Unraveling the Water Degradation Mechanism of CH₃NH₃PbI₃

Instability of perovskite photovoltaics is still a topic that is currently under intense debate, especially the role of the water environment. Unraveling the mechanism of this instability is urgent to enable practical application of perovskite solar cells. Here, ab initio metadynamics is employed to investigate the initial phase of a dissolution process of CH3NH3PbI3 (MAPbI3) in explicit water. It is found that the initial dissolution of MAPbI3 is a complex multistep process triggered by the departure of I– ion from the CH3NH3I-terminated surface. Reconstruction of the free-energy landscape indicates a low energy barrier for water dissolution of MAPbI3. In addition, we propose a two-step thermodynamic cycle for MAPbI3 dissolution in water at a finite concentration that renders spontaneity to the dissolution process. The low energy barrier for the initial dissolution step and the spontaneous nature of MAPbI3 dissolution in water explain why water immediately destroys pristine MAPbI3. The dissolution thermodynamics of all-inorganic CsPbI3 perovskite is also analyzed for comparison. Hydration enthalpies and entropies of aqueous ions play an important role for the dissolution process. Our findings provide a comprehensive understanding to the current debate on water instability of MAPbI3

Unraveling the Water Degradation Mechanism of CH₃NH₃PbI₃

Instability of perovskite photovoltaics is still a topic that is currently under intense debate, especially the role of the water environment. Unraveling the mechanism of this instability is urgent to enable practical application of perovskite solar cells. Here, ab initio metadynamics is employed to investigate the initial phase of a dissolution process of CH3NH3PbI3 (MAPbI3) in explicit water. It is found that the initial dissolution of MAPbI3 is a complex multistep process triggered by the departure of I– ion from the CH3NH3I-terminated surface. Reconstruction of the free-energy landscape indicates a low energy barrier for water dissolution of MAPbI3. In addition, we propose a two-step thermodynamic cycle for MAPbI3 dissolution in water at a finite concentration that renders spontaneity to the dissolution process. The low energy barrier for the initial dissolution step and the spontaneous nature of MAPbI3 dissolution in water explain why water immediately destroys pristine MAPbI3. The dissolution thermodynamics of all-inorganic CsPbI3 perovskite is also analyzed for comparison. Hydration enthalpies and entropies of aqueous ions play an important role for the dissolution process. Our findings provide a comprehensive understanding to the current debate on water instability of MAPbI3

A DFT Study on Rh-Catalyzed Asymmetric Dearomatization of 2‑Naphthols Initiated with C–H Activation: A Refined Reaction Mechanism and Origins of Multiple Selectivity

Comprehensive DFT calculations have been performed to pursue deeper understandings on the mechanism of Rh-catalyzed asymmetric dearomatization of 2-naphthols initiated with C–H activation, which was developed by our group recently. A refined reaction mechanism is described here to account for the experimentally observed high enantio- and regioselectivity. Although the C–H activation was suggested to be involved in the turnover-limiting step, the enantioselectivity of the reaction was found to be determined during the migratory insertion of the alkyne. Different from the originally proposed mechanism, the final dearomatized product is afforded via a [1,3′]-reductive elimination directly from the eight-membered rhodacyclic intermediate generated from the migratory insertion step. In addition, the π–π interaction between the phenyl substituent on the alkyne and 2-naphthol might partially contribute to the high regioselectivity when unsymmetrical alkynes were employed as the substrates

Biomimetic Total Syntheses of Linderaspirone A and Bi-linderone and Revisions of Their Biosynthetic Pathways

Simple exposure to sunlight is sufficient for triggering photochemical [2 + 2] cycloaddition−Cope or radical rearrangement cascades in the naturally occurring methyl linderone, leading to efficient biomimetic total syntheses of linderaspirone A and bi-linderone, two recently discovered bioactive spirocyclopentenedione natural products

A Combined Theoretical and Experimental Investigation into the Highly Stereoselective Migration of Spiroindolenines

This paper describes a combined theoretical and experimental investigation into the acid-catalyzed migration of spiroindolenines to the corresponding fused cyclic products. It is suggested that the “three-center-two-electron”-type transition state is the crucial reason accounting for the highly stereoselective phenomenon. Further studies demonstrated that the electronic property of the migratory group as well as the ring size may have a major influence on the reaction profile of the migration process. Some predictions based on the computational results were supported by additional experiments

Catalytic C6 Functionalization of 2,3-Disubstituted Indoles by Scandium Triflate

We report herein an unprecedented direct catalytic C6 functionalization reaction of 2,3-disubstituted indoles with various N–Ts aziridines catalyzed by Sc­(OTf)₃ under mild conditions. Mechanistic studies revealed that a kinetically favored but reversible formal [3 + 2] annulation occurs initially. The direct C6 functionalization, although having a relatively higher energetic barrier, delivers the thermodynamically favorable products

Bridging the Adsorption Data and Adsorption Process by Introducing a Polynomial Structure To Accurately Describe IUPAC Isotherms, Stepwise Isotherms, and Stepwise Breakthrough Curves

Porous heterogeneous adsorbents, those composed of multiple pore structures and surface chemical adsorption sites, can result in various gas or vapor adsorption isotherms, including five types of IUPAC adsorption isotherms and stepwise adsorption isotherms that have been difficult to model using a single adsorption equilibrium model. The limitation of the above equilibrium model further restricts the calculations of complex stepwise breakthrough curves. To bridge the adsorption data and adsorption process, it is important to first develop a simple model or method to describe these isotherms of various complex adsorption systems. In this work, assuming that the effect of the diffusion rate can be neglected under the static condition and the adsorption process is discontinuous, the number of adsorption isotherm inflection points can be used to represent the changed number of adsorption interactions. With the introduction of the polynomial structure, a series of empirical or semi-empirical polynomial adsorption models were developed. The N-site polynomial Langmuir–Freundlich equation could accurately fit common type I, II, III, IV, and V adsorption isotherms and complex stepwise adsorption isotherms covering various adsorbates, such as volatile organic compounds (VOCs), toxic industrial chemicals (TICs), water vapor, and carbon dioxide, as well as different adsorbents, such as metal/covalent organic frameworks (MOFs/COFs), zeolites, and porous carbons. Similarly, the introduction of a polynomial structure, such as the N-site polynomial Yoon–Nelson equation, was also successful in the description of interesting stepwise breakthrough curves. This work provides a more accurate adsorption equilibrium model to characterize all types of isotherms. As a foundation model, it is expected to be used to simulate the gas–solid adsorption process inside the fixed and fluidized beds packed with porous adsorbents

Asymmetric Dearomatization of Naphthols via a Rh-Catalyzed C(sp²)–H Functionalization/Annulation Reaction

A Rh-catalyzed enantioselective dearomatization of 1-aryl-2-naphthols with internal alkynes via C–H functionalization reaction was achieved. In the presence of a chiral Cp/Rh catalyst and combined oxidants of Cu­(OAc)₂ and air (oxygen), various highly enantioenriched spirocyclic enones bearing an all-carbon quaternary stereogenic center could be synthesized in 33–98% yields with up to 97:3 er

Synthesis and Application of Chiral Spiro Cp Ligands in Rhodium-Catalyzed Asymmetric Oxidative Coupling of Biaryl Compounds with Alkenes

The vastly increasing application of chiral Cp ligands in asymmetric catalysis results in growing demand for novel chiral Cp ligands. Herein, we report a new class of chiral Cp ligands based on 1,1′-spirobiindane, a privileged scaffold for chiral ligands and catalysts. The corresponding Rh complexes are shown to be excellent catalysts in asymmetric oxidative coupling reactions, providing axially chiral biaryls in 19–97% yields with up to 98:2 er

Facet-Dependent Atomic Distances Shape Vanadate Adsorption Complexes on Hematite Nanocrystals

The environmental fate of vanadate (V(V)) is significantly influenced by iron oxide nanocrystals through adsorption. Nevertheless, the underlying driving force controlling V(V) adsorption on hematite (Fe2O3) facets is poorly understood. Herein, V(V) adsorption on the {001}, {110}, and {214} Fe2O3 facets was explored using batch adsorption experiments, spectroscopic studies, and density functional theory (DFT) calculations. Adsorption experiments suggested that the order of V(V) adsorption capacity followed {001} > {110} > {214}. However, the affinity of V(V) to the {001} facet was the weakest, as evidenced by its least resistance to phosphate and sulfate competition. Our extended X-ray absorption fine structure (EXAFS) study indicated the formation of the inner-sphere monodentate mononuclear (1V) complex on the {001} facet and bidentate corner-sharing (2C) complexes on the {110} and {214} facets. Density functional theory (DFT) calculations showed the 1V complex is preferable when the adjacent Fe–Fe atomic distance is significantly larger than the O–O atomic distance of V(V). Otherwise, the 2C complex is formed if the distance is comparable. This determining factor in surface complex formation can be safely extended to other oxyanions that the compatibility in the atomic distance of Fe–Fe on Fe2O3 facets and O–O in oxyanions shapes the surface complex. The molecular-level understanding of the facet-dependent adsorption mechanism provides the basis for the design and application of oxyanion adsorbents