全文获取类型
收费全文 | 33928篇 |
免费 | 228篇 |
国内免费 | 290篇 |
专业分类
化学 | 14024篇 |
晶体学 | 330篇 |
力学 | 1451篇 |
综合类 | 12篇 |
数学 | 9151篇 |
物理学 | 9478篇 |
出版年
2020年 | 80篇 |
2019年 | 77篇 |
2018年 | 1241篇 |
2017年 | 1482篇 |
2016年 | 779篇 |
2015年 | 634篇 |
2014年 | 580篇 |
2013年 | 885篇 |
2012年 | 3363篇 |
2011年 | 2556篇 |
2010年 | 1939篇 |
2009年 | 1682篇 |
2008年 | 828篇 |
2007年 | 827篇 |
2006年 | 845篇 |
2005年 | 4658篇 |
2004年 | 4135篇 |
2003年 | 2469篇 |
2002年 | 670篇 |
2001年 | 430篇 |
2000年 | 247篇 |
1999年 | 254篇 |
1998年 | 168篇 |
1997年 | 128篇 |
1996年 | 103篇 |
1995年 | 83篇 |
1994年 | 107篇 |
1993年 | 109篇 |
1992年 | 263篇 |
1991年 | 226篇 |
1990年 | 197篇 |
1989年 | 161篇 |
1988年 | 171篇 |
1987年 | 131篇 |
1986年 | 115篇 |
1985年 | 140篇 |
1984年 | 122篇 |
1983年 | 65篇 |
1982年 | 87篇 |
1981年 | 92篇 |
1980年 | 79篇 |
1979年 | 89篇 |
1978年 | 76篇 |
1977年 | 82篇 |
1976年 | 131篇 |
1975年 | 77篇 |
1974年 | 89篇 |
1973年 | 78篇 |
1972年 | 56篇 |
1966年 | 56篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
11.
Could simple intraannular-arm macrocyclic systems exist in enantiopure stable forms? The effective synthesis of two representative compounds of such a class, their resolution into enantiomers, and experiments justifying their stability toward racemization are presented. 相似文献
12.
Akira Yanagisawa Prof. Dr. Takahiro Suzuki Takanori Koide Shogo Okitsu Takayoshi Arai Prof. Dr. 《化学:亚洲杂志》2008,3(10):1793-1800
A Barbier‐type regioselective propargylation of aldehydes and ketones with (3‐bromobut‐1‐ynyl)trimethylsilane has been achieved using reactive barium as a low‐valent metal in THF. Especially in the case of ketones, the corresponding homopropargylic alcohols form almost exclusively. In the reaction of α,β‐unsaturated carbonyl compounds, only 1,2‐adducts have been observed. This method is also applicable to propargylation of imines, and the corresponding homopropargylic amines are obtained regiospecifically in good yields with diastereomeric ratios of up to 87:13. 相似文献
13.
Ultrasonic longitudinal velocity and adsorption have been measured in ceramic superconductors YBa2Cu3Oy with various porosity and also in BiSrCaCu2Oy. A velocity drop of about 400 ppm was found at Tc only in the measurements on cooling. The magnitude of the velocity drop is anomalously large compared with the value expected from the thermodynamics. A hysteresis of velocity with respect to temperature was observed in all the samples studied. It is suggested that some structural change at pore size level is responsible for this phenomenon. 相似文献
14.
Micha? Chmielewski 《Tetrahedron letters》2004,45(31):6007-6010
Comparison of the anion binding properties of a series of uncharged macrocyclic tetraamides reveal significant effects of the receptor's size on the strength of its anion complexes. This study allowed for estimation of the optimal size of a macroring for complexation of common anions. 相似文献
15.
Sándor G?r?g 《Analytical sciences》2004,20(5):767-782
The development during the last 15 years and the state-of-the-art in the analysis of bulk steroid hormone drugs and hormone-like structures and pharmaceutical formulations made thereof are summarized. Other steroids (sterols, bile acids, cardiac glycosides, vitamins D) as well as biological-clinical aspects and pharmacokinetic and metabolic studies are excluded from this review. The state-of-the-art is summarized based on comparisons of monographs in the latest editions of the European Pharmacopoeia, United States Pharmacopoeia and the Japanese Pharmacopoeia. This is followed by sections dealing with new developments in the methodology for the fields of spectroscopic and spectrophotometric, chromatographic, electrophoretic and hyphenated techniques as well electroanalytical methods. The review is terminated by two problem-oriented sections: examples on impurity and degradation profiling as well as enantiomeric analysis. 相似文献
16.
The electrochemical reduction of 1-([(4-halophenyl)imino]methyl)-2-naphthols on graphite electrodes was studied using cyclic voltammetry, chronoamperometry, constant-potential coulometry and preparative constant-potential electrolysis techniques. The data revealed that the reduction on graphite was irreversible and followed an EC mechanism. The diffusion coefficients and the number of electrons transferred were determined using the chronoamperometric Cottrell slope and the ultramicro disc Pt-electrode steady-state current. The number of electrons was also determined by bulk electrolysis. The compounds were subjected to constant-potential preparative electrolysis and the electrolysis products were purified and identified by spectroscopic methods. Based on these findings, a mechanism for the electro-reduction process is proposed. 相似文献
17.
Yoshio Suzuki Noriyuki Tanji Chikako Ikeda Aki Honda Kenji Ookubo Daniel Citterio Suzuki Koji 《Analytical sciences》2004,20(3):475-482
Novel labeling reagents, called MS probes, which possess a positively charged quaternary amine moiety and can transform a neutral analyte into a charged compound by simply mixing with the analyte and allowing the mixture to stand from several minutes to 30 min at room temperature or while heating to 50 degrees C, were designed and synthesized for the highly sensitive detection of carbonyl, alcohol, carboxylic acid and primary amine samples by electrospray ionization mass spectrometry (ESI-MS). The positively charged products can be detected with high sensitivity in an ESI-MS system, which is the most popular liquid MS instrument. All of the labeled products showed a remarkably large increase in the molecular-ion peak abundance detection sensitivity of over 500-fold at picomolar concentration levels compared to that of unlabeled analytes in an ESI-MS system. These MS probes, used together with liquid MS detection, are widely applicable as a convenient method for the highly sensitive detection of less than picomolar levels of analytes, and therefore greatly enhance the power of ESI-MS analysis. 相似文献
18.
JingXiaWANG ChengYouKAN DeShanLIU 《中国化学快报》2004,15(10):1161-1163
Two novel polymerizable dyes with anthraquinone residue have been prepared, and their structures were characterized by FTIR, ^1HNMR and EA. 相似文献
19.
20.
Paramagnetic pyrroline and 1,2,3,6-tetrahydropyridine derivatives of BODIPY and their diamagnetic analogs have been synthesized and characterized as novel redox double sensor and cation sensitive reagents. 相似文献