Statistical Comparison of Multiple Methods for the Determination of Dissolved Oxygen Levels in Natural Waters

The following experiment reinforces students working knowledge of statistics by utilizing the t test to compare the results of two independent methods for the determination of dissolved oxygen (DO). In this experiment students utilize a dissolved oxygen probe to determine the levels of DO in natural waters at two sampling locations while obtaining samples of water from the laboratory for analysis using the classic Winkler titration. The importance of using proper sampling methods and techniques to obtain representative samkples is a large focus of the prelaboratory discussion and is continually stressed during fieldwork. After analyzing the water samples by the DO mete and the Winkler titration, students pool the class data and are asked to determine if the two methods for dissolved oxygen agree at each sampling location. The students are then asked to determine if the DO levels at the different sampling locations are statistically different or not. The students are asked to consider why their results agree or differ from the theoretical value they calculate using Henrys law.     

Supramolecular control over donor-acceptor photoinduced charge separation

A novel donor-bridge-acceptor system has been synthesized by covalently linking a p-phenylene vinylene oligomer (OPV) and a perylene diimid (PERY) at opposite ends of a m-phenylene ethynylene oligomer (FOLD) of twelve phenyl rings, containing nonpolar (S)-3,7-dimethyl-1-octanoxy side chains. For comparison, model compounds have been prepared in which either the donor or acceptor is absent. In chloroform, the oligomeric bridge is in a random coil conformation. Upon addition of an apolar solvent (heptane) the oligomeric bridge first folds into a helical stack and subsequently intermolecular self-assembly of the stacks into columnar architectures occurs. Photoexcitation in the random coil conformation, where the interaction between the donor and acceptor chromophores is small, results only in long-range intramolecular energy transfer in which the OPV singlet-excited state is transformed into the PERY singlet-excited state. In the folded conformation of the bridge, donor and acceptor are closer and their enhanced interaction favors the formation the OPV(*)(+)-FOLD-PERY(*)(-) charge-separated state upon photoexcitation. As a result, the extent of photoinduced charge separation depends on the degree of folding of the bridge between donor and acceptor and therefore on the apolar nature of the medium. As a consequence, and contrary to conventional photoinduced charge separation processes, the formation of the OPV(*)(+)-FOLD-PERY(*)(-) charge-separated state is more favored in apolar media.     

Guanidinium-Type resonance stabilization and its biological implications. 2. The doubly-extended-guanidine series

π-Electron delocalization in neutral and protonated “doubly-extended-guanidine,” (H₂N)₂C?N? CH?N—CH?NH, has been studied by ab initio methods at the self-consistent field (SCF) STO -3G and 3-21G levels for a large number of tautomeric, rotameric, pseudocyclic, and monocyclic (disubstituted triazine) forms. These π systems have been characterized in terms of a number of structural and energetic parameters: degree of single/bond character from bond lengths and π bond orders, electron distributions, and tautomer, rotamer, and protonation energies. The acyclic neutral forms exhibit largely alternant single–double bond patterns as predicted by classical bonding structures but with, however, significant deviations due to conjugation. The acyclic protonated forms exhibit bond patterns consistent with resonance delocalized structures extending over the whole molecule (“doubly-extended guanidinium”) or part of the molecule (“extended-guanidinium”) or guanidinium . All systems showed alternant charge distributions with electron-deficient carbons. The energy results have been analyzed in terms of possible contributions from steric interactions, lone-pair repulsions, purportive electrostatic interactions in pseudocyclic forms, overall π-system conformation (extended, kinked, or folded), and specific through-space π-overlap interactions in some pseudocyclic forms. It was found that these other interactions usually dominate the specifically π effects so that the general concept of preferential π delocalization in straight lines does not hold for the acyclic systems. Some interesting examples of pseudocyclic forms exhibiting strongly stabilizing intramolecular interactions attributed to π through-space coupling are identified. These systems with incipient-ring characteristics present intermediate bonding models between the acyclic and closed-ring π systems. The extent of stabilization of the guanidinium-type cations by resonance delocalization in cyclic systems depended on whether it reinforced or interfered with the overall ring delocalization.     

Theory of system zones in capillary zone electrophoresis

In the last years, it has been shown that the formation and migration of system zones is an inherent feature of capillary zone electrophoresis (CZE) and that it depends predominantly on the composition of an actual background electrolyte (BGE). In most of the currently used BGEs, the SZs are invisible by the UV absorbance detection system, however, the comigration of SZs with the zones of analytes deteriorates the analytical performance of CZE and may be fatal for its utilization. Therefore, the theoretical predictions of the existence and migration of SZs is of key importance for the expediency of CZE. This is a review of the theoretical treatments of SZs which reveals the origin and the properties of SZs and shows how to cope with them. Also, a table of some typical BGEs is presented where the existence and mobilities of SZs are given.     

Nonlinear differential equations and lie superalgebras

It is shown how the theory of nonlinear ordinary differential equations with superposition formulas can be generalized to the case of superequations involving anticommuting Grassmann variables. As an example, equations based on the osp(1, 2) superalgebra are analyzed in detail. They turn out to be super-Riccati equations and a super-superposition formula is obtained for their solutions.

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Résumé Nous montrons comment la théorie des équations différentielles ordinaires non-linéaires admettant des lois de superposition peut être généralisée au cas de super-équations contenant des variables de Grassmann anticommutantes. Comme exemple les équations basées sur la super-algèbre osp(1, 2) sont analysées en détail. Nous obtenons des super-équations de Riccati et leurs formules de superposition sont mises en évidence.

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Chargé de recherches du FNRS.     

Reference materials for monitoring nutrients in sea water environment

VKI, the Danish national reference laboratory for environmental chemistry, has prepared a range of reference materials for quality control of nutrient analyses in environmental samples. The relevant concentration levels and potential matrix interference effects vary with the sample type. In sea water, nutrient concentrations are in the g/L range and salt concentrations can cause matrix interferences. Two certified reference materials for nutrient analyses were prepared representing the salinities in the Kattegat (20–25 parts per thousand) and the Baltic sea (below 10 parts per thousand). The preparation and certification were an integrated part of a larger project conducted by VKI on the optimisation of nutrient analyses in sea water. Part of the project has been participation in a European interlaboratory trial exercise QUASIMEME [1], which has enabled the Danish reference material data to be traced back to other European data. Denmark has now a certified reference material that will help to ensure the quality of marine analyses that are part of the ongoing national water monitoring plan.     

A scalable synthesis of a histamine H3 receptor antagonist

Starting from 1-methylimidazole, a concise, scalable, three-step synthesis of the title compound is described. The required 2-chloroimidazole was prepared in very good yield by halogen-metal exchange between the 2-lithio derivative and hexachloroethane.     

Parametrization of an anharmonic Kirkwood-Keating potential for AlxGa1-xAs alloys

We introduce a simple semiempirical anharmonic Kirkwood-Keating potential to model A(x)B(1-x)C-type semiconductors. The potential consists of the Morse strain energy and Coulomb interaction terms. The optical constants of pure components, AB and BC, were employed to fit the potential parameters such as bond-stretching and -bending force constants, dimensionless anharmonicity parameter, and charges. We applied the potential to finite temperature molecular-dynamics simulations on Al(x)Ga(1-x)As for which there is no lattice mismatch. The results were compared with experimental data and those of harmonic Kirkwood-Keating model and of equation-of-motion molecular-dynamics technique. Since the Morse strain potential effectively describes finite temperature damping, we have been able to numerically reproduce experimentally obtained optical properties such as dielectric functions and reflectance. This potential model can be readily generalized for strained alloys.     

Structure and Dynamics of Adsorbed Dopamine on Solvated Carbon Nanotubes and in a CNT Groove

Advanced carbon microelectrodes, including many carbon-nanotube (CNT)-based electrodes, are being developed for the in vivo detection of neurotransmitters such as dopamine (DA). Our prior simulations of DA and dopamine-o-quinone (DOQ) on pristine, flat graphene showed rapid surface diffusion for all adsorbed species, but it is not known how CNT surfaces affect dopamine adsorption and surface diffusivity. In this work, we use molecular dynamics simulations to investigate the adsorbed structures and surface diffusion dynamics of DA and DOQ on CNTs of varying curvature and helicity. In addition, we study DA dynamics in a groove between two aligned CNTs to model the spatial constraints at the junctions within CNT assemblies. We find that the adsorbate diffusion on a solvated CNT surface depends upon curvature. However, this effect cannot be attributed to changes in the surface energy roughness because the lateral distributions of the molecular adsorbates are similar across curvatures, diffusivities on zigzag and armchair CNTs are indistinguishable, and the curvature dependence disappears in the absence of solvent. Instead, adsorbate diffusivities correlate with the vertical placement of the adsorbate’s moieties, its tilt angle, its orientation along the CNT axis, and the number of waters in its first hydration shell, all of which will influence its effective hydrodynamic radius. Finally, DA diffuses into and remains in the groove between a pair of aligned and solvated CNTs, enhancing diffusivity along the CNT axis. These first studies of surface diffusion on a CNT electrode surface are important for understanding the changes in diffusion dynamics of dopamine on nanostructured carbon electrode surfaces.     

Strong valid inequalities for the resource-constrained scheduling problem with uniform resource requirements

We consider the resource-constrained scheduling problem when each job’s resource requirements remain constant over its processing time. We study a time-indexed formulation of the problem, providing facet-defining inequalities for a projection of the resulting polyhedron that exploit the resource limitations inherent in the problem. Lifting procedures are then provided for obtaining strong valid inequalities for the original polyhedron. Computational results are presented to demonstrate the strength of these inequalities.