全文获取类型
收费全文 | 10500篇 |
免费 | 298篇 |
国内免费 | 31篇 |
专业分类
化学 | 7867篇 |
晶体学 | 31篇 |
力学 | 162篇 |
数学 | 1782篇 |
物理学 | 987篇 |
出版年
2020年 | 138篇 |
2019年 | 106篇 |
2018年 | 77篇 |
2017年 | 81篇 |
2016年 | 189篇 |
2015年 | 198篇 |
2014年 | 188篇 |
2013年 | 352篇 |
2012年 | 392篇 |
2011年 | 454篇 |
2010年 | 336篇 |
2009年 | 279篇 |
2008年 | 455篇 |
2007年 | 446篇 |
2006年 | 469篇 |
2005年 | 440篇 |
2004年 | 378篇 |
2003年 | 312篇 |
2002年 | 327篇 |
2001年 | 225篇 |
2000年 | 157篇 |
1999年 | 145篇 |
1998年 | 180篇 |
1997年 | 148篇 |
1996年 | 150篇 |
1995年 | 150篇 |
1994年 | 143篇 |
1993年 | 191篇 |
1992年 | 127篇 |
1991年 | 127篇 |
1990年 | 114篇 |
1989年 | 135篇 |
1988年 | 131篇 |
1987年 | 167篇 |
1986年 | 157篇 |
1985年 | 174篇 |
1984年 | 186篇 |
1983年 | 155篇 |
1982年 | 144篇 |
1981年 | 151篇 |
1980年 | 153篇 |
1979年 | 147篇 |
1978年 | 138篇 |
1977年 | 119篇 |
1976年 | 135篇 |
1975年 | 91篇 |
1974年 | 89篇 |
1973年 | 88篇 |
1971年 | 80篇 |
1970年 | 71篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
211.
Three silylated γ-alkynylfurans were prepared and subjected to both gold and platinum catalysts. The TMS- and the TBDMS-substituted furans reacted. With AuCl3 and the binuclear [(Ph3PAu)2Cl][BF4] catalyst a hydroarylation of the alkyne was observed. Na[AuCl4] gave phenols as the product, but these were formed only after in situ desilylation of the starting material by the gold catalyst and thus the wrong isomer dominated. Only with PtCl2(MeCN)2 phenols with a silyl group were formed. The TBDPS-substituted furan failed to react. Two alkynylsilanes were synthesized, but they also failed to react. 相似文献
212.
The title compound has been synthesized and its aromatisation to the Kekulé isomer has been measured. From the kinetic data a diradical process is deduced. 相似文献
213.
Nonracemic sec-alcohols of opposite absolute configuration were obtained either by asymmetric reduction of the corresponding ketone using 2-propanol as hydrogen donor or by enantioselective oxidation through kinetic resolution of the rac-alcohol using acetone as hydrogen acceptor employing whole lyophilized cells of Rhodococcus ruber DSM 44541. The microbial oxidation/reduction system exhibits not only excellent stereo- and enantioselectivity but also a broad substrate spectrum. Due to the exceptional tolerance of the biocatalyst toward elevated concentrations of organic materials (solvents, substrates and cosubstrates), the process is highly efficient. The simple preparation of the biocatalyst and its ease of handling turns this system into a versatile tool for organic synthesis. 相似文献
214.
Intensive studies on the diazomethane methylation of the common ribonucleosides uridine, cytidine, adenosine, and guanosine and its derivatives were performed to obtain preferentially the 2′-O-methyl isomers. Methylation of 5′-O-(monomethoxytrityl)-N2-(4-nitrophenyl)ethoxycarbonyl-O6-[2-(4-nitrophenyl)ethyl]-guanosine ( 1 ) with diazomethane resulted in an almost quantitative yield of the 2′- and 3′-O-methyl isomers which could be separated by simple silica-gel flash chromatography (Scheme 1). Adenosine, cytidine, and uridine were methylated with diazomethane with and without protection of the 5′ -O-position by a mono- or dimethoxytrityl group and the aglycone moiety of adenosine and cytidine by the 2-(4-nitrophenyl)ethoxycarbonyl (npeoc) group (Schemes 2–4). Attempts to increase the formation of the 2′-O-methyl isomer as much as possible were based upon various solvents, temperatures, catalysts, and concentration of the catalysts during the methylation reaction. 相似文献
215.
Baier J Maier M Engl R Landthaler M Bäumler W 《The journal of physical chemistry. B》2005,109(7):3041-3046
Singlet oxygen was generated by energy transfer from the photoexcited sensitizer, Photofrin or 9-acetoxy-2,7,12,17-tetrakis-(beta-methoxyethyl)-porphycene (ATMPn), to molecular oxygen. Singlet oxygen was detected time-resolved by its luminescence at 1270 nm in an environment of increasing complexity, water (H2O), pure phosphatidylcholine, phosphatidylcholine in water (lipid suspensions), and aqueous suspensions of living cells. In the case of the lipid suspensions, the sensitizers accumulated in the lipids, whereas the localizations in the cells are the membranes containing phosphatidylcholine. By use of Photofrin, the measured luminescence decay times of singlet oxygen were 3.5 +/- 0.5 micros in water, 14 +/- 2 micros in lipid, 9 +/- 2 micros in aqueous suspensions of lipid droplets, and 10 +/- 3 micros in aqueous suspensions of human colonic cancer cells (HT29). The decay time in cell suspensions was much longer than in water and was comparable to the value in suspensions of phosphatidylcholine. That luminescence signal might be attributed to singlet oxygen decaying in the lipid areas of cellular membranes. The measured luminescence decay times of singlet oxygen excited by ATMPn in pure lipid and lipid suspensions were the same within the experimental error as for Photofrin. In contrast to experiments with Photofrin, the decay time in aqueous suspension of HT29 cells was 6 +/- 2 micros when using ATMPn. 相似文献
216.
Wolfgang A. Herrmann 《Angewandte Chemie (International ed. in English)》1982,21(2):117-130
The Fischer-Tropsch Synthesis counts among the industrial-scale processes having a versatile and broad product range, and has for decades offered the most attractive possibility for the use of coal as a source of heating oil and fuels. This conceivably simple reaction, the catalytic hydrogenation of carbon monoxide, generally leads to simple hydrocarbons as well (i.e. short chain olefins) that have been sought as chemical feedstocks since the oil crisis of the seventies, but fails to provide the large-scale, economic process required, due in large part to the minimal selectivity of traditional Fischer-Tropsch processes. In an effort to solve this problem current research in this sector is concerned not only with the optimization of old and the development of new catalytic systems, but also increasingly with the elucidation of numerous relevant reaction mechanisms. This article will discuss, from the viewpoint of an organometallic chemist, the significance of typical model reactions, both with regard to some fundamental aspects of synthesis gas chemistry, and in comparison with previous views concerning the mechanism of the Fischer-Tropsch Synthesis. The importance of various unique classes of complexes that have been studied in the context of Fischer-Tropsch chemistry is also evaluated with regard to their importance in the synthesis of hydrocarbons from carbon monoxide and hydrogen. It emerges that the primary steps of the reductive oligomerization of carbon monoxide are best described by the carbide/methylene mechanism, as originally proposed by Hans Fischer and Franz Tropsch. 相似文献
217.
Mathematische Semesterberichte - Nicolaus Cusanus’ Beiträge zur Mathematik des 15. Jahrhunderts betreffen hauptsächlich Kreisquadraturen. Wir besprechen seinen... 相似文献
218.
219.
Wolfgang Schwack Bernd Brüger Steven Nyanzi 《Fresenius' Journal of Analytical Chemistry》1995,351(2-3):297-300
The differential pulse-polarographic (DPP) determination of both CS2 and COS gases, after their absorption in a methanolic piperidine reagent and the subsequent application of this technique to the residue analysis of dithiocarbamate fungicides is described. Rectilinear calibration curves for both CS2 and COS in the respective regions of 1.5–9.2 and 2.1–12.6 mol/l were obtained. The DPP method has been successfully applied to the determination of thiram residues on apples after a hot acid hydrolysis of the fruits. 相似文献
220.
We show that the conformal characters of various rational models ofW-algebras can be already uniquely determined if one merely knows the central charge and the conformal dimensions. As a side result we develop several tools for studying representations of SL(2,) on spaces of modular functions. These methods, applied here only to certain rational conformal field theories, may be useful for the analysis of many others. 相似文献