全文获取类型
收费全文 | 11818篇 |
免费 | 282篇 |
国内免费 | 95篇 |
专业分类
化学 | 8226篇 |
晶体学 | 225篇 |
力学 | 253篇 |
综合类 | 1篇 |
数学 | 2024篇 |
物理学 | 1466篇 |
出版年
2022年 | 84篇 |
2021年 | 105篇 |
2020年 | 126篇 |
2019年 | 118篇 |
2018年 | 93篇 |
2017年 | 96篇 |
2016年 | 227篇 |
2015年 | 194篇 |
2014年 | 201篇 |
2013年 | 574篇 |
2012年 | 541篇 |
2011年 | 637篇 |
2010年 | 321篇 |
2009年 | 279篇 |
2008年 | 588篇 |
2007年 | 618篇 |
2006年 | 597篇 |
2005年 | 559篇 |
2004年 | 490篇 |
2003年 | 388篇 |
2002年 | 457篇 |
2001年 | 130篇 |
2000年 | 139篇 |
1999年 | 134篇 |
1998年 | 126篇 |
1997年 | 156篇 |
1996年 | 203篇 |
1995年 | 134篇 |
1994年 | 123篇 |
1993年 | 132篇 |
1992年 | 133篇 |
1991年 | 115篇 |
1990年 | 132篇 |
1989年 | 116篇 |
1988年 | 119篇 |
1987年 | 101篇 |
1986年 | 106篇 |
1985年 | 184篇 |
1984年 | 206篇 |
1983年 | 143篇 |
1982年 | 230篇 |
1981年 | 197篇 |
1980年 | 169篇 |
1979年 | 154篇 |
1978年 | 148篇 |
1977年 | 134篇 |
1976年 | 145篇 |
1975年 | 127篇 |
1974年 | 153篇 |
1973年 | 133篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
trans-PtH2[As(t-Bu)3]2 was prepared in very good yield by afacile reaction of K2PtC14 with As(t-Bu)3 in alkaline ethanol. Treatment of trans-PtH2[As(t-Bu)3]2 with CF3CO2H or HCI afforded trans-PtH(O2CCF3)[As(t-Bu)3]2 or trans-PtHCl[As(t-Bu)3]2. respectively, in almost quantitative yield. 相似文献
122.
Zhu H Chai J Chandrasekhar V Roesky HW Magull J Vidovic D Schmidt HG Noltemeyer M Power PP Merrill WA 《Journal of the American Chemical Society》2004,126(31):9472-9473
The reaction of beta-diketiminated aluminum(I) monomer LAl with a large bulky azide N3Ar' (L = HC(CMeNAr)2, Ar' = 2,6-Ar2C6H3, Ar = 2,6-iPr2C6H3) in the temperature range from -78 degrees C to room temperature affords two different isomers 2 and 3, which have been characterized by spectroscopic and X-ray structural analyses, as well as elemental analysis. The variable-temperature 1H NMR kinetic studies of this reaction indicate the existence of the monomer LAlNAr' (1) at low temperature and the thermal stability of the compounds increases in the order of 1 < 2 < 3. 相似文献
123.
A systematic optimization strategy for reverse-phase h.p.l.c. is used to produce complete separation of all components of a mixture containing two coumarins, umbelliferone and scopoletin, and the cis-trans isomers of two cinnamic acids, p-coumaric and ferulic. A series of experiments gives retention time data for each of these substances in four citric acid-buffered mobile phases. A mathematical model fitted to this data gives model parameters that provide the basis for a window diagram of relative retentions vs. mobile phase pH. The window diagram gives an estimate of the mobile phase pH required for optimum separation. 相似文献
124.
Marshall M. Siegel Rushung Tsao William Bouley Theodore Kornas Glen Brubaker 《Journal of the American Society for Mass Spectrometry》1993,4(6):504-506
An inexpensive multisample fast-atom bombardment (FAB) probe assembly was designed for high-throughput analysis of samples on a VG ZAB-SE mass spectrometer. The system consists of a vacuum lock system and a FAB probe whose tip contains five or more sample wells. The probe enters the mass spectrometer source region perpendicular to the secondary ion beam axis, The probe is maintained at high voltage on contact with a spring clip attached to the screen plate of the source block. Sample throughput with the multisample probe is twice that of a coaxial probe, with about twice the sensitivity and no sample cross contamination. 相似文献
125.
2,5-Dihydro-5-oxo-1,2,4-triazine and some of its alkylated derivatives have been prepared. Nmr spectroscopic analysis has established that the 2,5-dihydro-5-oxo tautomers are preferred over the 4,5-dihydro-5-oxo ones. This preference, and the behavior of 1,2,4-triazines in some other chemical reactions has been interpreted in terms of electron-electron repulsions between the lone pairs of electrons of N1 and N2 in this ring system. 相似文献
126.
William R. Dolbier Jr. Jian-Xin Duan Xiao X. Rong 《Journal of fluorine chemistry》2007,128(10):1091-1093
Selective, high yield partial fluorination of p-bis-(trichloromethyl)benzene to p-bis-(chlorodifluoromethyl)benzene has been accomplished by warming a slurry of the p-bis-(trichloromethyl)benzene in anhydrous HF which also contains a small quantity of inert solvent, such as 1,2-dichloroethane. 相似文献
127.
A method of analysing steady-state fluorescence quenching in compartmentalized systems is proposed which allows the evaluation of partition and binding constants where both association processes occur simultaneosly. The number of binding sites is shown to be dependent on the nature of the quencher. 相似文献
128.
A simplex procedure is shown to be an efficient approach for solving separation problems. The best solvent ratio for the separation of phosphatidylcholine and sphingomyelin has been found by means of a two-dimensional simplex capable of expansion and contraction. A successful high-pressure liquid chromatography separation of lysophosphatidylcholine, phoaphatidylethanolamine, phosphatidylinositol, phosphatidylserine, phosphatidylcholine, sphingomyelin and phosphatidic acid was achieved with a 180-cm column packed with Corasil II and a solvent mixture of chloroform—methanol—ammonia (50.0:35.9:7.0, v/v/v). 相似文献
129.
Solubilities are reported for carbazole in three binary chloroalkane + dibutyl ether solvent mixtures at 25°C. Results of these measurements are compared with solution models developed for solubility in systems containing specific solute-solvent interactions. A simple model based on a single 1:1 carbazole:dibutyl ether complex described the solubility data, though the calculated equilibrium constant was about one-half of values published previously. A more sophisticated solution model, which assumes both carbazole:dibutyl ether and carbazole:chloroalkane complexes, was needed to thermodynamically describe the systems studied. Equilibrium constants for three presumed carbazole:chloroalkane complexes are calculated from measured carbazole solubilities. 相似文献
130.
Wang D Schwinden MD Radesca L Patel B Kronenthal D Huang MH Nugent WA 《The Journal of organic chemistry》2004,69(5):1629-1633
The reaction of a variety of methyl esters with dimethylsulfoxonium methylide at 0-25 degrees C affords the chain-extended beta-keto dimethylsulfoxonium ylides. Subsequent treatment with hydrogen chloride in THF proceeds with loss of DMSO to afford the corresponding alpha-chloroketones. This sequence has been utilized to convert the methyl esters of CBZ-protected alanine and valine to the anti N-protected alpha-amino epoxides, which are important pharmaceutical intermediates. When the same protocol is applied to BOC-protected phenylalanine methyl ester, epimerization occurs so that the use of a more reactive aryl ester is required. This chemistry provides a practical route to alpha-chloroketones that avoids the use of toxic and explosive diazomethane. 相似文献