The H2O2+OH-->HO2+H2O reaction in aqueous solution from a charge-dependent continuum model of solvation

We applied our recently developed protocol of the conductorlike continuum model of solvation to describe the title reaction in aqueous solution. The model has the unique feature of the molecular cavity being dependent on the atomic charges in the solute and can be extended naturally to transition states and reaction pathways. It was used to calculate the reaction energetics and reaction rate in solution for the title reaction. The rate of reaction calculated using canonical variational transition state theory in the context of the equilibrium solvation path approximation, and including correction for tunneling through the small curvature approximation, was found to be 3.6 x 10(6) M(-1) s(-1), significantly slower than in the gas phase in accord with experiment. These results suggest that the present protocol of the conductorlike continuum model of solvation with the charge-dependent cavity definition captures qualitatively and quantitatively the solvation effects at transition states and allows for quantitative estimates of reaction rates in solutions.     

Wetting-induced formation of quasiperiodical wrinkling patterns in alginate-based coatings

We present the results of systematic model experimental investigations on wrinkling instabilities which develop on alginate-based coatings when they are wetted by swelling electrolyte drops. The wrinkles first appear randomly within the wet spot, before they selectively protrude out around the periphery in a quasiperiodical wrinkle pattern. We discuss the critical parameters that drive the emergence (ionic strength and swelling rate) and spatial feature (size and periodicity) of these swelling-induced structures on such complex functional coatings. Beyond their relative aesthetics and their fundamental interest related to morphological instabilities, these reconstruction structures which are invisible to the naked eye can develop in a variety of technological processes (inkjet printing for instance), affecting irreversibly the quality of the products.     

A Dual Palladium and Copper Hydride Catalyzed Approach for Alkyl–Aryl Cross-Coupling of Aryl Halides and Olefins

We report an efficient means of sp²–sp³ cross coupling for a variety of terminal monosubstituted olefins with aryl electrophiles using Pd and CuH catalysis. In addition to its applicability to a range of aryl bromide substrates, this process was also suitable for electron-deficient aryl chlorides, furnishing higher yields than the corresponding aryl bromides in these cases. The optimized protocol does not require the use of a glovebox and employs air-stable Cu and Pd complexes as precatalysts. A reaction on 10 mmol scale further highlighted the practical utility of this protocol. Employing a similar protocol, a series of cyclic alkenes were also examined. Cyclopentene was shown to undergo efficient coupling under these conditions. Lastly, deuterium-labeling studies indicate that deuterium scrambling does not take place in this sp²-sp³ cross coupling, implying that β-hydride elimination is not a significant process in this transformation.     

Guiding in the visible with "colorful" solid-core Bragg fibers

We report on the fabrication and characterization of solid-core all-polymer Bragg fibers consisting of a large-diameter polymethyl methylacrylate (PMMA) core surrounded by 50 alternating PMMA/Polystyrene (PS) polymer layers. By modifying the reflector layer thickness we illustrate that bandgap position can be adjusted at will in the visible. Moreover, such fibers are intensely colored in both the transmission and the outside reflection modes. Potential applications of such fibers are discussed.     

Atomistic simulation of nafion membrane: I. Effect of hydration on membrane nanostructure

We used classical molecular dynamics simulation with the DREIDING force field to characterize the changes in the nanostructure of Nafion membrane brought about by systematically changing the hydration level. We calculated the relative percentages of free, weakly bound, and bound water in hydrated Nafion membranes. At low hydration levels, coordination of hydronium ions by multiple sulfonate groups prevents vehicular transport and impedes structural transport of protons through steric hindrance to hydration of the hydronium ions. Our results provide insights into the nanostructure of hydrated Nafion membrane and are in excellent agreement with experimental observations by neutron scattering of changes in the percentage of non diffusing hydrogen atoms.     

Insight from molecular modelling: does the polymer side chain length matter for transport properties of perfluorosulfonic acid membranes?

We present a detailed analysis of the nanostructure of the short side chain (SSC) perfluorosulfonic acid membrane and its effect on H(2)O clustering, H(3)O(+) and H(2)O diffusion, and mean residence times of H(2)O near SO(3)(-) groups based on molecular dynamics simulations. We studied a range of hydration levels (λ) at temperatures of 300 and 360 K, and compare the results to our findings in the benchmark Nafion? membrane. The water cluster diameter is nearly the same in the two membranes, while the extent of SO(3)(-) clustering is more in the SSC membrane. The calculated cluster diameter of about 2.4 nm is in excellent agreement with the recently proposed cylindrical water channel model of these membranes. The diffusion coefficients of H(2)O and H(3)O(+) are similar in SSC and Nafion membranes. Raising the temperature of the SSC membrane from 300 to 360 K provides a much bigger increase in proton vehicular diffusion coefficient (by a factor of about 4) than changing the side chain length. H(3)O(+) ions are found to exchange more frequently with SO(3)(-) partners at the higher temperature. Our key findings are that (a) the hydrophobic-hydrophilic separation in the two membranes is surprisingly similar; (b) at all hydration levels studied, the long side chain of Nafion is bent and is effectively equivalent to a short side chain in terms of extension into the water domain; (c) vehicular proton transport occurs mainly between SO(3)(-) groups; and (d) changing the size of the simulation cell does not change the results significantly. The simulations are validated in good agreement with the corresponding experimental values for the simulated membrane density and diffusion coefficients of H(2)O.     

Analysis of an interacting particle method for rare event estimation

We present a large deviations analysis for the performance of an interacting particle method for rare event estimation. The analysis is restricted to a one-dimensional setting, though even in this restricted setting a number of new techniques must be developed. In contrast to the large deviations analyses of related algorithms, for interacting particle schemes it is an occupation measure analysis that is relevant, and within this framework many standard assumptions (stationarity, Feller property) can no longer be assumed. The methods developed are not limited to the question of performance analysis, and in fact give the full large deviations principle for such systems.