全文获取类型
收费全文 | 2668篇 |
免费 | 61篇 |
国内免费 | 16篇 |
专业分类
化学 | 1815篇 |
晶体学 | 30篇 |
力学 | 43篇 |
数学 | 427篇 |
物理学 | 296篇 |
无线电 | 134篇 |
出版年
2022年 | 15篇 |
2021年 | 23篇 |
2020年 | 43篇 |
2019年 | 44篇 |
2018年 | 32篇 |
2017年 | 18篇 |
2016年 | 56篇 |
2015年 | 60篇 |
2014年 | 65篇 |
2013年 | 93篇 |
2012年 | 144篇 |
2011年 | 156篇 |
2010年 | 83篇 |
2009年 | 76篇 |
2008年 | 148篇 |
2007年 | 126篇 |
2006年 | 133篇 |
2005年 | 132篇 |
2004年 | 119篇 |
2003年 | 125篇 |
2002年 | 92篇 |
2001年 | 39篇 |
2000年 | 44篇 |
1999年 | 24篇 |
1998年 | 25篇 |
1997年 | 36篇 |
1996年 | 36篇 |
1995年 | 39篇 |
1994年 | 28篇 |
1993年 | 28篇 |
1992年 | 26篇 |
1991年 | 22篇 |
1990年 | 20篇 |
1989年 | 20篇 |
1988年 | 28篇 |
1987年 | 36篇 |
1986年 | 23篇 |
1985年 | 49篇 |
1984年 | 39篇 |
1983年 | 45篇 |
1982年 | 49篇 |
1981年 | 42篇 |
1980年 | 47篇 |
1979年 | 27篇 |
1978年 | 35篇 |
1977年 | 23篇 |
1976年 | 22篇 |
1975年 | 23篇 |
1974年 | 18篇 |
1973年 | 17篇 |
排序方式: 共有2745条查询结果,搜索用时 9 毫秒
21.
The preparation of the η4-4-2,3,5,6-tetramethyl-1,4-benzoquinonecomplex [CO(C5Me5)(C10H12O2)] (I) is reported. Complex I undergoesreversible protonation to yield the 2-6-η-4-hydroxy-1-oxo-2,3,5,6-tetramethylcyclohexadienyl complex [Co(C5Me5)(C10H13O2)BF4 (II) and diprotonation to yield the η6-6-1,4-dihydroxy-2,3,5,6-tetramethylbenzene complex [Co(C5Me5)(C10H14O2)] (BF4)2 (III). Methylation of complex I with MeI/AgPF6 gives the 2---6-η-4-methoxy-1-oxo-2,3,5,6-tetramethylcyclohexadienyl complex [Co(C5Me5)(C11H15O2])PF6 (IV). In trifluoroacetic acid solution complex IV is protonated to form the η6-1-hydroxy-4-methoxy-2,3,5,6-tetramethylbenzene cation [Co(C5Me5)-(C11H16O2)]2+ 相似文献
22.
The geometry of the linear molecule HBO, has been investigated within the restricted Hartree-Fock LCAO-MO-SCF approximation. The calculated bond lengths for the near Hartree-Fock calculation were R(H-B)=2.1913 bohr, R(B-O)=2.2284 bohr. Several one electron properties have been calculated for the minimum energy configuration. 相似文献
23.
[reaction: see text] The title alkaloid was synthesized in racemic form from 3,7-diallyl-2,4,6,8-tetraoxo-3,7-diazabicyclo[3.3.1]nonane (7) by a regioselective diallylation reaction followed by double ring-closing olefin metathesis and exhaustive reduction. Tetraoxobispidine 7 was itself prepared in three simple operations from dimethyl malonate. The entire sequence to alpha-isosparteine was conducted on a multigram scale and proceeded without recourse to chromatography. 相似文献
24.
Jones NA Nepogodiev SA MacDonald CJ Hughes DL Field RA 《The Journal of organic chemistry》2005,70(21):8556-8559
Described herein is the synthesis of 3-C-carboxy-5-deoxy-L-xylose (aceric acid), a rare branched-chain sugar found in the complex pectic polysaccharide rhamnogalacturonan-II. The key synthetic step in the construction of aceric acid was the stereoselective addition of 2-trimethylsilyl thiazole to 5-deoxy-1,2-O-isopropylidene-alpha-L-erythro-pentofuran-3-ulose (2), which was prepared from L-xylose. The thiazole group was efficiently converted into the required carboxyl group via conventional transformations. Aceric acid was also synthesized by dihydroxylation of a 3-C-methylene derivative of 2 followed by oxidation of the resulting hydroxylmethyl group. The C-2 epimer of aceric acid was also synthesized using thiazole addition chemistry, starting from L-arabinose. 相似文献
25.
Piasecki E Bresson S Lott B Bougault R Colin J Crema E Galin J Gatty B Genoux-Lubain A Guerreau D Horn D Jacquet D Jahnke U Jastrzebski J Kordyasz A Le Brun C Lecolley JF Louvel M Morjean M Paulot C Pienkowski L Pouthas J Quednau B Schröder WU Schwinn E Skulski W Töke J 《Physical review letters》1991,66(10):1291-1294
26.
Unstable transition metal compounds formed from hydridosilacyclobutanes are described: 1-methyl-1-silacyclobutane reacts with nonacarbonyldiiron to give the complexes [Fe(CO)4(H){H2}] and [(H)Me}(CO)4], and with bis(triphenylphosphine)(ethylene)platinum(0) to give [Pt(H)(PPh3)2{H2}]. 相似文献
27.
D. Colin Phillips Sandra Spewock William M. Alvino 《Journal of polymer science. Part A, Polymer chemistry》1976,14(5):1137-1150
4-Aminophthalic acid has been polymerized in almost quantitative yield to low molecular weight polymer by electrochemical initiation at a platinum anode. Systems involving the acid, amine salt, and the ammonium salt have been explored; the acid and/or the amine salt of the acid provides the best means of obtaining polymer. Maximum molecular weight is attained in a very short time at low current densities (~2 hr at 50 mA). Thermogravimetric data indicate a temperature stability comparable to polyamide-imide polymers (400°C). Long reaction times and high current densities decrease molecular weight and polymer yield. 相似文献
28.
Franken A Kilner CA Thornton-Pett M Kennedy JD 《Chemical communications (Cambridge, England)》2002,(18):2048-2049
Reaction of the [1-Ph-closo-1-CB9H(4)-6,7,8,9,10-I5]- anion with 4-MeC6H4MgBr in the presence of [PdCl2(PPh3)2] gives the [Pd2I2(P(C6H(4)-4-Me)3)4]2+ salt of the [1-Ph-closo-1-CB9H(4)-10-I-6,7,8,9-(C6H(4)-4-Me)4]- anion, which exhibits an unusual neutral supramolecular assembly in the solid state, in which the dipalladium dication is encapsulated by two four-armed 'tetrapus' anionic units; the anion also has potentialities for four-fold dendrimer construction. 相似文献
29.
In this work, the up-down counter detector for the acquisition of GPS signals is modified to use dual-thresholds for each trial. Closed form expressions for the mean and variance of the dwell time and the probabilities of detection and false alarm are derived. The new detector has a significantly smaller mean dwell time than the conventional single-threshold up-down counter and a lower dwell time variance when signals are weak. Further, the new detector has a low computational load, when compared to the optimal sequential probability ratio test (SPRT), whose main disadvantage is its computational complexity. 相似文献
30.