Atomistic simulations of graphene origami:Dynamics and kinetics

Origami offers two-dimensional(2D)materials with great potential for applications in flexible electronics,sensors,and smart devices.However,the dynamic process,which is crucial to construct origami,is too fast to be characterized by using state-of-the-art experimental techniques.Here,to understand the dynamics and kinetics at the atomic level,we explore the edge effects,structural and energy evolution during the origami process of an elliptical graphene nano-island(GNI)on a highly ordered pyrolytic graphite(HOPG)substrate by employing steered molecular dynamics simulations.The results reveal that a sharper armchair edge is much easier to be lifted up and realize origami than a blunt zigzag edge.The potential energy of the GNI increases at the lifting-up stage,reaches the maximum at the beginning of the bending stage,decreases with the formation of van der Waals overlap,and finally reaches an energy minimum at a half-folded configuration.The unfolding barriers of elliptical GNIs with different lengths of major axis show that the major axis should be larger than 242 A to achieve a stable single-folded structure at room temperature.These findings pave the way for pursuing other 2D material origami and preparing origami-based nanodevices.

Hot deformation behavior and dynamic recrystallization kinetics of AZ61 and AZ61+Sr magnesium alloys

In this study,the effect of strontium addition on hot deformation of AZ61 alloy was investigated by hot compression tests.A reference alloy(AZ61)and an Sr-containing alloy(AZ61+Sr)was cast while their average initial grain size were supposed to be about 140 and 40μm,respectively.In AZ61+Sr alloy,the Sr-containing precipitations were stable at homogenization temperature.Analysing the hot compression curves,it was revealed that dynamic recrystallization phenomenon had occurred and controlled the thermomechanical behaviour of the alloys.The derived constitutive equations showed that the hot deformation parameters(n and Q)in AZ61+Sr alloy is smaller than those of AZ61 alloy;this can be related to the small initial grain size and the lower amounts of solute aluminium atoms.The analysis of DRX kinetics along with the micrographs of the deformed microstructures showed that at the same condition the development of DRXed microstructure in AZ61+Sr alloy was faster than AZ61 alloy.The increased recrystallized microstructure was interpretated to be attributed to(1)the more grain boundaries present and(2)the existance of the Al-Mg-Sr precipitations assisted the PSN mechanism.Also,the attenuated intensity of the basal texture of AZ61+Sr was related to the DRX fraction of microstructure.

Hot-deformation kinetics analysis and extrusion parameter optimization of a dilute rare-earth free magnesium alloy

The fundamental research on thermo-mechanical conditions provides an experimental basis for high-performance Mg-Al-Ca-Mn alloys.However, there is a lack of systematical investigation for this series alloys on the hot-deformation kinetics and extrusion parameter optimization. Here, the flow behavior, constitutive model, dynamic recrystallization(DRX) kinetic model and processing map of a dilute rare-earth free Mg-1.3Al-0.4Ca-0.4Mn(AXM100, wt.%) alloy were studied under different hot-compressive conditions. In addition, the extrusion parameter optimization of this alloy was performed based on the hot-processing map. The results showed that the conventional Arrhenius-type strain-related constitutive model only worked well for the flow curves at high temperatures and low strain rates. In comparison, using the machine learning assisted model(support vector regression, SVR) could effectively improve the accuracy between the predicted and experimental values. The DRX kinetic model was established, and a typical necklace-shaped structure preferentially occurred at the original grain boundaries and the second phases. The DRX nucleation weakened the texture intensity, and the further growth caused the more scattered basal texture. The hot-processing maps at different strains were also measured and the optimal hot-processing range could be confirmed at the deformation temperatures of 600~723 K and the strain rates of 0.018~0.563 s^(-1). Based on the optimum hot-processing range, a suitable extrusion parameter was considered as 603 K and 0.1 mm/s and the as-extruded alloy in this parameter exhibited a good strength-ductility synergy(yield strength of ~ 232.1 MPa, ultimate strength of ~ 278.2 MPa and elongation-to-failure of ~ 20.1%). Finally, the strengthening-plasticizing mechanisms and the relationships between the DRXed grain size, yield strength and extrusion parameters were analyzed.

N-Heterocyclic carbene-catalyzed enantioselective(dynamic) kinetic resolutions and desymmetrizations

N-heterocyclic carbene-catalyzed enantioselective kinetic resolutions,dynamic kinetic resolutions,and desymmetrization reactions are systematically reviewed.The content is organized according to the activation modes involved in these transformations.Future advances within this highly active research field are discussed from our perspectives on the topic.

Asymmetric synthesis of syn-aryl-(2S,3R)-2-chloro-3-hydroxy esters via an engineered ketoreductase-catalyzed dynamic reductive kinetic resolution

We report here a generic,green synthesis of 17 valuable syn-aryl-(2S,3R)-2–chloro-3–hydroxy esters(syn-(2S,3R)-1)in 73%-99%isolated yields along with 6.1:1–83:1 dr and 31%~>99%ee,through dynamic reductive kinetic resolution of racemic arylα–chloroβ-keto esters(2)catalyzed by an engineered ketoreductase which was obtained via ep PCR-based directed evolution.The hectogram scale synthesis of syn-(2S,3R)-1b at a substrate concentration of 120 g/L showcased the application potential of the biocatalytic method developed presently.

Recent advance of chemoenzymatic catalysis for the synthesis of chemicals: Scope and challenge

Chemoenzymatic catalysis can give full play to the advantages of versatile reactivity of chemocatalysis and excellent chemo-,regio-,and stereoselectivities of biocatalysis.These chemoenzymatic methods can not only save resource,cost,and operating time but also reduce the number of reaction steps,and avoid separating unstable intermediates,leading to the generation of more products under greener circumstances and thereby playing an indispensable role in the fields of medicine,materials and fine chemicals.Although incompatible challenges between chemocatalyst and biocatalyst remain,strategies such as biphasic system,artificial metalloenzymes,immobilization or supramolecular host,and protein engineering have been designed to overcome these issues.In this review,chemoenzymatic catalysis according to different chemocatalysis types was classifiably described,and in particular,the classic dynamic kinetic resolutions(DKR)and cofactor regeneration were summarized.Finally,the bottlenecks and development of chemoenzymatic catalysis were summarized,and future development was prospected.

Atropoenantioselective synthesis of heterobiaryl N-oxides via dynamic kinetic resolution

A highly efficient enantioselective construction of heterobiaryl N-oxides was developed.A series of axially chiral heterobiaryl N-oxides were generated via the cascade reaction of aminobenzamides with heterobiaryl aldehydes in the presence of chiral phosphoric acids.A number of atropisomers were afforded in moderate to good yields with excellent enantioselectivities and diastereoselectivities.Preliminary results demonstrate that the heterobiaryl N-oxides can be utilized as efficient chiral ligands in asymmetric catalysis.

Carbene-catalyzed activation of cyclopropylcarbaldehydes for mannich reaction andδ-lactam formation:remote enantioselecitvity control and dynamic kinetic asymmetric transformation

An N-heterocyclic carbene(NHC)-catalyzed enantioselective Mannich reaction of the remoteγ-carbon of cyclopropylcarbaldehydes is disclosed for the first time.Diastereo-and enantiomerically enriched multicyclicδ-lactam compound is afforded as the main product from 8 possible stereo-specific isomers through dynamic kinetic asymmetric transformation(DYKAT)processes.Multiple chiral functional molecules can be afforded from the lactam products through simple protocols with retentions of the optical purities.

Organocatalytic dynamic kinetic resolution of N-arylindole lactams:atroposelective construction of axially chiral amino acids bearing a C–N chiral axis

Organocatalytic dynamic kinetic resolution of configurationally labile cyclic molecules represents one of the most efficient methods for the atroposelective construction of axially chiral molecules bearing a tetra-ortho-substituted chiral axis.Notably,this privileged strategy is limited to constructing a C–C chiral axis.Herein,organocatalytic dynamic kinetic resolution of configurationally labile N-arylindole lactams has been successfully achieved at the first time,allowing for access to a structurally diverse set of axially chiral N-arylindole amino esters with a tetra-ortho-substituted C–N chiral axis in excellent yields and atroposelectivities.In addition to the N-arylindole skeleton,N-aryl thieno[3,2-b]pyrrole,furo[3,2-b]pyrrole,and pyrrolo[2,3-b]pyridine skeletons are also compatible with this transformation.This transition-metal-free facile strategy features a broad substrate scope,mild reaction conditions,easy scale-up and excellent atom economy.Several potentially valuable molecules,such as axially chiral peptides,were efficiently generated from the resulting configurationally stable axially-chiral N-arylindole amino esters,demonstrating the power of this strategy.

Kinetic characterization of a slow chemical exchange between two sites in N,N-dimethylacetylamide by CEST NMR spectroscopy

Nuclear magnetic resonance(NMR)spectroscopy has provided many powerful tools for the study of dynamic processes.Among the reported methods,chemical exchange saturation transfer(CEST)is more suitable for systems with slow exchange rates,and there will be promising in the detection and dynamic mechanism of metastable substances.It has been widely used in magnetic resonance imaging(MRI),however whether it is applicable in the field of chemical kinetics needs more examples.Here we studied,as a proof of concept,the kinetics of the slow chemical exchange between the two N-methyl protons of N,N-dimethylacetylamide(DMA),exploiting QUantifying Exchange using Z-spectrum(QUEZS)and QUantifying Exchange using Saturation Time(QUEST)methods.It turned out that both of QUEZS and QUEST could give the corresponding exchange rates,showcasing the capability of this method to provide accurate kinetic data under a range of temperatures.Our results clearly demonstrated the reliability of CEST-based techniques as a tool for dynamic kinetics by NMR.

Recent progress in single-molecule fluorescence technology in nanocatalysis

Nanoparticles(NPs)play a vital role in the energy catalysis process,so understanding the heterogeneous catalytic properties of nanocatalysts is of great significance for rationally guiding the design of catalysts.However,the traditional method obtains the average information based on the whole and cannot study the catalytic activity of a single nanoparticle.It is critical to investigate the catalytic activity of individual nanoparticles using in situ techniques.This review summarizes some of Prof.Xu's recent accomplishments in studying the catalytic behavior of nanoparticles at the single-particle level using single-molecule fluorescence microscopy(SMFM).These achievements include revealing the effect of size,shape,and surface atoms of Pd nanoparticles on catalytic kinetics and dynamics as well as obtaining the activation energy of single Au nanoparticles for catalytic reactions by single-molecule methods.It is the first time to study the kinetics and dynamics of single-atom Pt catalysts.Furthermore,the method was extended to study the Pt deactivation process for hydrogen oxidation reactions as well as the catalytic kinetics of two-electron oxygen reduction reactions of individual Fe3O4 nanoparticles in electrocatalysis.Finally,single-molecule super-resolution techniques were used to observe the evolution of the activity of single Sb doped TiO2 nanorod domains.These studies are of guiding significance for in-depth understanding and realization of rational design of optimal catalysts.

Spin polarization strategy to deploy proton resource over atomic-level metal sites for highly selective CO_(2) electrolysis

Unlocking of the extremely inert C=O bond during electrochemical CO_(2) reduction demands subtle regulation on a key“resource”,protons,necessary for intermediate conversion but also readily trapped in water splitting,which is still challenging for developing efficient single-atom catalysts limited by their structural simplicity usually incompetent to handle this task.Incorporation of extra functional units should be viable.Herein,a proton deployment strategy is demonstrated via“atomic and nanostructured iron(A/N-Fe)pairs”,comprising atomically dispersed iron active centers spin-polarized by nanostructured iron carbide ferromagnets,to boost the critical protonation steps.The as-designed catalyst displays a broad window(300 mV)for CO selectivity>90%(98%maximum),even outperforming numerous cutting-edge M–N–C systems.The well-placed control of proton dynamics by A/N-Fe can promote*COOH/*CO formation and simultaneously suppress H2 evolution,benefiting from the magnetic-proximity-induced exchange splitting(spin polarization)that properly adjusts energy levels of the Fe sites’d-shells,and further those of the adsorbed intermediates’antibonding molecular orbitals.

Stereochemical editing:Catalytic racemization of secondary alcohols and amines

Chiral alcohols and amines are important structural units widely existing in pharmaceuticals,agrochemicals,and food additives.Dynamic kinetic resolution(DKR)is an efficient strategy to deliver optically active alcohols and amines from their racemates.For the development of DKR method,racemization catalyst plays as a crucial element with the requirement of compatibility with the kinetic resolution(KR)system.In this paper,recent advance in the catalytic racemization of secondary alcohols and amines is summarized based on different types of racemizing intermediates,which are redox racemization via ketone/imine intermediates,racemization via radical intermediates,and racemization via carbocation intermediates.Enzymatic racemization of secondary alcohols and amines is also enclosed.