Comparative study of the cope rearrangement of hexa-1,5-diene and barbaralane

The classical rules for Cope rearrangements predict a transition state with chair form to be favored over the boat form. On the other hand, bridged homotropylidenes, which allow only a boat-form transition state by steric reasons, have extremely low barriers. A controversy about the correct pathway and the different possible intermediates and transition states of the reaction has gone on for years. In this work, the hypersurfaces of barbaralane, in comparison with the boat- and chair-form of hexa-1,5-diene, are computed by the ab inito CASSCF (6,6)/6-31G** method starting with UMP2/6-31G** natural orbitals (NO's). All three hypersurfaces show characteristic features, and, moreover differ from each other. A hitherto undiscussed intermediate, bicyclo[2.2.0]hexane, was localized on the boat-hexa-1,5-diene pathway. So it is noteworthy that our transition state for the boat-hexa-1,5-diene does not correspond to the transition states found by other authors for this conformation. The computed enthalpies of activation of boat- and chair-hexa-1,5-diene, and barbaralane are in good agreement with the experimental data.     

Simulation of aerated lagoon using artificial neural networks and multivariate regression techniques

The aim of this study was to develop an empirical model that provides accurate predictions of the biochemical oxygen demand of the output stream from the aerated lagoon at International Paper of Brazil, one of the major pulp and paper plants in Brazil. Predictive models were calculated from functional link neural networks (FLNNs), multiple linear regression, principal components regression, and partial least-squares regression (PLSR). Improvement in FLNN modeling capability was observed when the data were preprocessed using the PLSR technique. PLSR also proved to be a powerful linear regression technique for this problem, which presents operational data limitations.     

Phase transfer of metal cations by induced dynamic carrier agents: biphasic extraction based on dynamic covalent chemistry

In contrast to the classical method where a single molecule is designed to extract metal cations under specific conditions, dynamic covalent chemistry provides an approach based on the implementation of an adaptive dynamic covalent library for inducing the generation of the extractant species. This approach has been applied to the liquid–liquid extraction of copper(ii) nitrate based on a dynamic library of acylhydrazones constituents that self-build and distribute through the interface of a biphasic system. The addition of copper(ii) cations to this library triggers a modification of its composition and the up-regulation of the ligand molecules driven by coordination to the metal cations. Among these, one species has proven to be sufficiently lipophilic to play the role of carrier agent and its formation by component exchange enables the partial extraction of the copper(ii). The study of different pathways to generate the dynamic covalent library demonstrates the complete reversibility and the adaptability of the system. The detailed analytical investigation of the system provides a means to assess the mechanism of the dynamic extraction process.

Phase transfer of Cu(ii) cations is achieved by component exchange in a dynamic covalent library of acylhydrazone ligands. B1/B2 component exchange leads to the generation of a lipophilic carrier agent that extracts Cu(ii) into chloroform.     

Physicochemical and structural properties of bacteriostatic sulfonamides: Theoretical study

A theoretical study at the Hartree–Fock and density functional theory levels is performed on sulfonamide‐type bacteriostatic compounds with the aim to provide an insight into their structure–activity relationship. The basicity of the p‐amino group is analyzed by means of the proton affinities and the protonation energies, showing that molecules presenting bacteriostatic activity are less basic, i.e., they are characterized by larger protonation energies and smaller proton affinities. The acidity of the amide group is analyzed through the deprotonation energy. The results reveal that the more acidic molecules present a larger bacteriostatic activity. This result is also confirmed from a study of bond orders. A bond order analysis of the amide group suggests that the electron attracting group in these molecules is responsible for the increase in acidity. The charge of the SO₂ group is also shown to be affected by the presence of the electron attracting group and consequently related to the acidity of the molecules. A geometric analysis shows that structures in which the amino group is more coplanar with respect to the benzenic ring possess larger bacteriostatic activity. A conformational analysis of these molecules illustrates that active molecules have relatively larger torsion energy barriers. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 165–172, 2003     

Adsorption of anionic dyes on chitosan beads. 1. The influence of the chemical structures of dyes and temperature on the adsorption kinetics

In this work, chitosan beads were synthesized in acidic medium and cross-linked in 1% glutaraldehyde solution. The characterization of the materials using TG/DTG, XRD, and BET surface areas showed that the beads did not modify their characteristics after the cross-linking reaction. The cross-linked beads were utilized as adsorbents for the removal of the yellow-, blue-, and red-anionic reactive dyes from aqueous solutions at pH 2.0. Adsorption of the yellow-dye increased from 25 to 50 degrees C. However, adsorption of the blue-dye decreased from 25 to 50 degrees C. Interestingly, the adsorption of the red-dye decreased from 25 to 35 degrees C and increased from 45 to 50 degrees C. The kinetic data were evaluated using an Avrami kinetic model, where the parameter n was related to the determination of changes in the adsorption mechanisms. Adsorption data of the dyes in relation to the contact time, the chemical structures of the dyes, and temperature were presented and were discussed.     

Study of the acidity of H3PW12O40 supported on activated carbon by microcalorimetry and methanol dehydration reaction

H₃PW₁₂O₄₀/activated carbon catalysts have been studied by microcalorimetry and by the dehydration of methanol to dimethyl ether. It has been shown that the acidity of the polyacid is greatly reduced upon grafting on activated carbon. The decrease is so high that, at low polyanion loadings, the catalysts are relatively inactive in the dehydration of methanol to dimethyl ether.     

Inelastic incoherent neutron scattering measurements of intact cells and tissues and detection of interfacial water

We have previously used inelastic incoherent neutron scattering spectroscopy to investigate the properties of aqueous suspensions of biomolecules as a function of hydration. These experiments led to the identification of signals corresponding to interfacial (hydration) water at low water content. A prediction from these studies was that in the crowded environment inside living cells, a significant proportion of the water would be interfacial, with profound implications for biological function. Here we describe the first inelastic incoherent neutron scattering spectroscopy studies of living cells and tissues. We find that the interfacial water signal is similar to that observed for water interacting with purified biomolecules and other solutes, i.e., it is strongly perturbed in the librational and translational intermolecular optical regions of the spectrum at 20-150 meV. The ratio of interfacial water compared to total water in cells (approximately 30%) is in line with previous experimental data for hydration water and calculations based on simple assumptions.     

Binuclear Cu2+ complex mediated discrimination between L-glutamate and L-aspartate in water

L-glutamate and L-aspartate selectivity is achieved by the action of two Cu2+ metal ions rightly disposed in a cyclophane-type macrocyclic framework; electrochemical sensing of glutamate has been achieved by adsorption of the copper complexes on graphite electrodes.     

Synthesis of 2-carboxy-4-methylazetidine. A novel isomeric analog of dl-proline

L-Azetidine-2-carboxylic acid is a naturally occurring azetidine currently used in the synthesis of abnormally high molecular weight polypeptides. The synthesis of 2-carboxy-4-methylazetidine is now reported which is a novel isomeric analog of dl -proline. Reaction of α,β-dibromo carbonyl ester with three molar equivalents of benzylamine yields 1-benzyl-2-carbomethoxy-4-methyl-azetidine. Hydrolysis of the latter compound with barium hydroxide yields 1-benzyl-2-carboxyl-4-methylazetidine which, if subjected to catalytic hydrogenation., yields the title compound in practical yield. The cis configuration is tentatively assigned to the title compound on the basis of previously published arguments.     

Calorimetric determination of the acidic character of amorphous and crystalline aluminosilicates

Aluminosilicates can present different structures such as crystalline true zeolite molecular sieves or amorphous silica–aluminas. With a large surface area available, both can be involved as catalysts, adsorbents or catalyst supports, and the determination of their surface acidic properties is an important parameter in the study of such materials.

The number, strength and strength distribution of the acidic sites were determined using microcalorimetry linked to a volumetric line. Ammonia was used as a basic probe molecule. The adsorption temperatures ranged from 353 K up to 473 K. The samples consisted of two amorphous silica–aluminas (Si/Al ≈ 6.5) and three microporous zeolites H-β, H-ZSM-5 and H-MCM-22 with similar Si/Al ratios (Si/Al ≈ 13).

The differential heats of ammonia adsorption versus coverage and the corresponding isotherms are given. The H-ZSM-5, H-MCM-22, H-β samples display a plateau of constant adsorption heats near 150 kJ mol⁻¹, while the silica–alumina samples present continuously decreasing heats from 150 kJ mol⁻¹ at zero coverage to 40 kJ mol⁻¹ at high coverage, due to their surface heterogeneity. For amorphous silica–aluminas, the number of acid sites is dependent of the aluminum distribution at the surface.

The differences observed in the adsorption behavior of ammonia over the three zeolites arise from differences in their morphology, i.e. the total free volumes, pore geometries and electric field gradients at the adsorption sites. The adsorption isosteres have also been calculated from the adsorption isotherms, and the isosteric heats of adsorption have been compared with the heats measured by calorimetry.