Ionic rectification by electrostatically actuated tethers on single walled carbon nanotube membranes

Large electrostatically actuating charged tethers (~2.5 nm) at the tip entrances of single walled carbon nanotubes (i.d. ~ 1.5 nm) can dramatically enhance ionic gating at the CNT core entrance. Significant rectification of small ions at physiological ionic strengths is observed and this system closely mimics the function of protein channels.     

Regioselective reductions of β,β-disubstituted enones catalyzed by nonracemically ligated copper hydride

CuH-catalyzed 1,2-additions to β,β-disubstituted α,β-unsaturated ketones have been further explored. Asymmetric reductions of enones lacking an α-substituent can be achieved with CuH complexed by DTBM-SEGPHOS in Et₂O at ?25 °C leading to the generation of highly valuable nonracemic allylic alcohols. The corresponding 1,4-reductions can also be achieved using the same reaction conditions by switching the ligand to a JOSIPHOS analog affording nonracemic β,β-disubstituted ketones. DFT calculations of the enone conformations and transition-state energies for model 1,2- and 1,4-additions were carried out to clarify the factors affecting the product ratios.     

Importance of membrane selection in the development of immunochromatographic assays for low-molecular weight compounds

This study was performed to demonstrate the importance of selecting an appropriate membrane when developing immunochromatographic assays (ICAs) for the sensitive detection of low-molecular weight compounds. Based on our findings, we propose a theoretical basis for selecting such a membrane. When eluting the sample solution for the competitive ICA using colloidal gold label for low-molecular analytes, the degree of binding inhibition is proportional to the collision frequency between the antibody-colloidal gold (Ab–CG) and analyte before Ab–CG binding to the capture antigen and a higher concentration of pesticides around the Ab–CG leads to a greater degree of inhibition. Therefore, we propose that the relative migration speed of the analyte and Ab–CG on the test strip is critically important for selecting a membrane in the development of sensitive competitive ICAs. We developed a novel method to estimate such a relative migration speed. We demonstrated the applicability of this proposal by using it to select an appropriate membrane for the development of an ICA of the pesticide diazinon.     

An evolving framework for describing student engagement in classroom activities

Student engagement in classroom activities is usually described as a function of factors such as human needs, affect, intention, motivation, interests, identity, and others. We take a different approach and develop a framework that models classroom engagement as a function of students’ conceptual competence in the specific content (e.g., the mathematics of motion) of an activity. The framework uses a spatial metaphor—i.e., the classroom activity as a territory through which students move—as a way to both capture common engagement-related dynamics and as a communicative device. In this formulation, then, students’ engaged participation can be understood in terms of the nature of the “regions” and overall “topography” of the activity territory, and how much student movement such a territory affords. We offer the framework not in competition with other instructional design approaches, but rather as an additional tool to aid in the analysis and conduct of engaging classroom activities.     

N-heterocyclic phosphenium ligands as sterically and electronically-tunable isolobal analogues of nitrosyls

The coordination chemistry of an N-heterocyclic phosphenium (NHP)-containing bis(phosphine) pincer ligand has been explored with Pt(0) and Pd(0) precursors. Unlike previous compounds featuring monodentate NHP ligands, the resulting NHP Pt and Pd complexes feature pyramidal geometries about the central phosphorus atom, indicative of a stereochemically active lone pair. Structural, spectroscopic, and computational data suggest that the unusual pyramidal NHP geometry results from two-electron reduction of the phosphenium ligand to generate transition metal complexes in which the Pt or Pd centers have been formally oxidized by two electrons. Interconversion between planar and pyramidal NHP geometries can be affected by either coordination/dissociation of a two-electron donor ligand or two-electron redox processes, strongly supporting an isolobal analogy with the linear (NO(+)) and bent (NO(-)) variations of nitrosyl ligands. In contrast to nitrosyls, however, these new main group noninnocent ligands are sterically and electronically tunable and are amenable to incorporation into chelating ligands, perhaps representing a new strategy for promoting redox transformations at transition metal complexes.     

Utilization of phosphinoamide ligands in homobimetallic fe and mn complexes: the effect of disparate coordination environments on metal-metal interactions and magnetic and redox properties

A series of homobimetallic phosphinoamide-bridged diiron and dimanganese complexes in which the two metals maintain different coordination environments have been synthesized. Systematic variation of the steric and electronic properties of the phosphinoamide phosphorus and nitrogen substituents leads to structurally different complexes. Reaction of [(i)PrNKPPh(2)] (1) with MCl(2) (M = Mn, Fe) affords the phosphinoamide-bridged bimetallic complexes [Mn((i)PrNPPh(2))(3)Mn((i)PrNPPh(2))] (3) and [Fe((i)PrNPPh(2))(3)Fe((i)PrNPPh(2))] (4). Complexes 3 and 4 are iso-structural, with one metal center preferentially binding to the three amide ligands in a trigonal planar arrangement while the second metal center is ligated by three phosphine donors. A fourth phosphinoamide ligand caps the tetrahedral coordination sphere of the phosphine-ligated metal center. M?ssbauer spectroscopy of complex 4 suggests that the metals in these complexes are best described as Fe(II) centers. In contrast, treatment of MnCl(2) or FeI(2) with [MesNKP(i)Pr(2)] (2) leads to the formation of the halide-bridged species [(THF)Mn(μ-Cl)(MesNP(i)Pr(2))(2)Mn(MesNP(i)Pr(2))] (5) and [(THF)Fe(μ-I)(MesNP(i)Pr(2))(2)FeI (7), respectively. Utilization of FeCl(2) in place of FeI(2), however, leads exclusively to the C(3)-symmetric complex [Fe(MesNP(i)Pr(2))(3)FeCl] (6), structurally similar to 4 but with a halide bound to the phosphine-ligated Fe center. The M?ssbauer spectrum of 6 is also consistent with high spin Fe(II) centers. Thus, in the case of the [(i)PrNPPh(2)](-) and [MesNP(i)Pr(2)](-) ligands, zwitterionic complexes with the two metals in disparate coordination environments are preferentially formed. In the case of the more electron-rich ligand [(i)PrNP(i)Pr(2)](-), complexes with a 2:1 mixed donor ligand arrangement, in which one of the ligand arms has reversed orientation relative to the previous examples, are formed exclusively when [(i)PrNLiP(i)Pr(2)] (generated in situ) is treated with MCl(2) (M = Mn, Fe): (THF)(3)LiCl[Mn(N(i)PrP(i)Pr(2))(2)(P(i)Pr(2)N(i)Pr)MnCl] (8) and [Fe(N(i)PrP(i)Pr(2))(2)(P(i)Pr(2)N(i)Pr)FeCl] (9). Bimetallic complexes 3-9 have been structurally characterized using X-ray crystallography, revealing Fe-Fe interatomic distances indicative of metal-metal bonding in complexes 6 and 9 (and perhaps 4, to a lesser extent). All of the complexes appear to adopt high spin electron configurations, and magnetic measurements indicate significant antiferromagnetic interactions in Mn(2) complexes 5 and 8 and no discernible magnetic superexchange in Fe(2) complex 4. The redox behavior of complexes 3-9 has also been investigated using cyclic voltammetry, and theoretical investigations (DFT) were performed to gain insight into the metal-metal interactions in these unique asymmetric complexes.     

Population transfer and rapid passage effects in a low pressure gas using a continuous wave quantum cascade laser

A continuous wave quantum cascade laser (cw-QCL) operating at 10 μm has been used to record absorption spectra of low pressure samples of OCS in an astigmatic Herriott cell. As a result of the frequency chirp of the laser, the spectra show clearly the effects of rapid passage on the absorption line shape. At the low chirp rates that can be obtained with the cw-QCL, population transfer between rovibrational quantum states is predicted to be much more efficient than in typical pulsed QCL experiments. This optical pumping is investigated by solving the Maxwell Bloch equations to simulate the propagation of the laser radiation through an inhomogeneously broadened two-level system. The calculated absorption profiles show good quantitative agreement with those measured experimentally over a range of chirp rates and optical thicknesses. It is predicted that at a low chirp rate of 0.13 MHz ns(-1), the population transfer between rovibrational quantum states is 12%, considerably more than that obtained at the higher chirp rates utilised in pulsed QCL experiments.     

An isocratic HPLC method for the determination of sorbitol and glycerol in pharmaceutical formulations

Sorbitol and glycerol, along with other inactive ingredients such as preservatives and dyes, are commonly used in various pharmaceutical and personal care products. To accurately assess the effectiveness of various formulations containing sorbitol and/or glycerol, their quantitative determination is essential. In the current study, two types of detectors (a Varian evaporative light scattering detector and an Agilent ultraviolet-visible detector) are evaluated for the assay of working sample solutions. The two detection techniques are complimentary, and a comparison of the results obtained using the two detectors is presented here. The current method is shown to be stability-indicating and free from interference from any of the formulation excipients and potential degradation products. The method is reproducible, accurate, sensitive and selective. It provides enhanced detection sensitivity for sorbitol and comparable sensitivity for glycerol versus similar methods reported in the literature that utilize a refractive index detector for the analysis of either of the two polyols.     

An investigation of the mode of sorption of inositol hexaphosphate to goethite

Adsorption of inositol hexaphosphate (IP(6)) on goethite has been studied as a function of pH and concentration, and by use of Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR). While adsorption was highest at low pH, a significant amount remained adsorbed above pH 10 where, in the absence of IP(6), the surface is expected to have a net negative charge. The adsorption isotherm at pH 5.5 indicated strong binding to the surface with each adsorbed species occupying about 2.5 nm(2). ATR-FTIR spectra of IP(6) solutions in the pH range from 2 to 12 were fitted with a single set of IR bands which were assigned primarily by analogy with phosphate spectra. From its variation in intensity with pH the band at 1040 cm(-1) was assigned to the effect of hydrogen bonding on the PO vibration. No additional bands were required to fit the spectra of IP(6) adsorbed to goethite, indicating that adsorption occurs by outer-sphere complexation in this system. At all pH values studied the band associated with hydrogen bonding was more intense for the adsorbed species than in solution at the corresponding pH indicating that hydrogen bonding plays an important role in binding IP(6) to goethite.