全文获取类型
收费全文 | 3076篇 |
免费 | 46篇 |
国内免费 | 33篇 |
专业分类
化学 | 1774篇 |
晶体学 | 60篇 |
力学 | 67篇 |
数学 | 668篇 |
物理学 | 586篇 |
出版年
2022年 | 49篇 |
2021年 | 64篇 |
2020年 | 30篇 |
2019年 | 35篇 |
2018年 | 38篇 |
2017年 | 56篇 |
2016年 | 79篇 |
2015年 | 76篇 |
2014年 | 86篇 |
2013年 | 210篇 |
2012年 | 134篇 |
2011年 | 179篇 |
2010年 | 114篇 |
2009年 | 125篇 |
2008年 | 171篇 |
2007年 | 160篇 |
2006年 | 149篇 |
2005年 | 124篇 |
2004年 | 142篇 |
2003年 | 99篇 |
2002年 | 105篇 |
2001年 | 50篇 |
2000年 | 43篇 |
1999年 | 47篇 |
1998年 | 41篇 |
1997年 | 52篇 |
1996年 | 55篇 |
1995年 | 34篇 |
1994年 | 34篇 |
1993年 | 41篇 |
1992年 | 27篇 |
1991年 | 32篇 |
1990年 | 34篇 |
1989年 | 33篇 |
1988年 | 20篇 |
1987年 | 27篇 |
1986年 | 19篇 |
1985年 | 30篇 |
1984年 | 42篇 |
1983年 | 24篇 |
1982年 | 24篇 |
1981年 | 30篇 |
1980年 | 23篇 |
1979年 | 21篇 |
1978年 | 20篇 |
1977年 | 29篇 |
1976年 | 12篇 |
1975年 | 12篇 |
1974年 | 10篇 |
1972年 | 11篇 |
排序方式: 共有3155条查询结果,搜索用时 15 毫秒
51.
Quantum chemical evaluation of protein control over heme ligation: CO/O2 discrimination in myoglobin
Control of O2 versus CO binding in myoglobin (Mb) is tuned by a distal histidine residue through steric and H-bonding interactions. These interactions have been evaluated via Car-Parrinello DFT calculations, whose efficiency allows full quantum mechanical treatment of the 13 closest residues surrounding the heme. The small (8 degrees ) deviation of the Fe-C-O bond angle from linearity results from the steric influence of a distal valine residue and not the distal histidine. H-bond energies were evaluated by replacing the distal histidine with the non-H-bonding residue isoleucine. Binding energies for CO and O2 decreased by 0.8 and 4.1 kcal/mol for MbCO and MbO2, in good agreement with experimental H-bond estimates. Ligand discrimination is dominated by distal histidine H-bonding, which is also found to stabilize a metastable side-on isomer of MbO2 that may play a key role in MbO2 photodynamics. 相似文献
52.
Lead has been determined in three certified reference materials (aquatic plant, olive leaves, and mussel tissue) by direct slurry atomization-graphite furnace atomic absorption spectrophotometry. The importance of using ascorbic acid as matrix modifier has been demonstrated, particularly to overcome calcium interferences. Using 0.05 M ascorbic acid and aqueous calibration standards excellent agreement was found with certified values. The method is proposed for general application in environmental analysis. 相似文献
53.
Grzegorz WojciechowskiMa?gorzata Ratajczak-Sitarz Andrzej KatrusiakBogumil Brzezinski 《Journal of Molecular Structure》2002,612(1):59-64
The crystals of 5,5′-dibromo-3-diethylaminomethyl-2,2′-biphenol N-oxide were studied by X-ray and FT-IR spectroscopy. Within this molecule two short OHO intramolecular hydrogen bonds are formed. The NO?H+?O− bond between the OH and the N-oxide groups is very strong, of 2.419(7) Å between the oxygen atoms. The proton potential of this hydrogen bond is flat, broad and has probably no barrier—consequently it could not be located from X-ray diffraction data. The other hydrogen bond formed between two hydroxyl groups appears asymmetrical from FT-IR spectra, and shows also relatively limited proton polarizability. The molecular conformation is non-planar, due to strong overcrowding effect between the oxygen atoms involved in the hydrogen bonds. 相似文献
54.
A coupled Hartree-Fock calculation of the proton shielding constant in FHF− with a reasonably good basis set of contracted gaussian orbitals leads to an extremely large upfield H-bond chemical shift with respect to the HF molecule. An approximate scheme, based on the requirement of the gauge origin independence of the finite basis set coupled Hartree-Fock calculations of the magnetic susceptibility, provides a considerable improvement of the computed proton shielding constant and results in the required downfield H-bond chemical shift of the proton resonance. The computed H-bond shift for FHF− agrees with the experimental data. 相似文献
55.
Anna Kowalska Zygmunt Kowalski Andrzej Kotarba Andrzej Barański 《Mikrochimica acta》1990,100(5-6):349-353
The analytical procedure of determination of Fe(II) in the presence of Fe(III) in milligram samples is proposed. A solid sample is dissolved in hydrochloric acid in argon atmosphere. Subsequently, the pH-value of the solution is fixed and Fe(III) complexed by phosphoric buffer. At the next step Fe(II) is oxidized by -molybdosilicic acid. Finally, the intensity of colouring caused by the reduced form of the acid is measured at the wavelength 770 nm. The proper treatment of the sample during oxidation, and immediately before, is decisive for the accuracy of the analysis. The method was verified by means of the analysis of synthetic magnetite. The theoretical content of Fe(II) was 24.12%, the determined one was 24.02 ± 0.10%. The method was applied to the determination of the homogeneity of an iron catalyst for ammonia synthesis. 相似文献
56.
57.
Andrzej Olczak Marek L. Gwka Jolanta Goka Magorzata Szczesio Joanna Bojarska Krystyna Kozowska Henryk Foks Czesawa Orlewska 《Journal of Molecular Structure》2007,830(1-3):171-175
Crystal structures of two title compounds and several their relatives known earlier reveal conservative and characteristic features, which may be related to their tuberculostatic activity. The molecules are predominantly planar due to conjugation through five successive bonds in the zwitterionic fragment S−–C(sp2)–N–NH+–C(sp2)–NH2 and intramolecular hydrogen bonds, which prevent rotation of the adjacent pyrazine (or pyridine) ring. It has been suggested that in spatial sense such planar molecules resemble acridines intercalating with nucleic acids and that similar process may be responsible for tuberculostatic activity of the title pyrazine-2-carboxamide-N′-carbonothioyl-hydrazones. 相似文献
58.
59.
Andrzej Czerwiński Iwona Kiersztyn Michał Grdeń 《Journal of Solid State Electrochemistry》2003,7(6):321-326
The effect of temperature on hydrogen and deuterium electrosorption into a palladium LVE (limited-volume electrode) has been
investigated. A decrease in hydrogen capacity (H/Pd ratio) with increasing temperature has been observed. Temperature strongly
influences the plots of measured H(D)/Pd values vs. potential scan rate. In addition, hydrogen absorption was found to be
dependent on the composition of the surrounding electrolyte solution. These results have confirmed the hypothesis that two
different mechanisms of hydrogen desorption from the palladium electrode take place, namely electrochemical oxidation and
non-electrochemical recombination. Further, the ratio between the rate constants for these two processes has been found to
change with temperature.
Electronic Publication 相似文献
60.
Maj-Urawska M Ziemianek D Mikołajczuk A Mieczkowski J Lewenstam A Hulanicki A Sokalski T 《Analytical and bioanalytical chemistry》2003,376(4):524-526
The properties of the carbonate neutral carrier 4-( n-hexadecyl)-3-nitro-1-trifluoroacetylbenzene were compared with those of a similar carrier, without a nitro group, studied previously. In spite of differences in the Hammett constant of the carbonyl group responsible for interaction with the analyte, the analytical characteristics of both carriers, measured under the same conditions, were comparable. Special care was taken to avoid the presence of an excessive carbon dioxide level in the diffusion layer at the membrane-solution interface. The internal reference solution was prepared without carbonate components; the external solution was protected from contact with atmospheric carbon dioxide. Under such conditions the detection limit of both electrodes was extended to 10(-11 )mol L(-1), and the selectivity towards salicylate, chloride, and acetate was significantly improved. 相似文献