排序方式: 共有63条查询结果,搜索用时 31 毫秒
21.
Richmond Lee Lixin Li Dr. Yuanhang Pan Prof. Kuo‐Wei Huang Prof. Choon‐Hong Tan Prof. Zhiyong Jiang 《化学:亚洲杂志》2012,7(4):771-777
Through the cleavage of the C? C bond, the first catalytic tandem conjugate addition–elimination reaction of Morita–Baylis–Hillman C adducts has been presented. Various SN2′‐like C‐, S‐, and P‐allylic compounds could be obtained with exclusive E configuration in good to excellent yields. The Michael product could also be easily prepared by tuning the β‐C‐substituent group of the α‐methylene ester under the same reaction conditions. Calculated relative energies of various transition states by DFT methods strongly support the observed chemoselectivity and diastereoselectivity. 相似文献
22.
Zhao Y Lim X Pan Y Zong L Feng W Tan CH Huang KW 《Chemical communications (Cambridge, England)》2012,48(44):5479-5481
Chiral bicyclic guanidine catalyzes the asymmetric H-D exchange reactions. Up to 30% ee was achieved. DFT calculations were employed to elucidate and explain the origin of the reaction's stereoselectivity. 相似文献
23.
Significant advances in fragment-based electronic structure methods have created a real alternative to force-field and density functional techniques in condensed-phase problems such as molecular crystals. This perspective article highlights some of the important challenges in modeling molecular crystals and discusses techniques for addressing them. First, we survey recent developments in fragment-based methods for molecular crystals. Second, we use examples from our own recent research on a fragment-based QM/MM method, the hybrid many-body interaction (HMBI) model, to analyze the physical requirements for a practical and effective molecular crystal model chemistry. We demonstrate that it is possible to predict molecular crystal lattice energies to within a couple kJ mol(-1) and lattice parameters to within a few percent in small-molecule crystals. Fragment methods provide a systematically improvable approach to making predictions in the condensed phase, which is critical to making robust predictions regarding the subtle energy differences found in molecular crystals. 相似文献
24.
Ma T Fu X Kee CW Zong L Pan Y Huang KW Tan CH 《Journal of the American Chemical Society》2011,133(9):2828-2831
A new chiral entity, pentanidium, has been shown to be an excellent chiral phase-transfer catalyst. The enantioselective Michael addition reactions of tert-butyl glycinate-benzophenone Schiff base with various α,β-unsaturated acceptors provide adducts with high enantioselectivities. A successful gram-scale experiment at a low catalyst loading of 0.05 mol % indicates the potential for practical applications of this methodology. Phosphoglycine ester analogues can also be utilized as the Michael donor, affording enantioenriched α-aminophosphonic acid derivatives and phosphonic analogues of (S)-proline. 相似文献
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Anatase TiO2 samples with different ratios of {101} to {001} facets were prepared with hydrothermal method and further treated under three specific calcination atmospheres (air, H2, N2). The characterization results indicate that {001} facets may generate more oxygen vacancies and more Ti3+ species than {101} facets. 相似文献
27.
OH自由基作为大气氧化性的重要衡量指标,其去除途径仍无法完全准确地被量化,OH自由基总反应性作为描述OH自由基总去除能力和大气氧化性的重要参数,是OH自由基收支闭合分析的关键参数.OH自由基活性高、寿命短,其反应活性的准确测量具有挑战性.本综述总结了OH自由基总反应性的实地测量方法,并对移动注射-激光诱导荧光法、闪光光解-激光诱导荧光法、化学离子质谱法和相对反应活性法四种方法的测量原理、仪器架构等进行了系统阐述,综合分析认为闪光光解-激光诱导荧光法是目前OH自由基总反应性测量技术的发展方向.同时进一步总结了OH自由基总反应性在城市、森林和郊区等大气环境下的化学行为等,并对研究难点和可能的重点研究方向进行了探讨. 相似文献
28.
Organicperoxidesaretheintermediateproductsintheoxidationofhydrocarboncompounds(HC).MethaneisthemostabundantHCintheatmosphere,anditsconcentrationisstillincreasingattherateof0.8%annually.Hence,theoxidationofCH4maybeanimportantsourceoforganicperoxides.However… 相似文献
29.
Analogy teaching of calculation of packing absorption column and plate tower of distillation was studied in this paper. Firstly, a detailed analogy of the above calculation problems was studied according to the frame of calculation type, calculation basis, quantitative calculation and qualitative analysis. Secondly, the qualitative analysis of the above calculations was used as examples to illustrate the analysis methodology. Students could master very well the sameness and difference of the calculation of absorption and distillation through analogy teaching. 相似文献
30.
Bin Qi Zhongming Chen Kesheng Shao Min Hu Yuanhang Zhang Xiaoyan Tang 《中国科学B辑(英文版)》1998,41(5):488-493
The formation of organic peroxides in the Cl-atom-initiated photooxidation of CH4 in O2-N2 mixtures at 101 325 Pa and 298 K was studied with HPLC and FT-IR methods. Four peroxidic products were detected, which were
H2O2, hydroxymethyl hydroperoxide (HOCH2OOH; HMHP), methyl peroxide (CH300H; MHP) and dimethyl peroxide (CH300CH3). A chromatogram peak at retention time of 8.08 min was assigned tentatively to peroxyformic acid (HC(O)OOH). The identification
of HMHP in the reaction system showed that one of the reaction paths for the self-reaction of CH300. led to producing Criegee intermediate CH2OO. The formation mechanism of organic peroxide in the photooxidation of CH4 is more complicated than it was assumed before. Photooxidation of CH4 is probably an important source of organic peroxides in the troposphere.
Project supported by the State Scientific and Technological Commission of China (Grant No. E96-05) 相似文献