Variable-temperature FT-IR studies on the thermodynamics of carbon dioxide adsorption on a faujasite-type H-Y zeolite

Adsorption of carbon dioxide on a faujasite-type H-Y zeolite (Si:Al = 2.6:1) was studied by variable-temperature (200-290 K range) infrared spectroscopy. Adsorbed CO₂ molecules interact with the Brønsted acid Si(OH)Al groups located inside the zeolite supercage, bringing about a characteristic bathochromic shift of the O-H stretching mode from 3645 cm⁻¹ (free OH group) to 3540 cm⁻¹ (hydrogen-bonded CO₂ adsorption complex). Simultaneously, the asymmetric (ν₃) mode of adsorbed CO₂ is observed at 2353 cm⁻¹. From the observed variation of the integrated intensity of the 3645 and 2353 cm⁻¹ IR absorption bands upon changing temperature, corresponding values of standard adsorption enthalpy and entropy were found to be ΔH° = −28.5(±1) kJ mol⁻¹ and ΔS° = −129(±10) J mol⁻¹ K⁻¹. Comparison with the reported values of ΔH° for CO₂ adsorption on other zeolites is briefly discussed.     

Model of an asymmetric DPPC/DPPS membrane: effect of asymmetry on the lipid properties. A molecular dynamics simulation study

The study of asymmetric lipid bilayers is of a crucial importance due to the great number of biological process in which they are involved such as exocytosis, intracellular fusion processes, phospholipid-protein interactions, and signal transduction pathway. In addition, the loss of this asymmetry is a hallmark of the early stages of apoptosis. In this regard, a model of an asymmetric lipid bilayer composed of DPPC and DPPS was simulated by molecular dynamics simulation. Thus, the asymmetric membrane was modeled by 264 lipids, of which 48 corresponded to DPPS- randomly distributed in the same leaflet with 96 DPPC. In the other leaflet, 120 DPPC were placed without DPPS-. Due to the presence of a net charge of -1 for the DPPS- in physiological conditions, 48 Na+ were introduced into the system to balance the charge. To ascertain whether the presence of the DPPS- in only one of the two leaflets perturbs the properties of the DPPC in the other leaflet composed only of DPPC, different properties were studied, such as the atomic density of the different components across the membrane, the electrostatic potential across the membrane, the translational diffusion of DPPC and DPPS, the deuterium order parameters, lipid hydration, and lipid-lipid charge bridges. Thus, we obtained that certain properties such as the surface area lipid molecule, lipid head orientation, order parameter, translational diffusion coefficient, or lipid hydration of DPPC in the leaflet without DPPS remain unperturbed by the presence of DPPS in the other leaflet, compared with a DPPC bilayer. On the other hand, in the leaflet containing DPPS, some of the DPPC properties were strongly affected by the presence of DPPS such as the order parameter or electrostatic potential.     

Mobilizing histories in mathematics teacher education: memories, social practices, and discursive games

In the first part of this paper, we share and elucidate the way we mobilize histories in some disciplines that are part of the undergraduate courses in mathematics teacher education offered by State University of Campinas and Federal University of Rio Grande do Norte in Brazil. This way of mobilization can be featured as a set of collective indisciplinary problematizations occurring in a series of student investigations. Mobilizing practices of mathematics culture are the object of these investigations. These practices are performed by different communities both constituted by and constituent of different human activities. In the second part of this paper, we will discuss our way of mobilizing histories, contrasting it with the theoretical perspective of expansive learning, just as it has been defended by Yrjö Engeström, in his article Non scolae sed vitae discimus—towards overcoming the encapsulation of school learning. We will also attempt to highlight the role which this researcher has attributed to history in his model of expansive learning, a perspective based on the current research on activity theory.     

Locally definable homotopy

In [E. Baro, M. Otero, On o-minimal homotopy, Quart. J. Math. (2009) 15pp, in press (doi:10.1093/qmath/hap011)] o-minimal homotopy was developed for the definable category, proving o-minimal versions of the Hurewicz theorems and the Whitehead theorem. Here, we extend these results to the category of locally definable spaces, for which we introduce homology and homotopy functors. We also study the concept of connectedness in ?-definable groups — which are examples of locally definable spaces. We show that the various concepts of connectedness associated to these groups, which have appeared in the literature, are non-equivalent.     

Combined theoretical and FTIR spectroscopic studies on hydrogen adsorption on the zeolites Na-FER and K-FER

The interaction between molecular hydrogen and the alkali-metal-exchanged zeolites Na-FER and K-FER at a low temperature was investigated by combining variable-temperature infrared spectroscopy and theoretical calculations by using a periodic DFT model. The experimentally determined values of standard adsorption enthalpy, DeltaH degrees , were -6.0 (+/-0.8) and -3.5 (+/-0.8) kJ mol(-1) for Na-FER and K-FER, respectively. These results were found to be in agreement with corresponding DeltaH degrees values obtained from calculations on the periodic model. Two types of alkali-metal cation sites in FER were found: channel intersection sites and channel wall sites. Calculations showed a similar interaction energy for both site types, and similar structures of adsorption complexes. Up to two dihydrogen molecules can be physisorbed on the alkali-metal cation located on the intersection of two channels, while only one H2 molecule is physisorbed on the cation at the channel wall site. The adsorption enthalpies of H2 on alkali-metal-exchanged FER are significantly smaller than those found previously for the MFI-type zeolites Na-ZSM-5 and K-ZSM-5, which is likely due to a difference in the alkali-metal cation coordination in the two zeolite frameworks.     

Ordered vacancy network induced by the growth of epitaxial graphene on Pt(111)

We have studied large areas of (√3×√3)R30° graphene commensurate with a Pt(111) substrate. A combination of experimental techniques with ab initio density functional theory indicates that this structure is related to a reconstruction at the Pt surface, consisting of an ordered vacancy network formed in the outermost Pt layer and a graphene layer covalently bound to the Pt substrate. The formation of this reconstruction is enhanced if low temperatures and polycyclic aromatic hydrocarbons are used as molecular precursors for epitaxial growth of the graphene layers.     

Vibrational spectrum and internal rotation in 2,5-dimethylpyrazine

The infrared and Raman spectra of 2,5-dimethylpyrazine have been recorded and assigned on the basis of a C_2h molecular geometry previously determined by MINDO/3. The potential energy function corresponding to the internal rotation of both methyl groups has been used to solve the Schrödinger equation, and to obtain the energy levels of that motion on the basis of a molecular symmetry G₃₆. The rotation of each substituent is found to be almost independent of the other.     

IR-Spectroscopic Study of the Binding Isomerism of Adsorbed Molecules

This paper discusses the use of IR spectroscopy in the studies of isomerism in the binding of adsorbed molecules with a surface when a molecule may form several different surface species at the same site. Species whose geometry does not provide minimal adsorption energy can be considered as adsorption complexes in an excited state. The spectral manifestations of such a steric excitation are compared with the electronic and vibrational excitations of surface species. The sterically excited isomeric states existing in thermodynamic equilibrium with ordinary adsorption species are found and studied in detail. Examples are CO molecules bound through C and O atoms with metal cations in zeolites or with surface hydroxyl groups, the thiophene molecule via hydrogen bonding with silanol groups, and HD molecules dissociatively adsorbed on ZnO. A possible role of sterically activated isomeric states in catalysis is discussed.     

A clean and efficient cyclocondensation to pyrido[2,3-d]pyrimidine derivatives in aqueous media

A series of pyrido[2,3-d]pyrimidine derivatives were easily constructed by cyclocondensation reactions of 6-aminouracils or 6- aminothiouracil withα,β-unsaturated carbonyl compounds（aldehyde,ketone and ester） possessing a leaving group on theβposition,in H₂O under reflux conditions.