Adaptive explicit-implicit tau-leaping method with automatic tau selection

The existing tau-selection strategy, which was designed for explicit tau leaping, is here modified to apply to implicit tau leaping, allowing for longer steps when the system is stiff. Further, an adaptive strategy that identifies stiffness and automatically chooses between the explicit and the (new) implicit tau-selection methods to achieve better efficiency is proposed. Numerical testing demonstrates the advantages of the adaptive method for stiff systems.     

Effect of reactant size on discrete stochastic chemical kinetics

This paper is aimed at understanding what happens to the propensity functions (rates) of bimolecular chemical reactions when the volume occupied by the reactant molecules is not negligible compared to the containing volume of the system. For simplicity our analysis focuses on a one-dimensional gas of N hard-rod molecules, each of length l. Assuming these molecules are distributed randomly and uniformly inside the real interval [0,L] in a nonoverlapping way, and that they have Maxwellian distributed velocities, the authors derive an expression for the probability that two rods will collide in the next infinitesimal time dt. This probability controls the rate of any chemical reaction whose occurrence is initiated by such a collision. The result turns out to be a simple generalization of the well-known result for the point molecule case l=0: the system volume L in the formula for the propensity function in the point molecule case gets replaced by the "free volume" L-Nl. They confirm the result in a series of one-dimensional molecular dynamics simulations. Some possible wider implications of this result are discussed.     

Carboxymethyl Xylan - Control of Properties by Synthesis

The hemicellulose xylan is a polysaccharide that occurs in nature in enormous amount in various one year- and perennial plants having different structures and molecular masses. Versatile ways to generate bio-based functional polymers result from the chemical modification of this biopolymer. In our research, xylans from various resources like birch-, beech-, and eucalyptus wood and from oat husk, rye bran, and corn cob were used to investigate the important method of carboxymethylation in detail. Different activation procedures were elaborated to synthesize carboxymethyl xylan. One step reactions lead to products with a degree of substitution (DS) from 0.13 to 1.22 in dependence on the molar ratio of anhydroxylose unit (AXU) to reagent. Two step syntheses yielded DS values up to 1.65. Carboxymethyl xylans are water soluble at a DS of 0.3. The solutions have different clearness depending on the provenience. NMR spectroscopy and HPL chromatography were applied to characterize the carboxymethyl xylans in detail.     

Züchtung und Eigenschaften von Mangan-, Kobalt-und Zinkmanganatkristallen

Crystals of the manganates of manganese, cobalt, zinc have been grown from a flux solution of PbO-PbF₂ by slow cooling. Their lattice constants are compared with dates of sintered products (from literature). It resulted by comparison of the densities obtained by X-ray and by pycnometer that the deficit of oxygen is caused by anionic vacancies in the lattice.     

Light scattering study on the glass former o-terphenyl

Combining double monochromator, tandem Fabry-Pérot interferometry (TFPI), and photon correlation spectroscopy (PCS), the full range of the dynamic susceptibility of the glass former o-terphenyl is covered (240-440 K). A time resolution of 6 ns is reached by the PCS setup, and the measuring gap between PCS and TFPI has significantly shrunk. Combined time domain data are presented, which cover 0.01 ps-100 s. Regarding the primary α-process, no recognizable change in shape and amplitude is observed, i.e., frequency-temperature superposition holds over the entire temperature range investigated. The only change in the susceptibility is the emergence of the excess wing contribution, leading to a flattening of the susceptibility minimum close to T(g). Separating relaxation contribution from fast and slow dynamics, we determine the relaxation strength 1-f(rel)(T) of the fast dynamics, which shows a cusplike temperature dependence with a crossover temperature T(c)=316±10 K. Due to relative small changes of 1-f(rel)(T) the effect is difficult to identify in f(rel)(T) itself. The crossover temperature is larger than previously reported as no extrapolation scheme is applied.     

Regio- and Diastereoselective Dimerization of Diazo Carbonyls: A Cooperative Catalytic Approach to Complex Scaffolds with Four Contiguous Stereocenters

Starting from readily available o-diazoacyl-substituted arene carboxylates, scaffolds with the 5,9-epoxycyclohepta[b]pyran-2(3H)-one core were obtained by cooperative Rh^II, Lewis and Brønsted acid catalysis. Four new bonds, three functional groups (lactone, ketal, and alcohol) and four contiguous stereocenters are formed during this regio- and diastereoselective process in a single synthetic step. Intensive optimization and mechanistic studies, including the trapping, isolation, and elucidation of reaction intermediates, led to a plausible mechanistic scenario. The reaction is proposed to involve carbonyl ylides but also transient species of the ketocarbene equilibrium that undergo a cascade of cycloaddition and skeletal rearrangements.     

2-Methoxyhydroquinone from Vanillin for Aqueous Redox-Flow Batteries

We show the synthesis of a redox-active quinone, 2-methoxy-1,4-hydroquinone (MHQ), from a bio-based feedstock and its suitability as electrolyte in aqueous redox flow batteries. We identified semiquinone intermediates at insufficiently low pH and quinoid radicals as responsible for decomposition of MHQ under electrochemical conditions. Both can be avoided and/or stabilized, respectively, using H₃PO₄ electrolyte, allowing for reversible cycling in a redox flow battery for hundreds of cycles.