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排序方式: 共有78条查询结果,搜索用时 15 毫秒
1.
Effects of double substitution on the thermodynamic stabilities of nitrogen radicals 总被引:1,自引:0,他引:1
Studiesonkineticsofradicalreactions[1]andonCHbonddissociationenergies(BDE)[2]haveshownthatcarbonradicalsarestabilizedbyeitheranelectrondonatinggroup(EDG)oranelectronwithdrawinggroup(EWG),andthatifacarbonradicalissubstitutedwithbothanEDGandanEWGatthesamecen… 相似文献
2.
Chaoran Xu Jianglin Qiao Shunxi Dong Yuqiao Zhou Xiaohua Liu Xiaoming Feng 《Chemical science》2021,12(15):5458
Heterocycles have been widely used in organic synthesis, agrochemical, pharmaceutical and materials science industries. Catalytic three-component ylide formation/cycloaddition enables the assembly of complex heterocycles from simple starting materials in a highly efficient manner. However, asymmetric versions remain a yet-unsolved task. Here, we present a new bimetallic catalytic system for tackling this challenge. A combined system of Rh(ii) salt and chiral N,N′-dioxide–Sm(iii) complex was established for promoting the unprecedented tandem carbonyl ylide formation/asymmetric [4 + 3]-cycloaddition of aldehydes and α-diazoacetates with β,γ-unsaturated α-ketoesters smoothly, affording various chiral 4,5-dihydro-1,3-dioxepines in up to 97% yield, with 99% ee. The utility of the current method was demonstrated by conversion of products to optically active multi-substituted tetrahydrofuran derivatives. A possible reaction mechanism was provided to elucidate the origin of chiral induction based on experimental studies and X-ray structures of catalysts and products.Catalytic asymmetric tandem carbonyl ylide formation/[4 + 3]-cycloaddition of β,γ-unsaturated α-ketoesters, aldehydes and α-diazoacetates was achieved by using a bimetallic rhodium(ii)/chiral N,N′-dioxide–Sm(iii) complex catalyst. 相似文献
3.
Hui Mei Wenjia Han Jianglin Hu Se Xiao Yizhu Lei Rui Zhang Wanlin Mo Guangxing Li 《应用有机金属化学》2013,27(3):177-183
This work describes a highly efficient unstrained C(sp3)―N bond activation approach for synthesis of N,N‐dimethylacetamide (DMAc) via catalytic carbonylation of trimethylamine using a PdCl2/bipy (bipy = 2,2′‐bipyridine)/Me4NI catalyst system. A low Pd catalyst dosage (1.0 mol%) is sufficient for high selectivity (98.1%) and yield (90.8%), with a turnover number (TON) of 90.0 mmol of DMAc obtained per mmol of PdCl2 employed under mild reaction conditions. The influence of reaction parameters such as catalyst precursor dosage, ligand type and promoter on activity is investigated. This work also discusses in detail the halide promoter's role in the reaction, and provides a plausible mechanism based on the intermediates methyl iodide and acetyl iodide. Analyses indicate that the carbonylation of trimethylamine may proceed through an active intermediate acetyl iodide formed by carbonylation of methyl iodide generated from the decomposition of the promoter Me4NI under reaction conditions. The formation of acetyl iodide favors the cleaving efficiency of the inert unstrained C(sp3)―N bond of trimethylamine. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
4.
Jianglin Zhao Yong Yan Lizhuang Huang Run Yang 《Mathematical Methods in the Applied Sciences》2019,42(11):3831-3847
This paper deals with a diffusive toxin producing phytoplankton‐zooplankton model with maturation delay. By analyzing eigenvalues of the characteristic equation associated with delay parameter, the stability of the positive equilibrium and the existence of Hopf bifurcation are studied. Explicit results are derived for the properties of bifurcating periodic solutions by means of the normal form theory and the center manifold reduction for partial functional differential equations. Numerical simulations not only agree with the theoretical analysis but also exhibit the complex behaviors such as the period‐3, 5, 6, 7, 8, 11, and 12 solutions, cascade of period‐doubling bifurcation in period‐2, 4, quasi‐periodic solutions, and chaos. The key observation is that time delay may control harmful algae blooms (HABs). Moreover, numerical simulations show that the chaotic states induced by the period‐doubling bifurcation are purely temporal, which is stationary in space and oscillatory in time. The investigations may provide some new insights on harmful phytoplankton blooms. 相似文献
5.
Qi Cong Ding Zi Tu Jianglin Wang Yuxing Wang Yinjie 《Journal of Thermal Analysis and Calorimetry》2021,144(6):2269-2284
Journal of Thermal Analysis and Calorimetry - As high heat dissipation has increasingly become the primary factor restricting the capability of electronic elements, and the high temperature of the... 相似文献
6.
偶动画是动画领域一个重要的分支,与传统手绘动画同步出现,甚至更早。偶动画作品以其独特的拍摄方法和艺术魅力,为众多艺术家和观众所喜爱。偶动画影片设计周期长、工作量大、生产过程复杂、经济风险高,用传统手工工艺制作完成一部5 min的定格动画影片,通常需要数年。本文以丹麦知名动画导演克里多夫·皮拉丹的《柏树山上的风》及德国格里高利·祖坎导演的《龙焰火枪》为例,提出以故事情节( s t o r y l i n e ( i ) ) , ? i ∈ 1,2 , … , n ? ![]()
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与情景链(s c e n a r i o ? c h a i n ![]()
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)语法有限集的形式语言表达式分解偶动画电影的故事元素,通过叙事空间(n a r r a t i v e ? s p a c e ![]()
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)中镜头画面的组接逻辑,形成不同叙事结构,以分析定格动画电影的故事寓意(i m p l i e d ? m e a n i n g ![]()
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),从故事计算的角度将其转化为标准化设计流程,为构建偶动画电影故事建模理论以及辅助智能计算提供思路,方法提高了偶动画电影的设计与制作效率,为多学科协同创作提供了新模式,使普通偶动画爱好者也能参与创作。 相似文献
7.
Jianglin Wu Chrys Wesdemiotis 《Journal of the American Society for Mass Spectrometry》2001,12(12):106
The Li atom adducts of formaldehyde (LiOCH2) and formaldimine (LiNHCH2) are produced in the gas phase by neutralization of the corresponding cations. Subsequent reionization, ca. 0.3 μs later, shows that the nominally hypervalent complexes LiXCH2 (X=O or NH) are stable, residing in potential energy minima. In the time span between the neutralization and reionization events, the LiXCH2 molecules dissociate partly into their constituents, Li + XCH2, the fragmentation extent of LiNHCH2 being more extensive. Ab initio calculations reveal three bound states for both Li atom complexes. Two (states A and B) resemble C-centered radicals carrying an ion pair, Li+·−X---CH2·, and can be viewed as lithiated derivatives of the hydroxymethyl (HOCH2·) or aminomethyl (H2NCH2·) radical; the third state (C) represents a conventional, electrostatically bonded Li---X=CH2 complex with an essentially intact X=C double bond and the unpaired electron located at the metal atom. States A and B are bound more strongly than state C for LiOCH2; the opposite is true for LiNHCH2, where C is the most stable arrangement and B only marginally bound. The larger degree of dissociation observed for LiNHCH2 vis à vis LiOCH2 upon neutralization–reionization points out that the experiment samples a considerable amount of state B which is barely bound for LiNHCH2. 相似文献
8.
Sample preparation holds an important place in MS-based proteome analysis. For effective proteolysis and MS analysis, it is essential to eliminate the interferences while extracting the analytes of interest from complex mixtures. To address this, herein we describe a new dried polyacrylamide gel absorption method. In this method, the protein sample prepared using high concentration of SDS was directly and completely absorbed by vacuum-dried polyacrylamide gel, and then the interfering substances including SDS and some other salts were efficiently removed by in-gel washing steps while retaining the denatured proteins in the gel, thus offering a clean environment amenable to downstream buffer exchange, proteolytic digestion and digest recovery, etc. In combination with in-gel digestion and LC-MS/MS, the newly developed method was applied to the proteome analyses of membrane-enriched fraction and whole tissue homogenate. It was demonstrated that the method is suitable for the analysis of a complex biological sample and can be widely used for sample cleanup in shotgun proteome analyses. 相似文献
9.
采用慢挥发溶剂组装方法制备了一系列羧酸/吡啶氢键缔合的主链型超分子复合物,并采用FTIR,DSC以及偏光显微镜等对其相转变与热致液晶相行为进行了比较研究.研究表明,分别具有较短的6或10个亚甲基的烷烃间隔基的双苯甲酸衍生物4',4'-二羧酸-1,6-二酚氧基己烷(C6-2COOH)和4',4'-二羧酸-1,10-二酚氧基癸烷(C10-2COOH)的系列复合物具有较高的熔点和清亮点,一般都只出现结晶近晶相和多晶型转变现象.而具有柔顺性较好的四甘醇醚链间隔基的4,4'-二羧酸-α,ω-二酚氧基四甘醇醚(C8O4-2COOH)得到的系列复合物均在降温过程生成单致的流体近晶SA和/或向列N液晶相.可见,间隔基增长,相转变温度降低,最终导致真正的流体液晶相的产生.另一方面,对于从同一种二元羧酸得到的组装体系,从4,4'-联吡啶(4,4'-BPy)、4,4'-联吡啶乙烯撑(4,4'-BPyE)到对苯二酚二异烟酸酯(p-PhBPy),由于双键或酯基的引入,可变形性和极性增大,刚性依次减弱,尽管中心核部分持续长度增大,所得复合物的各向同性化温度降低,形成流体液晶相的趋势增强,液晶有序性降低,流动性增加.指出了早期文献报道的一些不一致甚至矛盾的结果.通过不同系列的对比研究,得出的一些规律性对氢键组装尤其对羧基/吡啶氢键缔合超分子体系设计与构筑具有一定指导意义. 相似文献
10.
The additive–multiplicative hazards (AMH) regression model specifies an additive and multiplicative form on the hazard function for the counting process associated with a multidimensional covariate process, which contains the Cox proportional hazards model and the additive hazards model as its special cases. In this paper, we study the AMH model with current status data, where the cumulative hazard hazard function is assumed to be nonparametric and is estimated using B-splines with monotonicity constraint on the functional, while a simultaneous sieve maximum likelihood estimation is proposed to estimate regression parameters. The proposed estimator for the parameter vector is shown to be asymptotically normal and semiparametric efficient. The B-splines estimator of the functional of the cumulative hazard function is shown to achieve the optimal nonparametric rate of convergence. A simulation study is conducted to examine the finite sample performance of the proposed estimators and algorithm, and a real data example is presented for illustration. 相似文献