全文获取类型
收费全文 | 2734篇 |
免费 | 63篇 |
国内免费 | 1篇 |
专业分类
化学 | 1643篇 |
晶体学 | 13篇 |
力学 | 33篇 |
数学 | 190篇 |
物理学 | 919篇 |
出版年
2021年 | 18篇 |
2020年 | 48篇 |
2019年 | 33篇 |
2018年 | 22篇 |
2016年 | 47篇 |
2015年 | 39篇 |
2014年 | 48篇 |
2013年 | 116篇 |
2012年 | 103篇 |
2011年 | 97篇 |
2010年 | 73篇 |
2009年 | 71篇 |
2008年 | 127篇 |
2007年 | 112篇 |
2006年 | 134篇 |
2005年 | 114篇 |
2004年 | 97篇 |
2003年 | 57篇 |
2002年 | 53篇 |
2001年 | 56篇 |
2000年 | 49篇 |
1999年 | 27篇 |
1998年 | 30篇 |
1997年 | 42篇 |
1996年 | 47篇 |
1995年 | 38篇 |
1994年 | 46篇 |
1993年 | 57篇 |
1992年 | 52篇 |
1991年 | 27篇 |
1990年 | 38篇 |
1989年 | 25篇 |
1988年 | 25篇 |
1987年 | 40篇 |
1986年 | 35篇 |
1985年 | 38篇 |
1984年 | 39篇 |
1983年 | 22篇 |
1981年 | 21篇 |
1980年 | 27篇 |
1978年 | 30篇 |
1977年 | 39篇 |
1976年 | 35篇 |
1975年 | 27篇 |
1974年 | 21篇 |
1973年 | 36篇 |
1970年 | 20篇 |
1957年 | 30篇 |
1955年 | 29篇 |
1954年 | 21篇 |
排序方式: 共有2798条查询结果,搜索用时 31 毫秒
121.
122.
Stephen D. Pastor David N. Rahni Nancy Khoury Stephen A. Koch 《Phosphorus, sulfur, and silicon and the related elements》2013,188(8):1951-1956
The X-ray crystallographic analysis of 6,6-dimethyl-2,4,8,10-tetra-tert-octyl-dibenzo[d,f][1,3,2]dioxagermepin, 1 is reported. In the solid-state conformation of 1, the dihedral angle about the C─C sp2-sp 2 σ bond connecting the two aryl rings is 50.1°. The observed C2 symmetry in the solid-state conformation of 1 is consistent with the previously suggested solution conformation. 相似文献
123.
R. Otto P. Hecht H.-G. Könnecke H. Koch H. J. Naumann W. Winzer 《Isotopes in environmental and health studies》2013,49(12):425-429
This paper reviews ways in which radioactive tracer techniques can he used to improve the efficiency of a caprolactam production plant, Residence time spectra and material distributions in different parts of the plant were measured by application of sodium 24, bromine 82 and technetium 99m. The investigations led to improved cumeneoxidators and gave useful informations on the catalytic decomposition of the phenolic tar and the hydrogenation process of phenol. It could he shown that the construction of the lactam extractor must he varied and the results of the tracerexperiments made in the distillation unit contributed to a higher quality of the final product. 相似文献
124.
Steffen Blaurock Thomas Koch Olaf Kühl Peter Lönnecke Felicite Majoumo René Sommer 《Phosphorus, sulfur, and silicon and the related elements》2013,188(6-7):1451-1456
Substituted bifunctional phosphorus-based ligands HX(CRR') n PR"H (or -PR" 2 ) [where X = O, S, NR', (substituted) cyclopentadienyl; n = 1, 2, 3; R, R', R" = alkyl, aryl, H] were employed as bridging ligands in the synthesis of early/late bridged transition metal complexes. Synthetic routes to the bifunctional ligands were also developed. First, mononuclear complexes, such as [TpZr(OCH 2 PPh 2 ) 3 ] (Tp = trispyrazolylborato), [Cp 2 Zr(1-O-2-PHR-C 6 H 10 )(Me)] (R = 2,4,6-Pr i 3 C 6 H 2 (Tipp)), [Cp 2 Zr(SCH 2 CH 2 PHR) 2 ] (R = Ph, Mes, Tipp), and phosphinoferrocene derivatives, were prepared. These complexes are suitable precursors for the introduction of a second metal (as in, for example, [TpZr( w -OCH 2 PPh 2 ) 3 Mo(CO) 3 ]). 相似文献
125.
Burkhard Costisella Iris Keitel 《Phosphorus, sulfur, and silicon and the related elements》2013,188(3-4):161-165
Abstract The 1-acetoxy-1-methylthiomethanephosphoryl compounds (4) are prepared from methylthiomethanephosphoryl derivatives (1) by electrolysis in 0.6 molar sodium acetate/acetic acid solution. The reaction of 1-acetoxy compounds (4) with halogen carriers gives the 1-halogen-1-methylthiomethane-phosphoryl derivatives 5, 7 and 8. The reaction of 4a under PTC-conditions results in cleavage of the P-C-bond. Die 1-Acetoxy-1-methylthiomethanphosphoryl-Verbindungen (4) wurden aus Methylthiomethanphos-phorylderivaten (1) durch Elektrolyse in 0.6 molarer Na-acetat/Essigsäure-Lösung dargestellt. Die Reaktion der 1-Acetoxyverbindungen mit Halogenüberträgern führt zu den 1-Halogen-1-methyl-thiomethanphosphorylderivaten 5, 7 und 8. Bei Reaktion von 4a unter PTC-Bedingungen beobachtet man eine Spaltung der P-C-Bindung. 相似文献
126.
Fabian Benz Christoph Roderburg David Vargas Cardenas Mihael Vucur Jérémie Gautheron Alexander Koch Henning Zimmermann J?rn Janssen Lukas Nieuwenhuijsen Mark Luedde Norbert Frey Frank Tacke Christian Trautwein Tom Luedde 《Experimental & molecular medicine》2013,45(9):e42
MicroRNA (miRNA) levels in serum have recently emerged as potential novel biomarkers for various diseases. miRNAs are routinely measured by standard quantitative real-time PCR (qPCR); however, the high sensitivity of qPCR demands appropriate normalization to correct for nonbiological variation. Presently, RNU6B (U6) is used for data normalization of circulating miRNAs in many studies. However, it was suggested that serum levels of U6 themselves might differ between individuals. Therefore, no consensus has been reached on the best normalization strategy in ‘circulating miRNA''. We analyzed U6 levels as well as levels of spiked-in SV40-RNA in sera of 44 healthy volunteers, 203 intensive care unit patients and 64 patients with liver fibrosis. Levels of U6 demonstrated a high variability in sera of healthy donors, patients with critical illness and liver fibrosis. This high variability could also be confirmed in sera of mice after the cecal ligation and puncture procedure. Most importantly, levels of circulating U6 were significantly upregulated in sera of patients with critical illness and sepsis compared with controls and correlated with established markers of inflammation. In patients with liver fibrosis, U6 levels were significantly downregulated. In contrast, levels of spiked-in SV40 displayed a significantly higher stability both in human cohorts (healthy, critical illness, liver fibrosis) and in mice. Thus, we conclude that U6 levels in the serum are dysregulated in a disease-specific manner. Therefore, U6 should not be used for data normalization of circulating miRNAs in inflammatory diseases and previous studies using this approach should be interpreted with caution. Further studies are warranted to identify specific regulatory processes of U6 levels in sepsis and liver fibrosis. 相似文献
127.
Dr. Roberta Pievo Brigitta Angerstein Dr. Alistair J. Fielding Dr. Christian Koch Prof. Dr. Ivo Feussner Prof. Dr. Marina Bennati 《Chemphyschem》2013,14(18):4094-4101
Electron paramagnetic resonance (EPR) spectroscopy in combination with the rapid freeze‐quench (RFQ) technique is a well‐established method to trap and characterize intermediates in chemical or enzymatic reactions at the millisecond or even shorter time scales. The method is particularly powerful for mechanistic studies of enzymatic reactions when combined with high‐frequency EPR (ν≥90 GHz), which permits the identification of substrate or protein radical intermediates by their electronic g values. In this work, we describe a new custom‐designed micro‐mix rapid freeze‐quench apparatus, for which reagent volumes for biological samples as small as 20 μL are required. The apparatus was implemented with homemade sample collectors appropriate for 9, 34, and 94 GHz EPR capillaries (4, 2, and 0.87 mm outer diameter, respectively) and the performance was evaluated. We demonstrate the application potential of the RFQ apparatus by following the enzymatic reaction of PpoA, a fungal dioxygenase producing hydro(pero)xylated fatty acids. The larger spectral resolution at 94 GHz allows the discernment of structural changes in the EPR spectra, which are not detectable in the same samples at the standard 9 GHz frequency. 相似文献
128.
The heterobimetallic complexes [MMoO2(L)(H2O)2] (where M = Zn2+ (1), Cu2+ (2), and Co2+ (4)) and [{MMoO3(H2L)(H2O)2}2] (where M = Ni2+ (3) and Mn2+ (5)) are synthesized from bis(2-hydroxy-1-naphthaldehyde)malonoyldihydrazone (H4L) using the monometallic precursor complex [MoO2(H2L)]·H2O in ethanol. The composition of the complexes is established based on the data obtained from the elemental analysis and molecular weight determinations. The structure of the complexes is discussed in the light of data obtained from molar conductance, magnetic moment, electronic, EPR and IR spectroscopic studies. 相似文献
129.
Peptides bearing an acryloyl residue at their N‐terminus were coupled with various C‐glycosides in an equimolar ratio via cross‐metathesis. The newly formed olefin was obtained with high E/Z selectivity in satisfying to high yields with low homodimerization of the starting materials. The posttranslational cross‐metathesis approach was shown to be suitable for the combinatorial synthesis of a small library of C‐glycopeptides. 相似文献
130.
Dimesitylmethane-based compounds 9–17, incorporating four groups capable of serving as hydrogen bonding sites, such as pyrazole, pyrimidine, imidazole, indole and aminoalkyl groups, were prepared and their ability to complex selected carbohydrates tested. The tetrasubstituted dimesitylmethane scaffold provides a cavity of a correct shape and size for disaccharide encapsulation and its aromatic units are able to participate in CH-π interactions with the sugar substrate. First binding studies confirmed the expected di- vs monosaccharide binding preference of this type of compounds and their tendency to form strong complexes with maltoside. 相似文献