全文获取类型
收费全文 | 3684篇 |
免费 | 142篇 |
国内免费 | 23篇 |
专业分类
化学 | 2545篇 |
晶体学 | 31篇 |
力学 | 82篇 |
数学 | 589篇 |
物理学 | 602篇 |
出版年
2023年 | 28篇 |
2022年 | 51篇 |
2021年 | 117篇 |
2020年 | 73篇 |
2019年 | 84篇 |
2018年 | 73篇 |
2017年 | 55篇 |
2016年 | 125篇 |
2015年 | 113篇 |
2014年 | 136篇 |
2013年 | 228篇 |
2012年 | 220篇 |
2011年 | 310篇 |
2010年 | 181篇 |
2009年 | 161篇 |
2008年 | 220篇 |
2007年 | 215篇 |
2006年 | 182篇 |
2005年 | 169篇 |
2004年 | 145篇 |
2003年 | 109篇 |
2002年 | 121篇 |
2001年 | 55篇 |
2000年 | 50篇 |
1999年 | 39篇 |
1998年 | 29篇 |
1997年 | 29篇 |
1996年 | 33篇 |
1995年 | 32篇 |
1994年 | 36篇 |
1993年 | 35篇 |
1992年 | 20篇 |
1991年 | 27篇 |
1990年 | 25篇 |
1989年 | 19篇 |
1988年 | 22篇 |
1987年 | 17篇 |
1986年 | 29篇 |
1985年 | 31篇 |
1984年 | 28篇 |
1983年 | 24篇 |
1982年 | 28篇 |
1981年 | 20篇 |
1980年 | 15篇 |
1979年 | 14篇 |
1978年 | 15篇 |
1977年 | 9篇 |
1976年 | 7篇 |
1975年 | 9篇 |
1974年 | 9篇 |
排序方式: 共有3849条查询结果,搜索用时 78 毫秒
71.
Laura M. Bruno José L. de Lima Filho Eduardo H. de M. Melo Heizir F. de Castro 《Applied biochemistry and biotechnology》2004,113(1-3):189-199
Mucor miehei lipase was immobilized on magnetic polysiloxane-polyvinyl alcohol particles by covalent binding with high activity recovered.
The performance of the resulting immobilized biocatalyst was evaluated in the synthesis of flavor esters using heptane as
solvent. The impact on reaction rate was determined for enzyme concentration, molar ratio of the reactants, carbon chain length
of the reactants, and alcohol structure. Ester synthesis was maximized for substrates containing excess acyl donor and lipase
loading of 25 mg/mL. The biocatalyst selectivity for the carbon chain length was found to be different concerning the organic
acids and alcohols. High reaction rates were achieved for organic acids with 8 or 10 carbons, whereas increasing the alcohol
carbon chain length from 4 to 8 carbons gave much lower esterification yields. Optimal reaction rate was determined for the
synthesis of butyl caprylate (12 carbons). Esterification performance was also dependent on the alcohol structure, with maximum
activity occurring for primary alcohol. Secondary and tertiary alcohols decreased the reaction rates by more than 40%. 相似文献
72.
Eduardo CortS Corts Carlos M. Alcocer Castrejn 《Journal of heterocyclic chemistry》1997,34(6):1809-1812
A series of twelve new 2,3-dihydro-4-(para-substituted-phenyl)-7-[(o-, m-, and p-substituted)phenoxy]-1H-1,5-benzodiazepine-2-thiones, which have potentially useful pharmacological properties, has been synthesized by condensing the 3,3-dimercapto-1-(para-substituted phenyl)-2-propen-1-one with 3,4-diaminophenyl-R-phenyl ethers. The structure of all products was corroborated by ir, 1H-nmr, 13C-nmr and ms. 相似文献
73.
A flow-through voltammetric cell with a hanging mercury drop electrode has been developed to fit the static mercury drop electrode (PAR 303). The design has resulted in a linear increase of sensitivity with flow rate and an enhancement of sensitivity by the wall-jet effect. The cell is used in a flow injection system in which samples are introduced with a R??i?ka—Hansen injector. The mercury drop is held at plating potentials while the sample peak passes through the cell. Stripping is done under stopped flow conditions, to reduce noise, after the sample has been washed completely from the cell. The stripping thus takes place into the carrier electrolyte which always has a constant composition independent of sample constituents. Film-forming interfering species will, however, remain on the surface of the mercury drop. The effect of medium exchange on films produced by l-cysteine is reported. The flow-through medium exchange simplifies deaeration, speeds up analysis and reduces contamination. 相似文献
74.
The flash photolysis of purine in acetonitrile and in water at different pH was studied. The transients produced on flash excitation of degassed aqueous solutions have been identified as the triplet excited state, the hydrated electron, a purine radical cation and radical anion on the basis of quenching experiments and comparison to transients observed in low temperature photolysis. 相似文献
75.
Eduardo Corts Corts Eduardo Corts Romero Alicia Domínguez Taylor 《Journal of heterocyclic chemistry》1994,31(4):725-727
The preparation of nine novel 6H-2,7,7-trimethyl-4(o-,p-R-phenylamino)-7,8-dihydrofuro[3,2-c]-azepines with possible pharmacological activity is described. The structure of all products was corroborated by ir, 1H-nmr and ms. 相似文献
76.
Y. BallesterosM.J. Gonzalez de la Huebra M.C. QuintanaP. Hernandez L. Hernandez 《Microchemical Journal》2003,74(2):193-202
The electrochemical behaviour of kinetin (6-furfurylaminopurine) on a carbon paste modified with OV-17 silicone electrode, is studied. The determination of kinetin is possible working in square wave voltammetric techniques, reaching limits of determination of 38.7 ng ml−1. The proposed method was successfully applied to determine the cytokinin in extracts of apples (previously spiked with kinetin) and the obtained results were in accordance with the results obtained with HPLC-UV. 相似文献
77.
Eduardo A. Véliz 《Tetrahedron letters》2006,47(18):3153-3156
Herein, we report the use of triisopropyl phosphite (TIP) as an effective substitute for triphenylphosphine in the Mitsunobu reaction of nucleoside analogs. In addition, the use of triphenyl phosphite as an alternative reagent for the expensive hexamethylphosphorous triamide (HMPT) in the Véliz-Beal bromination protocol is reported. 相似文献
78.
A. Alberola L. F. Antolin A. M. Gonzalez M. A. Laguna F. J. Pulido 《Journal of heterocyclic chemistry》1986,23(4):1035-1038
The base-induced ring cleavage of 4-nitro-(Ia), 4-ethoxycarbonyl- (Ib) and 4-acetyl-5-methylisoxazole (Ic) and the conversion of the resulting β-cyanoenolates and β-enaminonitriles into 5-aminoazoles and 4-cyanoazoles was studied. 相似文献
79.
De la Fuente JR Jullian C Saitz C Neira V Poblete O Sobarzo-Sánchez E 《The Journal of organic chemistry》2005,70(22):8712-8716
[reaction: see text] Photoreduction of oxoisoaporphine dyes occurs via a stepwise mechanism of electron-proton-electron transfer that leads to the N-hydrogen oxoisoaporphine anion. When triethylamine, TEA, was used as the electron donor in anaerobic conditions, 1-diethylaminobutadiene, DEAB, was one of the oxidation products of TEA, among diethylamine and acetaldehyde. DEAB was identified by (1)H NMR and GC-MS experiments by comparison with the authentic 1-diethylaminobutadiene. This is the first report of a butadienyl derivative formed in the dye-sensitized photooxidation of TEA. In addition, isotopic exchange experiments with TEA-d(15) and D(2)O show that the hydrogens at carbon-2 and carbon-4 of the butadienyl moiety are exchangeable. The observed isotopic exchange pattern could be explained by the head-to-tail coupling of an N,N-diethylvinylamine intermediate that exchanges hydrogens at the C-beta via the enammonium ion. 相似文献
80.
The synthesis, characteristics, properties and reactions with metallic ions of six aromatic derivatives of 2-thiohydantoin have been studied. The reagents exhibit two pK values in aqueous solution, and with Pd(II), Au(III), Ag(I) and Hg(I) and (II), especially with the first, form stable complexes with molar absorptivities adequate for spectrophotometric measurements. 相似文献