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181.
Jian‐Hua Zhao Hsuan‐Liang Liu Hsin‐Yi Lin Chih‐Hung Huang Hsu‐Wei Fang Shau‐Wei Tsai 《中国化学会会志》2007,54(4):835-842
In this study, we present the results from the semiempirical molecular orbital calculations for the acylation step in the lipase‐catalyzed ester hydrolysis. The results reveal that the lowest energy path for the formation of the tetrahedral intermediate is for the serine residue of the catalytic triad to attack the substrate, followed by coupling heavy atom movement and proton transfer. The calculations of four active site models show that the cooperation of the aspartate group and the oxyanion hole is capable of lowering the activation energy by about 16 kcalmol?1. Our results further suggest that the lipase‐catalyzed ester hydrolysis adopts the single proton transfer mechanism. 相似文献
182.
The graft copolymers of chitosan with polycaprolactone (PCL) were prepared through a protection-graft-deprotection route using phthaloylchitosan as intermediate. PCL macromonomers terminated with isocyanate groups reacted with hydroxyl groups of phthaloyl-protected chitosan regioselectively, and then phthaloyl groups were deprotected to give the free amino groups. The graft reaction was carried out in homogeneous system and yielded copolymers with high grafting content due to solubilization. FTIR, NMR and XRD were detected to characterize the resultant chitosan-graft-PCL copolymers. 相似文献
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185.
Synthesis of poly(thiocarbonate)s from the copolymerization of epoxides and carbon disulfide (CS2) remains a tough challenge, due to inevitable oxygen‐sulfur atom scrambling process. In this work, we utilized the oxygen‐sulfur exchange reaction (O‐S ER) to synthesize a random copolymer with monothiocarbonate and trithiocarbonate units from CS2 and phenyl glycidyl ether (PGE) via metal‐free Lewis pairs. The copolymers contained monothiocarbonate and trithiocarbonate units of which the molar fraction could be tuned by varying either the types of Lewis pairs or the reaction temperature. Keeping track on the intermediate provided an insight in the process of O‐S ER and thus gave a hint to control the product structure. Production and consumption of phenyl thioglycidyl ether was the key process to regulate the chain structure. Remarkably, the oxygen atoms of PGE could be excluded out of the chain, resulting in the nearly complete production of poly(trithiocarbonate)s. Correspondingly, the refractive index of this kind of copolymer could be regulated in a wide range of 1.73—1.79 (at 590 nm). 相似文献
186.
Tao He Ying‐Fang Zou Cai‐Yuan Pan 《Journal of polymer science. Part A, Polymer chemistry》2002,40(20):3367-3378
Comb‐shaped graft copolymers with poly(methyl methacrylate) as a handle were synthesized by the macromonomer technique in two steps. First, polytetrahydrofuran acrylate (A‐PTHF), prepared by the living cationic ring‐opening polymerization of tetrahydrofuran, underwent homopolymerization with 1‐(ethoxycarbonyl)prop‐1‐yl dithiobenzoate as an initiator under 60Co γ irradiation at room temperature; Second, the handle of the comb‐shaped copolymers was prepared by the block copolymerization of methyl methacrylate with P(A‐PTHF) as a macroinitiator under 60Co γ irradiation. The two‐step polymerizations were proved to be controlled with the following evidence: the straight line of ln[M]0/[M] versus the polymerization time, the linear increase in the number‐average molecular weight with the conversion, and the relatively narrow molecular weight distribution. The structures of the P(A‐PTHF) and final comb‐shaped copolymers were characterized by 1H NMR spectroscopy and gel permeation chromatography. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3367–3378, 2002 相似文献
187.
制备了对丙烯直接气相环氧化具有较好催化性能的Ag-MoO3催化剂, 采用原位FT-IR技术研究了丙烯、环氧丙烷及丙烯+氧气的混合气在Ag和Ag-MoO3催化剂表面上的吸附及反应行为. 研究表明, 丙烯在Ag和Ag-MoO3催化剂表面上吸附后, 均不发生化学反应, 而环氧丙烷吸附后在较高温度下则发生开环和聚合反应直至产生积炭. 与Ag催化剂相比, 在Ag-MoO3催化剂上, MoO3的加入在降低催化剂活性的同时, 在一定程度上抑制了产物环氧丙烷的开环及深度反应, 使环氧丙烷的选择性提高. 另外, 在较低的反应温度和较短的滞留时间下, 环氧丙烷发生深度反应的程度明显降低. 相似文献
188.
A structure and kinetic stability study on some complexes with the general formula MN5, where M are the alkali-metal atoms, Li, Na, K, and Rb, has been carried by using hybrid density functional methods. Complex B (C2v) with two points of attachment to the N5 ring is the most energetically favored for all metals considered here. Pyramidal structures A (C5v) are kinetically unstable and they rapidly rearrange to the most stable planar structures B. At the QCISD(T)/6-311 + G*//B3LYP/6-311 + G* + ZPE (B3LYP/6-311 + G*) level, the decomposition barrier heights of LiN5-B, NaN5–B, KN5-B, and RbN5-B are predicted to be 19.9, 22.0, 22.5, and 23.0 kcal/mol, respectively. In addition, the rate constants of the decomposition reaction MN5-B MN3 + N2 (M + Li, Na, K, and Rb) are also predicted using conventional transition state theory and canonical variational transition state theory, respectively. 相似文献
189.
高速逆流色谱与质谱联用在中药分析中的应用 总被引:10,自引:0,他引:10
高速逆流色谱 ( High- speed countercurrent chromatography,HSCCC)是新型的液 -液分配色谱技术 ,它利用多层螺旋管同步行星式离心运动 ,在短时间内实现样品在互不相溶的两相溶剂系统中的高效分配 ,从而实现样品分离[1] . HSCCC分离纯化后 ,若用 MS离线检测 ,此时须挥发原 HSCCC的溶剂体系 ,才能实现样品转移 ,还可能会带来样品污染 .HSCCC与 MS的联用克服了这些缺陷 ,具有很强的实用价值 .丹参酮 A( Tashinone A)是一种结晶性菲醌类化合物 [2 ] ,在丹参乙醚提取物中含量丰富 .本文采用正己烷 -乙醚 -乙醇 -水体系 ,将 HSCC… 相似文献
190.
The electrochemical carbon dioxide reduction reaction (CO2RR) offers a promising solution to mitigate carbon emission and at the same time generate valuable carbonaceous chemicals/fuels. Single atom catalysts (SACs) are encouraging to catalyze the electrochemical CO2RR due to the tunable electronic structure of the central metal atoms, which can regulate the adsorption energy of reactants and reaction intermediates. Moreover, SACs form a bridge between homogeneous and heterogeneous catalysts, providing an ideal platform to explore the reaction mechanism of electrochemical reactions. In this review, we first discuss the strategies for promoting the CO2RR performance, including suppression of the hydrogen evolution reaction (HER), generation of C1 products and formation of C2+ products. Then, we summarize the recent developments in regulating the structure of SACs toward the CO2RR based on the above aspects. Finally, several issues regarding the development of SACs for the CO2RR are raised and possible solutions are provided.The electrochemical carbon dioxide reduction reaction (CO2RR) offers a promising solution to mitigate carbon emission and at the same time generate valuable carbonaceous chemicals/fuels. 相似文献