Stabilizing Anionic Redox and Tuning Its Extent in Na-Rich Cathode Materials through Electronic Structure Engineering

The capacity of sodium-ion batteries (SIBs) can be enhanced by incorporating anionic redox into Na-rich cathode materials. However, excessive participation of oxygen in the redox process during the cycling often leads to several undesired issues including oxygen release. In this study, using first-principles computational methods through a systematic investigation and detailed analysis, we demonstrate an electronic structure tuning strategy through the aliovalent doping method to tune the amount of anionic redox in SIBs. Furthermore, we provide a method for achieving reversible anionic redox and emphasize that reversible anionic redox is not solely dependent on the covalent interaction between the transition metal and oxygen but is influenced by multiple factors that govern the electronic structure of the material. Using the aforementioned strategy, we identify an Al-doped Na-rich material, Na2Ru0.5Al0.5O3, which exhibits reversible cationic and anionic redox. Additionally, we rationalize the dominance of cationic redox in pristine Na2RuO3

Understanding the Gas Phase Chemistry of Alkanes with First-Principles Calculations

Alkyl radicals are key intermediates in multiple industrially important reactions, including the dehydrogenation of alkanes. Because of their diverse chemistry, alkyl radicals form various products via a number of competing reactions in the gas phase. Using Density Functional Theory (DFT) and accurate ab initio electronic structure calculations (CBS-QB3), we investigated the thermodynamics and kinetics of gas phase alkyl radical reactions. Specifically, we investigated the hydrogen abstraction, radical recombination, and alkene formation reactions of light alkyl radicals (C₁–C₈). We show that the hydrogen abstraction Gibbs energies are correlated with the relative Gibbs energies of the corresponding radicals. On the basis of the reaction energy calculations, we identified that the competition between radical recombination reactions and alkene formation reactions is governed by the stability of the alkene products, with the alkene formation being preferred when more substituted alkenes are formed. It was found that the radical recombination is preferred over alkene formation at 298 K, but at high temperatures (773 K) alkene formation becomes highly preferred. Importantly, owing to the competition of different reactions, we demonstrate a systematic methodology to select a computational method to investigate the complex chemistry of alkyl radicals. Overall, this study provides a rich database of reaction energies involving alkyl radicals and identifies their thermodynamic preference that can aid in the design of more efficient processes for the chemical conversion of alkanes

Chemical Control in the Battle against Fidelity in Promiscuous Natural Product Biosynthesis: The Case of Trichodiene Synthase

Terpene cyclases catalyze the highly stereospecific molding of polyisoprenes into terpenes, which are precursors to most known natural compounds. The isoprenoids are formed via intricate chemical cascades employing rich, yet highly erratic, carbocation chemistry. It is currently not well understood how these biocatalysts achieve chemical control. Here, we illustrate the catalytic control exerted by trichodiene synthase, and in particular, we discover two features that could be general catalytic tools adopted by other terpenoid cyclases. First, to avoid formation of byproducts, the enzyme raises the energy of bisabolyl carbocation, which is a general mechanistic branching point in many sesquiterpene cyclases, resulting in an essentially concerted cyclization cascade. Second, we identify a sulfur–carbocation dative bonding interaction that anchors the bisabolyl cation in a reactive conformation, avoiding tumbling and premature deprotonation. Specifically, Met73 acts as a chameleon, shifting from an initial sulfur−π interaction in the Michaelis complex to a sulfur–carbocation complex during catalysis

Mechanistic Studies on the Michael Addition of Amines and Hydrazines To Nitrostyrenes: Nitroalkane Elimination via a Retro-aza-Henry-Type Process

In this article we report on the mechanistic studies of the Michael addition of amines and hydrazines to nitrostyrenes. Under the present conditions, the corresponding <i>N</i>-alkyl/aryl substituted benzyl imines and <i>N</i>-methyl/phenyl substituted benzyl hydrazones were observed via a retro-aza-Henry-type process. By combining organic synthesis and characterization experiments with computational chemistry calculations, we reveal that this reaction proceeds via a protic solvent-mediated mechanism. Experiments in deuterated methanol CD₃OD reveal the synthesis and isolation of the corresponding deuterated intermediated Michael adduct, results that support the proposed slovent-mediated pathway. From the synthetic point of view, the reaction occurs under mild, noncatalytic conditions and can be used as a useful platform to yield the biologically important <i>N</i>-methyl pyrazoles in a one-pot manner, simple starting with the corresponding nitrostyrenes and the methylhydrazine

Classical and Quantum Modeling of Li and Na Diffusion in FePO₄

Lithium diffusion in olivine phosphates has been widely studied both experimentally and theoretically. However, nuclear quantum effects (NQEs) of the Li ions have not been accounted for in theoretical studies thus far. In the current work, we compared Li and Na diffusion in Li_0.25FePO₄ and Na_0.25FePO₄ by computing density functional theory based classical diffusion barriers in conjunction with NQEs for the Li and Na ions. The NQEs are computed using a novel three-dimensional wave function method based on a path integral formulation. The calculations of both the potential and free energy diffusion barriers suggest that Li diffusion is faster than Na diffusion, in agreement with recent experiments. The NQEs for lithium ions in Li_0.25FePO₄ are higher than those for sodium ions in Na_0.25FePO₄. Although the contribution of NQEs to the computed Li and Na ion diffusion rates is rather small, the quantum behavior of the Li ions is unusual. Indeed, we observe a reduction in the computed diffusion rate for Li ions due to quantization. We ascribe this effect to the ability of FePO₄ to tightly bind the Li ions in the transient tetrahedral transition state, which reduces the classical diffusion barrier but also enhances quantum confinement

Identification of Highly Promising Antioxidants/Neuroprotectants Based on Nucleoside 5′-Phosphorothioate Scaffold. Synthesis, Activity, and Mechanisms of Action

With a view to identify novel and biocompatible neuroprotectants, we designed nucleoside 5′-thiophosphate analogues, <b>6</b>–<b>11</b>. We identified 2-SMe-ADP­(α-S), <b>7A</b>, as a most promising neuroprotectant. <b>7A</b> reduced ROS production in PC12 cells under oxidizing conditions, IC₅₀ of 0.08 vs 21 μM for ADP. Furthermore, <b>7A</b> rescued primary neurons subjected to oxidation, EC₅₀ of 0.04 vs 19 μM for ADP. <b>7A</b> is a most potent P2Y₁-R agonist, EC₅₀ of 0.0026 μM. Activity of <b>7A</b> in cells involved P2Y_1/12-R as indicated by blocking P2Y₁₂-R or P2Y₁-R. Compound <b>7A</b> inhibited Fenton reaction better than EDTA, IC₅₀ of 37 vs 54 μM, due to radical scavenging, IC₅₀ of 12.5 vs 30 μM for ADP, and Fe­(II)-chelation, IC₅₀ of 80 vs >200 μM for ADP (ferrozine assay). In addition, <b>7A</b> was stable in human blood serum, <i>t</i>_1/2 of 15 vs 1.5 h for ADP, and resisted hydrolysis by NPP1/3, 2-fold vs ADP. Hence, we propose <b>7A</b> as a highly promising neuroprotectant

Improving Energy Density and Structural Stability of Manganese Oxide Cathodes for Na-Ion Batteries by Structural Lithium Substitution

We report excellent cycling performance for P2–Na_0.6Li_0.2Mn_0.8O₂, an auspicious cathode material for sodium-ion batteries. This material, which contains mainly Mn⁴⁺, exhibits a long voltage plateau on the first charge, similar to that of high-capacity lithium and manganese-rich metal oxides. Electrochemical measurements, X-ray diffraction, and elemental analysis of the cycled electrodes suggest an activation process that includes the extraction of lithium from the material. The “activated” material delivers a stable, high specific capacity up to ∼190 mAh/g after 100 cycles in the voltage window between 4.6–2.0 V versus Na/Na⁺. DFT calculations locate the energy states of oxygen atoms near the Fermi level, suggesting the possible contribution of oxide ions to the redox process. The addition of Li to the lattice improves structural stability compared to many previously reported sodiated transition-metal oxide electrode materials, by inhibiting the detrimental structural transformation ubiquitously observed with sodium manganese oxides during cycling. This research demonstrates the prospect of intercalation materials for Na-ion battery technology that are active based on both cationic and anionic redox moieties