全文获取类型
收费全文 | 2956篇 |
免费 | 43篇 |
国内免费 | 6篇 |
专业分类
化学 | 2022篇 |
晶体学 | 10篇 |
力学 | 45篇 |
综合类 | 1篇 |
数学 | 471篇 |
物理学 | 456篇 |
出版年
2016年 | 35篇 |
2015年 | 32篇 |
2014年 | 37篇 |
2013年 | 108篇 |
2012年 | 88篇 |
2011年 | 77篇 |
2010年 | 52篇 |
2009年 | 48篇 |
2008年 | 88篇 |
2007年 | 105篇 |
2006年 | 84篇 |
2005年 | 81篇 |
2004年 | 62篇 |
2003年 | 61篇 |
2002年 | 30篇 |
2001年 | 25篇 |
1999年 | 21篇 |
1997年 | 35篇 |
1996年 | 28篇 |
1995年 | 34篇 |
1994年 | 48篇 |
1993年 | 34篇 |
1992年 | 30篇 |
1991年 | 36篇 |
1990年 | 36篇 |
1989年 | 41篇 |
1988年 | 34篇 |
1987年 | 37篇 |
1986年 | 36篇 |
1985年 | 50篇 |
1984年 | 56篇 |
1983年 | 30篇 |
1982年 | 54篇 |
1981年 | 54篇 |
1980年 | 49篇 |
1979年 | 39篇 |
1978年 | 69篇 |
1977年 | 60篇 |
1976年 | 43篇 |
1975年 | 40篇 |
1974年 | 38篇 |
1973年 | 48篇 |
1972年 | 34篇 |
1971年 | 51篇 |
1970年 | 30篇 |
1969年 | 18篇 |
1966年 | 24篇 |
1932年 | 18篇 |
1927年 | 17篇 |
1923年 | 18篇 |
排序方式: 共有3005条查询结果,搜索用时 15 毫秒
1.
A cheap synthesis of the so‐called ‘decalin‐1,8‐diones’ started with the conjugate (1,4‐) addition of cyclohex‐2‐en‐1‐one derivatives to the γ‐position of the dilithium derivative (buta‐1,3‐diene‐1,1‐bis(olate)) of crotonic acid. Hydrogenation of these ‘1,4‐γ’ adducts and final cyclization afforded the enol tautomers of decalin‐1,8‐diones. Nucleophilic substitutions at these 3‐oxoenols by NH3 or primary amines created only monoamino products (namely, 3‐oxoenamines) whose reactions with OPCl3 yielded dihydro(1,3,2)oxazaphosphinin‐2‐one derivatives. The two regioisomers of a trimethyl‐3‐oxoenamine served as models for the constitutional assignments of the two rapidly interconverting (hence, individually NMR‐invisible), tautomeric trimethyl‐3‐oxoenols. Such methyl substitutions served to break the ‘pretended’ symmetry of ‘decalin‐1,8‐dione’. Hydrazine and 3‐oxoenols furnished oxygen‐free indazole derivatives whose N?H bonds exchanged with t1/2=ca. 0.00035 s at ca. ?58(9) °C. 相似文献
2.
3.
Prof. Dr. Antonín Klásek Assoc. Prof. Dr. Stanislav Kafka Dr. Ondřej Rudolf Prof. Dr. Antonín Lyčka Dr. Michal Rouchal Lukáš Bednář 《ChemistryOpen》2021,10(6):645-652
3-Chloroquinoline-2,4-diones react with cyanide ions in dimethyl formamide to give 3-cyanoquinoline-2,4-diones in small yields due to the strong hindrance of the substituent at the C-3 atom. Good yields can be achieved if the substituent at this position is the methyl group. In the methanol solution, the reaction proceeds by an addition mechanism to form 2-oxo-1a,2,3,7b-tetrahydrooxireno[2,3-c]quinoline-7b-carbonitriles, from which 4-hydroxy-3-methoxy-2-oxo-1,2,3,4-tetrahydroquinoline-4-carbonitriles are subsequently formed by opening of the epoxide ring with methanol. Some minor products of these reactions have also been isolated. The 1H, 13C and 15N NMR spectra of the prepared compounds were measured, and all resonances were assigned using appropriate two-dimensional spectra. 相似文献
4.
Dr. Michael Georg Hoesl M. Sc. Stefan Oehm Dr. Patrick Durkin Dr. Elise Darmon Dr. Lauri Peil Dr. Hans‐Rudolf Aerni Prof. Dr. Juri Rappsilber Prof. Dr. Jesse Rinehart Prof. Dr. David Leach Prof. Dr. Dieter Söll Prof. Dr. Nediljko Budisa 《Angewandte Chemie (International ed. in English)》2015,54(34):10030-10034
We have changed the amino acid set of the genetic code of Escherichia coli by evolving cultures capable of growing on the synthetic noncanonical amino acid L ‐β‐(thieno[3,2‐b]pyrrolyl)alanine ([3,2]Tpa) as a sole surrogate for the canonical amino acid L ‐tryptophan (Trp). A long‐term cultivation experiment in defined synthetic media resulted in the evolution of cells capable of surviving Trp→[3,2]Tpa substitutions in their proteomes in response to the 20 899 TGG codons of the E. coli W3110 genome. These evolved bacteria with new‐to‐nature amino acid composition showed robust growth in the complete absence of Trp. Our experimental results illustrate an approach for the evolution of synthetic cells with alternative biochemical building blocks. 相似文献
5.
6.
Reaction of 3‐Hydroxyquinoline‐2,4‐diones with Inorganic Thiocyanates in the Presence of Ammonium or Alkylammonium Ions: the Unexpected Replacement of a Hydroxy Group by an Amino Group 下载免费PDF全文
Antonín Klásek Ondřej Rudolf Michal Rouchal Antonín Lyčka 《Helvetica chimica acta》2015,98(3):318-335
3‐Hydroxyquinoline‐2,4‐diones react with KSCN in the presence of the NH$\rm{{_{4}^{+}}}$ ions to generate 2,3‐dihydro‐3‐thioxoimidazo[1,5‐c]quinazolin‐5(6H)‐ones, 2,3‐dihydro‐2‐thioxo‐1H‐imidazo[4,5‐c]quinolin‐4(5H)‐ones, and products of molecular rearrangement of the 3‐aminoquinolinedione intermediates. Starting compounds with a benzyl (Bn) group at C(3) afford 3‐aminoquinolinediones, even when only AcONH4 is used. The results of the reaction between 3‐hydroxyquinoline‐2,4‐diones and KSCN in the presence of BuNH2 show that replacing a OH group with a secondary NH2 group is also possible. 相似文献
7.
8.
Thomas D. Hettich Richard Rudolf Dr. Christoph M. Feil Nicholas Birchall Dr. Martin Nieger Prof. Dr. Dr. h. c. Dietrich Gudat 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(17):5412-5416
Unprecedented metalated phosphonite boranes were prepared from PH-substituted precursors and silyl amides. Although potassium derivatives were thermally stable and could even be isolated and structurally characterised, lithiated analogues proved to be unstable towards self-condensation under cleavage of LiOR at ambient temperature. Reaction studies revealed that the metalated phosphonite boranes exhibit ambiphilic character. Their synthetic potential as nucleophilic building blocks was demonstrated in the synthesis of the first stannylated phosphonite representing a new structural motif in phosphine chemistry. 相似文献
9.
Catalytic chain transfer polymerization of isobutylene: The role of nucleophilic impurities 下载免费PDF全文
Tota Rajasekhar Ujjal Haldar Jack Emert Philip Dimitrov Rich Severt Rudolf Faust 《Journal of polymer science. Part A, Polymer chemistry》2017,55(22):3697-3704
Fast polymerization of isobutylene (IB) initiated by tert‐butyl chloride using ethylaluminum dichloride·bis(2‐chloroethyl) ether complex (T. Rajasekhar, J. Emert, R. Faust, Polym. Chem. 2017, 8, 2852) was drastically slowed down in the presence of impurities, such as propionic acid, acetone, methanol, and acetonitrile. The effect of impurities on the polymerization rate was neutralized by using two different approaches. First, addition of a small amount of iron trichloride (FeCl3) scavenged the impurity and formed an insoluble · impurity complex in hexanes. The polymerization rate and exo‐olefin content were virtually identical to that obtained in the absence of impurities. Heterogeneous phase scavenger (FeCl3) exhibited better performance than homogenous phase scavengers. In the second approach, conducting the polymerization in wet hexanes, the fast polymerization of IB was retained in the presence of impurities with a slight decrease in exo‐olefin content. 1H NMR studies suggest that nucleophilic impurities are protonated in the presence of water, and thereby neutralized. Mechanistic studies suggest that the rate constant of activation (ka), rate constant of propagation (kp), and rate constant of β‐proton elimination (ktr) are not affected by the presence of impurities. To account for the retardation of polymerization in the presence of impurities, delay of proton transfer to monomer in the chain transfer step is proposed. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 3697–3704 相似文献
10.
Rudolf Holze 《Journal of Solid State Electrochemistry》2017,21(9):2601-2607
Terminology of electrodes and electrode materials used in supercapacitors as well as naming of electrode processes and devices prepared with these electrodes is confusing and rather unregulated. Consequently, misunderstanding in communication about research and development is somehow matched with an incomplete understanding of the reasons of the observed capacitive, pseudocapacitive, or Faradaic behavior. Observed and investigated phenomena relevant for supercapacitor electrodes are briefly reviewed and explained in terms of electrode processes and interfacial phenomena. Further research possibly useful in more fundamental understanding and rational improvement of materials is proposed. 相似文献