On the Mechanism Behind the Instability of Isoreticular Metal–Organic Frameworks (IRMOFs) in Humid Environments

Increasing the resistance to humid environments is mandatory for the implementation of isoreticular metal–organic frameworks (IRMOFs) in industry. To date, the causes behind the sensitivity of [Zn₄(μ₄‐O)(μ‐bdc)₃]₈ (IRMOF‐1; bdc=1,4‐benzenedicarboxylate) to water remain still open. A multiscale scheme that combines Monte Carlo simulations, density functional theory and first‐principles Born–Oppenheimer molecular dynamics on IRMOF‐1 was employed to unravel the underlying atomistic mechanism responsible for lattice disruption. At very low water contents, H₂O molecules are isolated in the lattice but provoke a dynamic opening of the terephthalic acid, and the lattice collapse occurs at about 6 % water weight at room temperature. The ability of Zn to form fivefold coordination spheres and the increasing basicity of water when forming clusters are responsible for the displacement of the organic linker. The present results pave the way for synthetic challenges with new target linkers that might provide more robust IRMOF structures.     

The Collatz conjecture and De Bruijn graphs

We study variants of the well-known Collatz graph, by considering the action of the 3n+1$3n+1$ function on congruence classes. For moduli equal to powers of 2, these graphs are shown to be isomorphic to binary De Bruijn graphs. Unlike the Collatz graph, these graphs are very structured, and have several interesting properties. We then look at a natural generalization of these finite graphs to the 2-adic integers, and show that the isomorphism between the resulting infinite graphs is exactly the conjugacy map previously studied by Bernstein and Lagarias. Finally, we show that for generalizations of the 3n+1$3n+1$ function, such as the family of an+b$an+b$ functions and Collatz-like functions, we get similar relations with 2-adic and p$p$-adic De Bruijn graphs respectively.     

Task-specific ionic liquid for solubilizing metal oxides

Protonated betaine bis(trifluoromethylsulfonyl)imide is an ionic liquid with the ability to dissolve large quantities of metal oxides. This metal-solubilizing power is selective. Soluble are oxides of the trivalent rare earths, uranium(VI) oxide, zinc(II) oxide, cadmium(II) oxide, mercury(II) oxide, nickel(II) oxide, copper(II) oxide, palladium(II) oxide, lead(II) oxide, manganese(II) oxide, and silver(I) oxide. Insoluble or very poorly soluble are iron(III), manganese(IV), and cobalt oxides, as well as aluminum oxide and silicon dioxide. The metals can be stripped from the ionic liquid by treatment of the ionic liquid with an acidic aqueous solution. After transfer of the metal ions to the aqueous phase, the ionic liquid can be recycled for reuse. Betainium bis(trifluoromethylsulfonyl)imide forms one phase with water at high temperatures, whereas phase separation occurs below 55.5 degrees C (temperature switch behavior). The mixtures of the ionic liquid with water also show a pH-dependent phase behavior: two phases occur at low pH, whereas one phase is present under neutral or alkaline conditions. The structures, the energetics, and the charge distribution of the betaine cation and the bis(trifluoromethylsulfonyl)imide anion, as well as the cation-anion pairs, were studied by density functional theory calculations.     

Probing redox proteins on a gold surface by single molecule fluorescence spectroscopy

The interaction between the fluorescently labeled redox protein, azurin, and a thin gold film is characterized using single-molecule fluorescence intensity and lifetime measurements. Fluorescence quenching starts at distances below 2.3 nm from the gold surface. At shorter distances the quantum yield may decrease down to fourfold for direct attachment of the protein to bare gold. Outside of the quenching range, up to fivefold enhancement of the fluorescence is observed on average with increasing roughness of the gold layer. Fluorescence-detected redox activity of individual azurin molecules, with a lifetime switching ratio of 0.4, is demonstrated for the first time close to a gold surface.     

Optimally dense packings for fully asymptotic Coxeter tilings by horoballs of different types

The goal of this paper is to determine the optimal horoball packing arrangements and their densities for all four fully asymptotic Coxeter tilings (Coxeter honeycombs) in hyperbolic 3-space ${\mathbb{H}^3}$ . Centers of horoballs are required to lie at vertices of the regular polyhedral cells constituting the tiling. We allow horoballs of different types at the various vertices. Our results are derived through a generalization of the projective methodology for hyperbolic spaces. The main result states that the known B?r?czky?CFlorian density upper bound for ??congruent horoball?? packings of ${\mathbb{H}^3}$ remains valid for the class of fully asymptotic Coxeter tilings, even if packing conditions are relaxed by allowing for horoballs of different types under prescribed symmetry groups. The consequences of this remarkable result are discussed for various Coxeter tilings.     

Shifting representation search for hybrid flexible flowline problems

This paper considers the hybrid flexible flowline scheduling problem with a set of additional restrictions and generalizations that are common in practice. These include precedence constraints, sequence dependent setup times, time lags, machine eligibility and release times. There are many potential solution representations for this problem, ranging from simple and compact, to more complex and complete. Typically, when choosing the degree of detail of the solution representation, a tradeoff can be found between efficiency of the algorithm and the size of the search space. Several adaptations of existing methods are introduced (memetic algorithm, iterated local search, iterated greedy), as well as a novel algorithm called shifting representation search (SRS). This new method starts with an iterated greedy algorithm applied to a permutation version of the problem and at a given time, switches to an iterated local search on the full search space. As far as we know, this shift of the solution representation is new in the scheduling literature. Experimental results and statistical tests clearly prove the superiority of SRS compared with classical and existing methods.     

Synthesis and polymerization of p-(p′-vinylbenzoyl)-peroxybenzoic acid tert-butylester: A monomer containing a photodissociable radical source

In a previous report we have shown that efficiently photodecomposable, but thermally stable, peresters can be made from a compound that contains both a triplet photosensitizer functionality and a perester group. This fundamental approach has been extended to make a novel vinyl monomer containing a triplet sensitizer and a perester group. Thus, the monomer p-vinylbenzoyl-p-tert-butyl perbenzoate (VBPE) has been synthesized and polymerized by normal procedures without loss of perester activity. The resulting polymer perester can be used as an efficient photolabile radical source by irradiating at the λ_max of the benzophenone carbonyl moiety. The homopolymer (PVBPE) and copolymers with other monomers, such as styrene, could be photocrosslinked with high efficiency. Copolymers of VBPE and styrene containing low percentages of VBPE have been used to initiate graft chains of poly(methyl methacrylate) by irradiating a solution of the copolymer in methyl methacrylate at 366 nm.     

Photoinitiated polymerization of vinyl monomers by benzophenone t-butyl peresters

Benzophenone peresters undergo efficient photodecomposition when irradiated at 366 nm. This article reports peresters derived from benzophenone p,p′-dicarboxylic acid 1 which may produce radicals at two different centers in the same molecule. These peresters represent a unique new example of an efficient photoinitiator for acrylate and styrene polymerization.      

In-depth compositional analysis of ceramic (Bi2O3)0.75(Er2O3)0.25 by AES and XPS

The chemical composition of dense ceramics of erbia-stabilized -Bi₂O₃ was analyzed by Auger electron spectroscopy (AES) depth profiling using Ar⁺ ion sputtering. The relative sensitivity factors (rsf) and sputter rates of bismuth and erbium in this material have been determined by electron probe microanalysis (EPMA) and chemical analysis. These results, supplemented by data from angle resolved X-ray photoelectron spectroscopy (ARXPS), shows a bismuth enrichment at the surface. Evidence has been found for reduction of the bismuth-oxide at the outermost part of the surface layer.Dedicated to Professor Günther Tölg on the occasion of his 60th birthday