitos para los tumores
Sacerdote de Lustig, Eugenia; Lauria de Cidre, Lilia Susana
En relación a los tumores, los linfocitos han sido la gran vedette del tratamiento biológico del cáncer en esta última década, mientras que otras células del sistema inmune han sido descuidadas.
Angiogenic activity by spleen cell supernatants from tumor‐bearing and tumor‐resected mice
Davel, Lilia; Miguez, Marta; Jasnis, María Adela; Eiján, Ana María; Oisgold Dagá, Sara; Sacerdote de Lustig, Eugenia
Supernatants of cultured spleen cells (SCS) from small tumor‐bearing mice (STBM) and large tumor‐bearing mice (LTBM) are able to enhance tumor growth as well as accelerate tumor takes in vivo when inoculated 24 h before tumor implant. After tumor resection enhancing activity disappears at a rate depending on the size of the resected tumor. Spleen cells from tumor‐bearing mice also evoke a complex vascular response, lymphocyte‐induced angiogenesis (LIA) elicited via the release of lymphokines from activated T cells. As angiogenic factors promote tumor development we investigated if SCS from tumor‐bearing and tumor‐resected mice contain factors capable of evoking a LIA response. Angiogenic activity was detected by SCS of small and large tumor‐bearing mice as well as in large tumor‐resected mice. On the other hand, SCS from small tumor‐resected mice are not able to evoke an angiogenic response. Possibly, the large antigenic burden in LTRM stimulate the immune system in a different way than in STRM. We can suggest that the different behavior of spleen cells after small and large tumor removal can be explained by quantitative or qualitative changes in the spleen subpopulations.
