全文获取类型
收费全文 | 8500篇 |
免费 | 755篇 |
国内免费 | 225篇 |
专业分类
化学 | 9106篇 |
晶体学 | 24篇 |
力学 | 3篇 |
综合类 | 23篇 |
数学 | 4篇 |
物理学 | 320篇 |
出版年
2024年 | 6篇 |
2023年 | 88篇 |
2022年 | 241篇 |
2021年 | 226篇 |
2020年 | 277篇 |
2019年 | 215篇 |
2018年 | 210篇 |
2017年 | 186篇 |
2016年 | 312篇 |
2015年 | 302篇 |
2014年 | 359篇 |
2013年 | 684篇 |
2012年 | 458篇 |
2011年 | 433篇 |
2010年 | 390篇 |
2009年 | 432篇 |
2008年 | 441篇 |
2007年 | 508篇 |
2006年 | 450篇 |
2005年 | 448篇 |
2004年 | 457篇 |
2003年 | 353篇 |
2002年 | 494篇 |
2001年 | 184篇 |
2000年 | 157篇 |
1999年 | 125篇 |
1998年 | 128篇 |
1997年 | 118篇 |
1996年 | 103篇 |
1995年 | 103篇 |
1994年 | 66篇 |
1993年 | 69篇 |
1992年 | 66篇 |
1991年 | 54篇 |
1990年 | 45篇 |
1989年 | 34篇 |
1988年 | 42篇 |
1987年 | 29篇 |
1986年 | 14篇 |
1985年 | 20篇 |
1984年 | 35篇 |
1983年 | 13篇 |
1982年 | 22篇 |
1981年 | 15篇 |
1980年 | 15篇 |
1979年 | 19篇 |
1978年 | 8篇 |
1972年 | 5篇 |
1969年 | 4篇 |
1968年 | 3篇 |
排序方式: 共有9480条查询结果,搜索用时 15 毫秒
91.
Stephan D. Stamatov 《Tetrahedron letters》2006,47(15):2543-2547
Glycidyl esters and ethers undergo a regioselective and stereospecific opening of the oxirane ring upon treatment in chloroform in the presence of pyridine with trimethylsilyl halide (TMSX, X = Cl, Br, or I) and a mixture of carboxylic acid (CA)-trifluoroacetic anhydride (TFAA), to produce the corresponding C2-O-acylated vicinal halohydrins in high yields. 相似文献
92.
A gas chromatographic method was developed for the determination of low-molecular-weight carboxylic acids in aqueous samples based on a derivatization procedure compatible with aqueous solutions. The technique uses nitrogen-selective detection with a thermionic-specific detector after derivatization of carboxylic acids as 2- nitrophenylhydrazides. The hydrazides were extracted with ethyl acetate prior to injection into the gas chromatograph. The derivatives appear to be stable in ethyl acetate at 0–5° C for long periods and, therefore, can be stored for analysis at a later date. The detection limits of different short-chain acids are in the range 0.8–1.4 pmol per injected sample. The relative standard deviation is less than 10% at the 1 μM level. Examples of the use of the method are given for the determination of carboxylic acids in anoxic marine sediment pore waters, coastal sea water and Black Sea water samples. 相似文献
93.
Bolei Yan Simon W. Lewis Paul J. Worsfold J.Steven Lancaster Anthony Gachanja 《Analytica chimica acta》1991
An overview of liquid phase chemiluminescence (CL) processes is presented and the potential for CL detection in liquid chromatography (LC) is discussed, with particular reference to the luminol and peroxyoxalate reactions. Post column ion displacement from a solid phase reagent [a cation exchange resin in the copper(II) form] followed by catalysis of the luminol reaction is used for the quantification of mixtures of weak acids after separation by ion-exclusion chromatography. Polyaromatic hydrocarbons (PAHs) released during the combustion of biomass fuels are separated by reversed-phase chromatography and quantified by their sensitizing effect on the peroxyoxalate reaction. This procedure is also used for the determination of carboxylic acids in non-aqueous media after selective pre-column derivatization with a fluorescent label (9-anthracenemethanol). 相似文献
94.
Sunil B. Jadhav 《Tetrahedron letters》2007,48(14):2485-2487
A rapid, mild and high yielding method for the synthesis of methylthiomethyl esters is reported from the corresponding aliphatic, aromatic and unsaturated carboxylic acids under Swern oxidation conditions using dimethylsulfoxide, oxalyl chloride and triethylamine at low temperature. 相似文献
95.
Zirconyl chloride promoted highly efficient solid phase synthesis of amide derivatives 总被引:1,自引:0,他引:1
Cherkupally Sanjeeva Reddy Adki Nagaraj Pochampally Jalapathi 《中国化学快报》2007,18(10):1213-1217
An efficient solid phase route for the synthesis of amide derivatives by the reaction of carboxylic acids with urea in the presence of catalytic amount of zirconyl chloride under microwave irradiation conditions was described.In this way,a range of interesting amide derivatives was obtained in good to excellent yields.The catalyst was recycled with fresh reactants and it gave almost similar results without significant loss of activity up to the third run. 相似文献
96.
Characteristic features of reactions involving esters of phosphorus-containing acids in highly organized media (micelles, liquid crystals, vesicles, and emulsions) are surveyed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 298–312, February, 1996. 相似文献
97.
The development of analytical methods for routine simultaneous identification and quantification of carboxylic fatty acids (CFAs) are required in different fields, such as, pharmaceutical cosmetics, food products and formulations of water–microemulsion–oil systems. Determination of CFAs has been developed mainly by gas chromatography (GC). As an alternative to GC, liquid chromatography (LC) has better sensitivity and selectivity. However, most CFAs show no useful absorption in ultraviolet–violet (UV–Vis) region, one of the more used detection technique in high-performance liquid chromatography (HPLC). In order to allow the use of UV–Vis detection, the use of pre-column derivatization has been reported to increase sensitivity and selectivity. Therefore, establishment of a simpler and faster on-line method with complete separation is needed for the screening of large numbers of samples. 2,4-Dinitrophenylhydrazine (2,4-DNPH.), benzoil chloride (BC), and phenylhydrazine (PH) were used for derivatization of different FAs by microwaves radiation (MW). After the on-line derivatization, products were separated and quantified by HPLC. Reactor coil was placed inside of microwaves oven at 450 W. Parameters as flow, amount of reagents, irradiation time, and chromatographic conditions were optimized. The continuous analysis using the MW–HPLC–UV system provided high sensitivity and reduced both the amount of reagent used and the analysis times. This proposed method can be used for the routine analysis of FAs contained in water–microemulsion–oil systems, to quantify the total acid fraction in each phase. 相似文献
98.
Chemistry and biology of the acylneuraminic acids 总被引:4,自引:0,他引:4
R Schauer 《Angewandte Chemie (International ed. in English)》1973,12(2):127-138
Acylneuraminic acids occupy an outstanding position both sterically and with respect to biological functions in gangliosides and in many glycoprotein molecules. Acylneuraminic acids are N-acylated and partly O-acylated derivatives of neuraminic acid, a polyhydroxyamino keto acid that has the properties of a carbohydrate and is therefore capable of forming a glycoside linkage. The present report provides a survey of our knowledge of the chemistry and biology of the acylneuraminic acids, which occur widely in particular in the animal kingdom. 相似文献
99.
The liquid chromatographic enantiomer separation of N-fluorenylmethoxycarbonyl (FMOC) protected alpha-amino acids and their ethyl ester derivatives was performed on polysaccharide-derived chiral stationary phases, Chiralcel OD, Chiralpak AD, and Chiralpak AS. In general, Chiralcel OD and Chiralpak AD showed good performance for resolution of N-FMOC alpha-amino acids and their ethyl esters, respectively. All investigated N-FMOC alpha-amino acid enantiomers were baseline separated on Chiralcel OD or Chiralpak AD, whereas N-FMOC alpha-amino acid ethyl ester enantiomers were baseline resolved (alpha = 1.15-3.03) on Chiralpak AD, except for two analytes. The L-enantiomers of all examined FMOC alpha-amino acid ethyl ester derivatives are preferentially retained on Chiralpak AD, while the elution orders of the other enantiomer separations are not consistent. 相似文献
100.
We have demonstrated that the readily available amido-keto compounds 5, with prearranged carbonyl and glycine moieties, under strongly basic conditions easily undergo complete and highly diastereoselective cyclization, affording a generalized and practical access to the conformationally constrained phenylserine derivatives 4. High chemical yields, virtually complete diastereoselectivity combined with the operational convenience of the experimental procedures render this method useful for preparation of these diastereomerically pure derivatives. 相似文献