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排序方式: 共有199条查询结果,搜索用时 15 毫秒
81.
J. Pfab J. Hager W. Krieger C. V. Boughton R. E. Miller H. Zacharias M. M. T. Loy P. A. Roland A. Sudbo B. E. Lehmann C. H. Chen G. S. Hurst M. G. Payne R. D. Willis S. D. Kramer E. E. Marinero C. T. Rettner R. N. Zare H. Rottke K. H. Welge C. C. Wang M. T. Myers D. Zhou J. W. Hudgens T. G. DiGiuseppe M. C. Lin E. Riedle H. J. Neusser E. W. Schlag J. Pfeifler P. G. Carrick R. F. Curl Jr. F. K. Tittel C. G. Atkins G. Hancock R. F. Menefee R. R. Hall M. J. Berry D. M. Burland 《Applied physics. B, Lasers and optics》1982,28(2-3):112-123
82.
Effects of a magnetic field on some photosensitised reactions: A possible theoretical interpretation
P.W. Atkins 《Chemical physics letters》1973,18(3):355-356
The photosensitised isomerisation and piperylenes is discusses in terms of a theory related to CIDEP. It is proposed that the magnetic field induces intersystem crossing within the exciplex by virtue of the different g-values of the doublet ionic components of the state. 相似文献
83.
84.
P.W. Atkins 《Chemical physics letters》1973,18(2):290-294
A simple extension of the model of a radical pair is described in which the relative translational motion of the radicals is described by an exponential translational correlation function for departure and re-encounter. A variety of polarisations can be accounted for without invoking re-encounters, although re-encounters can be important in particular situations. 相似文献
85.
86.
Foote KG Francis DT Atkins PR 《The Journal of the Acoustical Society of America》2007,121(3):1482-1490
The problem of calibrating parametric sonar systems at low difference frequencies used in backscattering applications is addressed. A particular parametric sonar is considered: the Simrad TOPAS PS18 Parametric Sub-bottom Profiler. This generates difference-frequency signals in the band 0.5-6 kHz. A standard target is specified according to optimization conditions based on maximizing the target strength consistent with the target strength being independent of orientation and the target being physically manageable. The second condition is expressed as the target having an immersion weight less than 200 N. The result is a 280-mm-diam sphere of aluminum. Its target strength varies from -43.4 dB at 0.5 kHz to -20.2 dB at 6 kHz. Maximum excursions in target strength over the frequency band due to uncertainty in material properties of the sphere are of order +/-0.1 dB. Maximum excursions in target strength due to variations in mass density and sound speed of the immersion medium are larger, but can be eliminated by attention to the hydrographic conditions. The results are also applicable to the standard-target calibration of conventional sonars operating at low-kilohertz frequencies. 相似文献
87.
为支持在并行设计过程中设计特征模型到加工特征模型的逐步转换,提出了局部特征识别的方法.在并行设计中,设计特征模型和加工特征模型通过面名历史图共享零件的实体模型,设计特征的变动通过局部特征识别自动地转换为相应的加工特征.局部特征识别是由基于最小条件子图特征识别方法改进来的,它以零件的局部区域为识别对象,通过搜索匹配局部区域构成的边界模式,识别出该局部区域中所包含的加工特征.局部特征识别方法的特点是只对设计中发生变动的区域进行识别. 相似文献
88.
Atkins P 《Angewandte Chemie (International ed. in English)》2011,50(37):8442-8443
89.
Ryan Atkins Jason Wilson Paul Zschack Corinna Grosse Wolfgang Neumann David C. Johnson 《ChemInform》2013,44(11):no-no
The title ferecrystals (fere: Latin, almost) with 1 ≤ n,m ≤ 6 and n and m varied independently are prepared by physical vapor deposition of the elements followed by annealing at 400 °C for 30 min. 相似文献
90.
CF2H groups are unique due to the combination of their lipophilic and hydrogen bonding properties. The strength of H-bonding is determined by the group to which it is appended. Several functional groups have been explored in this context including O, S, SO and SO2 to tune the intermolecular interaction. Difluoromethyl ketones are under-studied in this context, without a broadly accessible method for their preparation. Herein, we describe the development of an electrochemical hydrodefluorination of readily accessible trifluoromethylketones. The single-step reaction at deeply reductive potentials is uniquely amenable to challenging electron-rich substrates and reductively sensitive functionality. Key to this success is the use of non-protic conditions enabled by an ammonium salt that serves as a reductively stable, masked proton source. Analysis of their H-bonding has revealed difluoromethyl ketones to be potentially highly useful dual H-bond donor/acceptor moieties.The electrochemical hydrodefluorination of trifluoromethylketones under non-protic conditions make this single-step reaction at deeply reductive potentials uniquely amenable to challenging electron-rich substrates and reductively sensitive functionalities.The difluoromethyl group (CF2H) has attracted significant recent attention in medicinal chemistry,1,2 which complements the well-documented importance and growing use of fluorine in small molecule pharmaceuticals.3–6 The CF2H group is an H-bond donor7,8 that is also lipophilic,9,10 a unique combination that positions it as an increasingly valuable tool within drug-discovery.11 CF2H has been used as a bioisostere of OH and SH in serine and cystine moieties, respectively, as well as NH2 groups, where greater lipophilicity and rigidity provide advantages to pharmacokinetics and potency.12–14The hydrogen-bond acidity of CF2H groups is exceptionally dependent on the atom or group to which it is appended (Fig. 1A).1,2 The H-bond acidity of alkyl-CF2H groups is half that of O–CF2H and even a quarter of SO2–CF2H groups.1 This mode of control allows the H-bonding strength and, therefore its function, to be finely tuned. While much research has focused on the synthesis, behaviour and use of XCF2H groups, where X = O, S, SO, SO2, Ar, it is surprising that the corresponding carbonyl containing moiety (X = CO) has remained relatively elusive in these contexts. Not only would difluoromethyl ketones (DFMK) be expected to provide a relatively strong H-bond, but the carbonyl unit provides a complementary, yet proximal mode of intermolecular interaction (Fig. 1B). Indeed, the dual action of neighbouring H-bond donor and acceptor functionalities provides the fundamental basis for many biological systems, including in the secondary structure assembly mechanisms for proteins and DNA/RNA nucleobase pairing, as well as in enzyme/substrate complexes. Indeed, the DFMK functionality has demonstrated important utility in biological applications, including anti-malarial and -coronaviral properties.15 Finally, the carbonyl provides a useful synthetic handle for further derivatization.Open in a separate windowFig. 1H-Bonding in DFMKs and their synthesis via hydrodefluorination.While some progress has been made on the synthesis of DFMKs,16 there still remains a need for a general and more broadly accessible route to their preparation. Current strategies for DFMK preparation require multi-step processes, expensive reagents, installation of activating groups, or are inherently low yielding.15a,16–25 The hydrodefluorination of trifluoromethyl ketones (1) potentially represents the most accessible strategy, as the starting materials are most readily prepared through a high-yielding trifluoroacetylation of C–H or C–X bonds.26–29 In 2001, Prakash demonstrated the viability of this approach using 2 equivalents of magnesium metal as stoichiometric reductant to drive the defluorination, with a second hydrolysis step (HCl (3–5 M) or fluoride, overnight stirring) to reveal the product.30 The scope in this 2-step process (6 substrates) reflects the limitations of using a reductant, such as Mg, that has a fixed reduction potential, as well as incompatibilities arising from Mg/halide exchange with aryl halides. Similar limitations with the use of electron-rich substrates were revealed in related contributions from Uneyama.31In order to access more electron-rich and reductively challenging substrates, such as those containing medicinally relevant heterocycles, we postulated that electrochemical reduction could be employed (Fig. 1C). Electrosynthesis is becoming an increasingly valuable enabling technology and has seen a recent resurgence due to the precise control, unique selectivity, and the potential scalability and sustainability benefits that it offers.32–36 This strategy would avoid the undesirable use of stoichiometric metals and the ‘deep-reduction’ potentials required are readily accessed by simply selecting the applied potential. Pioneering early work from Uneyama on the cathodic formation of silylenol ether intermediate 2, suggested this approach could be viable.37,38 The fundamental challenge in designing a practical, single-step process under highly reducing potentials (<−2.0 V vs. Fc/Fc+), is to avoid the reduction of the proton source, which would otherwise compete to generate H2 gas and leave the starting material untouched. Uneyama does not demonstrate hydrodefluorination, presumably due to this problem. Additional challenges posed by ‘deep-reduction’ include a lack of tolerance for reduction-sensitive functionality (alkene, C–X bonds etc.), low mass balance due to substrate decomposition and the undesirable use of sacrificial metal anodes.39 Solving these problems should provide generally applicable, safe and scalable conditions for the hydrodefluorination of readily accessible trifluoromethyl ketones (1).Given the electron-rich nature of indoles, their ubiquity in bioactive compounds, and their ease of functionalisation, we chose indole 1a as the model substrate for optimisation. The highly reductive potentials required will render it a challenging substrate, which should lead to more general conditions suitable for other important substrate classes. Indeed, when we applied the Mg conditions of Prakash to this substrate, no silyl enol ether intermediate (2a) was observed, nor product 3a, and the starting material remained completely untouched ( Entry Conditions different from above Reductant Proton source 1a a/% (2a) 3aa/% 1 Mg 0, THF, no electricity (Prakash conditions for3) Mg0 — 100 (0) n/a 2b Undivided cell, TBAPF6 Sacrificial Mg anode — 100 (0) n/a 3b Pb:C (cath:an), 0 oC, 30 mA (Uneyama conditions for2) TBABr (4 eq.) — 33 (32) 0 4b — TBABr (2 eq.) (a) Acetic acid; (b) oxalic acid. 51; 100 0; 0 5b — TBABr (2 eq.) Dimethylurea 82 0 6b — TBABr (2 eq.) TEAPF6 (4 eq.) 49 45 7 TMSCl (0 eq.) TBABr (2 eq.) TEAPF6 (4 eq.) 83 0 8b TMSCl (6 eq.) TBABr (2 eq.) TEAPF6 (4 eq.) 49 49 9c TMSCl (3 + 3 eq.) TBABr (2 eq.) TEAPF 6 (4 eq.) 0 97 10c Entry 9, but Pt:Gr (cath:An) TBABr (2 eq.) TEAPF6 (4 eq.) 0 94 11c Entry 9, but Ni:Pt (cath:An) TBABr (2 eq.) TEAPF6 (4 eq.) 0 83 12c Entry 9, but Stainless Steel:Pt (cath:An) TBABr (2 eq.) TEAPF6 (4 eq.) 0 85 13c Entry 9, but Gr:Pt (cath:An) TBABr (2 eq.) TEAPF6 (4 eq.) 0 18