全文获取类型
收费全文 | 3307篇 |
免费 | 100篇 |
国内免费 | 21篇 |
专业分类
化学 | 2042篇 |
晶体学 | 5篇 |
力学 | 109篇 |
数学 | 621篇 |
物理学 | 651篇 |
出版年
2023年 | 20篇 |
2022年 | 24篇 |
2021年 | 49篇 |
2020年 | 64篇 |
2019年 | 59篇 |
2018年 | 41篇 |
2017年 | 45篇 |
2016年 | 77篇 |
2015年 | 56篇 |
2014年 | 102篇 |
2013年 | 152篇 |
2012年 | 196篇 |
2011年 | 258篇 |
2010年 | 152篇 |
2009年 | 122篇 |
2008年 | 192篇 |
2007年 | 220篇 |
2006年 | 175篇 |
2005年 | 148篇 |
2004年 | 134篇 |
2003年 | 119篇 |
2002年 | 103篇 |
2001年 | 49篇 |
2000年 | 50篇 |
1999年 | 47篇 |
1998年 | 40篇 |
1997年 | 45篇 |
1996年 | 44篇 |
1995年 | 32篇 |
1994年 | 31篇 |
1993年 | 31篇 |
1992年 | 39篇 |
1991年 | 24篇 |
1990年 | 20篇 |
1989年 | 21篇 |
1988年 | 15篇 |
1986年 | 22篇 |
1985年 | 23篇 |
1984年 | 22篇 |
1983年 | 16篇 |
1982年 | 25篇 |
1981年 | 24篇 |
1980年 | 32篇 |
1979年 | 19篇 |
1978年 | 25篇 |
1977年 | 20篇 |
1976年 | 21篇 |
1975年 | 15篇 |
1974年 | 18篇 |
1973年 | 22篇 |
排序方式: 共有3428条查询结果,搜索用时 15 毫秒
71.
Wang T Frederick KK Igumenova TI Wand AJ Zuiderweg ER 《Journal of the American Chemical Society》2005,127(3):828-829
The fast dynamics of protein backbones are often investigated by nuclear magnetic relaxation experiments that report on the degree of spatial restriction of the amide bond vector. By comparing calmodulin in the peptide-bound and peptide-free states with these classical methods, we observe little difference in the dynamics of the polypeptide main chain (average order parameter decrease of 0.01 unit upon binding). However, when using NMR methods that monitor the mobility of the CO-Calpha bond vector, we reveal a significant reduction of dynamics of the protein main chain (average order parameter decrease of 0.048 units). Previous investigations have suggested that the side-chain dynamics is reduced by an average of 0.07 order parameter units upon ligand binding (Lee, A. L.; Kinnear, S. A.; Wand, A. J. Nat. Struct. Biol. 2000, 7, 72-77). The current findings suggest that the change of the CO-Calpha bond vector dynamics is intermediate between the changes in NH and side-chain dynamics and report a previously undetected loss of main-chain entropy. Weak site-to-site correlations between the different motional indicators are also observed. 相似文献
72.
Erik Larsson 《Fresenius' Journal of Analytical Chemistry》1930,79(5-6):170-175
Zusammenfassung Auf Grund der Dissoziatio nskonstanten der Thioglykolsäure,- und-Thiomilchsäure,-Mercaptoisobuttersäure,-Thiomalamidsäure und Thioäpfelsäure wurde gezeigt, dass von diesen Säuren nur die-Mercaptoisobuttersäure und Thioäpfelsäure gut mit Phenolphthalein alkalimetrisch titriert werden können, aber auch nur dann, wenn man viel Indikator benutzt. Bei den anderen Säuren hat man Indikatoren mit saurerem Umschlag zu wählen. Für die ersten drei Säuren dürften Neutralrot und Phenolrot geeignete Indikatoren sein. Dies wird auch bei der Thioglykolsäure experimentell erwiesen. Für die alkalimetrische Titration von-Thiomalamidsäure soll man Bromthymolblau als Indikator verwenden. 相似文献
73.
Drahl C Cravatt BF Sorensen EJ 《Angewandte Chemie (International ed. in English)》2005,44(36):5788-5809
Researchers in the post-genome era are confronted with the daunting task of assigning structure and function to tens of thousands of encoded proteins. To realize this goal, new technologies are emerging for the analysis of protein function on a global scale, such as activity-based protein profiling (ABPP), which aims to develop active site-directed chemical probes for enzyme analysis in whole proteomes. For the pursuit of such chemical proteomic technologies, it is helpful to derive inspiration from protein-reactive natural products. Natural products use a remarkably diverse set of mechanisms to covalently modify enzymes from distinct mechanistic classes, thus providing a wellspring of chemical concepts that can be exploited for the design of active-site-directed proteomic probes. Herein, we highlight several examples of protein-reactive natural products and illustrate how their mechanisms of action have influenced and continue to shape the progression of chemical proteomic technologies like ABPP. 相似文献
74.
Jørgensen JM Erlacher K Pedersen JS Gothelf KV 《Langmuir : the ACS journal of surfaces and colloids》2005,21(23):10320-10323
The influence of preparation temperature on the size and size distribution of dodecylthiol monolayer protected gold clusters was studied. The monolayer protected clusters (MPCs) were synthesized by two different variations of the Brust-Schiffrin procedure. In all of the experiments, the stoichiometry of the reactants dodecylthiol, HAuCl(4), and sodium borohydride was kept constant, while the temperature was varied in the range of -18 to +90 degrees C. Two series were performed in which an aqueous solution of NaBH(4) was either added over 30 s or all in one portion. The size and size distribution of the MPCs were determined by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). It has been demonstrated that in general the MPC size increases with elevated preparation temperatures. 相似文献
75.
Rolf Gleiter Jens Spanget-Larsen Erik W. Thulstrup Ichiro Murata Kazuhiro Nakasuji Christian Jutz 《Helvetica chimica acta》1976,59(5):1459-1468
The photoelectron (PE.) spectra of azuleno[l, 2, 3-cd]phenalene ( 1 ) and azuleno- [5,6,7-cd]phenalene( 2 ) have been recorded. The first five bands of both compounds could be assigned to transitions corresponding to removal of electrons from 4a2, 6b1, 5b1, 3a2 and 4bl orbitals. This assignment is based mainly on a comparison between the observed ionization potentials and orbital energies calculated in a HMO and a PPP model. The UV./VIS. polarized absorption spectrum of 1 in the region 10000–45000 cm?1 has been measured by means of the stretched film technique. The measurements were performed in polyethylene sheets at 77°K. Several bands could be assigned to π* ← π transitions calculated by a PPP-CI method. A comparison between the electronic structures of 1 and 2 is made by means of a simple HMO diagram. 相似文献
76.
77.
Erik G.P. da Silva Ana Carolina do N. Santos Antnio C.S. Costa Dalva M. da N. Fortunato Ndia M. Jos Maria G.A. Korn Walter N.L. dos Santos Srgio L.C. Ferreira 《Microchemical Journal》2006,82(2):159-162
In the present work, a slurry sampling flame atomic absorption spectrometric method to determine directly manganese and zinc in powdered chocolate samples is proposed. The optimization step was performed using univariate methodology involving the following factors: nature and concentration of the acid solution, sonication time, and particle size. The established conditions led to the use of a sample mass of 150 mg, 2.0 mol L− 1 nitric acid solution, sonication time of 15 min, and a slurry volume of 50 mL. This method allows the determination of manganese and zinc with detection limit of 52 and 61 ng g− 1, respectively, and a precision expressed as relative standard deviation (RSD) of 2.6% and 3.2% (both, n = 10) for contents of manganese and zinc of 52.4 and 100.0 μg g− 1, respectively. The proposed method was applied for determination of manganese and zinc in five powdered chocolate samples. In these, the manganese content varied from 42.8 to 52.7 and from 88.6 to 102.4 μg g− 1 of zinc. The analytical results were compared with the results obtained by analysis of these samples after digestion using open vessel and acid bomb digestion procedures and determination using FS-FAAS. The statistical comparison by t-test (95% confidence level) showed no significant difference between these results. 相似文献
78.
Gar B. Hoflund Steven D. Gardner David R. Schryer Billy T. Upchurch Erik J. Kielin 《Reaction Kinetics and Catalysis Letters》1996,58(1):19-26
The influence of promoters on Pt/SnOx/SiO2 and Au/MnOx low-temperature CO oxidation catalysts has been investigated under stoichiometric reaction conditions with no CO2 added to the feed gas. The performance of Pt/SnOx/SiO2 catalysts is improved significantly by the addition of 1 wt.% Fe but reduced by the addition of 5 wt.%Fe, 1 wt.% Sb, 5 wt.% Sb, 1 wt.% As, 5 wt.%As and 1.8 wt.% P. The performance of Au/MnOx is improved significantly by the addition of 1 at.% Ce but reduced by the addition of 1 at.% Co. For the catalysts and conditions examined, the Au/MnOx catalysts are superior to the Pt/SnOx/SiO2 catalysts with respect to both activity and decay characteristics. 相似文献
79.
Pedersen H Sinning S Bulow A Wiborg O Falborg L Bols M 《Organic & biomolecular chemistry》2004,2(19):2861-2869
A combinatorial synthesis of benztropine analogues is presented. Radical azidonation of 3-benzyloxy-8-azabicyclo[3.2.1]octane-8-carboxylic acid tert-butyl ester 3 to 3-(1-azidobenzyloxy)-8-azabicyclo[3.2.1]octane-8-carboxylic acid tert-butyl ester 4 was used as a key step in the synthesis. This step was optimized by adding 10% DMF to the reaction. Reaction of 4 with phenyl magnesium bromide followed by Boc removal and N-methylation gave benztropine 1. Reaction of five-component Grignard reagents with 4 was used to create a two-dimensional library of 25 N-normethylbenztropine analogues. Further reaction of this library with five alkyl bromides was carried out to create a three-dimensional library containing 125 compounds. Screening of the libraries towards binding and inhibition of uptake of the human dopamine (hDAT), serotonin (hSERT) and norepinephrine transporters (hNET) was carried out. None of the synthesized compounds were found to be stronger than benztropine, and none were selective for inhibition of binding over monoamine uptake. 相似文献
80.
Zusammenfassung Der Schwefel in Wolframoxiden und Wolframpulvern wird im Wasserstoffstrom bei 800°C in H2S übergeführt, der in Cadmium-Zinkacetat-Lösung absorbiert und anschließend jodometrisch titriert wird. Die Anwesenheit von WO3 ermöglicht eine quantitative Umwandlung von Sulfatschwefel in H2S. Die Standardabweichung beträgt für ein Wolframoxid mit 310 ppm S±13 ppmS und für ein Wolfram-Metallpulver mit 45 ppm S±1 ppm S.
Herrn Prof. Dr.E. Hayek zum 60. Geburtstag gewidmet. 相似文献
Sulfur is converted into hydrogen sulphide by hydrogenation at 800°C. This H2S is absorbed in a cadmium-zinc acetate solution and titrated iodometrically. The presence of WO3 makes possible a quantitative conversion of sulphate sulphur into H2S. The standard deviation is ±13 ppm S for WO3 containing 310 ppm S, and ±1 ppm S for metallic tungsten powder containing 45 ppm S.
Herrn Prof. Dr.E. Hayek zum 60. Geburtstag gewidmet. 相似文献