ACS Physical Chemistry Au最新文献

Differing in pseudoatom, three macrocycles with isosteric substitutions (geminal dimethyl, cyclopropyl, cyclobutyl) can be described as choreoisosteres. Under ambient conditions, they share a dynamic hinge-like motion that can be described as fully revolute in solution. The barriers to hinging, ΔG ^‡, are identical within experimental error: ΔG ^‡ = 14.2-15.2 kcal/mol as judged by variable-temperature ¹³C NMR spectroscopy. Consistent with conserved dynamic behavior and isosterism, other physical properties including hydrophobicity and solution/membrane diffusion constants are amenable to prediction.

Differing in pseudoatom, three macrocycles with isosteric substitutions (geminal dimethyl, cyclopropyl, cyclobutyl) can be described as choreoisosteres. Under ambient conditions, they share a dynamic hinge-like motion that can be described as fully revolute in solution. The barriers to hinging, ΔG^‡, are identical within experimental error: ΔG^‡ = 14.2–15.2 kcal/mol as judged by variable-temperature ¹³C NMR spectroscopy. Consistent with conserved dynamic behavior and isosterism, other physical properties including hydrophobicity and solution/membrane diffusion constants are amenable to prediction.

A detailed investigation of the structural changes of lithium borate (LiB) glass 25Li₂O-(75 - x)B₂O₃ was conducted in the absence and presence of lead(II) oxide or aluminum oxide (x = 10 mol %) glass modifiers. X-ray diffraction (XRD), Fourier transform infrared (FTIR), and electron paramagnetic resonance (EPR) spectroscopy were used to explore the structural properties of LiB glass by incorporating trace amounts of manganese(III) oxide (0.00-0.25 mol %) as a probe. Differential thermal analysis and XRD results for the glasses and their ceramics confirmed the integration of aluminum atoms into the glass framework by forming a lithium aluminum boron oxide Li₂(AlB₅O₁₀) crystalline phase. Lead atoms were located interstitially, which disordered the borate glass structure and produced a lithium tetraborate crystalline phase. Semiempirical modeling of the glass structures was conducted to estimate the fundamental vibrational modes of the glass materials using a parametric method 3 (PM3MM) with molecular mechanics corrections to elucidate the geometry of the borate (BO₃) groups and their possible vibrational modes. Our analysis revised the conventional representation of the tetrahedral BO₄ units, which were not observed, to "distorted-trigonal" BO₃ groups and associated with nonbridging oxygen (NBO) atoms. EPR spectroscopy established a link between the NBO in oxides and the well-defined peak at g-factor ∼4.2 in glass materials, which had been assigned to iron(III) ions according to the literature.

The kinetics of oxygen exchange dictate the rate of redox reactions, which is crucial for electrochemical-based sustainable technologies. In this study, we use isotope exchange pulse responses to elucidate the oxygen exchange mechanism for (Bi_0.8Tm_0.2)₂O_3-δ (BTM)-(La_0.8Sr_0.2)_0.99MnO_3-δ (LSM) composites. With an optimized composition and microstructure, these composites can achieve polarization resistances below 0.01 Ω·cm² at 700 °C. Analysis of the oxygen exchange rate, , by splitting it into elementary processes using the serial two-step scheme, demonstrates that both the dissociative adsorption and incorporation of oxygen are accelerated in BTM-LSM compared to the parent phases. Dissociative adsorption of molecular oxygen is rate-limiting below 900 °C in the range 0.002-0.05 atm O₂ and below 850 °C in 0.21 atm O₂. Cation interdiffusion or changes in the electronic structure at the interface between the two materials create an electrocatalytically active region spanning 1-40 nm around the BTM-LSM phase boundary. Oxygen exchange coefficients within this region were estimated to be 2-3 orders of magnitude higher compared to those of the entire composite surface. We propose two potential pathways for oxygen exchange in BTM-LSM, with calculated p _O2 dependencies for each rate-determining step (rds). The p _O2 dependency of reveals that molecular oxygen is involved in the rds.

A detailed investigation of the structural changes of lithium borate (LiB) glass 25Li₂O-(75 – x)B₂O₃ was conducted in the absence and presence of lead(II) oxide or aluminum oxide (x = 10 mol %) glass modifiers. X-ray diffraction (XRD), Fourier transform infrared (FTIR), and electron paramagnetic resonance (EPR) spectroscopy were used to explore the structural properties of LiB glass by incorporating trace amounts of manganese(III) oxide (0.00–0.25 mol %) as a probe. Differential thermal analysis and XRD results for the glasses and their ceramics confirmed the integration of aluminum atoms into the glass framework by forming a lithium aluminum boron oxide Li₂(AlB₅O₁₀) crystalline phase. Lead atoms were located interstitially, which disordered the borate glass structure and produced a lithium tetraborate crystalline phase. Semiempirical modeling of the glass structures was conducted to estimate the fundamental vibrational modes of the glass materials using a parametric method 3 (PM3MM) with molecular mechanics corrections to elucidate the geometry of the borate (BO₃) groups and their possible vibrational modes. Our analysis revised the conventional representation of the tetrahedral BO₄ units, which were not observed, to “distorted-trigonal” BO₃ groups and associated with nonbridging oxygen (NBO) atoms. EPR spectroscopy established a link between the NBO in oxides and the well-defined peak at g-factor ∼4.2 in glass materials, which had been assigned to iron(III) ions according to the literature.

The kinetics of oxygen exchange dictate the rate of redox reactions, which is crucial for electrochemical-based sustainable technologies. In this study, we use isotope exchange pulse responses to elucidate the oxygen exchange mechanism for (Bi_0.8Tm_0.2)₂O_3−δ (BTM)–(La_0.8Sr_0.2)_0.99MnO_3−δ (LSM) composites. With an optimized composition and microstructure, these composites can achieve polarization resistances below 0.01 Ω·cm² at 700 °C. Analysis of the oxygen exchange rate, , by splitting it into elementary processes using the serial two-step scheme, demonstrates that both the dissociative adsorption and incorporation of oxygen are accelerated in BTM–LSM compared to the parent phases. Dissociative adsorption of molecular oxygen is rate-limiting below 900 °C in the range 0.002–0.05 atm O₂ and below 850 °C in 0.21 atm O₂. Cation interdiffusion or changes in the electronic structure at the interface between the two materials create an electrocatalytically active region spanning 1–40 nm around the BTM–LSM phase boundary. Oxygen exchange coefficients within this region were estimated to be 2–3 orders of magnitude higher compared to those of the entire composite surface. We propose two potential pathways for oxygen exchange in BTM–LSM, with calculated p_O2 dependencies for each rate-determining step (rds). The p_O2 dependency of reveals that molecular oxygen is involved in the rds.