Reactivity of the cadmium ion in concentrated phosphoric acid solutions

The solvation transfer coefficients which characterize the changes of ion reactivity with phosphoric acid concentration have been calculated for cadmium from the constants of the successive chloride complexes, and for silver and diethyldithiophosphate from potentiometric measurements. They evidence the strong desolvation of the cadmium species in concentrated phosphoric acid media, causing a remarkable increase of its reactivity. They allow the results of liquid-liquid extraction, precipitation and flotation reactions to be correctly interpreted and their changes to be foreseen when the reagents are modified.     

A Blueprint for Problem Posing

The purpose of this paper is to outline a plan of action designed to enhance growth in problem-posing skills. The “blueprint” provides a model for teachers and teacher educators who wish to integrate more problem posing into their curriculum. Specific activities and instructional strategies are cited to illustrate the development of problem posing as a process in the classroom.     

Influence of Water Uptake on Dynamic Fracture Behavior of Poly(Methyl Methacrylate)

A study was completed to assess the effects of various humidity levels and amount of sorbed water on the fracture behavior of notched poly(methyl methacrylate) (PMMA) samples subjected to stress pulses generated by the impact of a projectile launched from an air gun. Impact experiments were performed on six sets of samples conditioned in different environments: dry samples; samples exposed to three different relative humidity environments (11 %, 60 %, and 98 %) using saturated salt solutions (Lithium Chloride, Sodium Bromide, and Potassium Sulfate, respectively); and distilled water- and seawater-exposed samples. Experiments varied by immersion time and water content, while loading conditions were kept constant. The main goal of this study was to understand the effects of sorbed water on the fracture behavior of PMMA when subjected to high strain rate impacts. It was observed that when PMMA is subjected to strain rates of 10² s ^?1, the effect of water content is not a dominant mechanism on the crack initiation and crack-tip speed of PMMA.     

Conformational manifold of alpha-aminoisobutyric acid (Aib) containing alanine-based tripeptides in aqueous solution explored by vibrational spectroscopy, electronic circular dichroism spectroscopy, and molecular dynamics simulations

Replacement of the alpha-proton of an alanine residue to generate alpha-aminoisobutyric acid (Aib) in alanine-based oligopeptides favors the formation of a 3(10) helix when the length of the oligopeptide is about four to six residues. This research was aimed at experimentally identifying the structural impact of an individual Aib residue in an alanine context of short peptides in water and Aib's influence on the conformation of nearest-neighbor residues. The amide I band profile of the IR, isotropic and anisotropic Raman, and vibrational circular dichroism (VCD) spectra of Ac-Ala-Ala-Aib-OMe, Ac-Ala-Aib-Ala-OMe, and Ac-Aib-Ala-Ala-OMe were measured and analyzed in terms of different structural models by utilizing an algorithm that exploits the excitonic coupling between amide I' modes. The conformational search was guided by the respective 1H NMR and electronic circular dichroism spectra of the respective peptides, which were also recorded. From these analyses, all peptides adopted multiple conformations. Aib predominantly sampled the right-handed and left-handed 3(10)-helix region and to a minor extent the bridge region between the polyproline (PPII) and the helical regions of the Ramachandran plot. Generally, alanine showed the anticipated PPII propensity, but its conformational equilibrium was shifted towards helical conformations in Ac-Aib-Ala-Ala-OMe, indicating that Aib can induce helical conformations of neighboring residues positioned towards the C-terminal direction of the peptide. An energy landscape exploration by molecular dynamics simulations corroborated the results of the spectroscopic studies. They also revealed the dynamics and pathways of potential conformational transitions of the corresponding Aib residues.     

Methylrhenium trioxide revisited: mechanisms for nonredox oxygen insertion in an M-CH3 bond

Methylrhenium trioxide (MTO) has the rare ability to stoichiometrically generate methanol at room temperature with an external oxidant (H2O2) under basic conditions. In order to use this transformation as a model for nonredox oxidative C-O coupling, the mechanisms have been elucidated using density functional theory (DFT). Our studies show several possible reaction pathways to form methanol, with the lowest net barrier (DeltaH++) being 23.3 kcal mol-1. The rate-determining step is a direct "Baeyer-Villiger" type concerted oxygen insertion into MTO, forming methoxyrhenium trioxide. The key to the low-energy transition state is the donation of electron density, first, from HOO(-) to the -CH3 group (making -CH3 more nucleophilic and HOO- more electrophilic) and, second, from the Re-C bond to both the forming Re-O and breaking O-O bonds, simultaneously (thus forming the Re-O bond as the Re-C bond is broken). In turn, the ability of MTO to undergo these transfers can be traced to the electrophilic nature of the metal center and to the absence of accessible d-orbitals. If accessible d-orbitals are present, they would most likely donate the required electron density instead of the M-CH3 moiety, and this bond would thus not be broken. It is possible that other metal centers with similar qualities, such as PtIV or IrV, could be competent for the same type of chemistry.     

13C carbonyl chemical shielding tensors: Comparing SCF,MBPT (2), and DFT predictions to experiment

In this work, we calculate the ¹³C nuclear magnetic resonance chemical shielding tensors for 18 carbonyl-containing compounds. The many-body perturbation theory (MBPT), self-consistent field (SCF), and density functional theory (DFT) formalisms were used with gauge including atomic orbitals (GIAO) to calculate the shielding tensors. Our data suggest that shielding tensors can be efficiently estimated by performing one MBPT(2) correlated calculation (e.g., at a reference geometry) and SCF-level calculations at other geometries and taking the SCF-to-correlated tensor element differences to be geometry independent. That is, the correlation contribution to the chemical shielding seems to be relatively constant over a considerable range of distortions. Treatment of correlation using DFT methods is shown to not be as systematically reliable as with MBPT(2). Data on 18 carbonyl compounds show that the single largest influence on the shielding tensor is the presence of nearby electron-withdrawing or electron-donating groups. Finally, although good agreement with powder or single-crystal experimental data is achieved for two or three tensor eigenvalues, systematic differences remain for one element; the origins of these differences are discussed. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 63: 875–894, 1997     

Simultaneous determination of residues of chloramphenicol, florfenicol, florfenicol amine, and thiamphenicol in shrimp tissue by gas chromatography with electron capture detection

A gas chromatographic (GC) method is presented for determining residues of chloramphenicol (CAP), florfenicol (FF), florfenicol amine (FFa), and thiamphenicol (TAP) in shrimp tissues, with meta-nitrochloramphenicol (mCAP) as the internal standard. The composited shrimp is extracted with basic ethyl acetate, followed by an acetonitrile-basic ethyl acetate mixture. This extract is centrifuged, filtered, evaporated, and reconstituted in water; the reconstituted extract is acidified, defatted with hexane, and passed through a propylsulfonic acid (PRS) and C18 solid-phase extraction (SPE) system. The C18 SPE column is eluted with methanol, and the PRS SPE column is eluted with basic MeOH plus counter ion. The combined eluates are evaporated, reconstituted in acetonitrile, and derivatized with Sylon BFT. After derivatization, the addition of toluene directly to the sample, followed by the addition of basic water, quenches the derivatization process. After centrifugation, the organic layer is carefully removed, and the analytes are determined by GC with electron capture detection. Shrimp tissues were fortified with fenicols (i.e., CAP, FF, FFa, and TAP) at 5, 10, 20, 40, and 80 ng/mL. Overall recoveries were 88, 101, 91, and 84% with overall interassay (between-day) variabilities (i.e., relative standard deviations) of 5.3, 9.4, 12.8, and 7.4% for CAP, FF, FFa, and TAP, respectively. The method detection limits were calculated as 0.7, 1.4, 2.4, and 1.3 ng/g (ppb) for CAP, FF, FFa, and TAP, respectively, based on a 10 g sample. The quantitation limit as determined empirically by this method is the lower limit of the standard curve, which is about 5 ng/g (ppb) for each analyte.