全文获取类型
收费全文 | 762篇 |
免费 | 28篇 |
国内免费 | 6篇 |
专业分类
化学 | 589篇 |
晶体学 | 7篇 |
力学 | 12篇 |
数学 | 20篇 |
物理学 | 168篇 |
出版年
2023年 | 6篇 |
2022年 | 6篇 |
2021年 | 12篇 |
2020年 | 7篇 |
2019年 | 16篇 |
2018年 | 6篇 |
2017年 | 7篇 |
2016年 | 16篇 |
2015年 | 17篇 |
2014年 | 23篇 |
2013年 | 22篇 |
2012年 | 40篇 |
2011年 | 31篇 |
2010年 | 29篇 |
2009年 | 28篇 |
2008年 | 43篇 |
2007年 | 42篇 |
2006年 | 30篇 |
2005年 | 24篇 |
2004年 | 32篇 |
2003年 | 24篇 |
2002年 | 32篇 |
2001年 | 22篇 |
2000年 | 21篇 |
1999年 | 11篇 |
1998年 | 8篇 |
1997年 | 9篇 |
1996年 | 12篇 |
1995年 | 6篇 |
1994年 | 7篇 |
1993年 | 10篇 |
1992年 | 10篇 |
1991年 | 13篇 |
1990年 | 7篇 |
1989年 | 5篇 |
1988年 | 13篇 |
1987年 | 8篇 |
1986年 | 8篇 |
1985年 | 19篇 |
1984年 | 14篇 |
1983年 | 6篇 |
1982年 | 12篇 |
1981年 | 10篇 |
1980年 | 4篇 |
1979年 | 16篇 |
1978年 | 10篇 |
1977年 | 11篇 |
1975年 | 4篇 |
1974年 | 6篇 |
1968年 | 4篇 |
排序方式: 共有796条查询结果,搜索用时 31 毫秒
11.
A reliable and sensitive capillary gas chromatographic-mass spectrometric method was developed for the detection and determination of diazepam and its major metabolite, N-desmethyldiazepam, in human material. Medazepam served as the internal standard. Quantitative determination was achieved using mass fragmentography with selected ions of m/z 256 for diazepam and m/z 242 for N-desmethyldiazepam and medazepam. The limit of detection was 1 ng/g and the recoveries were 98.54 +/- 3.95% for diazepam and 98.66 +/- 6.48% for N-desmethyldiazepam. The calibration graph was linear over the concentration range from 1.0 ng/g to 1.0 microgram/g for diazepam and N-desmethyldiazepam. Using this method, trace amounts of diazepam and N-desmethyldiazepam were detected in the tissues of an autopsied individual. 相似文献
12.
Itoh T Nagata K Yokoya M Miyazaki M Kameoka K Nakamura S Ohsawa A 《Chemical & pharmaceutical bulletin》2003,51(8):951-955
Chiral 1-substituted isoquinoline derivatives, which were obtained by the reaction using alanine derivatives as chiral auxiliaries, were transformed to (S)-2,3,9,10,11-pentamethoxyhomoprotoberberine (7) and a synthetic intermediate for O-methylkreysigine (9) in good yields and high stereoselectivity. The corresponding chiral allyl derivative of isoquinoline was transformed to a pyrrolidinoisoquinoline (16) in a highly enantioselective manner. 相似文献
13.
Characterization of the non-stoichiometric and isomorphic hydration and solvation in FK041 clathrate
Hisashi Mimura Satoshi Kitamura Teruyuki Kitagawa Shigetaka Kohda 《Colloids and surfaces. B, Biointerfaces》2002,26(4):397-406
FK041 crystallizes as a non-stoichiometric hydrate or as solvated hydrates which were characterized as isomorphic clathrates by powder X-ray diffractometry. Moisture and organic solvent vapor sorption studies, differential scanning calorimetry and thermogravimetric analysis revealed that FK041 monohydrate forms a physically stable host crystal, which has lattice channels for guest water and/or organic solvent molecules. The hydration state varies non-stoichiometrically between dihydrate and tetrahydrate depending on the relative humidity and the mol content of the co-existing organic solvent, that is 2-propanol, ethanol, or acetone. These organic solvents are thought to replace a part of originally present water with a mol ratio of 1:3. 2-Propanol exhibited the most stable solvation, indicating that the size and shape of 2-propanol are the most preferable to the lattice channels. 相似文献
14.
Tokuyama R Takahashi Y Tomita Y Suzuki T Yoshida T Iwasaki N Kado N Okezaki E Nagata O 《Chemical & pharmaceutical bulletin》2001,49(4):347-352
A structure-activity relationship (SAR) study on 5-substituted oxazolidinones as an antibacterial agent is described. The oxazolidinones, of which 5-acetylaminomethyl moiety was converted into other functions, were prepared and evaluated for antibacterial activity. Elongation of the methylene chain (8) and conversion of the acetamido moiety into guanidino moiety (12) decreased the antibacterial activity. The replacement of carbonyl oxygen (=O) by thiocarbonyl sulfur (=S) enhanced in vitro antibacterial activity. Especially, compound 16, which had the 5-thiourea group, showed 4-8 stronger in vitro activity than linezolid. Our SAR study revealed that the antibacterial activity was greatly affected by the conversion of 5-substituent. 相似文献
15.
The reaction of 2-dialkylamino-5-phenyl-1,3-oxathiolium cation 1 with sulphur ylides 2 was investigated. The behavior of 1 was solvent-dependent. In CH2Cl2, carbamate ester 3 was obtained: in CH3CN, the intermediate sulphonium salt 5, which on hydrolysis gave 3, was isolated. On the other hand, reaction in MeOH gave the diphenacyl derivative 6. The reaction mechanism is discussed. 相似文献
16.
Tsujita H Ura Y Wada K Kondo T Mitsudo TA 《Chemical communications (Cambridge, England)》2005,(40):5100-5102
2-(1-Alkoxycarbonyl)alkylidenetetrahydrofurans were readily synthesized by the codimerization of 2,3- or 2,5-dihydrofurans with alpha,beta-unsaturated esters using a zerovalent Ru catalyst, Ru(cod)(cot), with high regio- and stereoselectivity. 相似文献
17.
Glatz Z Marini MV Wimmerová M Damborský J Nagata Y 《Journal of chromatography. A》2000,895(1-2):219-225
A new sensitive method has been developed for the determination of haloalkane dehalogenase activity. The enzymatic reactions were carried out directly in thermostatted autosampler vials and the formation of product - bromide or chloride ions - was monitored by sequential capillary zone electrophoresis runs. The determinations were performed in a 75 microm fused-silica capillary using 5 mM chromate, 0.5 mM tetradecyltrimethylammonium bromide (pH 8.4) as a background electrolyte, separation voltage 15 kV (negative polarity) and indirect detection at sample wavelength 315 nm, reference wavelength 375 nm for brominated and chlorinated substrates, respectively 0.1 M beta-alanine-HCl (pH 3.50) as a background electrolyte, separation voltage 18 kV (negative polarity) and direct detection at 200 nm for brominated substrates. The temperature of capillary was in both cases 25 degrees C. The method is rapid, can be automated, and requires only small amount of enzyme preparation and substrate. 相似文献
18.
Fe/M (M = Ag, Zn and Sn) multilayers prepared by a vacuum evaporation method are studied by Mössbauer spectroscopy (MS), Rutherford backscattering spectroscopy (RBS) and X-ray diffraction (XRD). For the case of an M = Ag multilayer, MS reveals that Fe in the multilayer remains as an-phase down to the layer thickness of 10 nm. This result is in agreement with the RBS result that Fe and Ag form a completely discrete layer structure without any mutual mixing. For the case of M = Zn and Sn, RBS reveals that a considerable mixing has taken place between Fe and Sn during the specimen preparation. MS on Fe/Sn specimens with different layer thickness shows that an alloy phase of about 5 nm thickness is formed at the interface. Structural as well as magnetic properties of the alloy phase are discussed based on MS at different temperatures and on reported results of the intermetallic compound FeSn. 相似文献
19.
20.
Sagara T Nagata K Tsuruta H Nakashima N 《Chemical communications (Cambridge, England)》2002,(18):2116-2117
Potential-dependent dynamic adsorption-desorption and reorientation of a 4-pyridyl modified PAMAM G2 dendrimer at a Au(111) electrode has been demonstrated. 相似文献