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排序方式: 共有1185条查询结果,搜索用时 218 毫秒
71.
Natalia A. Romanova Maria A. Fritz Dr. Kaichin Chang Dr. Nadezhda B. Tamm Dr. Alexey A. Goryunkov Prof. Lev N. Sidorov Dr. Chuanbao Chen Prof. Shangfeng Yang Prof. Erhard Kemnitz Prof. Sergey I. Troyanov 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(35):11707-11716
Trifluoromethylation of a higher fullerene mixture with CF3I was performed in ampoules at 550 °C. HPLC separation followed by crystal growth and X‐ray diffraction study resulted in the structure elucidation of nine CF3 derivatives of D2d‐C84 (isomer 23). The molecular structures of C84(23)(CF3)4, C84(23)(CF3)8, C84(23)(CF3)10, C84(23)(CF3)12, two isomers of C84(23)(CF3)14, two isomers of C84(23)(CF3)16, and C84(23)(CF3)18 were discussed in terms of their addition patterns and the relative formation energies. Extensive theoretical DFT calculations were performed to identify the most stable molecular structures. It was found that the addition of CF3 groups to the C84(23) fullerene is governed by two main rules: no additions in positions of triple hexagon junctions and predominantly para additions in C6(CF3)2 hexagons on the fullerene cage. The only exception with an isolated CF3 group in C84(23)(CF3)12 is discussed in more detail. 相似文献
72.
Dr. Christopher R. Emerson Dr. Lev N. Zakharov Prof. Dr. Paul R. Blakemore 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(48):16342-16356
Four putative functionalized α‐chloroakyllithiums RCH2CHLiCl, where R=CHCH2 ( 18 a ), CCH ( 18 b ), CH2OBn ( 18 c ), and CH[O(CH2)2O] ( 18 d ), were generated in situ by sulfoxide–lithium exchange from α‐chlorosulfoxides, and investigated for the stereospecific reagent‐controlled homologation (StReCH) of phenethyl and 2‐chloropyrid‐5‐yl ( 17 ) pinacol boronic esters. Deuterium labeling experiments revealed that α‐chloroalkyllithiums are quenched by proton transfer from their α‐chlorosulfoxide precursors and it was established that this effect compromises the yield of StReCH reactions. Use of α‐deuterated α‐chlorosulfoxides was discovered to ameliorate the problem by retarding the rate of acid‐base chemistry between the carbenoid and its precursor. Carbenoids 18 a and 18 b showed poor StReCH efficacy, particularly the propargyl group bearing carbenoid 18 b , the instability of which was attributed to a facile 1,2‐hydride shift. By contrast, 18 d , a carbenoid that benefits from a stabilizing interaction between O and Li atoms gave good StReCH yields. Boronate 17 was chain extended by carbenoids 18 a , 18 b , and 18 d in 16, 0, and 68 % yield, respectively; α‐deuterated isotopomers D ‐ 18 a and D ‐ 18 d gave yields of 33 and 79 % for the same reaction. Double StReCH of 17 was pursued to target contiguous stereodiads appropriate for the total synthesis of (?)‐epibatidine ( 15 ). One‐pot double StReCH of boronate 17 by two exposures to (S)‐D ‐ 18 a (≤66 % ee), followed by work‐up with KOOH, gave the expected stereodiad product in 16 % yield (d.r.~67:33). The comparable reaction using two exposures to (S)‐D ‐ 18 d (≤90 % ee) delivered the expected bisacetal containing stereodiad (R,R)‐DD ‐ 48 in 40 % yield (≥98 % ee, d.r.=85:15). Double StReCH of 17 using (S)‐D ‐ 18 d (≤90 % ee) followed by (R)‐D ‐ 18 d (≤90 % ee) likewise gave (R,S)‐DD ‐ 48 in 49 % yield (≥97 % ee, d.r.=79:21). (R,S)‐DD ‐ 48 was converted to a dideuterated isotopomer of a synthetic intermediate in Corey’s synthesis of 15 . 相似文献
73.
S. G. Barsov S. I. Vorob’ev V. P. Koptev S. A. Kotov S. M. Mikirtych’yants G. V. Shcherbakov L. A. Aksel’rod G. P. Gordeev V. N. Zabenkin I. M. Lazebnik 《Physics of the Solid State》2007,49(8):1492-1497
The Pd1?x Fe x )0.95Mn0.05 alloy with random competing interaction was studied by measuring the muon spin relaxation in an external transverse magnetic field and in a zero magnetic field. Using the measured temperature dependence of the dynamic relaxation rate λ and the characteristics of the distribution of local static fields, the phase states of the sample under study are refined. In particular, it is shown that the ferromagnetic and spin-glass states coexist simultaneously in the sample below 25 K. Combined studies of the sample using the μSR and neutron depolarization methods made it possible to determine the size of magnetic inhomogeneities to be 2–6 μm in the temperature range 5–40 K. 相似文献
74.
In Situ Electrodeposition of an Asymmetric Sol–Gel Membrane Based on an Octadecyltrimethoxysilane Langmuir Film
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Maria Hitrik Prof. Ovadia Lev Prof. Daniel Mandler 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(38):12104-12113
The unique properties of Langmuir film formation were utilized in assembling a thin skin of an asymmetric membrane. An octadecyltrimethoxysilane (ODTMS) Langmuir monolayer was formed at the air–water interface and served as the substrate for growing a bulky sol–gel polymer in situ. The latter was based on the electrochemical deposition of tetramethoxysilane dissolved in the water subphase by means of horizontal touch electrochemistry. The resultant asymmetric layer that consisted of a thin hydrophobic ODTMS Langmuir film connected to a bulk hydrophilic sol–gel network was studied in situ and ex situ by using various techniques, such as cyclic voltammetry, electrochemical impedance spectroscopy (EIS), scanning electron microscopy, transmission electron microscopy (TEM), and goniometry. We found that a porous hydrophilic film grew on top of a hydrophobic layer as was evident from TEM, contact angle, and EIS analyses. The film thickness and film permeability could be controlled by changing the deposition conditions such as the potential window applied and its duration. Hence, this method offers an alternative approach for assembling asymmetric films for various applications 相似文献
75.
Transalkylation of Higher Trifluoromethylated Fullerenes with C70: A Pathway to New Addition Patterns of C70(CF3)8
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Nikita M. Belov Marina G. Apenova Alexey V. Rybalchenko Eugenia V. Borkovskaya Dr. Natalia S. Lukonina Dr. Alexey A. Goryunkov Dr. Ilya N. Ioffe Prof. Dr. Sergey I. Troyanov Prof. Dr. Lev N. Sidorov 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(4):1126-1133
We report three new isomers of C70(CF3)8, structurally related to p7mp‐C70(CF3)10, that are inaccessible by direct trifluoromethylation, but can be easily identified among the products of the transalkylation of higher trifluoromethylfullerenes with C70. The reported compounds are characterized by UV/Vis, 1 D and 2 D COSY 19F NMR spectroscopy, and DFT calculations. A rather unusual addition pattern is observed in p6,i‐C70(CF3)8 in which one addend is attached remotely from the others; polarization of the adjacent unsaturated bonds by the addend makes the molecule readily oxidizable. 相似文献
76.
Atomic-level studies of protein activity represent a significant challenge as a result of the complexity of conformational changes occurring on wide-ranging timescales, often greatly exceeding that of even the longest simulations. A prime example is the elucidation of protein allosteric mechanisms, where localized perturbations transmit throughout a large macromolecule to generate a response signal. For example, the conversion of chemical to electrical signals during synaptic neurotransmission in the brain is achieved by specialized membrane proteins called pentameric ligand-gated ion channels. Here, the binding of a neurotransmitter results in a global conformational change to open an ion-conducting pore across the nerve cell membrane. X-ray crystallography has produced static structures of the open and closed states of the proton-gated GLIC pentameric ligand-gated ion channel protein, allowing for atomistic simulations that can uncover changes related to activation. We discuss a range of enhanced sampling approaches that could be used to explore activation mechanisms. In particular, we describe recent application of an atomistic string method, based on Roux's “swarms of trajectories” approach, to elucidate the sequence and interdependence of conformational changes during activation. We illustrate how this can be combined with transition analysis and Brownian dynamics to extract thermodynamic and kinetic information, leading to understanding of what controls ion channel function. © 2019 Wiley Periodicals, Inc. 相似文献
77.
Lev S. Zarkhin Leonid I. Manevitch Nikolai S. Enikolopian 《Macromolecular Symposia》1989,26(1):431-450
Elementary events in polymer fracture at the molecular level are the breakdowns of single polymeric chains. Breakdown of a stretched macromolecule is accompanied by its mechanical degradation. Comparison of experimental data on the molecular products of thermal and mechanical degradation points to significant distinctions between these two processes. Generation of highly excited (“hot”) molecular products of mechanical degradation by the growing crack cannot be explained in terms of the kinetic concept of polymer fracture. Elaborate theoretical analysis of polymeric chain breakdown and generation of “hot” molecular products in subpicosecond time range is made by numerical molecular dynamic simulation with a computer. The mechanisms of primary and secondary chain breakdowns are established. The principal initiators of breakdowns are localized nonlinear modes of the lattice soliton type. Generation of highly excited (“hot”) molecular products occurs at the initial stage of mechanical degradation (during≈1 picosecond after the first breakdown). 相似文献
78.
Physics of Particles and Nuclei Letters - Physicists usually understand that physics cannot (and should not) derive that $$c \approx 3 \times {{10}^{8}}$$ m/s and $$\hbar \approx 1.054 \times... 相似文献
79.
Two novel schemes have been developed for the synthesis of pyrrolocoumarin derivatives. A series of previously unknown 4,9-dimethylpyrano[3,2-e]indol-7(3H)-ones have been prepared.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No 10, pp. 1524–1532, October, 2004. 相似文献
80.
It is well known that the size of propositional classical proofs can be huge. Proof-theoretical studies discovered exponential
gaps between cut-free (or normal) proofs and the corresponding (non-normal) proofs with cuts (or modus ponens). The task of
automatic theorem proving is, on the other hand, usually based on the construction of cut-free or only atomic-cuts proofs,
since this procedure produces less alternative choices. There are familiar tautologies whose cut-free proofs are huge while
the non-cut-free ones are small. The aim of this paper is to discuss basic methods of weight and/or size reduction of deductions
by switching from traditional tree-structured deductions to circuit-structured deductions. A desired efficiency is achieved
by adding the standard weakening rule of inference upgraded by adding suitable (propositional) unifications modulo variable
substitutions. We show examples where such a unification provides strong (in fact, exponential) compression of cut-free deductions.
Bibliography: 10 titles.
Published in Zapiski Nauchnykh Seminarov POMI, Vol. 358, 2008, pp. 77–99. 相似文献