全文获取类型
收费全文 | 3506篇 |
免费 | 69篇 |
国内免费 | 16篇 |
专业分类
化学 | 2636篇 |
晶体学 | 37篇 |
力学 | 43篇 |
数学 | 176篇 |
物理学 | 699篇 |
出版年
2022年 | 18篇 |
2021年 | 20篇 |
2020年 | 38篇 |
2019年 | 54篇 |
2018年 | 43篇 |
2017年 | 20篇 |
2016年 | 60篇 |
2015年 | 52篇 |
2014年 | 74篇 |
2013年 | 162篇 |
2012年 | 169篇 |
2011年 | 195篇 |
2010年 | 115篇 |
2009年 | 108篇 |
2008年 | 187篇 |
2007年 | 184篇 |
2006年 | 174篇 |
2005年 | 183篇 |
2004年 | 152篇 |
2003年 | 155篇 |
2002年 | 145篇 |
2001年 | 87篇 |
2000年 | 92篇 |
1999年 | 57篇 |
1998年 | 39篇 |
1997年 | 38篇 |
1996年 | 49篇 |
1995年 | 31篇 |
1994年 | 35篇 |
1993年 | 44篇 |
1992年 | 40篇 |
1991年 | 46篇 |
1990年 | 48篇 |
1989年 | 33篇 |
1988年 | 43篇 |
1987年 | 45篇 |
1986年 | 38篇 |
1985年 | 68篇 |
1984年 | 50篇 |
1983年 | 27篇 |
1982年 | 40篇 |
1981年 | 40篇 |
1980年 | 24篇 |
1979年 | 37篇 |
1978年 | 35篇 |
1977年 | 42篇 |
1976年 | 31篇 |
1975年 | 26篇 |
1974年 | 19篇 |
1973年 | 23篇 |
排序方式: 共有3591条查询结果,搜索用时 46 毫秒
41.
The structural change from the porphyrin free base to monoacid and diacid by successive protonation has been studied by the IR, visible and NMR spectroscopy. The results have indicated that the cation and anion of the porphyrin diacid are strongly associated through H-bonding. The far IR spectra show especially marked differences in the free base, monoacid, and diacid due to the changes of the inner core of the porphyrin ring. 相似文献
42.
Masahiro Yoshida 《Tetrahedron letters》2004,45(29):5573-5575
The direct coupling of propargylic alcohols with arylboronic acids has been achieved using palladium catalyst. Various propargylic alcohols and arylboronic acids can be coupled to afford the corresponding allenic and propargylic arenes, which are selectively produced depending on the substituent on the propargylic alcohol, respectively. 相似文献
43.
Jun-ichi KawakamiMitsuhiro Takeba Ikuyo KamiyaNoboru Sonoda Akiya Ogawa 《Tetrahedron》2003,59(34):6559-6567
Highly regioselective thioformylation of terminal acetylenes with thiols and carbon monoxide has been developed by the use of rhodium(I) complexes as the catalyst: formyl and thio groups are introduced into the terminal and inner positions of acetylenes, respectively. The thioformylation is performed in the presence of a catalytic amount of rhodium(I) complexes, such as RhH(CO)(PPh3)3, RhCl(PPh3)3, and RhCl(CO)(PPh3)2, under the pressure of CO (3 MPa) at 120°C in CH3CN to provide β-thio-α,β-unsaturated aldehydes in good yields. This thioformylation can be applied to a variety of terminal acetylenes and aromatic thiols. A mechanistic proposal includes the formation of the rhodium sulfide complex as the key species. 相似文献
44.
Tokuyama R Takahashi Y Tomita Y Suzuki T Yoshida T Iwasaki N Kado N Okezaki E Nagata O 《Chemical & pharmaceutical bulletin》2001,49(4):347-352
A structure-activity relationship (SAR) study on 5-substituted oxazolidinones as an antibacterial agent is described. The oxazolidinones, of which 5-acetylaminomethyl moiety was converted into other functions, were prepared and evaluated for antibacterial activity. Elongation of the methylene chain (8) and conversion of the acetamido moiety into guanidino moiety (12) decreased the antibacterial activity. The replacement of carbonyl oxygen (=O) by thiocarbonyl sulfur (=S) enhanced in vitro antibacterial activity. Especially, compound 16, which had the 5-thiourea group, showed 4-8 stronger in vitro activity than linezolid. Our SAR study revealed that the antibacterial activity was greatly affected by the conversion of 5-substituent. 相似文献
45.
Jun-ichi Yamada Yasushi Nishimoto Satoru Tanaka Rie Nakanishi Kenji Hagiya Hiroyuki Anzai 《Tetrahedron letters》1995,36(52):9509-9512
The synthesis of BEDT-TTF derivatives fused with heterocycles (3–7) has been accomplished via the BF3-promoted reaction of organotin thiolates (8 and 9) with electrophiles (10 and 11). Electrical conductivities of radical cation salts derived from some of them were also investigated. 相似文献
46.
Woodfordin C, a macro-ring hydrolyzable tannin dimer with antitumor activity, and accompanying dimers from Woodfordia fruticosa flowers 总被引:1,自引:0,他引:1
T Yoshida T Chou A Nitta K Miyamoto R Koshiura T Okuda 《Chemical & pharmaceutical bulletin》1990,38(5):1211-1217
Three new dimeric hydrolyzable tannins, woodfordins A, B and C, along with seven known hydrolyzable tannins, including oenothein B, a dimer exhibiting marked host-mediated antitumor activity, were isolated from an Indonesian crude drug, Sidowayah [dried flowers of Woodfordia fruticosa (L.) Kurz (Lythraceae)]. The structures of the new tannins were elucidated based on chemical and spectral evidence. Woodfordin C, having a macro-ring structure, was also found to exhibit a significant antitumor activity. 相似文献
47.
48.
A novel biosensor for the determination of biochemical oxygen demand (BOD) was developed using potassium hexacyanoferrate(III) [HCF(III)] as a mediator. The sensor element consists of a three-electrode system, with both working and counter electrodes compactly integrated as a disposable using etching and electroplating processes. Pseudomonas fluorescens biovar V (isolated from a wastewater treatment plant) was immobilized on the surface of the working electrode using poly(vinyl alcohol)-quaternized stilbazol (PVA-SbQ) photopolymer gel. Synthetic wastewater described by the Organization for Economic Cooperation and Development (OECD) was used as a standard solution instead of glucose-glutamic acid synthetic wastewater. The conditions of amperometric measurement were optimized at +600 mV (vs. Ag/AgCl) operating potential, namely 40 mM HCF(III) in a 0.1 M phosphate buffer (pH 7.0) at 20 degrees C. The sensor response was linear from 15 up to 200 mg O l-1 BOD. The response time was 15 min at 200 mg O l-1 BOD. To demonstrate the wide metabolic range of activity of the sensor, the sensor response to 14 substances in four categories of organic compounds was investigated. Further, it was shown that the response of this BOD sensor was not influenced in samples with low concentrations of dissolved oxygen under the measuring conditions used. For real wastewaters, the BOD values were determined using the sensor and compared favorably with those determined by the conventional BOD5 method. 相似文献
49.
50.