Species likely to be present in methylaluminoxane (MAO) are studied via dispersion-corrected DFT, which we show is able to accurately predict thermochemical parameters for the dimerization of trimethylaluminum (TMA). Both cage-like, (AlOMe) n,c , and TMA-bound nanotubes, (AlOMe) n,t ·(AlMe 3 ) m , are found to be important components of MAO. The most stable structures have aluminum/oxygen atoms in environments whose average hybridization approaches sp 3 /sp 2 . The (AlOMe) n,t ·(AlMe 3 ) m isomers with the lowest free energies possess Al−μ-Me−Al bonds. At 298 K a novel T d -(AlOMe) 16,c oligomer is one of the most stable structures among the six stoichiometries with the lowest free energies: (AlOMe) 20,c ·(AlMe 3 ) 2 , T d -(AlOMe) 16,c , (AlOMe) 18,c , (AlOMe) 20,c ·(AlMe 3 ), (AlOMe) 10,t ·(AlMe 3 ) 4 , and (AlOMe) 20,c . As the temperature rises, the abundance of (AlOMe) n,t ·(AlMe 3 ) m decreases, and that of (AlOMe) n,c increases. Because the former are expected to be precursors for the active species in polymerization, this may in part be the reason why the cocatalytic activity of MAO decreases at higher temperatures.

Understanding the degradation mechanisms of zeolitic imidazolate frameworks (ZIFs) is crucial to improve their chemical stability and realize their potential industrial applications. Here, we conduct a comprehensive study utilizing dispersion-corrected density functional theory calculations to investigate the chemical stability of bulk ZIFs and their external surfaces under conditions of acid-gas exposure. We examine the influence of steric factors such as topology and ligand functionalization on the relative chemical stability of prototypical ZIFs (ZIF-2 and ZIF-8), including their hypothetical polymorphs. We find that defect formation is more thermodynamically and kinetically favorable at ZIF external surfaces versus the bulk, and that both topology and ligand functionalization impact defect formation. In addition, we provide a detailed mechanism for the reaction of ZIFs with sulfurous and sulfuric acids, of which the latter serves as a catalyst in potential degradation reactions of ZIFs. We also provide information about the adsorption strength of a range of acid gases to defective ZIF structures, which can inform potential strategies to regenerate ZIFs and/or achieve defect engineering in these materials.

Chiral saturated oxygen heterocycles are important components of bioactive compounds. Cyclization of alcohols onto pendant alkenes is a direct route to their synthesis, but few catalytic enantioselective methods enabling cyclization onto unactivated alkenes exist. Herein is reported a highly efficient copper-catalyzed cyclization of γ-unsaturated pentenols that terminates in C-C bond formation, a net alkene carboetherification. Both intra- and intermolecular C-C bond formations are demonstrated, yielding functionalized chiral tetrahydrofurans as well as fused-ring and bridged-ring oxabicyclic products. Transition state calculations support a cis-oxycupration stereochemistry-determining step.

Isolated defects induced by water and acid gases have been extensively characterized in ZIF-8, a prototypical metal− organic framework material, but there is little understanding regarding how these single bond-breaking events lead to the structural amorphization observed after prolonged experimental exposure in acidic environments. We use density functional theory calculations to provide the first analysis of defect propagation in a zeolitic imidazolate framework (ZIF) material. Given a single bondbreaking event (the first step in the formation of any defect state), we exhaustively explore the energetics of subsequent defect states and find strong preference for additional bond-breaking located adjacent to the previous defect in both two-defect and three-defect systems. This series of favorable reaction energies is more exothermic when we replace water with sulfuric acid as a protonating agent, in agreement with experimental observations that ZIF-8 degradation is accelerated in humid acid gas environments. To give initial insights into experimental signatures of defect propagation, we compare the simulated powder pattern in structures at varying levels of defect concentrations.

We present a study of the effects of oxygen vacancies (Ovac) on the oxygen evolution reaction (OER) on the TiO2-terminated (001) surface of cubic BaTiO3 (cBTO-TiO2) using spin-polarized DFT+U calculations and the standard (cation-based) four proton-coupled-electron-transfer methodology. We find that the excess electrons associated with Ovac’s are involved in charge transfer (CT) to the intermediate adsorbate species HO*, O*, and HOO* and/or new surface oxygen hole states that we identified. The CT is responsible for an increase in these species’ binding energies to the oxygen-deficient surface (cBTO-TiO2–x ) to an extent consistent with their electronegativity. The much stronger stabilization of HO* and O* compared to HOO* results in an increased overpotential ηOER on the reduced oxide. This result is at odds with experiment that shows a significantly increased efficiency for oxygen-deficient BTO, suggesting that a different mechanism and/or surface must be involved under the experimental conditions. We also identify heretofore unreported HO* and O* intermediate adsorbate structures whereby these species oxidize the surface and a surface oxygen hole is formed adjacent to the adsorption site. We assign the facile surface oxidation to the 2-fold coordination of the surface oxygen atoms in Ti–O–Ti surface moieties and a resulting low work function.