Johannes Eicher

On the mechanism of hydrogen activation by frustrated Lewis pairs.

We report herein a comprehensive theoretical study of the thermodynamics and kinetics of molecular hydrogen activation by frustrated Lewis pairs (FLPs). A series of intermolecularly combined boranes (Lewis acids) and phosphines (Lewis bases), with experimentally established different reactivities towards H2, have been subjected to DFT and (SCS-)MP2 calculations, and analyzed in terms of their structural properties, the energetics of association of the FLPs, and the kinetics of their interactions with H2 and hydrogenation to the ion-pair products. The analysis included the following steps: 1) assessment of the ability/inability of the Lewis species to preorganize into FLPs with an optimum arrangement of the acid and base sites for preconditioning the reaction with H2 , 2) comprehension of the different thermodynamics of hydrogenation of the selected FLPs by comparing the Gibbs energies of the overall reactions, and 3) estimation of the mechanism of the activation of H2 by identifying the reaction steps and the associated kinetic barriers. The results of our studies correlate well with experimental findings and have clarified the reasons for the observed different reactivities of the investigated systems, ranging from reversible or nonreversible activation to no reaction with H2. The derived predictions could assist the future design of Lewis acid-base systems with desired properties and applicability as metal-free hydrogenation catalysts.

MgF2 antireflective coatings by sol–gel processing: film preparation and thermal densification

Magnesium fluoride sols for the wet chemical processing of porous MgF2 antireflective coatings were prepared by the reaction of MgCl2 with HF. The formation and crystallisation of MgF2 nanoparticles were followed by 19F NMR spectroscopy, X-ray diffraction (XRD) and dynamic light scattering (DLS) in the liquid phase. The crystallization of the resulting films was monitored by XRD experiments. At temperatures exceeding 550 °C the film material and glass substrates undergo a chemical reaction, MgO is formed and SiF4 evaporates as a volatile product. Microstructure and optical properties were characterized as a function of the annealing temperature. The mechanical stability of MgF2 films was evaluated by the Crockmeter test using both felt and steel wool. It is shown that porous MgF2 films prepared by this synthesis have a vast potential for the large-area processing of antireflective coatings.

Porous MgF2 antireflective λ/4 films prepared by sol–gel processing: comparison of synthesis approaches

Porous MgF2 is a highly promising film material for the preparation of durable antireflective coatings. Synthesis routes based on Mg(OMe)2 and Mg(OAc)2 precursors are described, and the resulting sol–gel films and their thermal consolidation are compared using thermal analysis (TGA/DTA), scanning electron microscopy, ellipsometric porosimetry and X-ray diffraction. Due to their constant viscosity, Mg(OAc)2-based coating solutions are superior from the practical point of view. The porous films are stable against degradation under the steady-state temperature humidity life test (85/85), and they can successfully be integrated into TiO2–MgF2–TiO2 interference filters.Graphical AbstractPorous MgF2 antireflective λ/4 coatings were prepared by sol–gel processing. Synthesis routes based on Mg(OMe)2 and Mg(OAc)2 precursors are established, and the properties of the resulting film materials are compared, respectively.