全文获取类型
收费全文 | 378篇 |
免费 | 20篇 |
专业分类
化学 | 246篇 |
晶体学 | 3篇 |
力学 | 9篇 |
数学 | 87篇 |
物理学 | 53篇 |
出版年
2023年 | 1篇 |
2022年 | 9篇 |
2021年 | 9篇 |
2020年 | 12篇 |
2019年 | 7篇 |
2018年 | 13篇 |
2017年 | 13篇 |
2016年 | 22篇 |
2015年 | 18篇 |
2014年 | 16篇 |
2013年 | 29篇 |
2012年 | 21篇 |
2011年 | 20篇 |
2010年 | 10篇 |
2009年 | 18篇 |
2008年 | 33篇 |
2007年 | 16篇 |
2006年 | 19篇 |
2005年 | 13篇 |
2004年 | 14篇 |
2003年 | 24篇 |
2002年 | 14篇 |
2001年 | 9篇 |
2000年 | 4篇 |
1999年 | 5篇 |
1998年 | 1篇 |
1997年 | 2篇 |
1996年 | 4篇 |
1995年 | 4篇 |
1994年 | 1篇 |
1993年 | 3篇 |
1992年 | 2篇 |
1991年 | 2篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1978年 | 1篇 |
1975年 | 3篇 |
排序方式: 共有398条查询结果,搜索用时 460 毫秒
331.
Leonid G. Voskressensky Svetlana Kovaleva Tatiana N. Borisova Alexandr B. Eresko Valery S. Tolkunov Sergey V. Tolkunov 《合成通讯》2013,43(22):3337-3343
A novel domino reaction of tetrahydrobenzofuro[3,2-c] or [2,3-c]pyridines with dimethylacetylene dicarboxylate (DMAD) or methyl propiolate (MP) resulted in the formation of spirobenzofuranpyridines in moderate yields. The spirobenzofuran derivatives reported may be considered analogs of the antifungal drug griseofulvin. 相似文献
332.
Jinhua Sun Artem Iakunkov Igor A. Baburin Boby Joseph Vincenzo Palermo Alexandr V. Talyzin 《Angewandte Chemie (International ed. in English)》2020,59(3):1087-1092
COF‐1 has a structure with rigid 2D layers composed of benzene and B3O3 rings and weak van der Waals bonding between the layers. The as‐synthesized COF‐1 structure contains pores occupied by solvent molecules. A high surface area empty‐pore structure is obtained after vacuum annealing. High‐pressure XRD and Raman experiments with mesitylene‐filled (COF‐1‐M) and empty‐pore COF‐1 demonstrate partial amorphization and collapse of the framework structure above 12–15 GPa. The ambient pressure structure of COF‐1‐M can be reversibly recovered after compression up to 10–15 GPa. Remarkable stability of highly porous COF‐1 structure at pressures at least up to 10 GPa is found even for the empty‐pore structure. The bulk modulus of the COF‐1 structure (11.2(5) GPa) and linear incompressibilities (k[100]=111(5) GPa, k[001]=15.0(5) GPa) were evaluated from the analysis of XRD data and cross‐checked against first‐principles calculations. 相似文献
333.
Daniela im
íkov Petr Tma Alexandr Jegorov Petr imek Petr Heneberg 《Journal of separation science》2020,43(5):962-969
Beauverolides (beauveriolides) are abundant, biologically active cyclodepsipeptides produced by many entomopathogenic fungi, including those that are used as biopesticides. Beauverolides act as cholesterol acyltransferase inhibitors in humans; thus, their mode of action has been the subject of pharmacological and clinical research. The cost‐effective analytical methods are needed for fast, routine laboratory analysis of beauverolides. We isolated beauverolides from the fungal strain Isaria fumosorosea PFR 97‐Apopka and opened the rings of the isolated beauverolides using a pyridine alkaline medium. We separated fractions of cyclic and linearized beauverolides by thin‐layer chromatography, and found the chloroform–acetate (9:1, v/v) and chloroform–acetonitrile–acetate (8:1:1, v/v/v) mobile phases, respectively, to be the most efficient. We examined all the fractions by liquid chromatography–mass spectrometry using ion trap and Orbitrap high resolution mass spectrometry. For rapid screening of the contents of cyclic, and, particularly, linearized beauverolides, we developed a novel analytical method that consisted of using capillary electrophoresis coupled with contactless conductivity detection. Furthermore, we improved the separation of the peptides by applying capillary micellar electrokinetic chromatography with the N‐cyclohexyl‐2‐aminoethanesulfonic acid:SDS:NaOH buffer, pH 9.8 as the background electrolyte. The described novel methods allow fast and cost‐effective separation of chemically related groups of beauverolides. 相似文献
334.
Wei W. Chen Dr. Ana B. Cuenca Dr. Alexandr Shafir 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(38):16434-16449
Hypervalent organoiodane reagents are ubiquitous in organic synthesis, both as oxidants and as electrophilic group-transfer agents. In addition to these hallmark applications, a complementary strategy is gaining momentum that exploits the ability of λ3-iodanes to undergo iodine-to-arene group transfer, for example, via iodonio-Claisen-type rearrangement processes. This Minireview discusses recent advances in the use of this method to access a variety of the C−H-functionalized iodoarenes. While Section 2 is focused on the ortho C−H propargylation, allylation, and the more unusual para C−H benzylation, Section 3 is devoted to the C-arylation of enol and phenol substrates. The accompanying discussion includes mechanistic considerations and goes into the synthetic applications of the final iodoarene cores. The Minireview concludes with further conceptual extensions of the method, including the use of non-conventional coupling partners (for example, cyanoalkylation), improved access to λ3-iodane building blocks, and the development of iterative approaches to polysubstituted iodoarenes. 相似文献
335.
Acylstannanes: Cleavable and Highly Reactive Photoinitiators for Radical Photopolymerization at Wavelengths above 500 nm with Excellent Photobleaching Behavior 下载免费PDF全文
Dipl.‐Ing. Moritz Mitterbauer Dr. Patrick Knaack Dr. Sergej Naumov Dr. Marica Markovic Prof. Dr. Alexandr Ovsianikov Prof. Dr. Norbert Moszner Prof. Dr. Robert Liska 《Angewandte Chemie (International ed. in English)》2018,57(37):12146-12150
Within this work, a novel acylstannane‐based photoinitiator (PI) is presented. Tetrakis(2,4,6‐trimethylbenzoyl)stannane ( 1 ) displays outstanding properties compared to state‐of‐the‐art acylgermane‐based initiators. Most importantly, the initiator shows absorption up to 550 nm, which allows higher penetration depths, especially in highly filled photopolymers. Besides that, 1 shows extremely high photoinitiating activity towards (meth)acrylic double bonds, as well as very fast photobleaching. Furthermore, unlike many organotin compounds, 1 shows surprisingly low cytotoxicity. 相似文献
336.
Lynay Santacruz Silvia Donnici Albert Granados Alexandr Shafir Adelina Vallribera 《Tetrahedron》2018,74(48):6890-6895
Osmium and iridium metal nanoparticles were supported on a fluorous organic-inorganic hybrid material prepared by the sol-gel process. Moreover, we also found that the thermolysis of the Ir4(CO)12 cluster in simply diphenylether also gave Ir(0) nanoparticles. All the materials were studied as catalysts in oxidation processes. Fluoro-tagged iridium nanoparticles were particularly active in aerobic oxidation processes, whereby the catalytic activity could be greatly enhanced through a simple pre-activation procedure. With this material, benzylic alcohols could be oxidized under O2 balloon in the absence of a basic additive; the oxidative stopped selectively to the corresponding benzaldehyde. Promisingly, the same reaction conditions were used in a benzylic CH oxidation of xanthene. 相似文献
337.
Alexandr?JegorovEmail author Ladislav?Cvak Jan??ejka Bohumil?Kratochvíl Petr?Sedmera Vladimír?Havlí?ek 《Journal of chemical crystallography》2005,35(8):621-627
Mollugin was isolated from the extract of Rubia tinctorum roots. Lucidin was obtained by semisynthesis from xanthopurpurin. Mollugin crystallises in space group P-1 (No. 2), a = 8.5857(7), b = 11.473(4), c = 15.024(1) Å, α = 77.64(2)∘, β = 89.36(1)∘, γ = 89.71(2)∘, and V = 1445.5(5) Å3, lucidin crystallises in space group P21/c (No.14), a = 16.800(6), b = 9.637(2), c = 7.073(7) Å, β = 98.01(5)∘, and V = 1134(1) Å3. 相似文献
338.
Lyubov K. Papernaya Alexandra A. Shatrova Ekaterina P. Levanova Alexandr I. Albanov Lyudmila V. Klyba Elena V. Rudyakova Galina G. Levkovskaya 《Heteroatom Chemistry》2013,24(6):466-475
The reaction of 3,5‐dimethyl‐4‐formylpyrazoles, bearing various substituents at N‐1 atom, with propane‐1,3‐diselenol and 2‐hydroxypropane‐1,3‐diselenol in the presence of ТМSCl proceeds without heating to chemoselectively give hitherto unknown 2‐(pyrazol‐4‐yl)‐1,3‐diselenane hydrochlorides in high yields. The latter are easily transformed to the corresponding free bases—2‐(pyrazol‐4‐yl)‐1,3‐diselenanes. The 15N chemical shifts of the pyrazole ring in 2‐(pyrazol‐4‐yl)‐1,3‐diselenanes obtained by 2D HMBC‐gp (15N‐1H) technique are indicative of the N‐2 atom protonation in hydrochlorides. 相似文献
339.
Our main result states that for each finite complex L the category TOP of topological spaces possesses a model category structure (in the sense of Quillen) whose weak equivalences are precisely
maps which induce isomorphisms of all [L]-homotopy groups. The concept of [L]-homotopy has earlier been introduced by the first author and is based on Dranishnikov’s notion of extension dimension. As
a corollary we obtain an algebraic characterization of [L]-homotopy equivalences between [L]-complexes. This result extends two classical theorems of J. H. C. Whitehead. One of them – describing homotopy equivalences
between CW-complexes as maps inducing isomorphisms of all homotopy groups – is obtained by letting L = {point}. The other – describing n-homotopy equivalences between at most (n+1)-dimensional CW-complexes as maps inducing isomorphisms of k-dimensional homotopy groups with k ⩽ n – by letting L = S
n+1
, n ⩾ 0.
The first author was partially supported by NSERC research grant.
Received December 12, 2001; in revised form September 7, 2002
Published online February 28, 2003 相似文献
340.
Alexandr Kostochka 《Random Structures and Algorithms》2004,24(1):1-10
Let m(r, k) denote the minimum number of edges in an r‐uniform hypergraph that is not k‐colorable. We give a new lower bound on m(r, k) for fixed k and large r. Namely, we prove that if k ≥ 2n, then m(r, k) ≥ ?(k)kr(r/ln r)n/(n+1). © 2003 Wiley Periodicals, Inc. Random Struct. Alg., 2004 相似文献