PDLC films prepared by electron beam and ultraviolet curing: influence of curing conditions on the electro-optical properties

Polymer dispersed liquid crystal (PDLC) films were prepared by polymerization-induced phase separation processes using ultraviolet (UV) and electron beam (EB) radiation. A mixture of the nematic LC material E7, an aromatic polyester acrylate, and additional monomeric acrylates was exposed to the EB radiation. A photoinitiator was included in the initial mixture in the case of UV exposure. The electro-optical behaviour of the PDLC films obtained has been investigated as a function of the chosen radiation. The transmission versus voltage curves strongly depend on the curing conditions, and are highly reproducible. Threshold and saturation voltages continuously increase with increasing dose values for UV-cured films, whereas plateau values were reached for EB-cured samples. A small memory effect has been observed for UV-cured systems.     

Use of ionic liquids in the platinum- and gold-catalyzed cycloisomerization of enyne systems

The platinum- and gold-catalyzed cycloisomerization of enyne systems has been carried out in various ionic liquids (ILs). In some cases, better selectivities and shorter reaction times have been observed compared to conventional conditions.     

Absorbing boundary conditions for the one-dimensional Schrödinger equation with an exterior repulsive potential

Mathematical constructions and comparisons of accurate absorbing boundary conditions for the one-dimensional Schrödinger equation with a general variable repulsive potential are developed. Stable semi-discretization schemes are built for the associated initial boundary value problems. Finally, some numerical simulations give a comparison of the various absorbing boundary conditions and show that they yield accurate computations.     

DFT Study on the Recovery of Hoveyda–Grubbs‐Type Catalyst Precursors in Enyne and Diene Ring‐Closing Metathesis

DFT (B3LYP‐D) calculations have been used to better understand the origin of the recovered Hoveyda–Grubbs derivative catalysts after ring‐closing diene or enyne metathesis reactions. For that, we have considered the activation process of five different Hoveyda–Grubbs precursors in the reaction with models of usual diene and enyne reactants as well as the potential precursor regeneration through the release/return mechanism. The results show that, regardless of the nature of the initial precursor, the activation process needs to overcome relatively high energy barriers, which is in agreement with a relatively slow process. The precursor regeneration process is in all cases exergonic and it presents low energy barriers, particularly when compared to those of the activation process. This indicates that the precursor regeneration should always be feasible, unlike the moderate recoveries sometimes observed experimentally, which suggests that other competitive processes that hinder recovery should take place. Indeed, calculations presented in this work show that the reactions between the more abundant olefinic products and the active carbenes usually require lower energy barriers than those that regenerate the initial precatalyst, which could prevent precursor regeneration. On the other hand, varying the precursor concentration with time obtained from the computed energy barriers shows that, under the reaction conditions, the precursor activation is incomplete, thereby suggesting that the origin of the recovered catalyst probably arises from incomplete precursor activation.     

Simple versus composite nature of the magnetic coupling in copper and nickel-based metallic chains. Over- and underestimation of J

Extended metal atom chains (EMACs) are molecular linear arrangements of metal atoms featuring magnetic properties. By means of the density functional theory (DFT), we have studied the magnetic coupling constants for [Cu₃(dpa)₄Cl₂]⁺, Ni₃(dpa)₄Cl₂ and Ni₅(tpda)₄Cl₂ to understand which is the origin of the previously reported theoretical underestimation of J for nickel complexes. We have decomposed J = J_σ + J_δ, finding that the former contribution is underestimated and the latter part is overestimated at the DFT/B3LYP level of computation. Varying the amount of Hartree-Fock exchange, we show that the B3LYP functional fails to describe the σ interaction properly, whereas the δ coupling is exaggerated.     

Di­methyl N,N′‐ox­alamido­diethano­ate

The title compound, dimethyl 2,2′‐(oxalyldiimino)diethanoate, C₈H₁₂N₂O₆, exhibits a network of hydrogen bonds between amide and ester groups. Molecules lie on inversion centres and show a planar conformation for both the oxal­amide and ester groups. The glycine residues adopt a conformation close to the polyglycine II structure.