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排序方式: 共有269条查询结果,搜索用时 15 毫秒
181.
The photochemical behavior of various substituted epoxycarbonyl compounds consisting of more than one possible photo‐labile site (i.e. δ‐hydrogen, β‐hydrogen and epoxide ring) has been investigated. These compounds on photo‐irradiation produced the β‐hydroxyenones in an eco‐friendly green approach. Mechanistically, these photo‐transformations have been envisaged to occur via an intramolecular β‐hydrogen abstraction by the carbonyl group of benzoyl moiety to generate the 1,3‐biradical followed by epoxide ring opening that isomerizes into the photoproducts. The photolysis of the probed epoxy ketones didn’t furnish any photoproduct through δ‐hydrogen abstraction, whatsoever. This exclusive preference for β‐H abstraction over δ‐H abstraction by carbonyl group has been vindicated by the MM2 energy mini‐ mized program for the investigated photochemical substrates. The structures of these photoproducts were established from the analysis of their spectral parameters (IR, 1H/13C NMR and Mass) and single crystal X‐ray crystallography data. 相似文献
182.
A dynamic method is employed to study the reaction mechanisms of CH3CH2OCF3 with the hydrogen trioxy(HOOO) radical. In our paper, the geometries and harmonic vibrational frequencies of all the stationary points and minimum energy paths(MEPs) are calculated at the MPW1K/6-31+G(d,p) level of theory, and the energetic information along MEPs is further refined by the CCSD/6-31+G(df,p) level of theory. The rate constants are evaluated with the conventional transition-state theory(TST), the canonical variational transition-state theory(CVT), the microcanonical variational transition-state theory(μVT), the CVT coupled with the small-curvature tunneling(SCT) correction(CVT/SCT), and the μVT coupled with the Eckart tunneling correction(μVT/Eckart) based on the ab initio calculations in the temperature range of 200 ~ 3000 K. The theoretical results are important in determining the atmospheric lifetime and the feasible pathways for the loss of HFEs. 相似文献
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Dr. Daniel Werner Uwe Bayer Dr. Dorothea Schädle Prof. Dr. Reiner Anwander 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(53):12194-12205
The cerium(IV) pyrazolate complexes [Ce(Me2pz)4]2 and [Ce(Me2pz)4(thf)] initiate β-hydride abstraction of the bis(dimethylsilyl)amido moiety, to afford a heteroleptic cerium(IV) species containing a dimethylpyrazolyl-substituted silylamido ligand, namely [Ce(Me2pz)3(bpsa)] (bpsa=bis((3,5-dimethylpyrazol-1-yl)dimethylsilyl)amido; Me2pz =3,5-dimethylpyrazolato), along with some cerium(III) species. Remarkably, the nucleophilic attack of the pyrazolyl at the silicon atom and concomitant Si−H-bond cleavage is restricted to the tetravalent cerium oxidation state and appears to proceed via the formation of a transient cerium(IV) hydride, which engages in immediate redox chemistry. When [Ce(Me2pz)4]2 is treated with [Li{N(SiMe3)2}], that is, in the absence of the SiH functionality, any redox chemistry did not occur. Instead, the ceric ate complex [LiCe2(Me2pz)9] and the stable mixed-ligand ceric species [Ce(Me2pz)2{N(SiMe3)2}2] were obtained. 相似文献
186.
Ilia A. Guzei Brian S. Dolinar Nozipho Khumalo James Darkwa 《Acta Crystallographica. Section C, Structural Chemistry》2013,69(8):847-850
In (η6‐p‐cymene)(difluorophosphinato‐κO){2‐[(1H‐pyrazol‐1‐yl)methyl‐κN2]pyridine‐κN}ruthenium(II) 0.85‐hexafluorophosphate 0.15‐tetrafluoroborate, [Ru(PO2F2)(C10H14)(C9H9N3)](PF6)0.85(BF4)0.15, (I), the [PO2F2]− ligand exhibits positional disorder due to one F atom and one O atom sharing the same two positions related by a mirror reflection across the O—P—F plane. The correct composition of this coordinated anion was successfully determined to be [PO2F2]− by refining the complex with various tetrahedral anions in which terminal atoms have similar atomic form factors. The noncoordinated counter‐ion is compositionally disordered between [PF6]− and [BF4]−. The difficulty in determining the correct composition of this anion illustrates the importance of a crystallographer remaining impartial and open to encountering unexpected moieties in the process of elucidating a structure. 相似文献
187.
Dr. Aäron G. Vandeputte Prof. Dr. Marie-Françoise Reyniers Prof. Dr. Guy B. Marin 《Chemphyschem》2013,14(16):3751-3771
One of the requisites for the development of detailed reaction networks is the availability of accurate kinetic data. Group additivity based models linking the Arrhenius parameters to structural characteristics of the transition state have proven to be a valuable tool to obtain those data. In this work, group additivity values are presented to allow a broad range of C H and S H hydrogen abstraction reactions by S radicals to be modeled. Rate coefficients in the temperature range from 300 to 1500 K are obtained by using the CBS-QB3 method in the high-pressure limit and are corrected for tunneling and anharmonicity of rotation about the transitional bond. A total of 149 reactions are studied. From these reactions, a total of 52 group additivity values and 35 resonance corrections are derived. The general applicability of the group additivity method is demonstrated for a test set containing 25 reactions. At 300 K, rate coefficients are on average reproduced within a factor of 2.8. The mean absolute deviations on the Arrhenius parameters are 2 kJ mol−1 for the activation energy and 0.38 for log A in which A is the pre-exponential factor. 相似文献
188.
A novel one‐component type II polymeric photoinitiator, poly(vinyl alcohol)–thioxanthone (PVA–TX), is synthesized by a simple acetalization process and characterized. PVA–TX enables photopolymerization of methyl methacrylate and acrylamide in both organic and aqueous media. Photopolymerization proceeds even in the absence of a co‐initiator since PVA–TX possesses both chromophoric and hydrogen donating sites in the structure.
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浅谈在高等数学教学中如何将基本概念形象化 总被引:2,自引:0,他引:2
高等数学中的基本概念具有高度抽象性,是教师讲授的难点,也是学生学习的重点,而基本概念的掌握是学生学习理解高等数学的关键,也是培养学生学习兴趣的前提.本总结了多种方法,将基本概念形象化变为可“听”,可“看”,可“摸”,降低了基本概念的教与学难度,提高了学生学好高等数学的兴趣。 相似文献