Ab initio calculations of anharmonic vibrational spectroscopy for hydrogen fluoride (HF)n (n = 3, 4) and mixed hydrogen fluoride/water (HF)n(H2O)n (n = 1, 2, 4) clusters

Anharmonic vibrational frequencies and intensities are computed for hydrogen fluoride clusters (HF)n, with n = 3, 4 and mixed clusters of hydrogen fluoride with water (HF)n(H2O)n where n = 1, 2. For the (HF)4(H2O)4 complex, the vibrational spectra are calculated at the harmonic level, and anharmonic effects are estimated. Potential energy surfaces for these systems are obtained at the MP2/TZP level of electronic structure theory. Vibrational states are calculated from the potential surface points using the correlation-corrected vibrational self-consistent field method. The method accounts for the anharmonicities and couplings between all vibrational modes and provides fairly accurate anharmonic vibrational spectra that can be directly compared with experimental results without a need for empirical scaling. For (HF)n, good agreement is found with experimental data. This agreement shows that the M?ller-Plesset (MP2) potential surfaces for these systems are reasonably reliable. The accuracy is best for the stiff intramolecular modes, which indicates the validity of MP2 in describing coupling between intramolecular and intermolecular degrees of freedom. For (HF)n(H2O)n experimental results are unavailable. The computed intramolecular frequencies show a strong dependence on cluster size. Intensity features are predicted for future experiments.     

Thermoluminescence glow-curve characteristics of LiF phosphors at high doses of gamma radiation

High doses of ionising radiation are becoming increasingly common for radiation-processing applications of various medical, agricultural and polymer products using gamma and electron beams. The objective of this work was to study thermoluminescence (TL) glow-curve characteristics of commonly used commercial LiF TL phosphors at high doses of radiation with a view to use them in dosimetry of radiation-processing applications. The TL properties of TLD 100 and 700 phosphors, procured from the Thermo-Scientific (previously Harshaw) company, have been studied in the dose range of 1–60 kGy. The shift in glow peaks was observed in this dose range. Integral TL responses of TLD 100 and TLD 700 were found to decrease as a linear function of dose in the range of 5–50 kGy. The paper describes initial results related to the glow-curve characteristics of these phosphors.     

New protein-resistant coatings for water filtration membranes based on quaternary ammonium and phosphonium polymers

Several simple, lightly cross-linked quaternary phosphonium- and ammonium-based polymer coatings were found to effectively resist the non-specific adsorption of proteins (i.e., bovine serum albumin (BSA) and fibrinogen (Fg)) from aqueous solution under both static exposure and dynamic membrane fouling conditions. In some cases, their protein-resistance performance is comparable to, or even better than, cross-linked poly(ethylene glycol) (i.e., PEG)-based polymers, which are considered benchmark protein-resistant coating materials. Similarly, these quaternary phosphonium and ammonium polymers exhibit comparable or better resistance to protein adsorption compared to polymeric analogues of some of the best organic functional groups identified in prior self-assembled monolayer-based protein-resistance studies. In particular, initial results of dynamic membrane fouling experiments showed that lightly cross-linked poly[trimethyl-(4-vinyl-benzyl)-phosphonium bromide] has exceptional protein-fouling resistance and better water transport properties than a representative PEG-based polymer coating. In addition to surface functional group chemistry, it was also found that the sub-surface chemistry; the nature of the substrate that the coating is on (i.e., substrate type and morphology); and the protein exposure conditions (i.e., static adsorption vs. dynamic filtration testing) can greatly affect the overall protein adsorption-resistance behavior of the coating. Finally, preliminary studies show that the presence of a regular nanostructure on the polymeric coating surface can lead to enhancement of protein resistance under static exposure conditions even with the same functional groups present, similar to what has been observed with inorganic surfaces.     

A Cognitive Modeling Tutor Supporting Student Inquiry for Balancing Chemical Equations

Improved interactive tutoring capabilities in educational software for chemistry problem solving is an important need that has been clearly articulated by teachers and students. The purpose of this work is to examine the incorporation of new concepts from the field of artificial intelligence (AI) as a route to meaningful individualized tutoring. The basic shift is to replace specific foreknowledge of problems with a direct representation of chemical and pedagogical principles and then simulate reasoning using these principles to tutor students. To assess the potential of an AI-based approach, we have developed a prototype tutorial program for balancing chemical equations that contains two important advances. First, the system can create a worked-out solution with detailed explanations for any equation entered by the student or teacher. Unlike a conventional tutorial, this is done dynamically, without the equation being stored ahead of time. Second, the program can interactively answer a variety of detailed questions about its work at each step. Studying worked-out examples plays an important role in learning, and this approach to supporting interactive student inquiry is being investigated as a method of cognitive modeling and apprenticeship intended to foster the students own self-explanation and question-asking abilities.     

Induced subgraphs of prescribed size

A subgraph of a graph G is called trivial if it is either a clique or an independent set. Let q(G) denote the maximum number of vertices in a trivial subgraph of G. Motivated by an open problem of Erd?s and McKay we show that every graph G on n vertices for which q(G)≤ C log n contains an induced subgraph with exactly y edges, for every y between 0 and n^{δ (C)}. Our methods enable us also to show that under much weaker assumption, i.e., q(G) ≤ n/14, G still must contain an induced subgraph with exactly y edges, for every y between 0 and . © 2003 Wiley Periodicals, Inc. J Graph Theory 43: 239–251, 2003     

Path integration for velocity-dependent potentials

We extent the prodistribution definition of path integrals to include Lagrangians with velocity-dependent potentials. We use Cameron-Martin transformations to evaluate a large part of a path integral exactly and give techniques for evaluating the remaining terms of the semi-classical expansion of the path integral. The Fredholm determinants, associated with these transformations, are evaluated explicitly in terms of Jacobi matrices defined by the classical system.