Preparative isolation and purification of 12 main antioxidants from the roots of Polygonum multiflorum Thunb. using high‐speed countercurrent chromatography and preparative HPLC guided by 1,1′‐diphenyl‐2‐picrylhydrazyl‐HPLC

A hyphenated strategy by off‐line coupling of 1,1′‐diphenyl‐2‐picrylhydrazyl‐high‐performance liquid chromatography, high‐speed countercurrent chromatography, and preparative high‐performance liquid chromatography was established to screen and separate antioxidants from ethyl acetate fraction of the roots of Polygonum multiflorum. Under the targeted guidance of 1,1′‐diphenyl‐2‐picrylhydrazyl‐high‐performance liquid chromatography experiment, 12 compounds were identified as potential antioxidants and readily isolated by high‐speed counter‐current chromatography and preparative high‐performance liquid chromatography. Ultraviolet spectroscopy, mass spectrometry, and ¹H NMR spectroscopy were employed to identify their structures, which were assigned as gallic acid ( 1 , 6.2 mg, 98.28%), catechin ( 2 , 8.8 mg, 90.69%), epicatechin ( 3 , 4.1 mg, 96.71%), polydatin ( 4 , 5.3 mg, 94.91%), 2,3,5,4′‐tetrahydroxy stilbene‐2‐Ο‐β‐D‐glucoside ( 5 , 20.2 mg, 95.23%), piceatannol ( 6 , 5.3 mg, 96.85%), rutin ( 7 , 5.4 mg, 97.92%), resveratrol ( 8 , 5.2 mg, 96.94%), isorhapontigenin ( 9 , 11.4 mg, 94.81%), hyperoside ( 10 , 9.7 mg, 98.52%), rhein ( 11 , 4.9 mg, 97.46%), and emodin ( 12 , 8.2 mg, 95.74%). Notably, compounds 6 and 9 were isolated from Polygonum multiflorum for the first time. In addition, antioxidant activity of compounds 1–12 were evaluated, and compounds 1–8 and 10 exhibited stronger antioxidant activity than ascorbic acid (positive control). These results indicated that the proposed method is a highly efficient strategy to screen and isolate antioxidants from complex natural products.     

Core–Shell Structured Nanocomposites for Photocatalytic Selective Organic Transformations

Core–shell structured nanocomposites, a type of talented functional materials with unique microstructure and properties, have shown great promise as photocatalysts for various applications, including photocatalytic degradation of pollutants, water splitting for hydrogen production, and selective organic transformations. The synthesis and utilization of efficient core–shell nanoarchitectured photocatalysts for selective organic synthesis are at the center of our research efforts and the focus of this minireview. Specifically, semiconductor‐based core–shell nanocomposites, including metal–semiconductor, semiconductor–semiconductor, semiconductor–shell (graphene and SiO₂) as photocatalysts/cocatalysts for selective oxidation of alcohols, reduction of nitro organics and carbon dioxide for synthesis of fine chemicals, and redox‐combined selective synthesis of pipecolinic acid are summarized. It is hoped that this minireview can make a contribution to catalyzing the development of smart core–shell nanostructures in the field of photocatalytic selective organic transformations for solar energy conversion.     

Estimates of Invariant Metrics on Pseudoconvex Domains Near Boundaries with Constant Levi Ranks

Estimates of the Bergman kernel and the Bergman and Kobayashi metrics on pseudoconvex domains near boundaries with constant Levi ranks are given. As a consequence, a characterization of Levi-flatness in terms of boundary behavior of the Bergman and Kobayashi metrics is obtained.     

磷钨酸纳米ZSM-5复合催化剂的氧化脱硫反应性

采用浸渍法制备了纳米晶ZSM-5负载的磷钨酸复合固体催化剂,纳米晶ZSM-5上单独负载磷钨酸时,Keggin结构磷钨酸转变为Dawson结构多酸化合物,结合镍盐后Keggin结构保持不变。 以有机硫化物氧化脱硫为探针反应的研究结果表明,在低温(40 ℃)反应条件下,含二苯并噻吩有机硫组分的正辛烷模拟油品的脱硫率高达94.5%。     

非对称氧杂环丁烷的选择性开环

氧杂环丁烷的开环反应主要包括亲核性开环、亲电性开环、自由基开环、酸催化和还原开环等几大类。本文主要总结了非对称氧杂环丁烷开环反应的区域选择性。非对称氧杂环丁烷开环反应的区域选择性主要受空间效应和电子效应的影响。氧杂环丁烷的开环反应以亲核性开环为主,强亲核性亲核试剂的开环,受空间位阻控制,主要进攻氧杂环丁烷位阻小的氧邻位碳原子;主要进攻2-乙烯基氧杂环丁烷乙烯基的β-位碳原子,发生S_N2'开环反应。只有在酸性条件下,亲核性相对较弱的含氧和卤素亲核试剂才受电子效应控制,主要进攻氧杂环丁烷位阻大的氧邻位碳原子。亲电性开环、自由基开环、路易斯酸催化的开环和钯催化的氢解开环都是在氧杂环丁烷位阻大的氧邻位碳原子一侧开环。希望本文的结论能够为利用氧杂环丁烷开环反应的同行提供一些有价值的信息,促进氧杂环丁烷开环反应的应用。     

中等严重程度冠状动脉病变模型的血流动力学参数分析

基于中等严重程度冠状动脉病变模型,应用流固耦合方法数值研究了中等严重程度面积狭窄率(AS=50%,65%,75%)和病变长度(LL= 0 mm,15 mm,20 mm) 对血流动力学参数的影响.研究发现:随着AS与LL的增大,病变血管分支的壁面剪应力变化愈加剧烈,狭窄段下游的壁面剪应力值逐渐降低,狭窄段下游回流区的长度呈"S"型增长,模型最大剪切速率呈抛物线型增长, 压力分布曲线显著下降.血流动力学参数结果表明, 中等严重程度面积狭窄率和病变长度均是可能引发血栓的因素,临床上应予以重视.      

碗状双亲型ZSM-5分子筛负载金纳米粒子的制备及催化性能

通过不对称修饰法制备了一种碗状双亲型的ZSM-5分子筛, 其拥有一个非极性的外表面和一个可修饰的极性内表面; 进一步利用经典的氨基接枝-原位还原过程, 选择性地在其内表面生长金纳米粒子. 通过X射线粉末衍射、 扫描电子显微镜、 透射电子显微镜、 红外光谱和X射线光电子能谱对材料进行了表征, 并考察了其催化硝基苯加氢制苯胺反应的性能. 在非常温和(室温、 水作溶剂及无需氮气保护)以及更低反应物浓度(0.03 mol/L, 低于室温下硝基苯在水中的溶解度)条件下, 该催化剂展现出与其它使用有机溶剂的催化剂相当的催化活性和选择性, 而且其催化性能明显高于其它以水为溶剂的催化剂, 这表明载体的双亲型结构是提升催化性能的主要原因. 该催化剂循环使用6次后, 其催化活性和选择性仍保持在相同等级.     

Ag/NH2-MIL-125(Ti)的构建及可见光还原水中Cr(Ⅵ)

开发高效、稳定的复合光催化材料是当前环境领域的迫切需求。 本文采用溶剂热协同紫外光还原法成功制备了复合光催化材料Ag/NH₂-MIL-125(Ti),并通过X射线衍射(XRD)、扫描电子显微镜(SEM)、能谱(EDS)、紫外-可见漫反射光谱(UV-Vis DRS)及X射线光电子能谱(XPS)等技术手段对其形貌、结构及光学性质进行分析,考察了Ag/NH₂-MIL-125(Ti)可见光(λ≥420 nm)催化还原Cr(Ⅵ)的性能,并优化了催化剂用量、Cr(Ⅵ)浓度、空穴捕捉剂种类及用量等条件。 结果表明,在最佳条件下,Ag/NH₂-MIL-125(Ti)具有良好的吸附及光催化还原Cr(Ⅵ)性能,其吸附及光催化还原率是NH₂-MIL-125(Ti)的3.11倍,Ag/NH₂-MIL-125(Ti)特殊的“芝麻饼”形貌以及Ag⁰与NH₂-MIL-125(Ti)之间形成的异质结有助于增强复合材料的光催化还原Cr(Ⅵ)的性能。 同时,通过条件实验,提出了光催化还原过程中主要的活性物种以及Cr(Ⅵ) 的还原机理。 本研究将为金属有机骨架(MOFs)复合材料在可见光催化环境修复领域的应用提供理论依据和实验参考。     

Growth process and optical investigations of Nd:NaGd (MoO4)2 crystals with varying content of Nd and Gd

In this paper, [NaGd_1‐xNdx](MoO₄)₂ crystals with x = 0.005–0.05 were grown by Czochralski method at 950 ℃ for 10 hours and subsequently characterized by absorption spectroscopy and fluorescence spectroscopy. The intensity parameters of Nd³⁺ were calculated by Judd‐Ofelt theory, and the relationship between the parameters and Nd³⁺ concentration was analyzed. The results show that with the increasing Nd³⁺ concentration, the oscillator strength, stimulated emission cross‐section, Ω_t and spontaneous transition probability decrease slightly, while the fluorescence lifetime decreases significantly. However, the fluorescence branching ratio is almost unchanged with the increasing Nd³⁺concentration. The [NaGd_1‐xNd_x](MoO₄)₂ crystal with (x = 0.01) possesses the highest quantum efficiency of 88.98%, a good fluorescence line‐width of 23 nm, a large value of σ_em·τ_f up to 0.55 × 10⁻²² cm²·s and a stimulated emission cross‐section up to 3.747 × 10⁻¹⁹ cm² for the transition from ⁴F_3/2 to⁴I_11/2 at 1064 nm. Test results indicate that [NaGd_1‐xNd_x](MoO₄)₂ crystals are an excellent gain media for the solid state laser system.     

Pyridine dicarbanion-bonded Ag13 organometallic nanoclusters: synthesis and on-surface oxidative coupling reaction

Highly reactive organometallic nanoclusters in situ generated in metal-catalyzed reactions are pivotal in the comprehension of catalytic mechanisms. Herein, we develop a two-step synthetic method to achieve three unprecedented aryl dicarbanion-bonded Ag₁₃ nanoclusters by using protective macrocyclic ligands. Firstly, various aryl dicarbanion–Ag₄ cluster intermediates are acquired via a silver-mediated annulation reaction within a macrocyclic ligand. These Ag₄ cluster precursors are released from the surrounding macrocycle by protonation, and further undergo an inter-cluster coupling to generate bipyridine products and low-valence silver atoms. The remaining resurgent diide–Ag₄ clusters assemble with low-valence silver atoms to yield a series of organometallic Ag₁₃ nanoclusters. These Ag₁₃ nanoclusters feature a unique open-shell electronic structure as well as a chiral cluster architecture due to the asymmetric arrangements of surrounding aryl dianion ligands. Furthermore, the pyridyl diide ligands on the surface of the nanocluster further experience an intra-cluster oxidative coupling to produce bipyridine coupling products and large nanoparticles. The coupling reaction-driven cluster-to-cluster transformation is comprehensively tracked by high resolution mass spectroscopy. This work is not only reminiscent of the detailed evolution of cluster species upon the occurrence of coupling reactions, but also reproduces novel inter- and intra-cluster coupling steps at different reaction stages.

Unprecedented pyridine dicarbanion-bonded Ag₁₃ nanoclusters were constructed according to a macrocycle-involved two-step synthetic protocol.