全文获取类型
收费全文 | 8506篇 |
免费 | 740篇 |
国内免费 | 233篇 |
专业分类
化学 | 9114篇 |
晶体学 | 24篇 |
力学 | 3篇 |
综合类 | 23篇 |
数学 | 3篇 |
物理学 | 312篇 |
出版年
2024年 | 6篇 |
2023年 | 89篇 |
2022年 | 241篇 |
2021年 | 223篇 |
2020年 | 275篇 |
2019年 | 218篇 |
2018年 | 212篇 |
2017年 | 187篇 |
2016年 | 310篇 |
2015年 | 301篇 |
2014年 | 359篇 |
2013年 | 689篇 |
2012年 | 453篇 |
2011年 | 428篇 |
2010年 | 383篇 |
2009年 | 435篇 |
2008年 | 435篇 |
2007年 | 506篇 |
2006年 | 448篇 |
2005年 | 442篇 |
2004年 | 449篇 |
2003年 | 354篇 |
2002年 | 496篇 |
2001年 | 181篇 |
2000年 | 159篇 |
1999年 | 125篇 |
1998年 | 131篇 |
1997年 | 118篇 |
1996年 | 107篇 |
1995年 | 104篇 |
1994年 | 65篇 |
1993年 | 68篇 |
1992年 | 70篇 |
1991年 | 55篇 |
1990年 | 47篇 |
1989年 | 36篇 |
1988年 | 42篇 |
1987年 | 32篇 |
1986年 | 16篇 |
1985年 | 21篇 |
1984年 | 36篇 |
1983年 | 15篇 |
1982年 | 22篇 |
1981年 | 19篇 |
1980年 | 12篇 |
1979年 | 24篇 |
1978年 | 8篇 |
1972年 | 5篇 |
1970年 | 4篇 |
1969年 | 4篇 |
排序方式: 共有9479条查询结果,搜索用时 15 毫秒
121.
M. Katkevics E. Korchagova T. Ivanova V. Slavinska E. Lukevics 《Chemistry of Heterocyclic Compounds》2004,40(6):734-741
New 3-hydroximino-1-hydroxy-2-pyrrolidinones have been synthesized by the interaction of NH2OH and NH2OBn with the methyl esters of 2-oxo-3-butenoic acid derivatives. Some intermediate compounds have been isolated and identified and the reaction mechanism is discussed. 相似文献
122.
Zafar Ahmed 《Tetrahedron》2005,61(8):2055-2063
A number of pulvinic acid natural products were prepared based on Suzuki cross coupling reactions of α-hydroxy-γ-alkylidenebutenolides which are readily available by cyclization of 1,3-bis-silyl enol ethers with oxalyl chloride. The formal total synthesis of pulvinic acid, atromentic acid, gomphidic acid and of 4-hydroxypulvinic acid, 4′-hydroxypulvinic acid and iso-gomphidic acid are reported. In addition, total syntheses of pinastric acid, xerocomic acid and variegatic acid were accomplished. 相似文献
123.
Piyarat Thanakoses Nagat Abd Alla Mostafa Mark T. Holtzapple 《Applied biochemistry and biotechnology》2003,107(1-3):523-546
Using the MixAlco process, biomass can be converted into carboxylic acids, which can be chemically converted into mixed alcohol
fuels. This study focused on the use of countercurrent fermentation to anaerobically convert sugarcane bagasse and chicken
manure to mixed carboxylic acids using a mixed culture of mesophilic microorganisms from terrestrial and marine sources. Bagasse
was pretreated with lime to increase digestibility. The continuum particle distribution model (CPDM) simulated continuous
fermentors based on data collected from batch experiments. This model saves considerable time in determining optimum operating
conditions. For an 80% bagasse/20% chicken manure fermentation with terrestrial inoculum at a volatile solids loading rate
(VSLR) of 7.36 g/(L of liquid·d) and a liquid residence time (LRT) of 8.88 d, total carboxylic acid productivity, total acid
selectivity, and yield were 2.49 g/(L of liquid·d), 0.581 g of total acid/g of VS digested, and 0.338 g of total acid/g of
VS fed, respectively, at a concentration of 18.7 g of total acid/L. At the same VSLR and LRT, fermentation with marine inoculum
gave higher total acid productivity, total acid selectivity, and yield than fermentation with terrestrial inoculum. For an
80% bagasse/20% chicken manure fermentation with marine inoculum at a VSLR of 3.83 g/(L of liquid·d) and an LRT of 12.1 d,
total carboxylic acid productivity, total acid selectivity, and yield were 1.38 g/(L of liquid·d), 0.667 g of total acid/g
of VS digested, and 0.359 g of total acid/g of VS fed, respectively, at a concentration of 16.2 g of total acid/L. 相似文献
124.
V. G. Badelin E. Yu. Tyunina G. V. Girichev N. I. Giricheva O. V. Pelipets 《Journal of Structural Chemistry》2007,48(4):647-653
This paper reports on our mass spectrometric study of sublimation of glycine and DL-alanyl-glycine (Ala-gly). The sublimation enthalpy of Ala-gly has been determined by generalization of the data obtained and the results of AM1 quantum-chemical calculations. A relationship has been found between the sublimation enthalpy (ΔH subl), heat capacity (C P), and the sum of bond lengths (Σn i l i ) in 17 α-amino acid and 9 dipeptide molecules. Correlations are suggested for evaluating ΔH subl of amino acids and peptides. 相似文献
125.
Polysiloxane formation in dimethyldiethoxysilane (DMDES)-ethyl alcohol (EtOH(D))-oxalic acid (OA) (DMDES:EtOH(D):OA=1:2:0.5) and DMDES-dimethylsulfoxide (DMSO)-OA (DMDES:DMSO:OA=1:2:0.5) systems was investigated by gas chromatography-mass spectrometry and 29Si-nuclear magnetic resonance. While the DMDES-EtOH(D)-OA system was homogeneous, the DMDES-DMSO-OA system consisted of two immiscible phases. In both systems, ethoxy-terminated linear oligomers ((EtO)Me2SiO(Me2SiO)nSiMe2(OEt); n=0–4, Et = C2H5, Me = CH3) and cyclic tetramer ((Me2SiO)4) were identified. The reaction mechanism for polysiloxane formation is discussed. 相似文献
126.
Full details are given for a modified Mitsunobu approach to the formation of N-alkylated 1,2,4-dithiazolidine-3,5-diones 2 from a wide range of alcohols 10 with predominantly, inversion of configuration. The resulting products 2 can be regarded as protected isocyanates 6. 相似文献
127.
Summary An HPLC reversed-phase ion interaction reagent method is presented, which makes use of chiral compounds as the interaction
reagents. By employing the optical isomeric forms of malic, tartaric and mandelic acids as the interaction reagents, a good
separation of D(−) and L(+) ascorbic acid has been achieved. The method has also been applied to the identification of vitamin
C in some medical formulations. The separation of te enantiomeric forms of DL malic acid has been attempted and a shor discussion
is presented about the elution sequence in chromatographic separation of D- and L-enantiomers. 相似文献
128.
The carrier mediated transport of cations using model compounds of the verdinoid and rubinoid bile pigment structural type as ionophores is studied. Verdinoid bile pigments turn out to be very effective carriers for cations exhibiting a pronounced selectivity for certain transition metal ions like Cr+-+, Fe++, Cu++ and Zn++. The scope of this behaviour of verdinoid bile pigments is compared to [18]-crown-6 and meso-tetraphenylporphin and is discussed with regard to structural, biological, analytical and technical implications. 相似文献
129.
CuBr2-catalyzed three-component coupling of N-benzylallylamine, ethyl glyoxalate, and terminal alkynes afforded glycinate-tethered 1,6-enynes, which were further transformed into polycyclic pyrrole-2-carboxylates via iridium-catalyzed cycloisomerization/Diels-Alder cycloaddition/dehydrogenation sequence under conventional and microwave heating conditions. The corresponding β-amino acid analogues were obtained from the Ir-catalyzed reaction of enynes prepared from lithium amides of allylamines with methyl non-2-en-4-ynoate. The Cu-catalyzed Mannich-type condensation was further extended to the synthesis of the glycinate-tethered dienyne and cyclopropylenyne, which were subjected to Rh-catalyzed cycloisomerizations to furnish bicyclic amino acids. 相似文献
130.
S. Jaenicke 《Catalysis Surveys from Asia》2005,9(3):173-185
The following report gives an overview on work done in the Catalysis Laboratory of the Department of Chemistry, National University
of Singapore over the last 15 years (1989–2004). Much of this work can be described as “characterization of catalytically
active surfaces through test reactions”. The methods, systems studied and the reactions that we evaluated will be described.
The review will mostly concentrate on work from the authors’ laboratory, but other relevant work will also be cited. 相似文献